The South Tibet detachment shear zone in the Dinggye area: Time constraints on extrusion models of the Himalayas

We investigate the timing of end of motion along the South Tibet Detachment System (STDS), a major normal fault system that runs parallel to the Himalayan range for more than 1500 km. Near Dinggye (∼ 28°10′N, 87°40′E), the STD dips ∼ 10 ± 5° to the North and separates Paleozoic Tethyan series from Upper Himalayan Crystalline Series (UHCS). Immediately below the STD, the UHCS is highly deformed in the STD shear zone, lineations trend NNE and the shear senses are top to the NE. In micaschist, the P–T path constrained by pseudosection and garnet chemistry, shows successive metamorphic conditions of ∼ 0.6 GPa and ∼ 550 °C and 0.5 GPa and 625 °C. U/Pb dating of monazites and zircons in deformed and undeformed leucogranites suggests that ductile deformation lasted until at least ∼ 16 Ma but ended prior to ∼ 15 Ma in the STD shear zone ∼ 100 m below the detachment. Ar/Ar micas ages in the footwall span between ∼ 14.6 and 13.6 Ma, indicating rapid cooling down to ∼ 320 °C, and suggesting persistence of normal faulting, at that time. The STDS is cut and offset by the N–S trending Dinggye active normal fault which initiated prior to 11 Ma thus providing a minimum bound for the end of STDS motion. These data are interpreted as reflecting 0.3 GPa (11 km) to 0.6 GPa (22 km) of exhumation along the STDS starting prior to ∼ 16 Ma, ending between 13.6 and 11 Ma. The 1000 km long stretch of the STDS east of the Gurla Mandata probably stopped almost synchronously between 13 and 11 Ma ago, coevally with a sudden switch from NNE–SSW to E–W extension at the top of the accretionary prism, with a jump of the major thrust from the lower Main Central Thrust (MCTl) to the Main Boundary Thrust (MBT), and with a change in the India and Asia convergence direction. This synchronism is probably better explained in the frame of a thrust wedge or thrust system model than a lower channel flow model. West of the Gurla Mandata the STDS appears to stop 5 to 3 Ma earlier, possibly related to local interactions with the Karakorum fault in a way that needs to be understood.