Post 4 Ma initiation of normal faulting in southern Tibet. Constraints from the Kung Co half graben

The timing of E–W extension of the Tibetan plateau provides a test of mechanical models of the geodynamic evolution of the India–Asia convergence zone. In this work we focus on the Kung Co half graben (Southern Tibet, China), bounded by an active N–S normal fault with a minimum vertical offset of 1600 m. To estimate the onset of normal faulting we combined high and medium temperature (U–Pb, Ar/Ar) and low temperature ((U–Th)/He) thermochronometry of the Kung Co pluton, a two-mica granite of the northern Himalayan granitic belt that outcrop in the footwall of the fault. Biotite and muscovite Ar/Ar ages , are close from each other [∼16 Ma±0.2 (Ms) and ∼15±0.4 Ma (Bt)], which is typical of fast cooling. The zircon and apatite (U–Th)/He ages range from 11.3 to 9.6 Ma and 9.9 to 3.7 Ma respectively. These He ages are indicative of (1) fast initial cooling, from 11.3 to ∼9 Ma, gradually decreasing with time and (2) a high geothermal gradient (∼400 °C/km), close to the surface at ∼10 Ma.