CONSTRICTION AND PLIOCENE ACCELERATION OF EAST-WEST EXTENSION IN THE NORTH LUNGGAR RIFT REGION OF WEST-CENTRAL TIBET

The North Lunggar rift is an active, north-trending rift in west-central southern Tibet that exposes an extensional metamorphic core complex bounded by an east-dipping low-angle normal fault. Apatite and zircon (U-Th)/He thermochronology and thermal modeling of the North Lunggar rift show rift inception before 10 Ma, and acceleration in footwall exhumation post 5 Ma. Miocene footwall cooling rates were initially slow to moderate (> 50°C m.y.^-1), followed by increased Pliocene rates as high as 300 – 500 °C m.y.^-1. Footwall isotherms were likely significantly compressed during rapid exhumation resulting in an elevated transient geothermal gradient, with the highest values in the central to southern portion of the rift. Thermal modeling shows the North Lunggar footwall underwent a minimum of 5 – 9 km exhumation, which with a fault dip of 30° translates to 8.7 – 15.6 km of horizontal extension. The mean horizontal extension rate of the North Lunggar rift is similar in magnitude, rate and orientation of slip to the kinematically linked Lamu Co dextral strike-slip fault to the north, suggesting a state of constrictional strain during Pliocene time along this portion of the Bangong-Nujiang suture from which the Lamu Co fault emanates. The onset of extension in this region may be explained by crustal thickening and gravitational orogenic collapse, followed by accelerated rifting resulting from localized crustal stretching and increased magmatic activity driven by the position and northward extent of underthrusting Indian lithosphere.