Early evolution of the middle crust in continental collisions

大陆-大陆碰撞后，由于岩石圈显著增厚和中地壳（15-40公里深）熔融弱化，形成了大型热造山带。熔融弱化作用促进深部侧向扩张，控制地表地形地貌和气候，影响岩浆和流体在地壳中的运移，并影响造山构造。从薄，小，冷造山楔为主的机械变形过程中的厚，大，热造山高原为主的热变形过程中的过渡很少记录，并形成了一个主要的知识差距在构造研究。在特定的结构背景下，可以针对造山作用早期阶段熔融弱化的中地壳的发展，这是本提案的主要目标。在陆-陆碰撞过程中或之后被挖出地表的中地壳岩石在造山作用期间对中地壳的状态保持着重要的制约。基于现今造山带腹地（主山带）暴露岩石的数值模型，为我们提供了对中地壳热性质和力学性质的许多了解。然而，这些岩石很少保留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所覆盖。在这个建议中，我假设，远距离旅行，高变质等级的外部推覆体运输更接近前陆（前）的早期碰撞过程中的演化过程中被挖出，这样的岩石更有可能记录大陆碰撞的早期阶段。 两个标志性的大型热造山带是这个建议的目标：尼泊尔和印度的特征鲜明的活动喜马拉雅山，在那里的前陆中地壳单元的早期剥露是已知的，和老格伦维尔（即目前在魁北克省劳伦蒂德），在那里，相比之下，很少有现代岩石年代学和变质研究已经进行。格伦维尔（> 10亿年）提供了一个机会，比较喜马拉雅地球动力学模型的大陆碰撞，演变完成。HQP领导的项目，包括实地考察和新提供的多方面尖端实验室调查，将重建压力-温度-时间-变形路径，并表征这两个造山带中早期变质岩的性质。这一研究方案将对初始碰撞后的中地壳性质（温度、压力、结构）提供强有力的限制。这对于理解大陆-大陆构造板块边界如何容纳会聚、物质如何在地壳内运输以及中地壳如何塑造地球表面至关重要。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过程之间的反馈效应的严格检验提供了必要的基础。

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.