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. ■

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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

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. ■

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Thompson, D.A., J.O. Hammond, J.M. Kendall, G.W. Stuart, G.R. Helffrich, D. Keir, A. Ayele, B. Goitom, (2015), Hydrous upwelling across the mantle transition zone beneath the Afar Triple Junction. Geochem Geophys Geosys, 16(3), 834-846.
Weinstein, A., S.J. Oliva, C.J. Ebinger, S. Roecker, C. Tiberi, M. Aman, … S. Peyrat, (2017), Fault‐magma interactions during early continental rifting: Seismicity of the Magadi‐Natron‐Manyara basins, Africa. Geochem Geophys Geosys, 18, 3662-3686.
White, R., D. McKenzie, (1989), Magmatism at rift zones: The generation of volcanic continental margins and flood basalts. J Geophys Res -Solid Earth, 94, 7685-7729.

Reference information

Constraining variability in mantle CO2 flux along the East African Rift System. J.D. Muirhead, T.P. Fischer, A. Laizer, S.J. Oliva, E.J. Judd, H. Lee, E. Kazimoto, G. Kianji, C.J. Ebinger, Z.D. Sharp, J. Dufek
GeoPRISMS Newsletter, Issue No. 41, Fall 2018. Retrieved from http://geoprisms.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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Klitgord, K. D., and H. Schouten (1986), Plate kinematics of the central Atlantic, in The geology of North America, vol. M, The Western North Atlantic Region, edited by P. R. Vogt and B. E. Tucholke, 351-378, Geological Society of America, Boulder, Colorado.
Kneller, E. A., C. A. Johnson, G. D. Karner, J. Einhorn, and T. A. Queffelec (2012), Computers & Geosciences Inverse methods for modeling non-rigid plate kinematics : Application to mesozoic plate reconstructions of the Central Atlantic, Comput. Geosci., 49, 217–230, doi:10.1016/j.cageo.2012.06.019.
Labails, C., J. L. Olivet, D. Aslanian, and W. R. Roest (2010), An alternative early opening scenario for the Central Atlantic Ocean, Earth Planet. Sci. Lett., 297(3-4), 355-368.
Marzoli, A., P. R. Renne, E. M. Piccirillo, M. Ernesto, G. Bellieni, and A. De Min (1999), Extensive 200-million-year-old continental flood basalts of the Central Atlantic Magmatic Province, Science, 284, 616–618, doi:10.1126/science.284.5414.616.
Maus, S., U. Barckhausen, H. Berkenbosch, N. Bournas, J. Brozena, V. Childers, F. Dostaler, J. D. Fairhead, C. Finn, R. R. B. von Frese, C. Gaina, S. Golynsky, R. Kucks, H. Lühr, P. Milligan, S. Mogren, R. D. Müller, O. Olesen, M. Pilkington, R. Saltus, B. Schreckenberger, E. Thébault, and F. Caratori Tontini (2009), EMAG2: A 2-arc min resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne, and marine magnetic measurements, Geochem. Geophys. Geosyst., 10, 1-12, doi:10.1029/2009GC002471.
Talwani, M., J. Ewing, R. E. Sheridan, W. S. Holbrook, and L. Glover III (1995), The EDGE experiment and the US East Coast magnetic anomaly, in rifted ocean-continent boundaries, edited by E. Banda, M. Torné and M. Talwani, 155-181, Kluwer Academic Publishers, Dordrecht, The Netherlands.
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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

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.
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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

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

Vignettes from the Salton Seismic Imaging Project: Student Field Work Experiences


Kathy Davenport (Virginia Tech) and members of the SSIP field crew

Figure 1. SSIP Project map. Red lines are faults; symbols (see index) are seismic sources or seismographs.

Figure 1. SSIP Project map. Red lines are faults; symbols (see index) are seismic sources or seismographs.

In early 2011, the Salton Seismic Imaging Project (SSIP) descended on Southern California. The Salton Trough was part of the Gulf of California focus area for MARGINS, and processes in this setting also address issues of rift initiation and evolution (RIE) important to GeoPRISMS. Over the course of three weeks, we acquired refraction and low-fold reflection seismic data along 7 lines totaling over 750 km, two 3D grids, and an offshore array. About 130 people participated in the data acquisition, including students from 31 different colleges and universities. During this time, 126 shots were fired, totaling 33,329 kg of explosives, and a 3.4-liter GI airgun was fired 2330 times in the Salton Sea. These sources were recorded on land on 2595 single-component seismographs and186 three-component seismographs at 4235 unique sites, as well as 48 three-component ocean bottom seismographs at 78 sites in the Salton Sea. A 42 station broadband deployment was also live during this time. We deployed instruments in sand dunes and snow, on bombing ranges and golf courses, beneath windmills and Joshua trees. We hiked through mesquite, avoided cactus and endangered lizards, and endured the stench of the Salton Sea. It took the best efforts of all the people involved to accomplish this massive data acquisition in the Salton Trough!

On January 23, Steve Skinner and I went to survey station locations along the San Andreas Fault east of Mecca. In this area of the desert few people have passed, so there are very few roads. We drove through washes and desert, looking for the easiest paths possible to reach our tentative waypoints. Jack rabbits and lizards tried to run away from us. When we finally stepped on the fault, with one foot on the Pacific Plate and the other on the North America Plate, looking at Salton Sea and the sunset, at that moment I felt that I was a real geologist.Liang Han, Virginia Tech. January 23, 2011

The Salton Trough is a prime target for investigating rift initiation and evolution and earthquake hazards because it is the northernmost extent of the Gulf of California extensional province. The San Andreas Fault ends in southern California, and strike-slip plate motion is transferred to the Imperial Fault. This step-over created the Salton Trough, a basin extending from Palm Springs to the Gulf of California. Previous studies suggest that North American lithosphere has rifted completely in the central Salton Trough. However, rifting here has been strongly affected by rapid sedimentation from the Colorado River, preventing the onset of seafloor spreading as has occurred in the southern Gulf of California. The 20-25 km thick crust in the central Salton Trough apparently is composed entirely of new crust created by magmatism from below and sedimentation from above. Between the major transform faults, active rifting is manifested by faults observed in modern sediment, abundant seismicity, minor volcanism, very high heat flow, and corresponding geothermal energy production.

Figure 2. Shot gather. The 911 kg shot was at the Imperial Fault. The 1142 seismograms (from Texans, plus vertical components from RT130's) were recorded along Line 2 that extends from the San Diego and Tijuana suburbs across the Peninsular Ranges, Salton Trough and Chocolate Mountains, to the Colorado River.

Figure 2. Shot gather. The 911 kg shot was at the Imperial Fault. The 1142 seismograms (from Texans, plus vertical components from RT130’s) were recorded along Line 2 that extends from the San Diego and Tijuana suburbs across the Peninsular Ranges, Salton Trough and Chocolate Mountains, to the Colorado River.

Based on the paleoseismic record, the southern San Andreas Fault is considered overdue for an earthquake of magnitude >7.5, and other nearby faults have had historic earthquakes with magnitudes >7. Earthquake hazard models and strong ground motion simulations require knowledge of the dip of the faults and the geometry and wavespeed of the adjacent sedimentary basins, but these parameters are currently poorly constrained.

SSIP ultimately will constrain the initiation and evolution of nearly complete continental rifting, including the emplacement of magmatism, effects of sedimentation upon extension and magmatism, and partitioning of strain during continental breakup. To improve earthquake hazard models, we will image the geometry of the San Andreas, Imperial and other faults, the structure of sedimentary basins in the Salton Trough, and the three-dimensional seismic wavespeed of the crust and uppermost mantle.

Constraining all these targets in the Salton Trough requires good instrument coverage in areas that are not always easily accessible. For instance, the deserts of Southern California are home to multiple military training facilities. These include the El Centro Naval Air Facility, whose bombing ranges are the winter training grounds for the Blue Angels, and the Chocolate Mountain Gunnery Range, Marine lands used for live munitions training. The Navy and Marine Corp were very accommodating to our project, providing safety training and time windows where we could safely cross the bombing ranges to deploy and pick up instruments. Of course, we had to work around the daily operations of these facilities, and that was not always easy.

Figure 3. Deploying a Texan seismograph on a wind farm near Palm Springs.

Figure 3. Deploying a Texan seismograph on a wind farm near Palm Springs.

The military assured us they had done sweep along our route so there shouldn’t be any live munitions on the ground. For safety, however, we were warned to avoid anything that appeared to be man-made. It was my role to drive into the desert, drop off the cross-country hikers, then drive around and pick them up on the other side of the bombing range. When I checked in at the operations center I was told that the Blue Angels were flying that day, and they don’t like moving objects on the ground. When I saw them I was to stop driving until they passed by. It seemed like I could drive for no more than a few minutes before the Blue Angels flew overhead and I would have to stop driving. It was pretty awesome to see them flying and executing their performance maneuvers right over our heads! As I stood by the truck awaiting the hikers, a solitary Blue Angel flew by, absolutely directly over my head. In the rush of noise and vibration of the flight, his elevation seemed like it was barely 30 meters. I decided to assume his flight path at that moment was a salute for the good work he thought we were doing.Janet Harvey, Caltech. March 2, 2011. El Centro NAF

Our access to the Chocolate Mountain marine bombing range was scheduled around daily munitions training. This meant we could only be on the range during hours when there was no chance of encountering one of the training groups, making this our earliest deployment – beginning at 3 am! We left the warehouse in El Centro hours before sunrise to give us enough time to get on and off the range before the firing started. Due to the extremely limited access, we could not survey the station locations ahead of time and instruments had to be deployed without precise GPS locations. We scurried around in the dark, planting seismometers as quickly as we could by flashlight, and left the base just as the sun came up. When we returned to retrieve the instruments we only had approximate station coordinates, so we had to scramble around, searching through the brush by flashlight for the buried instruments, with the imposing deadline of live ammunition flying through the air motivating us to find our instruments and get out by our sunrise deadline.Steve Skinner, Caltech. March 2, 2011. Chocolate Mountain Gunnery Range

Much of our work in the Imperial and Coachella Valleys was outside the urban areas and farmlands where the population is concentrated. We worked in the desert, the mountains, and on the Sea. Very often we found ourselves driving in washes or hiking because there were no roads where we needed to be. Bushwhacking, boating, and travelling cross-country led to many adventures for our deployment crews.

During surveying along Hwy 78 towards the Algodones sand dunes we chose a small, sandy side trail that was much safer than the main road. We tested the utility vans we would be using for deployment and learned that carefully driven, empty vans could successfully navigate the sandy road. Unfortunately, on deployment day I was the one driving the van loaded with instruments on this section. As we approached the dunes I saw the access to the side trail, took a deep breath, and began turning the van off the main road. 100 meters later, I learned that through either my lack of utility van experience or the weight of the fully loaded van, our test had failed… we were stuck. When we were pulled free we opted to work from the narrow shoulder on the main road. Later the trail looked more manageable, and much safer than pulling over on the half-shoulder of Hwy 78, so I gave it a second go… and 200 meters later became stuck again. After being pulled out for the second time, we finished our deployment from the main road. I would not try the van on the sandy trail again.Erin Carrick, Virginia Tech. March 1, 2011
Figure 4. Deploying an OBS into the shallow Salton Sea.

Figure 4. Deploying an OBS into the shallow Salton Sea.

The Salton Trough is often a barren and desolate place. Working on the Salton Sea, however, redefines desolate. I never saw another vessel on the water, despite a warning sign at the marina advising in case of emergency to flag down a passing boat, as there are no 911 services or coast guard rescue. We deployed our sound source and streamers off of a ~100’ barge towed behind a dual engine 40’ vessel. The water in the Sea is unbelievably hard on boat engines, precipitating salt quickly and preventing the internal cooling system from working. The Salton Sea also ‘blows out’ very quickly, going from dead calm to ocean size waves in 15 minutes. One nerve-wracking day, the water was as rough as I have ever seen it, one engine was out completely, and the other was screaming with warning sirens, close to overheating too. One may expect that this would be scary for fear of personal injury or lost data or ruined equipment, but the mind changes priorities on the Salton Sea. During the 4-hour ride back to the marina, I was only fearful of how utterly disgusting it would be to be in the water with the millions of dead tilapia. I would surely die from disgust! This particular evening, in true Salton Sea form, the water returned to glass 20 minutes out from the launch, and we enjoyed one of the most beautiful sunsets we had ever seen.Annie Kell, University of Nevada, Reno. March, 2011

The day’s assignment was to deploy two-dozen seismometers and geophones across the southern tip of the San Andreas Fault. We would drive as far as possible, and then pack in the instruments and equipment the rest of the way. Our crew had two extra members on this trip – a reporter and photographer from the Los Angeles Times. We drove into the field area on a path we blazed through the brush a month earlier. On the hike both of the media men were good sports, following us across the dry powdered mud in the heat, asking questions about regional tectonics and the SSIP experiment. After deploying the instruments we began the hike back to the vehicles along an abandoned railroad. All of a sudden we were stopped instantly in our tracks. An overwhelmingly close rattle sounded from just a few yards away and the biggest rattlesnake I have ever seen was coiled right off the tracks. We all backed away slowly. The cameraman, however, jumped into action, switching lenses and approaching the snake head-on until he was no more than a foot from its venomous fangs. Its head bobbed forward and back while he got his shots. This man who had fought in an infantry unit in Vietnam, covered troops in Iraq and Afghanistan, and won a Pulitzer Prize for following undocumented workers from Central America to the USA, had managed to find excitement and danger with a few geoscientists in the Salton Sea, California.Frank Sousa, Caltech. March 13, 2011
Figure 5. Backpacking seismographs across a Naval bombing range. Each person is carrying about 8 Texan seismographs and deployment equipment.

Figure 5. Backpacking seismographs across a Naval bombing range. Each person is carrying about 8 Texan seismographs and deployment equipment.

Onshore SSIP principal investigators are John Hole (Virginia Tech), Joann Stock (Caltech), and Gary Fuis (USGS, Menlo Park), working with Mexican collaborators Antonio Gonzalez-Fernandez (CICESE) and Octavio Lazaro-Mancilla (Univ. Autonoma de Baja California). The onshore work was funded by the NSF MARGINS Program (GeoPRISMS predecessor), the NSF EarthScope Program, and the USGS MultiHazards Program. The marine component, Wet-SSIP, is funded by an NSF Marine Geology and Geophysics Program grant to Neal Driscoll and Alistair Harding (Scripps Inst. Oceanography) and Graham Kent (Univ. Nevada, Reno). Broadband-SSIP is led by Simon Klemperer (Stanford Univ.) with funding from the NSF Geophysics Program. Onshore seismometers were provided by the EarthScope FlexArray and IRIS PASSCAL instrument pools with field support from PASSCAL. The OBSs were supplied by the OBSIP.

“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
Vignettes from the Salton Seismic Imaging Project: Student Field Work Experiences, Davenport, K., and members of the SSIP field crew;

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

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


Anne Egger, Tyrone Rooney1, and Donna Shillington2

1Michigan State University, 2Lamont Doherty Earth Observatory

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

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

Conference Overview

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

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

Scientific Advances Related to GeoPRISMS Goals in East Africa

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Broader Impacts

Hazards

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

Resources

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

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

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

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

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

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

Future Opportunities and Challenges for GeoPRISMS

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

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

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

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

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

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

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

U.S. Earth Scientists Prepare for a Community Seismic Experiment at the ENAM Primary Site


Harm Van Avendonk1, Beatrice Magnani2

1University of Texas at Austin, 2University of Memphis

Figure 1: Map of Discovery Corridors in ENAM focus area. The red shaded area is the target of the USGS seismic program on the U.S. Extended Continental Shelf. ECMA = East Coast Magnetic Anomaly, BSMA = Blake Spur Magnetic Anomaly.

Figure 1: Map of Discovery Corridors in ENAM focus area. The red shaded area is the target of the USGS seismic program on the U.S. Extended Continental Shelf. ECMA = East Coast Magnetic Anomaly, BSMA = Blake Spur Magnetic Anomaly.

Eastern North America (ENAM) was chosen as a GeoPRISMS Rift Initiation and Evolution primary site because it represents a mature rifted continental margin in which the entire record of continental break-up and rifting is preserved. The rifting history along ENAM is well recorded in basin stratigraphy and the underlying crustal structure, although subsidence, sediment transport and fluid flow are presently the dominant geological processes along the margin. The study of old rifted margins is often challenged by a thick cover of sediments, which masks much of the deep crustal structure. This is also true for ENAM; however, over the next few years, unprecedented opportunities exist to carry out focused geophysical studies, revealing both shallow and deep structures of ENAM in greater detail.

The convergence of two activities along ENAM serves to frame data-gathering opportunities. In 2013, the EarthScope Transportable Array (TA) will arrive in ENAM, and the USGS is planning a marine seismic reflection and a limited refraction study of the Extended Continental Shelf (ECS) along ENAM onboard the seismic vessel R/V Marcus Langseth, possibly as early as 2014. In addition, there is renewed interest from energy companies in the exploration of ENAM . At the joint Earthscope-GeoPRISMS Science Workshop on Eastern North America, held at Lehigh University in October 2011, discussions among various academic, government and industry scientists led to the suggestion that a community active-source seismic experiment could improve our understanding of the deep structure and evolution of ENAM, and make the best use of existing resources and upcoming opportunities. The planned USGS active-source seismic operations over the ECS provide part of the immediate impetus for such an experiment; however, the possibility exists to extend some of the proposed USGS profiles landward to image deep margin structures and obtain important seismic velocity constraints. Given the limited mission of USGS ECS surveys, funding to extend these profiles and record air-gun shots on-land must come from NSF, possibly with some industry sponsorship.

A GeoPRISMS-sponsored luncheon was held in San Francisco on December 8, 2011, during the AGU Fall Meeting. About 30 scientists met to discuss further the conceptual framework of a community proposal for an ENAM active-source seismic experiment. Several scenarios were discussed, from minimum-cost to comprehensive coverage. The latter could include onshore-offshore operations, e.g., air-gun shots from the R/V Marcus Langseth recorded not only by its 8-km-long multichannel seismic streamer, but also by co-linear OBSs and by EarthScope Flexible Array seismometers, deployed along on-land extensions of selected marine seismic transects. In addition, land-based shots along these transects could be recorded by Flexible Array seismometers as well as by OBSs, providing reverse coverage. Additional PI-driven piggyback deployments offshore and onshore could be designed to take further advantage of the community seismic effort. The consensus at the luncheon was that such a joint seismic experiment is feasible and opportune; however, the timing may depend on the final schedule for the USGS seismic program.

The GeoPRISMS ENAM primary site spans much of the U.S. and Canadian Atlantic margins, from Charleston to Nova Scotia. However, budgetary and logistical constraints require that the target area of a community seismic experiment be much smaller. The area of interest for the planned USGS ECS seismic study lies between the Outer Blake Ridge offshore South Carolina in the south and Cape Cod to the north (Figure 1). Within this region, the planned ECS seismic survey consists of profiles spaced 60 nautical miles apart, spanning the interval from the continental shelf break to the 200 nautical mile limit. To meet GeoPRISMS objectives, some of these profiles would be extended landward across the shelf, and onshore, where air-gun shots would be recorded by land stations.

At the EarthScope-GeoPRISMS Science Workshop at Lehigh, participants identified a few major corridors where dense data acquisition would benefit integrated studies of rifted margin processes (Figure 1). The “Philadelphia” and “Richmond” corridors exhibit pronounced along-strike structural variations in the Appalachians; thus, seismic transects that cross the shoreline in these two areas may yield insights into the role of inherited orogenic structure on the development of rift half-grabens, such as the Culpeper and Hartford basins, and the nature of syn-rift magmatic wedges that define the continent-ocean transition offshore. To the south, a transect in the vicinity of Charleston, SC, would image the transition between the Carolina Trough and the Blake Plateau, clarifying the structure and origin of basement in this area. In addition, the gas hydrate province of Blake Ridge is an important site for the assessment of geohazards on the continental slope. Comparisons of the deep-seismic structures along the northern and southern corridors would provide a view of regional differences in extension and magmatism during the opening of the Atlantic, helping to explain the linkages between these processes.

To have a true community experiment, broad participation from the U.S. scientific community is necessary. Researchers interested in participating in an ENAM community seismic experiment are invited to help with the (a) design of the active-source seismic data acquisition plan, (b) proposal writing, and (c) staffing of the data acquisition teams on-land and offshore. The involvement of graduate students and postdocs in this effort is very important, as these early-career scientists represent the core of the future GeoPRISMS and EarthScope communities. In the sprit of community science, we envision rapid data release and open data access following the experiment, enabling many members of the scientific community to participate in seismic data analysis and interpretation. Science proposals to use the seismic data could be submitted to NSF once the data are collected.

Although funding of the USGS seismic study of the ECS is currently uncertain, this field program is tentatively being planned for 2014. To create a successful partnership with the USGS in 2014, collaborative proposals must be submitted to the NSF GeoPRISMS and EarthScope Programs solicitations in 2012, on July 2nd and July 16th, respectively. Over the next few months, we hope to engage our colleagues in discussions about ENAM science priorities, and we welcome insights and contributions to the ENAM community seismic experiment proposal. Consider contributing through the GeoPRISMS forum site or by contacting us directly.

 icon-chevron-right Go to the mini-workshop webpage

Reference information
U.S. Earth Scientists Prepare for a Community Seismic Experiment at the ENAM Primary Site, Van Avendonk H., Magnani B.;

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

Workshop Report: EarthScope – GeoPRISMS Science Workshop for Eastern North America (ENAM)


Frank Pazzaglia1, Dan Lizarralde2, Vadim Levin3, Martha Withjack3, Peter Flemings4, Lori Summa5, Basil Tikoff6, Maggie Benoit7

1Lehigh University; 2WHOI; 3Rutgers University; 4University of Texas, Austin; 5ExxonMobil; 6University of Wisconsin; 7The College of New Jersey

Background and Motivations

The joint EarthScope-GeoPRISMS Eastern North America (ENAM) workshop held at Lehigh University from 26-29 October, 2011, with an attendance of ≈100 participants (Figure 1). EarthScope and GeoPRISMS represent research communities of geoscientists who study the processes that build continents, open oceans, and erode, transport and deposit sediments, along with the associated natural hazards of earthquakes, tsunamis, sea level rise, and landslides, both on land and under water. EarthScope science is undertaken primarily, but not exclusively on land and involves a facility of transportable and flexible arrays of seismometers with the primary goal of imaging the lithospheric and sub-lithospheric foundation of the United States. GeoPRISMS conducts shoreline-crossing interdisciplinary research to probe the processes that form and modify continental margins. Collectively, EarthScope and GeoPRISMS research provides an integrated framework for understanding the breadth of processes that govern continental formation, break-up, and evolution in the unique ENAM setting, and for assessing associated natural hazards and natural resources, in the US and Canada.

Further motivations for the convergence of interests in ENAM include the arrival of the EarthScope transportable array (TA) in 2012-13, while GeoPRISMS has identified ENAM as a primary site for research focused on rift initiation and evolution (RIE). The USGS also has been contracted to conduct a marine seismic survey of the US Extended Continental Shelf (ECS), tentatively in 2013. Concurrently, energy companies are showing a growing interest in the evolution of deep-sea margins, such as those along the eastern margin of North America. These activities offer distinct opportunities to leverage planned and potential onshore (e.g., USArray, FlexArray) and offshore (USGS or industry marine seismic surveys) programs. Therefore the timing is now ideal to organize the two communities and to identify the crucial science targets, and to develop or modify the strategies needed for science implementation for ENAM.

The GeoPRISMS community identified ENAM as a primary site to investigate rift initiation and evolution, in part because of the wide range of opportunities the geologic and geophysical setting provides for studying rifting and post-rift processes (figure 2). These include an apparent south to north transition from magma-rich to magma-poor break-up, numerous exposed and buried rift basins, thick archives of post-rift sediments and sedimentary rocks in shelf-slope basins, and well-documented surface processes. Similarly, ENAM appeals to the EarthScope community because of a long debated north to south transition in Appalachian structure, the west to east transition from craton to continental margin, the opportunity to investigate tectonic heredity in the context of continental assembly and dispersal, the emerging appreciation that sub-lithospheric dynamic mantle flow impacts surface dynamics, and the characterization of active seismic zones in a passive-margin setting.

An important goal of the science workshop was to focus the broader community effort on cross-disciplinary learning and approaches to collaborative science dedicated to the aforementioned science topics embodied in the archetypal passive margin. The workshop provided a national and international forum of scientists from universities, national laboratories, federal and state agencies, and industry, and included a colloquium and field trip specifically designed for early-career researchers including masters, doctoral, and post-doctoral scientists (figure 3).

Workshop Overview and Narrative

The workshop was constructed around two and one-half days of plenary presentations, short reports on “hot topics”, break-out sessions, and plenary discussions and decision making. Presentations and break-out sessions were organized around topics presented in participant white paper reports, and included: (a) orogenic processes, (b) rifting processes, (c) post-rift processes, and (d) neotectonic and surface processes. The break-out group attendance was designed to ensure diversity of thought, geographic interest, and synergy among the GeoPRISMS and EarthScope communities. Subsequent break-out discussions were defined by evolving participant interest in the geographic regions best suited to pursue the process-oriented science relevant to their field of study. Throughout the workshop, lively discussion ensued on how to best leverage the respective approaches of the GeoPRISMS and EarthScope communities in ENAM research.

Early in the meeting, we reviewed the EarthScope and GeoPRISMS Science Plans with particular focus on their implication for the Eastern North American Margin (ENAM). The EarthScope science plan and accompanying presentations of the 2009 science plan workshop articulate the key science targets for EarthScope research. Many of these science targets have direct relevance to ENAM, and presentations at the 2011 EarthScope National Meeting highlighted a range of scientific results from the study of these targets. More specific to ENAM was a 2004 EarthScope conference that focused on research frontiers and opportunities (http://www.earthscope.org/workshops/archive).

Similarly, the GeoPRISMS science plan (http://www.GeoPRISMS.org/science-plan.html) identifies rift initiation and evolution (RIE) as one of its initiatives. The implementation plan identifies ENAM as one of two RIE primary sites where the processes of continental rifting and transition to a passive margin will be studied. At ENAM, GeoPRISMS asks several interrelated questions regarding the distribution of lithospheric deformation, the influence of magmatism and pre-existing structural and compositional heterogeneity, the variation of rift structure and magmatism, the mantle dynamics of the syn- and post-rift margin, the processes that accompany the transition from late-stage rifting to mature seafloor spreading, how the margin has been influenced by post-rift tectonics, the identification of the magnitudes, mechanisms and timescales of elemental fluxes between the Earth, oceans and atmospheres along a passive margin during and after rifting, and characterizing the scales and frequency of submarine landslides and related natural hazards.
The first day of the meeting was dominated by plenary and hot-topic presentations that focused on building a content- and knowledge-base for ENAM from the wide range of geoscientific perspectives present at the meeting. Afternoon breakout sessions followed with a focus on the introduction of key research ideas and consideration of research corridors where the science could best be performed. What emerged out of this exercise was the organization of ENAM into three geographic regions: (1) a Northern area encompassing Atlantic Canada and New England; (2) a Mid-Atlantic region stretching from New York City to North Carolina; and (3) a Southern area stretching south from the Carolinas and wrapping around to the Gulf Coast.

The second day opened with breakout reports that articulated the geographic organization of science topics, followed by a slate of short presentations that focused on active tectonics, geodynamic modeling, and reports from aligned facilities, government organizations, and international partners. At this point, workshop participants were fully informed of the major science topics, high-interest focus areas, and opportunities for research synergy with community and industry partners. These presentations showed that the collective interests of university scientists, the USGS, and energy companies could provide a basis for a collaborative active-source seismic study offshore of the eastern United States, perhaps in the form of a jointly funded community experiment.

In the second round of breakout sessions workshop participants were charged with self-organizing into the three break-outs defined by geographic area, based on the results of the Thursday discussions. Nearly equal numbers of scientists attended the Northern and Southern geographic area break-outs, with a slightly larger proportion of participants attending the Central break-out. GeoPRISMS and EarthScope interests were similarly well-distributed among the three break-outs. In all groups, there was synergy across the shoreline among the terrestrial-based and marine-based geologists and geophysicists.

The relative size of the three geographic regions and the composition of the break-out attendees influenced the break-out discussions and the level of science implementation detail. The Southern break-out group restricted their consideration to the Atlantic margin to allow a purposeful overlap with the EarthScope TA. Similarly the Central group explored a number of potential shoreline-spanning projects because of the relatively restricted geographic area. In contrast, the Northern group was challenged with a greater diversity of interests and possible projects given its larger size. The deliverable from this third break-out exercise were focus areas, defined by polygons drawn on copies of the GSA Geologic Map of North America for the ENAM region (Figure 4).
Breakout reports followed that defined and presented the research corridors. The Southern group settled on a swath that stretched from eastern Tennessee, through South Carolina centered on Charleston, and out onto the shelf on the Blake Plateau. The justification for this line includes a classic cross section of the southern Appalachians, .incorporation of two seismic zones, including one that generated a historic M 7 earthquake, a traverse of rift basins that may contain the oldest syn-rift and post-rift sediments, a swath of the shelf that is underlain by potentially the oldest ocean crust, alignment with a funded mid-continent EarthScope project (OINK), and alignment with the Cape Fear Slide (CFS), perhaps the largest slide complex on the U.S. Atlantic margin.

The Central group defined two northwest-to-southeast mid-Atlantic focus areas, one in the south centered on Richmond, VA and one in the north centered on Philadelphia, PA. Both focus areas provide numerous opportunities for studying Appalachian structures, including the transition in deformation style from the northern Appalachians to southern Appalachians, Mesozoic rift basins, active seismic zones, and regions of documented recent deformation indicated by offset of deformed stratigraphic and geomorphic markers. They also take advantage of the thickest, richest, and best studied shelf-slope basin (the Baltimore Canyon Trough). The Richmond focus area has the added advantage of traversing early Cenozoic intrusive rocks. Given the close spatial position of the Richmond and Philadelphia focus areas, participants discussed the possibility of orienting a focus area parallel to the coast, centered more or less on the Fall Zone in an effort to take advantage of key features spanning the coastline in both the Philadelphia and Richmond areas. A north-south-oriented marine seismic line was also proposed that would link the extensive seismic and borehole data present across the continental shelf. As the U.S. Mid-Atlantic margins encompass the densest populations centers in ENAM, understanding the array of onshore and offshore geohazards are of particular concern for this region.

The Northern group defined a focus area centered on Nova Scotia that is positioned to take advantage of the well-known south to north transition from magma-rich to magma-poor continental margin. This focus area enjoys public access to an excellent Nova Scotia government-sourced database of industry seismic and well data for the Scotian basin, crosses the well-exposed Fundy rift basin, and shares a well-studied conjugate margin with Morocco. Notably, the EarthScope TA would have to be extended into Nova Scotia to take full advantage of onshore-offshore synergy. Nova Scotia is not currently part of the planned TA deployment, and modification to that plan will take effort and leadership by those individuals interested in studying this part of ENAM. The Northern group also defined a more narrow focus area stretching from the Adirondacks through southern New England and out onto the southern Georges Bank basin. There was considerable EarthScope geologic interest for study in this region, but it was not paired with equal enthusiasm for offshore research in the GeoPRISMS community, largely because the New England seamounts may overprint rift-related structure on the margin here.

Saturday morning opened with break-out reports for science implementation for the focus areas defined and supported on the previous day. There was lively discussion regarding how best to integrate field studies and data collection with several of the numerical models that had been presented. Discussion also ensued on which focus areas were best suited to leverage available resources and synergy with industry and community partners. There was an emerging sense that all of the focus areas had merit, but that there was greater potential for EarthScope-GeoPRISMS synergy in the Charleston and Nova Scotia focus areas, although lying outside the EarthScope study area challenged the latter.

At this point, the students were asked to give their perspective on the meeting, which included an independent evaluation of the science goals and prioritization of the focus areas based on those goals, inferred likelihood of success, and best opportunities for EarthScope-GeoPRISMS collaboration. The student report provided an objective summary of the workshop prepared by a group that was fully engaged in the process. They offered a rank order of the focus areas, with the best potential for EarthScope-GeoPRISMS collaboration as follows: Charleston, Nova Scotia, Richmond, Philadelphia, New England.

The student report was followed by short presentations and a panel discussion of ENAM broader impacts led by representatives of the GeoPRISMS and EarthScope outreach offices as well as David Smith, representing the Allentown, PA-based DaVinci Science Center. Collaborative EarthScope-GeoPRISMS research along the ENAM offers important opportunities to address a range of societal issues that can impact the most densely populated part of the nation. Natural hazard catastrophes are not in the collective memory of the nation with respect to ENAM, but in recorded history there have been very large, damaging earthquakes, and there is emerging, albeit controversial evidence for tsunamis. Other, related hazards include submarine landslides, potentially catastrophic clathrate degassing, fluid venting, sedimentation and erosion, flooding, and sea level rise. Infrastructure built along the North Atlantic margin range from wind power to telecommunications, and would be affected by such catastrophic events, as well as long-term sea level change. ENAM research also will contribute to the geotechnical considerations of siting the next generation of nuclear power plants, a dozen of which are operating, under construction, or ordered as of 2009-11. The Atlantic margin is a prime target for hydrocarbon exploration, motivating an improved understanding of past and present processes of the ENAM. Onshore and offshore basins and basalt flows are actively being evaluated as targets for carbon sequestration.

Finally, focusing efforts on the North Atlantic margins, particularly in eastern North America, opens the door for extensive education and outreach to US schools and universities active in Earth Science research.

Several opportunities were identified during the workshop for carrying out ENAM-wide synoptic studies, with a focus on those that would provide regional data sets that would benefit a wide range of GeoPRISMS and EarthScope researchers, i.e., the broader community. Specifically, there was discussion of the fate of the EarthScope TA once the planned deployment ends in 2015. Three main ideas were floated and discussed: (1) Plan to leave one in four TA instruments in ENAM and have these instruments adopted by state surveys, the NRC, and universities. This would provide for a widely spaced backbone (≈250 km) of instruments that could be densified by an FA for future EarthScope projects and OBS deployment for GeoPRISMS projects; (2) leave a 70-km spaced TA in place at one of the focus areas for more detailed, long-term studies of that region; (3) remove the TA completely and reassign the instruments to the FA pool for greater access and shortened wait times for smaller, more focused studies. The majority opinion was to exercise option (1), which is already taking place. A shorter discussion noted the opportunities for a parallel extension of a PBO GPS network. One EarthScope RAPID project has subsequently been successful in installing two PBO receivers on either side of the fault that ruptured in the 2011 VA earthquake.

A similar discussion was devoted to the possibility of a regional MCS and wide-angle survey along ENAM, leveraging planned USGS operations to conduct a seismic survey of the Extended Continental Shelf along the mid-Atlantic margin (see page 9, this issue). In addition, there was discussion about the future deployment of ocean bottom sensors as part of the Amphibious Array Facility (AAF) currently deployed along the Cascadia margin. The consensus was that the GeoPRISMS community needs to act now to demonstrate the interest to have these instruments move to ENAM when the facility leaves Cascadia. In the cases of future OBS or TA redeployment in ENAM, all participants agreed that one or more “heroes” will have to take up the cause and work closely with the community, NSF, IRIS, the USGS, and others to insure that there is lasting facility infrastructure in ENAM.