Tectonophysics 612–613 (2014) 26–39
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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,⁎, Rui Gao b,⁎⁎, Haiyan Li a, Hesheng Hou b, Huaichun Wu 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, China
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
b
a r t i c l e
i n f o
Article history:
Received 10 October 2012
Received in revised form 4 September 2013
Accepted 23 November 2013
Available online 4 December 2013
Keywords:
Seismic reflection
Crustal structure
Tectonics
Central Asian Orogenic Belt
Solonker suture zone
a b s t r a c t
The Solonker suture zone is one of the most important tectonic boundaries in the southeastern part of the Central
Asian Orogenic Belt (CAOB). An ~630 km-long reflection seismic profile across this suture was recently completed
by the Chinese SinoProbe Project. The processed seismic data show clear crustal structures and provide new
constraints on the tectonic and crustal evolution models. The Moho is delineated as a relatively flat boundary between 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,
the profile images an orogen that appears bivergent with, and approximately centered on, the Solonker suture
zone. The southern portion of this profile is dominated by a crustal-scale, cratonward propagating fold-andthrust system that formed during the late Permian and Triassic through collision and subsequent convergence
in a post-collisional stage. The major thrust faults are truncated by Mesozoic granitoid plutons in the upper
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
Eurasia, the largest continent on the Earth, was formed by multiple
phases of continental accretion and collision since the late Neoproterozoic.
The Central Asian Orogenic Belt (CAOB) occupies approximately 30% of
the land area in Asia. It contains a complex geological record of amalgamated accretionary zones and collisional sutures between the major
cratons, namely Baltica, Siberia, Tarim, and North China (NCC), as well
as numerous tectono-stratigraphic terranes with unknown tectonic
affiliations (variably termed massifs or microcontinental blocks,
Fig. 1). This huge tectonic collage has, in turn, been modified by younger
deformations resulting from the closure of the Mongol–Okhotsk Ocean,
collisions in the Tibetan Plateau, and subduction in the western Pacific
⁎ Corresponding author. Tel.: +86 10 82322257; fax: +86 10 82321983.
⁎⁎ Corresponding author. Tel.: +86 10 68999730.
E-mail addresses: [email protected] (S. Zhang), [email protected] (R. Gao).
0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.tecto.2013.11.035
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.,
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 between the Angaran (or Siberian) and Cathysian tectonic domains, the
timing and position of crustal thickening and thinning, and the proportion of juvenile crust versus ancient crust within the CAOB. The solution
to such problems requires a better understanding of deep structure of
the crust and mantle.
In this paper, we report the new findings on crustal structure
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
late Paleozoic collisional suture between the Archean-floored NCC
and more northerly terranes of the CAOB (Figs. 1 and 2). The high-
S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39
27
Siberia
Siberia
CAOB
SinoProbe
Seismicprofile
Fig.2
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.
tectonic elements defined by their geological characteristics and history
and by crustal compositions (Figs. 2, 3). The Solonker suture zone is generally considered to be the most important tectonic element crossed by
the SinoProbe traverse, but its location has been a controversial topic for
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
Manchurides by Sengör et al. (1993), is composed of displaced fragments of the Paleozoic northern active margin of the NCC. The Northern
Orogen, being part of the Altaids (Sengör et al., 1993), is composed of
tectonic fragments with affiliations to the Angaran (or Siberian) craton.
resolution seismic images acquired along this profile provide important
new deep structural constraints on tectonic and crustal evolution
models of this region.
2. Geological background
From the Huailai Basin near Beijing, our ~ 630 km seismic profile
continues northwestward via Zhangjiakou in northern Hebei Province,
crosses the poorly exposed grassland of Inner Mongolia, and ends at
the China–Mongolia boundary (Fig. 1). This region contains many
110
112
l t118
116
Fau
Dongwuqi
bo
n O
a
g
a
Ch
114
Tectonic units in the Northern Orogen (Altaids)
Solonker suture zone
Tectonic units in the Southern
Orogen (Manchurides)
U li a
25926
24000
Heg
North China Craton (NCC)
Chagan Obo
22000
Ophiolite,
Mafic-Ultramafic complex
Sonid Zuoqi
Ductile shear zone
20000
Seismic profile with CMP Erenhot
18000
44
1
0
Solonker
50
sta i
100km
Mandula
ensh
S
Ondo
rS
B a in a
10000
Huade
Bayan Obo
8000
Jining
im ia o
o
inh
oron
112
114
F a u lt
F a u lt
(1)
Chifeng Fault
Longhua
(2)
Chengde
(3)
Chicheng
Huailai
(4)
1
110
lt
au
tF
44
Chifeng
Weichang
Kangbao
2000
40
ult
L in x i
Xar M
B e lt
6000
Zhangbei
North China Craton (NCC)
Linxi
e lt
um B
Shangyi
4000
Zhangjiakou
Hohhot
a
t F
Xil
Xilinhot
o B e lt
14000
Ondor Sum
Bainaimiao
ho
e lt
an B
one
tu re Z
er Su
o lo n k
12000
42
n
Ere
B a o lid a
16000
Sonid Youqi
B e lt
40
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 subduction–accretion complexes of different polarities in Paleozoic, and, the
Solonker suture zone is generally considered to define the final collision
between 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 the world. It has
an Archean to Paleoproterozoic metamorphic basement that was
cratonized at ~1.85 Ga (Wang et al., 2005; Zhao et al., 2011, and references herein) and is covered by sedimentary and volcanic successions
ranging in age from ~ 1.78 Ga to Early Triassic (Li et al., 2013a; Lu
et al., 2008; Su et al., 2008; Wang et al., 2005). In our study region, the
basement rocks of the northern NCC are largely exposed (Fig. 3) and
are intruded by igneous rocks resulting from multiple magmatic events,
including the 1.75–1.68 Ga anorthosite–mangerite–alkali graniterapakivi granite suites (Zhao et al., 2011) in the Yanshan region (north
of 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 dikes
and sills (Zhang et al., 2009), the ~ 1.30 Ga bimodal magmatic rocks
(Zhang et al., 2012b) and the contemporaneous A-type granite and granitic porphyry (Shi et al., 2012). The NCC then experienced a long hiatus
in magmatic activity. However, a granite and volcanic belt of late
Carboniferous age (~ 320–300 Ma) have recently been recognized
along the northern margin of the NCC. The calc-alkaline geochemical
and I-type signatures of these rocks indicate an Andean-style continental arc (Zhang et al., 2007a,b), but Zhou and Wang (2012) proposed a
syntectonic magmatic flow model for the origin of this plutonic belt,
based on their field structural, micro-structural, lithological and U–Pb
chronological analysis.
The late Mesozoic was a time of decratonization for the eastern NCC.
This was likely due, in part, to subduction of the Pacific plate in the Early
Cretaceous, and is manifested by lithospheric thinning, lithospheric
mantle modification, extensive intracrustal ductile deformation, and
magmatic activity (Liu et al., 2005, 2012; Zhu et al., 2011, and references
herein). Northeast-trending extensional basins containing late Mesozoic and Cenozoic sedimentary and volcanic strata developed in an even
larger region in NE Asia (Lin et al., 2013 and references herein). A relationship between the volume of the these strata and the thickness of
the upper crust has been recognized in northern China, i.e. thicker strata
corresponding to thinner upper crust, and vice versa (Zhang et al.,
2011). Widespread regional unconformities and widespread exposures
of granite batholiths (Zhang et al., 2007b; Zhou and Wang, 2012) indicate 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 and
separating the complex CAOB from the NCC (Xiao et al., 2003, Fig. 4e).
At Huade on the seismic profile, south-vergent folding, thrusting and
ductile shear zones occur in pre-Permian granites and strata along the
Kangbao shear zone near the Chifeng fault (Zhou and Wang, 2012;
Wang et al., 2013, Fig. 4d). Continuation of the Chifeng fault beneath
Mesozoic and younger strata was mainly inferred from aeromagnetic
anomaly mapping (BGMRIM, 1991). In addition, exposures of NCC
Archean basement rocks are unusual, as illustrated by geological maps
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
2.2. Tectonic units in the Southern Orogen
The Southern Orogen is considered to reflect Paleozoic growth of the
NCC (Jian et al., 2008). It lies on the northern side of the Chifeng fault
and 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
29
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
(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 have
been well dated recently by the SHRIMP zircon U–Pb method (Zhang
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
dacite (453 ± 7 Ma, 436 ± 9 Ma); (2) deformed migmatitic sillimanite paragneiss (462 ± 11 Ma) and plagioclase–hornblende gneiss
(437 ± 5 Ma), and metadiorite (438 ± 2 Ma); (3) undeformed or
weakly foliated diorite–granodiorite plutons (419 ± 10 Ma); an undeformed pegmatite dike cutting the gneiss assemblage has an age of
411 ± 8 Ma. These isotopic ages are in good agreement with geological
observations that the arc complex is overlain by late Silurian strata
(BGMRIM, 1991).
Outcrops in the Ondor Sum belt are rare, but ophiolitic rocks exposed
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
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.,
2008), suggesting that the ophiolite complex is contemporaneous with
the Bainaimiao arc. The seismic profile passed through the Ondor Sum
region where the subduction–accretion complex is well documented
(Shi et al., 2013; Tang, 1992; Wang and Liu, 1986; Xiao et al., 2003). Structural mapping suggests two major phrases of regional deformation. The
kinematics of the earlier phrase of deformation indicate top-to-the-NW
folding and thrusting and suggest that southeast-directed subduction
occurred in the early Paleozoic. This earlier fabric is overlain unconformably by Devonian–Carboniferous strata that, in turn, are cut by a later
north-dipping thrusting fault (Shi et al., 2013, Fig. 4c). In the Ulan valley
near Ondor Sum, three litho-tectonic assemblages were juxtaposed in a
north-dipping thrust stack (Xiao et al., 2003), in which sheared pillow
lava and thrust-imbricated chert and pelagic strata occur in the structural
lower position, with folded and thrusted arc lavas in the middle,
and thrusted mylonitic high-pressure metamorphic rocks containing
glaucophane and phengite at the top of the tectonic sequence (Xiao
et al., 2003, 2009).
2.3. Tectonic units in the Northern Orogen
The Northern Orogen is considered to reflect the growth of the
southern Mongolia. Mongolia occupies a large part of the Altaids between the NCC and the Siberia craton. Its geology can be subdivided
into southern and northern parts, separated by the Main Mongolian
lineament (Badarch et al., 2002; Tomurtogoo et al., 2005; Wilhem
et al., 2012). The northern part is predominately composed of early
Paleozoic orogenic belts and a considerable area of Precambrian massifs,
whereas the southern part mainly consists of late Paleozoic accreted
S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39
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 three
tectonic belts, namely Uliastai, Hegenshan, and Baolidao belts, from
north to south (Figs. 2 and 3). The major tectonic character of these
units is summarized below.
2.3.1. Uliastai active continental margin
It is reported that the development of this belt is based on a
Precambrian–Cambrian passive margin (Badarch et al., 2002). An active
continental margin may have appeared for the first time in the Ordovician and diachronically developed in the western part. The Devonian is
dominated by a basalt, andesite and pyroclastic succession, and this, in
turn, was intruded by Carboniferous arc-type granitoid plutons. It is
commonly agreed that the subduction zone dipped northwards (Xiao
et al., 2003).
2.3.2. Hegenshan belt
This belt contains numerous outcrops of mafic–ultramafic complexes 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 the
Hegenshan mafic–ultramafic assemblages have recently been reported
by Jian et al. (2012). These authors recognized two lithologic belts
with significantly different ages. Lherzolite-dominant assemblages in
the north have early Carboniferous ages (ca.354–333 Ma), whereas
harzburgite-dominant assemblages in the south have Early Cretaceous
ages (ca. 142–125 Ma). Jian et al. (2012) questioned the previously published zircon U–Pb ages and suggested that the mafic–ultramafic
magmas all formed in the mantle and were emplaced at crustal levels
during two periods of extension that occurred between periods of compression. Field mapping depicts a post-Jurassic thrust system dipping to
both the north and south across the Hegenshan region (Wang, 1996;
Xiao et al., 2003). In the western part of this belt, the outcrops consist
mainly of Permian and Mesozoic granites. They intruded into Paleozoic
strata in which north-vergent thrusting and folding were observed
across the Erenhot Fault (Figs. 2 and 4a).
2.3.3. Baolidao belt
This belt is well exposed between Sonid Zuoqi and Xilinhot. The
tectonic setting of the Xilinhot gneiss complex, likely being the continuation of the Hutag Uul metamorphic complex, is still uncertain. Instead
of the interpretation as a Precambrian basement of a microcontinental
block (Xu et al., 2013), Chen et al. (2009) inferred the Xilinhot complex
to represent metamorphic fore-arc sediments. Two rock types were
dated by the SHRIMP U–Pb zircon method (Chen et al., 2000, 2009), a
gabbro-diorite sample from the deformed Baolidao arc yielded an age
of 310 ± 5 Ma, and a sample of the undeformed Halatu granite yielded
an age of 234 ± 7 Ma. These ages were interpreted as subduction-
31
related and post-collisional, respectively (Chen et al., 2000, 2009). The
structures 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 Xilinhot
fault in the south and the Erenhot Fault in the north (Fig. 2), respectively.
Both faults are inferred, based on aeromagnetic and gravity anomaly
mapping (BGMRIM, 1991) and lineaments on remote sensing images
(Li, 2012). Geological profile “b” crosses near Xilinhot (Fig. 2) and depicts
a multiple, predominantly north-dipping thrust system (Fig. 4b, Xu et al.,
2013).
The Hutag Uul terrane in southern Mongolia is likely the continuation of the Baolidao belt (Badarch et al., 2002). It contains three lithological assemblages. One is a Precambrian metamorphic complex of
gneiss, schist, migmatite, marble, quartzite, stromatolitic limestone
and quartzite. The second assemblage consists mainly of Devonian to
Carboniferous lava, tuff and volcaniclastic rocks. The third assemblage
is composed of subduction-related plutons, including tonalite, diorite
and granodiorite. However, ages of these rocks are poorly constrained.
2.4. Solonker suture zone
The Solonker suture is marked by a narrow belt between the
Xilinhot and Linxi faults (Fig. 2). This belt was also named Erdaojing
accretion complex by Xiao et al. (2003). Our seismic profile passes
through the central segment of this belt that is completely covered by Cenozoic strata, whereas the western and eastern segments are well exposed (Figs. 2 and 3). In its western segment near the China–Mongolia
border, ophiolitic fragments are exposed around Solonker (and in the
Sulinheer Mts.), consisting of serpentinite, dunite and gabbro. Based on
SHRIMP U–Pb dating and geochemical analyses, Jian et al. (2010) proposed a Permian arc–trench system in this area with subduction towards
the south. The preserved tectonic record includes pre-subduction extension (ca. 299–290 Ma), initial subduction (ca. 294–280 Ma), ridge–trench
collision (ca. 281–273 Ma) and slab break-off (ca. 255–248 Ma).
There is a paleontological and paleogeographic boundary along the
Solonker suture zone (Deng et al., 2009; Huang, 1980, 1993; Shi,
2006; Wang et al., 2005). A Silurian Tuvaella brachiopod fauna is widely
distributed north of this boundary but does not occur south of the
boundary (Rong and Zhang, 1982; Rong et al., 1995). In the early
Permian, the northern region was characterized by a cold water boreal
fauna in the ocean and a temperate Angara flora on land, whereas the
southern region was characterized by a tropic–subtropic Cathaysia
fauna in the ocean and flora on land. In the middle Permian, these
faunas began to mix. Immigration, intrusion and mixture between the
Angara and Cathaysia floras widely occurred in the Late Permian. It
was reported that the proportion of intruders decreased with distance
from the boundary (Fig. 5, Deng et al., 2009). Paleontologists favor the
interpretation that the oceans that once separated the Cathaysian
bioprovince from the Angaran bioprovince closed during the Permian
(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,
2007; Li et al., 2012; Pisarevsky et al., 2006; Pruner, 1987, 1992; Van
der Voo, 1993; Wu, 1988; Zhao et al., 1990) provide an independent
constraint for the tectonic evolution of this region. Paleomagnetic
poles from the NCC, Mongolia and Siberia are distinctly separate in the
Carboniferous (Fig. 6) and earlier (Zhang et al., 2000, 2006, 2012a).
Permian poles from the NCC, Uliastai belt and Mongolia are close but
are still distinguishable (Fig. 6). These include two newly obtained
poles from well dated and correlated earliest Permian formations on
each side of the Solonker boundary (Li et al., 2012), suggesting that
the NCC and its accretionary terranes collided with the Altaid terranes
during or soon after the Permian. There are no enough reliable data
for Triassic. But the late Jurassic data (Fig. 6) indicate that the united
NCC and southern Mongolia had already joined the Siberia continent
and 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 (common
midpoint, or common depth point, CDP) method, based on recording
near-vertical seismic reflections. The shot depth was 25 m; the shot
size was 24 kg with a 250 m nominal shot interval. In addition, 96 kg
charges were set off every 1 km, and 1 ton shots were placed at intervals of 50 km as part of the accompanying wide-angle reflection and refraction profile (Li et al., 2013b). A Sercel 408 XL recording system and
2000 strings of SM-24 geophones were deployed at a spacing of 50 m
for 24 kg with 600 traces and 96 kg with 720 traces, shot in the middle.
Recording was at a 2 ms sample interval for a total of 30 s.
Standard oil-industry software packages were used for data processing. The pre-stack processing stream included crooked-line binning,
refraction and tomographic statics, static corrections for wave-field continuation, true-amplitude recovery, frequency analysis, filter-parameter
tests, surface-consistent de-convolution, high-precision Radon transform, detailed velocity analyses, residual statics corrections and NMO
stack. An iterative procedure was followed to obtain the optimal parameters 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 upper
image in each section is a geological cross-section (Fig. 7a) that was
compiled based on geological maps at a scale of 1:200,000, our field observations, the literature reviewed above, and the structural profiles in
Fig. 4. We marked our most important observations in Fig. 7b. Because
the seismic image is cut into three sections to fit the page size, we cite
the CMPs (common middle points) as the location markers in the
discussion below. The uninterpreted image is displayed in Fig. 7c for
comparison. A electronic version single-sheet Fig. 7 providing a higher
resolution seismic profile can also be found in Appendix A.
There are three types of seismic features along the profile, strong reflectors and reflector stacks in the crust, transparent regions in the crust
and mantle, and areas with moderate reflectivity. Our geological interpretation was based on comparing the seismic images with the surface
geology and on theoretical analysis. In the southernmost part of the profile, between CMPs 1 and 3000, the Huailai basin can only be traced at a
very shallow level of the crust, and the Mohorovicic discontinuity
(Moho) is somewhat visible, but the reflection signal in most parts of
the crust is too faint to be interpreted so far. We thus ignore this segment in our further discussion. The basic observations from this profile
are described below.
4.1. Mohorovicic discontinuity
The Mohorovicic discontinuity, or the Moho, is marked by strong
reflections in the seismic reflection profile (“Mh” in Fig. 7). It also serves
as the boundary between the strongly reflective lower crust and a relatively transparent mantle (Cook, 2002; Cook et al., 2010; Mints et al.,
2009). The Moho is fairly continuous and flat in most parts of the profile
at an average depth that requires a two-way travel time (Twt) around
~14.5 s. This observation is in good agreement with the interpretation
of refraction data coincidently recorded along the same profile
(Li et al., 2013b) and is basically consistent with the refraction Moho
depths compiled from regional deep seismic sounding (DSS) data
(Li et al., 2006).
One important phenomenon is that the Moho cuts off most reflective
fabrics in crust (Fig. 7b), suggesting that it may have been tectonically
reformed.
4.2. Transparent zones in the crust
Numerous seismically transparent zones in the upper part of the
crust 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 those
with large scale and irregular shape) or plutons (for those at small
scale and oval- or lens-shape). The undeformed granitoids and plutons
are widely distributed in the study region and have been interpreted
as post-collisional intrusions (Fig. 3, Chen et al., 2000; Tong et al.,
2010; Wu et al., 2002). Because of their relatively homogeneous appearance in the seismic profile, non-deformed granitoids and intrusive complexes commonly show transparent images (e.g., Cook et al., 2004;
Dong et al., 2013a; Hammer et al., 2010; Mints et al., 2009). This interpretation is confirmed by matching granite outcrops and transparent
areas in the seismic profile (Fig. 7b). The well matching segments include the Uliastai and Hegenshan belts in the northernmost part of the
S. 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 that
are totally covered, recovered samples from numerous boreholes
support our interpretations (e.g., that at CMP 19000).
4.3. Crustal reflective fabrics
The seismic pattern of the crustal reflective fabric varies significantly
along the profile from south to north. In the southern portion of the profile, the crust contains several large north-dipping reflector stacks. Each
stack is composed of parallel or near-parallel reflectors of relatively
short extent (“LR” in Fig. 7b) and is separated by very strong reflective
boundaries (“T” in Fig. 7b) from other stacks. The reflector stacks may
represent tectonic sheets that were thrust on top of each other towards
the south during the last crustal-scale deformation event. They are
squeezed together and form a crustal wedge that becomes deeper and
thinner towards the north, with a tip reaching the area beneath the
Solonker suture zone and merging into the Moho. These reflections
are cut off by the Moho, indicating that the Moho is a younger tectonic
boundary rather than the original floor decollement of this thrust package. At some localities in the southernmost segment of the profile, the
reflector stacks seem to extend upwards into the shallow crust which
corresponds to outcrops of metamorphic Precambrian rocks of the
NCC (Fig. 7b, CMPs 3000–8400). We thus interpret this crustal wedge
as part of the deeper crust of the NCC. The short reflections within
each reflector stack likely represent gneissic banding and schistosity in
the Precambrian basement rocks. This seismic pattern is comparable
33
to other reflection images of Precambrian crust, such as Karelia (Mints
et al., 2009) and the Canadian shield (Cook et al., 2004).
The strong reflective boundaries (“T”, in Fig. 7b) may represent
major thrust faults or ductile shear zones between the tectonic sheets.
At least two of these interpreted major ductile shear zones can be traced
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).
This E–W striking ductile shear zone extends over 200 km near the
Chifeng Fault (around latitude N42°), with a width of up to 3 km. In
the outcrops, the shear zone cuts through Precambrian gneiss, schist,
sedimentary rock and Carboniferous granodiorite. It consists of
mylonite in which the foliation dips north at angles between 45° and
60°. All kinematic criteria, including S–C fabrics, inclined and recumbent
folds and sigma-type rotated porphyroclasts, demonstrate a top-to-thesouth sense of shear (Wang et al., 2013). Furthermore, Wang et al.
(2013) dated syndeformation minerals and suggested that this shear
zone was formed at ~ 270 Ma. Another fault is the Chicheng ductile
shear zone, that is exposed near CMP 3000. It appears in the seismic
image, extending north and down through the entire crust before
being cut by the Moho near CMP 12000. Geological evidence shows
that the Chicheng shear zone also represents a south-vergent foldand-thrust zone (Wang et al., 2013). However, synkinematic muscovite
in this belt yielded a 40Ar–39Ar age of ~230 Ma that was interpreted as
the time of deformation (Wang et al., 2013). This is significantly younger than the deformation age of the Kangbao shear zone. Between the
Kangbao and Chicheng shear zones there is another north-dipping
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
was 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 where
our profile crosses. As mentioned above, in the profile near longitude
110°E, this fault is depicted as a north-dipping thrust fault (Fig. 4e,
after Xiao et al., 2003). This interpretation is consistent with many
geological maps in central Inner Mongolia (BGMRIM, 1991). We thus
speculate that a major reflector, north of the Kangbao ductile shear
zone, is the continuation of the Chifeng fault in the middle and lower
crust (Fig. 7b).
In the central part of the profile, the Bainaimiao belt seems to represent a large tectonic sheet sandwiched between the crustal wedge of the
NCC underlying it in south and the Ondor Sum belt overlying it in north
(Fig. 7b). The southern and northern boundary faults of the Bainaimiao
belt can be traced into the surface and correspond to the Chifeng and
Xar Moron 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 the
Solonker suture zone are characterized by a compressional structural
style. Between CMPs 18000 and 14800, diverging reflectors (named
crocodile reflections by DEKORP Research Group et al., 1990) are imaged in the middle crust (“Cr” in Fig. 7b), indicating crustal shortening
in this area. A similar seismic pattern is common beneath the Variscan
orogenic belt of Europe, and a recent example was observed in the
Tianshan–Tarim reflection profile in western China (Gao et al., 2013).
In addition, there are some short, curved strong reflectors in the
upper crust of the Ondor Sum belt (“UR” in Fig. 7b). These match the
outcrops of the folded Paleozoic strata and the deformed subduction–
accretionary complex, which are conventionally named “Ondor Sum
Group” (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) is
marked as a north-dipping reflector that is truncated by a subhorizontal layer (“Bs” in Fig. 7b) near the base of crust. A series of
south-dipping reflectors is visible in the upper and middle crust, between CMPs 20400 and 17200, and these reflectors, in turn, are truncated by the Linxi Fault in the south and are cut by interpreted granitoid
bodies beneath the Baolidao and Hegenshan belts. Another series of
south-dipping reflectors occurs beneath the Bayanhonggeer area, at
the northern end of the profile. These reflectors again are truncated by
north-dipping reflectors in the south and cut by granite in the north.
The Xilinhot Fault is not visible in our seismic image, but is clearly
depicted as a north-dipping thrust fault in a structural profile compiled
previously (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 Sonid
Zuoqi (profile “b” in Figs. 2, and 4b) where the Xilinhot Fault may be
located. Their work indicates that pre-Devonian thrust faults were
superimposed by post-Permian thrust fault, but both are southvergent thrust systems. The Erenhot Fault is well traced from the surface
(CMP 20800) into the deep crust. It is truncated by a sub-horizontal
reflection (“Bs” in Fig. 7b) near the base of crust.
These horizontal or sub-horizontal layers near the base of crust are
an interesting seismic feature in the northern segment of the profile.
Their reflective character is obviously different to that of layered reflections in the lower crust of the NCC. Their geometry is like sills. These
layers are parallel or sub-parallel to the Moho and truncate the regional
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
in the upper crust in this area, these layers probably represent mafic–
ultramafic sills derived from mantle. We realize that this speculation
is weak because there is no surface geological evidence, but we come
back to this question in the Discussion section.
4.4. Mantle reflectors
A few mantle reflectors were also observed. A north-dipping reflector group is visible beneath the Moho near the northern end of the profile, north to the CMP 23600 (“MR” in Fig. 7b). These are compatible
with north-dipping reflectors in the lower crust beneath the Uliastai
belt and are more evident in a short, parallel profile some 60 km to
the east that is still in a data processing stage. Another group of mantle
reflectors was observed in the southern segment of the profile, between
CMPs 6900 and 4400. However, these are south-dipping, short and faint,
obviously different from the crustal reflectors in this region.
Compared with mantle reflectors reported from other seismic reflection profiles worldwide (Abramovitz et al., 1997; Balling, 2000; Calvert
et al., 1995; Hammer et al., 2010; Warner et al., 1996), we think the
mantle reflectors in our profile may be remnants of oceanic crust and
represent a relict subduction zone, or they may reveal sinking lower
crustal fragments from a delamination procession in this region.
5. Discussion
The salient north-dipping thrust system is apparent in the southern
portion of the seismic profile, yet numerous south-dipping reflectors are
observed in the northern portion. Therefore, in a broad view (Fig. 8), the
orogen appears to be bivergent with a center approximately at the
Solonker suture zone.
The southern portion likely represents a foreland fold-and-thrust
belt. Form the Kangbao ductile shear zone to the south, the deformation
age of the shear zones becomes younger, from late Permian to late
Triassic (Wang et al., 2013, Figs. 2, 4d and 7b). This age pattern may suggest that the north-dipping thrust system propagated cratonwards,
exhibiting a protracted Himalayan-type thrust system. This thrustand-ductile shear zone system cuts late Carboniferous granitic plutons,
is overlain unconformably by latest Early Jurassic strata and is
superimposed by the Late Jurassic–Cretaceous Yanshan thrusts that
have an opposite vergence (Davis et al., 2001; Wang et al., 2011; Zhao,
1990). This suggests that the pre-middle Jurassic south-directed
thrust-and-fold system formed during collision and subsequent convergence in the post-collisional stage. The Bainaimiao arc was overthrust
onto the NCC crustal wedge as a tectonic slice and was, in turn,
overthrust by the Ondor Sum accretionary complex (Fig. 8). This architecture indicates that considerable crustal shortening and thickening
has occurred when these tectonic fragments were squeezed together.
This interpretation does not contradict tectonic models suggesting
south-directed Paleozoic subduction. The seismic profile is only a snapshot of the tectonic history. If the south-dipping mantle reflectors beneath the NCC (Fig. 7b, between CMPs 6800 and 4400) are remnants
of subduction, they may be significantly older than the upper crustal
north-dipping fold-and-thrust system. In this case, the north-dipping
fold-and-thrust system probably represents the structure of collisional
secondary vergence, as depicted in Fig. 15B of Xiao et al. (2003).
In the northern portion of the profile, some syncollisional structures
may have been obliterated by episodic but extensive post-collisional
36
22800
19600
18000
16400
Ordor Sum Belt
Sonid Youqi
Fault
Bainaimiao Belt
Xianghuangqi
14000
11600
10000
Fault
Fault
Huade
SE
North China Craton
Zhangjiakou
6800
Zhangbei
3600
2400
Huailai
0
4
12
8
24
12
36
Moho
Moho
16
20
Possible relict subduction
Craton basement
40 km
Possible relict subduction
Major thrust fault
Mesozoic-Cenozoic basin
Mesozoic granitoid pluton
Paleozoic granitoid pluton
Underplating mafic sills
Fig. 8. Major crustal structures revealed from the SinoProbe deep seismic reflection profile across the Solonker suture zone of the CAOB.
48
60
6800
CMP of seismic profile
Approximate Depth (km)
Two-way time (s)
24400
Chifeng Kangbao
Xar Moron
Linxi
Fault
Baolidao Belt &
Solonker suture zone
Belt
Bayanhonggeer
0
Erenhot
Fault
Fault
Hegenshan
S. Zhang et al. / Tectonophysics 612–613 (2014) 26–39
Chagan Obo
Uliastai Belt
S. 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 structures
still remain clear on the seismic image and geological maps (Figs. 4a and
7b), for example, the Erenhot Fault and south-dipping reflectors near it
(“t” in Fig. 7b). The Solonker suture zone and the Baolidao belt together
appear to represent the central portion of the bivergent collisional
orogen. If the mantle reflectors (between CMPs 25200 and 23800) reflect relict northward subduction, the Erenhot Fault zone and many
south-dipping crustal reflectors in this area seem to be the structures
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
models in which the Southern Orogen (Manchurides) experienced
south-directed subduction whereas the Northern Orogen (part of the
Altaids) underwent north-directed subduction. The two subduction–
accretion systems finally collided, leading to a bivergent crustal
structure as illustrated in Fig. 8.
On geological maps, granitoid rocks of varying age are distributed in
Central Asia, but few were mapped in a vertical dimension in crust. We
attempt to recognize their vertical dimension from the seismic profile. It
is apparent that most granitoid bodies in this region truncate the
interpreted fold and thrust structures (Fig. 7b), suggesting that these
plutons formed during a post-collisional event. Episodic extensional
magmatic events were reported in this region, such as early Carboniferous and early Cretaceous mantle melting episodes in the Hegenshan
area (Jian et al., 2012), late Permian bimodal volcanism in the Xilinhot
area (Zhang et al., 2008), a Permian alkaline granite in the Uliastai belt
and a Triassic alkaline granite along the northern margin of the NCC
(Tong et al., 2010), and a post-collisional granitoid in the Sonid Zuoqi
area (Chen et al., 2000). These examples suggest that considerable
mantle-derived melts and heat entered the crust. In this case, repeated
episodes of post-collisional magmatic underplating may have renewed
the Moho. The horizontal and sub-horizontal layers immediately overlying the Moho are likely basaltic sills derived from the mantle.
6. Conclusions
(1) The seismic reflection profile reveals a fairly continuous and flat
Moho at a depth that requires ~ 14.5 s two-way travel time,
which is in agreement with the refraction data recorded along
the 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, approximately centered on the Solonker suture. The southern portion
of this profile is characterized by a crustal-scale cratonward
propagating fold-and-thrust system that formed during the late
Permian and Triassic by collision and continued convergence in
post-collisional setting. In the northern portion of the profile,
although partially obliterated by post-collisional magmatic
bodies, major south-dipping fold-and-thrust structures are still
traceable.
(3) Bands of layered reflectors immediately overlying the Moho are
indicative of basaltic sills derived from the mantle. Episodic magmatic underplating may have occurred in this region during the
post-collisional evolution and may have renewed the Moho.
(4) The overall geometry of the mantle reflectors, if they are really
relicts of subduction, supports tectonic models that the southern
orogen (Manchurides) experiences south-directed subduction
whereas the northern orogen (Altaids) reflects north-directed
subduction prior to collision along the Solonker suture zone.
Acknowledgments
This work was jointly supported by SinoProbe Project 02, the 973 Program (2013CB429800), the China Geological Survey (1212011120754),
and NSFC projects 40921062, 40974035, and 41104060. The authors are
37
grateful for discussions with Profs. Aimin Xue, Ganqing Jiang, Greg
Davis, Bei Xu, Bin Chen, Shaofeng Liu, Yu Wang and An Yin, and greatly
appreciate reconstructive comments by Profs. Alfred Kröner and Wenjiao
Xiao.
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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