TRANSIENT CHARACTER OF THE SOUTH TIBETAN DETACHMENT: MICROTECTONIC DOCUMENTATION FROM THE BHUTAN HIMALAYA

The Greater Himalayan Sequence (GHS) of the Bhutan Himalaya is a complex, highly deformed unit of gneisses and migmatites intruded by Miocene-age leucogranites. Several shear zones played 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 MCT is a major intracontinental shear zone with rocks of the Lesser Himalayan Sequence (LHS) in the footwall and the GHS in the hanging wall. The STD is a north-dipping north directed shear zone with Tethyan-sequence sedimentary rocks in the hanging wall. In the LHS and GHS the metamorphic sequence is inverted, increasing from structurally lower to structurally higher levels. Metamorphic isograds in the GHS begin at upper greenschist facies in the south and reach upper amphibolite-granulite facies in the north. In the uppermost part of the GHS, the metamorphic sequence reverts to normal. The metamorphic grade decreases rapidly across the STD into the base of the Tethyan sequence.

In the GHS of Bhutan, low-grade sedimentary rocks are found in cores of open upright synforms. Some of these structures are very near to the MCT, with less than a kilometre of GHS between them. Field observations indicate that a ductile shear zone soles the metasediments. Microtectonic analysis in this study provides the data used to determine the sense and type of shear within the shear zone. Because the shear zone at the bottom of the sediments contains top-to-the-north shear sense indicators, and because of their lithology, the sedimentary units are interpreted as klippen of the Tethyan sedimentary sequence. It is proposed that the STD in the Bhutan Himalaya extended further south than its modern trace and that the down-dip extent may exceed 240 kilometres. As the GHS became an active orogenic channel (Beaumont et al. 2001, Grujic et al. 2002), its southward flow caused opposite sense of shear at its boundaries: top-to-the-south thrusting at the MCT and top-to-the-north shearing at the STD. This and previous studies in southern Tibet indicate that south-directed shear preceded north-directed shear along the STD, and provide a new understanding of the geometry and kinematics of the STD, the inverted metamorphic sequence of the GHS, and the complex relationship between the STD and the MCT.