Fault Kinematics and Seismic Anisotropy Patterns in the Natron-Magadi Basins, Africa

Abstract:

Early-stage continental rift zones provide important insights into the deformation behavior of crust and mantle lithosphere, and its modification by the migration of magma and volatiles. In East Africa, lower crustal earthquakes provide opportunities to probe the deformation behavior of the entire crust. We use a catalogue of 3068 earthquakes of 1 < ML < 4.5 recorded on a 39-station seismic array spanning three 3 rift segments ( Magadi-Natron-Manyara) of the Eastern rift, Africa to determine kinematics of large offset border faults, their along-strike linkage, and their possible interactions with tomographically imaged magma conduits and reservoirs beneath active and dormant volcanoes. Earthquake focal mechanisms are predominantly NS-striking normal faults with steep dips from near surface to 25 km in the Natron and Magadi basins, whereas the strike of normal faults locally rotates to N60E at the northern tip of the Manyara border fault. This rift-oblique structure links the Manyara border fault to Gelai shield volcano via Oldoinyo Lengai volcano, and may be a zone of magma transfer. Crustal anisotropy measurements from lower crustal earthquakes provide information on the orientation of fluid-filled cracks and any strain fabric. We compare our new crustal splitting observations with the rift parallel anisotropy determined by ambient noise tomography, and with mantle anisotropy patterns determined from SKS-splitting. Initial results of SKS-splitting (> 1 s) show both the NS and NE fast directions at different stations, suggesting that aligned melt-filled cracks contribute to the observed patterns, as in more evolved rift sectors, like the Ethiopian and Afar rifts.