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集中在造山作用如何在非碰撞辐合边缘开始。野外研究和力学模拟结果表明，造山前覆岩板块的强度和流变是控制这些环境中构造变异性的关键因素。在安第斯山脉，典型的非碰撞造山带，是造山前弱化的主要原因，涉及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转变如何控制科迪勒拉从海平面以下开始上升的地质记录。甚至更少的地区保存着深层地壳如何通过这些不断变化的构造机制进化的记录。该项目的目的是确定高温、薄地壳和流变分层是如何由板内伸展引起的，在随后的缩短过程中影响了深部地壳的热演化和结构演化。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的热与结构演化的近乎完整的记录。初步结果表明，安第斯山脉最南端的盆地闭合导致大陆岩石圈在非碰撞的板内环境中大规模逆冲和/或部分俯冲。本项目利用构造地质学、变质岩石学、U-Pb年代学、热年代学等方法确定达尔文杂岩的变形年龄以及构造和热演化性质。该项目是对南美洲最南端一个重要但鲜为人知的地区的密集地质勘探。在这里，在一次构造活动中被拉伸的地壳随后在造山活动中经历了收缩。因此，该区域提供了在一个构造旋回中建立的力学特性如何影响后来事件的力学行为的研究。这项研究的结果将揭示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挖掘出来的。该项目还涉及美国、智利和澳大利亚机构之间的实质性合作。

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.