Receiver functions in central Tibet, implications for crustal structure and anisotropy

The dense INDEPTH III seismic array, which extended from the Lhasa terrane across the Banggong-Nujjang suture (BNS) into the Qiangtang terrane in central Tibet, yielded a large, high quality teleseismic data set. Results from stacking and migration of receiver functions with various frequencies reveal that (1) the crustal thickness ranges between 65-70 km. (2) A small ( ∼5 km) Moho offset is observed when using a constant crustal Vp/Vs ratio of 1.73 for the entire profile. On the other hand, using a higher Vp/Vs ratio of 1.80 for the Qiangtang terrane, as suggested from the timing of multiples, produces a nearly flat Moho. (3) For higher frequencies, the amplitudes of the P to S conversions at the Moho (PmS) diminish along a ∼100 km wide zone across the BNS, which implies a gradational velocity contrast. (4) A shallow ( ∼10 km) low-velocity zone (LVZ) appears ∼40 km south of BNS and is coincident with high conductivity observed from magnetotelluric studies. (5) In the mid-crust a strong continuous LVZ is present at ∼30 km depth in the southern section and becomes shallower ( ∼15 km) and more pronounced to the north. The northward increasing trend of the Vp/Vs ratio, the low Q values, and the large depth change of the mid-crustal layer observed from wide-angle seismic data are spatially consistent with this LVZ. (6) In general, the lower crust has more intra-crustal arrivals with large amplitudes under the Qiangtang terrane than in the Lhasa terrane. (7) The presence of dipping layers and/or seismic anisotropy is evident in the crust near the BNS based on azimuthally varying amplitudes and large tangential energy. The sharp onset of strong SKS splitting suggests crustal anisotropy with fast polarization directions varying from E-W to NE-SW. These splitting observations align well with the direction of shear in the present-day crustal strain field in Tibet. In order to constrain the origin and geometry of crustal anisotropy, observed variations in receiver functions with backazimuth will be modeled for hexagonal anisotropy parameters by using a global minimization technique. This approach will provide additional information on crustal fabrics, and in conjunction with geologic data, will enable us to distinguish crustal flow from fossilized fabrics of older tectonic events.