Characterization of High Velocity Layer in the Lower Crust of Southern Tibet

A high velocity layer (HVL) has been observed at the base of the crust in numerous deployments of temporary broadband seismic experiments in southern Tibet. In receiver functions it appears as a strong phase with positive amplitude 1 to 2 seconds before the Ps converted phase from the Moho. The HVL is first observed north of High Himalaya when crustal thickness approaches ~65 km and it continues uninterrupted beneath the Yarlung-zanbo suture (YZS) and Lhasa block. It is widely distributed laterally but terminates both to the north (at ~31°N) and east (at ~94.5°E) in the eastern Lhasa block. In the western Lhasa block (at ~80°E) it is observed extending beneath the Bangong-Nujiang suture to 34°N. The HVL is generally interpreted as eclogite reflecting high grade metamorphism as pressure and temperature increase with depth and/or as underplated Indian crust.and has potential geodynamic implications.

 

We are systematically investigating the HVL at the base of the crust to characterize its depth, thickness, lateral extent, Vp, Vs, and Vp/Vs, applying consistent processing across the various datasets where the layer is observed. We first carry out a joint inversion of receiver function and surface wave dispersion observations to determine a lithospheric shear velocity model. Initial results indicate significant complexity as the layer first emerges in the south, gradually increasing in thickness to a maximum of 15km before thinning northward and eventually terminating. The arrival time and amplitude of the converted phase from the layer shows complex variation and azimuthal dependence .

 

To further quantitatively explore the HVL properties in detail, we are developing a layer stripping approach using receiver functions and HK stacking methods. . Starting with the lithospheric velocity model,, we create synthetic receiver functions. The Moho related phases in the synthetics are identified, extracted and then subtracted from the observed receiver function. Subsequent HK stacking of the corrected receiver function is then able to resolve the top of HVL without the interference of Moho phases.