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Thermochronological results from 100-70 Ma granitoids of the Gangdese batholith in
southeastern Tibet show evidence of thrusting. The granitoids were buried during the Miocene
beneath the north-directed (~30— dip) Renbu Zedong thrust (RZT). Near Lian Xian, its hanging
wall consists of Tethyan metasediments derived from the Indian shelf that have experienced upper
greenschist to lower amphibolite facies metamorphism. The footwall granitoids and associated
wallrocks adjacent to the RZT exhibit considerable recrystallization to greenschist facies assemblages.
Footwall samples from a northeast-southwest traverse extending 15 km away from the trace of the
RZT all yield 9-12 Ma ages for the initial ~20% of gas release. Biotite and K-feldspar 40Ar/39Ar
ages increase systematically away from the RZT. All possible thermal histories consistent with
the measured K-feldspar age and kinetic properties were computed using a variational method
and contoured plots of probability density calculated from the best fit solutions constrain the full
range of temperature-time histories afforded by the sample. Samples far-removed from the RZT
are well described by regional slow cooling from 300 to 275—C until about 10 Ma, but thermal
histories for samples adjacent to the thrust show evidence for rapid cooling between 19 and 11
Ma which we interpret as indicating the timing of thrusting along the RZT. In conjunction with
a numerical thermal model, these results constrain the minimum average slip rate and
displacement along the ramp during this period to be 2 mm/yr and 12 km, respectively.
A cooling episode recorded by 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.
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