Abstract

North-dipping, gravitationally driven extensional structures that unroof the high-grade core of the Himalayas have been recognized in several parts of the orogen. Recent work in west-central Nepal demonstrates that the extensional history at the top of the Greater Himalayan metamorphic sequence in this region is laterally variable. In the upper Marsyandi valley there is evidence for three generations of early to middle Miocene west-directed (orogen-parallel) extensional structures. The study area consists of two tectonostratigraphic sequences. The lower sequence consists of a 12-km-thick section of high-grade metasedimentary rocks of the Greater Himalayan sequence. The higher sequence consists of a 4-km-thick section of medium-grade marble of the Tibetan sedimentary sequence; it overlies the Greater Himalayan sequence in the Marsyandi valley along a northwest-dipping ductile shear zone (the Chame detachment). Mylonitic fabrics in footwall rocks show evidence of west-directed displacement at sillimanite-grade through greenschist-grade conditions. A cataclastic zone indicates progressively more brittle deformation at the eastern end of the outcrop trace of the Chame detachment. In the Dudh Khola valley, 15 km west of the Marsyandi valley, the Tibetan sedimentary sequence is juxtaposed with the Greater Himalayan sequence by an early Miocene north-dipping ductile shear zone (the Dudh Khola detachment) that has well-developed, north-trending stretching lineations formed during sillimanite-grade metamorphism. North-directed (orogen perpendicular) extension in the Dudh Khola region is interpreted as coeval with west-directed (orogen-parallel) extension in the Marsyandi region. Along-strike variations in the orientation of extensional strain at the top of the Greater Himalayan sequence may be explained by lateral crustal thickness gradients.

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