Kinematics of linkage between the Main Ethiopian and Eastern rifts in the Turkana Depression

Abstract:

The ~300 km-wide Turkana Depression between the Ethiopian and East African plateau is the site of East Africa’s earliest volcanism at ~40 Ma, and the earliest extensional faulting south of the Afar depression at ~25 Ma. Within the Turkana Depression, the East Africa rift structures and magmatic centers are superposed on Mesozoic and Palaeogene rift basins, producing a broad zone of basins and tilted fault blocks that expose Mesozoic-Present sedimentary strata. Geodetic data indicate that that the modern Turkana lake basin roughly encompasses the zone of active extension, but linkage across the many sub-parallel rifts at the southern end of the Main Ethiopian rift remains unclear, and seismicity levels were largely unknown. We use new seismicity data from the Turkana Rift Arrays to Investigate Lithospheric Structure (TRAILS) array deployed in January 2019 to evaluate spatial variations in the distribution of seismicity and magmatism, and in the kinematics of active faults. Within the Turkana basin, seismicity is localized to Holocene eruptive centers and large faults beneath southern Lake Turkana, as well as shield complexes east of Lake Turkana. A Mw4.9 earthquake occurred in this area in May 2020, and we hope to access instruments to analyze this event. Earthquakes occur across the sub-parallel Omo, Chew Bahir, and Segen basins north of Turkana. Well-recorded earthquakes within the array are used determine focal mechanism solutions. The solutions include N-S to NNE- striking normal and oblique-slip faults, as well as strike-slip earthquakes in the northern Omo and Chew Bahir rifts. The strike-slip faulting may facilitate strain localization to the southern Main Ethiopian rift from the three rift arms. The new data enable re-interpretation of the seven Global Centroid Moment Tensor solutions for the region. We use a grid search algorithm to invert for principal stresses within different tectonic domains of this complex region. TRAILS results suggest that the original location of strain and magmatism was near the eastern edge of the Tanzania craton above a steep lithosphere-asthenosphere gradient, and that rifting has migrated eastward to form a more contiguous zone between the Main Ethiopian and Eastern rift zones.