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.org

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.org

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

 

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
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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.org

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. ■

References

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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.org

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. ■

References

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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.
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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.
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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.org

Late Stage Rifting and Early Seafloor Spreading History of the Eastern North American Margin


Anne Bécel
Lamont-Doherty Earth Observatory, Columbia University

During September-October 2014, the NSF-GeoPRISMS-funded Eastern North American Margin (ENAM) Community Seismic Experiment (CSE) collected deep penetration multichannel seismic (MCS) reflection profiles covering a 500 km wide section of the Mid-Atlantic passive margin offshore North Carolina, which formed after the Mesozoic breakup of supercontinent Pangea The ENAM-CSE data extend farther offshore than previous seismic surveys conducted in this area and encompass the full transition from continental breakup to mature seafloor spreading while specifically providing unique constraints on the events surrounding the final stage of continental rifting and the initial stage of seafloor spreading, which remain poorly understood. The results shown here demonstrate the ability of MCS data to image four distinct domains that highlight different basement characteristics and provide new insights on the degree of extensional strain localization experienced during continental breakup and how the earliest oceanic crust was formed after rifting.

Introduction

The Eastern North American Margin (ENAM) is a passive continental margin that was formed by the rifting of the Pangaea supercontinent and the opening of the Atlantic Ocean during the Late Triassic and Early Jurassic.

Figure 1. a) Elevation Map (Andrews et al., 2016) contoured every 500 m showing the location of the ENAM Community Seismic Experiment. Line 1 and ENAM Line 2 were chosen to characterize the deep structure of the Carolina Trough south of Cape Hatteras, and the Baltimore Canyon Trough north of Cape Hatteras, respectively whereas the Line 3 was chosen to characterize the structure of the crust and uppermost mantle to better understand the origin of the Blake Spur Magnetic anomaly. b) Magnetic anomaly map (Maus et al., 2009) of the North American Margin. ECMA: East Coast Magnetic Anomaly; BSMA: Blake Spur Magnetic Anomaly; IMQZ: Inner Magnetic Quiet Zone.

From offshore Nova Scotia to Florida, the ENAM has been classified as a volcanic-type margin (Marzoli et al. 1999). Multichannel seismic profiles have imaged seaward dipping reflectors (SDRs) that have been attributed to the subaerial eruption and subsequent subsidence of volcanic flows emplaced during the final phase of rifting (Austin et al., 1990). Seismic refraction profiles beneath the volcanic wedges have revealed a thick sequence of high seismic velocity lower crust rocks interpreted as igneous/magmatic underplating (Holbrook et al., 1994). The East Coast magnetic anomaly (ECMA) is a high-amplitude positive magnetic anomaly running along the length of the margin (Fig. 1) (Keller et al., 1954). The source of the ECMA has been primarily attributed to seaward dipping reflectors in the upper crust (Austin et al., 1990) and is interpreted as the limit between the continental crust and the normal oceanic crust. However, the exact nature and the width of the zone between the continental crust and normal oceanic crust remain uncertain. This zone is thought to either represent a new anomalously thick magmatic crust with higher velocity than lower oceanic crust with no continental crust present (Talwani et al., 1995) or a zone with volcanics on top of magmatic material intruded into extended continental crust or underplated beneath. The nature and the width of this zone are of fundamental importance to understanding the late stage rifting processes and over what time period the continental breakup occurred at this volcanic margin. Margins that experience a voluminous magmatism during rifting tend to have a more rapid continental breakup with a smaller zone of crustal extension (i.e. strain localization) and tend to develop more symmetric conjugate margins.

The Blake Spur magnetic anomaly (BSMA) is a positive, linear magnetic anomaly located 150-250 km to the east of the ECMA (Fig. 1). The BSMA is of lower amplitude than the ECMA but also consists of segments with several magnetic peaks separated by troughs. The age of BSMA is unknown but extrapolated ages range between 168-173 Ma. The nature and origin of this magnetic anomaly is still debated and different models have been proposed. BSMA is either thought to mark a ridge jump (Vogt, 1973), magmatic pulse associated to a plate re-organization (Klitgord and Schouten, 1986; Kneller et al., 2012) or a change in spreading rate/direction and asymmetry of incipient seafloor spreading during the early opening of the Central Atlantic (Labails et al., 2010). In the ridge jump scenario, the BSMA is thought to represent a sliver of West African rifted continental crust that experienced continental breakup magmatism and that was left on the Eastern North American margin after the early spreading center jumped east of the BSMA. This model implies that a now extinct mid-ocean ridge lies between ECMA and BSMA.

The Inner Magnetic (Jurassic) Quiet zone (IMQZ) lies between the ECMA and the BSMA (Bird et al., 2007). Because the magnetic anomalies are of very low amplitudes and variable in shape, the correlation of magnetic anomalies with magnetic reversals remains challenging in this zone (Fig. 1). Timing and location of breakup at the ENAM thus remain uncertain and the spreading rate of the earliest normal oceanic crust in the IMJQ is not well constrained.

Data acquisition and project goals

This project aims to extract information on the late-stage continental rifting including the relationship between the timing of rifting and the occurrence of offshore magmatism and early seafloor history of the Central Atlantic using multichannel (MCS) data from the ENAM-CSE. The MCS data were acquired on R/V Marcus Langseth using a 6600 cu. in. tuned airgun array and 636 channel 8-km-long streamer. The source and the streamer were both towed at a depth of 9 m for deep imaging. This project involves the multichannel seismic processing and interpretation of two offshore margin normal profiles (450-km-long and 370-km-long, respectively), spanning from continental crust ~50 km off the coast to mature oceanic crust 110 km east of the BSMA and a ~350-km-long MCS profile along the BSMA (Fig. 1). These primary MCS lines are also coincident with the ENAM seismic refraction profiles recorded on ocean bottom seismometers.

Results

The high-resolution MCS data provide detailed structure of the sedimentary cover and crust (Fig. 2 and Fig. 3). The initial images of the two margin normal profiles reveal several major changes in the basement character and roughness between the ECMA and the BSMA (Fig. 2) that have not been previously described. The four domains described below correspond to distinct magnetic anomalies that suggest that magnetization contrasts exist between those domains. The interpretation of the new observations from MCS data give new important insights into the late stage of rifting and rift to drift transition.

Figure 2. a) Magnetic anomaly profile coincident to the seaward part of ENAM-MCS Line 2 (Maus et al., 2009). b) Post-stack time migrated profile of the seaward part of ENAM-MCS Line 2 c) d) e) f) zooms into the four different domains discussed in the text and that display different basement characteristics.

From CDP 26500 to CDP 32500 (Fig. 2a and 2c), the top of the basement is smooth and less reflective than on the seaward part of the profile and it is also less distinguishable from the sedimentary layers above. The top basement characteristics suggest that it could correspond to smooth volcanic flows emplaced in shallow water conditions and coincide with the landward onset of the ECMA.

From CDP 32500 to CDP 41700 (Fig. 2a and 2d), there is a step up in the basement and a drastic change in the basement roughness. In this area, the crust is highly tectonized by normal faulting forming tilted, faulted crustal blocks. This crust could be interpreted as highly extended continental crust due to the geometry of syn-rift sedimentary sequences in the basement half-grabens. This interpretation would be in conflict with the zone between the continent and the oceanic being purely magmatic and would suggest that continental crust could have been thinned by faulting before being intruded by igneous material. Alternatively, this crust could be oceanic crust formed at very slow spreading rates (<15 mm/yr). Very slow-spreading crust is known to be fragmented by normal faulting with large crustal blocks (long wavelengths). On the sole basis of basement architecture, we cannot fully support either of the two proposed hypotheses. Ocean-bottom seismometer (OBS) refraction data acquired during the ENAM-CSE and coincident with the MCS data used in this project will help to decipher the nature of the crust where tilted basement blocks are imaged.

From CDP 41700 to CDP 51100 (Fig. 2a and 2e), the basement roughness appears to be that of a typical oceanic crust formed at a steady state slow spreading ridge.
From CDP 51100 to CDP 62000 (Fig. 2a and 2e), starting at the BSMA anomaly and seaward, the top basement is very smooth and reflective and the BSMA anomaly appears to coincide with a step-up in top basement.

Along the BSMA, clear Moho (Mohorovic Discontinuity) reflections are observed 2.5-3 s (8.12-9.75 km assuming an average crustal velocity of 6.5 km/s) beneath the top basement (Fig. 3) and are relatively continuous. Abundant intracrustal reflections, primarily restricted within the oceanic lower crust, are also observed over crust formed at BSMA time but also in younger crust.
In the ridge jump scenario, the BSMA would represent thinned continental crust intruded by igneous material. However, the top basement is very reflective indicating a strong impedance contrast between the sediment layers and the top basement. This would be more in agreement with a top basement produced by submarine seafloor spreading at a mid-ocean ridge than subaerial or shallow water emplacement of volcanics within sediments that would reduce the impedance contrast as in Fig. 2c.

Figure 3. Part of pre-stack time migrated profile (ENAM-MCS Line 3) along the Blake Spur Magnetic Anomaly.

The layering imaged within the lower crust (Fig. 2f) could indicate magmatic intrusives but the well-developed Moho would suggest no underplating. In addition, lower crustal reflections persist in younger crust beyond the BSMA suggesting that this crust is not continental crust that experienced pervasive melt migration during extension. There is also no evidence of a fossil spreading center between ECMA and BSMA.

A drastic increase in seafloor spreading rate and a change in the spreading in the vicinity of the BSMA could explain the change of the basement smoothness from rough to smooth and the basement relief but would not explain the thicker than normal oceanic crust. A magmatic pulse at BSMA time would produce a strongly magnetized upper oceanic crust and could explain the magnetic anomaly. The magnetic pulse would also be in agreement with the thicker than normal oceanic crust and smooth basement topography observed in the data.

The outcomes of the project described above clearly show that the MCS data from the ENAM-CSE provide important information for the study of late-stage rifting processes at this margin. Ultimately, results will be integrated with the landward part of the profiles (not shown here).

This project involves collaboration with Brandon Shuck and Harm van Avendonk at UTIG who are working on the offshore wide-angle reflection/refraction modeling coincident to the multichannel seismic lines used in this project. By combining constraints from the multichannel seismic profiles, refraction modeling and potential field studies, we hope to better understand implications for variations in crustal structures, faulting and magmatism seen in the MCS data at this margin and at a broader scale expand our knowledge of the continental breakup and early seafloor spreading at passive margins worldwide. Results from this project will also be integrated with two others GeoPRISMS projects recently awarded that aim to examine other datasets from the ENAM-CSE. ■

References

Andrews, B.D., J.D. Chaytor, U.S. ten Brink, D.S. Brothers, J.V. Gardner, E.A Lobecker, and B.R.Calder, (2016), Bathymetric terrain model of the Atlantic margin for marine geological investigations (ver. 2.0, May 2016): U.S. Geological Survey Open-File Report 2012–1266, 12 p., 1 pl.
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Reference information
Late Stage Rifting and Early Seafloor Spreading History of the Eastern North American Margin. A. Bécel
GeoPRISMS Newsletter, Issue No. 38, Spring 2017. Retrieved from http://geoprisms.org

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.org

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

Followed by the STEPPE Workshop:”Lake Tanganyika: A Miocene-Recent Source-to-Sink Laboratory in the African Tropics”

Thank you for your participation in the Mini-Workshop From rifting to drifting: evidence from rifts and margins worldwide at the 2015 AGU Fall Meeting! Pictures of all GeoPRISMS activities at AGU are available here.

Download the participant list

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)

AnnouncementPresentation archiveVenueSTEPPE WorkshopRead the report

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)

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

9:30-10:00 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

Grand Hyatt, Union Square Room, 36th floor

STEPPE Workshop: “Lake Tanganyika: A Miocene-Recent Source-to-Sink Laboratory in the African Tropics”

Conveners: Michael McGlue (University of Kentucky) and Christopher Scholz (Syracuse University)

2pm – 8:30pm

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).

Volcanoes of Virginia: A Window into the Post Rift Evolution of the Eastern North American Margin


Sarah E. Mazza1, Esteban Gazel1, Elizabeth A. Johnson2, Brandon Schmandt3
1 Department of Geosciences, Virginia Tech, Blacksburg, VA, 2Department of Geology and Environmental Sciences, James Madison University, Harrisonburg, VA, 3Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM

Figure 1. A) Simplified geologic map showing sample locations of Eocene magmas. Note the orientation of the Mesozoic Central Atlantic Magmatic Province dikes towards the northwest and Eocene dikes towards the northeast. Cross section X-Y is shown in Figure 2. ENAM – Eastern North American margin; VA – Virginia; N. – New. B) Example of basaltic dike found in Highland County, Virginia. C) Trimble Knob, an example of a diatreme in Highland County, Virginia.

Figure 1. A) Simplified geologic map showing sample locations of Eocene magmas. Note the orientation of the Mesozoic Central Atlantic Magmatic Province dikes towards the northwest and Eocene dikes towards the northeast. Cross section X-Y is shown in Figure 2. ENAM – Eastern North American margin; VA – Virginia; N. – New. B) Example of basaltic dike found in Highland County, Virginia. C) Trimble Knob, an example of a diatreme in Highland County, Virginia.

The Eastern North American Margin (ENAM) developed into a passive margin following the breakup of Pangea at the Triassic-Jurassic boundary. However, the definition of “passive” no longer fits traditional tectonic thinking, as is evident from topographic rejuvenation of the central Appalachians since the late Cenozoic (e.g. Rowley et al., 2013). The recent 2011 Mineral, VA earthquake (M5.8) reminded us that the ENAM is not as stable as we would like it to be. Multiple tectonic events have shaped the ENAM and the Appalachians into a complex, lithologically diverse mountain range. The geologic record encompasses several Wilson Cycles, including the Grenville (~1.2-0.9 Ga), Taconic (~550-440 Ma), Acadian (~420-360 Ma), and Alleghanian (~320-260 Ma) orogenic events. The Appalachians and Piedmont has seen its share of magmatic activity, from Alleghanian granitic plutons to the massive Central Atlantic Magmatic Province (CAMP) occurring at 200 Ma (e.g. Blackburn et al., 2013).

The youngest known magmatic rocks in the ENAM are Mid-Eocene (Southworth et al., 1993; Tso and Surber, 2006; Mazza et al., 2014), located in the Valley and Ridge Province of Virginia and West Virginia (Fig. 1a). Over the past three years we have been conducting extensive field work, sampling over 50 different locations thus far. The Eocene volcanic rocks occur as dikes, sills, plugs, and diatremes, up to ~400 m in diameter (Fig. 1b, and c). The volcanic rocks are bimodal in composition, including mostly basalt and trachydacite. Mafic end members are generally fresher, with well-preserved mafic minerals, and some carrying lower crustal and mantle xenoliths. The felsic samples are typically rich in amphibole and biotite, both of which are useful for 40Ar/39Ar age dating.

New 40Ar/39Ar age dates have confirmed that the Virginia/West Virginia volcanics are the youngest magmatic event in the ENAM at ~48 Ma (Mazza et al., 2014). The Eocene magmatic pulse is an example of continental intraplate volcanism. Intraplate volcanism can be explained by mantle plume activity, lithospheric delamination, or simple extension. Plume-generated volcanism has elevated productivity, high mantle temperatures, and geochemical signatures indicative of deep sources (e.g. Hawaii; Herzberg et al., 2007). Lithospheric delamination can explain similar geochemical signatures as plume-derived volcanism, but with lower melting temperatures and productivity (e.g. New Zealand; Hoernle et al., 2006).

Continental extension can also produce intraplate magmas, thinning the lithosphere and allowing for decompression melting. In the case of extension, melting temperatures are expected to be close to ambient mantle and the geochemical signature would be less enriched compared to those magmas produced from mantle plume or delamination (e.g. the Basin and Range, western US; Gazel et al., 2012).

Our results show that the Eocene magmatic pulse is mantle derived and record an equilibration temperature of 1412 ± 25 °C at a pressure of 2.32 ± 0.31 GPa. Thus, melting conditions of the Eocene magmatic pulse indicates that conditions were too cold to be mantle plume derived (>1500 °C; Herzberg et al., 2007) and too hot to be related to the mantle at mid-ocean ridge systems (~1350 °C; McKenzie et al., 2005).

In order to determine a mechanism for melting, we turned to the available geophysical data. Prior to the arrival of the USArray to the east coast, Wagner et al. (2012) proposed the presence of a fossilized slab beneath North Carolina. From their Appalachian Seismic Transect, they found evidence for a westward dipping fossilized slab, which they interpret as an eclogized remnant of a west-vergent subduction zone associated with the accretion of Carolinia. However, contrasting seismic data from the TEENA Array (Test Experiment for Eastern North America; Benoit and Long, 2009) suggests a single Moho below the Shenandoah Valley of Virginia (at a depth of ~40 km). Thus, between Virginia and North Carolina, the remnant eclogized slab is lost.

Based on the geochemistry, average temperatures and pressures of melting (Mazza et al., 2014), the presence of a thickened, eclogized root in North Carolina (Wagner et al., 2012), and the lack of a thick crust in the Shenandoah Valley of Virginia (Benoit and Long, 2009) leads us to suggest that the ENAM Eocene magmatism was the result of localized upwelling in response to delamination (Fig.  2; Mazza et al., 2014).

Figure 2. Schematic model of melting mechanism by lithospheric delamination and possible mantle sources of Virginia (VA) volcanoes. Line of cross-section X-Y is shown in Figure 1A.

Figure 2. Schematic model of melting mechanism by lithospheric delamination and possible mantle sources of Virginia (VA) volcanoes. Line of cross-section X-Y is shown in Figure 1A.

A recent seismic study using seismic waveforms initiated from the 2011 Mineral, VA earthquake and the USArray in the Midwestern US suggested that a hidden hotspot trail may exist beneath the ENAM (Chu et al., 2013). They modeled the possibility of a thermal anomaly’s retention over the course of tens of millions of years and predicted that it is possible for a thermal anomaly from ~50-75 Ma to still exist today. However, Chu and coauthors suggest that this thermal anomaly was the result of a plume track that passed under Virginia 60 Ma, which is ~12 m.y. too early based on the new age evidence. Our ages are younger and our calculated mantle potential temperatures are lower than expected for a plume environment. Because of these discrepancies, the data Chu et al. (2013) presented could also be interpreted as a delaminated lithosphere. Recent tomography of the ENAM using the newly arrived USArray (up to May 2014) from Schmandt and Lin (2014) shows a low-velocity anomaly at ~60-300 km depths beneath the central Appalachians (Fig. 3). Schmandt and Lin (2014) agree with our interpretation of delamination, suggesting that the Eocene delamination could have left the “thermal scar”.

If the Eocene intraplate magmatism was produced by delamination and localized mantle upwelling, then one would expect to see localized change with the topography in response to the influx of a hotter mantle. There is well documented Neogene landscape rejuvenation along the ENAM passive margin (Rowley et al., 2013 and references within), from elevated erosion, increased sedimentation rates, and alteration of drainage patterns. Due to the thermal potential of mantle derived Eocene magmas in the Virginias, there could have been a larger pulse of dynamic topographic change in the central Appalachians. Unfortunately, no indication of Eocene landscape rejuvenation has yet been identified.

With further collaboration between geochemists, geophysicists, and geomorphologists, we plan to continue to evolve our understanding of the post-rifted ENAM. Not only do we aim to better understand the evolution of the ENAM, but we hope that our future work will expand our knowledge of the mantle beneath cratons and passive margins worldwide. This project has the potential to be an excellent teaching aid, showing the complexity of the physical world we live in and thus sparking interests in the next generation of geoscientists.

Figure 3. S wave tomography at 200 km depth from Schmandt and Lin (2014). White arrow points to the location of the Virginia Eocene magmatism.

Figure 3. S wave tomography at 200 km depth from Schmandt and Lin (2014). White arrow points to the location of the Virginia Eocene magmatism.

Education & Outreach

Virginia Science Festival Exhibit “Volcanoes form the inside out”. PhD Student Pilar Madrigal in the inner exhibit  about melt generation and formation with examples form the VA Eocene Volcanoes and dikes in the Santa Elena Ophiolite in Costa Rica.

Virginia Science Festival Exhibit “Volcanoes form the inside out”. PhD Student Pilar Madrigal in the inner exhibit about melt generation and formation with examples form the VA Eocene Volcanoes and dikes in the Santa Elena Ophiolite in Costa Rica.

We have been striving to use the story of the “youngest volcanoes in the ENAM” as a teaching example. Just recently, we participated in the Virginia Science Festival with the goal of furthering the general public’s understanding of geologic processes right in their own backyard. From volcanic diking experiments to hands on exhibits, we have been encouraging the public’s interest in the geologic processes that helped shape the state they live in. At the college level, this project has funded several undergraduate research projects at James Madison University. Several of these undergraduates have been able to present their research at national and regional conferences. Reaching a broader, non-scientific audience can be challenging. We have been able to overcome the hurdle by communicating with the press, through organizations such as NPR, Scientific American, LiveScience, and the Washington Post.

References
Benoit, M.H., Long, M.D. (2009). The TEENA experiment: a pilot project to study the structure and dynamics of the eastern US continental margin: AGU Fall Meeting Abstracts.
Blackburn, T.J., Olsen, P.E., Bowring, S.A., McLean, N.M., Kent, D.V., Puffer, J., McHone, G., Rasbury, E.T., Et-Touhami, M. (2013). Zircon U-Pb Geochronology Links the End-Triassic Extinction with the Central Atlantic Magmatic Province. Science, 340(6135), 941–945.
Chu, R., Leng, W., Helmberger, D.V., Gurnis, M. (2013). Hidden hotspot track beneath the eastern United States. Nat. Geosci., 6, 963–966.
Gazel, E., Plank, T., Forsyth, D.W., Bendersky, C., Lee, C.T.A., Hauri, E.H. (2012). Lithosphere versus asthenosphere mantle sources at the Big Pine Volcanic Field, California. Geochem. Geophys. Geosys., 13(6).
Herzberg, C., Asimow, P.D., Arndt, N., Niu, Y., Lesher, C.M., Fitton, J.G., Cheadle, M.J., Saunders, A.D. (2007). Temperatures in ambient mantle and plumes: Constraints from basalts, picrites, and komatiites. Geochem. Geophys. Geosys., 8(2).
Hoernle, K., White, J.D.L., van den Bogaard, P., Hauff, F., Coombs, D.S., Werner, R., Timm, C., Garbe-Schönberg, D., Reay, A., Cooper, A.F. (2006). Cenozoic intraplate volcanism on New Zealand: Upwelling induced by lithospheric removal. Earth Planet. Sci. Lett., 248(1-2), 350–367.
Mazza, S.E., Gazel, E., Johnson, E.A., Kunk, M.J., McAleer, R., Spotila, J.A., Bizimis, M., Coleman, D.S. (2014). Volcanoes of the passive margin: the youngest magmatic event in Eastern North America. Geology, 42, 483–486.
McKenzie, D., Jackson, J., Priestley, K. (2005). Thermal structure of oceanic and continental lithosphere. Earth Planet. Sci. Lett., 233(3), 337–349.
Rowley, D.B., Forte, A.M., Moucha, R., Mitrovica, J.X., Simmons, N.A., Grand, S.P. (2013). Dynamic topography change of the Eastern United States since 3 million years ago. Science, 340, 1560–1563.
Schmandt, B., Lin, F.C. (2014). P and S wave tomography of the mantle beneath the United States. Geophys. Res. Lett., 41(18), 6342–6349.
Southworth, C. S., Gray, K., Sutter, J.F. (1993). Middle Eocene Intrusive Igneous Rocks of the Central Appalachian Valley and Ridge Province.Setting, Chemistry and Implications for Crustal Structure.
Tso, J.L., Surber, J.D. (2006). Eocene igneous rocks near Monterey, Virginia; A field study. Virginia Minerals, 49(3-4), 9–24.
Wagner, L.S., Stewart, K., Metcalf, K. (2012). Crustal-scale shortening structures beneath the Blue Ridge Mountains, North Carolina, USA.Lithosphere, 4(3), 242–256.Audet, P., Schwartz, S.Y. (2013). Hydrologic control of forearc strength and seismicity in the Costa Rican subduction zone, Nature Geosci., 6, 852–855. doi:10.1038/ngeo1927.
Reference information
Volcanoes of Virginia: A Window into the Post Rift Evolution of the Eastern North American Margin, Mazza, S.E., Gazel, E., Johnson, E.A., Schmandt, B.
GeoPRISMS Newsletter, Issue No. 33, Fall 2014. Retrieved from http://geoprisms.org

From the Mudline to the Mantle: Investigating the Eastern North American Margin


Deployment of a SCRIPPS Ocean Bottom Seismometer from the R/V Endeavor

Deployment of a SCRIPPS Ocean Bottom Seismometer from the R/V Endeavor

Brandon Dugan (Rice University), Kathryn Volk (University of Michigan), Dylan Meyer (UT, Austin), Kristopher Darnell (UT, Austin), Afshin Aghayn (Oklahoma State), Pamela Moyer (University of New Hampshire), Gary Linkevitch (Rice University)

The NSF-GeoPRISMS-funded Eastern North America Margin (ENAM) Community Seismic Experiment (CSE) is a community-driven research project aimed to study continental breakup and the evolution of rifted margins. The ENAM CSE includes acquisition of passive and active-source data from broadband ocean bottom seismometers (OBSs), short-period OBSs, multi-channel seismics (MCS), and onshore seismometers (Fig.1). Data are augmented by the onshore EarthScope USArray seismometers. Together they provide coverage across the shoreline and over a range of length scales. Project data will facilitate detailed studies of the early rifting between eastern North America and northwest Africa in the Mesozoic including processes associated with the Central-Atlantic Magmatic Province (CAMP), the East Coast Magnetic Anomaly (ECMA), and the Blake Spur Magnetic Anomaly (BSMA), as well as high-resolution studies of shallow sedimentary and fluid-flow processes including Quaternary landslides and gas hydrate systems.
Another component of the ENAM CSE was engaging young scientists in the field geophysical program so they could study the eastern North America margin and be educated about the planning and implementation of a multi-investigator, multi-component research program. To accomplish this, we included young researchers (undergraduate and graduate students, post-docs, and assistant professors) in all onshore and offshore field programs. The final stage of training and education will be seismic processing workshops for the OBS and the MCS data in summer 2015. Information for applying will be distributed via GeoPRISMS and other community list-servers.
In this phase of the ENAM CSE we conducted onshore and offshore operations in September 2014. Onshore activities (led by Beatrice Magnani and Dan Lizarralde) included deploying 80 short-period seismic stations to record our offshore shots and recovering the instruments. Offshore activities included deploying and recovering 94 short-period OBSs from the R/V Endeavor (led by Harm Van Avendonk and Brandon Dugan) and shooting MCS seismic data and providing active sources for the short-period OBSs and land seismic stations from the R/V Marcus G. Langseth (led by Donna Shillington, Matt Hornbach, and Anne Becel). Together these activities yielded high quality seismic reflection and refraction data across the shoreline and down to the mantle.

Figure 1. Idealized instrument layout and transects of the ENAM Community Seismic Experiment.

Figure 1. Idealized instrument layout and transects of the ENAM Community Seismic Experiment.

When I first heard about the ENAM CSE, I was very excited by the available cruise opportunities. I have been on several cruises before, ranging from 5 days to 5 weeks, and had been aching to get back out to sea again. Considering my prior experience collecting, processing, and interpreting MCS data, I decided it would be a good idea to expose myself to an alternative data type so I applied for the OBS deployment cruise on the R/V Endeavor. From getting accepted to actually boarding the vessel was really a blur. The next thing I knew, we were casting off the deck lines and heading out into the wild blue yonder. We all settled into our daily routine during the first week and it was great getting to know the crew and research staff. Sadly, the 12-hour watch schedule made it difficult to cross over with those on the other watch, but we were still able to see them at some meals and during watch changes. As the cruise went on and days blurred together, morale and energy remained elevated. We enjoyed our primary task of deploying and recovering OBSs and we filled our free time with reading, card games, and mingling. I had read all the information available on the ENAM CSE website and had chatted with the chief scientists about the project, but lacked the tangible connection between the activities that controlled every day of our lives at sea and the research goals of the ENAM CSE. Then, approximately two weeks after starting the voyage, we started getting data back from the OBSs we had deployed. The link between the physical (data collection) and theoretical (objectives and hypotheses) composition of the ENAM CSE research goals began to take form. Kathryn Volk, Gary Linkevich, and I met with Dr. Harm Van Avendonk in the main lab soon after the first data from the deployment became available. As a result of my past experience with port-processed MCS data I found that I had difficulty readjusting my perspective to data showing migrated time once the velocity structure been applied to convert time into depth. Through careful explanation, it became apparent that the data could be used to identify structure marking large changes in seismic velocity – so large that material with a velocity of 7 km/s would display as a horizontal layer. The purpose of this was to confirm that the seismic source had penetrated to the crust-mantle boundary. These data helped us identified the direct arrival, along with the position and depth of the OBS, the seismic multiple, and additional arrivals with increasing seismic velocity (a more in-depth description of these interpretations can be found in the ENAM CSE blog post put up on 9/30/14). From this conversation, the theory behind the data we were collecting and the physics behind the instrumentation we were working with became clear to me: combining the data from each line together will produce a seismic velocity model down to the crust-mantle boundary beneath the ENAM CSE study area. This will allow us to infer information concerning the crustal structure within the study region. With this connection drawn, we continued our work with a better-informed sense of purpose and finished the cruise in high spirits knowing that we helped obtain a dataset that will prove to be very important for the scientific community. My experience aboard the R/V Endeavor was very rewarding. Beyond the excitement of being out on an adventure at sea, I had a unique experience, from learning the construction and operation of OBSs to the important interpretations that can derive from the data. I am looking forward to the data workshops that are being offered next year to continue my education in this area. Dylan Meyer, University of Texas at Austin

Six students from across the country came together to participate in the R/V Endeavor cruise, and I was one of them. I had never been out to sea before in my life, so I was both excited and nervous for what was to come as we pulled away from port. We started our shifts right away, three students – including me – working the noon to midnight shift, and three other students working from midnight to noon. It took a few days to get to our first line where we would start deploying ocean bottom seismometers. The first task we learned, and one we would repeat many times, was the ocean bottom seismometer assembly. We would work with our shift to attach the metal grate, the instrument box, the ratchet on the side floats, and finally we would secure the top float. The final touch to the assembly included a strobe light, a radio, and a reflective flag to detect the instrument once at the surface. When assembled, the OBS was ready to be deployed off the side of the ship, or as the Captain referred to it, ‘pick her up and put her in’. At night, we could distinguish the flashes of the strobe light before the instrument disappeared under the waves. We would repeat this task, moving from one site to the next until we finished a line. Once the R/V Langseth had shot active-source seismic across a line, we had to go back and recover the OBSs by fishing them back out of the Atlantic. We would first return to the drop site and send a remote command telling the OBS to start burning through the wire attaching the metal grate to the buoyant OBS. Fifteen minutes later, the metal grate would detach allowing the OBS to rise back up to the surface. In extra deep water (~5000 m depth) it could take an OBS over an hour to surface. Just before the instrument reached the surface, the students would head up to the bridge, grab a pair of binoculars, and start looking around to locate it, which was harder than expected! Sometimes, the OBS would surface far from the ship, the bright orange flag being no more than a small, orange dot on the horizon, bobbling in and out of view. Fortunately, the combination of radio, flag, and strobe light, along with a handful of eyes was helpful to spot the instrument. The task was then up to the Captain or the First and Second Mate to drive the boat right towards it and the OBS technicians or the students would retrieve the OBS using six feet long pools equipped with hooks at the end. It usually took a bit of strength and good hand eye coordination to snag the OBS with the hook. The knuckle boom would finally drag the instrument up out of the water and onto the deck. And then move onto the next site. One of the most valuable things I learned on this cruise was what it takes to collect data. We needed a team of people willing to spend a month together in the ocean, repeating a task over a hundred times in rain or shine, calm seas or stormy, to acquire large amounts of new data that will generate new research, publications, and discoveries, and that’s pretty cool. Kathryn Volk, University of Michigan

My first time at sea and I will never forget the sight of the vast ocean and endless sky – there were more colors, sounds, and motions than I ever imagined.Pamela Moyer, University of New Hampshire
Record sections of hydrophone (top) and geophone (bottom) of OBS207. This was an instrument from the WHOI OBSIP group.

Record sections of hydrophone (top) and geophone (bottom) of OBS207. This was an instrument from the WHOI OBSIP group.

My first few hours aboard the R/V Langseth were spent walking in circles trying to identify the rooms of the ship and trying to navigate from my bunk to the galley, then from the galley to the lab, then to the muster deck, and finally back to my bunk. It seemed that the combination of identical walls and floors, narrow stairwells, and tight turns created a maze. After a few days, the ship started to look more like a structured, intimate home. Once I began my midnight shift (12am-8am) a set routine developed. My primary job was to maintain watch—that is, stay awake during my shift and report data losses, animal interferences, equipment malfunctions, science-related decisions, and major changes. I performed this job in front of the ship’s 30 computer monitors alertly glancing between monitors at the continuously streaming data. The science mission was to collect seismic data on the ship’s 8 kilometer-long streamer, a cable containing hydrophones (Fig.2). We did this by generating a large source of pressure directed towards the seafloor. This pressure pulse travelled towards the seafloor and reflected some energy back towards the hydrophones at every significant sediment interface. However, the science team did little to alter the fundamental operation of the ship. Instead, we simply modified many small parameters. For instance, the streamer was sometimes 11 m deep, while other times it was 9 m deep. Sometimes, pressure pulses were fired every 90 sec and at other times were fired every 20 sec. These little tweaks kept the work interesting. But, much of what was happening aboard the ship was repetitive, and it was easy to sink into a lull. Yet, the cruise progressed and we processed more and more data, and built an increasingly complex image of the subsurface. I became interested in the Cape Fear Slide, and entered into intense discussions with Derek Sawyer, Matt Hornbach, and Ben Phrampus. While simultaneously looking at the processed seismic data, we started piecing together maps, background literature, pore-pressure model predictions, and BSR estimates. My experience became active and exciting with the inclusion of real-time data acquisition and interpretation. Suddenly, we were really focused on internal reflectors within the main portion of the slide and we kept asking if we were seeing faults or sediment waves. It was this basic science question that helped translate our terrabytes of data into a rewarding and focused experience. Back on land now, I’m helping to piece together the puzzle and seeing the value of the data that I helped collect. It’s this tangible portion of my experience that seems most important. The beauty, though, is that with such a large project and so much data across varied sedimentary structures, there are little nuggets of excitement for us all to find.Kristopher Darnell, University of Texas at Austin

You can learn so much from the PIs and the other students being in a such a stimulating research environment.Gary Linkevich, Rice University

“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.org. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

 Reference information
From the Mudline to the Mantle: Investigating the Eastern North American Margin , Dugan B., Volk K., Meyer D., Darnell K., Aghayn A., Moyer P., Linkevich G.
GeoPRISMS Newsletter, Issue No. 33, Fall 2014. Retrieved from http://geoprisms.org