Miocene exhumation of northeast Pamir: Deformation and geo/thermochronological evidence from western Muztaghata shear zone and Kuke ductile shear zone

Abstract

The West Muztaghata Shear Zone (WMSZ) and Kuke ductile shear zone (KS) are located at the western and eastern margins, respectively, of the Muztaghata gneiss dome (MD) in the northeast Pamir. We report new geological data, including quartz fabric analyses, zircon U-Pb ages, and mica 40Ar-39Ar ages, to constrain the geometry, kinematics and evolution time of the WMSZ and KS. The presence of garnet and kyanite in the mylonites from the WMSZ indicate that the shear zone roots into the lower crust. The deformation characteristics of mylonites of the WMSZ show top-to-W or SW shear sense, and formed at mid-high temperatures. Zircons from high-grade mylonites and synkinematic leucogranites record a consistent U-Pb ages of ∼20 Ma, interpreted to reflect high-grade metamorphism and partial melting in the northeast Pamir in the early Miocene prior to ductile shearing along the WMSZ. The adjacent undeformed 12 Ma Kuzigan pluton, indicates deformation along the WMSZ ceased by then. 40Ar-39Ar dating results for mica show that the WMSZ underwent continued exhumation to the middle-upper crust until ∼12–8 Ma. To the east, the KS has E-dipping foliations with a top-to-E normal shear sense and preserves low to medium deformation temperature fabrics. Previous data and new biotite dating from the KS yield 40Ar-39Ar ages of ∼12–8 Ma, indicating broadly synchronous activity along the KS in the middle Miocene. Integrated with previous studies, our results suggest that continuous India-Asia convergence led to thickening of the Pamir crust, which propagated from south to north, initiating in the South Pamir in the late Eocene and advanced through the North Pamir around 20 Ma. This was immediately followed by rapid exhumation during east-west stretching and dome formation, broadly coeval but slightly outlasting exhumation of the other Central Pamir gneiss domes.