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

地壳深处的变质作用和韧性流——控制和构造后果

• 批准号: RGPIN-2015-06196
• 负责人: Jamieson, Rebecca
• 金额: $ 2.4万
• 依托单位: Dalhousie University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=684833
• 关键词: Metamorphism; ductile; flow; deep; crust

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.**

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，岩浆进入凉爽的近地表区域可以深刻地改变周围岩石的热平衡，强度和行为。这一建议侧重于融化和造山过程之间的相互作用。为了解决无法进入的问题，我结合了对深度侵蚀的山区地带的实地观察和模拟造山运动物理过程的计算机模型。在挪威西部，以前俯冲的大陆地壳显示出广泛的熔融证据，但对于岩石是否在地幔深处还是在返回地表时开始熔融仍存在争议。在安大略中部，一个有十亿年历史的山脉带的暴露的根部为研究融化和造山作用之间的关系提供了一个理想的天然实验室。在这种情况下，熔融开始相对较早，导致地壳的弱化和堆积，并将分解的高压变质岩向地表输送。在世界上最大的活动山脉带----喜马拉雅---西藏系，据推测，中地壳的熔融驱动了弱地壳的侧向流动，作为向山前延伸数百公里的中地壳通道。这个“通道流”假说在首次提出十多年后仍然存在争议，但是已经出现了新的概念和方法来测试竞争的想法，这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绝佳证据。使用岩石学和计算机方法的新组合，拟议的研究将解决一些关于变质作用，熔融和造山过程的原因与影响的持久问题。