Internal Flow, Extrusion and Exhumation History of the Greater Himalayan Slab

大喜马拉雅板片的内部流动、挤压和折返历史

• 批准号: 0711207
• 负责人: Richard Law
• 金额: $ 33.2万
• 依托单位: Virginia Polytechnic Institute and State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-08-15 至 2012-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0711207&HistoricalAwards=false
• 关键词: Internal; Flow; Extrusion; Exhumation; History

The Greater Himalayan Slab is a 5-30 km thick northward-dipping tectonic unit of mid-crustal rocks that forms the crystalline core of the Himalaya and is bounded along its base by the Main Central Thrust Zone and along the top by the South Tibetan Detachment System of normal faults. Assuming simultaneous or overlapping movement along these crustal-scale bounding shear zones, the Greater Himalayan Slab is often modeled as a north-dipping channel or wedge/slab of mid-crustal rocks that, beginning in Early Miocene times, was extruded southward from beneath the Tibetan plateau. Identification of a pure shear component is critically important because a significant pure shear component will itself act as a driver for crustal extrusion and exhumation, resulting in: 1) thinning and dip-parallel extension of the slab itself, 2) relative to strict simple shear, an increase in both strain rates and extrusion/exhumation rates of the Himalayan crystalline core. Testing of channel flow/extrusion models requires that spatial and temporal distributions of vorticity domains be mapped out across the slab, and ultimately also requires a close integration between kinematic and pressure-temperature-time analyses in order to constrain progressive deformation and exhumation paths. Results from previous work the Mt. Everest region (along the top of the Greater Himalayan Slab) suggests that, in contrast to predictions from channel flow and extrusion models, the highest pure shear components are located towards the base of the slab, possibly indicating the importance of lithostatic loading. This project will complete transport-parallel sampling traverse across the Greater Himalayan Slab in the Everest region and undertake a series of similar transport-parallel traverses in key areas along the length of the Himalaya in order to make a first order assessment of: a) along-strike spatial and temporal variations in flow and, b) how these variations in flow may be related to along-strike changes in the structural evolution and exhumation history of the Greater Himalayan Slab. In each traverse suites of oriented samples will be collected for laboratory-based vorticity analyses using all appropriate microstructural and crystal fabric/strain techniques. Data provided by these different analytical techniques will be linked to deformation temperatures indicated by associated microstructures and crystal fabrics, hence enabling changes in vorticity of flow to be tracked during progressive exhumation/cooling.The Himalaya is frequently cited as the classic example of a mountain chain produced by continent-continent collision, with the chain progressively evolving as sheets of rock are stacked up on top of one another during continued collision between India and Asia. It has been proposed that rocks forming the metamorphic core of the Himalaya, originally located beneath the Tibetan Plateau, were being squeezed and extruded southwards as a slab-shaped body towards the Earth's surface, driving upwards the crest of the Himalaya, and that this movement is driven by horizontal gradients in vertical load. The greater the degree of vertical squeezing and shortening in this flowing channel, the greater the amount of material extruded towards the surface. If surface erosion cannot keep pace with extrusion, then the greater the amount of extrusion the greater the amount of surface uplift, hence explaining why the highest Himalayan peaks always coincide with the outcrop position of this slab of mid-crustal rocks. If the material is deforming by simple shear, a mechanism similar to shuffling a pack of playing cards, then the rocks won't lengthen parallel to the shearing motion, just as a playing card doesn't lengthen when the deck is shuffled. So, in simple shear, rocks from the middle crust won't move very far towards the surface. However, if material is both sheared and vertically shortened (pure shear) then the rocks will lengthen parallel to the shearing motion and move towards the surface. This research project aims to determine if the pure shear driven extrusion processes have operated along the length of the Himalaya, or if they are unique to the currently highest part of the mountain chain.

纯剪切分量的识别是至关重要的，因为一个重要的纯剪切分量本身将作为地壳挤压和挖掘的驱动因素，导致：1)板本身变薄和倾斜平行延伸，2)相对于严格的简单剪切，喜马拉雅结晶岩心的应变速率和挤压/挖掘速率都增加。通道流动/挤压模型的测试需要绘制出整个平板上涡度域的空间和时间分布，最终还需要运动学和压力-温度-时间分析之间的紧密结合，以约束渐进变形和挖掘路径。先前对珠穆朗玛峰地区（沿着大喜马拉雅板块顶部）的研究结果表明，与河道流动和挤压模型的预测相反，最高的纯剪切分量位于板块的底部，这可能表明静岩载荷的重要性。该项目将完成珠穆朗玛峰地区大喜马拉雅板块的运输平行采样穿越，并在沿喜马拉雅长度的关键区域进行一系列类似的运输平行穿越，以便对以下方面进行一级评估：a)沿走向的流动空间和时间变化，b)这些流动变化如何与大喜马拉雅板块的构造演化和挖掘历史的沿走向变化相关。在每个遍历过程中，将收集定向样品进行实验室涡度分析，使用所有适当的显微结构和晶体织物/应变技术。这些不同的分析技术提供的数据将与相关的微观结构和晶体结构所指示的变形温度联系起来，因此能够在逐步挖掘/冷却过程中跟踪流动涡度的变化。喜马拉雅山脉经常被认为是大陆与大陆碰撞形成的山脉链的经典例子，在印度和亚洲之间持续碰撞的过程中，随着岩石的层层堆积，山脉链逐渐演变。在这个流动通道中垂直挤压和缩短的程度越大，向表面挤压的材料量就越大。如果地表侵蚀不能跟上挤压的速度，那么挤压的程度越大，地表隆起的程度就越大，这就解释了为什么喜马拉雅山脉的最高峰总是与这块中地壳岩石板的露头位置一致。如果材料是通过简单的剪切变形，一种类似于洗牌的机制，那么岩石就不会平行于剪切运动而变长，就像洗牌时纸牌不会变长一样。所以，在简单剪切中，来自地壳中部的岩石不会向地表移动很远。然而，如果材料既被剪切又被垂直缩短（纯剪切），那么岩石将平行于剪切运动而变长，并向表面移动。这个研究项目的目的是确定纯剪切驱动的挤压过程是否沿着喜马拉雅山脉的长度运行，或者它们是否是目前山脉的最高部分所独有的。