SHEAR-WAVE ANISOTROPY BENEATH THE TIBETAN PLATEAU

Eleven broadband digital seismic stations were deployed across the cen tral Tibetan Plateau in the first extensive passive-source experiment attempted within the Tibetan Plateau. One year of recording resulted i n 186 event-station pairs which we analyze to determine the characteri stics of shear wave splitting in the upper mantle beneath the array. M easurements of the fast polarization direction (phi) and delay time (d eltat) for SKS and direct S arrivals reveal systematic variations alon g the north-south oriented array. In the north central region of the p lateau, very large delay times are observed at three stations, the lar gest of which is BUDO with deltat=2.4 s. However, at TUNL, which is of f the northern edge of the plateau and 110 km from BUDO, and at sites in the south central plateau, deltat decreases by nearly a factor of 3 . We also observe a systematic rotation of phi from about 45-degrees ( NE) to 90-degrees (E-W) from south to north along the array. A previou sly identified zone of inefficient Sn propagation correlates well with our region of large deltat observations. The large delay times sugges t that a relateively high number of anisotropic crystals are preferent ially alligned within the mantle-lid, beneath the north central portio n of the Tibetan Plateau. In most cases, fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional def ormation that has produced the Tibetan Plateau.