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

下陆壳的行为控制着山脉带的生长和衰败。特别是，造山过程中的地壳增厚会导致地壳深部融化，导致地壳强度大幅下降。然后，熔融减弱的地壳可以响应与造山相关的压力梯度向外或向上流动，并可能作为地壳中部的水道横向输送数百公里，或重新加工成大型韧性褶皱。被俯冲到地幔深处(=100公里)的大陆地壳的融化甚至可能有助于其返回地表。尽管无法直接观察到深部地壳，但这些过程的地质证据来自深度侵蚀的大陆地壳中的变质片麻岩地体和最近暴露在年轻山脉带上的地区。这些岩石含有矿物组合和结构，其特征是在高温和高压下在数十至数百公里的尺度上流动，结构表明在变形过程中存在熔融。在大陆地壳上部，岩浆向凉爽的近地表地区输送可以深刻地改变周围岩石的热平衡、强度和行为。*这一建议侧重于熔融和造山过程之间的相互作用。为了解决难以进入的问题，我使用了来自严重侵蚀的山带的实地观察和模拟山体建筑物理的计算机模型的组合。在挪威西部，以前俯冲的大陆地壳显示出广泛的熔融证据，但关于岩石是在地幔深处开始融化还是在岩石返回地表时开始融化，仍存在持续的争论。在安大略省中部，一条有10亿年历史的山脉带的根部暴露得很好，为研究熔融和造山之间的关系提供了一个理想的天然实验室。在这种情况下，熔融开始得相对较早，导致地壳的弱化和堆积，并将解体的高压变质岩向地表输送。在世界上最大的活动山带-喜马拉雅-藏系中，中地壳的融化被认为驱动了弱地壳作为中壳通道的侧向流动，向山前延伸了数百公里。这一“渠道流”假说在首次提出十多年后仍存在争议，但用于检验相互竞争的想法的新概念和新方法已经出现，应该有助于解决这场争论。新斯科舍省南部和纽芬兰东北部的岩石保存了岩浆侵入如何影响上地壳的极好证据。利用岩石学和计算机方法的新组合，这项拟议的研究将解决一些关于变质作用、熔融和造山过程中因果关系的长期问题。

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.***This proposal focuses on interactions between melting and mountain-building processes. To get around the problem of inaccessibility I use a combination of field observations from deeply eroded mountain belts and computer models that simulate the physics of mountain building. In western Norway, formerly subducted continental crust displays widespread evidence of melting, but there is continuing debate about whether melting started when the rocks were still at mantle depths or during their return to the surface. In central Ontario, the well-exposed roots of a billion-year-old mountain belt offer an ideal natural laboratory for examining the relationship between melting and orogenesis. In this case, melting began relatively early, leading to weakening and stacking of the crust and transport of dismembered high-pressure metamorphic rocks toward the surface. In the world's largest active mountain belt, the Himalayan-Tibetan system, melting of the middle crust is postulated to have driven lateral flow of weak crust as a mid-crustal channel for hundreds of kilometres toward the mountain front. This "channel flow" hypothesis is still controversial more than a decade after it was first proposed, but new concepts and methods for testing competing ideas have emerged that should help to resolve the debate. Rocks in southern Nova Scotia and northeastern Newfoundland preserve excellent evidence of how magma intrusions affect the upper crust. Using a novel combination of petrological and computer methods, the proposed research will resolve some persistent questions concerning cause vs. effect with respect to metamorphism, melting, and mountain-building processes.**