Oroclinal Bending and Slab-Break-Off Causing Coeval East-West Extension and East-West Contraction in the Pamir-Nanga Parbat Syntaxis in the Past 10 m.y.

Active deformation of the Pamir-Nanga Parbat syntaxis system is dominated by east-west contraction in the south and east-west extension in the north on map view and a narrow zone of deep earthquakes up to 200 km beneath central Pamir and the Hindu Kush region on cross-sectional view. The deep seismicity shows a prominent gap between 15 and 70 km. E-W contractional tectonics is exemplified by the development of the north-trending Nanga Parbat synform and north-striking thrusts, while E-W extension is expressed by the Karakul rift in the north-central Pamirs of Tajikistan and the Kongur Shan normal fault in the eastern Pamir of China. Recent geochronologic and structural studies by several research groups show that east-west contraction in the Nanga Parbat region initiated at about 10 Ma, associated with extensive crustal melting. Our own investigations along the Kongur Shan normal fault indicate that extension across this system initiated between 10 and 8 Ma, coeval with the initiation of the Nanga Parbat contractional structure. The concentration of extensional and contractional strain along the north-south axis of the Pamir arc and location of extension in the outer arc and contraction in the inner arc of the ayntaxis system lead us to suggest that the coeval extension and contraction are controlled by oroclinal bending of the Pamir arc caused by torque exerted by the left-slip Cheman fault during the northward indentation of India. However, this explanation raises the question of why coeval extension and contraction occurred so late in the past 10 m.y. while the Indo-Asian collision and northward indentation of the Pamir promontory started at least 60 m.y. ago. One possible explanation is that the aseismic gap in the northern Pamir and Hindu Kush region represents a zone of ductile deformation along which a subducted oceanic (?) slab of either the Indian or Asian (Tadjik) plate was detached into the mantle. This slab break-off event may have occurred as a relatively slow process between ~25 and 15 Ma and caused gradual regional upwelling of mantle flow as expressed by widespread crustal melting during this time period in the region. Heating due to slab break-off may have induced thermal weakening by reducing the effective elastic thickness of the Pamir-Nanga Parbat crust. The combination of thermal weakening and oroclinal bending of the Pamir-Nanga Parbat syntaxis system eventually led to buckling of the Nanga Parbat crust in the south and synchronous extension in the northern Pamir at about 10 Ma. Because no east-west extension occurs in the Tian Shan north of the south-dipping Main Pamir thrust, this fault must have served as a decoupling zone above which oroclinal bending and crustal extension occur.