Crustal Shear Wave Splitting Patterns in the southern Tanganyika-Rukwa Rift Zone, East Africa

There remain outstanding questions on how strain is localized in old, thick, and cold continental lithosphere, such as those characterizing the basement of the western branch of the East African Rift System. One of the major challenges in addressing these questions is the lack of sufficiently dense geophysical surveys necessary to probe the deeper crustal structure. We use a local seismicity catalog developed from the TANGA14 array of 13 broadband seismographs to obtain crustal shear wave splitting measurements in the magma-poor southern Tanganyika-Rukwa Rift Zone. The results show that the distribution of fast polarization directions measured at each seismic station is diverse, yet dominant trends that correlate with observable tectonic features exist. In the Tanganyika Rift, Rukwa Rift, and the intervening rift interaction zone, station-event path-average dominant splitting directions trend NW-SE, subparallel to the trends of inherited basement shear zones, rift faults, maximum compressive horizontal stress direction, and mantle SKS splitting directions recently interpreted as deflected mantle flow. However, we also observe a NE-SW rift-perpendicular splitting direction most prominent in the Tanganyika-Rukwa rift interaction zone, possibly related to the anisotropic signature of frozen mafic intrusions in the crust. Although source localization of the anisotropic signatures is nontrivial, interpretations of the spatial patterns of the path-integrated anisotropy may be vital to deciphering the main drivers of strain localization in evolving magma-poor continental rift systems.