图书馆知识库

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 similar to 72 and similar to 60 Ma. The elevation dependence of the Rb/Sr ages suggests that cooling was controlled by erosion, which proceeded at a rate of similar to 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 similar to 0.3 km/My and accelerated to similar 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 similar to 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.