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Pressure-temperature-structural distance relationships within the Greater Himalayan zone, eastern Himalaya, eastern Bhutan
AuthorAgustsson, Kenjo S.
AdvisorGordon, Stacia M.
Geological Sciences and Engineering
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Studies of exhumed mid- to lower-crustal rocks within continental collisional orogens provide insight into the evolution of active mountain belts. In the Himalayan orogen, the Greater Himalayan zone (GHZ) was buried to mid- to lower-crustal depths, but is now exposed throughout the orogenic belt, positioned between two opposite-sense shear zones: the top-to-the-north South Tibetan Detachment above and the top-to-the-south Main Central Thrust (MCT) below. The GHZ consists of predominately amphibolite-facies orthogneiss and metasedimentary rocks, as well as large-scale (>100 km2) leucogranite bodies. Within the eastern quarter of the orogen in the kingdom of Bhutan, the GHZ is divided into structurally-lower (GHZl) and upper (GHZu) levels by the Kakhtang thrust (KT), which has been defined by the upsection crossing of the 2nd sillimanite isograd and a significant increase in the volume of crystallized melt. In central and eastern Bhutan, previous studies presenting quantitative estimates of peak P-T conditions have focused on the GHZl, whereas the P-T and exhumation history of the GHZu is by comparison not well studied. In order to better characterize the P-T conditions and the significance of major structures, including the MCT and KT, in the exhumation of the GHZ, pressure-temperature-structural distance relationships within two transects, one in north-central Bhutan and one in northeastern Bhutan, are compared against predicted relationships for three competing end-member models for the dominant exhumation mechanism of the GHZ: emplacement as a ductile channel, or as a ductilely-deformed thrust sheet or multiple thrust sheets under a system governed by critical taper dynamics. Pressure-temperature estimates determined via THERMOCALC, and supplemented by titanium-in-biotite temperature estimates, show different pressure-temperature-structural distance trends across each transect. A gradual decrease in temperatures structurally-upsection (740 to 650 °C) in the eastern transect is observed for the structurally lowest four samples, and the peak pressures attained decrease from ca. 10 to 5 kbar. Peak temperatures in the western transect range from 750 to 600 °C and show a similar decrease structurally-upsection for the lowest five samples. However, rocks from the western transect record a highly-compressed upright pressure gradient, with a 4 kbar decrease in pressure over a 5.5 km structural distance crossing the KT, corresponding to a super-lithostatic pressure gradient (0.74 ± 0.16 kbar/km). The results do not show a significant change in P-T conditions moving across the previously-mapped location of the KT, and the presence of foliation-parallel muscovite in equilibrium with the peak metamorphic assemblage in samples above and below the KT on both transects argues against the thrust being defined by the crossing of the 2nd-sillimanite isograd. The data from both transects combined argue that the KT is not a 1st-order, intra-GHZ shear zone. Furthermore, the super-lithostatic pressure gradients observed in central and eastern Bhutan favor large-scale flattening of the GHZ during emplacement and exhumation as a hinterland, ductile thrust sheet.