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

摘要： 深部岩石的弹性波各向异性是人们了解地壳深部构造特征,分析其成因,探讨其动力学意义的重要岩石物理参数。实验结果表明由矿物晶格定向分布(LPO)所引起的地壳岩石平均各向异性强度通常不超过5%,远不足以解释在青藏高原地壳中所观测到的弹性波各向异性之强度。模拟结果显示,熔体的定向分布(MPO)能够引起强烈的弹性波各向异性。例如,当熔体的形态因子(α)值介于0.1～0.5之间,熔融程度为5%～10%时,由定向分布的酸性熔体囊所产生的各向异性强度可以达到2%～10%(P波)、2.2～40%(S波)。众多研究资料显示,青藏高原—川滇西部具有加厚的中、上地壳和高地热梯度,低度部分熔融作用在其深部地壳中广泛存在。低度熔体在构造应力作用下的定向分布可能是造成该地区深部地壳存在异常强的各向异性层的重要原因。这暗示目前在青藏高原—川滇西部探测到的异常强的区域性各向异性层是具有部分熔融成因的强烈构造变形带。该构造变形带具有潜在的"解耦"功能,并作为地壳浅部刚性层(块体)的底界协调着块体与其下伏地壳或岩石圈地幔的差异运动。

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