再论岩石圈地幔蘑菇云构造及其深部成因

Mushroom structure of the lithospheric mantle and its genesis at depth: revisited

根据地球物理资料的分析得到,除了东北吉辽地区太平洋板块向大陆俯冲外,当今中国东部其他地区大陆下都没有俯冲太平洋板块的证据。中生代以来中国东部发生的岩石圈巨变不是太平洋板块向中国大陆的俯冲所造成,而是由于软流圈物质上涌形成蘑菇云构造对岩石圈地幔的改造而引发的重大地质事件。软流圈物质的上涌使岩石圈地幔成为新生地幔与残剩地幔并存的结构,岩石圈被激活,稳定的克拉通地壳褶皱变形,地壳减薄,地震频发,岩浆活动活跃,地面沉降,并形成广袤的西太平洋边缘海。但是岩石圈厚度并未减薄,只是由于岩石圈地幔形成了蘑菇云结构使它的速度降低,与周遍地区的岩石圈结构产生明显差别。蘑菇云地幔发育的地区覆盖整个东南亚西太平洋地区。包括中国东北的中东部、华北的东部、华南的东南缘、马来半岛、印尼、菲律宾、日本海、黄海、东海、南海,加罗林盆地和菲律宾海盆。与这个东南亚西太平洋低速岩石圈地幔相对应,还存在一个巨大的大地椭球面正异常,它应是核幔边界的质量过剩所引起。它与印度洋—西藏地区核幔边界质量亏损引起的负大地椭球面异常孪生,并形成一个控制中国大陆构造的深层动力系统,这个动力系统产生的时代可能为中生代或晚古生代。它们之间的地幔环流造成了东南亚西太平洋岩石圈巨变,并驱动印度板块的北移和青藏高原的隆升。

Analysis of geophysical data shows that there is no evidence for subduction of the Pacific plate beneath the continent of present eastern China except the Jilin-Liaoning （Jiliao） area, Northeast China. The tremendous change that has taken place in eastern China since the Mesozoic did not result from the subduction of the Pacific plate beneath continental China but is a major geological event caused by the reworking of the lithospheric mantle by mushroom structure formed by upwelling of asthenospheric materidals. The upwelling of asthenospheric materials resulted in the formation of a structure of coexistence of the new mantle and remnant mantle, reactivation of the lithosphere, folding of the crust of stable cratons, crustal thinning, repeated occurrence of earthquakes, active magmatism, ground subsidence and formation of vast marginal seas of the western Pacific. However, the thickness of the lithosphere did not decrease, and only the rate of lithospheric thinning decreased owing to the formation of the mushroom structure of the mantle. As a resuk, the lithospheric structure in the region is notably different from those in its surrounding regions. The region with the mushroom-shaped mantle covers the western Pacific region of the whole of Southeast Asia, including the east-central part of Northeast China, eastern North China, the southeastern margin of South China, Malaysia Peninsula, Indonesia, the Philippine, the Sea of Japan, Yellow Sea, East China Sea, South China Sea, the Caroline basin and the Philippine sea basin. There also exists a huge geodesic ellipsoidal positive anomaly corresponding to the low-velocity lithospheric mantle in the western Pacific region of Southeast Asia, which should be caused by mass surplus at the core-mantle boundary. It and its twin-the negative geodesic eUipsoidal anomaly caused by mass deficiency at the core-mantle boundary in the Indian Ocean-Tibet region-form a deep-level dynamic system controlling the structures of continental China. The age of this dynamic system might be Mesozoic or Late Paleozoic. The mantle circulation between brought about a tremendous change of the lithosphere below the western Pacific of Southeast Asia and drove the northward migration of the Indian plate and uplift of the Qinghai-Tibet Plateau.