Collaborative Research: Lithospheric Weakening, Deep Crustal Flow and the Initiation of Orogenesis at a Noncollisional Convergent Margin in the Andes

大陆构造学的一个基本问题是造山作用如何在非碰撞会聚边缘开始。野外研究和力学模拟结果表明，前造山覆盖板块的强度和流变性是控制这些环境中结构变异性的关键因素。在安第斯山脉，原型非碰撞造山带，前造山作用弱化的主要原因涉及到中生代期间大型伸展盆地的形成和破坏。然而，在大多数地区，现代安第斯造山带的影响掩盖了这些从伸展到收缩的过渡如何控制科迪勒拉从海平面以下最初上升的地质记录。甚至更少的地区保留了地壳深部如何通过这些不断变化的构造机制演变的记录。该项目的目的是确定高温，薄地壳，以及流变分层，板内伸展的结果如何影响在随后的缩短过程中的地壳深部的热和结构演化。这种观测方法是可能的，因为智利最南端的达尔文杂岩保存了一个大型伸展盆地形成和破坏期间中地壳热演化和结构演化的几乎完整的记录。初步结果表明，在最南端的安第斯山脉盆地关闭导致大规模的俯冲和/或部分俯冲的大陆岩石圈在非碰撞，板内设置。该项目利用构造地质学、变质岩石学、U-Pb地质年代学和热年代学来确定达尔文杂岩的变形年龄以及构造和热演化的性质。在这里，地壳在一次构造活动中被拉伸，随后在一次造山活动中经历了收缩。因此，该区域提供了一个构造循环中建立的力学性质如何影响后期事件中的力学行为的检查。这一研究结果将有助于阐明以厚皮构造为特征的狭窄造山带是如何形成的，以及深埋的大陆岩石是如何俯冲和折返的。该项目还涉及美国，智利和澳大利亚的机构。

A fundamental problem in continental tectonics centers on how orogenesis initiates at noncollisional convergent margins. Field studies and the results of mechanical modeling show that the strength and rheology of the pre-orogenic overriding plate is a crucial factor that controls structural variability in these settings. In the Andes, the archetypal noncollisional orogen, a leading cause of pre-orogenic weakening involved the formation and destruction of large extensional basins during the Mesozoic. However, in most areas, the effects of the modern Andean orogen has obscured the geologic record of how these the transition from extension to contraction controlled the initial rise of the Cordillera from below sea level. Even fewer areas preserve a record of how the deep crust evolved through these changing tectonic regimes. The objective of this project is to determine how the high temperatures, thin crust, and rheological stratifications that result from intraplate extension affected the thermal and structural evolution of the deep crust during subsequent shortening. This observational approach is possible because the Darwin Complex in southernmost Chile preserves a nearly complete record of the thermal and structural evolution of the middle crust during the formation and destruction of a large extensional basin. Preliminary results indicate that basin closure in the southernmost Andes led to the large-scale underthrusting and/or partial subduction of continental lithosphere in a noncollisional, intraplate setting. The project utilizes structural geology, metamorphic petrology, U-Pb geochronology, and thermochronology to determine the ages of deformation as well as the nature of the tectonic and thermal evolution of Darwin Complex.This project is an intensive geological exploration of an important, yet poorly understood, region in southernmost South America. Here, crust that had been stretched in one tectonic event subsequently experienced contraction during a mountain building episode. As such, this region affords examination of how mechanical properties established in one tectonic cycle influence the mechanical behavior in later events. Results of this study should shed light on how narrow orogenic belts characterized by thick skinned tectonics are formed and how deeply buried continental rocks are subducted and exhumed. This project also involves a substantial collaborations among U.S., Chilean, and Australian institutions.