Crustal Structures Revealed from a Deep Seismic Reflection Profile Across the Solonker Suture Zone of the Central Asian Orogenic

Tectonophysics 612–613 (2014) 26–39

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Tectonophysics

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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 thrusting 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-

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. including the 1.75–1.68 Ga anorthosite–mangerite–alkali granite- The northern boundary of the NCC is defined by the Chifeng fault rapakivi granite suites (Zhao et al., 2011)intheYanshanregion(north (Fig. 2). An earlier deep structural section near longitude 110°E (profile of Beijing, Fig. 3), the 1.6–1.2 Ga Zhaertai–Bayan Obo rift complex “e” in Figs. 2 and 4) depicted the Chifeng fault as north-dipping and (Wang et al., 1991; Zhao et al., 2004, 2011), the ~1.35 Ga mafic dikes separating the complex CAOB from the NCC (Xiao et al., 2003, Fig. 4e). and sills (Zhang et al., 2009), the ~1.30 Ga bimodal magmatic rocks At Huade on the seismic profile, south-vergent folding, thrusting and (Zhang et al., 2012b) and the contemporaneous A-type granite and gra- ductile shear zones occur in pre-Permian granites and strata along the nitic porphyry (Shi et al., 2012). The NCC then experienced a long hiatus Kangbao shear zone near the Chifeng fault (Zhou and Wang, 2012; in magmatic activity. However, a granite and volcanic belt of late Wang et al., 2013, Fig. 4d). Continuation of the Chifeng fault beneath Carboniferous age (~320–300 Ma) have recently been recognized Mesozoic and younger strata was mainly inferred from aeromagnetic along the northern margin of the NCC. The calc-alkaline geochemical anomaly mapping (BGMRIM, 1991). In addition, exposures of NCC and I-type signatures of these rocks indicate an Andean-style continen- Archean basement rocks are unusual, as illustrated by geological maps tal arc (Zhang et al., 2007a,b), but Zhou and Wang (2012) proposed a in areas to the north of this fault.

Fig. 3. Geological map of the study region. Compiled on a basis of existing regional geological and geophysical maps, literature cited in the text and ﬁeld observations, the geological map at a scale of 1:1,000,000 (BGMRIM, 1991) being used as starting-point. Numbers along the seismic proﬁle are CMPs. S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39 29

2.2. Tectonic units in the Southern Orogen Fault (e.g., Xiao et al., 2003, Figs. 2 and 3), which is poorly exposed and is mainly inferred from aeromagnetic and gravity anomaly maps The Southern Orogen is considered to reﬂect Paleozoic growth of the (BGMRIM, 1991) and lineaments on remote sensing images (Li, 2012). NCC (Jian et al., 2008). It lies on the northern side of the Chifeng fault The Bainaimiao arc (Jian et al., 2008; Tang, 1990, 1992; Xiao et al., and is composed of two tectonic belts, namely the Bainaimiao arc 2003) contains three major litho-tectonic assemblages that have (belt) in the south and the Ondor Sum subduction–accretion complex been well dated recently by the SHRIMP zircon U–Pb method (Zhang (belt) in the north. These two belts are separated by the Xar Moron et al., 2013): (1) a weakly metamorphosed volcanic and sedimentary

(a)

(b)

(c)

(d)

Fig. 4. Geological profiles referred to in the interpretation of the seismic profile. CMPs: common middle points in seismic profile. (a) Erenhot profile, (this study), (b) Baiyanbolidao profile (Xu et al., 2013), (c) Ondor Sum profile (modified from Shi et al., 2013; Xiao et al., 2003), (d) Kangbao–Huailai profile (compiled after Wang et al., 2013; Zhou and Wang, 2012), (e) Mandula profile (simplified from Xiao et al., 2003), (f) Kalaqinqi–Sihetang profile (simplified from Xiao et al., 2003), (g) Hegenshan–Ongniud profile (simplified from Lu and Xia, 1993; Xiao et al., 2003). 30 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

Fig. 4 (continued). assemblage of calc-alkaline basalt, andesite, rhyolite (474 ± 7 Ma) and occurred in the early Paleozoic. This earlier fabric is overlain unconform- dacite (453 ± 7 Ma, 436 ± 9 Ma); (2) deformed migmatitic silliman- ably by Devonian–Carboniferous strata that, in turn, are cut by a later ite paragneiss (462 ± 11 Ma) and plagioclase–hornblende gneiss north-dipping thrusting fault (Shi et al., 2013, Fig. 4c). In the Ulan valley (437 ± 5 Ma), and metadiorite (438 ± 2 Ma); (3) undeformed or near Ondor Sum, three litho-tectonic assemblages were juxtaposed in a weakly foliated diorite–granodiorite plutons (419 ± 10 Ma); an unde- north-dipping thrust stack (Xiao et al., 2003), in which sheared pillow formed pegmatite dike cutting the gneiss assemblage has an age of lava and thrust-imbricated chert and pelagic strata occur in the structural 411 ± 8 Ma. These isotopic ages are in good agreement with geological lower position, with folded and thrusted arc lavas in the middle, observations that the arc complex is overlain by late Silurian strata and thrusted mylonitic high-pressure metamorphic rocks containing (BGMRIM, 1991). glaucophane and phengite at the top of the tectonic sequence (Xiao Outcrops in the Ondor Sum belt are rare, but ophiolitic rocks exposed et al., 2003, 2009). separately at Tulinkai and Linxi (Fig. 2, Jian et al., 2008; Tang, 1992; Wang and Liu, 1986; Xiao et al., 2003) are convincing evidence of the existence of a subduction zone in this region. The age of the ophiolites in the 2.3. Tectonic units in the Northern Orogen Tulinkai area is well constrained by SHRIMP zircon ages for a tonalite (490.1 ± 7.1 Ma) and a metagabbro (479.6 ± 2.4 Ma) (Jian et al., The Northern Orogen is considered to reflect the growth of the 2008), suggesting that the ophiolite complex is contemporaneous with southern Mongolia. Mongolia occupies a large part of the Altaids be- the Bainaimiao arc. The seismic profile passed through the Ondor Sum tween the NCC and the Siberia craton. Its geology can be subdivided region where the subduction–accretion complex is well documented into southern and northern parts, separated by the Main Mongolian (Shi et al., 2013; Tang, 1992; Wang and Liu, 1986; Xiao et al., 2003). Struc- lineament (Badarch et al., 2002; Tomurtogoo et al., 2005; Wilhem tural mapping suggests two major phrases of regional deformation. The et al., 2012). The northern part is predominately composed of early kinematics of the earlier phrase of deformation indicate top-to-the-NW Paleozoic orogenic belts and a considerable area of Precambrian massifs, folding and thrusting and suggest that southeast-directed subduction whereas the southern part mainly consists of late Paleozoic accreted S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39 31 belts with some Precambrian rocks present (Badarch et al., 2002; related and post-collisional, respectively (Chen et al., 2000, 2009). The Demoux et al., 2009; Wang et al., 2005; Windley et al., 2007). structures in this belt are dominated by a north-dipping thrust system In the area where the seismic profile passed through, there are three (Xiao et al., 2003; Xu et al., 2013). The belt is bounded by the Xilinhot tectonic belts, namely Uliastai, Hegenshan, and Baolidao belts, from fault in the south and the Erenhot Fault in the north (Fig. 2), respectively. north to south (Figs. 2 and 3). The major tectonic character of these Both faults are inferred, based on aeromagnetic and gravity anomaly units is summarized below. mapping (BGMRIM, 1991) and lineaments on remote sensing images (Li, 2012). Geological profile “b” crosses near Xilinhot (Fig. 2) and depicts 2.3.1. Uliastai active continental margin a multiple, predominantly north-dipping thrust system (Fig. 4b, Xu et al., It is reported that the development of this belt is based on a 2013). Precambrian–Cambrian passive margin (Badarch et al., 2002). An active The Hutag Uul terrane in southern Mongolia is likely the continua- continental margin may have appeared for the first time in the Ordovi- tion of the Baolidao belt (Badarch et al., 2002). It contains three litholog- cian and diachronically developed in the western part. The Devonian is ical assemblages. One is a Precambrian metamorphic complex of dominated by a basalt, andesite and pyroclastic succession, and this, in gneiss, schist, migmatite, marble, quartzite, stromatolitic limestone turn, was intruded by Carboniferous arc-type granitoid plutons. It is and quartzite. The second assemblage consists mainly of Devonian to commonly agreed that the subduction zone dipped northwards (Xiao Carboniferous lava, tuff and volcaniclastic rocks. The third assemblage et al., 2003). is composed of subduction-related plutons, including tonalite, diorite and granodiorite. However, ages of these rocks are poorly constrained. 2.3.2. Hegenshan belt This belt contains numerous outcrops of mafic–ultramafic com- 2.4. Solonker suture zone plexes that were previously interpreted as ophiolitic rocks (Miao et al., 2008; Nozaka and Liu, 2002; Robinson et al., 1999; Xiao et al., 2003, The Solonker suture is marked by a narrow belt between the 2009). However, a different tectonic origin and new ages of the Xilinhot and Linxi faults (Fig. 2). This belt was also named Erdaojing Hegenshan mafic–ultramafic assemblages have recently been reported accretion complex by Xiao et al. (2003). Our seismic profile passes by Jian et al. (2012). These authors recognized two lithologic belts through the central segment of this belt that is completely covered by Ce- with significantly different ages. Lherzolite-dominant assemblages in nozoic strata, whereas the western and eastern segments are well ex- the north have early Carboniferous ages (ca.354–333 Ma), whereas posed (Figs. 2 and 3). In its western segment near the China–Mongolia harzburgite-dominant assemblages in the south have Early Cretaceous border, ophiolitic fragments are exposed around Solonker (and in the ages (ca. 142–125 Ma). Jian et al. (2012) questioned the previously pub- Sulinheer Mts.), consisting of serpentinite, dunite and gabbro. Based on lished zircon U–Pb ages and suggested that the mafic–ultramafic SHRIMP U–Pb dating and geochemical analyses, Jian et al. (2010) pro- magmas all formed in the mantle and were emplaced at crustal levels posed a Permian arc–trench system in this area with subduction towards during two periods of extension that occurred between periods of com- the south. The preserved tectonic record includes pre-subduction exten- pression. Field mapping depicts a post-Jurassic thrust system dipping to sion (ca. 299–290 Ma), initial subduction (ca. 294–280 Ma), ridge–trench both the north and south across the Hegenshan region (Wang, 1996; collision (ca. 281–273 Ma) and slab break-off (ca. 255–248 Ma). Xiao et al., 2003). In the western part of this belt, the outcrops consist There is a paleontological and paleogeographic boundary along the mainly of Permian and Mesozoic granites. They intruded into Paleozoic Solonker suture zone (Deng et al., 2009; Huang, 1980, 1993; Shi, strata in which north-vergent thrusting and folding were observed 2006; Wang et al., 2005). A Silurian Tuvaella brachiopod fauna is widely across the Erenhot Fault (Figs. 2 and 4a). distributed north of this boundary but does not occur south of the boundary (Rong and Zhang, 1982; Rong et al., 1995). In the early 2.3.3. Baolidao belt Permian, the northern region was characterized by a cold water boreal This belt is well exposed between Sonid Zuoqi and Xilinhot. The fauna in the ocean and a temperate Angara flora on land, whereas the tectonic setting of the Xilinhot gneiss complex, likely being the contin- southern region was characterized by a tropic–subtropic Cathaysia uation of the Hutag Uul metamorphic complex, is still uncertain. Instead fauna in the ocean and flora on land. In the middle Permian, these of the interpretation as a Precambrian basement of a microcontinental faunas began to mix. Immigration, intrusion and mixture between the block (Xu et al., 2013), Chen et al. (2009) inferred the Xilinhot complex Angara and Cathaysia floras widely occurred in the Late Permian. It to represent metamorphic fore-arc sediments. Two rock types were was reported that the proportion of intruders decreased with distance dated by the SHRIMP U–Pb zircon method (Chen et al., 2000, 2009), a from the boundary (Fig. 5, Deng et al., 2009). Paleontologists favor the gabbro-diorite sample from the deformed Baolidao arc yielded an age interpretation that the oceans that once separated the Cathaysian of 310 ± 5 Ma, and a sample of the undeformed Halatu granite yielded bioprovince from the Angaran bioprovince closed during the Permian an age of 234 ± 7 Ma. These ages were interpreted as subduction- (Deng et al., 2009; Shi, 2006; Wang et al., 2005).

Fig. 5. Distribution of latest Permian ﬂora in northern China, showing mixture between the Angara and Cathaysia ﬂoras (after Deng et al., 2009). 32 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

Fig. 6. Published paleomagnetic poles from Europe (EUR), Siberia (SIB), Tarim (TAR), Kazakhstan (KAZ), Mongolia (MON), Inner Mongolia (INM) and the North China craton (NCC). Data selection after Zhao et al. (1990) and Li et al. (2012). Gray image in the equal-area projections shows present position of the NCC.

Available paleomagnetic data (Chen et al., 1997; Cocks and Torsvik, comparison. A electronic version single-sheet Fig. 7 providing a higher 2007; Li et al., 2012; Pisarevsky et al., 2006; Pruner, 1987, 1992; Van resolution seismic profile can also be found in Appendix A. der Voo, 1993; Wu, 1988; Zhao et al., 1990) provide an independent There are three types of seismic features along the profile, strong re- constraint for the tectonic evolution of this region. Paleomagnetic flectors and reflector stacks in the crust, transparent regions in the crust poles from the NCC, Mongolia and Siberia are distinctly separate in the and mantle, and areas with moderate reflectivity. Our geological inter- Carboniferous (Fig. 6) and earlier (Zhang et al., 2000, 2006, 2012a). pretation was based on comparing the seismic images with the surface Permian poles from the NCC, Uliastai belt and Mongolia are close but geology and on theoretical analysis. In the southernmost part of the pro- are still distinguishable (Fig. 6). These include two newly obtained file, between CMPs 1 and 3000, the Huailai basin can only be traced at a poles from well dated and correlated earliest Permian formations on very shallow level of the crust, and the Mohorovicic discontinuity each side of the Solonker boundary (Li et al., 2012), suggesting that (Moho) is somewhat visible, but the reflection signal in most parts of the NCC and its accretionary terranes collided with the Altaid terranes the crust is too faint to be interpreted so far. We thus ignore this seg- during or soon after the Permian. There are no enough reliable data ment in our further discussion. The basic observations from this profile for Triassic. But the late Jurassic data (Fig. 6) indicate that the united are described below. NCC and southern Mongolia had already joined the Siberia continent and had become a coherent part of the Eurasian continent. 4.1. Mohorovicic discontinuity

The Mohorovicic discontinuity, or the Moho, is marked by strong 3. Seismic data acquisition and data processing reflections in the seismic reflection profile (“Mh” in Fig. 7). It also serves as the boundary between the strongly reflective lower crust and a rela- The seismic reflection data were collected using the CMP (common tively transparent mantle (Cook, 2002; Cook et al., 2010; Mints et al., midpoint, or common depth point, CDP) method, based on recording 2009). The Moho is fairly continuous and flat in most parts of the profile near-vertical seismic reflections. The shot depth was 25 m; the shot at an average depth that requires a two-way travel time (Twt) around size was 24 kg with a 250 m nominal shot interval. In addition, 96 kg ~14.5 s. This observation is in good agreement with the interpretation charges were set off every 1 km, and 1 ton shots were placed at inter- of refraction data coincidently recorded along the same profile vals of 50 km as part of the accompanying wide-angle reflection and re- (Li et al., 2013b) and is basically consistent with the refraction Moho fraction profile (Li et al., 2013b). A Sercel 408 XL recording system and depths compiled from regional deep seismic sounding (DSS) data 2000 strings of SM-24 geophones were deployed at a spacing of 50 m (Li et al., 2006). for 24 kg with 600 traces and 96 kg with 720 traces, shot in the middle. One important phenomenon is that the Moho cuts off most reflective Recording was at a 2 ms sample interval for a total of 30 s. fabrics in crust (Fig. 7b), suggesting that it may have been tectonically Standard oil-industry software packages were used for data process- reformed. ing. The pre-stack processing stream included crooked-line binning, refraction and tomographic statics, static corrections for wave-field con- 4.2. Transparent zones in the crust tinuation, true-amplitude recovery, frequency analysis, filter-parameter tests, surface-consistent de-convolution, high-precision Radon trans- Numerous seismically transparent zones in the upper part of the form, detailed velocity analyses, residual statics corrections and NMO crust are distinct from the reflective background of the entire profile stack. An iterative procedure was followed to obtain the optimal param- (“Gr” in Fig. 7b). We interpret these as undeformed magmatic bodies, eters for stacking and post-stack-noise attenuation. most likely late Permian and Mesozoic granitoid batholiths (for those with large scale and irregular shape) or plutons (for those at small 4. Results of the seismic profiling scale and oval- or lens-shape). The undeformed granitoids and plutons are widely distributed in the study region and have been interpreted The final seismic image is shown in three sections in Fig. 7. The upper as post-collisional intrusions (Fig. 3, Chen et al., 2000; Tong et al., image in each section is a geological cross-section (Fig. 7a) that was 2010; Wu et al., 2002). Because of their relatively homogeneous appear- compiled based on geological maps at a scale of 1:200,000, our field ob- ance in the seismic profile, non-deformed granitoids and intrusive com- servations, the literature reviewed above, and the structural profiles in plexes commonly show transparent images (e.g., Cook et al., 2004; Fig. 4. We marked our most important observations in Fig. 7b. Because Dong et al., 2013a; Hammer et al., 2010; Mints et al., 2009). This inter- the seismic image is cut into three sections to fit the page size, we cite pretation is confirmed by matching granite outcrops and transparent the CMPs (common middle points) as the location markers in the areas in the seismic profile (Fig. 7b). The well matching segments in- discussion below. The uninterpreted image is displayed in Fig. 7c for clude the Uliastai and Hegenshan belts in the northernmost part of the S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39 33 profile (CMPs 19000–24400), the Bainaimiao belt (CMPs 8400–12400) to other reflection images of Precambrian crust, such as Karelia (Mints and the northern NCC region (near CMPs 7000, 5200). For regions that et al., 2009) and the Canadian shield (Cook et al., 2004). are totally covered, recovered samples from numerous boreholes The strong reflective boundaries (“T”,inFig. 7b) may represent support our interpretations (e.g., that at CMP 19000). major thrust faults or ductile shear zones between the tectonic sheets. At least two of these interpreted major ductile shear zones can be traced 4.3. Crustal reflective fabrics to their outcrops. One is the Kangbao shear zone (near CMP 8750 of the seismic profile) that was documented in detail by Wang et al. (2013). The seismic pattern of the crustal reflective fabric varies significantly This E–W striking ductile shear zone extends over 200 km near the along the profile from south to north. In the southern portion of the pro- Chifeng Fault (around latitude N42°), with a width of up to 3 km. In file, the crust contains several large north-dipping reflector stacks. Each the outcrops, the shear zone cuts through Precambrian gneiss, schist, stack is composed of parallel or near-parallel reflectors of relatively sedimentary rock and Carboniferous granodiorite. It consists of short extent (“LR” in Fig. 7b) and is separated by very strong reflective mylonite in which the foliation dips north at angles between 45° and boundaries (“T” in Fig. 7b) from other stacks. The reflector stacks may 60°. All kinematic criteria, including S–C fabrics, inclined and recumbent represent tectonic sheets that were thrust on top of each other towards folds and sigma-type rotated porphyroclasts, demonstrate a top-to-the- the south during the last crustal-scale deformation event. They are south sense of shear (Wang et al., 2013). Furthermore, Wang et al. squeezed together and form a crustal wedge that becomes deeper and (2013) dated syndeformation minerals and suggested that this shear thinner towards the north, with a tip reaching the area beneath the zone was formed at ~270 Ma. Another fault is the Chicheng ductile Solonker suture zone and merging into the Moho. These reflections shear zone, that is exposed near CMP 3000. It appears in the seismic are cut off by the Moho, indicating that the Moho is a younger tectonic image, extending north and down through the entire crust before boundary rather than the original floor decollement of this thrust pack- being cut by the Moho near CMP 12000. Geological evidence shows age. At some localities in the southernmost segment of the profile, the that the Chicheng shear zone also represents a south-vergent fold- reflector stacks seem to extend upwards into the shallow crust which and-thrust zone (Wang et al., 2013). However, synkinematic muscovite corresponds to outcrops of metamorphic Precambrian rocks of the in this belt yielded a 40Ar–39Ar age of ~230 Ma that was interpreted as NCC (Fig. 7b, CMPs 3000–8400). We thus interpret this crustal wedge the time of deformation (Wang et al., 2013). This is significantly youn- as part of the deeper crust of the NCC. The short reflections within ger than the deformation age of the Kangbao shear zone. Between the each reflector stack likely represent gneissic banding and schistosity in Kangbao and Chicheng shear zones there is another north-dipping the Precambrian basement rocks. This seismic pattern is comparable shear zone near CMP 4400. It is likely the western continuation of the

Fig. 7. (a) Geological profile compiled on the basis of geological profiles in Fig. 4 (legend colors the same as in Fig. 3), for more structural readings see Fig. 4; (b) interpretation annotated profile. B—Mesozoic and Cenozoic basin in the shallow crust, Bs—basaltic sills, Cr—crocodile structure, Gr—granitoid batholith or pluton, red cross representing age b~270 Ma, black cross representing age N~270 Ma; LR—crustal reflector that extends into lower crust, Mh—Moho, MR—mantle reflector, T—major north-dipping thrust fault, t—south-dipping thrust, UR—reflector limited to upper crust. (c) Processed seismic profile. Approximate depth was estimated using an average velocity of 6 km/s. Numbers on the top of the seismic images are CMPs. 34 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

Fig. 7 (continued).

Longhua ductile shear zone (Figs. 2 and 4d), whose deformation age accretionary complex, which are conventionally named “Ondor Sum was determined as ~250 Ma (Wang et al., 2013). Group” (between CMPs 14700 and 12200). The Chifeng fault is not clear in the upper crust in the seismic image. In the northern part of the profile (Fig. 7b, CMPs 25926–15200), A Mesozoic granite pluton probably obliterated it in the location where crustal reflectors are more complex. The Linxi Fault (at CMP 15800) is our profile crosses. As mentioned above, in the profile near longitude marked as a north-dipping reflector that is truncated by a sub- 110°E, this fault is depicted as a north-dipping thrust fault (Fig. 4e, horizontal layer (“Bs” in Fig. 7b) near the base of crust. A series of after Xiao et al., 2003). This interpretation is consistent with many south-dipping reflectors is visible in the upper and middle crust, be- geological maps in central Inner Mongolia (BGMRIM, 1991). We thus tween CMPs 20400 and 17200, and these reflectors, in turn, are truncat- speculate that a major reflector, north of the Kangbao ductile shear ed by the Linxi Fault in the south and are cut by interpreted granitoid zone, is the continuation of the Chifeng fault in the middle and lower bodies beneath the Baolidao and Hegenshan belts. Another series of crust (Fig. 7b). south-dipping reflectors occurs beneath the Bayanhonggeer area, at In the central part of the profile, the Bainaimiao belt seems to repre- the northern end of the profile. These reflectors again are truncated by sent a large tectonic sheet sandwiched between the crustal wedge of the north-dipping reflectors in the south and cut by granite in the north. NCC underlying it in south and the Ondor Sum belt overlying it in north The Xilinhot Fault is not visible in our seismic image, but is clearly (Fig. 7b). The southern and northern boundary faults of the Bainaimiao depicted as a north-dipping thrust fault in a structural profile compiled belt can be traced into the surface and correspond to the Chifeng and previously (profile “g” in Figs. 2, 4g, after Xiao et al., 2003). Xu et al. Xar Moron faults, respectively. According to surface structural mapping, (2013) mapped a south-vergent thrust zone in the south of Sonid both are north-dipping thrust faults (Figs. 2 and 4c, e, and g). Zuoqi (profile “b” in Figs. 2, and 4b) where the Xilinhot Fault may be Crustal reflectors beneath the Ondor Sum accretionary belt and the located. Their work indicates that pre-Devonian thrust faults were Solonker suture zone are characterized by a compressional structural superimposed by post-Permian thrust fault, but both are south- style. Between CMPs 18000 and 14800, diverging reflectors (named vergent thrust systems. The Erenhot Fault is well traced from the surface crocodile reflections by DEKORP Research Group et al., 1990) are im- (CMP 20800) into the deep crust. It is truncated by a sub-horizontal aged in the middle crust (“Cr” in Fig. 7b), indicating crustal shortening reflection (“Bs” in Fig. 7b) near the base of crust. in this area. A similar seismic pattern is common beneath the Variscan These horizontal or sub-horizontal layers near the base of crust are orogenic belt of Europe, and a recent example was observed in the an interesting seismic feature in the northern segment of the profile. Tianshan–Tarim reflection profile in western China (Gao et al., 2013). Their reflective character is obviously different to that of layered reflec- In addition, there are some short, curved strong reflectors in the tions in the lower crust of the NCC. Their geometry is like sills. These upper crust of the Ondor Sum belt (“UR” in Fig. 7b). These match the layers are parallel or sub-parallel to the Moho and truncate the regional outcrops of the folded Paleozoic strata and the deformed subduction– deep faults such as the Erenhot and Linxi Faults, indicating that they S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39 35

Fig. 7 (continued). have a younger age. Compared with the transparent granitoid batholith orogen appears to be bivergent with a center approximately at the in the upper crust in this area, these layers probably represent mafic– Solonker suture zone. ultramafic sills derived from mantle. We realize that this speculation The southern portion likely represents a foreland fold-and-thrust is weak because there is no surface geological evidence, but we come belt. Form the Kangbao ductile shear zone to the south, the deformation back to this question in the Discussion section. age of the shear zones becomes younger, from late Permian to late Triassic (Wang et al., 2013, Figs. 2, 4dand7b). This age pattern may sug- 4.4. Mantle reflectors gest that the north-dipping thrust system propagated cratonwards, exhibiting a protracted Himalayan-type thrust system. This thrust- A few mantle reflectors were also observed. A north-dipping reflec- and-ductile shear zone system cuts late Carboniferous granitic plutons, tor group is visible beneath the Moho near the northern end of the pro- is overlain unconformably by latest Early Jurassic strata and is file, north to the CMP 23600 (“MR” in Fig. 7b). These are compatible superimposed by the Late Jurassic–Cretaceous Yanshan thrusts that with north-dipping reflectors in the lower crust beneath the Uliastai have an opposite vergence (Davis et al., 2001; Wang et al., 2011; Zhao, belt and are more evident in a short, parallel profile some 60 km to 1990). This suggests that the pre-middle Jurassic south-directed the east that is still in a data processing stage. Another group of mantle thrust-and-fold system formed during collision and subsequent conver- reflectors was observed in the southern segment of the profile, between gence in the post-collisional stage. The Bainaimiao arc was overthrust CMPs 6900 and 4400. However, these are south-dipping, short and faint, onto the NCC crustal wedge as a tectonic slice and was, in turn, obviously different from the crustal reflectors in this region. overthrust by the Ondor Sum accretionary complex (Fig. 8). This archi- Compared with mantle reflectors reported from other seismic reflec- tecture indicates that considerable crustal shortening and thickening tion profiles worldwide (Abramovitz et al., 1997; Balling, 2000; Calvert has occurred when these tectonic fragments were squeezed together. et al., 1995; Hammer et al., 2010; Warner et al., 1996), we think the This interpretation does not contradict tectonic models suggesting mantle reflectors in our profile may be remnants of oceanic crust and south-directed Paleozoic subduction. The seismic profile is only a snap- represent a relict subduction zone, or they may reveal sinking lower shot of the tectonic history. If the south-dipping mantle reflectors be- crustal fragments from a delamination procession in this region. neath the NCC (Fig. 7b, between CMPs 6800 and 4400) are remnants of subduction, they may be significantly older than the upper crustal 5. Discussion north-dipping fold-and-thrust system. In this case, the north-dipping fold-and-thrust system probably represents the structure of collisional The salient north-dipping thrust system is apparent in the southern secondary vergence, as depicted in Fig. 15B of Xiao et al. (2003). portion of the seismic profile, yet numerous south-dipping reflectors are In the northern portion of the profile, some syncollisional structures observed in the northern portion. Therefore, in a broad view (Fig. 8), the may have been obliterated by episodic but extensive post-collisional 36 .Zage l etnpyis612 Tectonophysics / al. et Zhang S.

Chagan Obo Erenhot Linxi Xar Moron Chifeng Kangbao Fault Fault Baolidao Belt & Fault Fault Fault Fault SE Uliastai Belt Hegenshan Ordor Sum Belt Bainaimiao Belt North China Craton Solonker suture zone Bayanhonggeer Belt Zhangjiakou Sonid Youqi Xianghuangqi Huade Zhangbei 24400 22800 19600 18000 16400 14000 11600 10000 6800 3600 2400 Huailai 0 0 4 12 h (km) 8 24 12 Dept te Moho 36 Moho 16 –

Two-way time (s) time Two-way 48 40km 26 (2014) 613 20 Possible relict subduction Possible relict subduction 60 Approxima Craton basement Major thrust fault Mesozoic-Cenozoic basin Mesozoic granitoid pluton Paleozoic granitoid pluton Underplating mafic sills 6800 CMP of seismic profile

Fig. 8. Major crustal structures revealed from the SinoProbe deep seismic reflection profileacrosstheSolonkersuturezoneoftheCAOB. – 39 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39 37 magmatic activity (Chen et al., 2000, 2009; Jian et al., 2012; Wu et al., grateful for discussions with Profs. Aimin Xue, Ganqing Jiang, Greg 2002; Zhang et al., 2008). Nevertheless, south-dipping crustal structures Davis, Bei Xu, Bin Chen, Shaofeng Liu, Yu Wang and An Yin, and greatly still remain clear on the seismic image and geological maps (Figs. 4aand appreciate reconstructive comments by Profs. Alfred Kröner and Wenjiao 7b), for example, the Erenhot Fault and south-dipping reflectors near it Xiao. (“t” in Fig. 7b). The Solonker suture zone and the Baolidao belt together appear to represent the central portion of the bivergent collisional Appendix A. Supplementary data orogen. If the mantle reflectors (between CMPs 25200 and 23800) reflect relict northward subduction, the Erenhot Fault zone and many Supplementary data to this article can be found online at http://dx. south-dipping crustal reflectors in this area seem to be the structures doi.org/10.1016/j.tecto.2013.11.035. of collisional secondary vergence, or reflect underthrusting of the Northern Orogen in post-collisional stage (Jian et al., 2010). The overall geometry of the mantle reflectors supports tectonic References models in which the Southern Orogen (Manchurides) experienced Abramovitz, T., Thybo, H., Berthelsen, A., 1997. Proterozoic sutures and terranes in the south-directed subduction whereas the Northern Orogen (part of the southeastern Baltic Shield interpreted from BABEL deep seismic data. Tectonophysics Altaids) underwent north-directed subduction. 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