STRUCTURAL AND KINEMATIC EVOLUTION OF EASTERN BHUTAN: THE INTERPLAY BETWEEN THRUST FAULTING AND NORMAL FAULTING IN AN OROGENIC WEDGE

The High Himalayan Crystalline Belt (HHC) of eastern Bhutan preserves a near continuous section of high-grade gneisses, leucogranites, and Tethyan facies metasedimentary rocks that were deformed by different styles of faulting during the Late Tertiary collision between India and Asia. Early top-up-to-the-south ductile thrust faulting along the Main Central Thrust (MCT) placed garnet-kyanite-sillimanite-bearing paragneisses and leucogranites on top of less deformed garnet-staurolite-bearing rocks of the Late Precambrian Daling-Shumar Group near Tashigang. This deformation produced a penetrative north-dipping foliation and north-plunging mineral lineations within the high-grade rocks of the HHC. Top-up-to-the-south thrusting also occurred either simultaneously with or preceding development of a regional-scale ductile normal fault that accommodated top-down-to-the-north displacements structurally above the MCT. Together, the MCT and this normal fault form the lower and upper boundaries, respectively, of an orogenic wedge. Following development of this wedge, both the MCT and normal fault were folded by tight inclined folds that form a type 1 interference pattern (domes and basins) at the scale of the mountain range. These folds form three sets that plunge gently and moderately to the northwest, northeast, and east. We have identified domains where each of these sets are dominant and mutually affect one another. Following folding, a major out-of-sequence thrust fault (the Kakhtang thrust) was emplaced across the top of the structural pile. The Kakhtang thrust is not folded. Finally, top-down-to-the-north displacements along the South Tibetan Detachment Fault and top-up-to-the south displacements along the Main Boundary Thrust also post-date folding of the MCT and emplacement of the Kahkthang thrust. Our data suggest that periodic top-down-to-the-north normal faulting alternated in time with top-up-to-the-south thrust faulting and constriction-related folding during growth of an orogenic wedge in Bhutan. The styles of thrust faulting and normal faulting we observed are kinematically compatible with the southward extrusion of deep crustal rocks between slabs of colder crustal material composed of Tethyan facies cover rocks.