Early evolution of the middle crust in continental collisions

由于岩石圈的显著增厚和中地壳(深度15-40公里)的熔融减弱，大陆-大陆碰撞后形成了大型热造山带。熔体的弱化促进了深部的侧向扩张，控制了地表的地形起伏和气候，影响了地壳中的岩浆和流体的运移，并影响了造山构造。从以机械变形过程为主的薄、小、冷造山楔体向以热机械变形过程为主的厚、大、热造山高原的转变，文献很少，形成了构造研究中的一个重大知识鸿沟。在造山作用早期阶段，熔融减弱的中地壳的发育可以在特定的构造环境中进行，这是这一提议的主要目标。 在大陆-大陆碰撞期间或之后被挖掘到地表的中地壳岩石，在造山作用期间保留了对中地壳状态的关键制约。基于现今造山带腹地(主山带)暴露的岩石的数值模型为我们提供了许多关于中地壳热和力学性质的理解。然而，这些岩石很少保存有关大陆碰撞初始阶段的任何信息，因为它们通常被随后的变质事件叠加。在这一提议中，我假设，距离前陆(前缘)较近的远行、高变质级别的外部推覆构造在碰撞演化过程中很早就被挖掘出来，这些岩石更有可能记录大陆碰撞的早期阶段。 这一建议针对的是两个标志性的大型热造山带：尼泊尔和印度的特征良好的活动喜马拉雅，已知前陆中地壳单元的早期折返；以及老格伦维尔(即魁北克的今天的劳伦蒂斯)，相比之下，那里进行的现代岩石年代学和变质研究很少。格伦维尔(&gt；10亿年)提供了将喜马拉雅地球动力学模型与演化到完成的大陆碰撞进行比较的机会。HQP主导的项目，包括野外工作和新获得的、多方面的尖端实验室调查，将重建压力-温度-时间-变形路径，并表征这两个造山带中涉及的早期变质岩的性质。 这项研究计划将对初始碰撞后的中地壳性质(温度、压力、结构)提供强有力的约束。这是理解大陆-大陆构造板块边界如何融合，物质如何在地壳内运输，以及中地壳如何有助于塑造地球表面的基础。这些对板块构造理论的改进为严格检验大陆碰撞的数值模型以及地质和表面过程之间的反馈效应提供了必要的基础。

Large-hot orogens form following continent-continent collision as a result of significant thickening of the lithosphere and melt weakening of the middle crust (15-40 km depth). Melt weakening facilitates lateral spreading at depth, controls the topographic relief and climate at surface, affects magma and fluid migration in the crust, and influence orogenic architecture. The transition from thin, small and cold orogenic wedges dominated by mechanical deformation processes to thick, large and hot orogenic plateaus dominated by thermomechanical deformation processes is poorly documented and forms a major knowledge gap in tectonic research. The development of the melt-weakened middle crust during the early stages of orogenesis can be targeted in specific structural settings and is the primary objective of this proposal. Mid-crustal rocks that are exhumed to the surface during or after continent-continent collisions preserve crucial constraints on the state of the middle crust during orogenesis. Numerical models based on rocks exposed in the present-day hinterland (main mountain belt) of orogens provide much of our understanding of mid-crustal thermal and mechanical properties. However, these rocks rarely preserve any information on the initial stages of continental collision because they are generally overprinted by subsequent metamorphic events. In this proposal, I hypothesize that far-travelled, high metamorphic grade external nappes transported closer to the foreland (front) are exhumed early during the collision evolution, and that such rocks are more likely to record the early stages of continental collision. Two iconic large-hot orogens are targeted by this proposal: the well-characterized active Himalaya of Nepal and India, where early exhumation of mid-crustal units in the foreland is known, and the old Grenville (i.e. the current Laurentides in Québec), where, in comparison, scarce modern petrochronologic and metamorphic studies have been conducted. The Grenville (> 1 billion years old) offers the opportunity to compare Himalayan geodynamic models to a continental collision that evolved to completion. HQP-led projects, including fieldwork and newly available, multifaceted cutting-edge laboratory investigations, will reconstruct the pressure-temperature-time-deformation path and characterize the nature of early-stage metamorphic rocks involved in these two orogens. This research program will provide robust constraints on mid-crustal properties (temperature, pressure, structure) following initial collision. This is fundamental to understand how convergence is accommodated at continent-continent tectonic plate boundaries, how material is transported within the crust, and how the middle crust contributes to shape the earth surface. These refinements to plate tectonic theory provide the necessary foundation for a critical examination of numerical models of continental collision and feedback effects between geological and surface processes.