Tectonophysics 612–613 (2014) 26–39

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Tectonophysics

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Crustal structures revealed from a deep seismic reflection profile acrossthe Solonker suture zone of the Central Asian Orogenic Belt, northernChina: An integrated interpretation

Shihong Zhang a,⁎, Rui Gao b,⁎⁎, Haiyan Li a, Hesheng Hou b, HuaichunWu a, Qiusheng Li b, Ke Yang a, Chao Li a,Wenhui Li b, Jishen Zhang b, Tianshui Yang a, G.R. Keller c, Mian Liu d

a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, Chinab Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, Chinac University of Oklahoma, Norman 73019, USAd University of Missouri, Columbia 48063, USA

⁎ Corresponding author. Tel.: +86 10 82322257; fax: +⁎⁎ Corresponding author. Tel.: +86 10 68999730.

E-mail addresses: shzhang@cugb.edu.cn (S. Zhang), ga

0040-1951/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.tecto.2013.11.035

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 October 2012Received in revised form 4 September 2013Accepted 23 November 2013Available online 4 December 2013

Keywords:Seismic reflectionCrustal structureTectonicsCentral Asian Orogenic BeltSolonker suture zone

The Solonker suture zone is one of themost important tectonic boundaries in the southeastern part of the CentralAsianOrogenic Belt (CAOB). An ~630 km-long reflection seismic profile across this suturewas recently completedby the Chinese SinoProbe Project. The processed seismic data show clear crustal structures and provide newconstraints on the tectonic and crustal evolution models. The Moho is delineated as a relatively flat boundary be-tween a strongly reflective lower crust and a transparent mantle at a depth of ~40–45 km (~14.5 s two-waytravel time), which is in agreement with the refraction data recorded along the same profile. In a broad view,the profile images an orogen that appears bivergent with, and approximately centered on, the Solonker suturezone. The southern portion of this profile is dominated by a crustal-scale, cratonward propagating fold-and-thrust system that formed during the late Permian and Triassic through collision and subsequent convergencein a post-collisional stage. The major thrust faults are truncated by Mesozoic granitoid plutons in the uppercrust and by theMoho at the base of the crust. This geometry suggests that theMohowas 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 Mohoare interpreted as basaltic sills derived from the mantle. Episodic mafic underplating may have occurred in thisregion, giving rise to post-collisional magmatic events and renewal of the Moho. A few mantle reflectors arealso visible. The overall geometry of these mantle reflectors supports the tectonic models that the southernorogen (Manchurides) experienced south-directed subduction and the northern orogen (Altaids) underwentnorth-directed subduction prior to collision along the Solonker suture zone.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Eurasia, the largest continent on the Earth, was formed by multiplephases of continental accretion and collision since the late Neoproterozoic.The Central Asian Orogenic Belt (CAOB) occupies approximately 30% ofthe land area in Asia. It contains a complex geological record of amal-gamated accretionary zones and collisional sutures between the majorcratons, namely Baltica, Siberia, Tarim, and North China (NCC), as wellas numerous tectono-stratigraphic terranes with unknown tectonicaffiliations (variably termed massifs or microcontinental blocks,Fig. 1). This huge tectonic collage has, in turn, beenmodified by youngerdeformations resulting from the closure of theMongol–Okhotsk Ocean,collisions in the Tibetan Plateau, and subduction in the western Pacific

86 10 82321983.

orui@cags.ac.cn (R. Gao).

ghts reserved.

region. Many models, often conflicting, have been proposed to explainthe tectonic evolution of the CAOB (e.g., Chen et al., 2009; Jian et al.,2008, 2010; Kröner et al., 2007, 2013; Li, 2006; Sengör and Natal'in,1996; Sengör et al., 1993; Windley et al., 2007; Xiao et al., 2003, 2009;Xu et al., 2013, among others). Disagreement includes the polarity(ies)of subduction and accretion, the timing and location of collision be-tween the Angaran (or Siberian) and Cathysian tectonic domains, thetiming and position of crustal thickening and thinning, and the propor-tion of juvenile crust versus ancient crust within the CAOB. The solutionto such problems requires a better understanding of deep structure ofthe crust and mantle.

In this paper, we report the new findings on crustal structurerevealed from a deep seismic reflection profile recently completed bythe Chinese SinoProbe Project (Dong et al., 2013b). The NW–SE profilecrosses a large region that is widely considered to contain the terminallate Paleozoic collisional suture between the Archean-floored NCCand more northerly terranes of the CAOB (Figs. 1 and 2). The high-

SouthChinaIndia

Tarim

NorthChina

Siberia Siberia

Major Cratons

Terranes

Central AsiaOrogenic Belt(CAOB)

Central China and Tethyan Orogens

Mongol-Okhotsk and W.Pacific Orogens0 500km

Beijing

SolonkerSuture

CAOB

SinoProbeSeismicprofileFig.2

Fig. 1. Tectonic positions of the Solonker suture and the SinoProbe reflection seismic profile.

27S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

resolution seismic images acquired along this profile provide importantnew deep structural constraints on tectonic and crustal evolutionmodels of this region.

2. Geological background

From the Huailai Basin near Beijing, our ~630 km seismic profilecontinues northwestward via Zhangjiakou in northern Hebei Province,crosses the poorly exposed grassland of Inner Mongolia, and ends atthe China–Mongolia boundary (Fig. 1). This region contains many

112 1

110 112 1

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40

110

25926

24000

20000

18000

14000

10

16000

22000

12000

North China Craton (NCC)

Tectonic units in the Northern Orogen (Altaids)

Tectonic units in the SouthernOrogen (Manchurides)

Solonker suture zone

North China Craton (NCC)

Ophiolite,Mafic-Ultramafic complex

Ductile shear zone

Seismic profile with CMP1

Chagan Obo

Sonid Zuoqi

Erenhot

Sonid YouqiOndor S

Huade

Jining

Shangyi

Z

ZHohhot

Bayan Obo

SolonkerMandula

Bainaimiao

0 50 100km

44

Fig. 2. Tectonic subdivision of the study region (modified from Xiao et al., 2003). Deformatio(1) Kangbao ductile shear zone, ~270 Ma; (2) Longhua ductile shear zone, ~250 Ma; (3) Chicheprofiles labeled (a) to (g) are depicted in Fig. 4.

tectonic elements defined by their geological characteristics and historyand by crustal compositions (Figs. 2, 3). The Solonker suture zone is gen-erally considered to be themost important tectonic element crossed bythe SinoProbe traverse, but its location has been a controversial topic formany years. The suturewas named by Sengör et al. (1993) as separatingtwo orogens (Fig. 2). The Southern Orogen (Jian et al., 2008), namedManchurides by Sengör et al. (1993), is composed of displaced frag-ments of the Paleozoic northern activemargin of the NCC. The NorthernOrogen, being part of the Altaids (Sengör et al., 1993), is composed oftectonic fragments with affiliations to the Angaran (or Siberian) craton.

14 118

11614 118

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(1)

(2)

(3)

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Chifeng Fault

1

000

8000

4000

2000

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Chagan Obo Fault

Erenhot Fault

Xilinhot Fault

Linxi Fault

Hegenshan Belt

Solonker Suture Zone

Bainaimiao Belt

Uliastai Belt

Baolidao Belt

Ondor Sum Belt Xar Moron Fault

um

Kangbao

hangbei

hangjiakou

Huailai

ChichengChengde

Longhua

WeichangChifeng

Linxi

Xilinhot

Dongwuqi

Beijing

n ages for numbered ductile shear zones are determined as follows (Wang et al., 2013):ng ductile shear zone, ~230 Ma; (4) unnamed ductile shear zone, ~210 Ma. The geological

28 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

It is widely believed that these two orogens represent coeval subduc-tion–accretion complexes of different polarities in Paleozoic, and, theSolonker suture zone is generally considered to define the final collisionbetween the two orogens (Chen et al., 2000, 2009; Jian et al., 2008, 2010,2012; Sengör and Natal'in, 1996; Sengör et al., 1993; Xiao et al., 2003,2009; Xu et al., 2013).

2.1. The northern NCC

The NCC is one of the oldest Precambrian cratons in theworld. It hasan Archean to Paleoproterozoic metamorphic basement that wascratonized at ~1.85 Ga (Wang et al., 2005; Zhao et al., 2011, and refer-ences herein) and is covered by sedimentary and volcanic successionsranging in age from ~1.78 Ga to Early Triassic (Li et al., 2013a; Luet al., 2008; Su et al., 2008; Wang et al., 2005). In our study region, thebasement rocks of the northern NCC are largely exposed (Fig. 3) andare intruded by igneous rocks resulting frommultiple magmatic events,including the 1.75–1.68 Ga anorthosite–mangerite–alkali granite-rapakivi granite suites (Zhao et al., 2011) in the Yanshan region (northof Beijing, Fig. 3), the 1.6–1.2 Ga Zhaertai–Bayan Obo rift complex(Wang et al., 1991; Zhao et al., 2004, 2011), the ~1.35 Ga mafic dikesand sills (Zhang et al., 2009), the ~1.30 Ga bimodal magmatic rocks(Zhang et al., 2012b) and the contemporaneous A-type granite and gra-nitic porphyry (Shi et al., 2012). TheNCC then experienced a long hiatusin magmatic activity. However, a granite and volcanic belt of lateCarboniferous age (~320–300 Ma) have recently been recognizedalong the northern margin of the NCC. The calc-alkaline geochemicaland I-type signatures of these rocks indicate an Andean-style continen-tal arc (Zhang et al., 2007a,b), but Zhou and Wang (2012) proposed a

Fig. 3.Geological map of the study region. Compiled on a basis of existing regional geological ana scale of 1:1,000,000 (BGMRIM, 1991) being used as starting-point. Numbers along the seism

syntectonic magmatic flow model for the origin of this plutonic belt,based on their field structural, micro-structural, lithological and U–Pbchronological analysis.

The lateMesozoic was a time of decratonization for the eastern NCC.Thiswas likely due, in part, to subduction of the Pacific plate in the EarlyCretaceous, and is manifested by lithospheric thinning, lithosphericmantle modification, extensive intracrustal ductile deformation, andmagmatic activity (Liu et al., 2005, 2012; Zhu et al., 2011, and referencesherein). Northeast-trending extensional basins containing late Mesozo-ic and Cenozoic sedimentary and volcanic strata developed in an evenlarger region in NE Asia (Lin et al., 2013 and references herein). A rela-tionship between the volume of the these strata and the thickness ofthe upper crust has been recognized in northern China, i.e. thicker stratacorresponding to thinner upper crust, and vice versa (Zhang et al.,2011). Widespread regional unconformities and widespread exposuresof granite batholiths (Zhang et al., 2007b; Zhou and Wang, 2012) indi-cate that extensive and deep erosion has occurred in the northern NCC.

The northern boundary of the NCC is defined by the Chifeng fault(Fig. 2). An earlier deep structural section near longitude 110°E (profile“e” in Figs. 2 and 4) depicted the Chifeng fault as north-dipping andseparating the complex CAOB from the NCC (Xiao et al., 2003, Fig. 4e).At Huade on the seismic profile, south-vergent folding, thrusting andductile shear zones occur in pre-Permian granites and strata along theKangbao shear zone near the Chifeng fault (Zhou and Wang, 2012;Wang et al., 2013, Fig. 4d). Continuation of the Chifeng fault beneathMesozoic and younger strata was mainly inferred from aeromagneticanomaly mapping (BGMRIM, 1991). In addition, exposures of NCCArchean basement rocks are unusual, as illustrated by geological mapsin areas to the north of this fault.

d geophysicalmaps, literature cited in the text and field observations, the geological map atic profile are CMPs.

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2.2. Tectonic units in the Southern Orogen

The Southern Orogen is considered to reflect Paleozoic growth of theNCC (Jian et al., 2008). It lies on the northern side of the Chifeng faultand is composed of two tectonic belts, namely the Bainaimiao arc(belt) in the south and the Ondor Sum subduction–accretion complex(belt) in the north. These two belts are separated by the Xar Moron

(a)

(b)

(c)

(d)

Fig. 4.Geological profiles referred to in the interpretation of the seismic profile. CMPs: common(Xu et al., 2013), (c) Ondor Sum profile (modified from Shi et al., 2013; Xiao et al., 2003), ((e) Mandula profile (simplified from Xiao et al., 2003), (f) Kalaqinqi–Sihetang profile (simpli1993; Xiao et al., 2003).

Fault (e.g., Xiao et al., 2003, Figs. 2 and 3), which is poorly exposedand is mainly inferred from aeromagnetic and gravity anomaly maps(BGMRIM, 1991) and lineaments on remote sensing images (Li, 2012).

The Bainaimiao arc (Jian et al., 2008; Tang, 1990, 1992; Xiao et al.,2003) contains three major litho-tectonic assemblages that havebeen well dated recently by the SHRIMP zircon U–Pb method (Zhanget al., 2013): (1) a weakly metamorphosed volcanic and sedimentary

middle points in seismic profile. (a) Erenhot profile, (this study), (b) Baiyanbolidao profiled) Kangbao–Huailai profile (compiled after Wang et al., 2013; Zhou and Wang, 2012),fied from Xiao et al., 2003), (g) Hegenshan–Ongniud profile (simplified from Lu and Xia,

Fig. 4 (continued).

30 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

assemblage of calc-alkaline basalt, andesite, rhyolite (474 ± 7 Ma) anddacite (453 ± 7 Ma, 436 ± 9 Ma); (2) deformed migmatitic silliman-ite paragneiss (462 ± 11 Ma) and plagioclase–hornblende gneiss(437 ± 5 Ma), and metadiorite (438 ± 2 Ma); (3) undeformed orweakly foliated diorite–granodiorite plutons (419 ± 10 Ma); an unde-formed pegmatite dike cutting the gneiss assemblage has an age of411 ± 8 Ma. These isotopic ages are in good agreementwith geologicalobservations that the arc complex is overlain by late Silurian strata(BGMRIM, 1991).

Outcrops in theOndor Sumbelt are rare, but ophiolitic rocks exposedseparately at Tulinkai and Linxi (Fig. 2, Jian et al., 2008; Tang, 1992;Wangand Liu, 1986; Xiao et al., 2003) are convincing evidence of the existenceof a subduction zone in this region. The age of the ophiolites in theTulinkai 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.,2008), suggesting that the ophiolite complex is contemporaneous withthe Bainaimiao arc. The seismic profile passed through the Ondor Sumregion where the subduction–accretion complex is well documented(Shi et al., 2013; Tang, 1992;Wang and Liu, 1986; Xiao et al., 2003). Struc-tural mapping suggests two major phrases of regional deformation. Thekinematics of the earlier phrase of deformation indicate top-to-the-NWfolding and thrusting and suggest that southeast-directed subduction

occurred in the early Paleozoic. This earlier fabric is overlain unconform-ably by Devonian–Carboniferous strata that, in turn, are cut by a laternorth-dipping thrusting fault (Shi et al., 2013, Fig. 4c). In the Ulan valleynear Ondor Sum, three litho-tectonic assemblages were juxtaposed in anorth-dipping thrust stack (Xiao et al., 2003), in which sheared pillowlava and thrust-imbricated chert and pelagic strata occur in the structurallower position, with folded and thrusted arc lavas in the middle,and thrusted mylonitic high-pressure metamorphic rocks containingglaucophane and phengite at the top of the tectonic sequence (Xiaoet al., 2003, 2009).

2.3. Tectonic units in the Northern Orogen

The Northern Orogen is considered to reflect the growth of thesouthern Mongolia. Mongolia occupies a large part of the Altaids be-tween the NCC and the Siberia craton. Its geology can be subdividedinto southern and northern parts, separated by the Main Mongolianlineament (Badarch et al., 2002; Tomurtogoo et al., 2005; Wilhemet al., 2012). The northern part is predominately composed of earlyPaleozoic orogenic belts and a considerable area of Precambrianmassifs,whereas the southern part mainly consists of late Paleozoic accreted

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belts with some Precambrian rocks present (Badarch et al., 2002;Demoux et al., 2009; Wang et al., 2005; Windley et al., 2007).

In the area where the seismic profile passed through, there are threetectonic belts, namely Uliastai, Hegenshan, and Baolidao belts, fromnorth to south (Figs. 2 and 3). The major tectonic character of theseunits is summarized below.

2.3.1. Uliastai active continental marginIt is reported that the development of this belt is based on a

Precambrian–Cambrian passivemargin (Badarch et al., 2002). An activecontinental margin may have appeared for the first time in the Ordovi-cian and diachronically developed in the western part. The Devonian isdominated by a basalt, andesite and pyroclastic succession, and this, inturn, was intruded by Carboniferous arc-type granitoid plutons. It iscommonly agreed that the subduction zone dipped northwards (Xiaoet al., 2003).

2.3.2. Hegenshan beltThis belt contains numerous outcrops of mafic–ultramafic com-

plexes that were previously interpreted as ophiolitic rocks (Miao et al.,2008; Nozaka and Liu, 2002; Robinson et al., 1999; Xiao et al., 2003,2009). However, a different tectonic origin and new ages of theHegenshan mafic–ultramafic assemblages have recently been reportedby Jian et al. (2012). These authors recognized two lithologic beltswith significantly different ages. Lherzolite-dominant assemblages inthe north have early Carboniferous ages (ca.354–333 Ma), whereasharzburgite-dominant assemblages in the south have Early Cretaceousages (ca. 142–125 Ma). Jian et al. (2012) questioned thepreviously pub-lished zircon U–Pb ages and suggested that the mafic–ultramaficmagmas all formed in the mantle and were emplaced at crustal levelsduring two periods of extension that occurred between periods of com-pression. Field mapping depicts a post-Jurassic thrust system dipping toboth the north and south across the Hegenshan region (Wang, 1996;Xiao et al., 2003). In the western part of this belt, the outcrops consistmainly of Permian and Mesozoic granites. They intruded into Paleozoicstrata in which north-vergent thrusting and folding were observedacross the Erenhot Fault (Figs. 2 and 4a).

2.3.3. Baolidao beltThis belt is well exposed between Sonid Zuoqi and Xilinhot. The

tectonic setting of the Xilinhot gneiss complex, likely being the contin-uation of theHutagUulmetamorphic complex, is still uncertain. Insteadof the interpretation as a Precambrian basement of a microcontinentalblock (Xu et al., 2013), Chen et al. (2009) inferred the Xilinhot complexto represent metamorphic fore-arc sediments. Two rock types weredated by the SHRIMP U–Pb zircon method (Chen et al., 2000, 2009), agabbro-diorite sample from the deformed Baolidao arc yielded an ageof 310 ± 5 Ma, and a sample of the undeformed Halatu granite yieldedan age of 234 ± 7 Ma. These ages were interpreted as subduction-

Fig. 5. Distribution of latest Permian flora in northern China, showing mixt

related and post-collisional, respectively (Chen et al., 2000, 2009). Thestructures in this belt are dominated by a north-dipping thrust system(Xiao et al., 2003; Xu et al., 2013). The belt is bounded by the Xilinhotfault in the south and the Erenhot Fault in the north (Fig. 2), respectively.Both faults are inferred, based on aeromagnetic and gravity anomalymapping (BGMRIM, 1991) and lineaments on remote sensing images(Li, 2012). Geological profile “b” crosses near Xilinhot (Fig. 2) and depictsamultiple, predominantly north-dipping thrust system (Fig. 4b, Xu et al.,2013).

The Hutag Uul terrane in southern Mongolia is likely the continua-tion of the Baolidao belt (Badarch et al., 2002). It contains three litholog-ical assemblages. One is a Precambrian metamorphic complex ofgneiss, schist, migmatite, marble, quartzite, stromatolitic limestoneand quartzite. The second assemblage consists mainly of Devonian toCarboniferous lava, tuff and volcaniclastic rocks. The third assemblageis composed of subduction-related plutons, including tonalite, dioriteand granodiorite. However, ages of these rocks are poorly constrained.

2.4. Solonker suture zone

The Solonker suture is marked by a narrow belt between theXilinhot and Linxi faults (Fig. 2). This belt was also named Erdaojingaccretion complex by Xiao et al. (2003). Our seismic profile passesthrough the central segment of this belt that is completely covered by Ce-nozoic strata, whereas the western and eastern segments are well ex-posed (Figs. 2 and 3). In its western segment near the China–Mongoliaborder, ophiolitic fragments are exposed around Solonker (and in theSulinheer Mts.), consisting of serpentinite, dunite and gabbro. Based onSHRIMP U–Pb dating and geochemical analyses, Jian et al. (2010) pro-posed a Permian arc–trench system in this area with subduction towardsthe south. The preserved tectonic record includes pre-subduction exten-sion (ca. 299–290 Ma), initial subduction (ca. 294–280 Ma), ridge–trenchcollision (ca. 281–273 Ma) and slab break-off (ca. 255–248 Ma).

There is a paleontological and paleogeographic boundary along theSolonker suture zone (Deng et al., 2009; Huang, 1980, 1993; Shi,2006;Wang et al., 2005). A Silurian Tuvaella brachiopod fauna is widelydistributed north of this boundary but does not occur south of theboundary (Rong and Zhang, 1982; Rong et al., 1995). In the earlyPermian, the northern region was characterized by a cold water borealfauna in the ocean and a temperate Angara flora on land, whereas thesouthern region was characterized by a tropic–subtropic Cathaysiafauna in the ocean and flora on land. In the middle Permian, thesefaunas began to mix. Immigration, intrusion and mixture between theAngara and Cathaysia floras widely occurred in the Late Permian. Itwas reported that the proportion of intruders decreased with distancefrom the boundary (Fig. 5, Deng et al., 2009). Paleontologists favor theinterpretation that the oceans that once separated the Cathaysianbioprovince from the Angaran bioprovince closed during the Permian(Deng et al., 2009; Shi, 2006; Wang et al., 2005).

ure between the Angara and Cathaysia floras (after Deng et al., 2009).

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). Dataselection after Zhao et al. (1990) and Li et al. (2012). Gray image in the equal-area projections shows present position of the NCC.

32 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

Available paleomagnetic data (Chen et al., 1997; Cocks and Torsvik,2007; Li et al., 2012; Pisarevsky et al., 2006; Pruner, 1987, 1992; Vander Voo, 1993; Wu, 1988; Zhao et al., 1990) provide an independentconstraint for the tectonic evolution of this region. Paleomagneticpoles from the NCC, Mongolia and Siberia are distinctly separate in theCarboniferous (Fig. 6) and earlier (Zhang et al., 2000, 2006, 2012a).Permian poles from the NCC, Uliastai belt and Mongolia are close butare still distinguishable (Fig. 6). These include two newly obtainedpoles from well dated and correlated earliest Permian formations oneach side of the Solonker boundary (Li et al., 2012), suggesting thatthe NCC and its accretionary terranes collided with the Altaid terranesduring or soon after the Permian. There are no enough reliable datafor Triassic. But the late Jurassic data (Fig. 6) indicate that the unitedNCC and southern Mongolia had already joined the Siberia continentand had become a coherent part of the Eurasian continent.

3. Seismic data acquisition and data processing

The seismic reflection data were collected using the CMP (commonmidpoint, or common depth point, CDP) method, based on recordingnear-vertical seismic reflections. The shot depth was 25 m; the shotsize was 24 kg with a 250 m nominal shot interval. In addition, 96 kgcharges were set off every 1 km, and 1 ton shots were placed at inter-vals of 50 kmas part of the accompanyingwide-angle reflection and re-fraction profile (Li et al., 2013b). A Sercel 408 XL recording system and2000 strings of SM-24 geophones were deployed at a spacing of 50 mfor 24 kgwith 600 traces and 96 kgwith 720 traces, shot in the middle.Recording was at a 2 ms sample interval for a total of 30 s.

Standard oil-industry software packageswere used for data process-ing. The pre-stack processing stream included crooked-line binning,refraction and tomographic statics, static corrections forwave-field con-tinuation, true-amplitude recovery, frequency analysis, filter-parametertests, surface-consistent de-convolution, high-precision Radon trans-form, detailed velocity analyses, residual statics corrections and NMOstack. An iterative procedurewas followed to obtain the optimal param-eters for stacking and post-stack-noise attenuation.

4. Results of the seismic profiling

The final seismic image is shown in three sections in Fig. 7. The upperimage in each section is a geological cross-section (Fig. 7a) that wascompiled based on geological maps at a scale of 1:200,000, our field ob-servations, the literature reviewed above, and the structural profiles inFig. 4. We marked our most important observations in Fig. 7b. Becausethe seismic image is cut into three sections to fit the page size, we citethe CMPs (common middle points) as the location markers in thediscussion below. The uninterpreted image is displayed in Fig. 7c for

comparison. A electronic version single-sheet Fig. 7 providing a higherresolution seismic profile can also be found in Appendix A.

There are three types of seismic features along the profile, strong re-flectors and reflector stacks in the crust, transparent regions in the crustand mantle, and areas with moderate reflectivity. Our geological inter-pretation was based on comparing the seismic images with the surfacegeology andon theoretical analysis. In the southernmost part of the pro-file, between CMPs 1 and 3000, the Huailai basin can only be traced at avery shallow level of the crust, and the Mohorovicic discontinuity(Moho) is somewhat visible, but the reflection signal in most parts ofthe crust is too faint to be interpreted so far. We thus ignore this seg-ment in our further discussion. The basic observations from this profileare described below.

4.1. Mohorovicic discontinuity

The Mohorovicic discontinuity, or the Moho, is marked by strongreflections in the seismic reflection profile (“Mh” in Fig. 7). It also servesas the boundary between the strongly reflective lower crust and a rela-tively transparent mantle (Cook, 2002; Cook et al., 2010; Mints et al.,2009). TheMoho is fairly continuous and flat inmost parts of the profileat an average depth that requires a two-way travel time (Twt) around~14.5 s. This observation is in good agreement with the interpretationof refraction data coincidently recorded along the same profile(Li et al., 2013b) and is basically consistent with the refraction Mohodepths compiled from regional deep seismic sounding (DSS) data(Li et al., 2006).

One important phenomenon is that theMoho cuts offmost reflectivefabrics in crust (Fig. 7b), suggesting that it may have been tectonicallyreformed.

4.2. Transparent zones in the crust

Numerous seismically transparent zones in the upper part of thecrust are distinct from the reflective background of the entire profile(“Gr” in Fig. 7b). We interpret these as undeformed magmatic bodies,most likely late Permian and Mesozoic granitoid batholiths (for thosewith large scale and irregular shape) or plutons (for those at smallscale and oval- or lens-shape). The undeformed granitoids and plutonsare widely distributed in the study region and have been interpretedas post-collisional intrusions (Fig. 3, Chen et al., 2000; Tong et al.,2010;Wu et al., 2002). Because of their relatively homogeneous appear-ance in the seismic profile, non-deformed granitoids and intrusive com-plexes commonly show transparent images (e.g., Cook et al., 2004;Dong et al., 2013a; Hammer et al., 2010; Mints et al., 2009). This inter-pretation is confirmed by matching granite outcrops and transparentareas in the seismic profile (Fig. 7b). The well matching segments in-clude the Uliastai and Hegenshan belts in the northernmost part of the

33S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

profile (CMPs 19000–24400), the Bainaimiao belt (CMPs 8400–12400)and the northern NCC region (near CMPs 7000, 5200). For regions thatare totally covered, recovered samples from numerous boreholessupport our interpretations (e.g., that at CMP 19000).

4.3. Crustal reflective fabrics

The seismic pattern of the crustal reflective fabric varies significantlyalong the profile from south to north. In the southern portion of the pro-file, the crust contains several large north-dipping reflector stacks. Eachstack is composed of parallel or near-parallel reflectors of relativelyshort extent (“LR” in Fig. 7b) and is separated by very strong reflectiveboundaries (“T” in Fig. 7b) from other stacks. The reflector stacks mayrepresent tectonic sheets that were thrust on top of each other towardsthe south during the last crustal-scale deformation event. They aresqueezed together and form a crustal wedge that becomes deeper andthinner towards the north, with a tip reaching the area beneath theSolonker suture zone and merging into the Moho. These reflectionsare cut off by the Moho, indicating that the Moho is a younger tectonicboundary rather than the original floor decollement of this thrust pack-age. At some localities in the southernmost segment of the profile, thereflector stacks seem to extend upwards into the shallow crust whichcorresponds to outcrops of metamorphic Precambrian rocks of theNCC (Fig. 7b, CMPs 3000–8400). We thus interpret this crustal wedgeas part of the deeper crust of the NCC. The short reflections withineach reflector stack likely represent gneissic banding and schistosity inthe Precambrian basement rocks. This seismic pattern is comparable

Fig. 7. (a) Geological profile compiled on the basis of geological profiles in Fig. 4 (legend colorsprofile. B—Mesozoic and Cenozoic basin in the shallow crust, Bs—basaltic sills, Cr—crocodile strurepresenting age N~270 Ma; LR—crustal reflector that extends into lower crust, Mh—Moho, MR—limited to upper crust. (c) Processed seismic profile. Approximate depth was estimated using an

to other reflection images of Precambrian crust, such as Karelia (Mintset al., 2009) and the Canadian shield (Cook et al., 2004).

The strong reflective boundaries (“T”, in Fig. 7b) may representmajor thrust faults or ductile shear zones between the tectonic sheets.At least two of these interpretedmajor ductile shear zones can be tracedto their outcrops. One is the Kangbao shear zone (near CMP 8750 of theseismic profile) that was documented in detail by Wang et al. (2013).This E–W striking ductile shear zone extends over 200 km near theChifeng Fault (around latitude N42°), with a width of up to 3 km. Inthe outcrops, the shear zone cuts through Precambrian gneiss, schist,sedimentary rock and Carboniferous granodiorite. It consists ofmylonite in which the foliation dips north at angles between 45° and60°. All kinematic criteria, including S–C fabrics, inclined and recumbentfolds and sigma-type rotated porphyroclasts, demonstrate a top-to-the-south sense of shear (Wang et al., 2013). Furthermore, Wang et al.(2013) dated syndeformation minerals and suggested that this shearzone was formed at ~270 Ma. Another fault is the Chicheng ductileshear zone, that is exposed near CMP 3000. It appears in the seismicimage, extending north and down through the entire crust beforebeing cut by the Moho near CMP 12000. Geological evidence showsthat the Chicheng shear zone also represents a south-vergent fold-and-thrust zone (Wang et al., 2013). However, synkinematic muscovitein this belt yielded a 40Ar–39Ar age of ~230 Ma that was interpreted asthe time of deformation (Wang et al., 2013). This is significantly youn-ger than the deformation age of the Kangbao shear zone. Between theKangbao and Chicheng shear zones there is another north-dippingshear zone near CMP 4400. It is likely the western continuation of the

the same as in Fig. 3), for more structural readings see Fig. 4; (b) interpretation annotatedcture, Gr—granitoid batholith or pluton, red cross representing age b~270 Ma, black crossmantle reflector, T—major north-dipping thrust fault, t—south-dipping thrust, UR—reflectoraverage velocity of 6 km/s. Numbers on the top of the seismic images are CMPs.

Fig. 7 (continued).

34 S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

Longhua ductile shear zone (Figs. 2 and 4d), whose deformation agewas determined as ~250 Ma (Wang et al., 2013).

The Chifeng fault is not clear in the upper crust in the seismic image.A Mesozoic granite pluton probably obliterated it in the location whereour profile crosses. As mentioned above, in the profile near longitude110°E, this fault is depicted as a north-dipping thrust fault (Fig. 4e,after Xiao et al., 2003). This interpretation is consistent with manygeological maps in central Inner Mongolia (BGMRIM, 1991). We thusspeculate that a major reflector, north of the Kangbao ductile shearzone, is the continuation of the Chifeng fault in the middle and lowercrust (Fig. 7b).

In the central part of the profile, the Bainaimiao belt seems to repre-sent a large tectonic sheet sandwiched between the crustalwedge of theNCC underlying it in south and the Ondor Sum belt overlying it in north(Fig. 7b). The southern and northern boundary faults of the Bainaimiaobelt can be traced into the surface and correspond to the Chifeng andXarMoron faults, respectively. According to surface structural mapping,both are north-dipping thrust faults (Figs. 2 and 4c, e, and g).

Crustal reflectors beneath the Ondor Sum accretionary belt and theSolonker suture zone are characterized by a compressional structuralstyle. Between CMPs 18000 and 14800, diverging reflectors (namedcrocodile reflections by DEKORP Research Group et al., 1990) are im-aged in the middle crust (“Cr” in Fig. 7b), indicating crustal shorteningin this area. A similar seismic pattern is common beneath the Variscanorogenic belt of Europe, and a recent example was observed in theTianshan–Tarim reflection profile in western China (Gao et al., 2013).In addition, there are some short, curved strong reflectors in theupper crust of the Ondor Sum belt (“UR” in Fig. 7b). These match theoutcrops of the folded Paleozoic strata and the deformed subduction–

accretionary complex, which are conventionally named “Ondor SumGroup” (between CMPs 14700 and 12200).

In the northern part of the profile (Fig. 7b, CMPs 25926–15200),crustal reflectors are more complex. The Linxi Fault (at CMP 15800) ismarked as a north-dipping reflector that is truncated by a sub-horizontal layer (“Bs” in Fig. 7b) near the base of crust. A series ofsouth-dipping reflectors is visible in the upper and middle crust, be-tween CMPs 20400 and 17200, and these reflectors, in turn, are truncat-ed by the Linxi Fault in the south and are cut by interpreted granitoidbodies beneath the Baolidao and Hegenshan belts. Another series ofsouth-dipping reflectors occurs beneath the Bayanhonggeer area, atthe northern end of the profile. These reflectors again are truncated bynorth-dipping reflectors in the south and cut by granite in the north.

The Xilinhot Fault is not visible in our seismic image, but is clearlydepicted as a north-dipping thrust fault in a structural profile compiledpreviously (profile “g” in Figs. 2, 4g, after Xiao et al., 2003). Xu et al.(2013) mapped a south-vergent thrust zone in the south of SonidZuoqi (profile “b” in Figs. 2, and 4b) where the Xilinhot Fault may belocated. Their work indicates that pre-Devonian thrust faults weresuperimposed by post-Permian thrust fault, but both are south-vergent thrust systems. The Erenhot Fault iswell traced from the surface(CMP 20800) into the deep crust. It is truncated by a sub-horizontalreflection (“Bs” in Fig. 7b) near the base of crust.

These horizontal or sub-horizontal layers near the base of crust arean interesting seismic feature in the northern segment of the profile.Their reflective character is obviously different to that of layered reflec-tions in the lower crust of the NCC. Their geometry is like sills. Theselayers are parallel or sub-parallel to the Moho and truncate the regionaldeep faults such as the Erenhot and Linxi Faults, indicating that they

Fig. 7 (continued).

35S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

have a younger age. Compared with the transparent granitoid batholithin the upper crust in this area, these layers probably represent mafic–ultramafic sills derived from mantle. We realize that this speculationis weak because there is no surface geological evidence, but we comeback to this question in the Discussion section.

4.4. Mantle reflectors

A few mantle reflectors were also observed. A north-dipping reflec-tor group is visible beneath theMoho near the northern end of the pro-file, north to the CMP 23600 (“MR” in Fig. 7b). These are compatiblewith north-dipping reflectors in the lower crust beneath the Uliastaibelt and are more evident in a short, parallel profile some 60 km tothe east that is still in a data processing stage. Another group of mantlereflectors was observed in the southern segment of the profile, betweenCMPs 6900 and 4400. However, these are south-dipping, short and faint,obviously different from the crustal reflectors in this region.

Comparedwithmantle reflectors reported fromother seismic reflec-tion profiles worldwide (Abramovitz et al., 1997; Balling, 2000; Calvertet al., 1995; Hammer et al., 2010; Warner et al., 1996), we think themantle reflectors in our profile may be remnants of oceanic crust andrepresent a relict subduction zone, or they may reveal sinking lowercrustal fragments from a delamination procession in this region.

5. Discussion

The salient north-dipping thrust system is apparent in the southernportion of the seismic profile, yet numerous south-dipping reflectors areobserved in the northern portion. Therefore, in a broad view (Fig. 8), the

orogen appears to be bivergent with a center approximately at theSolonker suture zone.

The southern portion likely represents a foreland fold-and-thrustbelt. Form the Kangbao ductile shear zone to the south, the deformationage of the shear zones becomes younger, from late Permian to lateTriassic (Wang et al., 2013, Figs. 2, 4d and 7b). This age patternmay sug-gest that the north-dipping thrust system propagated cratonwards,exhibiting a protracted Himalayan-type thrust system. This thrust-and-ductile shear zone system cuts late Carboniferous granitic plutons,is overlain unconformably by latest Early Jurassic strata and issuperimposed by the Late Jurassic–Cretaceous Yanshan thrusts thathave an opposite vergence (Davis et al., 2001; Wang et al., 2011; Zhao,1990). This suggests that the pre-middle Jurassic south-directedthrust-and-fold system formed during collision and subsequent conver-gence in the post-collisional stage. The Bainaimiao arc was overthrustonto the NCC crustal wedge as a tectonic slice and was, in turn,overthrust by the Ondor Sum accretionary complex (Fig. 8). This archi-tecture indicates that considerable crustal shortening and thickeninghas occurred when these tectonic fragments were squeezed together.This interpretation does not contradict tectonic models suggestingsouth-directed Paleozoic subduction. The seismic profile is only a snap-shot of the tectonic history. If the south-dipping mantle reflectors be-neath the NCC (Fig. 7b, between CMPs 6800 and 4400) are remnantsof subduction, they may be significantly older than the upper crustalnorth-dipping fold-and-thrust system. In this case, the north-dippingfold-and-thrust system probably represents the structure of collisionalsecondary vergence, as depicted in Fig. 15B of Xiao et al. (2003).

In the northern portion of the profile, some syncollisional structuresmay have been obliterated by episodic but extensive post-collisional

SE

ZhangjiakouXianghuangqiSonid Youqi

Bayanhonggeer

Chagan Obo

Fault

Erenhot

FaultLinxi

Fault

Xar Moron

Fault

Chifeng

Fault

Huade Zhangbei Huailai

Uliastai Belt HegenshanBelt

Ordor Sum Belt Bainaimiao Belt North China CratonBaolidao Belt &Solonker suture zone

Kangbao

Fault

0

12

24

36

48

0

4

8

12

16

20 60

24400 22800 19600 18000 16400 14000 6800 3600 2400

Possible relict subduction Possible relict subduction

Moho

1000011600

mk04

Craton basement Paleozoic granitoid plutonMesozoic granitoid pluton Underplating mafic sillsMesozoic-Cenozoic basinMajor thrust fault CMP of seismic profile6800

Ap

pro

xim

ate

De

pth

(km

)

Tw

o-w

ay

tim

e (s

)

Moho

Fig. 8. Major crustal structures revealed from the SinoProbe deep seismic reflection profile across the Solonker suture zone of the CAOB.

36S.Zhang

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–613(2014)

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37S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39

magmatic activity (Chen et al., 2000, 2009; Jian et al., 2012; Wu et al.,2002; Zhang et al., 2008). Nevertheless, south-dipping crustal structuresstill remain clear on the seismic image and geologicalmaps (Figs. 4a and7b), for example, the Erenhot Fault and south-dipping reflectors near it(“t” in Fig. 7b). The Solonker suture zone and the Baolidao belt togetherappear to represent the central portion of the bivergent collisionalorogen. If the mantle reflectors (between CMPs 25200 and 23800) re-flect relict northward subduction, the Erenhot Fault zone and manysouth-dipping crustal reflectors in this area seem to be the structuresof collisional secondary vergence, or reflect underthrusting of theNorthern Orogen in post-collisional stage (Jian et al., 2010).

The overall geometry of the mantle reflectors supports tectonicmodels in which the Southern Orogen (Manchurides) experiencedsouth-directed subduction whereas the Northern Orogen (part of theAltaids) underwent north-directed subduction. The two subduction–accretion systems finally collided, leading to a bivergent crustalstructure as illustrated in Fig. 8.

On geological maps, granitoid rocks of varying age are distributed inCentral Asia, but few were mapped in a vertical dimension in crust. Weattempt to recognize their vertical dimension from the seismic profile. Itis apparent that most granitoid bodies in this region truncate theinterpreted fold and thrust structures (Fig. 7b), suggesting that theseplutons formed during a post-collisional event. Episodic extensionalmagmatic events were reported in this region, such as early Carbonifer-ous and early Cretaceous mantle melting episodes in the Hegenshanarea (Jian et al., 2012), late Permian bimodal volcanism in the Xilinhotarea (Zhang et al., 2008), a Permian alkaline granite in the Uliastai beltand a Triassic alkaline granite along the northern margin of the NCC(Tong et al., 2010), and a post-collisional granitoid in the Sonid Zuoqiarea (Chen et al., 2000). These examples suggest that considerablemantle-derived melts and heat entered the crust. In this case, repeatedepisodes of post-collisional magmatic underplating may have renewedthe Moho. The horizontal and sub-horizontal layers immediately over-lying the Moho are likely basaltic sills derived from the mantle.

6. Conclusions

(1) The seismic reflection profile reveals a fairly continuous and flatMoho at a depth that requires ~14.5 s two-way travel time,which is in agreement with the refraction data recorded alongthe same profile. The Moho truncates most crustal reflections,suggesting that it is a relatively new feature.

(2) In a broad view, the profile shows a bivergent orogen, approxi-mately centered on the Solonker suture. The southern portionof this profile is characterized by a crustal-scale cratonwardpropagating fold-and-thrust system that formed during the latePermian and Triassic by collision and continued convergence inpost-collisional setting. In the northern portion of the profile,although partially obliterated by post-collisional magmaticbodies, major south-dipping fold-and-thrust structures are stilltraceable.

(3) Bands of layered reflectors immediately overlying the Moho areindicative of basaltic sills derived from themantle. Episodicmag-matic underplating may have occurred in this region during thepost-collisional evolution and may have renewed the Moho.

(4) The overall geometry of the mantle reflectors, if they are reallyrelicts of subduction, supports tectonic models that the southernorogen (Manchurides) experiences south-directed subductionwhereas the northern orogen (Altaids) reflects north-directedsubduction prior to collision along the Solonker suture zone.

Acknowledgments

This workwas jointly supported by SinoProbe Project 02, the 973 Pro-gram (2013CB429800), the China Geological Survey (1212011120754),and NSFC projects 40921062, 40974035, and 41104060. The authors are

grateful for discussions with Profs. Aimin Xue, Ganqing Jiang, GregDavis, Bei Xu, Bin Chen, Shaofeng Liu, Yu Wang and An Yin, and greatlyappreciate reconstructive comments by Profs. Alfred Kröner andWenjiaoXiao.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tecto.2013.11.035.

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