Seismological constraints on the density, thickness and temperature of the lithospheric mantle in southwestern Tibet

Abstract

We constrain the lithospheric mantle in southwest Tibet to be cold, thick, and dense by considering seismological observations, isostasy, and gravity anomalies. First, virtual deep seismic sounding (VDSS) indicates that the thickness of the crust increases from 50 ± 4 km beneath the Himalaya to 70 ± 4 km in the Lhasa terrane. This implies a ‘residual topography’ (difference between isostatic elevation of crust and true elevation) of −2.4 ± 1.5 km. Taking into account deviations from isostasy, the lithospheric mantle must be dense enough to depress the surface by 0.9 to 4.5 km. Our joint inversion of fundamental-mode Rayleigh wave dispersion and receiver functions suggests that the vertically-polarised shear-wave speed (Vsv) is 4.6 ± 0.1 km s−1 at depths of 120 to 300 km. From the shear-wave speed profile, we estimate the geotherm, which is on average 200 °C below the 1350 °C adiabat, and suggest that the base of the lithosphere is at a depth of 290 ± 30 km. To match the negative buoyancy, the lithospheric must be denser, on average, than ‘normal’ fertile adiabatic mantle, which rules out a depleted (harzburgite) composition. The density excess can be explained solely by thermal contraction, but we cannot rule out additional density increases due to composition. Our observations are not consistent with a depleted Indian slab underthrusting Tibet in this region, which would result in a lower average density and lower temperatures.