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部，最高级别的变质岩出露在高喜马拉雅板块中，高喜马拉雅板块是一种20-30公里厚的向北倾斜的深部地壳岩石楔体，变质深度为14-56公里，变质时间为35-15 Ma。该板块沿底部以南向中央逆冲(MCT)为界，沿顶部以南向正断层(STDS)为界，该正断层将喜马拉雅变质核和深熔核与青藏高原的未变质岩石隔开。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形的上部相对应。围绕着板块的逆冲断层和非NAL断层的区域尺度几何形状现在已经相当清楚，并且已经做了大量工作来记录沿着板块上下表面的剪切传感指示器，并约束板块内岩石的早期PTT路径。然而，在我们对流动的运动学(涡度)以及流动与挤压楔体内的渐进折返之间的关系的理解方面，仍然存在着至关重要的差距。弄清沿楔形边界断裂的位移时间和大小之间的时空关系，以及演化楔形构造内水流的运动学，对于理解造山带的地壳增厚、折返和剥蚀历史至关重要。例如，在最近发表的挤压模型中，板的内部是以纯剪切变形为主并以拉张断层为边界，还是像其他模型所说的那样是以简单剪切为主的流经整个板的流动。纯剪切分量的识别是至关重要的，因为显著的纯剪切分量的运行将导致：1)板材本身变薄和倾角平行延伸，2)相对于严格的简单剪切，应变速率和膨胀/折返速率都增加。挤压模型的测试需要绘制出整个板材的运动学(涡度)域的空间和时间分布，还需要运动学和PTT分析之间的紧密结合，以限制渐进变形和折返路径。只有一种已发表的定量涡度分析沿着高喜马拉雅板块的底部进行，而对于板块的中上部还没有这样的研究。中国地质局建议沿着一条横穿该板块的N-S导线，对高喜马拉雅板块的运动学演化和折返历史进行综合研究，并选择了珠穆朗玛峰地区进行这一研究。在珠穆朗玛峰地区，横跨板块的N-S样带长约60-80公里，考虑到研究所需的详细野外工作，在美国国家科学基金会资助的标准项目的2-3年内完成整个横跨板块的样带是不可能的。因此，PI建议将样带分成两个独立出资的阶段，从这项建议开始，在样带的北端，岩石位于暴露在山体北侧和东侧的荣布山谷和康胜山谷的性传播疾病的直接底部。分别是珠穆朗玛峰。对于该项目的涡度部分，他将采用一系列不同的分析技术，使他能够在结果之间进行交叉检查。用三种不同的分析方法对荣北地区进行的调查研究表明，平均运动涡度数(Wm)在0.73-0.98之间。这些数据表明，尽管通常以简单剪切为主，特别是在与STD相邻的样品中，但在滑脱下方400-600的样品中也存在主要的纯剪切成分(wk=0.75时，纯剪切和简单剪切对流动的贡献相等)。然而，紧密间隔的采样点对于确定运动涡度数中可能存在的潜在阶跃函数是必不可少的，这些阶跃函数可能存在于STD之下的深度中。该区域几乎连续的暴露非常适合这项工作，并将允许PI采样到3-4000英寸深的性传播疾病。涡度的时间变化将通过显微构造和岩组标准与变形温度相关联，这些标准将反过来与通过当时的气压测量和准分子激光40Ar/39Ar微探针分析确定的折返路径相关联。

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.