Doctor of Philosophy (PhD)
Geology and Geophysics
This dissertation focuses on two extremes of orogenic development in the Himalaya: the timing of early ultra-high pressure related tectonics and the subsequent emplacement of the high grade Greater Himalayan Crystallines. The Himalayan orogeny is one of if not the best example of ongoing collisional systems, marked by the ongoing convergence of the Indian and Asian continents. The presence of coesite in the Tso Morari complex respresents subduction of the Indian continental crust to ultrahigh-pressure (UHP) conditions. However, the timing of UHP metamorphism is debated, creating an uncertainty in the calculation of subduction and exhumation rates. Petrologic and geochronologic analyses of eclogitic zircon and rutile from two samples—a kyanite bearing white mica schist and a garnet-biotite schist were conducted to constrain the timing and duration of the UHP metamorphic event. Titanite analyses from quartzofeldspathic gneiss constrain timing of exhumation and record the regional amphibolite-facies metamorphism. Petrology and U-Pb analyses and observations reveal peak metamorphic histories in rutile-bearing metapelites. Ages in rutile bearing samples are 50.3 ± 0.85 Ma and 47.60 ± 0.52 Ma. This geochronology provides insight into the suite of geochronometers already published in previous Tso Morari studies, but additional thermobarometry is needed to strongly correlate potential UHP ages with UHP conditions. How the Greater Himalayan Crystalline was emplaced; and how much shortening was accomplished since the initial collision are debated. Three models; the Channel Flow model, Tectonic Wedging and Wedge extrusion models all have specific predictions of the spatial setting of the Greater Himalayan Crystalline relative to overlying stratigraphy. Here we show that the emplacement of the Greater Himalayan Crystalline is consistent with the predictions of Tectonic Wedging through structural mapping and sampling. The shortening budget of the Himalaya experiences approximately a 2,000 km deficit between plate convergence and crustal shortening recorded in the geologic record. It has recently been proposed in the Greater Indian Basin model that this can be attributed by 2675 ± 700 km of North-South extension of the Greater Indian margin in the late Mesozoic. The extension resulted into creating a Tethyan subcontinent that collided with Asian around 50 Ma followed by India around 25 Ma. Our field mapping is inconsistent with the two collision sequence predicted by the Greater Indian Basin hypothesis. Additionally the amount of shortening predicted by the Greater Indian Basin hypothesis greatly exceeds the shortening recorded by our line-length balancing of the Tethyan stratigraphy.
Document Availability at the Time of Submission
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Donaldson, Dennis Girard, "Geologic Extremes of the NW Himalaya: Investigations of the Himalayan Ultra-high Pressure and Low Temperature Deformation Histories" (2016). LSU Doctoral Dissertations. 3295.