Crustal structure of the central Qaidam basin imaged by seismic wide-angle reflection/refraction profiling Junmeng Zhao a, b, ⁎, Zhijun Jin c , Walter D. Mooney d , Nihal Okaya d , Shangxu Wang c , Xing Gao a , Liangjie Tang c , Shunping Pei a , Hongbing Liu a , Qiang Xu a a Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China b Institute of Geology, China Earthquake Administration, Beijing, 100029, China c Petroleum University (Beijing), Beijing, 102249, China d U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 90425, USA abstract article info Article history: Received 19 October 2011 Received in revised form 15 August 2012 Accepted 8 September 2012 Available online 14 September 2012 Keywords: Qaidam basin Tibet plateau P- and S-wave velocity structure Poisson's ratio V p /V s ratio Crustal composition We present the results of a seismic wide-angle reflection/refraction profile across the central Qaidam basin, the largest basin within the Qinghai-Tibetan plateau. The 350-km‐long profile extends from the northern margin of the East-Kunlun Shan to the southern margin of the Qilian Shan. The P- and S-wave velocity structure and Poisson's ratio data provide constraints on composition. The crust here consists of a near-surface sedimentary layer and a four-layered crystalline crust having several significant features. (1) The sedimentary fill of the Qaidam basin reaches a maximum thickness of 8 km, and the basin shape mirrors the uplifted Moho. (2) Within the four layers of the crystalline crust, P- (S-) wave velocities increase with depth and fall within the following velocity ranges: 5.9–6.3 km/s (3.45–3.65 km/s), 6.45–6.55 km/s (3.7 km/s), 6.65 km/s (3.8 km/s), and 6.7–6.9 km/s (3.8–3.9 km/s), respectively; (3) low-velocity zones with a 3–5% reduction in seismic velocity are detected in the lower half of the crust beneath the Qaidam basin and its transition to the Qilian Shan. (4) The crystalline crust is thickest beneath the northern margin of the basin towards the Qilian Shan (58–62 km) and thinnest beneath the center of the basin (52 km). Variations in crustal thickness are caused most pronouncedly by thick- ness variations in the lowermost layer of the crust. (5) Poisson's ratio and P-wave velocity values suggest that the Qaidam crust has an essentially felsic composition with an intermediate layer at its base. Based on the crustal structure reported here, we suggest that late Cenozoic convergence is accommodated by thick-skinned tectonic deformation with thickening involving the entire crust across the Kunlun–Qaidam–Qilian system. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The rhomb-shaped Qaidam basin, the largest basin within the Qinghai-Tibetan plateau, has a surface area of ~120,000 km 2 . It lies at an average elevation of ~ 3000 m above sea level and is surrounded by mountain belts that rise to ~5000 m above sea level including the Altyn Tagh range to the northwest, the Qilian Shan to the northeast, and the Qiman Tagh and East-Kunlun Shan to the southwest (Fig. 1). As one of the three largest oil and gas producing basins in western China, the Qaidam basin attracts many researchers world- wide and has been extensively studied using geophysics, geology, sedimentology, neotetonics and thermal chronology. Both thin- and thick-skinned tectonic deformation styles have been proposed for the late-Cenozoic deformation of the Kunlun–Qaidam– Qilian system (Chen et al., 1999; Métivier et al., 1998; Meyer et al., 1998; Tapponnier et al., 2001; Wang et al., 2004). The deep structure of the crust gives insight into how late-Cenozoic, N30°E-convergence between India and Asia has been accommodated across the Qaidam basin and its margins. The crust in the area of the Qaidam basin has been investigated in the scope of several active-source seismic surveys (Cui et al., 1995; Galvé, 2002; Jiang et al., 2006; Wang et al., 2013; Zhao et al., 2006). Although other regional profiles exist, no detailed image of the crustal structure across the center of the Qaidam basin has been previously reported. We present the results of the Golmud to Huahaizi seismic wide angle reflection/refraction profile that was re- cently completed as part of the “973 National Research Project” (Fig. 1). The high-quality P- and S-wave data allows us to determine the detailed two-dimensional seismic velocity structure of the crust and to infer crustal composition using the Poisson's ratio σ (equivalently, the V p /V s ratio). This profile provides important insight into the unique tectonic role that the modern Qaidam basin plays as the largest intermontane basin in the India–Asia convergence system. Tectonophysics 584 (2013) 174–190 ⁎ Corresponding author at: Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. Tel./fax: +86 10 62849699. E-mail address: zhaojm@itpcas.ac.cn (J. Zhao). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2012.09.005 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto