Early evolution of the middle crust in continental collisions

大陆碰撞中地壳的早期演化

• 批准号: RGPIN-2020-05916
• 负责人: SoucyLaRoche, Renaud
• 金额: $ 1.82万
• 依托单位: Institut national de la recherche scientifique
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2022
• 资助国家: 加拿大
• 起止时间: 2022-01-01 至 2023-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=751373
• 关键词: Early; evolution; middle; crust; continental

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.

大热造山带是在大陆-大陆碰撞后形成的，这是岩石圈显著增厚和中地壳（深度15-40 km）融化减弱的结果。熔体减弱有利于深部横向扩张，控制地表地形起伏和气候，影响地壳内岩浆和流体的迁移，影响造山构造。从以机械变形为主的薄、小、冷造山楔向以热、机械变形为主的厚、大、热造山高原的转变，文献记载甚少，是构造研究的一大知识空白。在造山作用的早期阶段，熔融弱化的中地壳的发育可以在特定的构造环境中进行定位，这是本建议的主要目标。在大陆与大陆碰撞期间或之后被挖出地表的中地壳岩石，对造山运动期间中地壳的状态保留了至关重要的约束条件。基于现今造山带腹地（主山带）裸露岩石的数值模型，提供了我们对中地壳热学和力学性质的许多认识。然而，这些岩石很少保留任何关于大陆碰撞初始阶段的信息，因为它们通常被随后的变质事件覆盖。在这一建议中，我假设在碰撞演化的早期，远行的、高变质等级的外部推覆体被挖掘出来，这些岩石更有可能记录了大陆碰撞的早期阶段。这一建议的目标是两个标志性的大热造山带：尼泊尔和印度的活动喜马拉雅山脉，在那里已知的前陆中地壳单元的早期发掘，以及古老的Grenville（即qusamubec的当前Laurentides），相比之下，很少进行现代岩石年代学和变质学研究。格伦维尔（10亿年前）提供了将喜马拉雅地球动力学模型与大陆碰撞过程进行比较的机会。hqp领导的项目，包括实地考察和最新的、多方面的前沿实验室调查，将重建这两个造山带的压力-温度-时间-变形路径，并表征这两个造山带的早期变质岩的性质。该研究项目将提供初始碰撞后中地壳性质（温度、压力、结构）的可靠约束。这是理解大陆-大陆构造板块边界如何适应汇聚、物质如何在地壳内运输以及中地壳如何塑造地球表面的基础。这些对板块构造理论的改进为大陆碰撞的数值模型以及地质和地表作用之间的反馈效应的批判性研究提供了必要的基础。