The South Tibetan Detachment System, Himalayan Orogen: Extension Contemporaneous With and Parallel to Shortening in a Collisional Mountain Belt
Burrell C. Burchfiel, Chen Zhiliang, Kip V. Hodges, Liu Yuping, Leigh H. Royden, Deng Changrong, Xu Jiene, 1992. "The South Tibetan Detachment System, Himalayan Orogen: Extension Contemporaneous With and Parallel to Shortening in a Collisional Mountain Belt", The South Tibetan Detachment System, Himalayan Orogen: Extension Contemporaneous With and Parallel to Shortening in a Collisional Mountain Belt, Burrell C. Burchfiel, Chen Zhiliang, Kip V. Hodges, Liu Yuping, Leigh H. Royden, Deng Changrong, Xujiene
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Six transects mapped across the boundary between the Tibetan sedimentary sequence and the underlying Greater Himalayan metamorphic sequence in southern Tibet demonstrate that a series of gently north-dipping normal faults, the South Tibetan detachment system, separates these two rocks sequences. Down-to-the-north movement on the detachments was Miocene to perhaps Pliocene in age and contemporaneous with structurally lower south-vergent thrusting within the Himalayan orogen to the south; thus, shortening and extension were contemporaneous and parallel at two different levels within the Himalayan and south Tibetan crust. Mapping during this study indicates that the South Tibetan detachment system continues for at least 700 km along strike in the physiographic Higher Himalaya, and regional relations suggest that the detachment system may traverse nearly the entire 2,000 km length of the Himalaya.
Rocks in the footwall of the detachment system contain mylonitic fabrics and show evidence of progressively more brittle deformation as the detachment system evolved. Footwall rocks are juxtaposed against weakly metamorphosed sedimentary rocks, some of which contain conodonts that yield coloration indices corresponding to temperatures no higher than 350°C. Petrologic data from some transects suggest that roughly 10 km of crust was eliminated by movement on the detachment system. At least 35 km of northward displacement is demonstrable along the profile at Qomolangma (Everest).
The hanging wall of the detachment system contains mostly north-dipping normal faults, many of which are thought to sole into the detachment at depth. In the western-most profile, a large, north-vergent synformal anticline has the geometry of a typical retrocharriage structure and the fold is interpreted to be part of a broad zone of down-to-the-north normal shear. The two eastern profiles indicate at least two periods of movement on the South Tibetan detachment system, and the easternmost profile contains a north-dipping normal fault that cuts the detachment system.
North of the South Tibetan detachment system, normal faults extend for about 100 km, normal fault-bounded Neogene basins are present, and at least one Miocene metamorphic core complex is recognized. Thus, the Higher Himalaya and southernmost Tibet contain late Cenozoic structures typical of areas of regional extension, although they were formed in an area of regional shortening. We interpret these relations as the result of topographic collapse of the southern edge of Tibet, driven by gravity acting on the high-standing plateau and triggered by melting and leucogranite production within the midcrust. At the same time as extension was occurring at higher crustal levels, convergence and crustal shortening were occurring at lower crustal levels. Between the South Tibetan detachment fault and the main subduction zone a wedge of midcrustal material, bounded by a thrust fault at its base and a normal fault at its top, was extruded relatively southward out from beneath the topographically high Miocene Tibetan plateau. Extension and southward extrusion of this midcrustal wedge appear to be reflected in the modern topography of southern Tibet.