Woods Hole Oceanographic Institution

Dr Rob. L. Evans

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» MT Survey of the East African Rift


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» Coastal Change: Drowning of Barrier Beaches

» Mariana Subduction System MT

» Archean Craton Studies: The SAMTEX Experiment

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An MT Study of the East African Rift

Overall Project Leader: Estella Atewkwana (Oklahoma State) MT: Alan Jones, Mark Muller (DIAS) Active Source Seismics: Pablo Canales, Dan Lizarralde (WHOI), Steve Harder (UTEP) Passive Seismics: Steve Gao, Kelly Liu (Missouri University of Science and Technology (MST)) Geochemistry: Alison Shaw (WHOI), Elliot Atekwana (Oklahoma State) Geodynamic Modeling: Mark Behn (WHOI), Roger Buck (Lamont) Geology: Abdel Salam, John Hogan  (MST)

Continental rifting plays a key role in global tectonics.  Rifting enables continental fragments to mobilize, promoting subsequent continental growth via collision, and modulating upper-mantle convection and the release of heat from Earth's interior through variation in the surface distribution of continents.  Continental rifting remains poorly understood, however, largely because it is a transient geodynamic process.  Most rift systems provide only a snapshot in time of the entire process, and existing studies have tended to focus on rift segments at or near completion.  As a result, the earliest stages of continental rifting, when key processes such as localization occur, remain largely unstudied in the field.  The East African Rift System (EARS), because of the proximity to the pole of rotation, exhibits a strong gradient in rift evolution along its length.  This provides a unique opportunity to investigate the processes that drive rift initiation and control early rift localization.  We are carrying out a multidisciplinary investigation of the southwest branch of the EARS, which includes the very early stage Okavango Rift Zone, where classic geomorphic rift features are just beginning to emerge, and the Mweru, Luangwa (Zambia) and Malawi Rifts, where geomorphic features are fully developed but magma (if present) has yet to breach the surface. Our project thus fills a key observational gap in rift evolution, enabling us to address the following questions related to early-stage continental rifting:

1) What are the dynamic processes driving rift initiation and how do preexisting structures influence rift location?

2) What driving mechanisms promote and sustain strain localization during early rifting?

3) How does rift initiation impact the crustal and upper mantle structure beneath young rifts and how do these impacts facilitate further rift growth?

These questions speak to longstanding, competing models and hypotheses of rift initiation, including: edge-driven mantle convection resulting in lithospheric thinning and possibly early-stage magmatism;  lithospheric rupture enabled by magmatic intrusion; and tectonic stretching by far-field extensional stresses.  We will apply a combination of geophysical, geochemical and geodynamic techniques across the southwest branch of the EAR system to test the predictions of those hypotheses. Passive seismic data will constrain lithospheric-scale structure and upper-mantle flow patterns. Wide-angle seismic profiling, along with gravity data, will constrain variations in crustal and uppermost mantle structure. Magnetotelluric measurements will provide system-scale constraints on lithospheric thin spots and the presence of melt. Geochemical analysis of hot spring fluids will identify the presence of mantle-derived melts.  Geodynamic modeling will synthesize the new geophysical observations and geochemical results to develop the next generation of continental rifting models.

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