Thermal evolution and slip history of the Renbu Zedong Thrust, southeastern Tibet

Abstract

Cretaceous granitoids of the Gangdese batholith, southeastern Tibet, were overthrust by upper greenschist to epidote‐amphibolite facies Tethyan rocks derived from the Indian shelf along the north directed (∼30° dip) Renbu Zedong Thrust (RZT). Thermochronological results obtained from a NE‐SW transect near Lian Xian show evidence of thermal effects related to thrusting. Granitoids immediately beneath the RZT exhibit considerable recrystallization to greenschist facies assemblages. Biotite and K‐feldspar 40Ar/39Ar ages measured along the traverse into the footwall increase systematically away from the RZT. The timing of initial upward displacement of the RZT hanging wall is constrained to have occurred at ∼18 Ma from the hornblende ages and the two K‐feldspar samples closest to the fault. K‐feldspars up to 15 km from the thrust yield 9–12 Ma ages for the initial ∼20% of 39Ar release. Distal samples yield 40Ar/39Ar ages nearly as old as the 70–110 Ma ion microprobe 206Pb*/238U ages determined for coexisting zircons. We have integrated our thermal history results with numerical heat flow models and found that while reheating to 320–280°C at shallow (∼7 km) levels due to rapid (>15 mm/yr) slip along the RZT at ∼10 Ma is capable of explaining the initial portion of the K‐feldspar age spectra, a prior common thermal history experienced by all samples cannot satisfactorily account for all the 40Ar/39Ar results. Instead, we find our thermal history constraints to be more completely explained by a numerical model in which (1) rocks currently at the surface originated from different depths,(2) footwall samples in close proximity to the RZT experienced fault drag from 19 to 15 Ma and (3) postthrusting denudation of the region involving localized tilting occurred at ∼10 Ma. The minimum average slip rate and displacement along the ramp during this period are 2 mm/yr and 12 km, respectively, but are likely to have been greater. The cooling episode recorded in all the K‐feldspar age spectra beginning at ∼10 Ma may either reflect denudation following regional uplift due to displacement along the ramp of the Main Himalayan Thrust or topographic collapse following cessation of RZT thrusting.