Kinematic Evolution and Exhumation History of the South Tibetan Detachment System, Everest Massif, Tibet

在喜马拉雅造山带的中东段，最高品位的变质岩暴露在高喜马拉雅板块中，这是一个20-30公里厚的北倾楔形深地壳岩石，在35-15 Ma的深度为14-56公里。该板块的底部受南向的中央主逆冲构造（MCT）的限制，顶部受北向的正断层的南藏拆离系统（STDS）的限制，这些正断层将喜马拉雅的变质和非变质岩心与青藏高原的未变质岩石分开。早中新世开始，高喜马拉雅板块的南向挤压作用对喜马拉雅地区的地质地貌演化产生了深远的影响，最高的地形和最深的侵蚀与挤压楔的上部相对应。断层的区域几何形状和非nal断层的边界现在已经相当清楚，并且已经做了大量的工作来记录沿断层的上下表面的剪切感指标，并限制了断层内岩石的早期PTt路径。然而，在我们对流动的运动学（涡度）和挤压楔内流动与渐进挖掘之间的关系的理解上，仍然存在至关重要的差距。确定沿楔边界断层位移的时间和大小与楔演化中的流动运动学之间的时空关系，对于理解地壳增厚、挖掘和造山带的侵蚀历史至关重要。例如，是否像最近发表的一个挤压模型所表明的那样，板的内部以纯剪切变形为主，并以拉伸断层为界，或者像其他模型所表明的那样，整个板的流动以简单剪切为主。纯剪切构件的识别是至关重要的，因为一个显著的纯剪切构件的操作将导致：1)薄板本身变薄和倾斜平行延伸，2)相对于严格的简单剪切，应变率和挤压/挖掘率都增加。挤压模型的测试需要在整个平板上绘制出运动学（涡度）域的空间和时间分布，还需要在运动学和PTt分析之间进行紧密集成，以约束渐进变形和挖掘路径。只有一篇已发表的关于高喜马拉雅板块基底部分涡度的定量分析，而对高喜马拉雅板块中上部分的涡度分析则没有。PI建议对高喜马拉雅板块沿N-S方向的运动演变和挖掘历史进行综合研究，并选择了珠穆朗玛峰地区进行这项研究。在珠穆朗玛峰地区，横贯平板的南北向样带长度约为60-80公里，考虑到研究所需的详细现场工作，在2-3年的标准nsf资助项目中，不可能完成横贯平板的整个样带。因此，PI建议将样带分成两个独立的资助阶段，从这个建议开始，在样带的北端，岩石位于直接下盘，分别暴露在珠穆朗玛峰北部和东部的Rongbuk和Kangshung山谷中。对于项目的涡度部分，他将采用一系列不同的分析技术，使他能够交叉检查结果。利用三种不同的分析技术，在绒北地区进行了侦察研究，表明平均运动涡量（Wm）在0.73-0.98之间。这些数据表明，虽然简单剪切分量通常占主导地位，特别是在靠近STDS的样品中，但在剥离下方400-600处的样品中也存在纯剪切分量（纯剪切和简单剪切在Wk=0.75时对流动的贡献相等）。然而，紧密间隔的采样点对于识别运动涡度数中的潜在阶跃函数是必不可少的，这些阶跃函数可能存在于性传播疾病下的深度。该区域几乎连续的暴露非常适合这项工作，并将允许PI在STDS下方3-4000英尺的深度进行采样。根据微观结构和岩石结构标准，涡度的时间变化将与变形温度相关联，而这些又将与当时的无气压测量和准分子激光4OAr/39Ar微探针分析确定的挖掘路径相关联。

Within the central-eastem sector of the Himalayan orogen the highest grade metamorphic rocks are exposed in the High Himalayan slab, a 20-30 km thick northward-dipping wedge of deep crustal rocks metamorphosed at 14-56 km depth at 35-15 Ma. The slab is bounded along the base by the south-vergent Main Central Thrust (MCT), and along the top by the South Tibetan Detachment System (STDS) of north-vergent normal faults which separate the metamorphic and anatectic core of the Himalaya from unmetamorphosed rocks of the Tibetan plateau. Beginning in Early Miocene time, southward extrusion of the High Himalayan slab has had a profound influence on the geologic and geomorphic evolution of the Himalaya, and the highest topography and deepest erosion corresponds with the upper part of the extruding wedge. The regional scale geometries of the thrust and non-nal faults bounding the slab are now reasonably well known, and much work has been done on documenting shear sense indicators along the upper and lower surfaces of the slab and constraining the early stage PTt paths of rocks within the slab. However, critically important gaps remain in our understanding of both the kinematics (vorticity) of flow and relationships between flow and progressive exhumation within the extruding wedge. Deten-nining the spatial and temporal relationships between timing and magnitude of displacement along the wedge-bounding faults and the kinematics of flow within the evolving wedge are crucial to understanding of the crustal thickening, exhumation, and erosional history of the orogen. For example, is the interior of the slab dominated by pure shear deformation and bounded by stretching faults as suggested in one recently published extrusion model, or is flow throughout the slab dominated by simple shear as suggested in other models. Identification of a pure shear component is critically important because operation of a significant pure shear component would result in: 1) thinning and dip-parallel extension of the slab itself, 2) relative to strict simple shear, an increase in both strain rates and extr-usion/exhumation rates. Testing of extrusion models requires that spatial and temporal distributions of kinematic (vorticity) domains be mapped out across the slab, and also requires a close integration between kinematic and PTt analyses in order to constrain progressive deformation and exhumation paths. Only one published quantitative vorticity analysis has been made along a basal section of the High Himalayan slab, and no such studies exist for the upper-middle sections of the slab. The PI proposes to undertake an integrated study of the kinematic evolution and exhumation history of the High Himalayan slab along a N-S traverse across the slab, and has chosen the Everest region for this study. In the Everest region the N-S transect across the slab is some 60-80 km in length and, given the detailed fieldwork required for the study, it would be impossible to complete the entire transect across the slab in the 2-3 years of a standard NSF-funded project. The PI therefore proposes to break the transect into two separately funded stages starting in this Proposal at the northern end of the transect with rocks lying in the immediate footwall to the STDS that are exposed in the Rongbuk and Kangshung valleys on the north and east sides of Mt. Everest respectively. For the vorticity part of the project he will employ a range of different analytical techniques allowing him to cross-check between results. The reconnaissance studies in the Rongbuk area, using three different analytical techniques, demonstrate that mean kinematic vorticity numbers (Wm) range between 0.73-0.98. These data indicate that although a simple shear component is generally dominant, particularly in samples adjacent to the STDS, there is also a major component of pure shear in samples located at 400-600 in beneath the detachment (pure and simple shear make equal contributions to flow at Wk=0.75). Closely spaced sampling sites are essential, however, for identifying potential step functions in the kinematic vorticity number that may exist with depth beneath the STDS. The almost continuous exposure in the region is ideally suited for this work, and will allow the PI to sample to depths of 3-4000 in beneath the STDS. Temporal variations in vorticity will be correlated with deformation temperatures using microstructural and petrofabric criteria, and these will in turn be linked to exhumation paths determined by then-nobarometry and excimer laser 4OAr/39Ar microprobe analyses.