Metamorphism and ductile flow in the deep crust - controls and tectonic consequences

下大陆地壳的行为控制着山脉的生长和衰退。特别是，造山过程中的地壳增厚可能导致地壳深部熔化，导致地壳强度大幅下降。熔融弱化的地壳随后可以响应与造山作用相关的压力梯度而向外或向上流动，并可能作为中地壳通道横向迁移数百公里，或重新形成大型韧性褶皱。已经俯冲到地幔深处（=100公里）的大陆地壳的熔融甚至可能有助于它返回到表面。虽然无法直接观测到地壳深部，但这些过程的地质证据来自深度侵蚀大陆地壳中的变质片麻岩和最近暴露的年轻山脉地区。这些岩石含有矿物组合和高温高压下数十至数百公里尺度的流动结构特征，其纹理表明在变形期间存在熔融。在上陆壳，岩浆向冷的近地表区域的迁移可以深刻地改变周围岩石的热平衡、强度和行为。

The behaviour of the lower continental crust controls the growth and decay of mountain belts. In particular, crustal thickening during mountain-building can lead to melting in the deep crust, causing a profound reduction in crustal strength. The melt-weakened crust can then flow outward or upward in response to the pressure gradients associated with mountain-building, and may be transported hundreds of kilometres laterally as mid-crustal channels or reworked into large ductile folds. Melting of continental crust that has been subducted to mantle depths (=100 km) may even assist its return to the surface. Although the deep crust is inaccessible to direct observation, geological evidence for these processes comes from metamorphic gneiss terranes in deeply eroded continental crust and recently exposed regions of younger mountain belts. These rocks contain mineral assemblages and structures characteristic of flow on scales of tens to hundreds of kilometres at high temperature and pressure, with textures indicating that melt was present during deformation. In the upper continental crust, transport of magma into cool near-surface regions can profoundly change the heat budget, strength, and behaviour of the surrounding rocks.