How Does the Mid-crust Accommodate Deformation in Large, Hot Collisional Orogens? Insight from the Himalaya-Tibet System

热的、弱的地壳是关于大陆-大陆碰撞演化的新假设的一个中心组成部分，特别是可能在过去5000万年中容纳喜马拉雅-西藏碰撞内超过3000公里的辐合中起重要作用。暗示半粘性地壳中岩石向南流向喜马拉雅造山带前缘的模型，即“河道流”，能够解释喜马拉雅地区的许多地质观测。然而，另一种碰撞模型，特别是“推楔锥”模型，表明所观察到的地质情况可能是在没有低粘度地壳中层的情况下形成的。通道流和推楔锥模型是否相互排斥，或者它们是否代表了在单一碰撞系统中时间和/或空间上的连续变形，这对于解释喜马拉雅造山带的演化，进而理解地球的演化至关重要。美国有许多大陆与大陆的碰撞区。这些对比模式的关键区别在于喜马拉雅前陆逆冲断层的位置、性质和相互关系；具体地说，主要的中央逆冲构造和拉玛尔格逆冲构造的空间和运动学关联。在喜马拉雅的河道流动模型中，被挖掘出来的地壳中岩石被预测为一个普遍变形的构造包，它的底部被一条主要断层所包围；拉姆加尔和主中央逆冲构造是该弥散构造的合成和同步组成部分。逆冲楔锥模型解释了主中央逆冲构造和拉姆加尔逆冲构造在运动学上是不同的，它们是在前陆扩展褶皱和逆冲带内顺序形成的。这些预测是通过对喜马拉雅中部到东部三个关键地点的样品进行分析来验证的，方法是结合结构分析、铀-钍-铅辅助相岩石年代学与热气压测定法、碎屑热年代学和块岩钕同位素地球化学。这些数据提供了关于主要中央逆冲和拉姆加尔逆冲的时间、中地壳变形和变质的时间以及岩石的来源特征的重要信息，这些特征为它们的前喜马拉雅起源提供了线索。这些数据还有助于深入了解在核心内记录的深地壳腹地型变形与喜马拉雅前缘附近记录的浅前陆型变形之间过渡的时间和空间演变。该项目支持研究生和本科生，为他们提供实地调查方法和各种分析设施的实践研究经验，特别是利用加州大学圣巴巴拉分校最先进的高分辨率和高精度电子和离子束仪器。该项目还支持两名早期职业研究人员的研究工作，并有助于扩大代表性不足的群体对地球科学的参与。该项目是一项国际科学合作的一部分，汇集了来自加州大学圣巴巴拉分校、加拿大、尼泊尔和印度的研究人员和学生。

Hot, weak crust is a central component of new hypotheses about the evolution of continent-continent collisions, and in particular may play an important role in accommodating the greater than 3000 kilometers of convergence within the Himalaya-Tibetan collision over the last 50 million years. Models that implicate flow of semi-viscous midcrustal rocks south towards the front of the Himalayan orogen, 'channel flow', are able to account for many geologic observations in the Himalaya. However, alternative models of collision, particularly 'thrust-wedge taper', demonstrate that the observed geology could have formed in the absence of a low-viscosity midcrustal layer. Whether channel flow and thrust-wedge taper models are mutually exclusive or whether they represent a continuum of deformation in time and/or space within a single collisional system is of crucial importance for explaining the evolution of the Himalayan orogen and, by extension, for understanding the evolution of Earth?s many continent-continent collision zones. A key difference in these contrasting models is the location, nature and relationships of thrust faults within the Himalayan foreland; specifically, the spatial and kinematic association of the Main Central Thrust and the Ramargh Thrust. In the channel flow model of the Himalaya, exhumed midcrustal rocks are predicted to act as a pervasively-deformed tectonic package bounded at its base by a single major fault; the Ramgarh and Main Central thrust are synthetic and coeval components of that diffuse structure. In contrast, thrust-wedge taper models interpret the Main Central thrust and Ramgarh thrust to be kinematically distinct, and formed in sequence within a foreland propagating fold and thrust belt. These predictions are being tested by analyzing samples at three key sites across the central to eastern Himalaya via a combination of structural analysis, uranium-thorium-lead accessory phase petrochronology coupled to thermobarometry, detrital thermochronology and bulk rock neodymium isotope geochemistry. These data provide essential information about the timing of the Main Central Thrust and the Ramgarh Thrust, the timing of deformation and metamorphism in the midcrust, and source characteristics of the rocks that give clues as to their pre-Himalaya origins. These data also yield insight into the evolution in time and space of the transition between deep crustal hinterland-style deformation recorded within the core and the shallow foreland-style deformation recorded near the front of the Himalaya. This project supports both graduate and undergraduate students, providing them with hands-on research experience in both field investigation methods and a variety of analytical facilities, particularly taking advantage of state-of-the-art high-resolution and high-precision electron- and ion-beam instrumentation at the University of California, Santa Barbara. The project is also supporting the research efforts of two early career researchers, and is contributing to the broadening of participation of underrepresented groups in the earth sciences. This project is part of an international scientific collaboration, bringing together researchers and students from the University of California, Santa Barbara, Canada, Nepal and India.