UNRAVELLING THE ROLES OF LITHOSPHERIC HETEROGENEITY, MANTLE UPWELLINGS, AND MAGMATISM ON RIFT DEVELOPMENT IN THE TURKANA DEPRESSION, E AFRICA (Invited Presentation)

Lateral heterogeneities in lithospheric structure and mantle dynamics influence the distribution of strain and magmatism in continental rift zones. The seismically and volcanically active Turkana Depression between the broad uplifted Ethiopia-Yemen and E. African plateaux is an ideal locale to test models: it was the site of the earliest flood magmatism at 45 Ma, the earliest rifting south of the Afar triple junction, and it hosts some of the most voluminous Quaternary magmatism in E Africa. The Eastern rift is kinematically connected to the Main Ethiopian rift ~300 km to the east across the Turkana depression. The unusual breadth and low elevation may be related to lateral heterogeneities in lithospheric structure associated with Mesozoic rifting and/or with terranes accreted during the Pan-African orogeny. We synthesize results of the 2019-2021 TRAILS seismic and geodetic project that characterizes the distribution of active faulting and magmatism, and images lithospheric and upper mantle structure beneath this critical part of the East African rift system. Upper mantle imaging reveals mantle lithospheric thinning beneath the Suguta-Turkana basins, and a broader zone of mantle thinning and heating beneath the southern Main Ethiopian rift. A zone of relatively higher velocity mantle lithosphere separates the two areas, and it corresponds to a ~200 km wide zone of normal and strike-slip faulting between sub-parallel, en echelon basins. Quaternary-Recent magmatism occurs within the active rift zones, and along the largely unfaulted eastern flank where the lowest asthenospheric velocities are found. The crustal strain field, therefore, appears to be controlled by pre-existing lithospheric structure, whereas mantle melting and resulting Quaternary shield complexes show an eastward migration, and potential interaction between two upper mantle upwellings. We present SKS-splitting measurements, stress inversions of earthquake source mechanisms, and double-difference earthquake locations to evaluate the influence of mantle flow patterns, fluid-filled cracks (e.g., dikes), and pre-existing mantle lithosphere strain fabrics on earthquake patterns, and their implications for earthquake hazards.