Alia Bigio

The Greater Himalayan Sequence (GHS) of the Bhutan Himalaya is a complex, highly deformed unit of gneisses and migmatites intruded by leucogranites. Several shear zones play a major role in the deformation of the GHS. From south to north, these are the Main Central Thrust (MCT), the out-of-sequence Kakhtang thrust, and the South Tibetan Detachment (STD). The metamorphic grade in the GHS begins at upper greenschist in the south, reaching upper amphibolite-granulite in the north. This is an inversion of the normal metamorphic sequence. In the uppermost part of the GHS, the metamorphic sequence reverts to right-way-up as the metamorphic grade decreases rapidly across the STD.

This study provides a structural analysis of contacts between GHS rocks and low grade sedimentary rocks in central Bhutan. Field observations indicate that the metasediments are soled by a ductile shear zone. Because the shear zone at the bottom of the sediments contains top-to-the-north shear sense indicators, the sedimentary units are interpreted as klippen. Accordingly, it is proposed that the STD in the Bhutan Himalaya extended further to the south than its current trace at the border to Tibet. Previous studies indicate that the north-directed shearing along the STD was preceded by a south-directed shearing. These observations have major implications in interpretation of tectonics of the Himalaya during the Miocene, when the STD was active as a ductile shear zone and leucogranites were emplaced and deformed along the STD. This new kinematic data helps to explain the inverted metamorphic sequence of the GHS and the complex relationship between the STD, GHS, and MCT.

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Supervisor: Djordje Grujic