Seismological Investigations of Crustal and Mantle Structure and Dynamics beneath North-central Africa

Abstract:

Here we report results of mantle azimuthal anisotropy from shear-wave splitting (SWS) analysis, crustal structure obtained using H-k stacking, and mantle transition zone discontinuity depths beneath the interior of north central Africa, including Libya and adjacent regions using recently available broadband seismic data obtained from stations managed by the Libyan Center for Remote Sensing and Space Science, and those from several other stations at which data are publicly accessible. A total of 583 pairs of high-quality SWS measurements utilizing the PKS, SKKS, and SKS phases demonstrate primarily N-S fast orientations with an average splitting delay time of approximately 1.2 s. An absence of periodic azimuthal variation of the observed splitting parameters indicates the presence of simple anisotropy, and lack of correlation between surficial features and the splitting parameters suggests that the origin of the observed anisotropy is primarily asthenospheric. Receiver function H-k stacking reveals crustal thicknesses ranging from 25 to 36 km, and Vp/Vs measurements ranging from 1.73 to 1.93, suggesting a spatially heterogeneous crustal structure and composition. The apparent depths of the seismic discontinuities (d410 and d660) bordering the mantle transition zone (MTZ) increase beneath Cenozoic volcanoes in central Libya, suggesting lower-than-normal upper mantle velocities. One of the most significant features in the resulting MTZ thickness map is a region in eastern Libya which has a MTZ thickness of about 270 km, indicating a colder than normal MTZ probably associated with subducted slabs.