Thermal evolution of the Gangdese batholith, southern Tibet: A history of episodic unroofing

Abstract

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]...

Tertiary structural evolution of the Gangdese Thrust System, southeastern Tibet

“…This event is observed not only at Zedong, but also 60 and 120 km further west along strike, respectively, at Samye and Quxu (Plate l) [Copeland et al, 1994]. This concordance of ages within regime 1 is perhaps the best evidence that thrusting was underway at 27 Ma.…”

“…This map relation indicates differential unroofing along strike: the structural level of pluton exposure near Lhasa is between 10 to 15 km as indicated by thermochronometry and barometry [Copeland et al, 1987[Copeland et al, , 1994 , whereas the widespread preservation of volcanic rocks in this belt suggests much less denudation west of Renbu. Spatially corresponding to the preservation of volcanic cover in the Gangdese Shan is the appearance, to the south, of the Xigaze Group (Plate 1).…”

“…Qiuwu strata have been substantially shortened [Wang et al, 1983], at least in part since the collision began, the amount of displacement could be significantly greater. If we consider that 10-15 km of crust has been eroded away from much of the Gangdese batholith east of Renbu as a consequence of late Oligocene-Early Miocene thickening [Copeland et al, 1987[Copeland et al, , 1994Harrison et al, 1992a] along a thrust dipping 30±5°, an additional 17-35 km of slip along the GT is indicated. Thus we estimate a minimum displacement on the thrust of 46±9 km.…”

Laser 40Ar/39Ar age constraints on Miocene sequences from the Bengal basin: Implications for middle Miocene denudation of the eastern Himalayas

“…The plutonic rocks of the Gangdese batholith, comprising granite, granodiorite, and tonalite, have a range of crystallization ages from ∼120 to 40 Ma [Harris et al, 1988]. The batholith also shows two distinct periods of plutonism that are broadly 120-90 and 70-40 Ma [Copeland et al, 1995]. Chung et al [1998] found evidence for tectonic and magmatic activity in the Tibetan plateau (as recorded by potassic volcanic rocks) from 30 to 40 Ma.…”

“…Many of these tectonic units may potentially have contributed to the Bengal basin detritus used in this study. Copeland et al [1995] studied the rocks of the Gangdese batholith in the southern Tibetan plateau (Figure 1) and found crystallization ages in the range of 94 to 42 Ma. The plutonic rocks of the Gangdese batholith, comprising granite, granodiorite, and tonalite, have a range of crystallization ages from ∼120 to 40 Ma [Harris et al, 1988].…”