THE METAMORPHIC SIGNATURE OF CONTEMPORANEOUS EXTENSION AND SHORTENINGIN THE CENTRAL HIMALAYAN OROGEN - DATA FROM THE NYALAM TRANSECT, SOUTHERN TIBET

Geological relationships and geochronological data suggest that in Mio cene time the metamorphic core of the central Himalayan orogen was a w edge-shaped body bounded below by the N-dipping Main Central thrust sy stem and above the N-dipping South Tibetan detachment system. We infer that synchronous movement on these fault systems expelled the metamor phic core southward toward the Indian foreland, thereby moderating the extreme topographic gradient at the southern margin of the Tibetan Pl ateau. Reaction textures, thermobarometric data and thermodynamic mode lling of pelitic schists and gneisses from the Nyalam transect in sout hern Tibet (28-degrees-N, 86-degrees-E) imply that gravitational colla pse of the orogen produced a complex thermal structure in the metamorp hic core. Amphibolite facies metamorphism and anatexis at temperatures of greater-than-or-equal-to 950 K and depths of at least 30 km accomp anied the early stages of displacement on the Main Central thrust syst em. Our findings suggest that the late metamorphic history of these ro cks was characterized by high-T decompression associated with roughly 15 km of unroofing by movement on the South Tibetan detachment system. In the middle of the metamorphic core, roughly 7-8 km below the basal detachment of the South Tibetan system, the decompression was essenti ally isothermal. Near the base of the metamorphic core, roughly 4-6 km above the Main Central thrust, the decompression was accompanied by a bout 150 K of cooling. We attribute the disparity between the P-T path s of these two structural levels to cooling of the lower part of the m etamorphic core as a consequence of continued (and probably accelerate d) underthrusting of cooler rocks in the footwall of the Main Central thrust at the same time as movement on the South Tibetan detachment sy stem.