青藏高原的构造分区及其边界的变形构造特征

宏观构造特征的确立对青藏高原隆升和“动力学建模”具有重要意义。青藏高原是由来自塔里木-中朝板块的北昆仑-阿尔金-祁连地体,华南-东南亚板块的南昆仑地体、可可西里-巴颜喀拉地体和冈瓦纳古陆的羌塘地体、冈底斯地体及喜马拉雅地体等3大板块(或古陆)的6个地体经多次裂解、会聚和陆内俯冲作用拼合而成的巨型“会聚-陆内俯冲型”岩石圈块体,它以青藏高原南缘结合带、青藏高原北缘结合带和青藏高原东缘结合带依次与印度岩石圈块体、塔里木-阿拉善-鄂尔多斯岩石圈块体和扬子岩石圈块体相隔。按现今动力学特征,这一巨型岩石圈块体(一级构造单元)又可进一步划分为喜马拉雅、藏北、青南和昆仑-阿尔金-祁连等4个二级构造单元(地块),它们依次以雅鲁藏布江结合带、西金乌拉-金沙江结合带、中昆仑结合带为界。4个地块又可进一步划分为若干以断裂为界的三级构造单元(地体)。组成青藏岩石圈块体的各构造单元处于统一的地球动力学系统,它总的表现为:在印度板块向欧亚板块持续、强烈俯冲和热的、具柔性流变学特征的青藏块体整体向北北东方向移动的区域构造背景上,其南、北两侧的喜马拉雅地块、昆仑-阿尔金-祁连地块分别向冷的、刚性的印度岩石圈块体和塔里木- 阿拉善-鄂尔多斯岩石圈块体不对称逆冲叠覆。位于青藏高原腹部的藏北地块和青南地块,在深部存在大量低速体向上涌动和整体自西向东扩展的区域构造背景上,前者叠置近南北向挤压,形成以南北向断陷带及北西和北东向共轭走滑为主的构造格局,而青南地块除松潘-甘孜地体显示自北而南的逆冲叠覆外,可可西里-巴颜喀拉地体以逐一向东挤出的左行走滑作用为主,以致整个青南地块呈现向扬子岩石圈块体逆冲扩展和向三江构造带平移扩展。因此,就现今动力学而言,青藏高原在随着时间推移、隆升速度不断加快的同时,还逐渐向外缘的刚性地块扩展,即高原面积在不断增大。因此青藏高原的边界具有扩展性质,按扩展机制可区分两类扩展型动力边界:走滑型扩展边界和逆冲型扩展边界。典型的走滑型扩展边界位于青藏高原北缘的阿尔金山和青藏高原东缘的三江地区,青藏高原南缘的动力边界属典型的逆冲型扩展边界,而位于祁连山和龙门山的动力边界兼有逆冲和走滑双重扩展性质。

Tectonic divisions of the Qinghai-Tibet Plateau and structural characteristics of deformation on their boundaries

The determination of macroscopic structural characteristics has great significance for the study of the uplift of the Qinghai-Tibet Plateau and its "dynamic model construction". The Qinghai-Tibet Plateau represents a gigantic "convergence-intracontinental subduction type" lithospheric block formed by amalgamation of six terranes of three plates (or old land), i.e. the North Kunlun-Altyn Tagh-Qilian terrane of the Tarim-Sino-Korean plate, the South Kunlun terrane and Hoh Xil-Bayan Har terrane of the South China-Southeastern Asian plate and the Qiangtang terrane, Gangdise terrane and Himalayan terrane of Gondwanaland, through multiple breakups, convergences and intracontinental subductions. It is separated from the Indus lithospheric block, Tarim-Alxa-Ordos lithospheric block and Yangtze lithospheric block by the south Qinghai-Tibet Plateau marginal junction zone, north Qinghai-Tibet Plateau marginal junction zone and east Qinghai-Tibet Plateau marginal junction zone respectively. According to the present-day dynamic characteristics, this gigantic lithospheric block (first-order tectonic unit) may be subdivided into four second-order tectonic units, namely, the Himalayan block, northern Tibet block, southern Qinghai block and Kunlun-Altyn Tagh-Qilian block, which are bounded by the Yarlung Zangbo junction zone, Xijir Ulan-Jinsha River junction zone and Central Kunlun junction zone successively. The four blocks may be further divided into several fault-bounded third-order tectonic units (terranes). The various tectonic units making up the gigantic Qinghai-Tibet lithospheric block are situated in a unifying geodynamic system. This geodynamic system is generally manifested as follows:the Himalayan block and Kunlun-Altyn Tagh-Qilian block on the southern and northern sides of the Qinghai-Tibet lithospheric block are asymmetrically thrust toward and stacked on the cold and rigid Indus lithospheric block and Tarim -Alxa -Ordos lithospheric block respectively in the tectonic setting that the Indian plate is continuously and strongly subducted beneath the Eurasian plate and the hot Qinghai-Tibet lithospheric block with ductile rheological properties moves en bloc in a NNE direction. Beneath the northern Tibet block and southern Qinghai block in the interior of the Qinghai-Tibet Plateau, large numbers of low-velocity bodies at depth upwell and propagate horizontally from west to east. In this tectonic setting, the former is superimposed by nearly N-S compression, forming a tectonic framework marked mainly by N-S -trending downfaulted zones and NW - and NE -trending conjugate strike -slip motions; whereas for the latter, except for the Songpan-Garze terrane that shows south -vergent thrust stacking, the Hoh Xil-Bayan Har terrane is mainly manifested by sinistral strike-slip motion leading to successive eastward extrusion, so that the whole southern Qinghai block experiences thrust propagation toward the Yangtze block and spreading toward the Sanjiang tectonic zone. Therefore, with increasing rate of uplift with time, the plateau also propagates gradually toward the rigid blocks at its peripheries, i.e. the area of the plateau expands progressively. So the plateau boundaries have the nature of propagation. On the basis of the propagation mechanism, two types of dynamic boundary may be distinguished: strike -slip type propagation boundary and thrust type propagation boundary. The typical propagation boundary is located in the Altyn Tagh Mountains on the northern margin of the Qinghai -Tibet Plateau and the Sanjiang area on the eastern margin of the plateau. The dynamic boundary on the southern margin of the plateau belongs to the typical thrust type propagation boundary, while the dynamic boundary in the Qilian and Longmen mountains has both the nature of thrust propagation and the nature of strike-slip propagation.