Crustal Strain Patterns in Magmatic and Amagmatic Early Stage Rifts: Border Faults, Magma Intrusion, and Volatiles

Thursday, 17 December 2015: 14:25
304 (Moscone South)
Cynthia J Ebinger1, Derek Keir2, Steven W Roecker3, Christel Tiberi4, Michael Aman5, Alexander Weinstein1, Catherine Lambert1, Connor Drooff1, Sarah Jaye Coros Oliva6, Katherine Peterson1, James Bourke7, Anastasia Rodzianko8, Ryan James Gallacher9, Aude Lavayssiere10, Donna J Shillington11, Mtelela Khalfan12, Gabriel Daudi Mulibo13, Richard Ferdinand-Wambura13, Avery Palardy1, Julie Albaric14, Stephanie Gautier15, James Muirhead16 and Hyunwoo Lee17, (1)University of Rochester, Rochester, NY, United States, (2)University of Southampton, Southampton, United Kingdom, (3)Rensselaer Polytechnic Inst, Troy, NY, United States, (4)University of Montpellier II, Montpellier Cedex 05, France, (5)University of Texas at Austin, Austin, TX, United States, (6)Abdus Salam International Center for Theoretical Physics, Trieste, Italy, (7)SUNY at Binghamton, Binghamton, NY, United States, (8)University of Chile, Santiago, Chile, (9)National Oceanography Centre, Glasgow, United Kingdom, (10)University of Southampton, Southampton, SO14, United Kingdom, (11)Columbia University of New York, Palisades, NY, United States, (12)University of Dar-es-Salaam, Dar-es-Salaam, Tanzania, (13)University of Dar es Salaam, Dar es Salaam, Tanzania, (14)NOSAR, Kjeller, Norway, (15)Géosciences Montpellier, Montpellier Cedex 05, France, (16)University of Idaho, Moscow, ID, United States, (17)University of New Mexico, Albuquerque, NM, United States
Rift initiation in thick, strong continental lithosphere challenges current models of continental lithospheric deformation, in part owing to gaps in our knowledge of strain patterns in the lower crust. New geophysical, geochemical, and structural data sets from youthful magmatic (Magadi-Natron, Kivu), weakly magmatic (Malawi, Manyara), and amagmatic (Tanganyika) sectors of the cratonic East African rift system provide new insights into the distribution of brittle strain, magma intrusion and storage, and time-averaged deformation. We compare and contrast time-space relations, seismogenic layer thickness variations, and fault kinematics using earthquakes recorded on local arrays and teleseisms in sectors of the Western and Eastern rifts, including the Natron-Manyara basins that developed in Archaean lithosphere. Lower crustal seismicity occurs in both the Western and Eastern rifts, including sectors on and off craton, and those with and without central rift volcanoes. In amagmatic sectors, lower crustal strain is accommodated by slip along relatively steep border faults, with oblique-slip faults linking opposing border faults that penetrate to different crustal levels. In magmatic sectors, seismicity spans surface to lower crust beneath both border faults and eruptive centers, with earthquake swarms around magma bodies. Our focal mechanisms and Global CMTs from a 2007 fault-dike episode show a local rotation from ~E-W extension to NE-SE extension in this linkage zone, consistent with time-averaged strain recorded in vent and eruptive chain alignments. These patterns suggest that strain localization via widespread magma intrusion can occur during the first 5 My of rifting in originally thick lithosphere. Lower crustal seismicity in magmatic sectors may be caused by high gas pressures and volatile migration from active metasomatism and magma degassing, consistent with high CO2 flux along fault zones, and widespread metasomatism of xenoliths. Volatile release and migration may be critical to strength reduction of initially cold, strong cratonic lithosphere. Our comparisons suggest that large offset border faults that develop very early in rift history create fluid pathways that maintain the initial along-axis segmentation until magma (if available), reaches mid-crustal levels.