西藏及其周围地区地壳、地幔地震层析成像——印度板块大规模俯冲于西藏高原之下的证据

TOMOGRAPHIC IMAGING OF THE TIBET AND SURROUNDING REGION: EVIDENCE FOR WHOLESALE UNDERTHRUSTING OF INDIAN SLAB BENEATH THE TIBETAN PLATEAU

文中介绍有关西藏—喜马拉雅碰撞带的一项地震层析成像研究。根据一个用天然地震数据产生的全球波速模型 ,印度板块有可能以近水平状俯冲于整个西藏高原之下至 16 5～ 2 6 0km深度。西藏岩石圈具有低波速地壳和高波速下岩石圈 (75～ 12 0km深 )。在 12 0～ 16 5km深度范围 ,西藏岩石圈与俯冲的印度板块之间有一层低速软流圈物质。高原中部从地表到 310km深处有一低速体 ,说明地幔物质有可能穿过俯冲板块的脆弱部位上隆。这些结果以及野外实测的地壳缩短值说明高原的抬升得助于印度板块的近水平俯冲。我们推论俯冲印度板块的升温上浮以及上覆软流层的存在是造成西藏高原高海拔抬升以及内部地表仍相对平坦的主要原因。2 0 0 1年 1月 2 6日在印度西部发生的毁灭性大地震有可能是俯冲应力在印度板块后缘薄弱处引发的岩石圈大断裂。

We report an analysis of the Tibet-Himalayan collision zone based on a global seismic tomography model using earthquake data. The result indicates a possibility that the Indian lithospheric slab has been subducted subhorizontally beneath almost the entire Tibetan plateau to depths of 165～260 km. Tibetan lithosphere has a low-velocity crust and high-velocity lithospheric mantle at depths between 75～120 km. At depths between 120～165 km beneath the Tibetan plateau, there is an interpreted asthenospheric layer overlies the subducted Indian slab. Beneath the central portion of the plateau a low-velocity anomaly exists from the crust down to 310 km depth, indicating possible mantle upwelling through a weakened portion of the subducted slab. These results, coupled with existing field-based crustal shortening estimates, suggest that uplift of the Tibetan plateau was assisted by subhorizontal subduction of the Indian lithosphere. We hypothesize that the buoyancy due to heating of the subducted Indian slab and the existence of the asthenospheric layer contribute to the high elevation and flatness of the Tibetan plateau. The destructive earthquake that occurred in western India on 26/01/2001 is probably attributable to a failure of the weakest part of the Indian lithosphere in response to the stress related to its subduction beneath Tibet.