Characterizing the Lower Crust in Southern Tibet by a New Layer-stripping Method

Abstract:

A widespread high velocity layer (HVL) has been detected by numerous seismic experiments at the base of crust in southern Tibet and northern Himalaya. Its general explanation is eclogitization of Indian lower crust under Tibet. However, its specific property is elusive. Eclogitization and the behavior of felsic to intermediate lower crust are significantly different from those of mafic lower crust. Determining the geophysical properties (thickness, Poisson’s ratio, density, and velocity structure) of the HVL could greatly improve our understanding about geodynamic process of an ongoing continental collision zone.

 

We present a new receiver function (RF) layer stripping approach to remove the Moho phase and isolate the HVL phase in each RF. We first carry out a joint inversion of RFs and ambient noise dispersion observations to determine the shear velocity structure. We use the velocity contrast across the Moho to create synthetic RFs that consists of only the Moho phases. Next we fix the values of accurate arrivals and amplitude of the observed Moho phases on the basis of HK stacking results plus a local search. We calibrate the synthetic Moho phases to the observed values and subtract from the observed RFs. Finally we perform two-step HK stacking method to search the thickness (H) and Vp/Vs ratio (K) of first layer (from surface to the top of HVL) and then the underlying HVL. The average Vs of each layer obtained from joint inversion are used in the HK staking instead of an assumed Vp.

We test this method on 23 Hi-CLIMB stations that exhibit HVL. The “doublets” merge together once the HVL is thinner than 10km, which is the resolution limit of this method. We found the average H and K of first layer are about 59km and 1.76, the southernmost station show highest K value (1.88) which is in agreement with presence of lower velocity zone in the middle crust. The average Vs of first layer ranges from 3.3 to 3.45 km/s. The HVL shows complex variation from station to station in H (with a maximum thickness 20.7 km in the center of profile) and K (mean: 1.69). Average Vs ranges from 3.89 to 4.09 km/s. Previous gravity anomaly study further shows the density of HVL is 3300kg/m³. All these evidences suggest the property of the HVL is felsic eclogite rather than mafic eclogite.