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Gourisanka Ghatak, Presidency College, Kolkata (India)
Na Na, Presidency College, Kolkata (India)
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To understand the origin of the apparent inversion of geotherm in several collision belts---the phenomenon deemed to be a paradox---it has been the general approach to consider the possible heat source(s) responsible for the observed spatio-temporal distribution of PT condition and then to construct models to simulate the observed picture. Downward decrease of peak-metamorphic temperatures in structural sections, as manifest in inverted isograds, cannot result only from the 'iron effect' (a hotter hanging wall thrust over a cooler footwall), even combined with shear heating. Nor does post-metamorphic folding of isograds satisfactorily explain the phenomenon. Besides, the reliability of geothermo-barometric data in regard to applicability in this problem has also been questioned. Geobarometric breaks across post-metamorphic thrust faults correspond to structural throw only if metamorphic conditions preserved in the rocks near the fault are the PT conditions of these rocks just prior to fault movement. But such cannot come to happen, as shown by numerical models, except when rapid uplift follows thrusting.
In the instance of the Himalayan orogen, syn-convergence extension is an additional, connected paradox. Over greater lengths of this collision regime, the major thrust faults, namely, the Main Frontal Thrust (MFT), the Main Boundary Thrust (MBT), and the Main Central Thrust (MCT), are inferred to sole at depth into a single major shear zone, the Main Himalayan Thrust (MHT), along which the Indian crust is believed to be subducting beneath the Central Asian crust. The MCT is a major ductile thrust zone that is associated with inverted isograds both above (in the Higher Himalayan Crystalline, HHC) and below (in the Lesser Himalay, LH). In the HHC, the inverted metamorphic sequence is characterised by a general superposition of garnet, staurolite, kyanite, sillimanite+muscovite and sillimanite+k-feldspar isograds from the base to the top of the unit (peak temperatures increasing upwards from 350 C to 550 C). Studies show that coeval movements along both the MCT and the South Tibetan Detachment System (STDS) were related to a tectonically controlled exhumation of high grade rocks. Kinematic flow analyses of the rocks demonstrate coexistence of both simple shear and pure shear during ductile deformation. Simultaneous southward shearing within the HHC near and above the MCT and normal faulting across the STDS may be explained by a tectonically induced extrusion of a ductily deforming wedge.
Two empirical interpretations are current at present: 1. inverted metamorphic sequence is the result of rotation of isograds due to simple shear non-coaxial flow, and this inversion is real; 2. inverted sequence is due to deformation and stretching of earlier formed peak-metamorphic isograds corresponding to a normal thermal gradient, the inversion is apparent. Wide ranging studies of mineral parageneses and microstructures lend support to the latter view.
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