The uplift of high-pressure-low-temperature metamorphic rocks
The uplift of high-pressure-low-temperature metamorphic rocks
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高压低温变质岩的隆升
DOI：
10.1098/rsta.1987.0006
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发表时间：
1987
期刊：
影响因子：
--
通讯作者：
J. Platt
中科院分区：
文献类型：
--
作者：
J. Platt
高P-低T变质岩的隆起被归因于浮力、底辟作用或水动力驱动的回流。只有当俯冲减慢或停止时，浮力才能使俯冲到地幔中的物质返回，从而减少向下的牵引力。由于高磷组合密度的增加，浮力在地壳内部将发生逆转，因此不能使俯冲物质上升到地壳底部之外。底辟作用和水动力流动过程需要低密度、低粘度的基质，并且只能解释流动物质中夹带的相对较小的高磷岩石体的就位。连贯的区域高P-低T地形的构造背景可以解释的力学行为的增生楔可以忽略不计的屈服强度，底侵是主要模式的增生。底镀从下面加厚楔形体并增加其表面坡度。这导致楔形体的上部水平延伸，即使收敛仍在继续。持续的底侵作用和以上的延伸可以使最古老的高磷岩石上升到一个中等量的侵蚀范围内的时间尺度上的顺序为10马。只要俯冲作用在楔下继续，地热梯度就不会松弛到正常值。这一过程解释了（a）当会聚持续时，高P-低T岩石通常被抬升的证据;（B）在许多情况下，没有明显的高T组合的叠加;（c）最古老和最高压力岩石的位置在造山楔的上后部;（d）上覆岩石缺乏足够的构造厚度来解释变质作用;变质后断层的普遍出现，切除了部分变质分带。
The uplift of high-P-low-T metamorphic rocks has been attributed to buoyancy, diapirism, or hydrodynamically driven return flow. Buoyancy forces can return material subducted into the mantle only if subduction slows or ceases, reducing the downward traction. The buoyancy forces will be reversed within the crust, because of the increased density of high-P assemblages, and therefore can not cause the subducted material to rise beyond the base of the crust. Diapirism and hydrodynamic flow processes require a low-density, low-viscosity matrix, and can only explain the emplacement of relatively small bodies of high-P rock entrained in the flowing material. The tectonic setting of coherent regional high-P—low- T terrains can be explained in terms of the mechanical behaviour of an accretionary wedge with negligible yield strength, where underplating is the dominant mode of accretion. Underplating thickens the wedge from beneath and increases its surface slope. This causes the upper part of the wedge to extend horizontally, even though convergence is continuing. Continued underplating beneath and extension above can allow the oldest high-P rocks to rise to within reach of a moderate amount of erosion on a time scale of the order of 10 Ma. As long as subduction continues beneath the wedge, the geothermal gradient will not relax to a normal value. This process explains (a) the evidence that high-P-low-T rocks are commonly uplifted while convergence is continuing; (b) the absence in many cases of significant overprinting by higher- T assemblages; (c) the position of the oldest and highest pressure rocks in the upper rear of orogenic wedges; (d) the lack of adequate tectonic thicknesses of overlying rock to explain the metamorphism; and (e) the common occurrence of post-metamorphic faults that excise parts of the metamorphic zonation.