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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.

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.

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.

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.

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.

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

Reference information

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.nineplanetsllc.com

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

Anne Egger, Tyrone Rooney¹, and Donna Shillington²

¹Michigan State University, ²Lamont Doherty Earth Observatory

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

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.

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.

Future Opportunities and Challenges for GeoPRISMS

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

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

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

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

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

Reference information

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

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

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.

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.

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.nineplanetsllc.com

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.nineplanetsllc.com/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.

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

Erin DiMaggio (Arizona State University)

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

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

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

All Under One Tent (1/21/2013)

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

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

The Wild Life (1/22/2013)

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

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

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

Ancient Rock Art (1/26/2013)

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

Geology at ELG (1/27/2013)

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

Figure 7. Local Afar children.

Afar Kids (2/1/2013)

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

The Fossil Hunt (2/2/2012)

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

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

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

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

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

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

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

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

Reference information

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

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

Workshop Report | GeoPRISMS Planning Workshop for East African Rift System

Morristown, NJ – October 25 – 27 2012

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

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

Background and Motivation

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

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

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

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

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

Student Symposium

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

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

Post-workshop field trip

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

Workshop Plan

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

Topic 1: How does the presence or absence of an upper-mantle plume influence extension?

Seismological imaging of plumes and associated magmatism in rifts – Gabriel Mulibo and JP O’Donnell.
Origin of magmas from geochemical perspective – Tyrone Rooney
Plume dynamics and surface uplift – D. Sarah Stamps

Topic 2: How does the mechanical heterogeneity of continental lithosphere influence rift initiation, morphology, and evolution?

Mechanisms for thinning the lithosphere, including thermal/chemical erosion, and interaction with large scale lithospheric structures –Ben Holtzman
Control of pre-existing structures on early rifting –Aubreya Adams
Geochemical heterogeneity of the lithosphere – Wendy Nelson

Topic 3: How is strain accommodated and partitioned throughout the lithosphere, and what are the controls on strain localization and migration?

Magmatism during rifting events – David Ferguson
Modeling and observations of faulting and magmatism during rifting – Juliet Biggs
Active deformation processes – Becky Bendick

Topic 4: What factors control the distribution and ponding of magmas and volatiles, and how are they related to extensional fault systems bounding the rift?

Geochemical studies of magmas and volatiles – Tobias Fischer
Geophysical imaging of magmas and fluids (MT, seismic): Derek Keir
Shallow dynamics of magma chambers/dikes and eruptions – Manahloh Bechalew

Topic 5: How does rift topography, on either the continental- or basin-scale, influence regional climate, and what are the associated feedback processes?

Climate and tectonics and feedbacks – Manfred Strecker
Modeling perspective – Joellen Russell
Tectonics and sedimentation at basin scale – Chris Scholz

Topic 6: Hazards and Resources in the EAR and Links to Rifting

Seismic hazard – Ataley Ayele
Volcanic hazard – Nicolas d’Oreye
Oil/gas exploration – Dozith Abeinomugisha

Topic 7: Synergies with other agencies / international projects

Topic 8: African partnerships panel

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

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

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

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

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

Workshop Outcomes

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

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

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

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

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

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

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

Reference information

GeoPRISMS Planning Workshop for East African Rift System – Report, Arrowsmith, R., Atekwana, E., Benoit, M., Cohen, A., Evans, R., Pritchard, M., Rooney, T., Shillington, D.

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

Rupturing Continental Lithosphere in the Gulf of California & Salton Trough

Rebecca J. Dorsey¹, Paul J. Umhoefer², and Michael E. Oskin³

¹University of Oregon, ² North Arizona State University, ³University of California, Davis

Figure 1. Map of topography, bathymetry, faults, and geophysical transects (Gonzalez-Fernandez et al., 2005; Lizarralde et al., 2007) in the Gulf of California – Salton Trough region. Systematic shallowing of water depth from south to north along the plate boundary is due to voluminous input of sediment from the Colorado River (Col. R.) in the north. Bold dashed line shows area of high-velocity anomaly at a depth of 100 km that indicates the presence of a stalled fragment of the Farallon plate in the upper mantle; purple color shows areas of post-subduction high-Mg andesites (Wang et al., in press). AB: Alarcón basin; BTF: Ballena transform fault; CaB: Carmen basin; CB: Consag basin; CPF: Cerro Prieto fault; DB: Delfin basin; EPR: East Pacific Rise FB: Farallon basin; GB: Guaymas basin; GF: Garlock fault; Gmp: Guadalupe microplate; IT: Isla Tiburón; Mmp: Magdalena microplate; PB: Pescadero basin; SAF: San Andreas fault; T.A.F.Z.: Tosco-Abreojos fault zone; TB: Tiburón basin; WB: Wagner basin.

How and why do continents break apart? Under what conditions does rifting progress to rupture of the lithosphere and formation of a new ocean basin? Can we identify the state parameters, physical properties, and forces that control this process? The Rupturing Continental Lithosphere (RCL) initiative of the NSF-MARGINS program was implemented to address these and related questions through integration of onshore-offshore geophysical, geological, and modeling studies. After marine investigations of the Red Sea rift became impractical due to geopolitical factors, the Gulf of California and Salton Trough became the sole focus site for the RCL initiative.

In this report, we highlight some of the key findings that have emerged from 10 years of RCL research along the Gulf of California – Salton Trough oblique divergent plate boundary (Fig. 1). A central goal of these studies was to better understand the spatial and temporal evolution of rifting and rupturing processes by linking data and observations with insights from numerical models and experiments. Researchers addressed questions regarding: forces and processes that govern rift initiation, localization, and evolution; key controls on deformation as it varies in time and space; physical and chemical evolution of the crust as rifting proceeds to sea-floor spreading; and the role of fluids and magmatism in continental extension. The following summary highlights results of recent studies, many of which have changed the way we think about continental rifting, rupture, and the underlying controls on these processes.

Upper-Mantle Structure

Complex upper-mantle structure beneath the Gulf of California – Salton Trough region reflects evolution of the plate boundary from a convergent-margin subduction zone and magmatic arc to the modern system of short spreading centers linked by long transform faults. Using Rayleigh-wave tomography, recent studies identify a fast anomaly in seismic velocity beneath the central Baja California peninsula and western Gulf (Wang et al., 2009, in press; Zhang et al., 2009). This anomaly is interpreted to be a fragment of the former Farallon plate that became stranded by slab detachment at a depth of ~100 km during failed subduction of the Farallon-Pacific spreading center. A discontinuous belt of post-subduction high-Mg andesites (bajaites) coincides with the landward edge of the stranded slab segment (Fig. 1), and is interpreted to record partial melting of ocean crust and upper mantle due to upwelling associated with opening the Gulf of California and/or replacement of detached lithosphere with hot asthenosphere at the end of the broken slab (Burkett and Billen, 2009; Wang et al., in press). Brothers et al. (2012) used seismic refraction data to identify another, shallower segment of stalled ocean crust at ~20 km depth beneath the southern peninsula. They concluded that slab detachment at ~12 Ma, and subsequent isostatic and thermal response, controlled the late Neogene history of uplift, erosion, subsidence and sedimentation on the Magdalena shelf off southern Baja California.
Receiver function studies show that continental crust of the Peninsular Ranges and Baja California microplate thins dramatically from about 40 km in the west to 15-20 km in the east, at the western margin of the Gulf Extensional Province (Lewis et al., 2000, 2001; Persaud et al., 2007). These results show that the eastern Peninsular Ranges lack an Airy crustal root, and that high topography in this area is instead supported by upper mantle buoyancy and a thinned mantle lithosphere. The geometry, distribution and post-Pliocene timing of rift-flank uplift suggest that removal or modification of mantle lithosphere is related to the modern phase of crustal extension driven by transform tectonics (Mueller et al., 2009), and is not inherited from an earlier period of Miocene extension. Mechanisms accommodating regional deformation of the lower crust and upper mantle are uncertain but may include lower crustal ductile flow, low-angle normal faulting, and convective instabilities in the lithosphere (Gonzalez-Fernandez et al., 2005; Persaud et al., 2007; Mueller et al., 2009).

Localization of Strain

One of the major questions that motivated RCL research was: how, where, and why does strain localize as rifting progresses to continental rupture (Umhoefer, 2011)? It has long been known that in some regions (such as the Basin and Range) the crust undergoes extension over large areas for 10’s of millions of years without breaking the continent. So why does strain rapidly become localized in some settings to rupture the lithosphere and form a new ocean basin? A decade of research in the Gulf of California – Salton Trough region generated new understanding of several key processes that control localization of strain in rift systems: (1) magmatism; (2) microplate coupling; (3) strike-slip faulting; and (4) sedimentation.

Figure 2. Seismic velocity models showing crustal-scale structure for 4 transects in the Gulf of California. The top, northernmost transect is from Gonzalez-Fernandez et al. (2005), and the lower 3 transects are from Lizarralde et al. (2007; PESCADOR experiment). Velocity contours in the lower 3 panels are color-coded and labelled in units of km/s. Yellow diamonds indicate instrument locations. COT is the interpreted continent/ocean transition. See Figure 1 for location of transects. The rift architecture seen in these transects alternates abruptly along the rift between wide-rift and narrow-rift mode. The observed variations in rift architecture likely reflect some combination of pre-rift magmatism and thickness of sediments in the basins.

Magmatism

Marine-seismic studies in the northern Gulf (Gonzalez‐Fernandez et al., 2005) and central to southern Gulf (Lizarralde et al., 2007) investigated crustal-scale structure and controls on rift architecture. Four transects reveal surprisingly abrupt variations in the geometry of rift segments and the width of extended continental crust (Figs. 1, 2). The northern Gulf transect is characterized by a broad diffuse crustal geometry, intermediate seismic velocities in the mid to lower crust, and lack of well defined ocean crust that may reflect the influence of thick sediments and lower crustal flow during extension (Gonzalez‐Fernandez et al., 2005). Rift segments in the central to southern Gulf alternate between wide- and narrow-rift geometries that Lizarralde et al. (2007) proposed are controlled by the presence or lack of pre-rift magmatism. According to this hypothesis, the upper mantle became chemically depleted in areas of early to middle Miocene, pre-rift ignimbrite eruptions. Chemically depleted mantle resulted in sparse syn-rift magmatism, thin basaltic crust, and a wide-rift architecture (Alarcon segment) that reflects the paucity of magma and a relatively strong lithosphere. Conversely, areas that were not affected by Miocene ignimbrite magmatism were inferred to have retained a fertile upper mantle that enhanced production of syn-rift magma, thus weakening the lithosphere and promoting a narrow-rift architecture (Lizarralde et al., 2007).

Behn and Ito (2008) used 2-D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Faulting is influenced by competing factors of lithospheric structure, rheology, and rate of magma accretion at the ridge axis, and that faulting typically follows a predictable pattern of initiation, growth, and termination. Fault growth in these models generates a strongly asymmetric thermal structure that can stabilize slip on large-offset normal faults, and may localize hydrothermal circulation into the footwall of evolving core complexes. Through integrated modeling and experimental studies, Takei and Holtzman (2009) found that, for a solid-liquid system in which solid grains deform by grain-boundary diffusion creep, addition of a very small amount of melt (phi < 0.01) results in significant reduction of effective bulk and shear viscosities. This means that very small melt fractions in the upper mantle will lead to substantial weakening and localization of strain. Bialas et al. (2010) used a 2-D numerical model to better understand how magma-filled dikes influence the evolution of fault stresses, heat, and lithospheric weakening. They found that only a small amount of magma is needed (<4 km of cumulative dike opening) to weaken the lithosphere such that strain may become localized and continue to ocean spreading by tectonic extension without input of additional magma.

Microplate Coupling and Strike-Slip Faults

Recent GPS studies provide new constraints on modern plate motions, plate rigidity, surface velocities, and kinematic boundary conditions in the Gulf of California – Salton Trough region. The Baja California microplate behaves as a rigid block that moves in approximately the same direction as the Pacific plate but ~10% slower than the Pacific plate (Plattner et al., 2007). Thus the microplate is incompletely coupled to the Pacific plate along the offshore Tosco-Abreojos fault zone (Fig. 1), and this “neighbor-driven” motion of the microplate drives northwest-directed rifting and seafloor spreading in the Gulf of California (Plattner et al. 2009). Mechanical coupling to the Pacific Plate is likely enhanced by the presence of shallow-dipping fragments of the former subducting Farallon plate beneath the Baja peninsula (Zhang et al., 2007; Wang et al., 2009; Brothers et al., 2012).

Existing regional seismic profiles run between and parallel to long transform faults that link short spreading centers (i.e. Gonzalez-Fernandez et al., 2005; Lizarralde et al., 2007), and therefore do not fully address questions about complex 3-D strain and regional strain partitioning in oblique rifts. A recent study by Brune et al. (2012) explored this question using a simple analytic mechanical model and advanced thermomechanical numerical techniques. They found that oblique extension is favored, and more efficient, than orthogonal rifting because it requires less force to reach the plastic yield limit of the lithosphere. This result suggests that oblique extension can exert a major control on localization of strain that evolves to lithospheric rupture, and may explain why continental extension progressed rapidly to rupture in the Gulf of California and Salton Trough (Umhoefer, 2011).

Figure 3. (A) Map of topography, bathymetry, faults and basins in the northern Gulf of California, compiled from numerous published sources. The northern Gulf contains several pull-apart basins bounded by large transform faults. Active diffuse deformation in the Delfin basin occurs on closely-spaced oblique-slip faults, and there is no evidence for existence of oceanic crust at depth. Much of the crust is sedimentary due to the high rate of input from the Colorado River. ABF, Agua Blanca fault; CDD, Canada David detachment; SPMF, San Pedro Martir fault. P, Puertecitos; SF, San Felipe. (B) Simplified tectonic model for late Miocene to modern kinematic evolution of the northern Gulf of California. Geologic relations in coastal Sonora record a period of NE-SW extension between about 10 and 6 Ma (black faults; Darin, 2010), and rapid focusing of strain into a narow zone of dextral transtensional deformation and related offshore faults at ca.7-8 Ma (red faults; Bennett et al., in press). Plate boundary motion now occurs on the Ballenas transform (blue faults).

The prediction that oblique rifting controls strain localization is supported by recent geologic mapping and structural studies in the northern Gulf of California and coastal Sonora region (Fig. 3). Geologic mapping and fault-kinematic analysis provide evidence for large magnitude (55-60%) NE-SW extension between about 10 and 6 Ma in the Sierra Bacha, immediately northeast of a major dextral shear zone (Darin, 2011). During this time, at ~7-8 Ma, strain became focused into a narrow zone of strong transtensional deformation and related transform faulting (up to ~100% local extension) in coastal Sonora and Isla Tiburon (Bennett et al., in press). These studies highlight the important role that strike-slip faults played in localizing transtensional strain into the northern Gulf of California shortly prior to lithospheric rupture. In contrast, Busch, et al., 2011 and 2013 and Umhoefer, et al., in review showed that normal faults remain active – but at low slip rates (<1 mm/yr) – along the Gulf Margin fault system at the latitude of La Paz.

Sedimentation

Recent studies call attention to the critical role that sediments play in continental rifting, lithospheric rupture, and formation of new ocean basins. Bialas and Buck (2009) developed a two dimensional mechanical model that explores the buoyancy effects of adding a load of non-locally derived sediment to an evolving rift system. In the absence of a sediment load, the buoyancy force contrast between areas of thinned and un-thinned crust hinders rift localization and promotes a wide-rift mode of extension. Conversely, if non-locally derived sediment is added to the rift zone, this reduces the contrast in buoyancy force and allows extension to persist within the rift, causing strain to become localized and hastening the time to rupture (Bialas and Buck, 2009). It is not clear, however, how the effect of buoyancy forces compares to the thermal effect of adding sediments, which may warm and weaken the lithosphere due to thermal blanketing (e.g. Lizarralde et al., 2007) or cool and strengthen a rift by adding a large volume of cold material to the crust.

Sediments and Crustal Recycling

It is now clear that voluminous input of sediment from the Colorado River exerts a first order control on rift architecture, crustal composition, and lithospheric rupture in the northern Gulf of California and Salton Trough region. We observe a pronounced change from sediment-starved, deep-marine seafloor spreading centers with thin basaltic crust and magnetic lineations in the southern Gulf, to overfilled shallow-marine and nonmarine pull-apart basins in the north that contain thick sediments above a quasi-continental lower crust (Fig. 1; Dorsey and Umhoefer, 2012; Fuis et al., 1984; Gonzalez-Fernandez et al., 2005; Lizarralde et al. 2007). Thus the degree to which basins have completed the transition from continental rifts to ocean spreading centers changes dramatically from south to north, even though there has been roughly the same amount of extension across the plate boundary since either ca. 6 Ma (Oskin and Stock 2003) or ~12 Ma (Fletcher et al., 2007). Although pre-rift continental lithosphere has ruptured completely in the north, as it has in the south, the northern rift segments lack normal basaltic spreading centers, and deep sediment-filled basins are floored by young crust composed of Colorado River-derived sediments and mantle-derived intrusions (Fuis et al., 1984).

Recent studies have tested and appear to confirm the crustal model of Fuis et al. (1984). Using Sp receiver functions, Lekic et al. (2011) found that the lithosphere-asthenosphere boundary (LAB) beneath the Salton Trough is very shallow (40 km), and that the lateral edges of shallow LAB coincide approximately with major active faults. They proposed that the entire pre-Tertiary lithosphere beneath the Salton Trough has been replaced, and that the LAB represents the base of newly formed mantle lithosphere generated by rift-related dehydration and mantle melting. New results from the Salton Seismic Imaging Project provide additional constraints on crustal and upper mantle structure beneath the Salton Trough. Seismic velocity models reveal a ~40 km-wide basin bounded by the San Jacinto fault zone on the southwest and paleo San Andreas fault on the northeast (Han et al., 2012a,b). Crystalline “basement” at depths of ~4 to 10-12 km consists of metamorphosed Plio-Pleistocene sediment on the basis of intermediate P-wave velocities (~5.0-6.2 km/s). High heat flow results in vigorous hydrothermal circulation and emplacement of Quaternary rhyolites produced by episodic remelting of hydrothermally altered basalts (Schmitt and Vazquez, 2006; Schmitt and Hulen, 2008).

Figure 4. Diagram illustrating a conceptual model for lithospheric rupture and sedimentation in the Salton Trough and northern Gulf of California (Dorsey, 2010). Deep basins are filled with synrift sediment derived from the Colorado River to form a new generation of recycled crust along the oblique-divergent plate boundary.

Crustal extension during mid to late Tertiary time led to collapse of a pre-existing orogenic plateau, reversal of regional drainages, and diversion of the Colorado River into subsiding basins along the fault-bounded tectonic lowland (Dorsey, 2010, and references therein). In this setting, continental crust is rapidly recycled by a linked chain of processes: erosion and fluvial transport of sediment off the Colorado Plateau, followed by deposition, burial, and metamorphism in deep rift basins (Fig. 4). Dorsey and Lazear (in press) found that the volume of sediment in the basins is, within error, equal to the volume of crust (ca. 310,000 km3) eroded from the Colorado Plateau over the past ~6 m.y., but only if the calculated sediment volume includes metasedimentary crust between 4-5 and 10-12 km deep in the basins. These studies challenge geologists to think about what the middle to lower crust will look like in a setting like this if the Salton Trough were uplifted and exhumed.

Recent insights from the northern Gulf of California and Salton Trough permit recognition of a new type of rifted continental margin (in addition to popular volcanic and non-volcanic end members): one where the continent-ocean transition consists of thick, largely non-volcanic crust constructed from syn-rift to post-rift sediments (Sawyer et al., 2007). This may help explain the origin of “transitional” crust at some ancient rifted margins. Recycled sedimentary crust of this type may be recognized by an overall geometry similar to that of volcanic rifted margins but with intermediate seismic velocities that are not consistent with a simple basaltic composition (e.g. Nova Scotia margin; Funck et al., 2004; Wu et al., 2006).

Conclusions

The past decade of research in the Gulf of California – Salton Trough focus site generated new insights into the processes that control continental rifting and transition to lithospheric rupture. Several key factors – upper mantle structure, magmatism, rift obliquity, and sedimentation – were found to be especially important. An unexpected result was the discovery of abrupt contrasts in rift architecture and evolution that reflect extreme variability in governing processes and conditions along the rift axis. For example, magmatism played a major role in the south, whereas sedimentation has strongly perturbed the system in the north due to voluminous input from the Colorado River. We see a change from large-scale simple shear and lower crustal flow associated with low-angle detachment faults in the north, to early localization of strain in the central Gulf (Guaymas basin) and southern Gulf (Cabo San Lucas), to protracted, pure-shear style extension and delayed continental rupture in the south. The role of upper mantle processes is one aspect that we expect will be more fully understood by tracking the complete evolution from active rifting through the thermal-subsidence phase at ancient rifted margins. ■

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Reference information

Rupturing Continental Lithosphere in the Gulf of California & Salton Trough, Dorsey R.J., Umhoefer P.J., Oskin M.E.

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

Mini-Workshop Report | Collaborative Efforts in the East African Rift System

AGU Fall Meeting 2013, San Francisco, USA

Conveners Part 1: Simon Kattenhorn¹, Cynthia Ebinger², Tobias Fischer³
Conveners Part 2: D. Sarah Stamps⁴, Wendy Nelson⁵, Robert Moucha⁶, Andy Nyblade⁷

¹University of Idaho; ²University of Rochester; ³University of New Mexico; ⁴MIT; ⁵University of Houston; ⁶Syracuse University; ⁷Penn State University

The AGU GeoPRISMS Mini-Workshop on Collaborative Efforts in the East African Rift System was held Thursday evening, 12 December 2013 at the Grand Hyatt hotel in San Francisco during the AGU Fall Meeting. It was well attended with 50 participants, 9 of whom were graduate students and 7 were postdoctoral researchers. This workshop had two sections sharing a common theme of bringing scientists together to discuss collaborative efforts underscored by the GeoPRISMS East African Rift System (EARS) implementation plan.

Part 1. The Eastern Branch Focus Site

Ongoing research programs in the Kenya-Tanzania rift sector were briefly reviewed as a foundation for scientific planning, and as experiential learning in terms of data acquisition and collaboration. These studies show high levels of seismicity, fault activity, and gas emissions in this magmatically-active region, which spans basins that formed at 25 Ma to < 1 Ma. The age span enables studies of rift initiation, propagation, and evolution within one sector. Presenters outlined existing data sets acquired by academics, petroleum, mineral, geothermal exploration, and governmental organizations.
Coordination of field programs and collaborative training opportunities enables fuller, more rewarding interactions with our international colleagues and provides more effective liaison with the relatively small EAR research community. For example, Fischer outlined strong support offered by geothermal exploration and production teams in Kenya.
Presenters outlined the procedure to obtain research permits in Kenya and Tanzania, as well as potential collaborating institutions. Attention was drawn to the USAID PEER program, which enables African collaborators to seek separate funding for enrichment of participation in NSF-funded research.

Figure 1. Simon Kattenhorn presents research background and planning studies of the Eastern Rift Focus Site

Figure 2. Moderators prepare their computers for real-time input during discussions

Part 2. Synoptic Studies of the East African Site

The GeoPRISMS initiative offers an unprecedented opportunity to synthesize EARS data and models for an improved understanding of the fundamental geodynamics of continental rifting. In 2012, during the GeoPRISMS EARS planning workshop, the community identified synoptic investigations along the entire EARS as a Collaborative Target of Opportunity. The initial questions posed in the implementation plan motivate studies of the mechanisms enabling rifting of cratonic lithosphere, the origin, composition, and timing of volcanism, the rate and distribution of strain along and across the rift systems, and large-scale pre-rift structure and dynamics underpinning the rift system.
Part 2 of the Mini-Workshop centered on obtaining feedback from participants in real-time. We presented three questions for discussion and report responses to each:
What questions are of interest to the community that concern synoptic studies of the EARS?
What datasets exist and what is needed to address system-wide studies of the EAR?
Is there interest in a community-driven proposal?
Given the recorded responses at the AGU Mini-Workshop, there is some interest within the GeoPRISMS community to develop a community-driven proposal to address synoptic studies of the EARS as evidenced by 50% of the responses.

Acknowledgements

We thank the GeoPRISMS Program of the National Science Foundation for funding this workshop, the moderators who volunteered to record participant responses in real-time to on-line documents using their personal computers, and the GeoPRISMS chair for providing Internet connectivity during the workshop.

Reference information

Collaborative Efforts in the East African Rift System, Kattenhorn, S., Ebinger, C., Fischer, T., Stamps, D.S., Nelson, W., Moucha, R., Nyblade, A.

GeoPRISMS Newsletter, Issue No. 32, Spring 2014. Retrieved from http://geoprisms.nineplanetsllc.com