Investigations of Libyan Upper Mantle Anisotropy and Crustal Structure Using Shear Wave Splitting and Receiver Function Analyses

Abstract:

This study represents the first investigation of crustal structure and mantle anisotropy using broadband seismic stations in the interior of Libya. Seismic anisotropy is an invaluable tool for interpreting the development of sub-surficial features and lithosphere-asthenosphere dynamics over the course of geologic time in response to the interaction of adjacent tectonic plates and the stresses. Similarly, spatial variations in crustal structure can provide constraints for the growth of terranes specific to local tectonism and the subsequent response. Here we report analysis of data obtained from 16 stations belonging to the Libyan Center for Remote Sensing and Space Science (LCRSSS) in addition to 3 publically available stations both in Libya and adjacent areas by employing both shear-wave splitting (SWS) and receiver function (RF) methods. A total of 449 pairs of high-quality SWS measurements utilizing PKS, SKKS, and SKS phases from 19 broadband stations demonstrate primarily NNE-SSW to NNW-SSE fast orientations. An absence of significant fast direction variation as a function of event back-azimuth indicates the presence of simple anisotropy. Lack of correlation between surficial features and the observed fast orientations suggests that the origin of the anisotropy is asthenospheric. Preliminary time-series stacking of about 1000 radial RFs indicates that the crust does not thin significantly from the continental margin, where the crustal thickness ranges from 30 to 35 km, to inland Libya where the crust is about 30 km thick excepting local thickening due to volcanic activity. Measurements of the P-to-S wave velocity ratio Vp/Vs, which range from 1.70 to 1.87, reveal short-order variations as the result of heavy sedimentary basin deposits and local magmatic emplacement.