OLIGOCENE-MIDDLE MIOCENE CRUSTAL THICKENING AND LATE MIOCENE EXHUMATION ALONG THE EASTERN PAMIR, CHINA

Field mapping, thermochronologic, geochronologic, and petrologic studies along the southern portion of the Kongur Shan extensional system in the eastern Pamir of China have documented a complex deformational history from the Oligocene to Miocene prior to initiation of east-west extension along the active Kongur Shan extensional system. Hanging wall and footwall schists and gneisses of the southernmost portion of the Kongur Shan normal fault preserve upper amphibolite facies metamorphic assemblages, with biotite-garnet-kyanite-K-feldspar schists recording peak conditions of 9-10 kbar, 700-750°C. In-situ SIMS geochronology of monazite inclusions in garnet yield a spread in ages with most analyses falling between 13-35 Ma, the younger ages overlapping with U-Pb zircon ages of 12-18 Ma from a large hanging wall granite pluton ~10 km to the west along the China-Tajikistan boarder. These results are interpreted to indicate initiation of tectonic burial and crustal thickening along the eastern margin of the Pamir in the Early Oligocene (related to initiation of motion along the Main Pamir Thrust?) with peak metamorphic conditions occurring in the Early to Middle Miocene. High grade metamorphism may have also resulted in partial anatexsis of the schists and gneisses, generating the Miocene granites to the west. Peak metamorphism was followed by rapid exhumation, recorded by biotite 40Ar/39Ar cooling ages of 8-9 Ma, along a mid-crustal ductile shear zone. Kinematic indicators along the presently south dipping shear zone (rotated porphoroblasts, asymmetric boudins, and S-C fabrics) show top to the south sense of shear, orthogonal to the active east-west extension along the Kongur Shan extensional system. Offset in biotite cooling ages of <1 Ma across the active range bounding normal fault show exhumation due to east-west extension along the southern portion of the fault system is minimal. These results point to dramatic shifts in kinematics along the eastern Pamir, from Oligocene-Middle Miocene crustal thickening, to Late Miocene north-south ductile shear and significant crustal thinning, to Pliocene-Recent east-west extension. The cause of the Late Miocene ductile shear zone is uncertain but may be related to growth of the Muztaghata gneiss dome and the right-slip Ghez ductile shear zone further to the north.