High-Angle Normal Faulting at the Tangra Yumco Graben (Southern Tibet) since ∼15 Ma

Several active graben systems in Tibet and the Himalaya are the expression of ongoing east-west extension, but the significance and history of normal faulting in this large region are still debated. Here, we present geo- and thermochronological data for a granite intrusion in the footwall of an active high-angle normal fault at the Tangra Yumco graben to constrain the onset and history of normal faulting. Crystallization of the granitic rocks occurred at $87±1$ Ma, as revealed by U/Pb zircon dating. After an initial phase of rapid cooling from magmatic temperatures, a later phase of slow cooling is recorded by Rb/Sr biotite ages between ∼72 and ∼60 Ma. The elevation dependence of the Rb/Sr ages suggests that cooling was controlled by erosion, which proceeded at a rate of ∼0.05 km/My during the latest Cretaceous and early Paleocene. The subsequent history of normal faulting is recorded by zircon (U-Th)/He ages of $12.5±1.1$ and $9.7±0.7$ Ma, apatite fission-track ages between $10.8±1.7$ and $7.8±1.2$ Ma, and apatite (U-Th)/He ages from $4.9±0.4$ to $3.0±0.2$ Ma. Thermokinematic modeling of these age data indicates that normal faulting started at $14.5±1.8$ Ma at a rate of ∼0.3 km/My and accelerated to ∼0.7 km/My in the Pliocene. Our age constraint for the initiation of faulting supports a widespread onset of rifting in Tibet at ∼15–10 Ma, as reported for other graben systems. Finally, we suggest that the distribution of high-angle and low-angle normal faults is controlled by their position relative to the India-Asia convergence vector and by lateral variations in the thermal state of the lithosphere.