Tectonophysics 420 (2006) 253–266 www.elsevier.com/locate/tecto Crustal structure of the northeastern margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin ⁎ Mingjun Liu a,b, Walter D. Mooney b, , Songlin Li a,b, Nihal Okaya b, Shane Detweiler b a Geophysical Exploration Center, China Earthquake Administration, 104 Wenhua Road, Zhengzhou, Henan 450002, China b U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA Received 19 January 2005; received in revised form 17 October 2005; accepted 4 January 2006 Available online 5 May 2006 Abstract The 1000-km-long Darlag–Lanzhou–Jingbian seismic refraction profile is located in the NE margin of the Tibetan plateau. This profile crosses the northern Songpan-Ganzi terrane, the Qinling-Qilian fold system, the Haiyuan arcuate tectonic region, and the stable Ordos basin. The P-wave and S-wave velocity structure and Poisson's ratios reveal many significant characteristics in the profile. The crustal thickness increases from northeast to southwest. The average crustal thickness observed increases from 42 km in the Ordos basin to 63 km in the Songpan-Ganzi terrane. The crust becomes obviously thicker south of the Haiyuan fault and beneath the West-Qinlin Shan. The crustal velocities have significant variations along the profile. The average P-wave velocities for the crystalline crust vary between 6.3 and 6.4 km/s. Beneath the Songpan-Ganzi terrane, West-Qinling Shan, and Haiyuan arcuate tectonic region P-wave velocities of 6.3 km/s are 0.15 km/s lower than the worldwide average of 6.45 km/s. North of the Kunlun fault, with exclusion of the Haiyuan arcuate tectonic region, the average P-wave velocity is 6.4 km/s and only 0.5 km/s lower than the worldwide average. A combination of the P-wave velocity and Poisson's ratio suggests that the crust is dominantly felsic in composition with an intermediate composition at the base. A mafic lower crust is absent in the NE margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin. There are low velocity zones in the West-Qinling Shan and the Haiyuan arcuate tectonic region. The low velocity zones have low S-wave velocities and high Poisson's ratios, so it is possible these zones are due to partial melting. The crust is divided into two layers, the upper and the lower crust, with crustal thickening mainly in the lower crust as the NE Tibetan plateau is approached. The results in the study show that the thickness of the lower crust increases from 22 to 38 km as the crustal thickness increases from 42 km in the Ordos basin to 63 km in the Songpan-Ganzi terrane south of the Kunlun fault. Both the Conrad discontinuity and Moho in the West-Qinling Shan and in the Haiyuan arcuate tectonic region are laminated interfaces, implying intense tectonic activity. The arcuate faults and large earthquakes in the Haiyuan arcuate tectonic region are the result of interaction between the Tibetan plateau and the Sino–Korean and Gobi Ala Shan platforms. © 2006 Published by Elsevier B.V. Keywords: China; Crustal structure; Deep seismic sounding; NE Tibetan plateau 1. Introduction The NE margin of the Tibetan plateau, located in an ⁎ Corresponding author. Tel.: +1 650 329 4764; fax: +1 650 329 5163. important tectonic junction, is an ideal site to study E-mail addresses: [email protected] (M. Liu), [email protected] continental dynamics. Much attention has been paid to (W.D. Mooney). the Tibetan plateau regarding its evolution as a product 0040-1951/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.tecto.2006.01.025 254 M. Liu et al. / Tectonophysics 420 (2006) 253–266 of the India–Asia collision (Molnar and Tapponnier, A 1000-km-long geophysical profile was conducted 1975). Since India collided with Asia ∼55 million years from Darlag–Lanzhou–Jingbian extending from the ago, the Tibetan plateau has dominantly been growing Songpan-Ganzi terrane to the Ordos basin in the NE toward the east and northeast (Tapponnier et al., 2001). margin of the Tibetan plateau to study the interaction Therefore, crustal structure and evolution in the NE between the Tibetan plateau and the Sino–Korean plateau are attracting more attention (Cui et al., 1995; platform, and to study the deep driving mechanisms of Herquel et al., 1995; Zhu and Helmberger, 1998; Meyer tectonic deformation (Fig. 1). It is a combination profile et al., 1998; Galve et al., 2002; Vergne et al., 2002; Pares that consists of seismic refraction, teleseismic observa- et al., 2003). tion, and magnetotelluric soundings (MTS). In this Fig. 1. Tectonic map of the NE Tibetan plateau (after Yin and Harrison, 2000). The seismic profile is indicated by a solid black line with circled numbers 1 through 9, indicating shot point locations. The southernmost shot point (SP1) is located in the Songpan-Ganzi terrane near Darlag. The northernmost shot point (SP9) is located in the Ordos basin near Jingbian. The seismic profile crosses the Qinling-Qilian fold system (SP2 to SP6) and the Haiyuan arcuate tectonic belt (SP7). M. Liu et al. / Tectonophysics 420 (2006) 253–266 255 paper, we present the 2-D velocity structure from P- and thrust system. Nowhere else does active overthrusting S-waves, we infer the composition of the crust using spread over a greater surface area (∼500,000 km2) Poisson's ratio, and we discuss the continental dynamics than in the NE corner of Tibet (Tapponnier et al., in the NE margin of the Tibetan plateau based on the 2001). This broad region of coeval mountain building seismic refraction data (Li et al., 2002; Liu et al., 2003). is delimited by the Kunlun fault to the south, the Altyn Tagh fault to the northwest, and the Haiyuan 2. Geologic setting fault to the northeast (Fig. 1). The most vigorously rising thrust-ranges along the eastern rim of Tibet lay The study area covers the NE margin of the Tibetan north of the Kunlun fault (Tapponnier et al., 2001). plateau, and has a complicated geological background. The Kunlun fault separates the Songpan-Ganzi It involves mainly three geological units, the northeast- terrane from the Qinling-Qilian fold system which ern Tibetan plateau, which includes the Songpan-Ganzi is divided into the West-Qinling Shan and the Qilian terrane, the Qinling-Qilian fold system, the Haiyuan Shan by the Qinling north margin fault. The West- arcuate tectonic region, the Ordos basin, in addition to Qinling Shan is part of the Central Orogenic Belt two tectonic belts. The NS belt divides the Chinese in China. continent into eastern and western tectonic provinces from the Helan Shan to the Sichuan-Yunnan (Li and 2.3. Haiyuan arcuate tectonic region (shot point SP7 in Mooney, 1998), and the Central Orogenic Belt that Fig. 1) divides the Chinese continent into northern and southern tectonic provinces along Kunlun–Qinling–Dabie (Jiang The Haiyuan arcuate tectonic region connects et al., 2000). Tectonic activity is very intense in the three tectonic units: the Tibetan plateau in the study area, where several M≥8 earthquakes have southwest, the Sino–Korean platform in the north- occurred in the past hundred years. Compression from east, and the Gobi Ala Shan platform in the north. It the Indian plate has not only caused the crustal is also a junction of three intraplate blocks, similar to thickening and uplift of the NE Tibetan plateau, but a triple junction that connects three plates (Tian and has also affected adjacent areas such as the Sino-Korean Ding, 1998). There are four arcuate faults in the platform and the Gobi Ala Shan platform. At the same region: the Haiyuan fault, the Xiangshan Northern time, the relatively solid blocks surrounding the plateau Margin fault, the Yantongshan fault, and the constrain the movement of the Tibetan plateau to some Qingtongxia-Guyuan fault. The faults converge to- degree. ward the southeast and diverge toward the northwest. Many large earthquakes occurred on the arcuate faults 2.1. Songpan-Ganzi terrane (shot point SP1 in Fig. 1) in the region, including the 1920 M=8.5 Haiyuan earthquake on the Haiyuan fault. The Haiyuan fault is The Songpan-Ganzi terrane, which occupies a large the boundary of the northeastern Tibetan plateau. The part of the central Tibetan plateau, is a triangular NS belt passes through the Haiyuan arcuate tectonic tectonic element between the Qinling-Qilian fold system region. in the north and the Qiangtang terrane in the south. It is characterized by a thick sequence of Triassic strata of 2.4. Ordos basin (Sino-Korean Platform) (between shot deep marine deposits. These Triassic strata are com- point SP8 and SP9 in Fig. 1) monly referred to as the Songpan-Ganzi flysch complex, which was intensely deformed by folding and thrusting The Ordos Basin is a Mesozoic basin in the during the Late Triassic and Early Jurassic (Yin and Sino-Korean platform. The Sino-Korean platform is Harrison, 2000). The southwestern end of the seismic considered to be one of the most stable tectonic profile extends into the Songpan-Ganzi terrane by about provinces in China (Yin, 2002). Its basement 50 km. consists of Archean metamorphic rocks. The upper Proterozoic and the lower Palaeozoic layers are 2.2. Qinling-Qilian fold system (West-Qinling Shan marine sediments, the Carboniferous layers are and Qilian Shan) (between shot point SP2 and SP6 in interbedded coal strata of marine and land sedi- Fig. 1) ments, and above the Permian layers are land sediments. The surface cover of the Ordos basin The Qinling-Qilian fold system is situated in the consists of thick Mesozoic and Cenozoic layers.