喜马拉雅地区深反射地震──揭示印度大陆北缘岩石圈的复杂结构
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摘要
喜马拉雅山的崛起和青藏高原的隆升被认作是印度板块和亚洲板块中、新生代以来汇聚、碰撞、挤压的结果，是典型的陆－陆碰撞地带。此文介绍了在喜马拉雅山区进行的第一次深反射地震试验的结果。试验剖面布置在北喜马拉雅地区内，从喜马拉雅山山脊南的帕里到康马南的萨马达共中１５点（ＣＭＰ）叠加剖面上表现出如下特点：①显示了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道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，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，发现了南部有双莫霍层的存在；⑤试验中还取得莫霍层下面３２ｓ、３８ｓ、４８ｓ等双程走时的多条反射，均向北倾斜，反射同相轴延续较长，信息丰富，反映了上地幔的成层结构。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。
he abrupt rise of Himalaya and the uplift of Qinghai-Tibet plateau have been considered as the consequence of convergence, collision and compression between Indian and Asian plates, since Mesozoic and Cenozoic. Here we report the results of our first deep seismic reflection survey in Himalayan mountain country. The test profile is located within North Himalaya region,from Pagri, south of the Himalayan crest, to Samada, south of Kangmar. On the CMP stacke section 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 undsrthrusts southern Tibet; 2) an abundance of reflections in the upper crust, suggestive of large-scale structural imbrications; 3) the reflections of the lower crust suggesting some plastic rheological features; 4)a distinct Moho reflection in the southern part of the profile at a depth of about 72-75 km; and an additional secondary Moho in the south; 5) informative reflections acquired from beneath Moho at 32s 36s, 48s. TWas, implying stratifications (dipping north) of the upper mantle. All these results give substantial support to such views, that the intact indian continental crust or its lower crust underthrusts South Tibetan Tethyan Himalaya crust, resulting in thickening of southernmost Tibetan crust.
he abrupt rise of Himalaya and the uplift of Qinghai-Tibet plateau have been considered as the consequence of convergence, collision and compression between Indian and Asian plates, since Mesozoic and Cenozoic. Here we report the results of our first deep seismic reflection survey in Himalayan mountain country. The test profile is located within North Himalaya region,from Pagri, south of the Himalayan crest, to Samada, south of Kangmar. On the CMP stacke section 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 undsrthrusts southern Tibet; 2) an abundance of reflections in the upper crust, suggestive of large-scale structural imbrications; 3) the reflections of the lower crust suggesting some plastic rheological features; 4)a distinct Moho reflection in the southern part of the profile at a depth of about 72-75 km; and an additional secondary Moho in the south; 5) informative reflections acquired from beneath Moho at 32s 36s, 48s. TWas, implying stratifications (dipping north) of the upper mantle. All these results give substantial support to such views, that the intact indian continental crust or its lower crust underthrusts South Tibetan Tethyan Himalaya crust, resulting in thickening of southernmost Tibetan crust.
引文
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