Western Gondwana imaged by S receiver-functions (SRF): New results on Moho, MLD (mid-lithospheric discontinuity) and LAB (lithosphere-asthenosphere boundary)

Abstract

We study the Moho, the mid-lithospheric discontinuity (MLD), and the lithosphere-asthenosphere boundary (LAB) from southern Africa to northern Arabia, from Archean cratons to active rifts, at 1° resolution using our comprehensive new database of shear-wave receiver functions (SRFs). The good agreement between the Moho depth obtained from our SRFs and published P-wave receiver function (PRF) results provides confidence that our images of deeper lithospheric discontinuities are robust, including boundaries not normally visible on PRFs. We map the Moho and a deeper negative velocity gradient (NVG) almost everywhere we have data coverage. Our synthetic tests and comparisons of SRFs processed with and without deconvolution, and with varying filter parameters, indicate the observed NVG represents earth structure, not a processing artifact. Depth comparisons with seismic tomography and tectonothermal age studies suggest the NVG represents the MLD beneath Archean cratons but represents the LAB beneath non-cratonic regions. Both preserved crustal thickness and lithospheric thickness in the Nubia-Somalia-Arabia plates are statistically thinner for Phanerozoic and late Proterozoic terranes and older regions reactivated during these eras, than for cratons not reworked since the early Proterozoic or Archean. In contrast, NVG depth is uniform for all tectonothermal ages, though with a possible increase in amplitude with age. The equivalence of NVG depth and LAB depth in Phanerozoic lithosphere suggests that low-wavespeed compositions are frozen into the lithosphere as it thickens by cooling, forming our observed MLD at the present day.