Category: EARS

Mini-Workshop Report | From rifting to drifting: evidence from rifts and margins worldwide

AGU Fall Meeting 2015, San Francisco, USA

Conveners: Rebecca Bendick1, Ian Bastow2, Tyrone Rooney3, Harm van Avendonk4, Jolante van Wijk5

1University of Montana, 2Imperial College London, 3Michigan State University, 4Univ. Texas Institute for Geophysics, UT-Austin, 5New Mexico Tech

On Sunday December 13, 2015, from 8am to 1:30pm, a representative cross section of researchers interested in rifting met in the Grand Hyatt San Francisco before the AGU Fall Meeting. Our primary focus was to facilitate discussion on the current state of research into continental extension. Our aim was to be broadly inclusive by bringing an audience with widely varying backgrounds to a common understanding of the state of the art in this field. Our ultimate goal was to initiate a discussion on future research challenges for the community and how these challenges align with the existing science plans for the GeoPRISMS Eastern North America and East African Rift Focus Sites. To facilitate community building and cross disciplinary linkages, the meeting was coordinated with the STEPPE consortium (Sedimentary Geology, Time, Environment, Paleontology, Paleoclimatology, Energy) workshop investigating source-to-sink processes of the Lake Tanganyika rift (East Africa), which took place directly following the GeoPRISMS workshop from 2 to 8pm.

The meeting was structured to allow for discussion under four broad subheadings:

Topic 1: Melt Generation in Extensional Environments

A 30 minute introduction to this topic was presented by Tyrone Rooney. The talk covered the historical context of rifting studies and then focused on the relationship between magma and lithospheric strength. The concept of magma within the lithosphere facilitating rifting was introduced. The presentation examined how magmas provide an important temporal record of mantle processes during extension. It was shown how thermochemical constraints of the upper mantle source region of rift magmas could be probed with erupted lavas. In particular, the dual challenges of mantle potential temperature and pyroxenites in the upper mantle were highlighted as important frontiers in our understanding of mantle melting processes. The role of volatiles in some rifting environments (Rio Grande Rift) was introduced. The role of magmas in influencing seismic images of the upper mantle and also acting as a mechanism of strain accommodation during late stage rifting was also discussed. Finally, an examination of the continental lithospheric mantle as a possible magma source was also presented.
The discussion, moderated by Harm van Avendonk, first explored the issue of the role of water in magma generation processes. In particular, there were questions asked about the storage of water in water-bearing phases but also the ability of olivine to store volatiles. Further discussions continued on the role of hydrous phases on lithospheric rheology. The first key question arising from these discussion was – where could volatiles reside and how much in the source of rift magmas (especially water and carbon dioxide). Suggestions on approaching this question through studies of xenoliths and reconstructing lithospheric architecture were made. The second key question focused on the role of structural inheritance. It was acknowledged that crustal heterogeneity and mantle lithosphere heterogeneity may not necessarily correspond. Finally the third key question related to the amount of melt generation with the timing and magnitude of stretching.

From rifting to drifting: evidence from rifts and margins worldwide | December 13 AGU 2015

Topic 2: Magma-lithosphere interaction

A 30 minute introduction to this topic was presented by Chris Havlin. This presentation first delivered an overview of the physics and thermodynamics of melt transport. This was further subdivided on the basis of porous flow within the mantle and lithosphere and in terms of crustal fractures and channels and how lithospheric inheritance influenced melt transport. The porous flow concept was expanded to examine the dependence on pressure gradients, buoyancy and dynamic pressure. The concept of a ‘freezing boundary’ was raised in terms of a melt focusing mechanism, which if dipping, could redistribute melt. Within the lithosphere the concept of lithospheric and crustal fabrics was raised. It was acknowledged that grain size may affect porosity and surface tension. As a result, melt is preferentially directed into smaller grain size domains. The presentation also examined end-member models of strain i.e. whole lithospheric heating, and basal heating and impact of the porosity front shallowing over time creating an effective thinning of the lithosphere. Finally, it was shown that there could be a growing zone of modified lithosphere whereby mechanically it behaves as does the asthenosphere but chemically it may still resemble the lithospheric mantle.
The discussion, moderated by Ian Bastow, first examined the concept of the background state of stress in rifting environments and how stress may change with changes in viscosity. It was noted that thinning does not require large extensional stresses. A point was raised on the competing grain size effects on porosity and surface area in relation to bulk permeability. Questions were raised by the group as what happens in relation to thinning and melt alteration of the lithosphere in seemingly amagmatic rift segments. It was acknowledged, however, that segments defined as amagmatic due to a lack of surface volcanism may still possess significant melt at depth within the lithosphere. As a result of these discussions, two key questions arose: (1) What is the role of melt in magmatic and amagmatic (in terms of surface volcanism) rift segments? and (2) What are the feedbacks between melt transport and lithospheric thinning and what are the mechanisms?

Topic 3: Stretching the lithosphere

A 30+ minute introduction to this topic was presented by Suzon Jammes. The presentation first examined the concept of mechanical stretching and the genetic relationship of stretching as an important factor in the Wilson Cycle. The factors controlling this mechanical stretching focused on exhumation, tectonic inheritance, and the control of rift and margin architecture. The topic of depth-dependant stretching was examined and how vertical decoupling was incompatible with pure and simple shear endmembers. An introduction to time-dependant stretching mechanisms followed with some idealized cross section of basinward migration of deformation. Dr. Jammes presented an evolutionary model whereby mechanical stretching was followed by the creation of a ‘necking zone’ for major crustal thinning and finally an exhumation phase. The discussion continued into a discussion of how rifting processes are determined by rheological layering of the lithosphere and the impact of structural inheritance and sensitivity to this vertical layering.

The discussion, moderated by Rebecca Bendick, was more limited due to time constraints but did establish a key question of how the feedbacks with melting might vary in terms of the recognized global variety of architectures of rifts and rifted margins.

Topic 4: Melt delivery and focusing

A 30 minute introduction to this topic was presented by Derek Keir. Dr. Keir showed how within the East African Rift changes in mantle potential temperature are probable first order controls on magma supply. It was also shown how variations in magmatism are multi-scalar with lateral variation at several scales both in the presence and absence of melt and melt chemistry. There was a view that melt pathways and focusing might represent the best mechanism for generating smaller scale variability and examples from the Black Sea and Afar were shown. Afar provided a particularly interesting case as in this region it was show that volcanism responded to increasing subsidence. That is, the more the thinning, the more melt and thus more melt focusing. Dr. Keir showed how a mantle potential temperature anomaly of at least 100 degrees could help explain observed seismic velocities and also the presence of melt throughout the region. A comparison was made between Afar and slow spreading ridges and also to Krafla (Iceland) between 1975 and 1984. The discussion continued as to the impact of melt focusing in time and space and how it is influenced by the temporal accumulations of tectonic stresses. The result of this was described as a general migration of volcanism from the rift flanks towards the rift axis with the competing tectonic and gravitational stresses.

The discussion, moderated by Jolante van Wijk, examined comparisons between the Havlin models discussed in topic 2 and those presented by Keir in topic 4. Some discussion centered on the concept of focusing at the lithosphere-asthenosphere boundary and then subsequent defocusing within the crust. It was acknowledged that geochemical data were critical to address these issues. It was noted that magmatic sources clearly differ along strike within the rift and thus are inconsistent with a single centralized source.

From rifting to drifting: evidence from rifts and margins worldwide | December 13 AGU 2015

Broad discussion

Following a break, the group reconvened to try and systematize some of the key concepts raised. The issues can be summarized as follows:

1. Rift Initiation
What is the role of mantle plumes?
How can mechanical heterogeneity facilitate initial rifting?
What role does chemical heterogeneity in the lithospheric mantle control initial extension?
What is the initial thermo-chemical structure of the lithosphere and asthenosphere in a nascent rift?
What does incipient rifting look like? Okavango suggests preexisting structure critical.
Is this a top down or bottom up process? How does extension propagate?

2. Evolution of rifting in time and space
Why do rifts ultimately fail?
What is the role of nonlinear feedbacks?
How can datasets from igneous petrology and the sedimentary record provide a temporal insight into rift evolution?
What is the time evolution of strain?

3. Rift Architecture
How do non-uniqueness issues create difficulties in creating global models of rift evolution?
How can real constrains be linked with ever more innovative and detailed simulations?
What variables control the strength of the lithosphere?
What is the role of far-field vs. local controls on strain and rift evolution?

4. Volatiles in extensional environments
What are the volatile pathways from depth to the surface?
How deep are the volatiles derived from?
What is the role of rift valley volcanoes in global production of volatiles (e.g., CO2, SO2)?
How can lithospheric heterogeneity and inheritance influence the volatile budget?

In summary the basic concepts on which the group agreed that were critical for GeoPRISMS were:

  1. What is the history of melt? Where is it formed, when is it formed, why is it formed, how is it focused, and what pathways does it take through the lithosphere?
  2. What is the material (thermal and chemical) heterogeneity in the rift lithosphere? How does inheritance play a role, is there spatial organization at play, and how can we assess the importance of these heterogeneities to rifting?
  3. Comparison of focus areas is needed. How do ENAM and the EAR differ and how are they similar? What can be learned from focused studies at both sites?

Go to the Mini-Workshop webpage

Reference information

From rifting to drifting: evidence from rifts and margins worldwide, R. Bendick, I. Bastow, T. Rooney, H. Van Avendonk, J. van Wijk

GeoPRISMS Newsletter, Issue No. 36, Spring 2016. Retrieved from http://geoprisms.nineplanetsllc.com

Workshop Report | NSF-GeoPRISMS Rift Initiation and Evolution Theoretical and Experimental Institute

Tobias Fischer1, Donna Shillington2
1University of New Mexico, 2LDEO, Columbia University

The GeoPRISMS Theoretical and Experimental Institute (TEI) for the Rift Initiation and Evolution (RIE) initiative was held February 8-10, 2017 in Albuquerque, NM. This meeting brought together 132 scientists with diverse expertise working on rifts and rifted margins around the world to discuss recent scientific advances, emerging questions, and to identify potential high-priority science for future GeoPRISMS RIE efforts. The meeting included a series of oral and poster presentations, pop ups and discussions. The workshop conveners have prepared a report that summarizes science results and future directions discussed at the workshop.

The GeoPRISMS Rift Initiation and Evolution (RIE) Theoretical and Experiment Institute (TEI) was held in Albuquerque, NM from February 7-10. The objectives of the meeting were to summarize progress and recent scientific advances related to the RIE initiative, identify high-priority science for future GeoPRISMS RIE efforts and promote community building and formation of new collaborations.

To meet those objectives, a diverse group of scientists was enlisted to serve on the convening team, give invited and contributed talks and to contribute to the meeting as attendees. The expertise of conveners, speakers and attendees spanned a broad range of interests connected with the RIE initiative, from deep geodynamical processes underlying rifting to surface processes controlling syn- and post-rift evolution. Scientists undertaking studies in the RIE primary sites (the East Africa Rift and the Eastern North American Margin) and working at other rifts and rifted margins were encouraged to contribute to all aspects of the workshop to ensure diverse perspectives. The meeting was attended by 133 participants, 59 of which were students and postdoctoral researchers. Besides attracting a large group of early career scientists, attendees included mid-career investigators who were relatively new to RIE science. Scientists from abroad were invited to attend to provide insights regarding the RIE primary sites and on rifts in general.

The meeting structure was designed to cover the broad spectrum of science included in the GeoPRISMS RIE science plan, to encourage interdisciplinarity and to bring in diverse perspectives. The main meeting had seven main oral sessions:

  1. Rift evolution from initiation to post rift architecture
  2. Geodynamics of rift and post-rift processes
  3. Magmatism and volatile exchanges
  4. Faulting and strain
  5. Surface processes & feedbacks between deep/surface processes
  6. Hazards associated with rifting environments

There was substantial time allocated for discussion and interaction; the meeting included several poster sessions at various times of day, two breakout sessions, one small-group discussion and plenary discussion after each oral session and throughout the meeting. As described in more detail below, the speakers successfully synthesized the state of knowledge on various aspects of rift evolution and of highlighting important outstanding questions. The breakouts and discussion were dynamic, generating excellent ideas and insights. The main meeting was preceded by a half-day student and postdoc symposium organized and led by three postdocs.

Overview of science presented at the meeting

Student-Postdoc Symposium

The student-postdoc symposium was held the afternoon before the main meeting and was led by Yelebe Birhanu (Bristol), James Muirhead (Syracuse), and Jean-Arthur Olive (LDEO). The organizers began the symposium with a presentation that provided an overview of the outstanding science questions related to RIE. These questions focused on the topics of rift initiation, the 4-D rift architecture, long- and short-term rift deformation mechanisms, rift volcanism, magmatism and volatile fluxes as well as surface processes at rifts and rifted margins. These topics were the focus of small group discussions later in the afternoon, and the discussion leaders summarized these discussions during the first day of the main meeting to all attendees. The symposium also included pop-ups by all participants on their RIE related research. Over sixty people attended the student-postdoc symposium, including nearly all students and postdocs at the meeting and a few representatives from the GeoPRISMS Office and GSOC, NSF and the convening team of the main meeting. The scientific discussions were followed by a career development panel discussion where students and postdocs had the opportunity to engage directly with scientists at a variety of stages in their careers.

Main meeting

The main part of the meeting began with a session on rift evolution from initiation to post-rift architecture. Roger Buck (LDEO) emphasized the role of magma throughout the life of rifts, from diking during rift initiation to the association of rifted margins with large magmatic outpourings and seaward dipping reflectors. Harm Van Avendonk (UTIG) reviewed insights on rifting processes from studies of both magma-poor and magmatic rifted margins, where recent studies show interesting variations in the distribution and timing of magmatism in relation with rifting, including provocative clues from ENAM on distribution of magmatism and highly thinned continental crust. Danny Brothers (USGS) focused on postrift evolution of rifted margins, including how sediment delivery and pre-failure configuration control evolution and evidence for active fluid venting, slope failure, and sediment compaction.

Session 2 focused on geodynamics. Jolante Van Wijk (NM Tech) provided an overview of numerical modeling approaches and the importance of testing and comparing models to both observations and other numerical solutions. Zach Eilon (Brown) synthesized geophysical observations from the Woodlark Rift in Papua New Guinea and showed evidence of limited melt, lithospheric removal and opening direction parallel anisotropy. Andrew Smythe (Penn State) showed how high-temperature thermochronology and diffusion speedometers can be used to assess mantle upwelling rates and how strain is vertically distributed during rifting. Robert Harris (Oregon State) showed high-resolution heat flow results from the Gulf of California and emphasized the role of fluid flow as well as conductive heat transfer. Colton Lynner (Arizona) showed new shear-wave splitting results from the ENAM community seismic experiment and suggests that 3-D edge driven flow at the edge of the margin can explain their observations.

Session 3 followed with talks on magmatism, volcanism and volatile exchanges. Cornelia Class (LDEO) gave an overview of the geochemical and petrological tools to identify magma and volatile sources in rift settings, highlighting the importance of using multiple geochemical systems to identify mantle components. Sara Mana (Salem State) showed chronological and geochemical data from the North Tanzania Convergence zone and highlighted the evidence for pulsed magmatism and a metasomatized mantle source. Juliane Hübert (Edinburgh) provided new insights on magma storage and pathways using magnetotelluric data in the Main Ethiopian Rift. Madison Meyers (U. of Oregon) emphasized the occurrence of large silicic volcanic centers in rift settings and showed how detailed work on volatiles recorded by melt inclusions allow for the quantification of magma ascent rates. Philip Kyle (NM Tech) ended the session with an overview of the magmatic history of the West Antarctic Rift.

Day 2 started off with the session on faulting and strain, where Cindy Ebinger (Tulane) provided a ‘recipe for rifting’ for cratonic and orogenic rifts where the difference in both crustal and mantle rheology are of critical importance for rift architecture and extension, including the possibly important but poorly known hydration state and distribution of volatiles at depth. Paul Umhoefer (Northern Arizona U.) showed how variations in inherited structures, strain partitioning, angle of obliquity and sediment input control extension in the Gulf of California – Salton trough plate boundary.

James Muirhead (Syracuse) showed the results from an interdisciplinary study in the East African Rift that better constrain the role and sources of fluids and mantle melting in the early stages of rifting and their connection to faulting. Hannah Mark (WHOI) provided new insights from modeling of observed seismic coupling coefficients that show how the thermal regime scales with seismic coupling in MOR and continental rifts. Elifuraha Saria (Ardhi) ended the session by providing an overview of geodetic constraints on crustal deformation in Africa emphasizing the fact that large parts of the continent are not adequately monitored geodetically.
Session 5 focused on surface processes and feedbacks in rifts, where Kyle Straub (Tulane) showed how geomorphology signals are stored in the stratigraphic and landscape record. His talk was followed by Jean-Arthur Olive (LDEO) who discussed the role of surface processes in the stabilization of half-graben structures. Erin DiMaggio (Penn State) talked about the connection between rift development as preserved in the stratigraphic record and the development of the Ledi-Geraru paleontological site. Liang Han (Virginia Tech) showed how rapid sedimentation in the Salton Trough resulted in the formation of new crust, delayed continental breakup and seafloor spreading, and how metamorphism of sediment can further delay final crustal breakup. Rob Gawthorpe (Colorado School of Mines) ended the session with insights on the evolution of the Corinth Rift, Greece from the onshore-offshore observations.

The final science session highlighted hazards in rifts and rifted margins. Karen Fontijn (Oxford) focused mainly on volcanic hazards in the East African Rift, emphasizing the low viscosity of rift magmas, the high potential for phreatic eruptions, and the abundance of large caldera systems as well as the role of hazardous CO2 degassing. Atalay Ayele (Addis Ababa U.) highlighted the challenges in disaster risk management in Africa that are due to limited capacity in equipment and human resources and the general level of understanding of potential risk. He also pointed to recent successes such as capacity building efforts, advances in real-time data flows, and national workshops. Maurice Lamontagne (GSC) showed how earthquakes and tsunamis related to rifting are the main hazards in Eastern Canada and how detailed mapping of ancient fault structures provides key insights on earthquake mechanisms and distributions in the region. Sang Mook Lee (SNU) highlighted the geohazards of the East Sea and the Sea of Japan and their potential to affect nuclear power plant safety.

Collaborative opportunities were discussed with presentations on the RiftVolc initiative, connections between rifting and hydrology, EarthScope and Africa Array updates.

Science themes with opportunities for near-term future studies

The TEI was designed to provide ample opportunities for participants to ask questions and discuss scientific issues related to the presentations. This was achieved through a panel discussion following each session including all speakers. Additional focused discussions occurred during two breakout sessions and small group discussions, which focused on the identification of high priority science questions and work needed to tackle these questions.

The following major science themes emerged from discussions at the TEI. For each of these themes, discussions focused on exciting recent findings and opportunities for near-term research progress through the GeoPRISMS RIE TEI initiative.

1. Tracking fluids (volatiles and magmas) through the lithosphere and with time

The importance of fluids for a spectrum of interconnected processes throughout the life of rifts and rifted margins was a topic of significant interest at the meeting. Meeting presentations covered recent results that have revealed strong, nonlinear interactions between volatiles and faults (e.g., talk by Muirhead), the important influence of prerift and synrift metasomatic events on magmatism (e.g., talk by Sana), and the capacity of fluids to advect heat and strongly modulate the thermal structure of rifts (e.g., talk by Harris). Geochemical tracers can be used to constrain the modification of the lithosphere by magmatic events (e.g., talk by Class). New studies of rifted margins also reveal unexpected mantle structure and magmatism, hinting at active processes long after rifting (e.g., talk by Lynner).
These new science results point to several exciting near-term future science directions:

  • Understanding the connections between deep volatiles and shallow observations, including constraining magma and volatiles residence times and pathways
  • Developing a quantitative understanding of the impact of volatiles/magmatism on strain localization and rheology (connects to theme 2)
  • Connecting general rheological models to morphological and process-based differences between magma-poor and magma-rich regions
  • Investigating the origin and significance of post-rift magmatism on rifted margins

2. Controls on deformation and localization at different temporal scales

Elucidating controls on deformation and localization are central to understanding rift processes, and were another major focus of meeting presentations and discussions. Magma is clearly a great localizer of strain (e.g., talks by Buck, Ebinger), but magma is not present everywhere, at least not in abundance. In magma-poor locations, fluids, pre-existing structures and/or chemical heterogeneity may be important factors (e.g., talks by Van Avendonk, Eilon). Volatiles appear to influence crustal rheology and fault behavior (e.g., talks by Muirhead, Ebinger), but are still poorly understood. The role of pre-existing lithospheric structure in strain localization appears to vary among rift systems and at different scale lengths (e.g., talks by Lynner, Eilon).

New numerical models and observations suggest that surface processes may also control strain localization (e.g., talks by Olive, Han; connects to theme 3). The slip behavior of rift faults (creeping, locked, etc.) is poorly known (e.g., talk by Mark), and there are few constraints on how it might change over time or with rift evolution (e.g., talk by Van Avendonk).

These new science results point to several exciting near-term future science directions:

  • Integration of rifting processes across a range of time scales from the earthquake cycle to geologic time
  • Characterization of slip behavior of faults over time and space
  • Understanding variations in temporal/spatial patterns of deformation between magmatic and magma-poor systems
  • Comparing transient behavior in rifts (creep, slow slip) to subduction and transform zones
  • Observing how volatiles are distributed through lithosphere (connects to theme 1) with an emphasis on how they impact rheology, faulting, and transient deformation
  • Constraining mantle rheology on a variety of time scales and as a function of volatile abundance, metasomatism and melt extraction processes (connects to theme 1).

3. Surface mass sedimentary fluxes and feedbacks with rifting

Recent studies have demonstrated strong connections between surface processes and all stages of rift evolution. These include the formation of new crust through rapid sedimentation (e.g., talk by Han), the impact of erosion on fault evolution (e.g., talk by Olive), the structural control of sediment pathways during and long after rifting (e.g., talk by Gawthorpe), and the structural control of slope failure (e.g., talks by Brothers and Lamontagne). The vertical displacements and crustal architecture associated with extensional tectonics strongly influence the spatial and temporal distribution of depositional domains (e.g., talks by Straub, Brothers).
These results point towards several important near-term future science directions:

  • Developing more comprehensive sedimentary histories of rifts to improve understanding of rift-related mass transport
  • Improving conceptual and numerical models of sediment influence on extensional processes, including thermal and mechanical feedbacks (connects to theme 2)
  • Utilizing the extant and paleolake systems for integrated investigations of landscape evolution.

Efforts needed to make progress on themes within GeoPRISMS

To address outstanding questions related to the themes above, the following future efforts were highlighted as particularly important.

Synthesis

Comparing among and within rifts is important to address many of the overarching RIE science questions and the specific questions within the themes above. A growing volume of data is now available in both primary sites and in other rift systems on everything from surface processes to magmatism and deep geodynamics. These observations include existing geophysical datasets on both EAR and ENAM from GeoPRISMS and other efforts, growing geochemical data and drilling data in various rifts. Particular themes discussed for syntheses were:

  • Geochemical variations along/across ENAM/EAR
  • Sediment mass fluxes from existing (limited) drilling data
  • Geochronological data on magmatic/volcanic events and surface processes
  • Crustal/lithospheric structure of rifts from existing geophysical imaging, with focus on comparisons between and within systems with variable magmatism
  • Geochemical data from geothermal exploration projects (drilling) in volcanic and non-volcanic settings.

New data collection and experimental/numerical work

From discussion at the meeting, it is clear that new data and experiments are required to tackle many important science themes, and several key gaps emerged from discussions at the meeting. Below are examples:

  • Studies of volatile systems to understand their distribution/abundance/residence time at various levels in the lithosphere. This would involve integrated geophysical imaging including but not limited to MT, seismic, and detailed geochemical studies such as melt inclusions, sampling volatiles at the surface, high density flux measurements, and other approaches.
  • Experimental and numerical modeling directed at the impact of volatiles and lower crust/mantle lithosphere hydration state/compositions on deformation throughout the lithosphere,
  • Observations to constrain the time scales of processes are needed. These include but are not limited to more geodetic observations to understand average rates and observe transient events as well as investigations of paleoseismology, deformed volcanic ash markers, and tectonic geomorphology to understand longer term accommodation of strain by events. On a longer time scale, better and improved geologic timing information is needed.
  • New constraints on sedimentary fluxes in rifts including but not limited to cosmogenic dating techniques, river incision rates, and obtaining data from new drill cores.
  • Advance the understanding of landscape evolution through better access to high resolution topographic data. ■

 icon-chevron-right For more information and browse the collection of archived presentations, please visit the TEI webpage

Report on the NSF-GeoPRISMS Rift Initiation and Evolution Theoretical and Experimental Institute . T. Fischer, D. Shillington

GeoPRISMS Newsletter, Issue No. 38, Spring 2017. Retrieved from http://geoprisms.nineplanetsllc.com

Spotlight | Complex upper mantle structure beneath the East African Rift System

Erica Emry1, Andrew Nyblade2, and Yang Shen3

1New Mexico Tech, 2Penn State University, 3University of Rhode Island

The East African Rift System (EARS) was one of the GeoPRISMS primary sites within the theme of Rift Initiation and Evolution, because of the variety of rifting stages and styles exhibited along this margin and because of the number of science questions that can be addressed there. Along this margin and in neighboring regions of Africa, Europe, and the Middle East, many broadband seismic instruments have been previously deployed, and numerous studies have explored the subsurface structure over a broad range of scales. However, there is often a disjoint between features that had been previously imaged through smaller-scale, regional tomographic inversions and those imaged by larger-scale inversions. In a recent tomographic study of the upper mantle beneath Africa, we used a full-waveform tomography method, constrained by long-period signal from ambient seismic noise to image the upper mantle beneath Africa to the top of the mantle transition zone (Emry et al., 2019). We found good agreement with prior models, at both large and regional scales, and we imaged new features in higher detail beneath more poorly resolved segments of the EARS. Here, we highlight the overall patterns along the EARS and focus on the complexity observed beneath the Turkana region.

What did we do?

We gathered continuous seismic data for more than 800 seismic stations and extracted Rayleigh waves from ambient seismic noise at periods as high as 340 seconds (Shen et al., 2012). Long period signal is valuable, because it is sensitive to structure deeper in the upper mantle and allows us to resolve down to about 350 to 400 km. Of the more than 800 seismic stations, we identified stations that provided clear signal at 40-340 seconds and used them to constrain our inversion (Fig. 1). This was a new set of data that had not yet been used to image the deeper lithosphere and asthenosphere beneath Africa.

Figure 1. Station map modified from Emry et al. (2019). Cratons are outlined in thick black lines. Blue triangles denote stations for which ambient noise data were collected and red triangles show stations that were used to invert for tomography. Abbreviations are as follows: AF-Afar, AP-Arabian Peninsula, DB–Damara Belt, KpC–Kaapvaal Craton, LR–Luangwa Rift, MER–Main Ethiopian Rift, MR–Malawi Rift, OR–Okavango Rift, RVP–Rugwe Volcanic Province, SS–South Sudan, TC–Tanzania Craton, TD–Turkana Depression, VVP–Virunga Volcanic Province, ZC–Zimbabwe Craton.

Figure 2. Two depth slices showing shear velocity at a) 165 km and b) 424 km, modified from Emry et al. (2019). For each depth, the color scale (m/s) is centered around the shear velocity in AK135 for that depth. Coastlines are shown as thin black lines, gray and blue lines indicate velocities that are 1.7% and 5% greater than AK135 model. Gray triangles show stations that inform the inversion. Abbreviations are as in Figure 1.

Although other seismic phases are often used to constrain full-waveform tomographic models, we used Rayleigh waves, as it is the principal phase extracted from seismic ambient noise. We used high-performance computing (HPC) clusters at the University of Rhode Island Graduate School of Oceanography to simulate waves propagating through a laterally variable Earth structure. Once synthetic waveforms were calculated for each seismic source in the model, we measured misfit between synthetic Rayleigh waves and those extracted from the data, determined the volume of Earth that influences the traveling wave, and inverted to identify a better-fitting model. For each new model, these steps were repeated until minimal change was made to the model. Our final results provide the absolute, isotropic, shear wave velocity (Fig. 2).

New results from the East African Rift System

There were many similarities between our results and prior studies of the EARS in regions where dense seismic or magnetotelluric arrays have been previously located (Benoit et al., 2006; Bastow et al., 2008; Adams et al., 2012; Mulibo and Nyblade, 2013; O’Donnell et al., 2013; Civiero et al., 2015; Gallacher et al., 2016; Accardo et al., 2017; Yu et al., 2017; Sarafian et al., 2018). As in prior models, we saw abundant indications for mantle upwellings or plumes as well as a pattern of lower velocities at shallow upper mantle depths in the northern EARS and higher velocities at shallow depths in the southern EARS. However, in our results, the patterns of low-velocities at middle upper mantle depths were laterally discontinuous along the full length of the EARS, and we imaged variable lithospheric topography that may influence the shallow flow of mantle upwellings.

Segmented upwellings beneath East Africa

Beneath the EARS, we imaged low-velocities at mantle transition zone (MTZ) depths, but at middle upper mantle depths, we imaged persistent patterns of separation between low-velocity features. While we have confidence in the pattern of separation within the upper mantle, we cannot resolve small features at deep depths and therefore cannot be certain whether the separation at shallower depths continues into the MTZ. At the shallowest upper mantle depths, the low-velocities appear to be overall more connected than at the middle upper mantle and are located mostly beneath the rift axis. In many regions, at shallow and middle upper mantle depths, the low-velocity anomalies are located adjacent to or between high-velocity features.

This pattern provides an overall sense that distinct buoyant upwellings, presumably of a thermal or thermochemical nature, are rising through the upper mantle and that their paths are likely influenced at shallow depths by rigid, presumably lithospheric, structures. Ultimately, it appears that these upwellings are sourced from MTZ depths. Such a pattern of secondary upwellings could be sourced by a deeper, ponded anomaly at or beneath the mantle transition zone, as has been previously suggested for the EARS from seismic and geochemical observations (Kieffer et al., 2004; Furman et al., 2006; Bastow et al., 2008; Huerta et al., 2009; Mulibo and Nyblade, 2013; Civiero et al., 2015). This pattern of buoyant upper mantle upwellings appears to be occurring along much of the EARS, and also in some other regions of Africa, however we note that fewer upwellings were imaged beneath the less evolved southern and western segments. In our view, this may be due to the history of upwellings or to the generally thicker lithosphere in the south and west that may act to divert upwellings.

Complex upper mantle beneath Turkana

One region that is most suggestive of a complex upwelling and diversion process is beneath the Turkana and South Sudan region. Here, the upper mantle has been difficult to image due to a lack of broadband seismic instrumentation. The Turkana segment is part of the primary EARS focus site and is particularly unique with regards to other segments of the EARS, because of the broad, diffuse rifting pattern and history of previous rifting oblique to the current-day trend (Brune et al., 2017; Ebinger et al., 2017).

Beneath this region, the indication of a low-velocity anomaly at deep upper mantle and mantle transition zone depths was most prominent (Fig. 3c). Directly above this, at middle upper mantle depths, a high-velocity feature was identified in the west beneath South Sudan, and the lowest velocities at these depths were located immediately adjacent to the high-velocity feature, to the north and to the southeast and southwest (Fig. 3b). Above this, at the shallowest upper mantle depths, the lowest velocities were imaged to the east beneath Lake Turkana. At these shallowest depths to the west beneath South Sudan, slightly slow to average upper mantle velocity was observed, while the fastest structure was located to the south and southwest beneath the laterally continuous Uganda and Bomu-Kibalan Cratons (Fig. 3a).

Figure 3. Three depth slices from the Turkana-South Sudan and Ethiopian Plateau regions showing shear velocity at a) 123 km, b) 260 km, and c) 424 km, modified from Emry et al. (2019). Cross-sections correspond to lines plotted on a. Abbreviations explained in Figure 1.

This pattern may suggest that rising asthenosphere is being diverted north and south around a lithospheric structure within the middle upper mantle beneath South Sudan. However, it is difficult to be certain of the spatial relationship and possible connection between this high-velocity feature and the Uganda and Bomu-Kibalan Cratonic roots located at shallower depths to the south and southwest. At this this point we can only speculate whether the structure is part of a stable, deep lithospheric root or whether it is sinking or foundering lithosphere (see discussion in Emry et al., 2019). However, we expect that this feature may affect the style of rifting, patterns of magma-rich vs. magma-poor extension, and connections between the Main Ethiopian Rift and the Eastern and Western Branches.

Summary

Overall, the EARS shows variability in lithospheric topography and reveals regions where the lithospheric structure may be affecting the path of upwellings at shallow and middle-upper mantle depths. However, there is also a clear sense of distinct upwellings within the upper mantle that might be sourced from a common, deeper anomaly. Our results of the upper mantle and mantle transition zone are useful in understanding the spatial relationships and possible connections between different segments and we hope that they will aid the overall goal of synthesis.

Acknowledgments

AfricaArray seismic stations in Dodoma, Tanzania. The photo was taken looking down into the 4-m deep vault at the Geological Survey in Dodoma, Tanzania. Photo credit: Andy Nyblade.
AfricaArray seismic stations in Zomba, Malawi. The photo was taken looking down into the 3-m deep seismic vault shown in photo at left at the Geological Survey in Zomba, Malawi. Photo credit: Andy Nyblade.

 

We thank the NSF Earth Sciences Postdoctoral Fellowship program for supporting this research (EAR-1349684). The shear velocity model from Emry et al. (2019) is available through the IRIS-EMC and through the GeoMapApp tool. We thank Manochehr Bahavar from the IRIS-EMC and Andrew Goodwillie from IEDA-GeoMapApp for helping to format the model and make it available.

Because our data came from ambient seismic noise, it was necessary that stations had temporally overlapping records. In this regard, the sparsely distributed long-term seismic deployments, such as the GSN, GEOSCOPE, AfricaArray (see photos), and MedNet were irreplaceable, and allowed us to also incorporate several 1-2 year (‘PASSCAL-type’) seismic deployments throughout Africa and the surrounding regions. ■

References

Accardo, N.J., J.B. Gaherty, D.J. Shillington, C.J. Ebinger, A.A. Nyblade, G.J. Mbogoni, P.R.N. Chindandali, R.W. Ferdinand, G.D. Mulibo, et al. (2017). Surface wave imaging of the weakly extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers. Geophys. J. Int., 209(3), 1892-1905
Adams, A., A.A. Nyblade, D. Weeraratne (2012). Upper mantle shear wave velocity structure beneath the East African plateau: Evidence for a deep, plateauwide low velocity anomaly. Geophys. J. Int., 189(1), 123-142
Bastow, I.D., A.A. Nyblade, G.W. Stuart, T.O. Rooney, M.H. Benoit (2008). Upper mantle seismic structure beneath the Ethiopian hot spot: Rifting at the edge of the African low-velocity anomaly. Geochem. Geophys. Geosys. 9(12), Q12022
Benoit, M.H., et al. (2006). Upper mantle P-wave speed variations beneath Ethiopia and the origin of the Afar hotspot. Geology, 34(5), 329-332.
Brune, S., et al. (2017). Controls of inherited lithospheric heterogeneity on rift linkage: Numerical and analog models of interaction between the Kenyan and Ethiopian rifts across the Turkana depression. Tectonics, 36, 1767-1786
Civiero, C., J.O.S. Hammond, S. Goes, S. Fishwick, A. Ahmed, A. Ayele, C. Doubre, et al. (2015). Multiple mantle upwellings in the transition zone beneath the northern East-African Rift system from relative P-wave travel-time tomography. Geochem. Geophys. Geosys. 16, 2949-2968
Ebinger, C.J., D. Keir, I.D. Bastow, K. Whaler, J.O.S. Hammond, A. Ayele, M.S. Miller, C. Tiberi, S. Hautot (2017). Crustal structure of active deformation zones in Africa: Implications for global crustal processes. Tectonics, 36, 3298-3332
Emry, E.L., Y. Shen, A.A. Nyblade, A. Flinders, X. Bao (2019). Upper mantle Earth structure in Africa from full-wave ambient noise tomography. Geochem. Geophys. Geosys. 20, 120-147
Furman, T., Bryce, J., Rooney, T., Hanan, B., Yirgu, G., Ayalew, D. (2006). Heads and tails: 30 million years of the Afar plume. In G. Yirgu, C. J. Ebinger, P. K. H. Maguire (Eds.), The Structure and evolution of the East African Rift System in the Afar Volcanic Province, Geological Society of London Special Publications, 259, 95-119. doi.org/10.1144/GSL.SP.2006.259.01.09
Gallacher, R.J., D. Keir, N. Harmon, G. Stuart, S. Leroy, J.O.S. Hammond et al. (2016). The initiation of segmented buoyancy-driven melting during continental breakup. Nature Communications. 7, 13110
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Mulibo, G.D., A.A. Nyblade (2013). The P and S wave velocity structure of the mantle beneath eastern Africa and the African superplume anomaly. Geochem. Geophys. Geosystems, 14(8)
O’Donnell, J.P., A. Adams, A.A. Nyblade, G.D. Mulibo, F. Tugume (2013). The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: Constraints on rift evolution. Geophys. J. Int., 194(2), 961-978
Sarafian, E., R.L. Evans, M.G. Abdelsalam, E. Atekwana, J. Elsenbeck, A. G. Jones, E. Chikambwe (2018). Imaging Precambrian lithospheric structure in Zambia using electromagnetic methods. Gondwana Research, 54, 38-49
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Reference information

Spotlight | Complex upper mantle structure beneath the East African Rift System. E. Emry, A. Nyblade, Y. Chen
GeoPRISMS Newsletter, Issue No. 42, Spring 2019. Retrieved from http://geoprisms.nineplanetsllc.com

Spotlight | A continent-scale geodetic velocity field for East Africa

Rebecca Bendick1, Mike Floyd2, Elias Lewi3, Gladys Kianji4, Robert King2, El Knappe1

1University of Montana, 2MIT, 3Addis Ababa University, 4University of Nairobi

The East African Rift System is a complicated set of extensional structures reaching from Malawi in the south to Eritrea and Djibouti in the north (Fig. 1)(e.g. Ebinger, 2005). These structures are broadly interpreted as the expression of the ongoing breakup of the African continent into a “Somali” block moving east or northeastward relative to a “Nubia” block, with perhaps additional smaller blocks (e.g. Saria et al., 2014) also involved. The details of the kinematics, the presence or importance of entrained microplates, and even the components of the force balance exciting relative block motions and extensional strains are all the subject of ongoing research and incompletely resolved scientific debates.

Figure 1. Overview of the EARS, with shaded topography in gray, major faults in red, recorded seismicity of Mw>5 as blue circles, and generalized kinematic velocities from Saria et al. (2014).

Several decades of geophysical and geologic research have contributed a large body of observational data related to the timing (Bosworth, 1992; Bosworth and Strecker, 1997; George et al., 1998; Wichura et al., 2010), chemistry (Aulbach et al., 2008; Bianchini et al., 2014; Chesley et al., 1999; Kaeser et al., 2009; Pik et al., 2006), mechanics (Buck, 2004; Calais et al., 2008; Corti et al., 2003; Weinstein et al., 2017), kinematics (Birhanu et al., 2015; Calais et al., 2008; Modisi et al., 2000; Saria et al., 2014), mantle involvement (Adams et al., 2012; Bastow et al., 2005; Bastow et al., 2008; Chang and Van der Lee, 2011; Fishwick, 2010; Hansen and Nyblade, 2013), magmatism (Bastow et al., 2010; Kendall et al., 2005) and natural hazards (Ayele, 2017) of continental extension in Africa. However, most of these studies are focused on a single “segment” of the larger rift system, hence on a distinct structural unit. Some work has been done to compare segments as a means of exploring the relative importance of contributing factors, such as the availability of fluids in magma-rich and magma-poor segments (Bialas et al., 2010; Hayward and Ebinger, 1996; Roecker et al., 2017; Rooney et al., 2011), the influence of total finite strain (Ebinger, 2005) on rift morphology, or the importance of sublithospheric plume impingement on the force balance (Ebinger and Sleep, 1998; Lin et al., 2005; Nyblade and Robinson, 1994). However, fully synoptic studies for the whole East African Rift System (EARS) are few in number.

A GeoPRISMS-supported collaboration between MIT and the University of Montana targeted the development of a comprehensive, consistent geodetic surface velocity solution for the entire EARS focus area (Fig. 2). This effort included several components:

  1. Collection of all publically available raw GPS observations from East Africa from 1992 to 2015;
  2. Negotiation for the release and inclusion of several additional restricted GPS observation data sets from European and African sources;
  3. Compilation and verification of all related metadata;
  4. Systematic assessment and quality control on all available data sets; and
  5. Processing of the merged data sets with a consistent processing strategy and reference frame.

Figure 2. The most recent community geodetic solution, using all available raw data from the EARS region, processed using GAMIT/GLOBK with a consistent quality standard and editing approach, in a single common reference frame. This solution, data sources, and relevant metadata are available from the GeoPRISMS data portal at http://www.marine-geo.org with doi:10.1594/IEDA/321764

The supported work addresses the GeoPRISMS Rift Initiation and Evolution (RIE) goal of synthesis, especially in the context of multiscale mechanics and controls on deformation and localization of strain.

During the period of support for this experiment, we also leveraged the NSF funding to invest in permanent geodetic instrumentation in Ethiopia and add new observations in the Turkana Depression of Ethiopia and Kenya, the part of the EARS with the fewest prior geodetic observations. In the first case, we extended operations of a previously-funded Ethiopian Highlands continuous GPS network for an additional year. That year allowed Addis Ababa University and the University of Montana to negotiate with several different stakeholders in the U.S. and Africa, with the end result that the Institute of Geophysics, Space Science, and Astronomy of Addis Ababa University adopted a fully operational, scientific-grade geodetic network of ten sites for permanent ongoing observations (Fig. 3). The network became the largest entirely African owned and operated geophysical system, and maintains operations and a fully open data policy to the present. In the second case, we added an additional epoch of campaign observations on six campaign GPS sites (Fig. 4) and added two continuous GPS systems in the Turkana Depression (Fig. 5). The continuous sites are located on either side of Lake Turkana and are hosted by the Turkana Basin Institute, a nonprofit entity supporting research through the region.

The primary purposes of the project were scientific and infrastructural capacity-building. The synoptic geodetic velocity field is intended for use by a wide range of researchers in many different disciplines within the rifting initiative and the EARS focus area. Many users will likely leverage the kinematic framework as boundary conditions, a priori constraints, or tectonic context for more focused studies without having to address data collection, standardization, quality control, metadata management, or processing strategies. We hope that the solution will inform other work and serve as an example of the value of a community commitment to open sharing of high-quality observations. In addition, the successful adoption of the instrumental array by African scientists sets a precedent for negotiated transfers of other instruments and capabilities throughout the region. African researchers and institutions can and should use such combinations of infrastructure and technical skills to pursue their own novel scientific targets and build indigenous training capabilities. Finally, the new Turkana Basin continuous sites are approaching a full year of operation, and will begin to yield usable scientific constraints on the most enigmatic part of the EARS very soon. ■

Figure 3. Birhanu Bekele (IGSSA MS student in geodesy) and El Knappe service continuous station SHIS (near Lalibela) supervised by children from a nearby farm. Photo Credit: Shannon Hall
Figure 4. Campaign GPS installation (SEOL) near Sereolipi, Kenya guarded by local wildlife rangers.
Figure 5. Welding the antenna mount for the Turkwel continuous site (XTBT) at the Turkana Basin Institute.
Sunset in Nekemt, Ethiopia after servicing the Highlands GPS network. Photo credit: Ellen Knappe
Central island in Lake Turkana, which has been active in the Holocene and currently has fumarolic activity. This photo was taken in February 2018 while flying between the Turkana Basin Institute field stations, which now host two new GPS stations. Photo credit: Ellen Knappe

References

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Ayele A., (2017), Probabilistic seismic hazard analysis (PSHA) for Ethiopia and the neighboring region. J Afr Earth Sci, 134, 257–264.
Bastow I., G. Stuart, J.-M. Kendall, C. Ebinger, (2005), Upper-mantle seismic structure in a region of incipient continental breakup: Northern Ethiopian rift. Geophys J Int, 162, 479–493. doi: 10.1111/j.1365246X.2005.02666.x.
Bastow I., A. Nyblade, G. Stuart, T. Rooney, M. Benoit, (2008), Upper mantle seismic structure beneath the Ethiopian hotspot: Rifting at the edge of the african low velocity anomaly. Geochem Geophys Geosyst, 9(12), Q12022. doi: 10.1029/2008GC002107.
Bastow I., S. Pilidou, J.-M. Kendall, G. Stuart, (2010), Melt-induced seismic anisotropy and magma assisted rifting in Ethiopia: Evidence from surface waves. Geochem Geophys Geosyst, 11, Q0AB05. doi: 10.1029/2010GC003036.
Bialas R., W. Buck, R. Qin, (2010), How much magma is required to rift a continent? Earth Planet Sci Lett, 292(1),68–78.
Bianchini G., J. Bryce, J. Blichert-Toft, L. Beccaluva, C. Natali, (2014), Mantle dynamics and secular variations beneath the East African Rift: Insights from peridotite xenoliths (Mega, Ethiopia). Chem Geol, 386, 49–58.
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Reference information

A continent-scale geodetic velocity field for East Africa. R. Bendick, M. Floyd, E. Lewi, G Kianji, R. King, E. Knappe
GeoPRISMS Newsletter, Issue No. 41, Fall 2018. Retrieved from http://geoprisms.nineplanetsllc.com

Spotlight | Constraining variability in mantle CO2 flux along the East African Rift System

James D. Muirhead1, Tobias P. Fischer2, Amani Laizer3, Sarah J. Oliva4, Emily J. Judd1, Hyunwoo Lee5, Emmanuel Kazimoto3, Gladys Kianji6, Cynthia J. Ebinger4, Zachary D. Sharp2, Josef Dufek7

1Syracuse University, 2University of New Mexico, 3University of Dar es Salaam, 4Tulane University, 5Seoul National University, 6University of Nairobi, 7University of Oregon

Figure 1. Annotated SRTM map showing the extent of the rift basins in the current study. Filled circles show the location of sampling regions within each basin, and the dashed brown line delineates the eastward-dipping surface boundary between the Tanzanian craton and Proterozoic mobile belt rocks (from the geological map of Thiéblemont et al. (2016)). Also included is the mean flux of magmatic CO2 from sampling sites in each basin. Inset in the top left shows the location of the DEM map on the African continent. Red lines show the extent of the Eastern (EB) and Western (WB) branches of the EARS.

In May and June 2018, our team of researchers completed the longest along-strike magmatic CO2 degassing survey in the East African Rift System (EARS) to date. Our CO2 flux data now extend over four rift basins, from the Magadi basin (Kenya) southward to the Balangida basin (Tanzania) (Fig. 1). During the 25-day field campaign, we collected over one thousand diffuse soil degassing flux measurements, and sampled hydrothermal spring systems along major fault zones to analyze the sources and fluxes of different volatile species. Here we present preliminary results of diffuse CO2 flux in zones within one hundred meters of observed spring discharge and use these values to examine variations in magmatic CO2 discharges between basins. The spatial variability of these data reveal how mantle CO2 fluxes in the EARS may evolve over the course of rift basin development, and are impacted by the initial composition and structure of the East African lithosphere.
Continental rifts are sites of lithospheric thinning and heating, which is commonly accompanied by magmatism and volatile transfer from Earth’s mantle to the lithosphere and atmosphere (White and McKenzie, 1989; Ebinger, 2005; Rooney, 2010; Lee et al., 2017; Foley and Fischer, 2017). They represent a key tectonic setting for natural CO2 emissions and possibly modulate Earth’s climate on geological timescales (Brune et al., 2017; Foley and Fischer, 2017). However, the total volume of mantle CO2 emitted at rift settings is poorly constrained, as are the mechanisms that control variations in CO2 flux over the lifetime of rifting.

The original carbon content of cratonic lithosphere is expected to be relatively low (~0.25 Mt C km–3 for 2-3 Ga lithosphere; Foley and Fischer, 2017). However, abundant carbon may be sequestered in the mantle lithosphere during the infiltration of both plume melts (e.g., Thompson et al., 2015) and carbon-rich hydrous-silicate melts generated during subduction (Foley and Fischer, 2017; Malusà et al., 2018).

These processes can potentially enrich carbon contents in the mantle lithosphere up to a hundred times above background values (Foley and Fischer, 2017). The resulting carbon accumulated during these events may be released during the generation and ascent of magma at continental rift settings (Malusà et al., 2018) (Fig. 2).

Although continental rifts represent potentially key sites of CO2 release, measuring the flux of CO2 from these settings is challenging and requires direct measurements and observations of CO2 discharge from zones of active rifting. The magma-rich Eastern branch of the East African Rift System (EARS) represents an ideal location to investigate these processes. Earlier degassing studies focused on direct measurements of volcanic plumes emitted from active volcanoes, such as Nyiragongo (Sawyer et al., 2008) and Oldoinyo Lengai (Brantley and Koepenick, 1995). In addition to these plume sources, EARS volcanoes release mantle volatiles to the atmosphere via springs, fumaroles, and zones of diffuse soil degassing, as well as during eruptive episodes (Darling et al., 1995; Fischer et al., 2009; Barry et al., 2013; de Moor et al., 2013; Hutchison et al., 2015; Lee et al., 2017). More recent studies in the EARS have shown that large volumes of mantle carbon are also released to the atmosphere along extensional fault systems situated away from volcanoes (Lee et al., 2016, 2017; Hunt et al., 2017). During this process, termed “tectonic degassing” (Burton et al., 2013; Lee et al., 2016), mantle carbon ascends to the surface along permeable fault zones and exits via springs, diffuse soil degassing zones, and gas vents (Muirhead et al., 2016; Lee et al., 2016, 2017; Hunt et al., 2017). This mantle carbon is primarily sourced from an enriched sub-continental lithospheric mantle and released into the crust and atmosphere by magmas emplaced at lower crustal depths (Lee et al., 2017; Roecker et al., 2017).

Figure 2. Production and transport of magmatic CO2 at continental rift settings modified from Hunt et al. (2017). White arrows represent zones of CO2 fluid flow, yellow stars are hydrothermal springs, and orange stars are deep earthquakes. The CO2 depicted exsolves from cooling upper and lower crustal magmas. The distribution of crustal magma (red polygons) is based on seismicity from Weinstein et al., (2017) and the seismic tomography model of Roecker et al. (2017).

Given the large aerial extent, pervasive faulting, and widespread magma emplacement occurring at depth in the EARS (e.g., Keranen et al., 2004; Roecker et al., 2017; Plasman et al., 2017), quantifying the volumes of CO2 released requires observations from a wide variety of structural settings along the rift system. Results of diffuse soil degassing surveys have thus far been reported from the northern and central Main Ethiopian Rift (Hunt et al., 2017) and Magadi-Natron basin (Lee et al., 2016), with estimates of 0.52-4.36 Mt yr-1 and 2.15-5.95 Mt yr-1 for each rift sector, respectively.

Extrapolation of these estimates point to potential CO2 fluxes on the order of 10-100 Mt yr-1, particularly when accounting for dissolved CO2 volumes transported in springs (Lee et al., 2017). However, these estimates do not consider the spatial and temporal variations of mantle CO2 discharge expected along any active rift system. The flux of CO2 within any rift basin should depend on a number of critical factors, such as the volume of carbon trapped within the underlying mantle lithosphere, rates of magma production, and the dissolved CO2 contents of ascending rift magmas (Foley and Fischer, 2017; Hunt et al., 2017). These variables are expected to vary both spatially and temporally within any continental rift setting, and quantifying their importance for mantle CO2 release requires extensive along-strike sampling of zones of volatile discharge.

Our recent field campaign was specifically designed to fill in these critical gaps in our understanding of rift CO2 fluxes, through an investigation of four segments of the Eastern branch of the EARS: the Magadi, Natron, Manyara, and Balangida basins (Fig. 1). These basins encompass a ~350 km-long stretch of continental rifting and range in age between 1 and 7 Ma, and are thus currently at different stages of development. Furthermore, these basins exhibit varying volcanic/magmatic fluxes and histories, and even cross the boundary between Proterozoic mobile belt rocks and the Archean Tanzania Craton (Fig. 1). Therefore, from these data we can assess:

  1. How mantle CO2 fluxes may evolve over the course of basin development; and
  2. How CO2 fluxes are impacted by the initial lithospheric composition and structure of the East African lithosphere.

Given the inherent variability of CO2 flux within individual rift basins (e.g., Hunt et al., 2017), when comparing CO2 discharges between basins it is critical to compare data from sites exhibiting similar structure, substrate, and hydrology. Therefore, we present here a subset of our collected data, focusing specifically on flux data (1) from rift-graben sediments, (2) in the vicinity of faults, and (3) in areas within 100 m of observed spring discharge.

The sources for diffuse soil CO2 discharges in volcano-tectonic settings are typically characterized as either biogenic or magmatic, with flux data in each population exhibiting a log-normal distribution and the highest mean flux observed in the magmatic population (e.g., Chiodini et al., 1998, 2008). Data from each study site, presented as probability plots in Figure 3, were sub-divided into two distinct populations by adapting the methodology of Sinclair (1974) into a newly designed MATLAB® code. This code iteratively fits biogenic and magmatic regression lines to the log-transformed data. Based on these functions, synthetic data sets are generated for each population and plotted against observed data, with the final solution being that which produces the highest R-squared and smallest root-mean-squared error values between the compared datasets. Outputs from this procedure provide an estimate of the percent contribution of biogenic and magmatic sources and their mean flux values.

Figure 3. Probability plots of diffuse soil CO2 fluxes for each rift basin in the study. Note that the overall CO2 flux values decrease from north (Magadi) to south (Balangida). Flux values below the equipment detection limit (<0.24 g m-2 d-1) cannot be presented on the plots, but still affect the probability distribution of flux values above the detection limit.

Comparing data between basins, we observe a north to south decrease in both the percent contribution of the magmatic flux population and the mean magmatic flux value (see mean flux values in Figure 1). Lower magmatic CO2 flux values also correspond with younger rift basins (e.g., the Manyara and Balangida basins). These younger basins also exhibit lower volcanic/magmatic inputs (Le Gall et al., 2008; Albaric et al., 2014), which may relate to the low degree stretching and related decompression melting during this earlier stage of rifting, or to the relatively dry nature of thick Archean mantle that enables its preservation (e.g., Currie and van Wijk, 2016). Finally, as the locus of rifting gradually transitions from the Proterozoic mobile belt in the Natron basin, to the Tanzanian craton in the Balangida basin, we observe a significant reduction in the mean magmatic CO2 flux.

These preliminary results suggest that the volume of mantle CO2 discharge in the Eastern branch of the EARS is strongly dependent on the degree of lithospheric thinning, mantle hydration state, and related magmatism. The greatest mantle CO2 discharges in the EARS likely occur in more evolved systems outside the Archaean craton, such as the Kenya Rift (Lee et al., 2016) and Main Ethiopian Rift (Hunt et al., 2017). Furthermore, basins in their earliest rift stages (the ~1 Ma Manyara and Balangida basins) within Proterozoic mobile belt rocks exhibit higher CO2 fluxes than those in the Archean craton. This observation suggests that the Proterozoic lithosphere in East Africa may contain greater volumes of sequestered carbon, with its structure and composition suited for volumetrically significant CO2 discharges compared to the thick and probably dehydrated cratonic lithosphere. ■

Measuring diffusely degassing soil CO2 in the Balangida basin using a CO2 accumulation chamber. Photos credit: James Muirhead
Graduate students Amani Laizer (University of Dar es Salaam) and Sarah Oliva (Tulane University) sampling gas from an actively degassing vent observed along a fault in the Natron basin. Photo credit: James Muirhead
The field team enjoying the view of Oldoinyo Lengai volcano from the top of the border fault to the Natron basin (from left to right: James Muirhead, Tobias Fischer, Amani Laizer, Sara Oliva). Photo credit: James Muirhead
Tobias Fischer samples an actively degassing vent on the western border fault to the Natron basin. The world’s only active carbonatite volcano, Oldoinyo Lengai, can be seen in the background. Photo credit: James Muirhead

References

Albaric, J., J. Deverchere, J. Perrot, A. Jakovlev, A. Deschamps, ( 2014), Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift. Geochem Geophys Geosys 15, 374-394.
Barry, P.H., D.R. Hilton, T.P. Fischer, J.M. de Moor, F. Mangasini, C. Ramirez, (2013), Helium and carbon isotope systematics of cold “mazuku” CO2 vents and hydrothermal gases and fluids from Rungwe Volcanic Province, southern Tanzania. Chem Geol, 339, 141-156.
Brantley, S.L., K.W. Koepenick, (1995), Measured carbon dioxide emissions from Oldoinyo Lengai and the skewed distribution of passive volcanic fluxes. Geology, 23, 933-936.
Brune, S., S.E. Williams, R.D. Müller, (2017), Potential links between continental rifting, CO2 degassing and climate change through time. Nat Geosci, 10, 941.
Burton, M.R., G.M. Sawyer, D. Granieri, (2013), Deep Carbon Emissions from Volcanoes. Carbon in Earth, 75, 323-354.
Chiodini, G., R. Cioni, M. Guidi, B. Raco, L. Marini, (1998), Soil CO2 flux measurements in volcanic and geothermal areas. Appl Geochem, 13, 543-552.
Chiodini, G., S. Caliro, C. Cardellini, R. Avino, D. Granieri, A. Schmidt, (2008), Carbon isotopic composition of soil CO2 efflux, a powerful method to discriminate different sources feeding soil CO2 degassing in volcanic-hydrothermal areas. Earth Planet Sci Lett, 274, 372-379.
Currie, C.A., J. van Wijk, (2016), How craton margins are preserved: Insights from geodynamic models. J Geodyn, 100, 144-158.
Darling, W.G., E. Griesshaber, J.N. Andrews, H. Armannsson, R.K. O’Nions, (1995), The origin of hydrothermal and other gases in the Kenya Rift Valley. Geochim Cosmochim Acta, 59, 2501-2512.
de Moor, J.M., T.P. Fischer, Z.D. Sharp, D.R. Hilton, P.H. Barry, F. Mangasini, and C. Ramirez, (2013), Gas chemistry and nitrogen isotope compositions of cold mantle gases from Rungwe Volcanic Province, southern Tanzania. Chem Geol, 339, 30-42.
Ebinger, C., (2005), Continental break-up: The East African perspective. Astronomy & Geophysics, 46, 16-21.
Fischer, T.P., P. Burnard, B. Marty, D.R. Hilton, E. Furi, F. Palhol, Z.D. Sharp, and F. Mangasini, (2009), Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites. Nature, 459, 77-80.
Foley, S.F., T.P. Fischer, (2017), An essential role for continental rifts and lithosphere in the deep carbon cycle. Nat Geosci, 10, 897.
Hunt, J.A., A. Zafu, T.A. Mather, D.M. Pyle, P.H. Barry, (2017), Spatially Variable CO2 Degassing in the Main Ethiopian Rift: implications for magma storage, volatile transport, and rift‐related emissions. Geochem, Geophys Geosys, 18, 3714-3737.
Hutchison, W., T.A. Mather, D.M. Pyle, J. Biggs, G. Yirgu, (2015), Structural controls on fluid pathways in an active rift system: A case study of the Aluto volcanic complex. Geosphere, 11, 542-562.
Keranen, K., S.L. Klemperer, R. Gloaguen, E.W. Group, (2004), Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian rift. Geology, 32, 949-952.
Le Gall, B., P. Nonnotte, J. Rolet, M. Benoit, H. Guillou, M. Mousseau-Nonnotte, J. Albaric, J. Deverchere, (2008), Rift propagation at craton margin. Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times. Tectonophysics, 448, 1-19.
Lee, H., J.D. Muirhead, T.P. Fischer, C.J. Ebinger, S.A. Kattenhorn, Z.D. Sharp, G. Kianji, (2016), Massive and prolonged deep carbon emissions associated with continental rifting. Nat Geosci, 9, doi. 10.1038/NGEO2622.
Lee, H., T.P. Fischer, J.D. Muirhead, C.J. Ebinger, S.A. Kattenhorn, Z.D. Sharp, G. Kianji, N. Takahata, Y. Sano, (2017), Incipient rifting accompanied by the release of subcontinental lithospheric mantle volatiles in the Magadi and Natron basin, East Africa. J Volcanol Geotherm Res, 346, 118-133.
Malusà, M.G., M.L. Frezzotti, S. Ferrando, E. Brandmayr, F. Romanelli, G.F. Panza, (2018), Active carbon sequestration in the Alpine mantle wedge and implications for long-term climate trends. Scientific reports, 8, 4740.
Muirhead, J.D., S.A. Kattenhorn, H. Lee, S. Mana, B.D. Turrin, T.P. Fischer, G. Kianji, E. Dindi, D.S. Stamps, (2016), Evolution of upper crustal faulting assisted by magmatic volatile release during early-stage continental rift development in the East African Rift. Geosphere, 12, 1670-1700.
Plasman, M., C. Tiberi, C. Ebinger, S. Gautier, J. Albaric, S. Peyrat, … & F. Wambura, (2017), Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa. Geophys J Int, 210, 465-481.
Roecker, S., C. Ebinger, C. Tiberi, G. Mulibo, R. Ferdinand-Wambura, K. Mtelela, K., … & Peyrat, S., (2017), Subsurface images of the Eastern Rift, Africa, from the joint inversion of body waves, surface waves and gravity: investigating the role of fluids in early-stage continental rifting. Geophys J Int, 210, 931-950.
Rooney, T.O., (2010), Geochemical evidence of lithospheric thinning in the southern Main Ethiopian Rift. Lithos, 117, 33-48.
Sawyer, G.M., S.A. Carn, V.I. Tsanev, C. Oppenheimer, M. Burton, (2008), Investigation into magma degassing at Nyiragongo volcano, Democratic Republic of the Congo. Geochem Geophys Geosys, 9.
Sinclair, A.J., (1974), Selection of threshold values in geochemical data using probability graphs. J Geochem Explor, 3, 129-149.
Thiéblemont, D., J-P. Liégeois, M. Fernandez-Alonso, A. Ouabadi, B. Le Gall, R. Maury, M. Jalludin, M. Vidal, C. Ouattara Gbélé, R. Tchaméni, A. Michard, P. Nehlig, P. Rossi, (2016), Geological Map of Africa: French Geological Survey (BRGM), scale 1:10 000 000.
Thompson, D.A., J.O. Hammond, J.M. Kendall, G.W. Stuart, G.R. Helffrich, D. Keir, A. Ayele, B. Goitom, (2015), Hydrous upwelling across the mantle transition zone beneath the Afar Triple Junction. Geochem Geophys Geosys, 16(3), 834-846.
Weinstein, A., S.J. Oliva, C.J. Ebinger, S. Roecker, C. Tiberi, M. Aman, … S. Peyrat, (2017), Fault‐magma interactions during early continental rifting: Seismicity of the Magadi‐Natron‐Manyara basins, Africa. Geochem Geophys Geosys, 18, 3662-3686.
White, R., D. McKenzie, (1989), Magmatism at rift zones: The generation of volcanic continental margins and flood basalts. J Geophys Res -Solid Earth, 94, 7685-7729.

Reference information

Constraining variability in mantle CO2 flux along the East African Rift System. J.D. Muirhead, T.P. Fischer, A. Laizer, S.J. Oliva, E.J. Judd, H. Lee, E. Kazimoto, G. Kianji, C.J. Ebinger, Z.D. Sharp, J. Dufek
GeoPRISMS Newsletter, Issue No. 41, Fall 2018. Retrieved from http://geoprisms.nineplanetsllc.com

Spotlight | Investigating mantle controls on volcano spacing along the East African Rift System

Eric Mittelstaedt & Aurore Sibrant

University of Idaho

Figure 1. Digital Elevation model of the eastern part of Africa showing the main part of the East African Rift System (EARS). Solid black lines show major faults bounding rift depressions. The white dashed square shows the location of the focus rift and the black dashed elliptical lines indicates Ethiopian and Kenya domes. The thick black lines indicate the boundaries of the Congo and Tanzanian Craton.

The spatial variation in magma supply within a continental rift may determine the mode of lithospheric extension (active or passive) and the eventual pattern of oceanic spreading center segmentation (e.g., Hammond et al., 2013). As continental rifts evolve, volcanic centers within rift valleys often develop a characteristic spacing, or wavelength, such as observed in the Red Sea Rift (e.g., Bonatti, 1985) and within the Afar depression, the Main Ethiopian Rift (MER), and the Kenya (Gregory) Rift of the East African Rift System (EARS) (Fig. 1, 2; e.g., Mohr and Wood, 1976). Based primarily on observations, the surprisingly regular spacing of the volcanic centers within the EARS has been attributed to lithosphere thickness (Vogt, 1974; Mohr and Wood, 1976), pre-existing fault systems, and mantle processes similar to those at island arc and mid ocean ridges (Keer and Lister, 1988). In this project, we investigate the processes that control the spacing of volcanoes in the EARS. We are using numerical experiments to investigate if the surface expression of volcanism is primarily controlled by melt production (e.g., localized mantle instability, variations in mantle temperature and/or buoyancy) or by melt extraction (e.g., thickness of the lithosphere, pre-existing fractures).

The EARS is a perfect natural laboratory to test relationships between volcanism and parameters such as mantle temperature, lithosphere thickness, rift extension rate, and the presence of pre-existing structures. For example, the presence of one or two mantle plumes (Ebinger and Sleep, 1998; George et al., 1998) located primarily under the eastern rather than the western branch (e.g., Mulibo and Nyblade, 2013) suggests a role for anomalously warm, perhaps volatile-rich, mantle controlling the development of volcanic structures beneath the eastern branch rift segments. Additionally, decompression melting of upwelling mantle should be greater beneath the MER, where the opening rate is ~5 mm/yr (Saria et al., 2014), than beneath the Kenya rift, where the opening rate is ~3 mm/yr (Jestin et al., 1994; Saria et al., 2014). Differences in such tectonic and mantle parameters likely regulate magma supply throughout the EARS.

To constrain our experiments, we first examined the distribution of volcanoes throughout the EARS. We find that the median spacing of volcanoes in the Ethiopian and Kenya Rifts are similar (25 km and 32 km, respectively) and relatively uniform (e.g., small inter-quartile ranges, 15-16 km; Fig. 2). The median spacing of volcanoes in the Western Rift is much larger (53 km) and more irregular with an inter-quartile range of 68 km. We also found that volcano spacing may have some correlation with edifice volume, which could indicate a contribution of lithosphere flexure (e.g., Hieronymus and Bercovici, 1999).

Figure 2. The active volcanoes during the last 10 ka of the (A) West, (B) Kenya, and (C) Ethiopian Rift axis. The red and white stars indicate volcanoes centered or offset from the rift axis, respectively. The white number indicates the spacing between volcanoes centered along the rift axis. (D) The median (marked) and inter-quartile range (boxes) in measured volcano spacing increases from the Ethiopian to West sections of the EARS. (E) No consistent relationship exists between spacing and the volume of each volcanoes of the Ethiopian Rift.

For example, spacing of volcanic centers in the MER decreases with increasing volume of the largest volcanoes. However, for smaller volcanoes this trend does not hold; the spacing between volcanoes with a volume ~<10 km3 shows no correlation with volcano volume. Thus, initial volcano formation is likely controlled by deeper processes.

The combination of regular volcano spacing in the Ethiopian and Kenyan Rifts and the presence of relatively warm plume mantle indicate that a Rayleigh-Taylor (RT) instability in the mantle could regulate magma supply along the rift axis. A RT instability occurs in the unstable situation where a dense fluid rests atop a less dense fluid and the interface between them is perturbed; this results in growth of an instability that forms regularly spaced upwelling and downwelling diapirs. The diapirs form at a dominant, or preferred wavelength (i.e., spacing) that is controlled by the fluid parameters (e.g., density contrast, viscosity contrast, layer thickness). For example, when both fluids are Newtonian a larger thickness of the lower fluid layer yields a larger preferred instability wavelength.

To test the hypothesis of a RT instability in the sub-EARS mantle, we developed numerical models of a less dense viscous material (e.g., warm plume mantle) underlying a relatively dense viscous fluid (e.g. non-plume mantle). Simulations are performed with the finite-difference, marker-in-cell code SiStER (Simple Stokes with Exotic Rheologies; e.g., Olive et al., 2016). We simulate the evolution of two fluid layers with different contrasts in density, temperature, and flow law exponent (Newtonian versus Non-Newtonian fluids). We initially perturb the layer interface by 1% of the imposed wavelength and set the box width to half of the imposed wavelength (Fig. 3). By examining a range of parameters, we will be able to address how variations in mantle properties along the Ethiopian and Kenyan rift and between East and West Rifts may control volcano spacing.

Figure 3. For numerical simulations of non-Newtonian fluids, the (A) viscosity is a strong function of the (B) second invariant of the strain rate field. In contrast to Newtonian cases, these sharp changes in viscosity yield a weak dependence on the (C) thickness of the lower layer (colors) for cases with intermediate layer thicknesses. Gray arrows in (A) are velocity vectors.

Our preliminary results with Non-Newtonian fluids demonstrate that the growth rate of instabilities is not controlled by the lower layer thickness as in Newtonian fluids, but by the characteristic distance over which viscosity changes away from the interface between the two fluids, in agreement with previous studies (Molnar et al., 1998; Miller and Behn, 2012). If the lower layer is significantly thicker than this characteristic distance, than the preferred wavelength of upwelling diapirs will not “feel” the effect of the layer limits. However, if the layer is smaller than the characteristic distance, layer thickness will alter the preferred wavelength. Thus, for relatively thick lower layers, the preferred wavelength depends upon other system parameters, such as the flow law exponent.

For values of the lower layer thickness (~10 km), flow law exponent (3-4), activation energy (E ~200-500 kJ.mol-1), and density anomaly (3000 kg.m-3 in the lower layer and 3200 kg.m-3 in the upper layer) that resemble possible mantle conditions beneath the EARS, we find wavelengths on the order of those for the Ethiopian Rift and portions of the Kenya Rift. Although we have not yet incorporated the effect of background strain rate due to rift extension, spatially variable temperature, and more complicated rheologies (e.g., incorporation of a viscous yield stress), our preliminary results suggest that a RT instability in the upper mantle could conceivably control the volcano spacing along the EARS rift segments. In addition to incorporating the above complexities into our simulations, we plan to compare our predictions to seismic, petrographic, and structural studies in the EARS to further constrain the properties that may be required to form RT instabilities in the sub-rift mantle. ■

References
Bonatti, E., 1985. Punctiform initiation of seafloor spreading in the Red Sea during transition from a continental to an oceanic rift. Nature 316, 33-37.
Ebinger, C.J., Sleep, N.H., 1998. Cenozoic magmatism throughout east Africa resulting from impact of a single plume. Nature 395, 788-791.
George, R., Rogers, N., Kelley, S., 1998. Earliest magmatism in Ethiopia: Evidence for two mantle plumes in one flood basalt province. Geology 26, 923-926.
Hammond, J.O.S., Kendall, J.-M., Stuart, G.W., Ebinger, C.J., Bastow, I.D., Keir, D., Ayele, A., Belachew, M., Goitom, B., Ogubazghi, G., Wright, T.J., 2013. Mantle upwelling and initiation of rift segmentation beneath the Afar Depression. Geology, doi:10.1130/G33925.1.
Hieronymus, C.F., Bercovici, D., 1999. Discrete alternating hotspot islands formed by interaction of magma transport and lithospheric flexure. Nature 397, 604-606.
Jestin, F., Huchon, P., Gaulier, J.M., 1994. The Somali plate and the East African Rift System: present-day kinematics. Geophysical Journal International 116, 637-654.
Keer, R.C., Lister, J.R., 1988. Island arc and mid-ocean ridge volcanism, modelled by diapirism from linear source regions. Earth and Planetary Science Letters 88, 143-152.
Miller, N.C., Behn, M.D., 2012. Timescales for the growth of sediment diapirs in subduction zones. Geophysical Journal International 190, 1361-1377.
Mohr, P.A., Wood, C.A., 1976. Volcano spacing and lithospheric attenuation in the Eastern rift of Africa. Earth and Planetary Science Letters 33, 126-144.
Molnar, P., Houseman, G.A., Conrad, C.P., 1998. Rayleigh–Taylor instability and convective thinning of mechanically thickened lithosphere: effects of non-linear viscosity decreasing exponentially with depth and of horizontal shortening of the layer. Geophysical Journal International 133, 568-584
Mulibo, G., Nyblade, A.A., 2013. The P and S wave velocity structure of the mantle beneath eastern Africa and the African superplume anomaly. Geochemistry Geophysics Geosystems 14, 2696-2715.
Olive, J.-A., M. D. Behn, E. Mittelstaedt, G. Ito, and B. Z. Klein (2016), The role of elasticity in simulating long-term tectonic extension, Geophysical Journal International, doi:10.1093/gji/ggw1044.
Saria, E., Calais, E., Stamps, D.S., Delvaux, D., Hartnady, J.H., 2014. Present-day kinematics of the East African Rift. Journal of Geophysical Research 119, doi:10.1002/2013JB010901.
Vogt, P.R., 1974. Volcano spacing, fractures, and thickness of the lithosphere. Earth and Planetary Science Letters 21, 235-252.
Reference information
Investigating mantle controls on volcano spacing along the East African Rift System. E. Mittelstaedt, A. Sibrant

GeoPRISMS Newsletter, Issue No. 37, Fall 2016. Retrieved from http://geoprisms.nineplanetsllc.com

From rifting to drifting: evidence from rifts and margins worldwide mini-workshop

 icon-map-marker Grand Hyatt San Francisco
345 Stockton Street, San Francisco, CA
Union Square Room – 36th Floor

Sunday December 13, 2015, 8 – 1:30pm

Conveners: Rebecca Bendick, Ian Bastow, Tyrone Rooney, Harm van Avendonk, Jolante van Wijk

 icon-file-text-o Participant list

AgendaMeeting objectivesSTEPPE WorkshopMeeting report

Topic 1: Melt Generation in Extensional Environments
8:00-8:30 | Overview talk by Tyrone Rooney
8:30-8:45 | Panel discussion moderated by Harm van Avendonk

Topic 2: Magma-lithosphere interaction
8:45-9:15: Magma-lithosphere interaction | Chris Havlin
9:15-9:30: Panel discussion moderated by Ian Bastow

9:30-10 | Coffee

Topic 3: Stretching the lithosphere
10:00-10:30 | Stretching of the lithosphere |  Suzon Jammes
10:30-10:45 | Panel discussion moderated by Rebecca Bendick

Topic 4: Rifting and Oceanic Spreading
10:45-11:15 | Rifting and oceanic spreading – the focusing of melt delivery in space and time – Derek Keir
11:15-11:30 | Panel discussion moderated by Jolante van Wijk

11:30-12:30 | Lunch outside the venue

Discussion
12:30-13:00 | Summary of current results
13:00-13:30 | Avenues for future study

The purpose of this workshop is to facilitate discussion on the current state of research into continental extension. Our aim is to be broadly inclusive by bringing an audience with widely varying backgrounds to a common understanding of the state of the art in this field. Our ultimate goal will then be to pursue a discussion on future research challenges for the community and how these challenges align with the existing science plans for the GeoPRISMS Eastern North America and East African Rift Focus Sites. We will organize this meeting around the following themes:

  1. Melt generation in extensional environments: Mantle decompression, thermal state and composition of the mantle.
  2. Magma-lithosphere interaction: diking, metasomatism, thermal weakening, changing the composition of the lithosphere, coupling between deformation and melt migration.
  3. Stretching of the lithosphere: Strain localization in brittle and ductile rheology,  rates of extension, punctuated events.
  4. Feedback loops – rifting and surface processes: sedimentation, margin architecture
  5. Rifting and oceanic spreading – the missing link: Lithospheric breakup, focusing of melt delivery,  evolution of mantle deformation
Conveners:
Rebecca Bendick (University of Montana)
Ian Bastow (Imperial College London)
Tyrone Rooney (Michigan State University)
Harm van Avendonk (Univ. Texas Institute for Geophysics, UT-Austin)
Jolante van Wijk (New Mexico Tech)

Conveners: Michael McGlue and Christopher Scholz

Description: This STEPPE workshop will investigate source-to-sink processes through an examination of the Lake Tanganyika rift (East Africa), which faithfully records profound signals of tectonics, climate variability, and surface processes in a high-continuity sedimentary archive. The workshop will bring together inter-disciplinary experts to discuss the geodynamic, atmospheric, hydrologic, and biological processes affecting the Tanganyika hinterland that influence sediment generation and transport, as well as the limnological and depositional processes influencing stratal architecture and the composition of sediment. Lake Tanganyika is widely considered to be the premier target to recover a long-term, high resolution record of tropical climate, evolutionary biology, and rift tectonics via scientific drilling, and it is also an active frontier petroleum basin. The goal of the workshop is to lay the framework for future scientific drilling and consider the best pathways for deconvolving forcing mechanisms from the depositional signal, potentially through the application of new analytical techniques, integration of large digital datasets, or process modeling. Interested participants (especially early career scientists – students, post-docs, etc.) are encouraged to participate and contact the conveners for more information (michael.mcglue@uky.edu or cascholz@syr.edu).

From rifting to drifting: evidence from rifts and margins worldwide mini-workshop

AGU Fall Meeting 2015, San Francisco, USA

Conveners: Rebecca Bendick1, Ian Bastow2, Tyrone Rooney3, Harm van Avendonk4, Jolante van Wijk5

1University of Montana, 2Imperial College London, 3Michigan State University, 4Univ. Texas Institute for Geophysics, UT-Austin, 5New Mexico Tech

On Sunday December 13, 2015, from 8am to 1:30pm, a representative cross section of researchers interested in rifting met in the Grand Hyatt San Francisco before the AGU Fall Meeting. Our primary focus was to facilitate discussion on the current state of research into continental extension. Our aim was to be broadly inclusive by bringing an audience with widely varying backgrounds to a common understanding of the state of the art in this field. Our ultimate goal was to initiate a discussion on future research challenges for the community and how these challenges align with the existing science plans for the GeoPRISMS Eastern North America and East African Rift Focus Sites. To facilitate community building and cross disciplinary linkages, the meeting was coordinated with the STEPPE consortium (Sedimentary Geology, Time, Environment, Paleontology, Paleoclimatology, Energy) workshop investigating source-to-sink processes of the Lake Tanganyika rift (East Africa), which took place directly following the GeoPRISMS workshop from 2 to 8pm.

The meeting was structured to allow for discussion under four broad subheadings:

Topic 1: Melt Generation in Extensional Environments

A 30 minute introduction to this topic was presented by Tyrone Rooney. The talk covered the historical context of rifting studies and then focused on the relationship between magma and lithospheric strength. The concept of magma within the lithosphere facilitating rifting was introduced. The presentation examined how magmas provide an important temporal record of mantle processes during extension. It was shown how thermochemical constraints of the upper mantle source region of rift magmas could be probed with erupted lavas. In particular, the dual challenges of mantle potential temperature and pyroxenites in the upper mantle were highlighted as important frontiers in our understanding of mantle melting processes. The role of volatiles in some rifting environments (Rio Grande Rift) was introduced. The role of magmas in influencing seismic images of the upper mantle and also acting as a mechanism of strain accommodation during late stage rifting was also discussed. Finally, an examination of the continental lithospheric mantle as a possible magma source was also presented.
The discussion, moderated by Harm van Avendonk, first explored the issue of the role of water in magma generation processes. In particular, there were questions asked about the storage of water in water-bearing phases but also the ability of olivine to store volatiles. Further discussions continued on the role of hydrous phases on lithospheric rheology. The first key question arising from these discussion was – where could volatiles reside and how much in the source of rift magmas (especially water and carbon dioxide). Suggestions on approaching this question through studies of xenoliths and reconstructing lithospheric architecture were made. The second key question focused on the role of structural inheritance. It was acknowledged that crustal heterogeneity and mantle lithosphere heterogeneity may not necessarily correspond. Finally the third key question related to the amount of melt generation with the timing and magnitude of stretching.

Topic 2: Magma-lithosphere interaction

A 30 minute introduction to this topic was presented by Chris Havlin. This presentation first delivered an overview of the physics and thermodynamics of melt transport. This was further subdivided on the basis of porous flow within the mantle and lithosphere and in terms of crustal fractures and channels and how lithospheric inheritance influenced melt transport. The porous flow concept was expanded to examine the dependence on pressure gradients, buoyancy and dynamic pressure. The concept of a ‘freezing boundary’ was raised in terms of a melt focusing mechanism, which if dipping, could redistribute melt. Within the lithosphere the concept of lithospheric and crustal fabrics was raised. It was acknowledged that grain size may affect porosity and surface tension. As a result, melt is preferentially directed into smaller grain size domains. The presentation also examined end-member models of strain i.e. whole lithospheric heating, and basal heating and impact of the porosity front shallowing over time creating an effective thinning of the lithosphere. Finally, it was shown that there could be a growing zone of modified lithosphere whereby mechanically it behaves as does the asthenosphere but chemically it may still resemble the lithospheric mantle.
The discussion, moderated by Ian Bastow, first examined the concept of the background state of stress in rifting environments and how stress may change with changes in viscosity. It was noted that thinning does not require large extensional stresses. A point was raised on the competing grain size effects on porosity and surface area in relation to bulk permeability. Questions were raised by the group as what happens in relation to thinning and melt alteration of the lithosphere in seemingly amagmatic rift segments. It was acknowledged, however, that segments defined as amagmatic due to a lack of surface volcanism may still possess significant melt at depth within the lithosphere. As a result of these discussions, two key questions arose: (1) What is the role of melt in magmatic and amagmatic (in terms of surface volcanism) rift segments? and (2) What are the feedbacks between melt transport and lithospheric thinning and what are the mechanisms?

Topic 3: Stretching the lithosphere

A 30+ minute introduction to this topic was presented by Suzon Jammes. The presentation first examined the concept of mechanical stretching and the genetic relationship of stretching as an important factor in the Wilson Cycle. The factors controlling this mechanical stretching focused on exhumation, tectonic inheritance, and the control of rift and margin architecture. The topic of depth-dependant stretching was examined and how vertical decoupling was incompatible with pure and simple shear endmembers. An introduction to time-dependant stretching mechanisms followed with some idealized cross section of basinward migration of deformation. Dr. Jammes presented an evolutionary model whereby mechanical stretching was followed by the creation of a ‘necking zone’ for major crustal thinning and finally an exhumation phase. The discussion continued into a discussion of how rifting processes are determined by rheological layering of the lithosphere and the impact of structural inheritance and sensitivity to this vertical layering.

The discussion, moderated by Rebecca Bendick, was more limited due to time constraints but did establish a key question of how the feedbacks with melting might vary in terms of the recognized global variety of architectures of rifts and rifted margins.

Topic 4: Melt delivery and focusing

A 30 minute introduction to this topic was presented by Derek Keir. Dr. Keir showed how within the East African Rift changes in mantle potential temperature are probable first order controls on magma supply. It was also shown how variations in magmatism are multi-scalar with lateral variation at several scales both in the presence and absence of melt and melt chemistry. There was a view that melt pathways and focusing might represent the best mechanism for generating smaller scale variability and examples from the Black Sea and Afar were shown. Afar provided a particularly interesting case as in this region it was show that volcanism responded to increasing subsidence. That is, the more the thinning, the more melt and thus more melt focusing. Dr. Keir showed how a mantle potential temperature anomaly of at least 100 degrees could help explain observed seismic velocities and also the presence of melt throughout the region. A comparison was made between Afar and slow spreading ridges and also to Krafla (Iceland) between 1975 and 1984. The discussion continued as to the impact of melt focusing in time and space and how it is influenced by the temporal accumulations of tectonic stresses. The result of this was described as a general migration of volcanism from the rift flanks towards the rift axis with the competing tectonic and gravitational stresses.

The discussion, moderated by Jolante van Wijk, examined comparisons between the Havlin models discussed in topic 2 and those presented by Keir in topic 4. Some discussion centered on the concept of focusing at the lithosphere-asthenosphere boundary and then subsequent defocusing within the crust. It was acknowledged that geochemical data were critical to address these issues. It was noted that magmatic sources clearly differ along strike within the rift and thus are inconsistent with a single centralized source.

From rifting to drifting: evidence from rifts and margins worldwide | December 13 AGU 2015

Broad discussion

Following a break, the group reconvened to try and systematize some of the key concepts raised. The issues can be summarized as follows:

1. Rift Initiation

  • What is the role of mantle plumes?
  • How can mechanical heterogeneity facilitate initial rifting?
  • What role does chemical heterogeneity in the lithospheric mantle control initial extension?
  • What is the initial thermo-chemical structure of the lithosphere and asthenosphere in a nascent rift?
  • What does incipient rifting look like? Okavango suggests preexisting structure critical
  • Is this a top down or bottom up process? How does extension propagate?

2. Evolution of rifting in time and space

  • Why do rifts ultimately fail?
  • What is the role of nonlinear feedbacks?
  • How can datasets from igneous petrology and the sedimentary record provide a temporal insight into rift evolution?
  • What is the time evolution of strain?

3. Rift Architecture

  • How do non-uniqueness issues create difficulties in creating global models of rift evolution?
  • How can real constrains be linked with ever more innovative and detailed simulations?
  • What variables control the strength of the lithosphere?
  • What is the role of far-field vs. local controls on strain and rift evolution?

4. Volatiles in extensional environments

  • What are the volatile pathways from depth to the surface?
  • How deep are the volatiles derived from?
  • What is the role of rift valley volcanoes in global production of volatiles (e.g., CO2, SO2)?
  • How can lithospheric heterogeneity and inheritance influence the volatile budget?

In summary the basic concepts on which the group agreed that were critical for GeoPRISMS were:

  1. What is the history of melt? Where is it formed, when is it formed, why is it formed, how is it focused, and what pathways does it take through the lithosphere?
  2. What is the material (thermal and chemical) heterogeneity in the rift lithosphere? How does inheritance play a role, is there spatial organization at play, and how can we assess the importance of these heterogeneities to rifting?
  3. Comparison of focus areas is needed. How do ENAM and the EAR differ and how are they similar? What can be learned from focused studies at both sites?

Conference Report | Magmatic Rifting and Active Volcanism, Afar Rift Consortium (Addis Ababa, Ethiopia)

Anne Egger, Tyrone Rooney1, and Donna Shillington2

1Michigan State University, 2Lamont Doherty Earth Observatory

Figure 1. Map of the Afar rift region showing major tectonic and magmatic features from Ebinger et al., 2008.

Figure 1. Map of the Afar rift region showing major tectonic and magmatic features from Ebinger et al., 2008.

Conference Overview

The Magmatic Rifting and Active Volcanism (MRAV) Conference took place in Addis Ababa, Ethiopia January 10-13, 2012, convened by members of the Afar Rift Consortium, an international team investigating active magmatism and deformation in the Afar region. Over 200 people from around the world attended. The conference participants primarily presented the results of work on ongoing rifting processes in Afar, but work was also presented that addressed other portions of the East African Rift, comparable rift settings elsewhere, rifting processes in general, and the hazards and resources associated with the East African Rift.

The scientific program outlined the current state of knowledge in the East African rift and placed recent discoveries within the broader context of rift-related research globally. Central to the meeting was the presentation of results from thematic, multi-collaborator, international programs (e.g. Afar Consortium, RiftLink, Actions Marges), individual research groups, and industrial partners. The rich detail and modern datasets presented at the meeting highlight the importance of the existing infrastructure of international research in East Africa, which should be leveraged by GeoPRISMS to effectively focus resources in the extensive East African Rift System primary site.

Scientific Advances Related to GeoPRISMS Goals in East Africa

What follows is a brief summary of scientific results reported at the MRAV conference. A complete volume of abstracts and the program can be found at http://www.see.leeds.ac.uk/afar/new-afar/conference/conference.html. We present these results in the context of the questions outlined in the GeoPRISMS science Implementation Plan for the East Africa Rift System (EARS).

How is strain accommodated and partitioned throughout the lithosphere, and what are the controls on strain localization and migration?

A significant focus of the conference was the 2005 Dabbahu rifting event, which was dominated by a series of 14 dike intrusions and 4 eruptions with an estimated 2.5 km3 of magma intruded since September 2005. The initial Dabbahu diking events affected a large portion (60 km) of the magmatic segment, while subsequent activity was more localized. Several lines of evidence (including InSAR and seismicity) indicate that diking preceded and drove seismicity in the Dabbahu events. Importantly, the seismic moment and the associated slip along faults accounts for only 10% of the geodetic moment, indicating that most deformation in this rifting event was taken up aseismically, through dike injection or other igneous intrusion. Many aspects of this rifting resemble the 1974-89 rifting event at Krafla, in Iceland.

Additional recent tectonic activity reported on at the conference included the 2010 Gulf of Aden seismic swarm, which occurred along three segments of the rift at depths of less than 10 km. The 1989 Dobi earthquake swarm in central Afar appears to have followed a “bookshelf faulting” model, with slip occurring on at least 14 different faults during the earthquake sequence. The Asal rift was imaged with RADARSAT from 1997-2008; this time series showed 2-3 m of opening, accompanied by subsidence in the rift itself and uplift on the flanks with some component of shear.

What factors control the distribution and ponding of magmas and volatiles, and how are they related to extensional fault systems bounding the rift?

The Dabbahu event was dominantly a diking phenomenon, with magma playing a key role in crustal deformation. Similar to other portions of the rift, fractional crystallization processes and magmatic plumbing systems differ between axial and off-axis magmas. Resistivity surveys, surface velocity models, and receiver functions in the Dabbahu area all suggest that some 3000 km3 of magma remains in the crust, possibly stored in elongated magma chambers parallel to the rift axis, and that these may erupt on ~40 ka cycles. At upper mantle and lower crustal depths, the resistivity structure of active and inactive segments of the Afar rift are similar. The most significant heterogeneity exists at mid-crustal depths and is related to the presence or absence of melt.

Very high-resolution seismicity obtained through deployment of seismometer arrays helps detail the relationship between magmatic activity and faulting. While normal faulting occurs during the diking process, regions where magmatism has occurred are less seismically active. More broadly in the region, rift basalts show expected age progression with the youngest basalts at the center of the rift, and pointing to a spreading rate of 12 ± 1 mm/yr. However, less clear is off-axis magmatism, which shows no simple age progressive trend.

How does the mechanical heterogeneity of continental lithosphere influence rift initiation, morphology, and evolution?

Many presentations addressed aspects of the rift beyond the Dabbahu event. Comparing the recent, well-studied and well-constrained rifting event in Afar with the longer geologic record highlights that these processes change over time. Primarily, the asymmetry of the Afar rift suggests that the locus of rifting has migrated eastward. The orientation of different fault sets in the Asal-Danakil rift indicate two different directions of tension between 1.35 Ma and 0.3 Ma. This could be due to magmatic loading and flexure of the crust in addition to extension. Paleomagnetic data suggest minor block rotation (~7°) in Afar. The marginal grabens on the western edge of Afar are enigmatic: still seismically active, on top of the steepest gradient of crustal thickness. They are likely developed over crustal flexure, and the variability from north to south is controlled by migration of a wave of erosion. Farther south, thermochronology from the Albertine section of the rift show a complex, multi-stage cooling history and differential uplift within mountain blocks.

Several geophysical results suggest that structures at the surface mimic and reflect structures at depth in the lithosphere. Crustal anisotropy (fast direction) and the geoelectric strike both match the orientation of surface structures, with a transition zone in Afar. Both also increase in the magmatic segments of the rift: anisotropy is sensitive to strain fabrics, and MT to presence of melt. Shear-wave splitting directions in the mantle are different below mid-ocean ridges and the East African Rift. Below the Main Ethiopian Rift, they are parallel to rift axis; below the EPR, they are perpendicular to the rift axis. At slower-spreading ridges (mid-Atlantic and Gakkel), they are more variable. Gravity profiles across Dabbahu suggest a Moho depth of 19 or 23 km, and that faults at the surface may continue at depth.

How does the presence or absence of an upper-mantle plume influence extension?

At a wider scale, discussions focused on the lithosphere-asthenosphere boundary and how the thermo-chemical state of the East African upper mantle impacted the rifting process in East Africa. The nature of the lithosphere-asthenosphere boundary differs on the rift flanks in comparison to the central part of the rift. Beneath the flanks, velocities decrease with depth, suggesting melt pockets at the lithosphere-asthenosphere boundary, whereas velocities increase with depth beneath the main rift. These properties mean that at ~70 km depth, the rift in Afar resembles the East Pacific Rise. These observations are consistent with observations that at 50-150 km depth, the lowest seismic velocities follow the ridge structure. However, at 300 km depth, there is a very broad anomaly that lacks structure and extends down to the transition zone. Elevated mantle potential temperatures are detected in Afar and throughout the East African rift, supporting seismic evidence of a deep upwelling. Despite these elevated temperatures, the magnitude of the observed seismic anomalies cannot be explained solely by a thermal means and requires a chemical component within the upwelling.
How does rift topography, on either the continental- or basin-scale, influence regional climate, and what are the associated feedback processes?

Rifting affects climate through the construction of topography, which can have a significant effect on the local distribution of precipitation. Results of modeling experiments suggest that both tectonic events (the development of high topography associated with rifting) and orbital forcing (variability in insolation) are likely to have affected climate in eastern Africa over the last 20 million years. The East African Rift is also an excellent location to explore the mesoscale affects of orography, due to the presence of multiple lakes. Lakes generate their own weather, and interact with prevailing winds and local topographic features. There are coring efforts underway in Lake Malawi to test these effects. Rift lake sediments preserve unique records of climate and tectonics, including key time intervals in hominid evolution.

Figure 2. A fissure on the edge of Lake Besaka. Fantale volcano is in the background; it last erupted 170,000 years ago.

Figure 2. A fissure on the edge of Lake Besaka. Fantale volcano is in the background; it last erupted 170,000 years ago.

Broader Impacts

Hazards

Volcanic hazard risks associated with Ethiopian volcanoes are unexpectedly high, largely due to the uncertainties associated with individual volcanic centers. In particular, the geologic record is temporally limited. Of concern is that InSAR observations have shown that there are far more volcanoes that are currently deforming than have erupted historically, suggesting significant potential for future eruptions. To more broadly assess volcanic hazard potential, the NERC-funded ‘Global Volcano Model’, in cooperation with 12 international partners, seeks to better characterize potentially hazardous volcanoes.
Remote volcanic hazard monitoring through SO2 emissions, InSAR, thermal imaging, and infrasound, provide means to monitor volcanoes in difficult to access areas. Eruptions in remote regions may not have an immediate hazard impact due to sparse habitation, however the Nabro event in Eritrea was determined to have been the largest SO2 producer since 1991. These remote sensing techniques therefore have further application for global SO2 models with obvious implications for climate change studies.

Resources

The economic potential of East Africa is substantial; energy, commodity and tourism resources are clear growth areas. Epithermal gold deposits in Afar that are associated with geologically modern hydrothermal systems linked to rift magmatism are targets of active exploration. The gold potential of these systems is enhanced by the relatively low salinity magmatic environment in the rift. The resources being devoted to this epithermal play speak to the resource potential of currently active rifts (i.e. we do not have to wait for them to fill with sediments and develop oil).

There is extensive oil exploration in Lake Albert region in Uganda, and many boreholes have been drilled. Little production is occurring at this time, due to transport constraints, although estimates of the resources are substantial (~1000 million barrels). Oil exploration has also focused on the Lake Turkana region, where very detailed gravity, magnetic surveys and mapping have been completed.

Significant challenges remain in the electrification of East Africa. Only 15% of East Africans have access to electricity with an average consumption of 68 KwH/yr (compared with ~2500 KwH/yr per person globally). With current production, every East African could light a 60W bulb 3 hours/day. Energy production needs to expand 33 fold. So far, only ~1% of the geothermal potential of the Ethiopian Rift has been exploited. And while geothermal energy is a key area of exploration, there are inherent problems with power generation and cost scaling – small facilities are more costly to operate. There is also a drive to construct more dams for hydropower in Ethiopia, but the selection of dams is complicated by seismic and volcanic activity, which may be episodic.

One particularly interesting presentation addressed geotourism as a growing industry that should be examined in more detail, including prioritizing the generation of digestible information and graphics for visitor centers.

Figure 3. Field trip participants examine 'blister cave' in a welded tuff in the southern Afar.

Figure 3. Field trip participants examine ‘blister cave’ in a welded tuff in the southern Afar.

Future Opportunities and Challenges for GeoPRISMS

Attendees expressed strong interest in continuing research in the Afar region, as well as other parts of the East African Rift. Several projects are continuing or planned, and there are multiple opportunities for GeoPRISMS. Close collaborations with African scientists, particularly, will be essential to the success of GeoPRISMS work in the EAR, and many scientists from Ethiopia and elsewhere who attended the meeting expressed enthusiasm for such interactions.

The conference was opened by the Ethiopian Minister for Mines, who emphasized her desire to engage international scientists and the need to translate the scientific knowledge gained through research into economically useful information. The logistical, cultural, and administrative challenges of working in East Africa require and benefit from close collaboration with scientists from the host countries. Many of the participants from Africa were directly involved in the energy, commodity, or tourism industries, or other efforts that closely link to the scientific research being undertaken in the region. Another opportunity for GeoPRISMS scientists is to build successful cooperative efforts by linking the fundamental research to applications in energy, resource development, and hazards mitigation that can yield tangible benefits to the host country.

The conference was closed by the Dean of Research at Addis Ababa University, who articulated the need for a better understanding of the rift and its consequences for hazards and announced a new 5-year, $10 M Ethiopian birr (over $500,000 USD) initiative focused on hazards. Representatives from energy companies (including geothermal and hydrocarbon) and mining companies also attended the meeting and expressed interest in collaborating with international academic teams to better understand the tectonics and their consequences for resources. In January 2013, the 24th Colloquium of African Geology will be held in Addis Ababa, with sessions dedicated to the East African Rift, providing an additional opportunity to focus GeoPRISMS’ efforts.

Numerous graduate students from around the world were present at the meeting, as well as several undergraduates from Addis Ababa University. The opportunities to build research capacity in Africa by involving graduate and undergraduate students from the host countries in research are tremendous, and should be a part of any GeoPRISMS effort.

Ultimately, GeoPRISMS must work closely with East African scientists and develop a strategy that complements and capitalizes on existing initiatives. The opportunities for meaningful collaborations are significant.

Reference information
Report from the Magmatic Rifting and Active Volcanism Conference, Afar Rift Consortium (Addis Ababa, Ethiopia), Egger, A., Rooney T., Shillington D.;

GeoPRISMS Newsletter, Issue No. 28, Spring 2012. Retrieved from http://geoprisms.nineplanetsllc.com

Report from the Field | Welcome to a field season at Ledi-Geraru, Afar, Ethiopia!

Erin DiMaggio (Arizona State University)

Figure 1. NASA MODIS imagery of the Afar Depression highlighting the location of the Ledi-Geraru Research Project.

Figure 1. NASA MODIS imagery of the Afar Depression highlighting the location of the Ledi-Geraru Research Project.

The 2013 Ledi-Geraru Research Project field season brought together geologists, paleontologists, and archaeologists from multiple universities to study the environmental context of human evolution in the Afar Depression, Ethiopia. Our field area is located in the southern Afar Depression, near the famous early hominin sites of Hadar and Dikika. This season we focused our efforts on the eastern portion of Ledi-Geraru (ELG) because of its fossiliferous sediments, presence of stone tools, and extensive outcrops. We also targeted this location because the time period represented by the sediments at ELG is scarcely represented in the sedimentological record in Ethiopia and in East Africa in general. As a result, we lack knowledge about important events during this time period including major changes in faunal (animal) populations, and the beginning of stone tool manufacture. Furthermore, the faulting history of the Afar Depression since the late Pliocene (<3.0 Ma) is captured in the structure and geomorphology of the region, all nicely exposed along tributaries of the Awash River. Below I organized some of my field notes to provide a short preview into the daily life and culture at the Ledi-Geraru Camp. Enjoy!

“I am dirty, smelly, and have obviously not showered in three days of field work, but I had a chocolate donut for breakfast. This camp is great!” I mentioned this to my advisor, Ramón Arrowsmith one morning after finishing a freshly made donut and gearing up for another day in the field. It’s true! Aside from the constant barrage of dust that coats anything left out for more than 30 seconds, and afternoon temperatures that make me want to join the Afar hiding under the Land Cruiser for a quick shady nap, I have to admit that our camp life is pretty plush. Our cooks are pros at setting up a fully functional and clean kitchen, including a bread baking station, a deep fryer for our nightly fill of fried potatoes, and a food storage system that somehow defies the laws of spoilage and bug infestation. I can’t even manage to keep a bottle of contact solution in my tent without somehow attracting a line of ants! We are served dinners that include a range of pasta dishes, fried eggplant, and my personal favorite, goat kebobs and tomato salad, all of which are served with soup, fried potatoes, fresh bread, and veggies.

All Under One Tent (1/21/2013)

Today, Brian wanted to better understand the geologic context for some of the fossils found in a particular region earlier in the day. Brian Villmoare (George Washington University) and Dominique Garello, a geology graduate student (Arizona State University), are sporting stylish red/blue 3D glasses because I do most of my geology mapping on anaglyphs created from aerial photographs. I also use high resolution (0.5 meter) satellite imagery for mapping faults or the extent of a volcanic ash deposit, which I later check in the field. It was not until I arrived in the field and completely immersed myself in multiple research worlds that I genuinely understood interdisciplinary research. Geologists, archeologists, and paleontologists all actively collected data within one shared field area and organized, planned, and analyzed results under one central work tent. For example, our geologic mapping helps to determine which archeology sites the archaeologists will focus their efforts on, and faulting patterns that we map one day may direct where paleontologists survey or collect fossils the next day. We are continually communicating our results and changing our plan for the following day based on what we have learned.

Figure 2. (left) Cook Getachew Senbeto is preparing a great dinner for 40 in our well-organized cook tent. Figure 3. (right) Brian (left), Dominique (right), and I (center) look over the days mapping using anaglyphs (hence, the 3D glasses) to investigate the geology of a fossiliferous area.

Figure 2. (left) Cook Getachew Senbeto is preparing a great dinner for 40 in our well-organized cook tent. Figure 3. (right) Brian (left), Dominique (right), and I (center) look over the days mapping using anaglyphs (hence, the 3D glasses) to investigate the geology of a fossiliferous area.

The Wild Life (1/22/2013)

This morning we were greeted by a large group of ostriches and gazelle hanging out by the road. There is nothing wilder than an early morning race to the field against a half dozen ostriches! We are also fortunate to see baboons, warthogs, and occasionally a hyena. Later in the day, Dominique and I were taking measurements of faults along a steep cliff outcrop on the banks of the Mille River. Our Afar friend, Ali Yasin, and our representative from the National Museum of Ethiopia, Tesfaye (ARCCH), informed us not to proceed further along the water. We were confused and thought it might be due to that fact we were on loose slopes. We were wrong. Ali had been watching a crocodile slowly approach where we were working. Needless to say, the strike and dip measurement I was after will have to wait for another day!

Figure 4. Land Cruiser vs. ostriches – a morning race to our field site.

Figure 4. Land Cruiser vs. ostriches – a morning race to our field site.

Figure 5. (top) Omar Abdullah showing us stone carvings in basalt boulders. Figure 6. (bottom) Tephra deposits (white and yellow layers) faulted by a beautifully exposed normal fault.

Figure 5. (top) Omar Abdullah showing us stone carvings in basalt boulders. Figure 6. (bottom) Tephra deposits (white and yellow layers) faulted by a beautifully exposed normal fault.

Ancient Rock Art (1/26/2013)

The Afar are proud of their heritage and were very excited to take us on a trip to show us a place near the Awash River where ancient people had created rock art along the sides of one of the basalt hills. In this photo, Omar Abdullah is pointing out a particularly beautiful etched rock with numerous animals including gazelle, camels, pigs, and monkeys! It was relaxing to take a day away from work and play tourist in this beautiful land guided by our Afar friends who were proud to share their land and its history with us.

Geology at ELG (1/27/2013)

The lack of vegetation at ELG, and in the Afar in general, is a blessing and a curse. A blessing because, well, I’m a geologist and there is no shortage of exposed rock! The only hindrance to acquiring a fault plane measurement or measuring and describing a 30 meter stratigraphy section is the sometimes thick cover of eroded sediment – all remedied by good shovel and geology pick. The curse is trying to find a suitable location for lunch, when “Me’e silalo” (good shade in the Afar language) is hard to come by mid-day, often taking the shape of a few feet of shade provided by the Land Cruiser. Today after lunch we found this beautifully exposed fault that slices through two volcanic ash deposits. There is no shortage of faults in the ELG thanks to its proximity to the Afar Triple Junction. In fact, sometimes it is hard to find a complete stratigraphy section to measure that is not interrupted by a fault! Luckily, there are also abundant volcanic ash deposits (or tephras) interbedded in the sediments (see white layers in the photo). Tephras are extremely valuable to the project because they serve as marker beds across the landscape, and some contain crystals that can be dated using 40Ar/39Ar methods, or fresh glass shards that we can use to ‘fingerprint’ the tephra for possible correlation to other areas within ELG or Afar.

Figure 7. Local Afar children.

Figure 7. Local Afar children.

Afar Kids (2/1/2013)

The Afar children living and working around our camp site (most Afar children have shepherd responsibilities) love to see what we are up to. Hiding behind trees at a set distance, the kids are curious but shy and quickly warm up when approached. Today we brought out our cameras and had some fun with them taking photos. I realized during my first field season in the Afar a few years back that the Afar kids didn’t seem to smile when I took their photo. Why? Well, simply, we teach kids to smile for the camera from the time they are born, and it becomes second nature. The Afar kids had huge grins on their faces after we took their photo, when they see themselves and their friends on the digital screen. They point at themselves and their friends and giggle, giggle, giggle!

The Fossil Hunt (2/2/2012)

Today the geologists all headed out with paleontologists Kaye Reed, Brian Villmoare (right), graduate student John Rowan (left), and the best Afar fossil hunters to survey and collect fossils (left photo). I learned quite a bit from them including that some fossils are more important than others. What do I mean by that? Because of their size, elephant fossils are commonly found throughout ELG. But collecting elephant fossils is laborious (they are huge!) and are not a very diagnostic species (in contrast to, say, pig fossils). While a few elephant fossils were collected (mostly teeth), elephant and hippopotamus fossil abundance is only noted so that it can be included in later descriptions the region’s paleoecology. In this photo, John and Kaye are holding small yellow ruggedized computers called Nomads. The Nomads are used to store and catalog information (GPS coordinates, element, genus, etc.) about each fossil to a centralized digital database.

Location, Location, Location (2/3/2013)

One of my tasks this season was to complete a gazetteer of Afar place names within the eastern Ledi-Geraru (ELG), including the most ‘correct’ Afar to English spelling and the meaning of the place name. Last night I spent an entire evening with our kind Afar Regional State Representative, Mohammed Hameddin, who did an excellent job of aiding in my not-so-easy quest. Hands down, my favorite place name (and story) is a location in the southern part of ELG, referred to as Dabali Isi. Mohammed spelled out the name for me, while our two best Afar geographers, Ali Yasin and Subudo Baro, explained the meaning of the place name to Mohammed. I knew this was going to be an amusing story because all three men had a smirk on their face during the exchange. Mohammed smiled, stood up and said, “The Afar are telling me that Dabali Isi is named after a woman who was passing through that area. She was very, very beautiful and had…” Mohammed stopped speaking and proceeded to caress his sides and top of his rear end. I was brought to laughter (Mohammed is a very funny man!) and awkwardly had to try and guess the meaning of his gestures. “Does it mean rear end?” Nope. “Sides?” Nope. “Shape?” Yes. As the story goes, Dabali Isi was a very beautiful woman passing through that particular area who had a very, very, memorable womanly form. The Beyoncé of the Afar Depression!

Figure 8. (left) John, Kaye, and Brian (left to right) head out for an exciting day of fossil collecting. Figure 9. (right) Mohammed Ahameddin (sitting, center), Kadar Mohammed, Mohammed Ibrahim, and Ali Yasin (left to right) spell and explain the meaning of local place names for our project gazetteer.

Figure 8. (left) John, Kaye, and Brian (left to right) head out for an exciting day of fossil collecting. Figure 9. (right) Mohammed Ahameddin (sitting, center), Kadar Mohammed, Mohammed Ibrahim, and Ali Yasin (left to right) spell and explain the meaning of local place names for our project gazetteer.

“Lucy Dinga”, a.k.a. Archaeology (2/1/2013)

The Afar people have played an integral role in the work that is conducted in the Afar, some as fossil hunters, others as guides and geology field assistants. Many of the same men return year after year to help in our project, and know well the history that surrounds the search for early humans and stone tools in the outcrops along the Awash that began in the 1970’s. In the Afar language “Dinga” means rock, and almost all of the Afar in the Mille and Elowa region know about the famous discovery of Lucy at Hadar in 1974. As a result the Afar refer to stone tools as “Lucy Dingas”. Today, Dominique and I wanted to see the process of a site excavation and so we spent the afternoon with the archaeologists learning about excavation techniques. We also learned about how to identify “Lucy Dingas” among a wealth of stream fractured cobbles that blanket surfaces across the Ledi-Geraru. In the photo above, from left to right, archaeologists, Will Archer, Yonatan Sahle, and David Braun (U. of Cape Town), prepare for excavation of a site in the Ledi-Geraru. The site is located on the slope on the right, while the total station is located across such that each point is visible and can be accurately measured.

Figure 10. Archaeologists prepare for excavation of a site in the Ledi-Geraru

Figure 10. Archaeologists prepare for excavation of a site in the Ledi-Geraru

Overall, we had a very successful field season – we collected new fossils and artifacts, geologic observations, and hundreds of pounds rocks to be analyzed! We hope that this brief glimpse into life at camp, the culture of the Afar people, and the work conducted by Ledi-Geraru researchers opened a door to the process and excitement of conducting field work in Ethiopia.

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.nineplanetsllc.com. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

Reference information
Welcome to a field season at Ledi-Geraru, Afar, Ethiopia! DiMaggio E.

GeoPRISMS Newsletter, Issue No. 30, Spring 2013. Retrieved from http://geoprisms.nineplanetsllc.com

Workshop Report | GeoPRISMS Planning Workshop for East African Rift System

Morristown, NJ – October 25 – 27 2012

Workshop conveners: Ramon Arrowsmith1, Estella Atekwana2, Maggie Benoit3, Andrew Cohen4, Rob Evans5, Matthew Pritchard6, Tyrone Rooney7, Donna Shillington8

1Arizona State University; 2Oklahoma State University; 3The College of New Jersey; 4University of Arizona; 5WHOI; 6Cornell University; 7Michigan State University; 8Lamont Doherty Earth Observatory

Background and Motivation

The planning workshop for the East African Rift System (EARS) GeoPRISMS primary site was held in Morristown, NJ, on October 25-27th 2012, mere days before Hurricane Sandy made landfall. An international group of ~115 attendees took part, including a gratifyingly large number of graduate students (~40). Overall, 15 different countries were represented, with a large number of participants from several African countries, including Tanzania, Uganda, Kenya, Ethiopia, Malawi, and the Democratic Republic of Congo (figure 1).

Figure 1. Participants at the GeoPRISMS EARS Workshop in Morristown, NJ, October 2012.

Figure 1. Participants at the GeoPRISMS EARS Workshop in Morristown, NJ, October 2012.

The East African Rift System was chosen as a primary site for GeoPRISMS because it offers significant opportunities to study a wide range of questions outlined in the GeoPRISMS Science Plan for the Rift Initiation and Evolution (RIE) Initiative, as outlined in the GeoPRISMS Science Plan and the Draft Implementation Plan (http://www.geoprisms.nineplanetsllc.com/science-plan.html); these documents served as the starting point for this workshop.

The main goals of the workshop were to clarify the community research objectives in the EARS, to discuss candidate focus areas for concentrated research, to identify opportunities for international partnerships, and to develop a detailed Implementation Plan for GeoPRISMS research in EARS to guide GeoPRISMS proposers and reviewers, considering the available resources and infrastructure.

Figure 2. Students gathed around Roy Schlische and Martha Withjack, from Rutgers University, leaders of the student field trip in the Newark Basin

Figure 2. Students gathed around Roy Schlische and Martha Withjack, from Rutgers University, leaders of the student field trip in the Newark Basin

Student Symposium

Prior to the formal meeting, a student symposium was organized by Maggie Benoit (The College of New Jersey). Interested students were given an introduction to the East African Rift System and associated projects, a chance to present their own research to their peers, and an opportunity to meet some of the meeting conveners in an informal setting. More than just providing information on the existing state of knowledge in the region, this event facilitated team building and critical thinking, which allowed the student participants to produce a well-thought out plan of their own during the formal meeting. A field trip to the Newark Basin, led by Martha Withjack and Roy Schlische (both of Rutgers University), visited some local rift features (figure 2).

Figure 3. Workshop attendees participate in post-workshop field trip to the northern part of the Newark Basin.

Figure 3. Workshop attendees participate in post-workshop field trip to the northern part of the Newark Basin.

Post-workshop field trip

The day after the workshop, Paul Olsen, from LDEO, led a field trip for all interested workshop attendees, exploring the northern part of the Triassic-Jurassic Newark basin. This trip provided an overview of this ancient rift basin, analogue to the active East African Rift basins, highlighting similarities and major differences between the two systems (figure 3).

Workshop Plan

The planning workshop itself was structured around 5 key questions from the RIE component of the GeoPRISMS science plan pertinent to the East African Rift. Talks were organized around these topics to give the audience an overview of what is known of the rift system and, more critically, what remains unknown. Presentations from selected talks below are available on the GeoPRISMS website (figure 4).

Topic 1: How does the presence or absence of an upper-mantle plume influence extension?
  • Seismological imaging of plumes and associated magmatism in rifts – Gabriel Mulibo and JP O’Donnell.
  • Origin of magmas from geochemical perspective – Tyrone Rooney
  • Plume dynamics and surface uplift – D. Sarah Stamps
Topic 2: How does the mechanical heterogeneity of continental lithosphere influence rift initiation, morphology, and evolution?
  • Mechanisms for thinning the lithosphere, including thermal/chemical erosion, and interaction with large scale lithospheric structures –Ben Holtzman
  • Control of pre-existing structures on early rifting –Aubreya Adams
  • Geochemical heterogeneity of the lithosphere – Wendy Nelson
Topic 3: How is strain accommodated and partitioned throughout the lithosphere, and what are the controls on strain localization and migration?
  • Magmatism during rifting events – David Ferguson
  • Modeling and observations of faulting and magmatism during rifting – Juliet Biggs
  • Active deformation processes – Becky Bendick
Topic 4: What factors control the distribution and ponding of magmas and volatiles, and how are they related to extensional fault systems bounding the rift?
  • Geochemical studies of magmas and volatiles – Tobias Fischer
  • Geophysical imaging of magmas and fluids (MT, seismic): Derek Keir
  • Shallow dynamics of magma chambers/dikes and eruptions – Manahloh Bechalew
Topic 5: How does rift topography, on either the continental- or basin-scale, influence regional climate, and what are the associated feedback processes?
  • Climate and tectonics and feedbacks – Manfred Strecker
  • Modeling perspective – Joellen Russell
  • Tectonics and sedimentation at basin scale – Chris Scholz
Topic 6: Hazards and Resources in the EAR and Links to Rifting
  • Seismic hazard – Ataley Ayele
  • Volcanic hazard – Nicolas d’Oreye
  • Oil/gas exploration – Dozith Abeinomugisha

Topic 7: Synergies with other agencies / international projects

Topic 8: African partnerships panel

Figure 4. Juli Morgan, GeoPRISMS Chair, introduces the GeoPRISMS Program.

Figure 4. Juli Morgan, GeoPRISMS Chair, introduces the GeoPRISMS Program.

Break-out discussions were interspersed with the plenary sessions, enabling more focused discussions about potential topics of future research. Breakout sessions on Day 1 focused on identifying the most compelling science, the highest priorities for GeoPRISMS program funds, and which types of experiments or observations might be needed. Participants were also asked to identify which themes, if any, require focusing of effort with concentrated, collaborative investigations at specific sites.

Recognizing that the East African Rift offers significant broader impacts, both in terms of hazards and resources, and in terms of education and capacity building opportunities, plenary sessions were organized to cover these topics. A session at the end of Day 1 focused on seismic and volcanic hazards, as well as opportunities that might arise from oil and gas exploration activities. On Day 2, a panel of African colleagues gave valuable insights into what needs to be considered when entering into partnerships with scientists in African nations, and thoughts on how to build successful collaborations.

The conveners also recognized that work in EARS will require PIs to initiate international collaborations and, in some cases, seek funds from other programs at NSF and elsewhere, in order to accomplish their goals, and the goals of the GeoPRISMS Program. Overviews of existing programs and other opportunities for funding were given both by invited speakers and through “pop-up”, sessions in which participants were given the opportunity to express their own thoughts and interests to the meeting. Student participants were also given the opportunity to highlight their own work through brief “pop-up” presentations.

Breakout sessions on Day 2 started to focus in on identifying target areas where the key questions could best be addressed, with the aim of narrowing in on a few locations. In addition, the student participants organized an additional session in the evening (and into the early hours) distilling the information they had gained throughout the workshop, into a decision matrix which they presented on Day 3. The final breakouts attempted to gauge interests in the various sites identified as candidates for focused effort.

Workshop Outcomes

Shortly following the meeting, the conveners distilled the feedback and outcomes of all the discussions and identified the following as the potential areas for GeoPRISMS effort (figure 6).
Primary focus area: The Eastern Rift, The Eastern Branch of the EARS was identified in breakout groups and by the graduate students as a location where a focused inter-disciplinary effort could substantially impact our understanding of rift processes and effectively address the majority of the science questions that form the core of the science plan. This region would encompass the rift from the Tanzanian divergence in the south to Lake Turkana and southern Ethiopia to the north. Particular opportunities highlighted by discussion and relevant to the science plan include (but are not limited to) the role/origin of a plume in this part of this rift; the interaction of the rift and plume with major lithospheric structures; an active magmatic system; along-strike variations in the amount of cumulative extension and lithospheric thickness (from thin in the north to thick in the south); the preservation of a record of the interplay of climate and tectonics. The existing studies characterizing this region provide a rich framework upon which GeoPRISMS science will build.

The conveners also identified what they termed “Collaborative Targets of Opportunity” where we recognize that efforts have been ongoing, offering leveraging opportunities for future programs.

Target 1: The Afar and Main Ethiopian Rift. This part of the rift system is the focus of intense recent and ongoing international and US efforts. Further GeoPRISMS studies that could enhance our understanding of rifting processes include (but are not limited to) efforts that examine strain localization, and studies probing the origin and role of a plume in rifting. The recent rifting and eruption in this region allows studies of active processes.

Target 2: The Western Rift and SW branch. This site would provide the opportunity to examine the role of magmatism in rifting by comparing this comparatively less magmatic system with the highly magmatic Eastern Rift. It also contains the most weakly extended portions of the rift and thus can be used to tackle questions concerning incipient rifting. Finally, deep lakes along the Western Rift contain the longest continuous record of climate/tectonic interactions available for the EARS. New GeoPRISMS studies in this area can leverage recently funded NSF programs and other previous and ongoing tectonic and climate investigations.

Target 3: Synoptic investigations along the entire rift. As identified in many discussions at the workshop, there are questions in the science plan that are best addressed by examining the rift as a whole.  These concern rift-wide variations in the origin and timing of volcanism, the strain field along and across the rift and large scale structure and dynamics underpinning the rift system.  Thus, key components of the implementation plan should include broad and open data assimilation efforts, strategic infilling of climatic, geochemical, and geophysical observations, and modeling and experimental work, which would provide a framework for the focused investigations along the rift.

The workshop conveners are currently in the process of wrapping up the first draft of the GeoPRISMS implementation plan for the East African Rift System primary site, which then will be disseminated to the community for input. The conveners thank all participants for their attendance and input to this plan, and the GeoPRISMS Office for coordinating a successful workshop.

Figure 5. A map of the East African Rift System (EARS) highlighting the primary focus area and the Collaborative Targets of Opportunity discussed in the Implementation Plan.comes attendees and introduces the GeoPRISMS Program.

Figure 5. A map of the East African Rift System (EARS) highlighting the primary focus area and the Collaborative Targets of Opportunity discussed in the Implementation Plan.comes attendees and introduces the GeoPRISMS Program.

 Reference information
GeoPRISMS Planning Workshop for East African Rift System – Report, Arrowsmith, R., Atekwana, E., Benoit, M., Cohen, A., Evans, R., Pritchard, M., Rooney, T., Shillington, D.
GeoPRISMS Newsletter, Issue No. 30, Spring 2013. Retrieved from http://geoprisms.nineplanetsllc.com