Tectonophysics 612–613 (2014) 26–39 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Crustal structures revealed from a deep seismic reflection profile across the Solonker suture zone of the Central Asian Orogenic Belt, northern China: An integrated interpretation Shihong Zhang a,⁎,RuiGaob,⁎⁎, Haiyan Li a, Hesheng Hou b,HuaichunWua,QiushengLib,KeYanga,ChaoLia, Wenhui Li b,JishenZhangb, Tianshui Yang a, G.R. Keller c,MianLiud a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China b Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China c University of Oklahoma, Norman 73019, USA d University of Missouri, Columbia 48063, USA article info abstract Article history: The Solonker suture zone is one of the most important tectonic boundaries in the southeastern part of the Central Received 10 October 2012 Asian Orogenic Belt (CAOB). An ~630 km-long reflection seismic profile across this suture was recently completed Received in revised form 4 September 2013 by the Chinese SinoProbe Project. The processed seismic data show clear crustal structures and provide new Accepted 23 November 2013 constraints on the tectonic and crustal evolution models. The Moho is delineated as a relatively flat boundary be- Available online 4 December 2013 tween a strongly reflective lower crust and a transparent mantle at a depth of ~40–45 km (~14.5 s two-way travel time), which is in agreement with the refraction data recorded along the same profile. In a broad view, Keywords: fi Seismic reflection the pro le images an orogen that appears bivergent with, and approximately centered on, the Solonker suture Crustal structure zone. The southern portion of this profile is dominated by a crustal-scale, cratonward propagating fold-and- Tectonics thrust system that formed during the late Permian and Triassic through collision and subsequent convergence Central Asian Orogenic Belt in a post-collisional stage. The major thrust faults are truncated by Mesozoic granitoid plutons in the upper Solonker suture zone crust and by the Moho at the base of the crust. This geometry suggests that the Moho was formed after the thrust- ing event. The northern portion of the profile, although partially obliterated by post-collisional magmatic bodies, shows major south-dipping folding and thrusting. Bands of layered reflectors immediately overlying the Moho are interpreted as basaltic sills derived from the mantle. Episodic mafic underplating may have occurred in this region, giving rise to post-collisional magmatic events and renewal of the Moho. A few mantle reflectors are also visible. The overall geometry of these mantle reflectors supports the tectonic models that the southern orogen (Manchurides) experienced south-directed subduction and the northern orogen (Altaids) underwent north-directed subduction prior to collision along the Solonker suture zone. © 2013 Elsevier B.V. All rights reserved. 1. Introduction region. Many models, often conflicting, have been proposed to explain the tectonic evolution of the CAOB (e.g., Chen et al., 2009; Jian et al., Eurasia, the largest continent on the Earth, was formed by multiple 2008, 2010; Kröner et al., 2007, 2013; Li, 2006; Sengör and Natal'in, phases of continental accretion and collision since the late Neoproterozoic. 1996; Sengör et al., 1993; Windley et al., 2007; Xiao et al., 2003, 2009; The Central Asian Orogenic Belt (CAOB) occupies approximately 30% of Xu et al., 2013, among others). Disagreement includes the polarity(ies) the land area in Asia. It contains a complex geological record of amal- of subduction and accretion, the timing and location of collision be- gamated accretionary zones and collisional sutures between the major tween the Angaran (or Siberian) and Cathysian tectonic domains, the cratons, namely Baltica, Siberia, Tarim, and North China (NCC), as well timing and position of crustal thickening and thinning, and the propor- as numerous tectono-stratigraphic terranes with unknown tectonic tion of juvenile crust versus ancient crust within the CAOB. The solution affiliations (variably termed massifs or microcontinental blocks, to such problems requires a better understanding of deep structure of Fig. 1). This huge tectonic collage has, in turn, been modified by younger the crust and mantle. deformations resulting from the closure of the Mongol–Okhotsk Ocean, In this paper, we report the new findings on crustal structure collisions in the Tibetan Plateau, and subduction in the western Pacific revealed from a deep seismic reflection profile recently completed by the Chinese SinoProbe Project (Dong et al., 2013b). The NW–SE profile crosses a large region that is widely considered to contain the terminal ⁎ Corresponding author. Tel.: +86 10 82322257; fax: +86 10 82321983. fl ⁎⁎ Corresponding author. Tel.: +86 10 68999730. late Paleozoic collisional suture between the Archean- oored NCC E-mail addresses: [email protected] (S. Zhang), [email protected] (R. Gao). and more northerly terranes of the CAOB (Figs. 1 and 2). The high- 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.11.035 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39 27 Siberia Siberia CAOB SinoProbe Fig.2 Seismicprofile Solonker Suture Tarim Beijing NorthChina Major Cratons SouthChina India Terranes Central Asia Orogenic Belt (CAOB) Central China and Tethyan Orogens 0 500km Mongol-Okhotsk and W.Pacific Orogens Fig. 1. Tectonic positions of the Solonker suture and the SinoProbe reflection seismic profile. resolution seismic images acquired along this profile provide important tectonic elements defined by their geological characteristics and history new deep structural constraints on tectonic and crustal evolution and by crustal compositions (Figs. 2, 3). The Solonker suture zone is gen- models of this region. erally considered to be the most important tectonic element crossed by the SinoProbe traverse, but its location has been a controversial topic for 2. Geological background many years. The suture was named by Sengör et al. (1993) as separating two orogens (Fig. 2). The Southern Orogen (Jian et al., 2008), named From the Huailai Basin near Beijing, our ~630 km seismic profile Manchurides by Sengör et al. (1993), is composed of displaced frag- continues northwestward via Zhangjiakou in northern Hebei Province, ments of the Paleozoic northern active margin of the NCC. The Northern crosses the poorly exposed grassland of Inner Mongolia, and ends at Orogen, being part of the Altaids (Sengör et al., 1993), is composed of the China–Mongolia boundary (Fig. 1). This region contains many tectonic fragments with affiliations to the Angaran (or Siberian) craton. 110 112 114 116 118 Dongwuqi Tectonic units in the Northern Orogen (Altaids) Chagan Obo Fault Solonker suture zone 25926 Uliastai Belt Tectonic units in the Southern Erenhot Fault Orogen (Manchurides) 24000 North China Craton (NCC) Hegenshan Belt Chagan Obo 44 Ophiolite, 22000 Xilinhot Fault44 Mafic-Ultramafic complex Xilinhot Sonid Zuoqi Baolidao Belt Ductile shear zone 20000 Linxi Fault Zone Linxi 1 Seismic profile with CM P Erenhot ker Suture Solon t 18000 Faul r Moron 0 50 100km Xa 16000 Ondor Sum Belt Solonker Sonid Youqi 14000 Ondor Sum Mandula Chifeng Bainaimiao Belt Weichang 42 12000 10000 Bainaimiao (1) Huade Chifeng Fault Kangbao Bayan Obo 8000 6000 Longhua Jining Zhangbei (2) Hohhot Shangyi 4000 Chengde Zhangjiakou (3) Chicheng 2000 (4) North China Craton (NCC) Huailai 1 40 40 110 112 114 Beijing 118 Fig. 2. Tectonic subdivision of the study region (modified from Xiao et al., 2003). Deformation ages for numbered ductile shear zones are determined as follows (Wang et al., 2013): (1) Kangbao ductile shear zone, ~270 Ma; (2) Longhua ductile shear zone, ~250 Ma; (3) Chicheng ductile shear zone, ~230 Ma; (4) unnamed ductile shear zone, ~210 Ma. The geological profiles labeled (a) to (g) are depicted in Fig. 4. 28 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39 It is widely believed that these two orogens represent coeval subduc- syntectonic magmatic flow model for the origin of this plutonic belt, tion–accretion complexes of different polarities in Paleozoic, and, the based on their field structural, micro-structural, lithological and U–Pb Solonker suture zone is generally considered to define the final collision chronological analysis. between the two orogens (Chen et al., 2000, 2009; Jian et al., 2008, 2010, The late Mesozoic was a time of decratonization for the eastern NCC. 2012; Sengör and Natal'in, 1996; Sengör et al., 1993; Xiao et al., 2003, This was likely due, in part, to subduction of the Pacific plate in the Early 2009; Xu et al., 2013). Cretaceous, and is manifested by lithospheric thinning, lithospheric mantle modification, extensive intracrustal ductile deformation, and 2.1. The northern NCC magmatic activity (Liu et al., 2005, 2012; Zhu et al., 2011, and references herein). Northeast-trending extensional basins containing late Mesozo- The NCC is one of the oldest Precambrian cratons in the world. It has ic and Cenozoic sedimentary and volcanic strata developed in an even an Archean to Paleoproterozoic metamorphic basement that was larger region in NE Asia (Lin et al., 2013 and references herein). A rela- cratonized at ~1.85 Ga (Wang et al., 2005; Zhao et al., 2011, and refer- tionship between the volume of the these strata and the thickness of ences herein) and is covered by sedimentary and volcanic successions the upper crust has been recognized in northern China, i.e. thicker strata ranging in age from ~1.78 Ga to Early Triassic (Li et al., 2013a; Lu corresponding to thinner upper crust, and vice versa (Zhang et al., et al., 2008; Su et al., 2008; Wang et al., 2005). In our study region, the 2011). Widespread regional unconformities and widespread exposures basement rocks of the northern NCC are largely exposed (Fig. 3) and of granite batholiths (Zhang et al., 2007b; Zhou and Wang, 2012)indi- are intruded by igneous rocks resulting from multiple magmatic events, cate that extensive and deep erosion has occurred in the northern NCC.