Onset timing of significant unroofing around Qaidam basin, northern Tibet, China: constraints from 40 Ar/ 39 Ar and FT thermochronology on granitoids Fei Wang a, * , Ching-Hua Lo b , Qi Li c , Meng-Wan Yeh d , Jinglin Wan c , Dewen Zheng c , Erqi Wang a a Ar/Ar Laboratory, Institute of Geology and Geophysics, Chinese Academy of Science, Beijing 100029 b Department of Geosciences, Taiwan University, Taipei, Taiwan c Institute of Geology, Seismological Bureau, Beijing d Department of Earth Science, National Taiwan Normal University, Taipei, Taiwan Received 13 September 2001; revised 20 May 2003; accepted 24 July 2003 Abstract 40 Ar/ 39 Ar and fission track thermochronological results from two granitoid suites in the mountain ranges along the northern and southern edges of Qaidam basin constrain the timing of a Tertiary denudation event and their tectonic association in northern Tibet. The cooling histories based upon 40 Ar/ 39 Ar and fission track ages suggest a rapid cooling event (7.5 – 10.7 8C/Ma) during Oligocene – early Miocene time. This cooling event is thought to reflect an increasing denudation rate associated with the rapid uplift and denudation of the North Qaidam and Kunlun Shan mountain ranges. With consideration to the field evidence, sedimentation rate of the Qaidam basin, and the propagation rate along the Altyn Tagh fault, the rapid uplifting and denudation are probably due to crustal thickening associated with Cenozoic propagation of the Altyn Tagh fault in accommodating the collision of India with Asia. q 2003 Elsevier Ltd. All rights reserved. Keywords: Qaidam basin,; 40 Ar/ 39 Ar dating; Fission track dating; Multi-domain diffusion model; Unroofing 1. Introduction Although Murphy et al. (1997) suggested that a significant portion of southern Tibet was elevated to 3–4 km during the Early Cretaceous, most features of the Himalayan belt and the Tibetan plateau are the result of India–Asia collision during the Cenozoic. Cenozoic activities in the Tibetan plateau and adjacent areas have long been the focus of a number of geological studies on lithospheric accommodation mechanisms, particularly the partitioning of thickened homogeneous crust with horizontal compression, and lateral extrusion along major strike-slip faults under continental collision (Tapponnier et al., 1986; Dewey et al., 1988; Avouac, 1991). Tapponnier et al. (1990) suggested that local crustal thickening would also occur under lateral extrusion due to (1) progressively filled sedimentary basins, and (2) formation of large anticlines by major thrusts ‘branching’ from strike-slip faults. One of the major strike-slip faults associated with the Cenozoic tectonics of this range is the Altyn Tagh fault zone. The Altyn Tagh fault zone separates the Tarim basin to the north from the Qaidam basin, Kunlun Shan, and the Tibetan plateau to the south (Fig. 1). Several studies (Peltzer et al., 1989; Tapponnier et al., 1990; Meyer et al., 1998) suggested that the propagation of the Altyn Tagh fault not only controlled the uplift and denudation, but also determined the geometry of mountain belts in this area (Fig. 1). Thus, if we want to have a further understanding of the accommodation mechanism and tectonic history of this region, precise reconstruction of the thermochronolo- gical history for the propagation of the Altyn Tagh fault is needed. Although the structural and stratigraphic evidence indicated left-lateral displacement of the Altyn Tagh fault (Molnar and Tapponnier, 1975; Molnar et al., 1987; 1367-9120/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2003.07.004 Journal of Asian Earth Sciences 24 (2004) 59–69 www.elsevier.com/locate/jaes * Corresponding author. Tel.: þ 86-10-62007357; fax: þ 86-10- 62028617. E-mail address: wfdzs@263.net (F. Wang).