Crustal structure and seismic anisotropy of rift basins in Somaliland

Rift margins provide insights into the processes governing the rupture of the continental lithosphere and the subsequence formation of sedimentary basins. The Proterozoic basement underlying Somaliland has been affected by multiple rifting; however, the crustal structure of these rifted basins remains unknown. This study utilized teleseismic receiver function analysis, Bayesian inversion, common conversion point imaging and 2D forward gravity modelling to examine the crust and upper mantle of Somaliland. The results indicate 36.8-38.2 km of crust in southern Somaliland, while the central and northern regions feature thinned crust (~ 21 km) with 5-6 km thick sediments. The joint analysis of radial and transverse components of receiver functions and shear wave splitting revealed fast axis directions trending to 50-56° in the upper mantle, indicating that azimuthal anisotropy is oriented in the regional Africa-Arabia plate motion. Such orientation may have resulted from lattice preferred orientation of olivine from the asthenospheric flow. Additionally, the fast polarization of the crust in central Somaliland is oriented at - 15°, indicating fossil deformation in the thinned crust related to the NW-SE trending Late Jurassic rift event. Further, the fast polarization for stations near the Gulf of Aden is oriented at 75-80°, suggesting crustal deformation associated with the Oligocene rift event. The crustal anisotropy at southern Somaliland revealed fast polarization oriented at - 85°, indicating a preserved far-field response of the WNW-ESE trending Late Cretaceous rift event. Overall, the study provides for the first-time insight into the rift-related extensional strain fabric in the crust and upper mantle anisotropy induced by asthenospheric flow in Somaliland.