Cenozoic evolution of the eastern Pamir: Implications for strain-accommodation mechanisms at the western end of the Himalayan-Tibetan orogen

Detailed field mapping, geochronologic and thermochronologic analyses, and petrologic investigations conducted along the southern segment of the late Cenozoic Kongur Shan extensional system provide new information on the Cenozoic tectonic evolution of the eastern Pamir at the western end of the Himalayan-Tibetan orogen. Field relations and cooling-age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of eastwest extension along the southern Kongur Shan extensional system, from 20 km or less along the Muztaghata massif in the north, to < 3 km along the Tashkorgan fault in the south. These results, in conjunction with previously published work on the northern segment of the Kongur Shan extensional system, show a general southward decrease in east-west extension along the entire length of the extensional system, consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the Pamir. Petrologic data, 40Ar/39Ar cooling ages, and monazite Th-Pb ages from schists and gneisses in the footwall of the southern Kongur Shan normal fault along the Muztaghata massif record two tectonic events that immediately preceded late Miocene initiation of east-west extension: (1) high-grade schists and gneisses experienced upper amphibolite facies metamorphic conditions (9-10 kbar, 700-750 degrees C) dated as late Oligocene to middle Miocene by in situ ion-microprobe analyses of monazite inclusions in garnet; and (2) high-grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late Miocene with 40Ar/39Ar biotite cooling ages of 7.5-9 Ma. Rapid exhumation was accommodated in part by the east-west-striking, south-dipping, Shen-ti normal fault. Field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the Central Pamir gneiss domes, and the antiformal Muztaghata massif is the eastward continuation of the Sares dome of the Central Pamir. These observations suggest that the antiformal gneiss domes of the Central Pamir have not been offset across the Karakorum right-slip fault from the Qiangtang anticlinorium in Tibet. Instead, we propose that the development of the Central Pamir gneiss domes may have been related to Oligocene-Miocene northward underthrusting and thickening of crust beneath the Pamir.