关键词: 部分熔融, 弹性波各向异性, 熔体定向分布(MPO), 壳内解耦带, 青藏高原

Abstract: It is commonly agreed that seismic anisotropy,most likely caused by aligned minerals,is a very important indicator of intracrustal deformation.Ultrasonic velocity measurements on the schists,gneisses,migmatites,amphibolites and mylonites from Higher Himalayan Crystallines(HHC) and Honghe strike-slip fault zone in the southwestern China show that the average anisotropic magnitude of them is about 5%,which is much less than that observed by a series of measurements of surface-wave dispersion inversions and waveform inversions of P to S conversions in these regions.Modeled results indicate that seismic anisotropy can be enhanced by alignment of melt.For instance,extra anisotropy of 2%～10% for P-wave velocity and of 2.2%～40% for S-wave velocity would be induced by melt pocket preferred orientation when aspect ratio of melt pockets and melt fraction range from 0.1 to 0.5 and from 5% to 10% respectively.Obviously,the contribution of aligned melt to the anisotropy is likely comparable to or larger than that induced by lattice preferred orientation of major minerals.Geophysical investigations demonstrate that Tibetan and Sichuan-Yunnan crust is characterized by high geothermal gradients and abnormal thickness.Low degree partial melting accounting for low average crustal velocity and low velocity zones is present in the crust pervasively.Hence,we attribute the extremely high anisotropy observed within Tibetan and Sichuan-Yunnan crust to aligned melt pocket induced by localized deformation.It implies that the anisotropy zones within Tibetan and Sichuan-Yunnan crust is one of candidates to cause the upper part of crust(crustal block)to decouple to the other part of lithosphere.

Key words: partial melting, seismic anisotropy, melt preferred orientation(MPO), intracrustal decoupling zone, Tibet