Mid-Miocene Rifting of the Karakul Graben, NE Tajikistan
Lake Karakul sits in a rift basin on the northern Pamir plateau, ~50 km south of the Alai valley and ~160 km northwest of the Kongur Shan extensional system. To evaluate the timing of rifting four ~1500 m vertical transects were collected on the flanks of the graben for apatite U-Th/He dating. Two transects from the northeast (E76.76 N39.1) and northwest (E73.25 N39.1) flanks of the graben suggest that the two lobes of Lake Karakul fill two N-S oriented grabens, separated by a down-dropped horst comprising islands in the lake. Cooling ages on the northern flanks of the graben range between 21 and 15 Ma, and suggest a mid-Miocene exhumation event. Two additional transects collected from the southern part of the lake (E73.39 N38.85; E73.56 N38.83) show older closure ages (60-30 Ma) consistent with the lesser magnitude of extension observed to the south. If this interpretation is correct then rifting at Karakul was earlier than the onset of the Kongur Shan extensional system (7-9 Ma), but consistent with the onset of rifting in central Tibet at >13.5 Ma (Robinson et al. 2004; Blisniuk et al. 2001). These constraints suggest that Tibet may have been pulling on the eastern Pamir plateau during the middle Miocene before the intervening Kongur Shan system began to absorb large amounts of extensional strain, thereby unzippering and kinematically isolating the Pamir plateau from the rest of Tibet.