INTRA-CONTINENTAL SHORTENING ALONG THE ALAI VALLEY, PAMIR-TIEN SHAN, CENTRAL ASIA

The Alai Valley between the Pamir frontal range and the southern Tien Shan is the remnant of a Cenozoic sedimentary basin that linked the Tajik basin to the west and the Tarim basin in the east. The ‘Main Pamir Thrust’ corresponds to a major thrust splay of the intra-continental subduction zone along which Asian lithosphere is subducting southward under the Pamir along a seismically active zone. Late Cenozoic deformation is separated into several segments and the lateral transitions are accommodated by complex areas of transfer faults and thrust systems. Map interpretation and evaluation of structural data indicates up to 40° clockwise rotation of the shortening direction from NNW-SSE to NNE-SSW since the late Miocene. Distinct differences in the fault pattern between several segments of the northern Pamir and between the northern Pamir and the southern Tien Shan are effects of local block rotations. Active deformation of the Pamir frontal range is obvious from Recent surface breaks along the western and central segments; the disastrous Mw 6.7 Nura earthquake on October 5^th, 2008 testifies to renewed activity along the eastern segment. The southern Tien Shan shows active deformation in the eastern part that interacts with the dextral Talas-Ferghana fault system.

Estimates on the age of deformation are based on the tectonic interpretation of facies variations in shallow marine and continental sedimentary rocks. Distributed north-south contraction occurred during the late Oligocene–early Miocene with shortening resumed in the late Miocene. According to new Ar-Ar geochronology, the southern Tien Shan cooled through 300°C at ~275 Ma, following a major intrusion event at 305-280 Ma (U-Pb zircon), and later through ~100°C at ~10-15 Ma. Sandstone samples from Paleogene and Miocene strata from a intra-montane basin in the southern Tien Shan yielded detrital apatite and zircon fission-track data with age peaks at ~ 330, 250, 170, 145, and 75 Ma that are not reset during the Cenozoic. Detrital apatite and zircon from the Pamir frontal range are also unreset, yielding age peaks at ~240 Ma and ~140 Ma. The only Cenozoic ages from the thrust system of the Pamir frontal range stem from Lower Cretaceous sedimentary and Paleozoic volcanoclastic rocks and suggest that the major shortening is ≤5 Ma.