Lithospheric structure of the Qiangtang Terrane, northern Tibetan Plateau, from complete regional waveform modeling: Evidence for partial melt

Abstract

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 observed complete regional waveforms. The data are three‐component broadband seismograms recorded by the 1991–1992 IRIS‐PASSCAL (Incorporated Research 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 thick 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 the 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±0.01. Attenuation in the crust and upper mantle is high (QP = 100–200 and QS = 44–89). Modeling of the broadband P waveforms suggests that a decrease in mantle velocity occurs at about 160 km depth in the mantle; however, this is not unambiguously supported by the data and modeling. The crust and uppermost mantle of the Qiangtang Terrane probably contains partial melt based on the high Poisson's ratio, low shear wave velocities, and low Q. The absence of high‐frequency Sn and the presence of volcanism of mantle lithospheric origin support 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% slower, respectively, in the Qiangtang Terrane relative to the Lhasa Terrane. 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 Terrane. Differences in the uppermost mantle P and S wave velocities and Poisson's ratios of these two adjacent terranes cannot be explained by temperature differences alone. Using the mantle temperature estimates of McNamara et al.. [1997] we suggest that partial melt of an ultramafic composition beneath the Qiangtang Terrane fits the velocity and Poisson's ratio estimates.