MR42A-08
The Importance of Magmatic Fluids in Continental Rifting in East Africa
Thursday, 17 December 2015: 12:05
301 (Moscone South)
James Muirhead, University of Idaho, Moscow, ID, United States, Simon A Kattenhorn, ConocoPhillips Company Houston, Houston, TX, United States, Cynthia J Ebinger, University of Rochester, Rochester, NY, United States, Hyunwoo Lee, University of New Mexico, Albuquerque, NM, United States, Tobias P Fischer, University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM, United States, Steven W Roecker, Rensselaer Polytechnic Inst, Troy, NY, United States and Gladys Kianji, University of Nairobi, Nairobi, Kenya
Abstract:
The breakup of strong continental lithosphere requires more than far-field tectonic forces. Growing evidence for early-stage cratonic rift zones points to the importance of heat, magma and volatile transfer in driving lithospheric strength reduction. The relative contributions of these processes are fundamental to our understanding of continental rifting. We present a synthesis of results from geological, geochemical and geophysical studies in one of the most seismically and volcanically active sectors of the East African Rift (Kenya-Tanzania border) to investigate the role of fluids during early-stage rifting (<10 Ma). Xenolith data indicate that rifting initiated in initially thick lithosphere. Diffuse soil CO2 flux maxima occur in the vicinity of faults, with carbon isotope values exhibiting a mantle-derived signature. These faults feed aligned sets of hydrothermal springs, which have N2-He-Ar relative abundances also indicating a mantle-derived source. Geochemical and surface faulting information are integrated with subsurface imaging and fault kinematic data derived from the 38-station CRAFTI broadband seismic array. Teleseismic and abundant local earthquakes enable assessment of the state-of-stress and b-values as a function of depth. High Vp/Vs ratios and tomographic imaging suggest the presence of fluids in the crust, with high pore fluid pressures driving failure at lower tectonic stress. Together, these cross-disciplinary data provide compelling evidence that early-stage rifting in East Africa is assisted by fluids exsolved from deep magma bodies, some of which are imaged in the lower crust. We assert that the flux of deep magmatic fluids during rift initiation plays a key role in weakening lithosphere and localizing strain. High surface gas fluxes, fault-fed hydrothermal springs and persistent seismicity highlight the East African Rift as the ideal natural laboratory for investigating fluid-driven faulting processes in extensional tectonic environments.
