Seismic velocities in Southern Tibet lower crust: a receiver function approach for eclogite detection

Beneath the Tibet plateau, the deficit of crustal thickening with respect to what is expected from the plate tectonic constraints is thought to be absorbed either by lateral extrusion or by vertical rock-mass transfer. To nourish the unsettled debate of the relative importance of these two processes, we propose a new approach, based on the S-to-P and the P-to-S wave conversions, enabling the precise determination of the seismic velocities. The weighted amplitudes of the direct conversion and of reverberations are stacked at their predicted arrival times for various values of layer thickness and v(P)/v(S) ratio separately for two sets of P- and S-receiver functions. For each set of receiver functions, coherent stack gives the v(P)/v(S) ratio and thickness for the considered layer (the grid search stacking method). The values of v(P)/v(S) ratio and layer thickness are functions of the velocity used for stacking the set of receiver functions, but using the P- and S-receiver functions allows us to solve this indetermination and to find the effective parameters of the layer: velocity v(S), v(P)/v(S) ratio and thickness. We use a bootstrap resampling of the receiver function data sets to estimate the parameters uncertainties. For the Southern Lhasa Block, the migrated sections of both P- and S-receiver functions (Hi-CLIMB experiment data) show a layer in the lower crust that may be related to the lower Indian crust underplated beneath Tibet. With the grid search stacking method, high shear wave velocities (v(S) similar to 4.73 km s(-1)) and low v(P)/v(S) ratios (similar to 1.69) are detected in this layer. Such values are typical for high-grade eclogites, and the low v(P)/v(S) ratio precludes the confusion with mafic granulites. There is no evidence for partial eclogitization near and south of the Yarlung-Tsangpo Suture, and the about 19 km thick eclogitic layer extends northwards only to about the middle of the Lhasa terrane.