青藏高原上地幔速度结构及其动力学性质

利用地震层析成像结果分析了中国西部地区的上地幔速度结构,发现青藏高原北部至东南边缘上地幔顶部速度普遍偏低;随着深度的增加,低速区主要分布在羌塘、松潘—甘孜和云南西部地区,而印度大陆、塔里木、柴达木、鄂尔多斯和四川盆地均显示出较高的速度.上述速度分布与青藏高原及周边地区的岩石层结构和深部动力性质密切相关:其中羌塘地区的低速异常反映了青藏北部的地幔上涌和局部熔融,起因于印度大陆岩石层的向北俯冲;松潘—甘孜地区的低速异常与青藏东部的深层物质流动及四川盆地刚性岩石层的阻挡有关;而滇西地区的低速异常可能受到印缅块体向东俯冲作用的影响.以上三个区域构成青藏高原和周边地区的主要地幔异常区.相比之下,印度大陆、塔里木、柴达木、鄂尔多斯和四川盆地的高速异常反映了大陆构造稳定地区的岩石层地幔特点.根据速度变化推测,地幔上涌和韧性变形并非贯穿整个青藏高原,而是主要集中在羌塘、松潘—甘孜和滇西地区,上述构造效应不仅导致岩石层厚度减薄且引发了火山和岩浆活动.

Upper mantle velocity structure and dynamic features of the Tibetan plateau

We analyze the upper mantle structure of western China based on tomographic results. The results show that the uppermost mantle of the Tibetan plateau and western Yunnan are widely dominated by slow seismic velocities. At greater depths, however, these slow velocities are mostly concentrated in the Qiangtang area of northern Tibet, the Songpan-Ganzi area of eastern Tibet and western Yunnan, which are surrounded by fast velocities in the India, Tarim, Qaidam, Ordos and Sichuan basins. A synthetic analysis indicates that the velocity variations at these depths are closely related to the lithospheric mantle structure and deep dynamic features of the Tibetan plateau and surrounding areas. Among them, the slow velocities beneath the Qiangtang area reflect the partial mantle upwelling produced by the northward penetration of the Indian continental lithosphere into the Tibetan plateau; the slow velocities beneath the Songpan-Ganzi terrain are related to the lateral deep flow of eastern Tibet and the obstruction of the rigid lithosphere of the Sichuan basin, while the slow velocities beneath western Yunnan is apparently affected by the eastward subduction of the India-Burma plate. These three areas are considered to be underlain by an anomalous mantle. On the contrary, the fast upper mantle velocities of the areas surrounding the Tibetan plateau, such as the India continent, Tarim, Qaidam, Ordos and Sichuan basins, reflect the features of the lithospheric mantle structure in tectonically stable continental blocks. We infer that the mantle upwelling and ductile deformation are not distributed throughout the Tibetan plateau, rather concentrated in the Qiangtang, Songpan-Ganzi and western Yunnan, which has resulted in the thinning of the lithosphere and volcanic and magmatic activities.