Collaborative Research: Testing Mechanical Models of Himalayan Orogenesis in NW India

喜马拉雅和西藏是大陆-大陆碰撞的典型例子，因此，印亚造山带是碰撞造山的范例和构造模式的试验场。一个压倒一切的构造问题是，是什么控制了造山带的构造演化。逆冲发展的经典模型(临界锥形模型)不排除地壳的薄弱部分，但表明它们不能协同工作来诱导流动；相比之下，最近的热-力学模型(沟道流动模型)认为中地壳和下地壳非常弱，并与侵蚀相一致，主导着变质和构造发展。该项目由阿拉巴马大学和博伊西州立大学的研究人员与瓦迪亚喜马拉雅地质研究所的科学家合作开展，旨在通过利用这些模型预测的截然不同的压力-温度-时间运动学历史来区分这两种截然不同的观点。具体地说，该小组将在印度西北部的喜马拉雅山-乌塔兰恰尔山进行综合的构造、岩石学、地层学和地质年代学调查，以解决关于喜马拉雅逆冲推覆构造运输和构造演化的三个基本问题：(1)沿尼泊尔走向，地层和构造包如何相互关联？野外测绘和构造分析将使从尼泊尔到印度的地层对比成为可能，并通过平衡的横截面确定推覆几何形状和缩短量。薄片、Nd同位素和碎屑锆石分析将有助于确定主要的岩石构造单位，这对准确定位不同的逆冲片至关重要。(2)喜马拉雅主要冲断带的压力-温度-时间演化特征；通过电子和离子探针对独居石颗粒的岩石学和年代学特征将与每种岩石的温度-时间演化、逆冲运动的屈服时间以及升温和冷却速率有关。使用新的更准确和更精确的痕量元素温度计来确定峰值压力-温度条件、分布和路径。(3)喜马拉雅-乌坦兰恰尔的运动学历史与尼泊尔相比如何？这项研究的主要目的是解决现代构造学中一个基本且有争议的问题：是用临界锥度模型更好地解释造山过程中地壳的整体行为，还是用最近的渠道流模型更好地解释这一问题。临界锥度理论认为，楔形变形岩块在刚性支撑体前面发育，导致向前传播的逆冲断层。通道流模型认为，山脉前缘的集中侵蚀迫使中下地壳弱通道发生变形，部分熔岩通向山前。该项目还支持研究生和本科生的研究教育，促进美国和印度境内机构之间更好的科学合作，并支持地球科学教授中代表性不足的群体。

The Himalaya and Tibet stand as the type example of a continent-continent collision, thus the Indo-Asian orogen serves as a paradigm for collisional orogenesis and a testing ground for tectonic models. One overriding tectonic issue is what controls the structural evolution of an orogen. Classic models of thrust development (the critical taper model) do not preclude weak portions of the crust but suggest that these do not work in concert to induce flow; in contrast recent thermal-mechanical models (channel flow models) treat the middle and lower crust as profoundly weak and in concert with erosion dominate metamorphic and structural development. This project, carried out by researchers from the University of Alabama and Boise State University in collaboration with scientists at the Wadia Institute of Himalayan Geology, aims to discriminate between these two opposing views by exploiting the radically different pressure-temperature-time kinematic histories predicted by these models. Specifically, the team will carry out integrated structural, petrologic, stratigraphic, and geochronologic investigations in the Himachal-Uttaranchal in northwestern India to address three basic questions regarding Himalayan thrust transport and tectonic evolution: (1) How do stratigraphic and structural packages correlate along strike from Nepal? Field mapping and structural analysis will permit stratigraphic correlations from Nepal to India and define thrust geometries and shortening amounts via balanced cross sections. Thin section, Nd isotope, and detrital zircon analysis will help define major lithotectonic units, which is critical to accurate placement of different thrust sheets. (2) What was the pressure-temperature-time evolution of the major Himalayan thrusts? Petrologic and chronologic characterization of monazite grains via electron and ion microprobes will be related to the temperature-time evolution of each rock, yielding timing of thrust movement and rates of heating and cooling. New more accurate and precise trace element thermometers are use to determine peak pressure-temperature conditions and distributions and paths. (3) How does the kinematic history in Himachal-Uttaranchal compare with Nepal? The main purpose of this research is to address a fundamental and controversial question in modern tectonics: Is the overall behavior of the crust during mountain building better explained by the critical taper model or the more recent channel flow model. The critical taper theory argues that a wedge-shaped mass of deformed rocks develops in front of a rigid backstop resulting in forward propagating thrust faults. The channel flow model argues that focused erosion at the mountain range front forces deformation in a weak channel in the mid- to lower crust and partially molten rocks tunnel toward the mountain front. This project also support the research education of graduate and undergraduate students, fosters improved scientific collaborations among institutions both within the United States and in India, and supports an underrepresented group among the geosciences professoriate.