苏鲁高压-超高压变质带折返抬升过程中构造界面和应力场的变化

Changes in tectonic boundaries and stress fields during exhumation and uplift of the Sulu HP-UHP metamorphic belt.

作为地质历史时期深俯冲作用产物的超高压变质岩,尽管有不同的形成演化历史,但其最终的折返作用无不是在逆冲扩展过程中完成的。因此,逆冲扩展作用及其数值模拟的研究对完整认识苏鲁高压．超高压变质带的折返机制和折返过程具有重要意义。逆冲扩展作用最显观的构造效应是不同层次构造界面和应力场的变化,对于地壳尺度的逆冲扩展作用而言,陆表面和Moho是最重要的活动性构造界面,陆表面的变化导致山体抬升和滞后伸展盆地的形成,出现盆-山相间的构造格局。初步的模拟计算表明,山体的抬升量、滞后伸展盆地的坳陷量和Moho的上拱量与逆冲地块的平移速度和逆冲扩展速度成正相关关系,而且随着时间的推移,山体的抬升速度、滞后伸展盆地的坳陷速度和Moho的上拱速度都有逐渐增大趋势。逆冲扩展过程中,构造应力场的变化总的表现为随着平移速度和逆冲扩展速度的增大和逆冲扩展作用的持续进行,逆冲块体内部由挤压应力状态逐渐向拉张应力状态转化。地块的平移速度是构造强度的一个重要标量,当高压-超高压变质带以仰冲块体为运动载体,沿断裂带向陆壳浅部折返时,构造界面的移动规律基本反映了高压-超高压变质带的折返过程,可见,构造作用的强度和性质应该是制约高压-超高压变质带折返的重要因素之一,在岩石密度差相同的条件下,拉张构造应力场更有利于折返作用的进行。苏鲁高压-超高压变质带的折返是通过多期构造作用完成的,根据数值模拟结果可以推测,每期构造作用都伴随有折返速度由慢到快的变化,在整个折返过程中,构造运动性质和强度的差异导致了折返速度的不均一,总体上,折返速度将随着逆冲地块由挤压向拉张状态的转化和拉张强度(构造作用强度)的不断增强而逐渐增大,最终在以拉张为主导的构造应力场中完成了高压-超高压变质带折返的全过程。

The final exhumations of UHP metamorphic rocks, as the product of deep subduction in geological times, were all completed during thrust propagation though they have different histories of formation and evolution. Therefore thrust propagation and its numerical modeling have great significance for a complete understanding of the mechanism and process of exhumation of the Sulu HP-UHP metamorphic belt. The most pronounced tectonic effect of thrust propagation is changes in tectonic boundaries and stress fields at different levels, and for the crustal-scale thrust propagation, the land surface and Moho are the most important active tectonic boundaries. The change of the land surface may result in the mountain uplift and formation of lagging extension basins, as well as appearance of a tectonic framework of alternately arrayed basins and ranges. Preliminary modeling calculation indicates that the magnitudes of mountain uplift and lagging extension basin subsidence are positively correlated with the magnitude of Moho uparching and translational and thrust propagation velocities of thrust blocks, and that both the velocities of mountain uplift and lagging extension basin subsidence and the velocity of Moho uparching have a trend of gradual increase with time. During thrust propagation, the change in tectonic stress field is generally manifested by the gradual transformation of the stress state inside a thrust block from the compressional one to extensional one with increasing translational and thrust propagation velocities and continuous thrust propagation. The translational velocity of a block is an important scalar quantity of the tectonic strength. When an HP-UHP metamorphic belt is exhumed to the shallow level of the continental crust along a fault with the obducted block as the movement carrier, the movement rule of the tectonic boundary essentially reflects the exhumation process of the HP-UHP metamorphic belt. Thus the strength and nature of tectonism should be one of the important factors for constraining the exhumation of the HP-UHP metamorphic belt, and the extensional stress field is more favorable for the proceeding of exhumation if the density contrasts of the rocks are the same. The exhumation of the Sulu HP-UHP metamorphic belt was completed through repeated tectonic processes. According to the results of numerical modeling, the following may be inferred ： The tectonism of each phase is accompanied by the change of exhumation velocity from slow to rapid and the differences in nature and strength of tectonic movements lead to exhumation velocity heterogeneity in the whole process of exhumation. In general, the exhumation velocity will increase progressively with the transformation from the compressional to extensional states of a thrust block and continuous increase in strength of extension （tectonism） , and finally the whole exhumation process of the HP-UHP metamorphic belt is accomplished in an extension-dominated tectonic stress field.