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

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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.