THE SOUTH LUNGGAR RIFT, WESTERN TIBET: RATES, TIMING AND EVOLUTION OF AN ACTIVE DETACHMENT SYSTEM FROM STRUCTURAL MAPPING AND (U-TH)/HE THERMOCHRONOLOGY

The north-trending South Lunggar rift in the western Lhasa block of Tibet accommodates active E-W extension of the southern Tibetan plateau. The rift is made up of a 45 km along-strike horst block, the South Lunggar range, bordered on the east and west by two 10-15 km wide rift basins. The South Lunggar range is composed of amphibolite-facies orthogneisses and greenschist-facies metavolcanic rocks that are intruded by km-scale leucogranite plutons. The eastern flank of the range is bounded by a moderately east-dipping normal fault that hosted a Mw 6.7 earthquake in 2008. The western side of the range is bound by a west-dipping moderate to high-angle normal fault in the south that changes to a low-angle (~20° dipping), well-developed mylonitic shear zone in the north. Kinematic indicators include S-C fabrics and chatter marks consistent with top to the west displacement. Map patterns and geologic cross-sections bracket the magnitude of horizontal extension to be between 7 and 20 km across the northern part of the rift, where extension is likely the highest. The west-dipping fault is considered to be active as indicated by Quaternary fault scarps that cut moraines at the range front. These increase in throw and distance from the range front from south to north, with the largest being up to 3 km from the range and having a cumulative throw of ~100 m west of the mylonitic shear zone. Zircon (U-Th)/He ages indicate the onset of exhumation before 10 Ma, with a northward increase in exhumation rates; the youngest zircon He ages (~3.5 Ma) are found at the mylonitic shear zone on the northwestern margin of the range. (U-Th)/He data from two east-west transects across the footwall suggest vertical uplift of the southern part of the range, and moderate amounts of horizontal-axis rotation of the northern portion of the range away from the west-dipping fault, consistent with models of evolving low-angle detachment fault systems incorporating back-rotation of the footwall. Preliminary analysis suggests horizontal extension rates of ~1 mm yr^-1 in the south, to ~2.5 mm yr^-1 in the north. These observations suggest an along-strike transition from a slower, high angle fault system in the south, to faster, core-complex-like extension in the north.