The Gangdese batholith, southern Tibet, was part of an Andean-type arc at the southern margin of Asia prior to the collision of India and Asia at approximately 50 to 40 Ma. Fission-track and 4øAr/39Ar analyses of 28 rocks from 10Gangdese granitoid plutons along an -250 km length of the batholith in the Lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks. These data suggest a range of ages for these plutons of 94 to 42 Ma, with the majority being of Tertiary age. The postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities. We conclude that most of these discontinuities, and all of them after 40 Ma, reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time. In addition to the initial cooling of hot magma against cold country rock, all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling, dropping many tens of degrees in a few million years. Conversely, these plutons all experienced intervals during which they cooled very slowly or not at all; these slow-cooling intervals lasted from 5 to 50 million years. Our data indicate that since the collision between India and Asia began, response to continued convergence has been quite variable in even this relatively small area. The data reported here are consistent with a recentlyproposed model of Oligo-Miocene crustal shortening along the Gangdese Thrust system in this area. Copyfight 1995 by the Amefiean Geophysical Union.Paper number 94TC01676. 0278-7407/95/94TC-01676510.00 of modes by which continents respond to plate motions places a premium on knowing the timing and duration of each deformation mechanism. The contributions of radiogenic isotope investigations to the understanding of the evolution of mountain belts are both direct, such as determining the timing of fault motion, and inferential, such as deducing the timing of crustal thickening from denudation rates. The Tertiary Indo-Asian orogeny presents a unique opportunity to use both direct and inferential evidence to examine active and ancient manifestations of the collision of two continental masses. Before the start of the collision betweenIndia and Asia at about 50 Ma the southern margin of Asia was marked by a Andean-type arc, known as the Gangdese or Transhimalayan batholith. The batholith marking the roots of this arc, which lies 100 to 200 km north of the Himalaya and stretches E-W for over 2000 km across southern Tibet and northern India, varies between 20 and 60 km wide (Figure 1). In many places the suture marking the closure of the Tethyan ocean lies immediately to the south of the batholith. The plutonic rocks of the Gangdese batholith have a range of crystallization ages from -120 Ma to 40 Ma [Harris et al., 1988b; Schiirer and Allbgre, 1984] and a wide variation in composition including gabbro, granite, and tonalite; the average composition is granodiorite [Debon et al., 1986]...

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