LITHOSPHERIC STRUCTURE OF THE QIANGTANG TERRANE, NORTHERN TIBETAN PLATEAU, FROM COMPLETE REGIONAL WAVE-FORM MODELING - EVIDENCE FOR PARTIALMELT

We report models of P and S wave velocity and attenuation for the the crust and uppermost mantle of the Qiangtang Terrane, northern Tibetan Plateau, inferred by fitting reflectivity synthetic seismograms to obs erved complete regional waveforms. The data are three-component broadb and seismograms recorded by the 1991-1992 WS-PASSCAL (Incorporated Res earch Institutions for Seismology - Program for Array Seismic Studies of the Continental Lithosphere) Tibetan Plateau Experiment and Global Seismic Network stations in the region. The Qiangtang Terrane has thic k crust (65 +/- 5 km) with P and S wave velocities of 6.1-6.3 and 3.34 -3.43 km/s, respectively, yielding an anomalously high crustal Poisson 's ratio of 0.29 +/- 0.02. Seismic velocities of the upper mantle of t he Qiangtang Terrane are normal for P waves and slow for S waves (8.10 and 4.35-4.41 km/s, respectively) with a high mantle Poisson's ratio of 0.29 +/- 10.01. Attenuation in the crust and upper mantle is high ( Q(P) = 100-200 and Q(S) = 44-89). Modeling of the broadband P waveform s suggests that a decrease in mantle velocity occurs at about 160 km d epth in the mantle; however, this is not unambiguously supported by th e data and modeling. The crust and uppermost mantle of the Qiangtang T errane probably contains partial melt based on the high Poisson's rati o, low shear wave velocities, and low Q. The absence of high-frequency Sn and the presence of volcanism of mantle lithospheric origin suppor t the presence of partial melt. Crustal and uppermost mantle structure in the Qiangtang Terrane is different from that for the Lhasa Terrane (immediately to the south) based on results of our previous studies. The average crustal P and S wave velocities are 4% faster and 2% slowe r, respectively, in the Qiangtang Terrane relative to the Lhasa Terran e. This yields a significant difference in the crustal Poisson's ratio with values of 0.29 for the Qiangtang Terrane and 0.25 for the Lhasa Tenane. Differences in the uppermost mantle P and S wave velocities an d Poisson's ratios of these two adjacent terranes cannot be explained by temperature differences alone. Using the mantle temperature estimat es of McNamara et al. [1997] we suggest that partial melt of an ultram afic composition beneath the Qiangtang Terrane fits the velocity and P oisson's ratio estimates.