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International Geology Review
ISSN: 0020-6814 (Print) 1938-2839 (Online) Journal homepage: http://www.tandfonline.com/loi/tigr20
Oblique wedge extrusion of UHP/HP complexes
in the Late Triassic: structural analysis and zircon
ages of the Atbashi Complex, South Tianshan,
Kyrgyzstan
Miao Sang, Wenjiao Xiao, Apas Bakirov, Rustam Orozbaev, Kadyrbek Sakiev
& Kefa Zhou
To cite this article: Miao Sang, Wenjiao Xiao, Apas Bakirov, Rustam Orozbaev, Kadyrbek Sakiev
& Kefa Zhou (2016): Oblique wedge extrusion of UHP/HP complexes in the Late Triassic:
structural analysis and zircon ages of the Atbashi Complex, South Tianshan, Kyrgyzstan,
International Geology Review, DOI: 10.1080/00206814.2016.1241163
To link to this article: http://dx.doi.org/10.1080/00206814.2016.1241163
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Oblique wedge extrusion of UHP/HP complexes in the Late Triassic: structural
analysis and zircon ages of the Atbashi Complex, South Tianshan, Kyrgyzstan
Miao Sang
a,b,c
, Wenjiao Xiao
a,c
, Apas Bakirov
d
, Rustam Orozbaev
d,e
, Kadyrbek Sakiev
d,e
and Kefa Zhou
a,c
a
Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, China;
b
University of Chinese Academy of Sciences, Beijing, China;
c
Xinjiang Key Laboratory of Mineral Resources and Digital Geology, Urumqi,
Xinjiang, China;
d
Institute of Geology, Kyrgyz National Academy of Science, Bishkek, Kyrgyzstan;
e
Research Center for Ecology and
Environment of Central Asia (Bishkek), Bishkek, Kyrgyzstan
ABSTRACT
The exhumation and tectonic emplacement of eclogites and blueschists takes place in forearc
accretionary complexes by either forearc- or backarc-directed extrusion, but few examples have
been well analysed in detail. Here we present an example of oblique wedge extrusion of UHP/HP
rocks in the Atbashi accretionary complex of the Kyrgyz South Tianshan. Our field mapping and
structural analysis demonstrate that the Atbashi Eclogite–Blueschist Complex is situated in a
complicated duplex formed by a northerly dextral transpression system and a southerly sinistral
transtension system. The two major shear systems suggest that the Atbashi Complex was
extruded obliquely southwestwards during eastward penetration of the southern tip of the Yili–
Central Tianshan Arc of the Kazakhstan Orocline during the Late Triassic. Also, we report new
zircon U–Pb metamorphic ages of four eclogites and one garnet-bearing quartz-schist from the
Atbashi complex of 217–221 Ma and 223.9 Ma, respectively, suggesting that the main extrusion
was later than previously proposed and that the final orogenesis was not completed until the Late
Triassic. The HP/UHP rocks have an oblique plunge to the NE and extrusion took place south-
westwards during escape tectonics along the South Tianshan accretionary wedge in the Late
Triassic. Our work shows that the movement of HP/UHP rocks had a 3D style with an arc-parallel
structure, and sheds light on earlier 2D models with either forearc- or backarc-directed extrusions,
which indicates that more systematic structural and geochronological work is needed to char-
acterize the accretionary tectonics of many orogens around the world. Our data on the timing of
extrusion and emplacement of the Atbashi Eclogite–Blueschist Complex also help to resolve the
long-standing controversy about the time of terminal orogeny of the Central Asian Orogenic Belt.
ARTICLE HISTORY
Received 26 June 2016
Accepted 22 September
2016
KEYWORDS
Kyrgyzstan Tianshan; Atbashi
Complex; eclogite;
garnet-bearing quartz-schist;
oblique extrusion;
Late Triassic terminal
orogeny
1. Introduction
Eclogites and Blueschists are important components of
many accretionary and collision-type orogenic belts, a sys-
tematic study of which provides key information on the
temporal and spatial framework and subduction polarity. It
is widely accepted that eclogite–blueschist units are
exhumed by a process of wedge extrusion, as thin (ca.
1 km wide), sub-horizontal slabs that are emplaced over
or into adjacent accretionary orogens (Figure 1(a))
(Maruyama 1997; Maruyama et al.1996,2010;Masago
et al.2005;Agardet al.2009), alternatively termed ‘channel
flow’in the Himalayas and equivalent collisional orogens
(Godin et al.2006). The main extrusion direction of the well-
studied accretionary orogens is mostly within the forearc
towards the ocean, thus showing a coupled relationship
with the subduction polarity (Figure 1(a)), for example, the
Sanbagawa Complex in Japan (Maruyama et al.1996;
Masago et al.2005), post-Cretaceous subduction systems
in the circum-Pacific (Maruyama et al.1996;Agardet al.
2009), accretionary complexes in Palaeozoic orogens such
astheBlueschistUnitonAnglesey–Lleyn, UK (Kawai et al.
2006,2007), and Precambrian orogens (Ring et al.2002;Ota
et al.2004a,2004b; Möller et al.2015).
However, the wedge and backstop shapes of the
accretionary complexes in different orogens differ in
several aspects (Ernst 2006; Warren 2012). Those
extrusions coupled with subduction polarities men-
tioned above may form in a closed wedge (Figure 1
(b)), or in an open wedge (Figure 1(c)), where the
extrusion of the HP/UHP rocks is mainly towards the
arc or backarc in contrast to the forearc (Brandon and
Calderwood 1990; Schwartz et al.2000;Bousquetand
CONTACT Wenjiao Xiao wj-xiao@mail.iggcas.ac.cn Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography,
Chinese Academy of Sciences, Urumqi 830011, China
Supplemental data for this article can be accessed here.
INTERNATIONAL GEOLOGY REVIEW, 2016
http://dx.doi.org/10.1080/00206814.2016.1241163
© 2016 Informa UK Limited, trading as Taylor & Francis Group
Meurs 2002). Rarely, between these two end-mem-
bers, high-pressure (HP) and/or ultra-high-pressure
(UHP) metamorphic rocks, as in the Hellenic forearc
ridge, formed by vertical extrusion (Marsellos et al.
2010), in a way we call a decoupling relationship.
Actually, many sub-horizontal or shallow-dipping
slabs of eclogite–blueschist underwent multiple defor-
mation events, which distorted their original geome-
tries. Accordingly, the mechanism of emplacement of
eclogite–blueschist units and their relationship to the
subduction polarity may be controversial and requires
constant re-appraisal.
Here we report our detailed mapping and structural
analysis with new age data of the South Tianshan accre-
tionary complex of the Central Asian Orogenic Belt
(CAOB) (or Altaids) in Kyrgyzstan (Figure 2(a–c)), in
which the HP/UHP rocks have a unique arc-parallel
structure indicating a shallow-dipping extrusion. These
relations help to better understand the emplacement
mechanism of high-pressure (HP) and/or ultrahigh-pres-
sure (UHP) metamorphic rocks in accretionary orogens.
2. Tectonic background and regional geology
The CAOB, located between the Eastern European and
the Siberian cratons to the north and the Tarim and
North China cratons to the south, is one of the largest
accretionary orogens recording considerable continen-
tal growth (Figure 2(a))(Şengör et al.1993; Jahn et al.
2004; Windley et al.2007; Xiao et al.2010,2015). The
South Tianshan Accretionary Complex (STAC) occupies
an important position in the western CAOB, tectonically
separating the Ili Central Tianshan Arc to the north from
the Tarim Craton to the south (Figure 2(b)). The STAC is
on the southern limb of the Kazakhstan Orocline, which
collided with the Tarim Craton and minor nearby con-
tinental slices. There are several HP/UHP metamorphic
rocks along the Atbashi–Inylchek fault in the STAC, the
Subducting plate
Subducting plate
Extrusion
forearc-ward
coupling with subduction
Forearc
Arc Arc
Extrusion
backarc-ward
decoupling with subduction
Backarc
Forearc
Accretionary
complex
Block B
Block A
Tectosphere
Arc
Figure 1. Various models to provide background data for wedge extrusion in accretionary orogens. (a) N–S cross-section of the
Shimanto accretionary complex in central Shikoku showing emplacement of the Sambagawa belt (Ota et al. 2004a; Maruyama et al.
2010). (b) Schematic model of the emplacement of HP/UHP rocks in an accretionary complex in the Franciscan of California (Cloos
1982), in Sulawesi (Parkinson 1996), and the Western Alps (Bousquet 2008). (c) Schematic model for the emplacement of HP/UHP
rocks by extrusion processes in evolving divergent wedges of accretionary complexes in the Central Alps (Bousquet et al.2002), the
southern Western Alps in Italy (Schwartz et al.2000), and the Olympic Mountains in Greece (Brandon and Calderwood 1990).
2M. SANG ET AL.
two major outcrops being well exposed in Kyrgyzstan
and China (Figure 2(a,b)) (Tagiri et al.1995; Gao et al.
1998; Volkova and Budanov 1999; Xiao et al.2013,
2015). A systematic investigation of the STAC will help
constrain not only the emplacement of HP/UHP rocks in
the accretionary complex, but also the formation of the
Ili–Central Tianshan Arc, and the timing of assembly of
the Tarim Craton.
Compared with the HP/UHP metamorphic rocks in
the southern Chinese Tianshan that have been exten-
sively studied (Gao et al.1995; Zhang et al.2007; Klemd
et al.2011; Scheltens et al.2015), the HP/UHP meta-
morphic rocks in the Tianshan of Kyrgyzstan have been
received less study largely due to their relative inacces-
sibility in high mountains and the inaccessibility for
foreigners for decades (Figure 2(c)) (Tagiri et al.1995;
Simonov et al.2008; Hegner et al.2010; Safonova et al.
2015). The eclogite–blueschist complex of the Atbashi
(Atbashy) Ridge (Figure 2(c)) is composed of pelitic to
siliceous schists alternating with basic UHP eclogitic
schists (Tagiri et al.1995). An early study has shown
that the core and mantle of garnets in eclogites are
Fig. 12
Figure 2. (a) Tectonic map of the main components of the Central Asian Orogenic Belt, showing the Kazakhstan Orocline and Tuva-
Mongol Orocline. (b) Geological and tectonic map of the Tianshan orogenic collage showing major Palaeozoic magmatic arcs and
the South Tianshan (Kokshaal–Kumishi) accretionary complex. Positions of Figures 2(c) and 12 are indicated. (c) Geological map of
the Tianshan orogenic collage in Kyrgyzstan showing the major Palaeozoic Ili-Central Tianshan magmatic arc and the South Tianshan
accretionary complex with emphasis on the Atbashi eclogite–blueschist complex (compiled after a Soviet Geological Map with a
scale of 1:200,000, 1971) (Soviet Union Institute of Geology 1971). Position of Figure 3 is marked.
INTERNATIONAL GEOLOGY REVIEW 3
rich in inclusions such as omphacite, quartz, calcite,
sphene, and pseudomorphs after coesite (Tagiri et al.
1995). Omphacite occurs in the matrix, and garnet cores
contain quartz pseudomorphs after coesite and talc-
albite intergrowths (Tagiri et al.1995). Simonov et al.
(2008) defined the PT-conditions of crystallization of the
high-pressure minerals in the eclogites as 23–25 kbar
and 510–570°C. Their trace and rare-earth elements
indicate that the protoliths of two established types of
eclogites with characteristics of N-MORB and OIB pla-
teau basalts formed as a result of the interaction of a
MORB magmatic system with plateau basalt–plume
magmatism.
The Atbashi eclogite has an Rb–Sr mineral-isochron
age of 267 ± 5 Ma, defined by garnet, omphacite,
phengite
1
, phengite
2
(low-Rb content), and the whole
rock (Tagiri et al.1995). The high-pressure mineral
assemblage of eclogites in the Atbashi Ridge has a HP
Sm–Nd isochron age of 319 ± 4 Ma (Hegner et al.2010).
Stepwise
40
Ar/
39
Ar dating of coexisting minerals from
phengite and glaucophane gave a narrow range of
324–327 Ma, and a spectra of glaucophanes from one
sample yielded a plateau age of 281 ± 11 Ma (Simonov
et al.2008). The above studies yielded new data mainly
about the genesis of the Atbashi eclogites: i.e. PT-con-
ditions, fluid regime, time of formation, and their pro-
toliths (Simonov et al.2008). However, there have been
no systematic field studies regarding the structural fra-
mework of the HP/UHP metamorphic rocks in the
Atbashi Ridge in the Kyrgyz Tianshan.
In order to resolve the problems of when and how the
HP/UHP metamorphic rocks in the Kyrgyz Tianshan were
emplaced in the South Tianshan accretionary orogen, a
new joint Sino-Kyrgyzstan structural and tectonic project
wasinitiated.Thisprojectismainlybasedonthethree
decades of mapping of Bakirov et al.(1984), and has been
launched with joint new lithological, field structural and
geochronological studies of our group with Kyrgyzstan
colleagues (Bakirov et al.1974,1998; Simonov et al.2008).
With the aim of investigating and documenting the accre-
tionary history of the Atbashi Eclogite–Blueschist Complex,
we carried out detailed field mapping, kinematic analysis,
and zircon geochronology, and finally we discuss its role in
the accretionary orogen and its implications for the tectonic
development of the CAOB.
3. Field geology
The Atbashi Eclogite–Blueschist Complex (AEBC) is a
conventional, formal name for the Atbashi Formation
that contains pelitic to siliceous schists alternating with
HP/UHP eclogites and blueschists (Figures 3 and 4).
The main belt of the AEBC strikes SW–NE mostly
parallel to the Atbashi–Inylchek Fault (Figure 2(b)). Our
study focused on the mountainous, north-facing slope
of the middle Atbashi Ridge, ca. 20 km south of Atbashi
village (Figures 2(b) and 3), which has a length of
2–3 km and a width of 0.8 km, and which is traversed
by the Kembel River and other unnamed rivers.
Figure 2. Continued.
4M. SANG ET AL.
Metamorphism and structural map of petrotectonic assemblage
along the Kembel River in Atbashi Range (modified after Bakirov, et al. 1984
Figure 4. Regional geological and structural map with rock assemblages in the Atbashi Range showing the metamorphic grade and
the locations of our new isotopic ages in the Atbashi eclogite–blueschist complex (modified after Bakirov et al.1984). The positions
of the photographs of Figures 5 and 8are marked. The dotted line and the arrow are the same as those in Figure 3. The yellow
arrow is the proposed wedge extrusion direction.
Figure 3. Regional geological and structural map of petrotectonic assemblages to the west and east of the Kembel River in the
Atbashi Ridge (modified after Bakirov et al.1984). The positions of Figures 6(a,b) and 7are marked. Lower hemisphere projections
show the orientations of the main foliations of the Atbashi eclogite–blueschist complex. The dotted line represents the dividing
plane separating the two duplex systems. DCA 1–20 are case study areas of duplexes. The green arrow is the proposed wedge
extrusion direction.
INTERNATIONAL GEOLOGY REVIEW 5
The metamorphic zones of the AEBC illustrate the
juxtaposition of greenschists, blueschists, and eclogites,
which generally increase in grade towards the centre of
the AEBC (Figure 4) within a complex duplex framework
(Figures 3 and 4).
In the northern foot of the Atbashi Ridge some Late
Palaeozoic conglomerates and felsic volcanic rocks were
interpreted to be deposited unconformably on the eclo-
gite-bearing complex (Hegner et al.2010). However, this
unconformity has no direct contact with the AEBC. The
contacts between the HP/UHP rocks and surrounding
rocks are tectonic (Figures 2(b) and 3), and therefore we
have concentrated on the AEBC.
The study area includes several large outcrops in the
northern slope of the Atbashi Ridge. For the ease of
description, we divide the field geology into eastern
and western parts.
The eastern part of the AEBC in the Atbashi Ridge is
0.8 km long and 0.6 km wide (Figure 3). North of the
AEBC there are conglomerates, thick limestones, sand-
stones, and siltstones. Farther north, there are fine-
grained sandstones and mudstones interbedded with
micaceous polymict sandstones, an ophiolitic mélange
as well as actinolite-chlorite schists. South of the AEBC
there are quartz-albite muscovite schists, and many
eclogites (some are carbonatized) that occur as layers
and blocks some tens of centimetres to 10 m across
together with glaucophane schists, cherts, ultramafic
serpentinites, and metagabbros within various schist
types. Some eclogites contain pillow structures
(Figure 5(d,e)), and some lenses of ultramafic rocks are
mixed with high-pressure metamorphic rocks. One glau-
cophane schist lens that is about 3 m long and 2 m
wide has a tectonic contact with adjacent eclogite
(Figure 5(g)).
The outcrops from the western part of the Atbashi
Region are up to ca. 2 km long and 0.6 km wide, divided
into three Segments I, II, and III; the biggest outcrop is
located in Segment I (Figure 3). As in the eastern part,
the AEBC is in tectonic contact with sub-vertical lime-
stones (Figure 5(a)), conglomerates, sandstones, and
siltstones. Farther north there are fine-grained sand-
stones and mudstones interbedded with micaceous
polymict sandstones. Similar to those eclogites in the
eastern part to the Kembel River, the southern part of
this section also has quartz-albite muscovite schists.
There are many large lenses of eclogites along the
ridges of Segment I (Figures 3 and 5(b)), but smaller
eclogitic lenses in Segments II and III (Figure 3). In
Segment II, which is approximately 1.5 km west to the
Segment I, two bodies of eclogites have been mapped
as blocks in the garnet-glaucophane schists and two
small bodies of eclogites in quartz-chlorite schists.
Father westward in Segment III eclogite bodies only
occur in garnet-glaucophane chlorite schists. No eclo-
gites have been mapped in quartz-muscovite chlorite
schists in Segment III.
Some eclogites contain large garnets that stand out
with high relief (Figure 5(c)). Well-preserved pillow
structures occur in eclogites along both sides of the
Kembel River, demonstrating their origin as pillow
basalts. Moreover, some pillows have well-preserved
cores of eclogite, and rims of blueschist (Figure 5(f)).
Most eclogites are massive, but some are foliated. All
exposed contacts between eclogites and schists appear
to be tectonic.
Along both flanks of the Kembel River the eclogites
and blueschists are mostly dark green or blue-green
(Figure 3). Microscopically the eclogites are mainly com-
posed of garnet, omphacite and glaucophane with sub-
ordinate quartz, albite, muscovite, chlorite, rutile,
titanite, and zoisite, and the host schists mostly have
high-grade assemblages (Figure 3).
4. Internal duplex structures of the AEBC
The structural relations of the HP rocks enable us to
decipher the mechanism of extrusion by means of two
large map-view geometries of en echelon and/or imbri-
cated sigmoidal duplexes in the AEBC. The general
duplex structural framework is characterized by a north-
ern dextral transpressional system and a southern sinis-
tral transtensional system, both of which contain a
series of strike-slip duplexes at several scales. The
duplexes are characterized by repetition of strata on
sigmoidal, imbricated cross-faults developed between
a floor fault and a roof fault. Duplexes may be sub-
divided into three major types: hinterland-dipping
duplex, antiformal stack, and foreland-dipping duplex
(Boyer and Elliott 1982). As various types of duplexes
may occur in nature, some complicated duplexes (for
instance, one with imbricate stacking of several distinct
duplex structures between a floor and a roof boundary
faults, and another in which two adjacent second-rank
duplexes share one boundary fault as the floor fault of
one duplex and the roof fault of the other) can be
termed ‘multiduplex’(Fermor and Price 1987;
McMechan 2001). On a regional scale the duplex struc-
tures are demonstrated by variations in tectono-strati-
graphy, which define 20 numbered case study areas of
duplexes (DCA) shown in Figure 3. In the sections below
we describe these duplexes from SW to NE along the
strike of the AEBC, and the basic structures are illu-
strated by cross-sections and block-diagrams (Figures
3,6, and 7).
6M. SANG ET AL.
4.1. Northern dextral transpressional system
The northern part of the AEBC is characterized by
duplexes on many different scales, which we describe
from west to east along strike (Figure 3).
DCA 1, located in Segment III, is a long, large-scale
duplex entirely composed of several horses of quartz-
muscovite-chlorite schists bound by two strike-slip
faults against garnet-glaucophane-muscovite-quartz-
chlorite schists to the north and south.
Figure 5. Field photos showing details of the eclogites. For locations of these photos see Figure 4. (a) Fault contact between
limestone and the eclogite–blueschist complex; (b) Eclogite lenses along the ridge; (c) Eclogite with large red garnets in a green
matrix of omphacite; (d) and (e) Eclogite with pillow structures; (f) An eclogite lens, the core of which has eclogite facies mineralogy,
whereas the rim has a blueschist facie assemblage; (g) Tectonic contact of an eclogite with glaucophane schists. Arrows in c, d and e
are referred to as the top of pillows.
INTERNATIONAL GEOLOGY REVIEW 7
DCA 2 duplex in Segment II has two major boundary
faults encompassing thrust-bound horses consisting of
garnet-glaucophane-muscovite-quartz-chlorite schists
that contain lenses of glaucophane-bearing eclogite.
Towards the south there is another duplex DCA 3,
mainly comprised of garnet-glaucophane-quartz-mus-
covite schists containing lenses of eclogite; the northern
tectonic boundary is defined by garnet-glaucophane-
muscovite-quartz-chlorite schists, whereas the southern
tectonic contact is bound by quartz-muscovite-chlorite
schists. All the thrust horses of duplexes 2 and 3 are
characterized by S-dipping foliations.
East of Segment II in DCA 4, a duplex is composed of
garnet-glaucophane-muscovite-quartz-chlorite schists bou-
nd on its northern and southern sides by S-dipping faults.
There are only minor angular differences between the
directions of dip of the foliations and of the boundary faults.
Next to DCA 4 duplex DCA 5 is comprised of garnet-glau-
cophane–quartz–muscovite schists with a moderate
N-dipping foliation. The SE-side of DCA 5 contains several
eclogite lenses within an area of ca. 50 m
2
(Figure 3). The
fact that duplex DCA 5 is thrust over duplex DCA 4 and that
there are the relatively large angular differences between
the strike directions of the foliations and boundary faults
show that duplexes DCA 4 and DCA 5 have the geometry of
an antiformal stack or a multiple duplex structure (Figures 3
and 6(a)). Farther south there are three complicated
duplexes (DCA 6, 7, and 8) marked by dextral strike-slip
faults. Duplex DCA 6 contains eclogites in an area of ca.
5km
2
within quartz-muscovite-chlorite schists; the roof and
floor thrusts of this duplex dip gently to the south and strike
sub-parallel to the Northern Transpressive Boundary. The
duplex DCA 7, which is located against the floor thrust of
duplex DCA 6, is narrow and long extending for 800 m, and
high-angle thrust faults define their horses, which are com-
posed of garnet-glaucophane-muscovite-quartz-chlorite
schists with two eclogite lenses. Two lenses of eclogite in
DCA 6 and 7 have en echelon geometry, showing an overall
a
b
Figure 6. Cross-sections of the Atbashi eclogite–blueschist complex to the west and east of the Kembel River in the Atbashi Ridge
(modified after Bakirov et al.1984). (a) A-A’section across the Northern Dextral Transpressional System showing unanimous duplex
structures with a general northward kinematic feature; (b) B–Bʹsection across both the Northern Dextral Transpressional and
Southern Sinistral Transtensional Systems. The green arrow is the proposed wedge extrusion direction. DCA with numbers are the
case study areas of duplexes marked in Figure 3. The positions of sections are marked in Figure 3.
8M. SANG ET AL.
dextral strike-slip and northward thrusting. Like the rela-
tionship between duplexes DCA 6 and 7, duplex DCA 8
occurs against the floor thrust of duplex DCA 7, it comprises
garnet-glaucophane–quartz–muscovite schist containing
four en echelon eclogite lenses, and its roof and floor
thrusts are both marked by garnet-glaucophane-musco-
vite-quartz-chlorite schists. In a cross-section the duplex of
DCA 8 is composed of several stacked eclogite-bearing
horses with a northward sense of thrusting (Figure 6(a)).
Duplex DCA 9, located structurally above duplex DCA 8, is
narrow, 600 m long and bound by high-angle thrust faults.
The thrust sheets within duplex 9 are discontinuous along
strike, and composed of quartz-muscovite-chlorite schist
containing one eclogite lens in an approximate area of
500 m
2
and another minor lens in the north. Both eclogites
strike NE–SW within dextral strike-slip packages, which help
to define en echelon horses with a northward thrust sense
in cross-section (Figure 6(a)).
Across the Kembel River to the northeast the AEBC
comprises several large duplexes (Figures 3 and 6(b)).
Duplex DCA 10 contains a 100 m-long eclogitic lens in
sub-vertical garnet-glaucophane schist that dips to the
S and E within a major marginal horse of the main
duplex. The map-view of these horses and their relation-
ships with their boundary thrusts faults indicate en
echelon structure and dextral strike-slip motions. Like
the structures in DCA 5, this duplex 10 shows a north-
ward thrust sense in the section (Figure 6(b)). Therefore,
we can deduce that the duplex has both dextral and
northward thrust directions. Also, there are duplexes
with similar senses of movement tectonically beneath
duplex DCA 10 as illustrated in Section 6.2 (Figure 6(b)).
Duplex DCA 11, which is adjacent to and tectonically
above DCA 10 (Figure 6(b)), is about 700 m long. The
duplex is composed of horses with well-preserved
jadeite-chlorite-bearing eclogite lenses in garnet-glau-
cophane-chlorite schists within adjacent horses that
have a mutual dextral shear sense. One horse in this
duplex is composed of NE-trending garnet-glauco-
phane-chlorite schist. In the western part of this duplex
Figure 7. Block-diagram showing the 3D structure of the Atbashi eclogite–blueschist complex in the Atbashi Ridge (modified after
Bakirov et al.1984). The green arrow is the proposed wedge extrusion direction.
INTERNATIONAL GEOLOGY REVIEW 9
is a lens composed of garnet-actinolite eclogite within
garnet-glaucophane-chlorite schists.
Farther south, duplex DCA 12 (Figures 3 and 6(b))
that is ca. 800 m long and ca. 60–80 m wide contains
several lenses of garnet-bearing glaucophane-chlorite
eclogites, glaucophane-bearing eclogites, and pre-
hnite-pumpellyite eclogites. The largest eclogite has
well-preserved pillow structures (Figure 3). The map-
view (Figure 3) illustrates the sense of dextral strike-
slip faulting and the cross-section (Figure 6(b)) the
northward thrusting.
On the southern side of duplex DCA 12, DCA 13 may
have formed as a multiduplex (Figure 3). The duplex is
more than 200 m wide in the west and becomes nar-
rower towards the east in map-view, appearing only ca.
60–70 m wide in the cross-section of Figure 6(b).
Recognition of horses in this multi-duplex may not be
as easy as in other duplexes, but the foliations of the
garnet-glaucophane schists and their relationships with
their enclosed eclogites indicate major sigmoidal or en
echelon structures, which we interpret as horses
(Figure 3). One large lens of glaucophane-bearing eclo-
gite contains well-preserved pillows suggesting deriva-
tion from pillow basalts. This multi-duplex may be
partially connected with horses in duplexes DCA 9
and/or DCA 8 across the Kembel River (Figure 3), but a
detailed link between them requires further study.
Just southeast of multi-duplex DCA 13, the sigmoi-
dal duplex of DCA 14 occupies a neck-zone in the
outcrops of the Atbashi Ridge, showing a dextral
shear sense (Figure 3). A narrow belt of chlorite-quartz
schists forms the northwestern horse of this duplex
(Figures 3 and 6(b)). A large lens of eclogite has well-
preserved pillow structures, indicative of its protolith.
The southeastern horse of the duplex is composed of
garnet-glaucophane schists. The whole duplex has a
south-verging thrust structure in cross-section
(Figure 6(b)).
The last duplex in the northern transpressional sys-
tem is DCA 15 (Figure 3), which is ca. 100 m wide and
300 m long. The thrust pile of DCA 15 is mainly com-
posed of garnet-glaucophane-muscovite-quartz-chlorite
schists with several eclogite lenses. The roof and floor
thrusts show a NW-dipping ramp-flat geometry. There
are two major horses containing garnet-glaucophane
quartz chlorite schists, which host one lens of eclogites
with pillow structures, and another lens is composed of
an alternation of eclogites and quartz chlorite schists.
The general structure and geometry in cross-section
and map-view indicate a dextral transpressional-thrust
movement pattern. But some minor structures (Figure 8
(a)) demonstrate a sense of extension, in horses contain-
ing quartz veins.
East of the Kembel River some eclogites have micro-
scopic duplex structures. Figure 8(b) exhibits a sinistral
strike-slip duplex in a well exposed, 0.8 m long eclogite.
Some eclogites contain tight chevron folds. Figure 8(c)
shows tight folds with NE-dipping axial planes, illu-
strated by similarly folded quartz veins.
Photomicrographs of samples 14AT10, 14AT13,
14AT53, and 14AT57 reveal duplex structures. Sample
14AT10 collected from the east bank of the Kembel
River is a garnet-glaucophane-muscovite-quartz schist,
in which a duplex formed by glaucophane and mica
displays dextral strike-slip (Figure 8(h)). Sample 14AT13,
collected from east bank of the Kembel River, is a
garnet-bearing glaucophane-chlorite eclogite, in which
the micas form an imbricate duplex with dextral strike-
slip (Figure 8(d)). In contrast, in Segment I, west of the
Kembel River, the photomicrograph (Figure 8(e)) of a
quartz–muscovite-chlorite schist in sample 14AT53
shows a mica fish fabric with right-lateral strike-slip.
Figure 8(f) of a garnet-glaucophane-quartz-muscovite
schist 14AT57 displays an imbricate duplex with dextral
strike-slip, in which glaucophane and mica define the
horses.
4.2. Southern sinistral transtensional system
There are regional-scale duplex structures in the south-
ern part of the AEBC. Duplexes numbered 16 to 20,
shown in Figure 3, have dominant sinistral strike-slip
shear sense (Figures 7 and 8).
Duplexes DCA 16–18 are located in Segment Ⅰwest
of the Kembel River. The thrust horses of duplex DCA 16
are composed of quartz-muscovite-chlorite schists and
garnet-glaucophane-muscovite-quartz-chlorite schists;
both schists form long-narrow horses with left-lateral
strike-slip sense and vertical to sub-vertical foliations.
The duplex in DCA 17 is dominantly composed of gar-
net-glaucophane quartz schist with minor garnet-glau-
cophane quartz-chlorite schists. Two or three horses can
be recognized by the foliations in the garnet-glauco-
phane quartz schists (Figure 3). Although not fully
exposed, we consider there are two other horses in
the western parts of this duplex (Figure 3). The geome-
tries of all these horses and their relationships to the
floor and roof boundary faults indicate a sinistral strike-
slip sense. Furthermore, they form a geometrical anti-
formal stack, as evidenced by their well-layered SE- and
SW-dipping flanks. Duplex DCA 18 is on the southern-
most side of the AEBC in the western Atbashi Ridge. It
consists entirely of quartz-muscovite-chlorite schists, in
which the roof and floor boundary faults are bound by
S-dipping glaucophane-bearing schists.
10 M. SANG ET AL.
Duplexes DCA 19 and 20, located east to the Kembel
River, show typical sinistral strike-slip. Brecciated eclo-
gites and jadeitite-carbonate-bearing glaucophane
schists constitute the horses of the duplex in DCA 19,
in which glaucophane schists form a ribbon around the
eclogites, both of which have together undergone left-
Figure 8. Field photos and photomicrographs showing folds and duplex structures of eclogites. Arrows indicate the sense of shear.
(a) and (b) Duplex showing sinistral strike-slip in eclogites; (c) Chevron folds with tight interlimb angles in eclogites; (d) Duplexes in
garnet-glaucophane-quartz-muscovite schist (sample 14AT51) indicating right-lateral strike-slip; (e) Mica fish fabric showing left-
lateral strike-slip in quartz-muscovite-chlorite schist 14AT53; (f) Imbricate thrust in garnet-glaucophane-muscovite-quartz-chlorite
schist 14AT57. Glaucophane and mica comprising the horses indicate dextral strike-slip; (g) Duplexes in garnet-glaucophane-quartz-
muscovite schist 14AT51 showing left-lateral strike-slip; and (h) Multi-duplexes in quartz-muscovite-chlorite schist 14AT10 with
dextral strike-slip movements. Locations are shown in Figure 4.
INTERNATIONAL GEOLOGY REVIEW 11
lateral strike-slip. Duplex DCA 20 is well exposed close
to the southern boundary fault, trends NE–SW, is ca.
400 m long, and contains an almost perfect sigmoidal
eclogitic lens. The horse is composed of garnet-glauco-
phane-quartz-muscovite schists with several eclogitic
lenses, one of which is 220 m long and has a complete
duplex structure, similar to the surrounding peripheral
duplex that has a sinistral strike-slip geometry.
Similar to the left-lateral structure of its duplex, gar-
net-glaucophane-quartz-muscovite schist 14AT51,
sampled from outcrops west of the Kembel River,
shows a microscale duplex structure with sinistral
strike-slip (Figure 8(g)).
In their cross-sectional views, all these duplexes in
the southern part of the AEBC are characterized by an
extensional component with S-dipping normal faults
(Figures 6(b) and 7).
5. Zircon geochronology
5.1. Sampling
There has been a long controversy about the age of the
Atbashi eclogites, because of errors created by using
different methods, different sampling techniques, and
more importantly the existence of a range of different
types of eclogites.
To constrain the extrusion of the AEBC and to place it
in its temporal framework during docking of the Tarim
Craton to the southern margin of the Ili–Tianshan Arc,
several eclogites and one quartz-schist were sampled
for separation of zircons for U–Pb analyses, in order to
document the timing event during extrusion.
Four eclogite samples (14AT47, 14AT52, 14AT54,
14AT56) and one quartz-schist (14AT55) were collected
from Segment Ⅰ(Figure 4). In order to extract a max-
imum number of zircon grains, at least 5–10 kg of each
sample was collected.
In terms of their different metamorphic grades and
mineral assemblages, the modes of occurrence of
minerals in the samples are shown in Figure 9 and
petrological description of them are presented below.
Sample 14AT47 mainly consists of garnet (30%), ompha-
cite (68%), glaucophane in disordered distribution (2%),
as well as a few rutiles, and trace amounts of actinolite,
epidote and chlorite. Sample 14AT52 is mainly com-
posed of garnet (30%), omphacite (40%), glaucophane
(10%), epidote (20%), and minor rutile. Sample 14AT54
is dominated by 63% garnet and 30% omphacite, as
well as 5% quartz and minor rutile. Sample 14AT56
comprises garnet (15%), omphacite (45%), glaucophane
(30%), 5% of both chlorite and epidote, minor musco-
vite and quartz, as well as a trace of rutile. Also, a
sample of garnet-bearing quartz-schist 14AT55 is com-
posed of 80% quartz, 15% muscovite and 5% garnet, as
well as minor rutile. The above mineral identifications
were made in the laboratory of the Bureau of Geology
and Mineral Resources of Xinjiang Uygur Autonomous
Region of China.
For their locations, all five samples were taken along
the NE/SW-trending mountain ridge, which has well-
preserved outcrops. Eclogite sample 14AT47 is sur-
rounded by greenschists, whereas eclogite lenses (sam-
ples 14AT52, 14AT54, and 14AT56) are bordered by
glaucophane schists. In addition, the garnet-bearing
quartz-schist sample 14AT55 is located ca. 30 m north
of eclogite sample 14AT56 (Figure 4).
5.2. Methods
Samples for U–Pb zircon analysis were processed by
conventional magnetic and density techniques to con-
centrate non-magnetic and heavy fractions. Zircon
grains, together with standard 91500 were mounted in
epoxy mounts, which were then polished to section the
crystals for analysis. All zircons were documented with
transmitted and reflected light micrographs as well as
cathodoluminescence (CL) images to reveal their inter-
nal structures. The mounts were then prepared for later
analyses.
The zircon ages were analysed by LA-ICP-MS at the
Institute of Geology and Geophysics, Chinese Academy
of Sciences, using an Agilent 7500a quadrupole ICP-MS
and a Thermo-Finnigan Neptune multi-collector ICP-MS
connected with a 193 nm Excimer ArF laser-ablation
system (Geolas plus). The analyses were carried out
with a beam diameter of 32 μm, 6 Hz repetition rate
and the ablation depth was 20–40 μm. Helium was used
as a carrier gas to enhance transport efficiency of the
ablated material. Harvard zircon 91500 was used as the
external standard for age calculation, and Australian
National University zircon NIST SRM 610 as the external
standard for concentration calculations. The detailed
analytical procedures for LA-ICP-MS can be found in
Xie et al.(2008). Isotopic data were analysed using the
software Glitter. Common lead was corrected for LA-ICP-
MS results using the correction proposed by Andersen
(2002). The weighted mean U–Pb ages and Concordia
plots were carried out by Isoplot/Ex-3.0 program
(Ludwig 2003).
5.3. Results
There are no published zircon U–Pb isotopic dates for
the eclogites and schists from the AEBC. We report here
new zircon U–Pb isotopic data for 70 grains from four
12 M. SANG ET AL.
eclogites and 11 grains from one garnet-bearing quartz-
schist, all documented in Supplementary Table 1. For
each sample, selected zircon CL images that reveal their
internal structures are shown in Figure 10. The CL
images of most zircons are characterized by magmatic
zonings (Figure 10). The U–Pb isochron age and histo-
gram statistics are illustrated in Figure 11.
In general, zircons in eclogites can be inherited from an
oceanic protolith or formed during metamorphism
(Rubatto and Hermann 2003). Hence we treat them in a
way similar to that of detrital zircon ages, in so far as the
youngest date may predate the age of the protolith of the
eclogites. For the garnet-bearing quartz-schist sample, we
use detrital zircons to constrain the age of the protolith.
Detrital zircon geochronology of sandstones or siliciclastic
sediments is a powerful method used to establish the age
distribution of magmatism within orogenic belts, which is
often pertinent to provenance analysis. It can also be used
to constrain the time of deposition, using the youngest
grain as a limiting factor (Fedo et al.2003), or it can
provide information on the lag time, the time between
the youngest zircon and an established deposition age.
Therefore, we use histograms with probability density
statistics to display the age results (Figure 10).
Sample 14AT47 provided 19 individual zircons with
variable grain shapes and sizes for analysis. The age
distribution shows prominent peaks at 395 Ma with
two small peaks at 370 and 420 Ma. The youngest age
Figure 9. Photomicrographs showing the occurrence of minerals in the eclogites and the garnet-bearing quartz-schist. (a) Euhedral
garnet is included in a matrix of abundant omphacite and minor disordered glaucophane and phengite. (b) Garnet, glaucophane,
and phengite in a fine-grained matrix of hornblende and epidote. (c) Omphacite and hornblende in a matrix containing garnet, as
well as minor scattered quartz and glaucophane. (d) Garnet rimmed by hornblende is included in a matrix of glaucophane and
omphacite. (e) A few garnets and muscovites with parallel alignments within an overwhelming majority of quartz. Car: carbonate; Ep:
epidote; Hb: hornblende; Gln: glaucophane; Gnt: garnet; Omp: omphacite; Q: quartz; Mu: muscovite.
INTERNATIONAL GEOLOGY REVIEW 13
is 221.5 ± 4.7 Ma, and the CL image shows this zircon
has distinct oscillatory zoning (Figure 9).
Only eight zircon grains could be dated from the sample
14AT52 because of poor zircon preservation. Five spots
from a zircon rim show ages of 217.1 ± 4.2 Ma,
219.4 ± 4.2 Ma, 232.7 ± 5.7 Ma, 280.2 ± 6.8 Ma,
396.9 ± 7.6 Ma, respectively, whereas two spots from zircon
cores have ages of 821.9 ± 16.3 Ma and 920.3 ± 18.2 Ma
(Figures 9 and 10), in which the grain with a younger age of
232.7 ± 5.7 Ma has clear oscillatory zones.
In sample 14AT54 most zircons have a white cor-
roded rim that possibly indicates a thermal
Figure 10. Cathodoluminescence (CL) images representing all dated zircons with their ages indicated. The yellow circles represent
the spots of the LA-ICP-MS analyses. The youngest age is highlighted in read.
14 M. SANG ET AL.
metamorphic event (Figure 10). We selected 24 zircon
grains for U–Pb analyses. The age distribution shows
major peaks concentrated at 320–380 Ma, and
298.1 ± 20.8 Ma is the youngest constraint for the
metamorphic age.
Nineteen spot analyses were made of 19 zircons from
sample 14AT56, which contains a significant number of
zircons (200 grains in 3 kg of rock). The major peaks are
focused at 370–400 Ma, whereas the youngest age is
362.9 ± 7.1 Ma (Figure 11).
Figure 11. U–Pb Concordia age diagrams and histograms from the Atbashi eclogites and blueschists. All analyses by LA-ICP-MS.
INTERNATIONAL GEOLOGY REVIEW 15
Sample 14AT55 is a garnet-bearing quartz-schist that
hosts eclogite lenses. Although it contains less zircons
(19 grains in 3 kg rocks), the CL images reveal that the
zircons are relatively large and have clear oscillatory
zoning, which indicates that the conditions are favour-
able for viable results. Nine individual zircon grains were
selected, of which 11 spots were analysed (Figure 10).
The age distribution exhibits a prominent peak at
255 Ma, whereas the youngest age is 223.9 ± 5.3 Ma
(Figure 11).
All the zircon analyses from four eclogites and one
garnet-bearing quartz-schist are summarized below: the
oldest age for the eclogites is latest Devonian
(362.9 ± 7.1 Ma), an intermediate age is early Permian
(298.1 ± 20.8 Ma), and the credible youngest meta-
morphic age of the eclogites is well constrained by
217.1 ± 4.15 Ma from 14AT52 and 221.5 ± 4.67 Ma
from 14AT47, whereas the youngest age of the garnet-
bearing quartz-schist is 223.9 ± 5.3 Ma from 14AT55.
Based on their ages we know that there are various
kinds of eclogites with different ages within the AEBC.
The youngest age of the eclogites and the garnet-bear-
ing quartz-schist may be Late Triassic for the Northern
Dextral Transpressional and the Southern Sinistral
Transtensional Systems, which, therefore, suggests that
the extrusion and emplacement of the AEBC into the
South Tianshan accretionary complex did not occur
until the Late Triassic.
6. Discussion
6.1. Oblique extrusion of the AEBC
Our work demonstrates that two major shear systems
contributed to the development of the AEBC. The north-
ern part of the AEBC comprises a transpressional system
in which several duplex structures show dextral strike-
slip movements with northward thrusting. In contrast,
the southern part of the Atbashi Complex is mainly
characterized by a transtensional sinistral system
(Figure 3).
The northern dextral transpressional and southern
sinistral transtensional shear systems suggest that the
Atbashi Complex underwent a unique oblique south-
westward extrusion with a general plunge to the NE, the
horizontal projection of which is sub-parallel to the
strike of the major structures.
Compared with other extrusion models, the extru-
sion direction in the Atbashi Complex shows a major
mountain-parallel structure with an oblique gentle NE–
plunge. With the Yili–Central Tianshan Arc to the north,
there is a component of backarc-directed thrusting, but
the main body of the AEBC was apparently extruded in
general along the strike of the orogen. The surface
occurrence of the AEBC is sub-vertical and slightly
southward-dipping; however, the deep occurrence of
the AEBC could have generally dipped to the north.
This is because the wedge shape of the South
Tianshan accretionary complex containing the AEBC
and the main faults within it have been well constrained
by combined geological and geophysical studies
(Alekseev et al.2007,2008; Makarov et al.2010), which
show a consistent northward-dip, interpreted as the
polarity of northward subduction of the South
Tianshan Oceanic plate beneath the Yili–Central
Tianshan Arc (Figure 12). Therefore, the whole wedge
is like a tilted, bent huge tabular toothpaste tube in
which the middle part of the toothpaste is obliquely
extruded. The major extrusion of the HP/UHP rocks in
the Kyrgyz Tianshan is dissimilar to that of previous
published models that have either ocean-ward or arc-
ward extrusions.
Figure 12. Combined geological and geophysical cross-section showing the tectonic units and zonation of the South Tianshan
orogen near the China–Kyrgyzstan border, explaining the extrusion of the Atbashi complex. The Muzduk Fault is roughly connected
with the North Tarim Fault marked in Figure 2(a). The line of section is marked in Figure 2(a). The yellow arrow points to the
extrusion direction of the Atbashi eclogite–blueschist complex. A half arrow is general subduction direction. See text for explanation.
16 M. SANG ET AL.
If the previous models show coupled (parallel, with
the extrusion either similar or opposite to the subduc-
tion polarity) or decoupled (parallel, but with the extru-
sion vertical, and no indication of the subduction
polarity) relationships with the subduction polarity
(Figure 1), our model shows no such relationship. The
subduction polarity of the South Tianshan Ocean plate
is demonstrated by the occurrence of the Ili–Central
Tianshan Arc to the north, and an accretionary wedge
to the south. Therefore, the extrusion of the AEBC is
almost perpendicular to, or at a large angle to, the
subduction polarity of the South Tianshan Ocean
plate. Accordingly, there is no direct indication of polar-
ity from the structures and kinematics of the HP/UHP
complexes. In fact, the movements of HP/UHP com-
plexes in the earlier models in Figure 1 seem to have
demonstrated only 2D structures. In those models, the
extrusions of UHP/HP rocks was regarded either as
coupled or decoupled with the subduction polarities,
in which they were both (extrusion and subduction
polarity) in one plane (Figure 1). Those so-called 2D
models were the results of previous investigations in
various orogens, and the simplified relationships
between the extrusions of UHP/HP rocks and the sub-
duction polarities need to be reconsidered. We envision
that the tectonic movements in accretionary complexes
have a 3D style and much more complicated structures
than those in the previous 2D models. First-hand data of
field structural geology and lithology is of fundamental
significance for us to better understand the extrusion
mechanism. Clearly more systematic structural and geo-
chronological studies need to be undertaken, together
with more digital simulations, to verify the 3D structures
of extrusion of HP/UHP rocks in orogens. However, the
extrusion of HP/UHP rocks should be used with care
when discussing and defining the subduction polarity
of orogens. In our case, the extrusion was more compli-
cated than just being paralleling to the strike of the
orogen, which was oblique, a unique situation that has
been unrecognized by the international community.
This is one of the most important implications of our
current work, which should shed light on the anatomy
of other orogens worldwide.
6.2. Mechanism and timing of extrusion and its
regional significance
Our work suggests that the subduction channel above
an oceanic subduction zone underwent more compli-
cated movements than previously described. The extru-
sion of the HP/UHP rocks could have been directed
towards the forearc or backarc, or it was mostly arc-
parallel. The complications of the extrusion of HP/UHP
rocks in forearc accretionary wedges may suggest that
more complicated exhumation mechanisms exist.
Rollback and/or retreat of a subducting slab may
account for the forearc- or backarc-directed extrusion
of the HP/UHP rocks, which may take place mainly in
normal convergent orogens. However, in the case of the
South Tianshan, the subduction of the South Tianshan
Ocean plate may have been oblique during the Late
Triassic (Figure 13).
The eclogites and the garnet-bearing quartz-schist
have yielded Triassic ages, some of which are Late
Triassic. Therefore, the Late Triassic age of the possible
protolith might place a lower time constraint for the
Figure 13. Tectonic model illustrating the 3D structure of the Atbashi eclogite–blueschist complex in the South Tianshan accre-
tionary complex. The yellow arrow is the proposed wedge extrusion direction, and small half arrows are general movements of
faults. Note that, although the wedge extrusion represented by the yellow arrow is oblique, the dip of the wedge across the strike is
generally sub-vertical with slightly southward dipping on the upper part, while the lower part dips to the north that is interpreted in
accordance with the N-dipping major faults based on geophysical features in combination with geological analysis.
INTERNATIONAL GEOLOGY REVIEW 17
extrusion. At that time, the formation of the northerly
Kazakhstan Orocline was almost finally complete with-
out further large-scale rotations, when its southern tip
(Ili–Central Tianshan Arc) moved eastwards to the
Junggar and Altai terranes (Wang et al.2007). In the
meantime, the Tarim Craton may have rotated clock-
wise from the N–S orientation of its present-day E–W-
oriented long axis (Xiao et al.2009). During those pro-
cesses the Ili–Central Tianshan Arc might have collided
with the eastern (present-day coordinates) promontory
of the Tarim Craton. These two boundary situations may
have jointly generated a westward-opening triangular
shape for the South Tianshan accretionary complex
(Figure 14). This could have actually generated a special
subduction channel along a narrow zone almost
between the backstop of the Ili–Central Tianshan Arc
and the South Tianshan Accretionary Complex. Along
this narrow zone the AEBC may have been obliquely
extruded southwestward, leaving a southern sinistral
and a northern dextral shear stress regime (Figure 14).
The major reason for the arc-parallel extrusion may have
been caused by a reorganization of plate convergence
during the eastward penetration of the southern tip of
the Yili–Central Tianshan Arc of the Kazakhstan Orocline
during the Triassic (Xiao et al.2010,2015).
Our results also shed light on the regional final
tectonic history of the CAOB, which has long been
controversial. Divergent opinions have suggested that
the final orogenesis and termination of the CAOB were
either in the Carboniferous (Gao et al.1998,2009;
Charvet et al.2007), or in the end-Permian to mid-
Triassic (Xiao et al.2009,2015). The South Tianshan
accretionary wedge records the final stage of orogen-
esis of the CAOB, as it occupies the southernmost
tectonic position in the orogen. As mentioned before,
the peak metamorphic ages of the eclogites have a
wide span from the Late Palaeozoic to the early
Mesozoic. These different ages might have been cre-
ated by seamounts or oceanic plateaus together with
the South Tianshan Ocean plate, when they were
subducted to the depth of eclogite-facies metamorph-
ism. However, the youngest ages of the eclogites and
blueschist are in the Late Triassic. Early studies also
reported some Permian ages for the eclogites and
blueschist (Tagiri et al.1995; Simonov et al.2008).
Therefore, the extrusion must have been later or
slightly younger than these youngest ages. If the
extrusion of the Atbashi HP/UHP Complex was in the
Late Triassic or even younger, one might reasonably
conclude that the final orogenesis did not take place
until the Late Triassic. Along the Atbashi–Inylchek
Fault in the Chinese South Tianshan, where eclogites
and blueschists occur, some early work from our team
also produced an Ar–Ar age of 251 ± 1 Ma for a
Figure 14. End-Permian to Triassic palaeogeographic reconstruction showing the position of the Atbashi eclogite–blueschist
complex in Kyrgyzstan together with comparable HP/UHP rocks along strike in the Tianshan of China. Note that the Kazakhstan
Orocline was finally formed and its southern tip, the Yili–Central Tianshan Arc, might have collided with the eastern (present-day
coordinates) promontory of the Tarim Craton whose present-day’sE–W-long axis orientation was mostly N–S-oriented before
240 Ma. The Tarim Craton might have gradually clockwise rotated. This process leaves a west-open triangle shape for the South
Tianshan accretionary complex that may have driven the oblique wedge extrusion of the Atbashi eclogite–blueschist complex. The
green arrow is the proposed wedge extrusion direction for the Atbashi eclogite–blueschist Complex in the Kyrgyz Tianshan. WJ:
West Junggar. The N in the Siberian craton is for the present-day North.
18 M. SANG ET AL.
blueschist, and a 243–225 Ma age for a dextral fault
separating the backstop of the Ili–Central Tianshan Arc
from the HP/UHP rocks (Scheltens et al.2015). Zhang
et al.(2007) also reported Middle to Late Triassic ages
(233 ± 4 –226 ± 4.6 Ma) for the eclogites there.
Therefore, to a first approximation we postulate that
the extrusion of the HP/UHP rocks started in the east
in the Chinese Tianshan. Slightly later, in the Late
Triassic, general escape tectonics along the South
Tianshan accretionary wedge forced the HP/UHP
rocks in the Kyrgyz South Tianshan to obliquely
extrude southwestward with a general plunge to the
NE. Therefore, this provides a piece of robust evidence
for the younger termination in the Late Triassic for the
CAOB.
Acknowledgments
We thank N. Pak, E. Evleva, S.J. Ao, W.D. Li, Z.X. Zhu, Q.Q. Qiao,
Z.X. Zhang, C. N. Uulu, and I. Erkibekov for their help during
the joint field expeditions to the Kyrgyz Tianshan. I. Safonova,
A. Kröner, and A.M.C. Şengör are acknowledged for useful
discussions and suggestions. We appreciate B.F. Windley for
his critical reading of the manuscript and three anonymous
reviewers for their constructive comments and suggestions.
This study was financially supported by the 973 Program
(2014CB440801), the Strategic Priority Research Program (B)
of the Chinese Academy of Sciences (XDB18020203), the Key
Research Program of Frontier Sciences, CAS (QYZDJ-SSW-
SYS012), and the National Natural Science Foundation of
China (41390441, 41230207, 41190075, and 41202150). This
is a contribution to IGCP 592.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the 973 Program [grant number
2014CB440801]; Strategic Priority Research Program (B) of the
Chinese Academy of Sciences [grant number XDB18020203];
Key Research Program of Frontier Sciences, CAS [grant num-
ber QYZDJ-SSW-SYS012]; National Natural Science Foundation
of China [grant numbers 41390441], [41230207], [41190075],
[41202150].
ORCID
Rustam Orozbaev http://orcid.org/0000-0003-0786-2992
Kadyrbek Sakiev http://orcid.org/0000-0001-6484-7081
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