KARAKORAM FAULT ZONE ROCKS COOL IN 2 PHASES

The Karakoram fault zone of Ladakh, India, is a major strike-slip boun dary along which the tectonic evolution of Tibet has been accommodated . In this paper Ar-40/Ar-39 isotopic age data are integrated with stru ctural and metamorphic data to infer an exhumation history for granite s and low- to intermediate-grade metamorphic rocks exposed adjacent to two strands of the fault zone. Near Tangtse, leucogranites which crys tallized c. 17 Ma ago are cut by the Karakoram fault zone, indicating that fault movement was initialed subsequent to 17 Ma. The Ar-40/Ar-39 data give temperature-lime histories which indicate that the <17 Ma m etamorphic rocks within the fault zone were exhumed differentially rel ative to the adjacent (and already cooled to <150 degrees C) Ladakh Ba tholith, while cooling progressively through amphibolite to below gree nschist facies temperatures. Two phases of rapid cooling of Karakoram fault zone rocks are evident, one at c. 17 Ma to c. 13 Ma, and another , following an intervening period of relatively slow cooling: starting at c. 8 Ma and continuing until at least 7 Ma. Uplift since c. 17 Ma via dextral oblique thrusting has resulted in exhumation from amphibol ite facies metamorphic conditions, possibly with a larger thrust compo nent from 17 to 13 Ma, followed by a change to dominant strike-slip mo tion at about 13 Ma. Rapid cooling starting at c. 8 Ma at greenschist facies temperatures is probably the result of renewed oblique thrustin g. It is notable that the two episodes of rapid cooling recorded in Ka rakoram fault zone rocks coincide with episodes of exhumation in the P akistani Karakoram and also in southern Tibet.