深反射地震揭示喜马拉雅地区地壳上地幔的复杂结构

DEEP SEISMIC REFLECTION IN HIMALAYA REQION REVEALS THE COMPLEXITY OF THE CRUST AND UPPER MANTLE STRUCTURE

报告了中、美两国在喜马拉雅山区进行的第一次深反射地震试验的结果．试验剖面南起喜马拉雅山山脊南亚东县的帕里镇，向北穿过喜马拉雅山脊的荡拉，到达康马南的萨马达．剖面长约１００ｋｍ．共中心点（ＣＭＰ）叠加剖面上显示出：１．在地壳中部有一强反射带，向北缓倾斜下去，延长达１００ｋｍ以上．它可能代表了一个活动的逆冲断裂或是一条巨大的拆离带，印度地壳整体或下地壳沿此拆离层俯冲到藏南之下．２．上部地壳的反射很丰富，显示了上地壳存在着大规模的叠瓦状结构．３．下地壳的反射同相轴呈现短而有规律的分布，显示了塑性流变特征．４．在测线南部莫霍反射明显，深度达７２－７５ｋｍ．发现南部有双莫霍层的存在．５．试验中还取得莫霍层下面３２，３８，４８ｓ等双程走时的多条反射，向北倾斜，反射同相轴延续较长，信息丰富，反映了上地幔的成层结构和变形特征．这些结果对印度大陆地壳整体或其下地壳俯冲到藏南特提斯喜马拉雅地壳之下，并导致西藏南端地壳增厚的观点，给予了实质性的支持．

The abrupt rise of Himalaya and the uplift of the Qinghai-Tibet plateau are considered as the consequence of convergence, collision and compression between Indian and Asian plates since Mesozoic and Cenozoic. The results of the first deep seismic reflection survey in Himalayan mountain area are reported here. The test profile is located within North Himalaya region, from Pagri, to the south of the Himalayan crest, to Samada, to the south of Kangmar. On the CMP stacked section the following features are observed: 1. a mid-crustal strong reflection zone, that extends as far as 100 km and more, dipping gently to the north; it probably represents an active thrust or a huge decollement zone, along which the intact Indian crust or its lower crust underthrusts beneath southern Tibet; 2. an abundance of reflections in the upper crust, indicating occurrence of large-scale imbrications in it; 3. the reflection of the lower crust suggesting some plastic-rlieological features; 4.a distinct Moho reflection in the Southern Part of the profile at a depth of about 72-75km and an additional secondary Moho in the south; 5. a series of north-dipping reflections of large extension acquired from beneath Moho at 32s, 36s, 48s of TWTT with abundant information, which may reflect the stratification and deformation features of the upper mantle. All these results give substantial support to such views that the intact Indian continental crust or its lower part underthrusts South-Tibetan Tethys-Himalayan crust, resulting in thickening of the southernmost Tibetan crust.