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Accepted Manuscript
Tectonostratigraphic history of the southern Tian Shan Mountains, Western
China, from seismic reflection profiling
Chao Li, ShengLi Wang, LiangShu Wang
PII:
DOI:
Reference:
S1367-9120(18)30358-4
https://doi.org/10.1016/j.jseaes.2018.08.017
JAES 3621
To appear in:
Journal of Asian Earth Sciences
Received Date:
Revised Date:
Accepted Date:
27 October 2017
30 July 2018
19 August 2018
Please cite this article as: Li, C., Wang, S., Wang, L., Tectonostratigraphic history of the southern Tian Shan
Mountains, Western China, from seismic reflection profiling, Journal of Asian Earth Sciences (2018), doi: https://
doi.org/10.1016/j.jseaes.2018.08.017
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Tectonostratigraphic history of the southern Tian Shan Mountains, Western China, from
seismic reflection profiling
Chao Lia, ShengLi Wanga?, LiangShu Wanga*
a
State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences,
Nanjing University, Nanjing 210093, China.
ABSTRACT
The modern Tian Shan range formed due to the India-Eurasia collision. Uplift of the
Tian Shan is recorded notably by the sedimentary sequences and subsidence history of
the coupling foreland basins. We analyzed a 353-km-long seismic profile and logging
data of four wells in the southern Tian Shan foreland area to decipher its tectonic,
stratigraphic, and subsidence history. The sedimentary sequences comprise the
Cambrian-Silurian, Devonian-Permian, Triassic, Jurassic-Cretaceous, and Paleogene
tectonostratigraphic units, overlain by the late Oligocene-Quaternary foreland basin
unit. The Jidike Formation is the oldest sedimentary sequence of the foreland
succession, deposited at ~26 Ma based on magnetostratigraphic constraints, indicating
that uplift of the southern Tian Shan Mountains was initiated by at least ~26 Ma. In
addition, the tectonic subsidence rate of the southern Tian Shan foreland basin
increased significantly since ~26 Ma due to lithospheric flexure caused by building of
?
Corresponding authors.
E-mail: wangsl@nju.edu.cn (S.-L. Wang), lswang@nju.edu.cn (L.-S. Wang).
1 / 53
the topography of the southern Tian Shan Mountains. The envelope line of the
forelandward onlaping points within the foreland unit indicates that the forebulge of
the southern Tian Shan foreland basin migrated southwards at 1.6�1 mm/yr between
~26.3 Ma and ~12 Ma and at 14.6�1 mm/yr after ~12 Ma. The increase of migration
rate at ~12 Ma suggests an accelerated convergence between the Tarim Basin and the
Tian Shan.
Keywords: Tian Shan, Tarim, foreland basin, seismic reflection profiling
1 Introduction
The modern Tian Shan range is the most significant geological structure in central
Asia (Fig. 1), extending east-west for ~2500 km, with the Tuomoer peak reaching
about 7400 m. The modern Tian Shan range is part of the intracontinental deformation
caused by the India-Eurasia collision (Molnar and Tapponnier, 1975; Tapponnier and
Molnar, 1979). The initiation and processes of mountain-building are still poorly
understood and hotly debated. How the deformation propagated from the southern
margin of Asia to remote intracontinental regions and the timing and process of initial
growth have been focused for a long time (Neil and Houseman, 1997; England and
Molnar, 1997; Yin et al., 1998). Recent magnetostratigraphic works along both flanks
of the Tian Shan indicate a range from the late Eocene to the late Miocene for initiation
of the Tian Shan (Huang et al., 2006; Ji et al., 2008; Zhang et al., 2014).
Low-temperature thermochronological data suggest that exhumation in the Tian Shan
started from the late Oligocene (Hendrix et al., 1994; Dumitru et al., 2001; Macaulay
2 / 53
et al., 2013; Yu et al., 2014a). Active thrusting and folding in the northern piedmont of
the Tian Shan indicate that its uplift occurs since the Miocene (Avouac et al., 1993) to
Quaternary (Burchfiel et al., 1999). Many of these studies worked on limited
section(s), rather than on a scale of the foreland basin. The discrepancy in the timing
probably relates to the various geological records across the mountains encountered
by different authors and a lack of comprehensive work on the foredeep sedimentary
sequences in the foreland basins.
The topographic evolution of mountain ranges and the development of foreland
basins are linked by the bending of the lithosphere related to the crustal thickened
wedge and the surface redistribution of mass due to the erosion of mountain
catchments and the sedimentation in the foreland basins. The sedimentary infill of
foreland basins is a major source of insight into the growth of mountain ranges
through geological time (Decelles and Giles, 1996; Sinclair, 2012). A number of
publications in the past 30 years have studied the interaction of thrust wedges and
foreland basins, and they unraveled and simulated the growth history of thrust wedge
based on the subsidence and sedimentary successions in foreland basin (Heller et al.,
1988; Flemings and Jordan, 1989; Sinclair et al., 1991; Liu et al., 2005). In recent
years, numerous high-quality seismic profiles have been acquired during hydrocarbon
exploration in the southern Tian Shan foreland areas, providing scientific insights into
the understanding of the foreland dynamics. Magnetostratigraphic studies (Huang et al.,
2006; Charreau et al., 2009) provide reliable chronostratigraphic constraints on the
foreland strata.
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In this paper, we present a ~353-km-long seismic profile (Fig. 1) across the
southern Tian Shan foreland area and the northern Tarim basin to track each seismic
reflector and to decipher the architecture and evolution of the foreland area. Six
tectonostratigraphic units have been identified, based on the architecture of seismic
reflectors? terminations. We deciphered the architecture of the foreland unit and its
implications for the growth of the modern southern Tian Shan Mountains. Ages of
reflectors in the foreland sedimentary sequences were determined based on
magnetostratigraphic studies (Huang et al., 2006; Charreau et al., 2009). We also
analyzed the Cenozoic subsidence history of the southern Tian Shan foreland basin
based on logging data of 4 wells.
2 Geological backgrounds
The Tian Shan orogen originated from accretion of several island arcs and collision
of continental blocks in the Paleozoic (Burtman, 1975; Windley et al., 1990; Xiao et al.,
2013), with large-scale dextral strike-slip occurring in the Permian (Bazhenov et al.,
1999; Laurent-Charvet et al., 2002; 2003; Wang et al., 2007; Choulet et al., 2011). In
the Mesozoic, the Tian Shan was reactivated in response to collisions of the Lhasa and
Qiangtang blocks with the southern margin of Asia (Dumitru et al., 2001; Chen et al.,
2011), recorded by apatite fission track analysis (Jolivet et al., 2010), sedimentary
deposits in the foreland area (Hendrix et al., 1992), and transpressional deformation in
the northeastern Tarim Basin (Wang et al., 2012). In the late Cenozoic, the Paleozoic
Tian Shan rejuvenated as an intracontinental orogen with high topography, caused by
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the ongoing India-Eurasia collision (Molnar and Tapponnier, 1975; Tapponnier and
Molnar, 1979; Avouac et al., 1993; Hendrix et al., 1994; Yin et al., 1998). GPS
measurements show that the present-day shortening rates across the Tian Shan are ~20
mm/yr at 78癊 and ~5?10 mm/yr at 84癊 (Wang et al., 2001; Zubovich et al., 2010),
indicating present growing of this orogen. The difference of shortening rates along the
strike of the Tian Shan may be caused by the rotation of the Tarim block (Zubovich et
al., 2010).
Building up of the modern Tian Shan range induces flexural subsidence of the
southern Junggar and the northern Tarim basins (Lu et al., 1994; Fig. 1). The Kuqa
foreland basin in the south piedmont of the Tian Shan has a maximum thickness of
~7 km of the Cenozoic rocks. The sequences include the Kumugeliemu (E1-2
km),
Suweiyi (E2-3S), Jidike (N1j; 26.3?13.4 Ma), Kangcun (N1k; 13.4?5.3 Ma), Kuqa (N2k;
5.3?1.7 Ma), and Xiyu ((N2-Q1) x; 1.7?0 Ma) formations in an ascending order (Fig.3).
Charreau et al. (2006; 2009) and Huang et al. (2006; 2010) constrained their ages using
magnetostratigraphic method.
Drillings in the northern Tarim basin reveal that the lower part of the Jurassic
consists of conglomerate intercalated with coal beds and that the upper part of the
Jurassic is of sandy conglomerate. The base of the Cretaceous also consists of
conglomerate, while the middle and upper parts of the Cretaceous are composed of
sand-shale interlayers. The Paleogene includes the Kumugeliemu and Suweiyi
formations. The former comprises purplish green mudstone and sandy mudstone
intercalated with gypsum; and the latter consists of red sandstone and mudstone,
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intercalated with pebble layers and gypsum (Fig.3; Jia et al., 2004). The Jidike
Formation consists of grayish green and light green mudstone and sandstone
intercalated with gypsum. The Kangcun Formation is mainly composed of gray and
purple mudstone, sandy mudstone, and sandstone. The Kuqa Formation consists of
yellow, gray, and taupe sandstone, sandy mudstone, and mudstone (Jia et al., 2004).
The Xiyu Formation consists of monolithic, clast-supported, poorly to weakly sorted,
massive to horizontally and cross-bedded conglomerates, made up of pebble- to
block-sized clasts in a coarse-sand matrix (Charreau et al., 2009).
Major geological structures in the Kuqa foreland basin have been constrained by
seismic profiles (Wang et al., 2011). Thrust faults and folds have been identified in the
Triassic?Quaternary strata. Three main folds have been recognized from the
geomorphology, outcrop studies, and seismic profiles (Fig. 2a; Hubert-Ferrari et al.,
2007). These folds detach in d閏ollement levels located in the Tertiary and Jurassic coal
beds (Fig. 2b), and accommodate the majority of the deformation of the thrust belt
(Burchfiel et al., 1999; Saint-Carlier et al., 2016). Shortening rate of these folds
observed from seismic profiles is estimated at ~5.4 mm/yr since 2.58 Ma (Li et al.,
2012), roughly half of the present-day shortening rates across the Tian Shan.
3 Ages of the foreland sequences
Recent magnetostratigraphic studies of the Cenozoic strata in the eastern part of
the Kuqa foreland basin provide a chronostratigraphic framework (Fig. 2a). The ages of
the Jidike, Kangcun, and Kuqa along the Yaha section are determined by Huang et al.
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(2006) and Charreau et al. (2009). The bases of these formations are dated at 26.3, 13.4,
and 5.3 Ma, respectively. These two magnetostratigraphic sections are close and
subparallel to our seismic profile (Fig. 2). Therefore, the detailed ages of the Cenozoic
strata can be correlated to the seismic profiles and extrapolated across the northern
Tarim (Figs. 2 and 5).
Charreau et al. (2009) demonstrated that the Xiyu Formation is a gravel wedge that
has prograded over the underthrusting forelands, as inferred from contrasting
magnetostratigraphic sections from the north and south piedmonts of the Tian Shan.
The low boundary of the Xiyu conglomerates is diachronous, ranging from ~5.2 to
~1.7 Ma. Accordingly, we infer that the age of the base of the Xiyu Formation in the
foredeep zone, far from the southern Tian Shan thrust belt, is younger than 1.7 Ma. The
base of the Xiyu Formation in the profile across northern Tarim was speculated as
~1.7 Ma (Figs. 3 and 5).
Assuming a constant sedimentation rate between any two reflectors in one
stratum, the age of the seismic reflectors in the Jidike thorough Kuqa formations (Fig.
4) were interpolated (Fig. 5b and Table 1).
4 Interpretation of the Seismic profile
A ~353-km-long and 6-second-deep (~12 km) seismic profile across the southern
Tian Shan foreland area, from the middle of the Kuqa foreland basin to the central
Tarim, is presented in Figure 4. The formations? boundaries in the seismic profile were
determined based on well logging, outcrop data near the profile and interpretations of
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seismic profiles in the Kuqa foreland basin from a previous publication (Li et al.,
2016). We tracked all the seismic reflectors in the profile and identified unconformities
based on seismic reflector continuity and terminations (Figs. 4 and 5). The SNR
(signal-to-noise ratio) of the seismic profile in the Kuqa fold belt is too low to track
reflectors; while the seismic profile to south of the Kuqa fold belt is imaged well and
therefore can be tracked reliably. We have tracked 88 reflectors in the Cenozoic strata
which were named R1 through R84 and R'1 through R'4 in an ascending order. R1 to
R84 extend or terminate southwards within the seismic profile, while R'1 to R'4
terminate northwards (Fig. 5). The lateral variation of the sedimentary sequences is
determined by positions of reflectors? terminations and their characteristics.
4.1 The structures in the profile
In the northern part, three north-dipping thrust faults were recognized (Ft1, Ft2 and
Ft3). These thrust faults extend downwards to the Precambrian metamorphic basement.
The Ft3, the southernmost one, thrusts upwards into the Cretaceous, whereas the Ft2
penetrates upwards into the Jidike Formation. The uppermost fault, the Ft1, reaches the
ground surface. A fault-related fold in the hanging wall of the Ft1 is cut by a
south-dipping back-thrust (Fig. 4).
In the south of the Kuqa fold belt, eight reverse faults (Fr1, Fr2, Fr3a, Fr3b, Fr4a,
Fr4b, Fr5 and Fr6) and a normal fault (Fn1) in the deep part of the seismic profile
extend into the Precambrian basement (Fig. 4). The Fr3a, Fr3b and Fr4b extend
upwards to the Carboniferous, and the Fr4a incised into the Devonian. The vertical
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offsets along the Fr3a, Fr3b, Fr4a and Fr4b are no more than ~60 msec (~200 m) (Figs.
4b and 4c). The inclinations of Fr3a and Fr4a are opposite to that of Fr3b and Fr4b.
The upper parts of Fr3a, Fr3b, Fr4a and Fr4b in the Devonian and Carboniferous
steepen close to the vertical (Fig. 4c). The Fr3a and Fr3b comprise a pop-up structure.
The Cambrian through the Jurassic between Fr3a and Fr3b are folded, which means
these two faults may stop moving in the Jurassic. Similar anticline structure occurs in
the Cambrian?Paleogene between the Fr4a and Fr4b. It implies that the Fr4a and Fr4b
remained moving until the Paleogene. Ren et al. (2011) and Lin et al. (2015)
demonstrate that the Fr3a, Fr3b, Fr4a and Fr4b began to move since the Cambrian.
In the north of the fault Fr3a, there are two reverse faults (Fr5 and Fr6) in the
Cambrian and the pre-Cambrian basement. These two faults of Fr5 and Fr6 cause the
sharp bending of the Cambrian and the lower Ordovician (Fig. 4), but don?t affect the
top of Ordovician. It indicates that these two reverse faults moved in the Ordovician.
The north-dipping reverse fault Fr2 penetrates upward to the lower Cretaceous.
The hanging wall of the Fr2 forms a fault-related fold. The maximum vertical offset
along the fault is about 30 msec (~120 m; Fig. 4). The Fr2, that is the Luntai fault,
also began to move since the Cambrian, based on previous analyses of gravitational
data, aeromagnetic data and seismic data (Ren et al., 2011; Lin et al., 2015). The offset
of the Cretaceous is obviously less than that of the Triassic?Jurassic along the Fr2,
which indicates two phases of fault movements during the Mesozoic: the fault moved
before the Cretaceous and folded the Cambrian?Jurassic in the hanging wall; then, it
stopped moving before the deposition of the Cretaceous in the Tarim basin; after that,
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the Fr2 reactivated and folded the Cambrian?Cretaceous in the hanging wall (Fig. 4d).
The south-dipping normal fault Fn1 extends from the pre-Mesozoic upwards to the
Cretaceous. The maximum vertical offset along the fault is about 100 msec (~400 m;
Fig. 4d). On top of Fn1, there exists a reversed fault (Fr1) with opposite inclination of
Fn1. Fr1 occurs within the Cretaceous, the Paleogene and the Jidike formation, and its
hanging wall is folded. The vertical offset along the Fr1 is no more than 60 msec
(~240 m; Fig. 4d). The southward thrusting, on the southern piedmont of the Tian
Shan since the Neogene (Yin et al., 1998; Zhao et al., 2003), could induce the Fr1.
4.2 Division of tectonostratigraphic units
The sedimentary sequences in the southern Tian Shan foreland area were divided
into six tectonostratigraphic units separated by unconformities between them identified
in the profile (Fig. 4). These unconformities were defined based on the architecture of
seismic reflectors? terminations, such as truncation.
4.2.1 The Cambrian-Silurian tectonostratigraphic unit
Conformable continuous reflectors of the Cambrian strata above the pre-Cambrian
basement reveal a constant thickness of Cambrian strata across the seismic profile.
These reflectors are truncated by the Cretaceous in the northern end of the profile
(Fig.4d), and extend southwards out of the profile (Fig.4c). Reflectors of the
Ordovician and Silurian are characterized by toplap under Devonian strata, which
indicates that the base of the Devonian is an unconformity. The Cambrian remains
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relatively uniform thickness through the profile, but the Ordovician and the Silurian
thins northward. The continuous and conformable seismic facies of the Cambrian
reveal that the strata deposited in a tectonically stable environment (Fig. 4). This
lowest tectonostratigraphic unit consists of the Cambrian, the Ordovician and the
Silurian.
4.2.2 The Devonian-Permian tectonostratigraphic unit
Horizontal reflectors in the Devonian terminate unconformably below the
Cretaceous strata in the northern end but extend out of the profile in the southern end.
The Carboniferous?Permian reflectors are parallel to the Devonian and are truncated by
the Triassic in the north. The Carboniferous?Permian layer thins northwards.
Therefore, the base of the Triassic is interpreted as an unconformity, and a northward
uplift appears to have occurred in the Carboniferous?Permian time. The second
tectonostratigraphic unit consists of the Devonian and Carboniferous?Permian strata
(Fig. 4).
4.2.3 The Triassic tectonostratigraphic unit
Conformable north-dipping reflectors in the Triassic terminate northwards under
the basal Jurassic, toplapping to the north, and extend southwards beyond the profile.
The northward-thinning Triassic is unconformably overlain by Jurassic strata. The
Triassic constitutes the third tectonostratigraphic layer (Fig. 4).
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4.2.4 The Jurassic-Cretaceous tectonostratigraphic unit
Reflectors in the Jurassic are characterized by toplap terminations in both north and
south ends of the profile, and therefore an upward decrease in reflector length. This
reveals a clear unconformity at the base of the Cretaceous. The maximum thickness of
the Jurassic occurs 150 km south of the Kuqa fold belt (Figs. 4 and 5). The Cretaceous
strata are present throughout the profile, and reflectors display, similarly to the
Jurassic, both southward and northward toplaps below the Paleogene surface. The
lengths of Cretaceous reflectors also decrease upwards. Therefore, the Cretaceous is
also covered unconformably by the basal Paleogene. The maximum thickness of the
Cretaceous occurs ~50 km north of the location of maximum Jurassic thickness. It
suggests that the Cretaceous depocenter moved northward compared to the Jurassic.
The Jurassic and the Cretaceous constitute the fourth tectonostratigraphic unit (Fig. 4).
4.2.5 The Paleogene tectonostratigraphic unit
The Paleogene strata have relatively constant thickness (~150 msec; ~300 msec)
throughout the profile. It consists in four continuous reflectors (R1, R2, R3 and R4),
that can be traced across the whole basin, and four reflectors (R'1, R?2, R?3 and R'4)
that terminate northwards at 251 km, 160 km, 188 km and 256 km sites in Figure 5
respectively. The Paleogene is overlain by the foreland sedimentary unit. The
Paleogene is the fifth tectonostratigraphic unit.
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4.2.6 The foreland unit
The foreland unit overlays the Paleogene tectonostratigraphic unit and includes
the Jidike (N1j), Kangcun (N1k), Kuqa (N2k), and Xiyu, ((N2-Q1)x) formations.
Reflectors in the foreland unit onlap to the south, and the areal extent of their
terminations migrate, in general, southwards from the bottom up, but they although
alternate back and forth (Fig. 5).
There are 7 reflectors in the Jidike Formation: R5, R6, R7, R8, R9, R10 and R11.
At the base of the Jidike Formation, R5 extends southwards and terminates at 27 km
site in the profile. The termination points of R6 and R9 are located between 37 and
40 km (Fig. 5), onlapping southward. The Jidike Formation progressively laps on the
Paleogene southward between 27 and 40 km in the profile (Fig. 5d and Table 1). The
Jidike Formation gradually thins southward, and the maximum thickness is located near
the southern margin of the southern Tian Shan foreland thrust belt. These indicate that
the depocenter of the northern Tarim block was located to the southern piedmont of the
Tian Shan during deposition of this formation.
There exist 14 reflectors in the Kangcun Formation: R12 to R25. R13 terminates
southwards onto the continuous reflector of R12 at the 53 km of the profile. R19
terminates at 8-km site of the profile. The termination point of R21 expands
southwards to 145 km site of the profile, whereas termination point of R25 retreats by
94 km in comparison with R21 (Fig. 5 and Table 1). Termination points in the Kangcun
Formation expand southwards to the middle of the profile, but ones of R19 and R25
retreat back to the northern margin of the profile. The seismic facies of the Kangcun
13 / 53
Formation indicate that the thickness gradually decreases southwards and then remains
constant to the south of the 145-km position (Fig. 5).
The Kuqa Formation comprises 46 reflectors (R26?71). The basal part of the Kuqa
Formation is characterized by sequential southward onlaps of R29 to R32. Their
terminations locate at 133 km, 182 km, 42 km and 195 km, respectively. Eleven onlaps
occur in the middle part of the Kuqa Formation above R34 and below R64. Among
these, R38 terminates at 229 km site, revealing the southernmost onlap in the foreland
unit. In the upper part of the Kuqa Formation, R67 to R71 terminate at 81 km, 43 km,
131 km, 26 km, and 73 km, respectively, reflecting southward migration of onlapping
points (Fig. 5 and Table 1). There are 20 onlaps in the Kuqa Formation in total,
including the southernmost onlap of R38 at 229 km site. The thickness of the Kuqa
Formation gradually decreases southwards along the profile.
The uppermost Xiyu Formation contains 12 reflectors (R73?R84). The middle and
upper Xiyu Formation are not imaged in the seismic profile, meaning that seismic
character and thickness variations cannot be determined (Figs. 4 and 5).
Considering that the seismic profile is not perpendicular to the Tian Shan range,
the perpendicular distances between these reflection terminations and Kuqa foreland
thrust belt were calculated based on reflector termination positions in this profile and
the angle between the direction of the profile and the trend of the Tian Shan (Fig. 6
and Table. 1). The areal extent and quantity of onlaps in the foreland unit increase
upwards. From R5 to R38, reflectors? terminations migrate southwards by ~157 km
(Fig. 6 and Table 1). The age of the lowermost reflector, R5, is ~26.3 Ma, according to
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Huang et al.?s work (2006). R11 and R25 are dated at ~13.4 and ~5.3 Ma according to
Huang et al. (2006) and Charreau et al. (2006), representing the base Kangcun and
Kuqa formations (Fig. 5b). The age of R38 is ~4.3 � 0.1 Ma according to the
interpolation result, with the southernmost termination.
According to the ages of seismic reflectors and distances between reflectors?
terminations and the southern Tian Shan orogenic wedge, the southward onlaps
migrated from the 25 to 50 km south of Kuqa fold belt at a rate of 1.6 � 0.1 mm/yr
between ~26.3 and 12 Ma (R5?14), but migrated from the 50-km site to the southern
end of the profile at a rate of 14.6 � 0.1 mm/yr after ~12 Ma (R13?71), indicating a
significant acceleration at ~12 Ma (Fig. 6). The southward migration rate of onlap
points in the Xiyu Formation was extrapolated based on the envelop line of southward
onlaps in Kangcun and Kuqa Formations.
5 Subsidence analysis of the southern Tian Shan foreland basin
Our subsidence analysis was based on logging data of four wells along the seismic
profile (Fig. 1). The total subsidence was calculated by using the methods of
decompaction, paleobathymetric correction, and eustatic correction (Allen and Allen,
2013). This methodology requires some basic information including the lithology, the
top and bottom depths, the age, the decompaction parameters (Table 2) and the
paleo-water depth of each stratigraphic unit. The top and bottom depths were extracted
from the wells. The paleobathymetry was estimated based on the sedimentary
environment analysis of each stratigraphic unit according to Guo et al. (2002), Li et al.
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(2004), Li et al. (2005) and Shao et al. (2006). The uncertainty on the estimated
sedimentary environment makes up most of the errors in the paleo-water depth. Two
backstripping methods were used to calculate tectonic subsidence, either 1) Airy
isostasy or 2) flexural backstripping (Allen and Allen, 2013). According to Aitken
(2011), flexural rigidity used for the calculation of flexure backstripping is 1.9�24.
Applying this methodology, we obtained the total subsidence (in red) and the
tectonic subsidence corrected for sediment loading by Airy isostasy (in yellow) or by
flexural backstripping (in black) for the four wells (Fig.7). All wells computations show
that the Cretaceous and Paleogene total and tectonic subsidence rates in the northern
Tarim Basin are very low. The subsidence rates remained approximately stable during
this time, although a slight increase is revealed by the data from well 1 and a slight
decrease is revealed by data from well 4. The total and tectonic subsidence curves from
wells 1?4 show a significant increase in subsidence rate after ~26 Ma (Fig. 7). The
acceleration in subsidence through ~26 Ma?1.7 Ma characterizes the basin associated
with flexure, and it may be indicative of the subsidence history of a pro-foreland basin
(Sinclair and Naylor, 2012). The maximum Cenozoic total and tectonic subsidence are
~4.3 km and ~2.0 km (Fig. 7), respectively, as revealed by the subsidence plot from the
well 2 located in the proximal part of the foredeep zone. The subsidence plots of well 4
in the distal part of foredeep zone show the least Cenozoic subsidence (Fig. 7).
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6 Discussion
The architectures of the southern Tian Shan foreland shown in the seismic profile
(Fig. 4) and its subsidence history (Fig. 7) suggest that the sedimentary sequences
comprise the Cambrian?Silurian, Devonian?Permian, Triassic, Jurassic?Cretaceous,
Paleogene and late Oligocene?Quaternary tectonostratigraphic units from bottom up.
6.1 Implications for Pre-Miocene history
The reverse faults of Fr5 and Fr6 cause the sharp bending of the Cambrian and the
lower Ordovician, but don?t affect the top of Ordovician. These imply that Fr5 and Fr6
moved during the Ordovician. Hence, the Cambrian in the Tarim Basin might be
deposited in a tectonically stable environment, but the basin experienced compressive
stress during the late Ordovician (Fig. 8; Jia et al., 1997; Lin et al., 2011; Lin et al.,
2015). The Silurian only occurs in the middle and southern parts of the seismic profile.
The limited extent may be due to compression and uplift of the northern Tarim Basin
since the end of the Ordovician (Fig. 8b; Lin et al., 2011).
The Devonian?Permian tectonostratigraphic unit is cut by Fr2, Fr3a, Fr3b, Fr4a
and Fr4b, but its thickness is not affected by these faults. The relative constant thickness
of the Devonian and its continuous and conformable seismic facies manifest a relatively
quiescent tectonic environment. The truncation of reflectors in the northward-thinning
Carboniferous?Permian strata indicates erosion in the northern Tarim Basin during the
deposition of the Carboniferous?Permian. It also reveals the southward migration of the
depocenter after the Devonian (Fig. 4), likely associated with the Paleozoic tectonic
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evolution of the Tian Shan range. The closure of the central Tian Shan Ocean and
subsequent closure of the South Tian Shan back-arc basin since the Late Devonian
produced the Central Tian Shan and the South Tian Shan suture zones (Charvet et al.,
2011; Xiao et al., 2013). The northern part of the Tarim Basin, which was positioned in
the south of the South Tian Shan suture zone, formed a foreland basin, and the central
Tarim
Basin
became
an
intracratonic
basin
(Jia
et
al.,
1997).
The
Carboniferous?Permian in the seismic profile were likely deposited in this central
intracratonic basin, so its main depocenter formed in the central area of the Tarim Basin
(Fig. 8c). The erosion in the northern Tarim Basin during this period is also a response
to uplift in the South Tian Shan.
The Triassic tectonostratigraphic unit thins northwards, and reflectors in the unit
terminate northward under the basal Jurassic. The Triassic strata in the profile are also
restricted to the central intracratonic basin, similar to the Permian. The truncation of
reflectors in this unit indicates that denudation still occurred in the northern Tarim
Basin due to uplift of the South Tian Shan during the Triassic (Fig. 8d; Matte et al.,
1996; Jia et al., 1997).
The Jurassic?Cretaceous tectonostratigraphic unit is cut by Fr2. The difference
between the offsets of the Cretaceous and the Triassic?Jurassic along the Fr2 indicates
two phases of fault movements: first phases before deposition of the Cretaceous and
second phase after it. The maximum thickness of the Jurassic occurs in 150 km south of
the Kuqa fold belt (Figs. 4 and 5), and the thickest Cretaceous strata are located ~50 km
north of the Jurassic depocenter. The thickness variations reveal a migration of the
18 / 53
depocenter towards the South Tian Shan during the Jurassic?-Cretaceous. In the
Mesozoic, the Tian Shan orogen was weakly reactivated and uplifted in response to
accretion at the southern margin of the Asian continent, especially in the Tibet area
(Hendrix et al., 1994; Dumitru et al., 2001; Du and Wang, 2007; Jolivet et al., 2010).
The mild uplift of the Mesozoic Tian Shan drove low-amplitude subsidence of the
foreland area, which increased tectonic subsidence along the northern Tarim Basin.
Hence, the depocenter migrated northward to the Tian Shan area during the
Jurassic?Cretaceous (Fig. 8e).
The Paleogene tectonostratigraphic unit has a relatively constant thickness in the
seismic profile. The R?1?R?4 northward onlaps suggest that a depocenter existed at the
southern end of the profile during the Paleogene. Seismic reflection profiling indicates
that the Paleogene sediments in the Junggar Basin (Wang et al., 2013) and Qaidam
Basin (Wang et al., 2008) are similar to those in the Tarim Basin, showing that the
Paleogene was a tectonically quiet interval responding to dynamic subsidence induced
by subduction of the ancient Indian plate beneath the Asian plate (Wang et al., 2013).
The normal fault of Fn1 in the northern Tarim basin may form during tectonically quiet
interval (Fig. 8f; Zhang et al., 2011; Li et al., 2016).
6.2 Foreland unit and its implications for uplift of the Tian Shan
6.2.1 Nature of the southern Tian Shan Foreland basin
The foreland sedimentary sequences are cut by Ft1, Ft2, and Ft3, accompanied by
the fault-related fold in the Kuqa fold belt. This unit includes the Jidike, Kangcun,
19 / 53
Kuqa, and Xiyu formations. The strata thin southwards from the proximal part of the
foredeep to the distal part. The seismic facies suggest that the onlaps to the south in the
unit extend ~157 km towards the foreland (Fig. 4). This reflector pattern records the
infill history of the southern Tian Shan foreland basin.
The subsidence rate in this foreland area increased significantly since the late
Oligocene based on well logging data (Fig. 6). Subsidence in the foreland area of the
southern Tian Shan Mountains since the late Oligocene was driven mainly by flexure of
lithosphere induced by the coeval reactivation of the Tian Shan, which is part of the
intracontinental deformation of the Asian continent driven by the India-Eurasia
collision (Tapponnier and Molnar, 1979; Neil and Houseman, 1997; Yin et al., 1998).
Wide-angle seismic reflection/refraction profiling and magnetotelluric probing
demonstrate that the Tarim block underthrusts beneath the Tian Shan (Zhao et al.,
2003). Therefore, the deep structures beneath the southern Tian Shan foreland basin are
characteristic of the plate setting of a pro-foreland basin (Fig. 9). The subsidence plots
from four wells in the southern Tian Shan foreland area also indicate the classic rapidly
accelerating subsidence history of a pro-foreland basin from ~26 Ma onwards (Fig. 7).
In a typical pro-foreland basin, such as the southern Pyrenees Basin and the north
Alpine Basin, the foreland unit onlaps directly onto the subducted cratonic margin
towards the foreland (Naylor and Sinclair, 2008). However, the foreland sediments
already covered the whole northern Tarim Basin at the base of the foreland unit, with
non-sequential southward onlaps within each stratigraphic unit (Fig. 5). The
Jidike?Kuqa formations are present throughout the profile and have a constant
20 / 53
thickness (~1.5 s; ~2000 m) in the southern part of the profile and in the central area of
the Tarim Basin (Fig. 4; Jia et al., 1997). The Tarim Basin is bounded by the Tian
Shan, the western Kunlun range, and the Altyn ranges (Fig. 1). Thermochronological
and magnetostratigraphic works provide the evidence of exhumations in the three
mountain ranges since the Miocene (Chen et al., 2001; Wang et al., 2006; Cao et al.,
2009; Dumitru et al., 2001; Zhang et al., 2014), which may have induced the
sedimentary overfilling in the Tarim basin since then due to a small amount of outflux
from the basin. This means extra accommodation for sediments due to ponding and a
regional drape, such as the Jidike?Kuqa formations, across the Tarim basin since the
Miocene.
6.2.2 Initiation of the Tian Shan range
The base of the foreland unit is a diachronous unconformable surface, becoming
younger towards the foreland, and a geological record of the oldest clasts in foreland
basin derived from the Tian Shan range. The bottom corresponds to R5, the base of the
Jidike Formation (Fig. 5). The age of R5, in the lowest part of the Jidike Formation, is
constrained at ~26 Ma by magnetostratigraphic study (Huang et al., 2006; Fig. 4 and
Table 1), which is the age of the oldest southward onlap of the foreland unit. This
oldest onlapping sequence, along with widespread onlapping recognized in the foreland
unit, represents more direct evidence of initiation of the modern Tian Shan orogen.
The pro-foreland basin infill records the most recent development of the mountain
belt, and the oldest sediments preserved in it are not older than the initial growth of the
21 / 53
orogenic wedge (Naylor and Sinclair, 2008). Considering the plate tectonic setting of
the southern Tian Shan foreland area, the Paleozoic Tian Shan orogen could have been
rejuvenated to build the modern southern Tian Shan Mountains no later than the age of
the base of the foreland unit (~26 Ma). The tectonic subsidence rate in the southern
Tian Shan foreland basin increased significantly at ~26 Ma based on well data, which
suggests that subsidence was initiated due to lithosphere flexure driven by uplift of the
southern Tian Shan Mountains (Fig. 9). The Tarim block rotated clockwise relative to
the Junggar and Kazakhstan blocks in the Cenozoic, based on the westward increase in
Cenozoic shortening across the northern Tian Shan thrust belt (Avouc et al., 1993).
GPS measurements in central Asia show that the western part of the Tarim Basin
currently converges with Eurasia at a rate of ~20 mm/yr, but the rate of convergence
between the eastern Tarim and Junggar basins is ~10 mm/yr (Zubovich et al., 2010).
Therefore, the initial uplift of the eastern and western parts of Tian Shan may not have
been synchronous. Further work is required to understand the age of initial uplift in
different parts of the Tian Shan range.
6.2.3 Migration of the forebulge
In this foreland unit, the envelope line of the southward onlap points represents the
trace of the southward migration of the forebulge (Figs. 4 and 5). According to the
migration rate of southward stratal onlaps, the forebulge of the southern Tian Shan
foreland basin migrated from the 25 km to the 50 km south of the southern Tian Shan
orogenic wedge at a rate of 1.6 � 0.1 mm/yr between~26 Ma and ~12 Ma, but from the
22 / 53
50 km site to the southern end of the profile at a rate of 14.6 � 0.1 mm/yr after ~12 Ma
(R13?71). The migration rate increases by a factor of ~9 at ~12 Ma (Fig. 6).
Migration rate of forelandward stratal onlap in the pro-foreland basin is equal to the
rate of plate convergence plus a component driven by the outward growth of the thrust
wedge, based on numerical modeling (Naylor and Sinclair, 2008). Therefore, the
migration rate of the southward onlaps in the southern Tian Shan foreland basin is the
upper limit of the convergence rate between the Tarim and Junggar blocks.
The acceleration of the southward migration rate at ~12 Ma was possibly due to
accelerated convergence between the Tarim block and the Tian Shan orogen, and an
increase in the outward growth of the thrust wedge driven by a fast subduction rate
(Fig. 9). Increases in exhumation and deformation at ~11 Ma have been identified
throughout the Tian Shan by low-temperature thermochronological works (Bullen et
al., 2001; Sobel et
al., 2006; Macaulay et al., 2013). In addition, a
magnetostratigraphic study on the southern flank of the Central Tian Shan shows an
increase in sedimentation rate at ~11 Ma, suggesting the coeval accelerated uplift and
erosion of the Tian Shan range (Charreau et al., 2006). Evidently, a significant event
occurred in the late Miocene that affected the deformation style in the Tian Shan belt.
The events in the Tian Shan occurring in ~12Ma could have been results of the tectonic
changes to the south in the Himalayan-Tibetan orogen. One hypothesis is that
increasing elevation of the Tibetan plateau during the Miocene led to radially oriented
compressive strain in the area surrounding Tibet, thereby driving deformation into the
Tian Shan (Molnar and Stock, 2009).
23 / 53
7 Conclusions
1. The sedimentary sequences in the southern Tian Shan foreland area comprise the
Cambrian?Silurian,
Devonian?Permian,
Triassic,
Jurassic?Cretaceous,
and
Paleogene tectonostratigraphic units, overlain by the late Oligocene?Quaternary
foreland unit.
2. The oldest sedimentary sequence in the foreland succession is the base of the Jidike
Formation, which deposited at ~26 Ma based on magnetostratigraphic constraints.
The age of the Jidike Formation suggests that uplift of the modern southern Tian
Shan Mountains and subsidence of the coupled foreland basin started no later than
~26 Ma.
3. The tectonic subsidence rate in the southern Tian Shan foreland basin increased
significantly at ~26 Ma due to lithosphere flexure driven by uplift of the southern
Tian Shan.
4. The forebulge of the southern Tian Shan foreland basin migrated southward at a
rate of 1.6�1 mm/yr between ~26 and ~12 Ma, and at 14.6�1 mm/yr after ~12
Ma. The increase in migration rate at ~12 Ma indicates accelerated convergence
between the Tarim block and the Tian Shan.
Acknowledgments
This work was supported by the National Science Foundation of China [grant
number 41372201] and the National Science and Technology Major Project of China
[grant number 2017ZX05008001]. This study benefited from discussions with Hugh
24 / 53
Sinclair and Mark Naylor. Gratitude is expressed to Gang Xue and Zhongqiang Shan
for helping us with presentation of the seismic profile. Prof. Jacques Charvet and an
anonymous reviewer are warmly thanked for their constructive reviews. We are also
grateful for the help by Prof. Michel Faure (the handling editor) and Miss Diane Chung.
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Xiao, W., Windley, B. F., Allen, M. B., & Han, C., 2013. Paleozoic multiple
accretionary and collisional tectonics of the Chinese Tianshan orogenic collage.
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Yin, A., Nie, S., Craig, P., Harrison, T.M., Ryerson, F.J., Qian, X., & Yang, G., 1998.
Late Cenozoic tectonic evolution of the southern Chinese Tian Shan. Tectonics
17, 1?27.
Yu, S., Chen, W., Evans, N. J., McInnes, B.A., Yin, J., & Sun, J., 2014a. Cenozoic
uplift, exhumation and deformation in the north Kuqa depression, China as
constrained by (U-Th)/He thermochronometry. Tectonophysics, 630, 166?182.
Yu, Y., Tang, L., Yang, W., Huang, T., Qiu, N., & Li, W., 2014b. Salt structures and
hydrocarbon accumulations in the Tarim Basin, northwest China. American
Association of Petroleum Geologists Bulletin, 98, 135?159.
Zhang, T., Fang, X., Song, C., Appel, E., & Wang, Y., 2014. Cenozoic tectonic
deformation and uplift of the south Tian Shan: implications from
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magnetostratigraphy and balanced cross-section restoration of the Kuqa
depression. Tectonophysics, 628, 172?187.
Zhang, Y. P., Ren, J. Y., Yang, H, Z., Hu, D, S., & Li, P., 2011. Structure features and
its evolution of Lunnan low uplift, the Tarim Basin. Oil & Gas Geology, 32,
440?460. (in Chinese with English abstract)
Zhao, J., Liu, G., Lu, Z., Zhang, X., & Zhao, G., 2003. Lithospheric structure and
dynamic processes of the Tianshan orogenic belt and the Junggar basin.
Tectonophysics, 376, 199?239.
Zubovich, A. V., Wang, X., Scherba, Y. G., Schelochkov, G. G., Reilinger, R., &
Reigber, C., Mosienko, O., Molnar, P., Michajljow, W., Makarov, V, I., Li, J.,
Kuzikov, S, I., Herring, T, A., Hamburger, M, W., Hager, B, H., Dang, Y. M.,
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Figure captions:
Figure 1. (a) Location of the modern Tian Shan intracontinental orogen within the
India-Tibet collision system (modified after Zhang et al., 2014). The movement
direction of the Indian plate is from Wang et al. (2001). (b) Shaded relief map and
thickness contours (unit: m) of the Cenozoic in the Tarim Basin (modified after Jia et
al., 1997). Locations of Figures 2b and 4 are marked with thick lines. Black dots
represent the four drill holes (Wells 1, 2, 3 and 4) in the northern Tarim Basin. (c)
36 / 53
Geological sketch map of the Tian Shan belt and main structural elements of the study
area (modified after Yu et al., 2014a). (d) The general cross-section across the entire
Tarim Basin (redrawn after Yu et al., 2014b), which illustrates basic structures in the
basin. 1 = Neogene; 2 = Paleogene; 3 = Cretaceous; 4 = Jurassic; 5 = Triassic; 6 =
Permian; 7 = Carboniferous; 8 = Devonian; 9 = Silurian; 10 = Middle and Upper
Ordovician; 11 = Lower Ordovician and Cambrian; 12 = Sinian. See Figure 1a for the
location of the section.
Figure 2. (a) Detailed geological map (Zhang et al., 2014) showing the locations of the
studied seismic profiles in the Kuqa foreland basin (see Fig.1b for location). Colorful
heavy traces indicate the locations of measured magnetostratigraphic sections by
Huang et al. (2006, 2010), Charreau et al. (2006; 2009), Sun et al. (2009), and Zhang et
al. (2014). Black full lines show the locations of seismic profiles. (b) Corresponding
relationship between the magnetostratigraphic ages and seismic reflectors. The
stratigraphic sketch of the seismic profile (Wang et al., 2011) is combined with the
magnetostratigraphic results from several sections along the Kuqa River (Charreau et
al., 2006, 2009; Huang et al., 2006).
Figure 3. Stratigraphic chart of the Kuqa River Section in the Kuqa foreland basin
modified from Li et al. (2005). Timescale is from Charreau et al. (2006) and Huang et
al. (2006). The sedimentary environment of each stratigraphic unit is estimated based
on the references of Guo et al. (2002), Li et al. (2004) and Shao et al. (2006).
37 / 53
Figure 4. Original (a) and interpreted (b) seismic profile across the southern Tian Shan
foreland area. See Figure 1 for location. Vertical exaggeration is ~5?. The formations?
boundaries in the seismic profile are determined based on well logging, outcrop data
near the profile and interpretations of seismic profiles in the Kuqa foreland basin from
Li et al. (2016). Black dots and arrows indicate terminations of seismic reflectors. The
slight anticline near the well 3 is an artifact caused by the direction change of the
profile. (c) & (d) Interpreted zoom of the profile delimited by the dashed square and
showing the characters of Fn1, Fr1, Fr2, Fr3a, Fr3b, Fr4a and Fr4b, without vertical
exaggeration.
Figure 5. (a) Interpreted seismic profile crossing the foredeep of the Kuqa foreland
basin. Reflectors are marked with dark blue lines and named R1?84 and R?1?4. (b)
Tracing lines of seismic reflectors in the Cenozoic and correlation between the
magnetostratigraphic ages and seismic reflectors. The arrows indicate reflector
terminations. The onlap points toward the foreland are marked with a blue dashed line.
The magnetostratigraphic column is from Charreau et al. (2006) and Huang et al.
(2006). See Figure 2 for the correlation between the seismic profiles and the
magnetostratigraphic results. (c) Reflectors in the Paleogene highlight the northward
trend in onlap. (d) Reflectors highlight the oldest onlap southward and the lateral
migration of the sedimentary sequence coupled with the Cenozoic uplift of the southern
Tian Shan Mountains.
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Figure 6. Plot of ages of reflectors of the Kuqa foreland sequences and distances from
their termination points to the southern Tian Shan orogenic wedge. The envelope line of
terminations indicates migration of the forebulge. An increase in the rate of onlap
towards the foreland is identified at ~12 Ma. All age constraints are determined by the
correlation between the seismic profile and magnetostratigraphic results (Charreau et
al., 2006; Huang et al., 2006).
Figure 7. Subsidence curves in wells 1, 2, 3 and 4. The total subsidence and tectonic
subsidence curves are corrected for sediment loading using Airy isostasy
(backstripping), and the tectonic subsidence curve is also corrected for sediment
loading by flexure backstripping using data from the wells in the northern Tarim Basin.
Well positions are shown in Figure 1. The dotted lines mark the age of the base of the
foreland unit. The solid red lines represent total subsidence without any correction for
sediment loading. The black solid lines show the tectonic subsidence corrected for
sediment loading by flexure backstripping, and the solid yellow lines show the tectonic
subsidence corrected for sediment loading based on Airy isostasy. The error bars of
subsidence due to paleobathymetric corrections and eustatic corrections are showed by
the slim black boxes. In the flexure backstripping calculation, the rigidity of the Tarim
plate was estimated at 1.9�24 Nm according to Aitken (2011).
Figure 8. Phanerozoic tectonic evolution in the southern Tian Shan foreland area and
the northern Tarim basin. Cross sections containing the Paleozoic evolution in the Tian
Shan area modified after Li et al. (2004), Charvet et al. (2011) and Xiao et al. (2013).
39 / 53
The history of faulting in the section is based on interpretation of the seismic profile and
previous analysis of gravitational and aeromagnetic data (Lin et al., 2015).
Figure 9. Schematic sketch of the Cenozoic evolution of filling sequence in the
southern Tian Shan foreland basin. The southward onlaps are marked by blue circles.
The length of arrow (V) represents velocity of the southward migration of the forebulge
in the Tarim basin. The numbers of ?1? through ?5? are equidistant markers in the
Tarim blocks.
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Table captions:
Table 1. Locations of seismic reflection terminations, distances between terminations
and the southern Tianshan orogenic wedge and ages of reflectors in the Cenozoic. All
age constraints are determined by the correlation between the seismic profile and
magnetostratigraphical studies (Charreau et al., 2006; Huang et al., 2006).
Table 2. Lithologies and decompaction parameters used in the tectonic subsidence
analysis (Sclater and Christie, 1980).
50 / 53
Tables:
Table 1
Perpendicular distance between
Number of
reflection terminations and the
Orientation of
Formation
Age(Ma)*
Reflector
southern Tianshan orogenic wedge
termination
(km)
R'1
250.7
197.5
Northward
Paleogene
R'2
160.0
133.2
Northward
Paleogene
R'3
188.0
153.1
Northward
Paleogene
R'4
256.2
201.3
Northward
Paleogene
R5
26.5
25.0
Southward
Jidike
26.3�1
R6
36.8
34.8
Southward
Jidike
24.2�1
R9
39.3
37.1
Southward
Jidike
17.9�1
R13
52.6
49.7
Southward
Kangcun
12.2�5
R19
7.7
7.3
Southward
Kangcun
8.9�5
R21
144.9
122.5
Southward
Kangcun
7.6�5
R25
50.6
47.8
Southward
Kangcun
5.3�5
R29
133.3
114.3
Southward
Kuqa
5.0�1
R30
182.4
149.1
Southward
Kuqa
4.9�1
R31
41.6
39.3
Southward
Kuqa
4.8�1
R32
195.0
158.0
Southward
Kuqa
4.7�1
R34
24.4
23.0
Southward
Kuqa
4.6�1
R38
229.2
182.2
Southward
Kuqa
4.3�1
R39
153.2
128.4
Southward
Kuqa
4.2�1
R46
6.1
5.8
Southward
Kuqa
3.7�1
R47
198.1
160.2
Southward
Kuqa
3.6�1
R49
149.6
125.9
Southward
Kuqa
3.4�1
R51
217.0
173.6
Southward
Kuqa
3.3�1
R56
222.6
177.6
Southward
Kuqa
2.9�1
R57
202.7
163.5
Southward
Kuqa
2.8�1
R59
136.6
116.7
Southward
Kuqa
2.6�1
R63
96.8
88.5
Southward
Kuqa
2.3�1
R67
81.3
76.8
Southward
Kuqa
2.0�1
R68
42.8
40.4
Southward
Kuqa
1.9�1
R69
131.1
123.8
Southward
Kuqa
1.9�1
R70
26.4
24.9
Southward
Kuqa
1.8�1
R71
72.8
68.8
Southward
Kuqa
1.7�1
* All age constraints are determined by the correlation between the seismic profile and magnetostratigraphic results
(Charreau et al., 2006; Huang et al.,2006)
Reflection
termination positions
in Fig.5(km)
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Table 2
Lithology
Compaction decay length
C*105/cm-1
Sandstone
Shale stone
Limestone
Shaly sand
0.27
0.51
0.71
0.39
Initial porosity
Sediment grain
?0
density ?s/g cm-3
0.49
0.63
0.7
0.56
2.65
2.72
2.71
2.68
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80�
100�
90�
Junggar
Junggar Basin
110�
45�
60� North 70�
Kazakhstan
Urumqi
West Kunlu n
35
-3
0
Arabian
Sea
ai
m
n
m
Bo
ud
/a
ar
y T
hrus
t
India Plate
i
g
ngta
a
sh
st
thru
M
n
Kuq
N
44癗
a fo
oun
db
relan
asin
ns
i
a
t
Turpan
Basin
Ta rim Bl oc k
(a)
Luntai
Korla
1
2
42癗
Kuqa
3
26 - 12 Ma
N
Akesu
belt
Tarim Block
Tarim
Basin
40癗
(b)
82癊
84癊
86癊
88癊
2
3
4
Before 26Ma
N
Low:-192 m
80癊
1
Southern
Tianshan
V
5
Scale
High:7070 m
78癊
Southern
Tianshan
V
Yanqi Basin
Kashi
76癊
12 - 0 Ma
Yili Block
Tibet Plateau
ian
Kep
am
25�
M
Er do s
35�
Qaid
Pamir
T
N
Tarim Block
1
(c)
2
3
4
5
Southern
Tianshan
Highlights:
?
The uplift of the modern southern Tianshan initiated by at least ~26 Ma.
?
The related foreland basin migrates southwards at 1.6 mm/a between 26.3 and
~12 Ma.
?
The foreland basin migrates southwards at 14.6 mm/a since ~12 Ma.
53 / 53
ting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tectonostratigraphic history of the southern Tian Shan Mountains, Western China, from
seismic reflection profiling
Chao Lia, ShengLi Wanga?, LiangShu Wanga*
a
State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences,
Nanjing University, Nanjing 210093, China.
ABSTRACT
The modern Tian Shan range formed due to the India-Eurasia collision. Uplift of the
Tian Shan is recorded notably by the sedimentary sequences and subsidence history of
the coupling foreland basins. We analyzed a 353-km-long seismic profile and logging
data of four wells in the southern Tian Shan foreland area to decipher its tectonic,
stratigraphic, and subsidence history. The sedimentary sequences comprise the
Cambrian-Silurian, Devonian-Permian, Triassic, Jurassic-Cretaceous, and Paleogene
tectonostratigraphic units, overlain by the late Oligocene-Quaternary foreland basin
unit. The Jidike Formation is the oldest sedimentary sequence of the foreland
succession, deposited at ~26 Ma based on magnetostratigraphic constraints, indicating
that uplift of the southern Tian Shan Mountains was initiated by at least ~26 Ma. In
addition, the tectonic subsidence rate of the southern Tian Shan foreland basin
increased significantly since ~26 Ma due to lithospheric flexure caused by building of
?
Corresponding authors.
E-mail: wangsl@nju.edu.cn (S.-L. Wang), lswang@nju.edu.cn (L.-S. Wang).
1 / 53
the topography of the southern Tian Shan Mountains. The envelope line of the
forelandward onlaping points within the foreland unit indicates that the forebulge of
the southern Tian Shan foreland basin migrated southwards at 1.6�1 mm/yr between
~26.3 Ma and ~12 Ma and at 14.6�1 mm/yr after ~12 Ma. The increase of migration
rate at ~12 Ma suggests an accelerated convergence between the Tarim Basin and the
Tian Shan.
Keywords: Tian Shan, Tarim, foreland basin, seismic reflection profiling
1 Introduction
The modern Tian Shan range is the most significant geological structure in central
Asia (Fig. 1), extending east-west for ~2500 km, with the Tuomoer peak reaching
about 7400 m. The modern Tian Shan range is part of the intracontinental deformation
caused by the India-Eurasia collision (Molnar and Tapponnier, 1975; Tapponnier and
Molnar, 1979). The initiation and processes of mountain-building are still poorly
understood and hotly debated. How the deformation propagated from the southern
margin of Asia to remote intracontinental regions and the timing and process of initial
growth have been focused for a long time (Neil and Houseman, 1997; England and
Molnar, 1997; Yin et al., 1998). Recent magnetostratigraphic works along both flanks
of the Tian Shan indicate a range from the late Eocene to the late Miocene for initiation
of the Tian Shan (Huang et al., 2006; Ji et al., 2008; Zhang et al., 2014).
Low-temperature thermochronological data suggest that exhumation in the Tian Shan
started from the late Oligocene (Hendrix et al., 1994; Dumitru et al., 2001; Macaulay
2 / 53
et al., 2013; Yu et al., 2014a). Active thrusting and folding in the northern piedmont of
the Tian Shan indicate that its uplift occurs since the Miocene (Avouac et al., 1993) to
Quaternary (Burchfiel et al., 1999). Many of these studies worked on limited
section(s), rather than on a scale of the foreland basin. The discrepancy in the timing
probably relates to the various geological records across the mountains encountered
by different authors and a lack of comprehensive work on the foredeep sedimentary
sequences in the foreland basins.
The topographic evolution of mountain ranges and the development of foreland
basins are linked by the bending of the lithosphere related to the crustal thickened
wedge and the surface redistribution of mass due to the erosion of mountain
catchments and the sedimentation in the foreland basins. The sedimentary infill of
foreland basins is a major source of insight into the growth of mountain ranges
through geological time (Decelles and Giles, 1996; Sinclair, 2012). A number of
publications in the past 30 years have studied the interaction of thrust wedges and
foreland basins, and they unraveled and simulated the growth history of thrust wedge
based on the subsidence and sedimentary successions in foreland basin (Heller et al.,
1988; Flemings and Jordan, 1989; Sinclair et al., 1991; Liu et al., 2005). In recent
years, numerous high-quality seismic profiles have been acquired during hydrocarbon
exploration in the southern Tian Shan foreland areas, providing scientific insights into
the understanding of the foreland dynamics. Magnetostratigraphic studies (Huang et al.,
2006; Charreau et al., 2009) provide reliable chronostratigraphic constraints on the
foreland strata.
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In this paper, we present a ~353-km-long seismic profile (Fig. 1) across the
southern Tian Shan foreland area and the northern Tarim basin to track each seismic
reflector and to decipher the architecture and evolution of the foreland area. Six
tectonostratigraphic units have been identified, based on the architecture of seismic
reflectors? terminations. We deciphered the architecture of the foreland unit and its
implications for the growth of the modern southern Tian Shan Mountains. Ages of
reflectors in the foreland sedimentary sequences were determined based on
magnetostratigraphic studies (Huang et al., 2006; Charreau et al., 2009). We also
analyzed the Cenozoic subsidence history of the southern Tian Shan foreland basin
based on logging data of 4 wells.
2 Geological backgrounds
The Tian Shan orogen originated from accretion of several island arcs and collision
of continental blocks in the Paleozoic (Burtman, 1975; Windley et al., 1990; Xiao et al.,
2013), with large-scale dextral strike-slip occurring in the Permian (Bazhenov et al.,
1999; Laurent-Charvet et al., 2002; 2003; Wang et al., 2007; Choulet et al., 2011). In
the Mesozoic, the Tian Shan was reactivated in response to collisions of the Lhasa and
Qiangtang blocks with the southern margin of Asia (Dumitru et al., 2001; Chen et al.,
2011), recorded by apatite fission track analysis (Jolivet et al., 2010), sedimentary
deposits in the foreland area (Hendrix et al., 1992), and transpressional deformation in
the northeastern Tarim Basin (Wang et al., 2012). In the late Cenozoic, the Paleozoic
Tian Shan rejuvenated as an intracontinental orogen with high topography, caused by
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the ongoing India-Eurasia collision (Molnar and Tapponnier, 1975; Tapponnier and
Molnar, 1979; Avouac et al., 1993; Hendrix et al., 1994; Yin et al., 1998). GPS
measurements show that the present-day shortening rates across the Tian Shan are ~20
mm/yr at 78癊 and ~5?10 mm/yr at 84癊 (Wang et al., 2001; Zubovich et al., 2010),
indicating present growing of this orogen. The difference of shortening rates along the
strike of the Tian Shan may be caused by the rotation of the Tarim block (Zubovich et
al., 2010).
Building up of the modern Tian Shan range induces flexural subsidence of the
southern Junggar and the northern Tarim basins (Lu et al., 1994; Fig. 1). The Kuqa
foreland basin in the south piedmont of the Tian Shan has a maximum thickness of
~7 km of the Cenozoic rocks. The sequences include the Kumugeliemu (E1-2
km),
Suweiyi (E2-3S), Jidike (N1j; 26.3?13.4 Ma), Kangcun (N1k; 13.4?5.3 Ma), Kuqa (N2k;
5.3?1.7 Ma), and Xiyu ((N2-Q1) x; 1.7?0 Ma) formations in an ascending order (Fig.3).
Charreau et al. (2006; 2009) and Huang et al. (2006; 2010) constrained their ages using
magnetostratigraphic method.
Drillings in the northern Tarim basin reveal that the lower part of the Jurassic
consists of conglomerate intercalated with coal beds and that the upper part of the
Jurassic is of sandy conglomerate. The base of the Cretaceous also consists of
conglomerate, while the middle and upper parts of the Cretaceous are composed of
sand-shale interlayers. The Paleogene includes the Kumugeliemu and Suweiyi
formations. The former comprises purplish green mudstone and sandy mudstone
intercalated with gypsum; and the latter consists of red sandstone and mudstone,
5 / 53
intercalated with pebble layers and gypsum (Fig.3; Jia et al., 2004). The Jidike
Formation consists of grayish green and light green mudstone and sandstone
intercalated with gypsum. The Kangcun Formation is mainly composed of gray and
purple mudstone, sandy mudstone, and sandstone. The Kuqa Formation consists of
yellow, gray, and taupe sandstone, sandy mudstone, and mudstone (Jia et al., 2004).
The Xiyu Formation consists of monolithic, clast-supported, poorly to weakly sorted,
massive to horizontally and cross-bedded conglomerates, made up of pebble- to
block-sized clasts in a coarse-sand matrix (Charreau et al., 2009).
Major geological structures in the Kuqa foreland basin have been constrained by
seismic profiles (Wang et al., 2011). Thrust faults and folds have been identified in the
Triassic?Quaternary strata. Three main folds have been recognized from the
geomorphology, outcrop studies, and seismic profiles (Fig. 2a; Hubert-Ferrari et al.,
2007). These folds detach in d閏ollement levels located in the Tertiary and Jurassic coal
beds (Fig. 2b), and accommodate the majority of the deformation of the thrust belt
(Burchfiel et al., 1999; Saint-Carlier et al., 2016). Shortening rate of these folds
observed from seismic profiles is estimated at ~5.4 mm/yr since 2.58 Ma (Li et al.,
2012), roughly half of the present-day shortening rates across the Tian Shan.
3 Ages of the foreland sequences
Recent magnetostratigraphic studies of the Cenozoic strata in the eastern part of
the Kuqa foreland basin provide a chronostratigraphic framework (Fig. 2a). The ages of
the Jidike, Kangcun, and Kuqa along the Yaha section are determined by Huang et al.
6 / 53
(2006) and Charreau et al. (2009). The bases of these formations are dated at 26.3, 13.4,
and 5.3 Ma, respectively. These two magnetostratigraphic sections are close and
subparallel to our seismic profile (Fig. 2). Therefore, the detailed ages of the Cenozoic
strata can be correlated to the seismic profiles and extrapolated across the northern
Tarim (Figs. 2 and 5).
Charreau et al. (2009) demonstrated that the Xiyu Formation is a gravel wedge that
has prograded over the underthrusting forelands, as inferred from contrasting
magnetostratigraphic sections from the north and south piedmonts of the Tian Shan.
The low boundary of the Xiyu conglomerates is diachronous, ranging from ~5.2 to
~1.7 Ma. Accordingly, we infer that the age of the base of the Xiyu Formation in the
foredeep zone, far from the southern Tian Shan thrust belt, is younger than 1.7 Ma. The
base of the Xiyu Formation in the profile across northern Tarim was speculated as
~1.7 Ma (Figs. 3 and 5).
Assuming a constant sedimentation rate between any two reflectors in one
stratum, the age of the seismic reflectors in the Jidike thorough Kuqa formations (Fig.
4) were interpolated (Fig. 5b and Table 1).
4 Interpretation of the Seismic profile
A ~353-km-long and 6-second-deep (~12 km) seismic profile across the southern
Tian Shan foreland area, from the middle of the Kuqa foreland basin to the central
Tarim, is presented in Figure 4. The formations? boundaries in the seismic profile were
determined based on well logging, outcrop data near the profile and interpretations of
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seismic profiles in the Kuqa foreland basin from a previous publication (Li et al.,
2016). We tracked all the seismic reflectors in the profile and identified unconformities
based on seismic reflector continuity and terminations (Figs. 4 and 5). The SNR
(signal-to-noise ratio) of the seismic profile in the Kuqa fold belt is too low to track
reflectors; while the seismic profile to south of the Kuqa fold belt is imaged well and
therefore can be tracked reliably. We have tracked 88 reflectors in the Cenozoic strata
which were named R1 through R84 and R'1 through R'4 in an ascending order. R1 to
R84 extend or terminate southwards within the seismic profile, while R'1 to R'4
terminate northwards (Fig. 5). The lateral variation of the sedimentary sequences is
determined by positions of reflectors? terminations and their characteristics.
4.1 The structures in the profile
In the northern part, three north-dipping thrust faults were recognized (Ft1, Ft2 and
Ft3). These thrust faults extend downwards to the Precambrian metamorphic basement.
The Ft3, the southernmost one, thrusts upwards into the Cretaceous, whereas the Ft2
penetrates upwards into the Jidike Formation. The uppermost fault, the Ft1, reaches the
ground surface. A fault-related fold in the hanging wall of the Ft1 is cut by a
south-dipping back-thrust (Fig. 4).
In the south of the Kuqa fold belt, eight reverse faults (Fr1, Fr2, Fr3a, Fr3b, Fr4a,
Fr4b, Fr5 and Fr6) and a normal fault (Fn1) in the deep part of the seismic profile
extend into the Precambrian basement (Fig. 4). The Fr3a, Fr3b and Fr4b extend
upwards to the Carboniferous, and the Fr4a incised into the Devonian. The vertical
8 / 53
offsets along the Fr3a, Fr3b, Fr4a and Fr4b are no more than ~60 msec (~200 m) (Figs.
4b and 4c). The inclinations of Fr3a and Fr4a are opposite to that of Fr3b and Fr4b.
The upper parts of Fr3a, Fr3b, Fr4a and Fr4b in the Devonian and Carboniferous
steepen close to the vertical (Fig. 4c). The Fr3a and Fr3b comprise a pop-up structure.
The Cambrian through the Jurassic between Fr3a and Fr3b are folded, which means
these two faults may stop moving in the Jurassic. Similar anticline structure occurs in
the Cambrian?Paleogene between the Fr4a and Fr4b. It implies that the Fr4a and Fr4b
remained moving until the Paleogene. Ren et al. (2011) and Lin et al. (2015)
demonstrate that the Fr3a, Fr3b, Fr4a and Fr4b began to move since the Cambrian.
In the north of the fault Fr3a, there are two reverse faults (Fr5 and Fr6) in the
Cambrian and the pre-Cambrian basement. These two faults of Fr5 and Fr6 cause the
sharp bending of the Cambrian and the lower Ordovician (Fig. 4), but don?t affect the
top of Ordovician. It indicates that these two reverse faults moved in the Ordovician.
The north-dipping reverse fault Fr2 penetrates upward to the lower Cretaceous.
The hanging wall of the Fr2 forms a fault-related fold. The maximum vertical offset
along the fault is about 30 msec (~120 m; Fig. 4). The Fr2, that is the Luntai fault,
also began to move since the Cambrian, based on previous analyses of gravitational
data, aeromagnetic data and seismic data (Ren et al., 2011; Lin et al., 2015). The offset
of the Cretaceous is obviously less than that of the Triassic?Jurassic along the Fr2,
which indicates two phases of fault movements during the Mesozoic: the fault moved
before the Cretaceous and folded the Cambrian?Jurassic in the hanging wall; then, it
stopped moving before the deposition of the Cretaceous in the Tarim basin; after that,
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the Fr2 reactivated and folded the Cambrian?Cretaceous in the hanging wall (Fig. 4d).
The south-dipping normal fault Fn1 extends from the pre-Mesozoic upwards to the
Cretaceous. The maximum vertical offset along the fault is about 100 msec (~400 m;
Fig. 4d). On top of Fn1, there exists a reversed fault (Fr1) with opposite inclination of
Fn1. Fr1 occurs within the Cretaceous, the Paleogene and the Jidike formation, and its
hanging wall is folded. The vertical offset along the Fr1 is no more than 60 msec
(~240 m; Fig. 4d). The southward thrusting, on the southern piedmont of the Tian
Shan since the Neogene (Yin et al., 1998; Zhao et al., 2003), could induce the Fr1.
4.2 Division of tectonostratigraphic units
The sedimentary sequences in the southern Tian Shan foreland area were divided
into six tectonostratigraphic units separated by unconformities between them identified
in the profile (Fig. 4). These unconformities were defined based on the architecture of
seismic reflectors? terminations, such as truncation.
4.2.1 The Cambrian-Silurian tectonostratigraphic unit
Conformable continuous reflectors of the Cambrian strata above the pre-Cambrian
basement reveal a constant thickness of Cambrian strata across the seismic profile.
These reflectors are truncated by the Cretaceous in the northern end of the profile
(Fig.4d), and extend southwards out of the profile (Fig.4c). Reflectors of the
Ordovician and Silurian are characterized by toplap under Devonian strata, which
indicates that the base of the Devonian is an unconformity. The Cambrian remains
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relatively uniform thickness through the profile, but the Ordovician and the Silurian
thins northward. The continuous and conformable seismic facies of the Cambrian
reveal that the strata deposited in a tectonically stable environment (Fig. 4). This
lowest tectonostratigraphic unit consists of the Cambrian, the Ordovician and the
Silurian.
4.2.2 The Devonian-Permian tectonostratigraphic unit
Horizontal reflectors in the Devonian terminate unconformably below the
Cretaceous strata in the northern end but extend out of the profile in the southern end.
The Carboniferous?Permian reflectors are parallel to the Devonian and are truncated by
the Triassic in the north. The Carboniferous?Permian layer thins northwards.
Therefore, the base of the Triassic is interpreted as an unconformity, and a northward
uplift appears to have occurred in the Carboniferous?Permian time. The second
tectonostratigraphic unit consists of the Devonian and Carboniferous?Permian strata
(Fig. 4).
4.2.3 The Triassic tectonostratigraphic unit
Conformable north-dipping reflectors in the Triassic terminate northwards under
the basal Jurassic, toplapping to the north, and extend southwards beyond the profile.
The northward-thinning Triassic is unconformably overlain by Jurassic strata. The
Triassic constitutes the third tectonostratigraphic layer (Fig. 4).
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4.2.4 The Jurassic-Cretaceous tectonostratigraphic unit
Reflectors in the Jurassic are characterized by toplap terminations in both north and
south ends of the profile, and therefore an upward decrease in reflector length. This
reveals a clear unconformity at the base of the Cretaceous. The maximum thickness of
the Jurassic occurs 150 km south of the Kuqa fold belt (Figs. 4 and 5). The Cretaceous
strata are present throughout the profile, and reflectors display, similarly to the
Jurassic, both southward and northward toplaps below the Paleogene surface. The
lengths of Cretaceous reflectors also decrease upwards. Therefore, the Cretaceous is
also covered unconformably by the basal Paleogene. The maximum thickness of the
Cretaceous occurs ~50 km north of the location of maximum Jurassic thickness. It
suggests that the Cretaceous depocenter moved northward compared to the Jurassic.
The Jurassic and the Cretaceous constitute the fourth tectonostratigraphic unit (Fig. 4).
4.2.5 The Paleogene tectonostratigraphic unit
The Paleogene strata have relatively constant thickness (~150 msec; ~300 msec)
throughout the profile. It consists in four continuous reflectors (R1, R2, R3 and R4),
that can be traced across the whole basin, and four reflectors (R'1, R?2, R?3 and R'4)
that terminate northwards at 251 km, 160 km, 188 km and 256 km sites in Figure 5
respectively. The Paleogene is overlain by the foreland sedimentary unit. The
Paleogene is the fifth tectonostratigraphic unit.
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4.2.6 The foreland unit
The foreland unit overlays the Paleogene tectonostratigraphic unit and includes
the Jidike (N1j), Kangcun (N1k), Kuqa (N2k), and Xiyu, ((N2-Q1)x) formations.
Reflectors in the foreland unit onlap to the south, and the areal extent of their
terminations migrate, in general, southwards from the bottom up, but they although
alternate back and forth (Fig. 5).
There are 7 reflectors in the Jidike Formation: R5, R6, R7, R8, R9, R10 and R11.
At the base of the Jidike Formation, R5 extends southwards and terminates at 27 km
site in the profile. The termination points of R6 and R9 are located between 37 and
40 km (Fig. 5), onlapping southward. The Jidike Formation progressively laps on the
Paleogene southward between 27 and 40 km in the profile (Fig. 5d and Table 1). The
Jidike Formation gradually thins southward, and the maximum thickness is located near
the southern margin of the southern Tian Shan foreland thrust belt. These indicate that
the depocenter of the northern Tarim block was located to the southern piedmont of the
Tian Shan during deposition of this formation.
There exist 14 reflectors in the Kangcun Formation: R12 to R25. R13 terminates
southwards onto the continuous reflector of R12 at the 53 km of the profile. R19
terminates at 8-km site of the profile. The termination point of R21 expands
southwards to 145 km site of the profile, whereas termination point of R25 retreats by
94 km in comparison with R21 (Fig. 5 and Table 1). Termination points in the Kangcun
Formation expand southwards to the middle of the profile, but ones of R19 and R25
retreat back to the northern margin of the profile. The seismic facies of the Kangcun
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Formation indicate that the thickness gradually decreases southwards and then remains
constant to the south of the 145-km position (Fig. 5).
The Kuqa Formation comprises 46 reflectors (R26?71). The basal part of the Kuqa
Formation is characterized by sequential southward onlaps of R29 to R32. Their
terminations locate at 133 km, 182 km, 42 km and 195 km, respectively. Eleven onlaps
occur in the middle part of the Kuqa Formation above R34 and below R64. Among
these, R38 terminates at 229 km site, revealing the southernmost onlap in the foreland
unit. In the upper part of the Kuqa Formation, R67 to R71 terminate at 81 km, 43 km,
131 km, 26 km, and 73 km, respectively, reflecting southward migration of onlapping
points (Fig. 5 and Table 1). There are 20 onlaps in the Kuqa Formation in total,
including the southernmost onlap of R38 at 229 km site. The thickness of the Kuqa
Formation gradually decreases southwards along the profile.
The uppermost Xiyu Formation contains 12 reflectors (R73?R84). The middle and
upper Xiyu Formation are not imaged in the seismic profile, meaning that seismic
character and thickness variations cannot be determined (Figs. 4 and 5).
Considering that the seismic profile is not perpendicular to the Tian Shan range,
the perpendicular distances between these reflection terminations and Kuqa foreland
thrust belt were calculated based on reflector termination positions in this profile and
the angle between the direction of the profile and the trend of the Tian Shan (Fig. 6
and Table. 1). The areal extent and quantity of onlaps in the foreland unit increase
upwards. From R5 to R38, reflectors? terminations migrate southwards by ~157 km
(Fig. 6 and Table 1). The age of the lowermost reflector, R5, is ~26.3 Ma, according to
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Huang et al.?s work (2006). R11 and R25 are dated at ~13.4 and ~5.3 Ma according to
Huang et al. (2006) and Charreau et al. (2006), representing the base Kangcun and
Kuqa formations (Fig. 5b). The age of R38 is ~4.3 � 0.1 Ma according to the
interpolation result, with the southernmost termination.
According to the ages of seismic reflectors and distances between reflectors?
terminations and the southern Tian Shan orogenic wedge, the southward onlaps
migrated from the 25 to 50 km south of Kuqa fold belt at a rate of 1.6 � 0.1 mm/yr
between ~26.3 and 12 Ma (R5?14), but migrated from the 50-km site to the southern
end of the profile at a rate of 14.6 � 0.1 mm/yr after ~12 Ma (R13?71), indicating a
significant acceleration at ~12 Ma (Fig. 6). The southward migration rate of onlap
points in the Xiyu Formation was extrapolated based on the envelop line of southward
onlaps in Kangcun and Kuqa Formations.
5 Subsidence analysis of the southern Tian Shan foreland basin
Our subsidence analysis was based on logging data of four wells along the seismic
profile (Fig. 1). The total subsidence was calculated by using the methods of
decompaction, paleobathymetric correction, and eustatic correction (Allen and Allen,
2013). This methodology requires some basic information including the lithology, the
top and bottom depths, the age, the decompaction parameters (Table 2) and the
paleo-water depth of each stratigraphic unit. The top and bottom depths were extracted
from the wells. The paleobathymetry was estimated based on the sedimentary
environment analysis of each stratigraphic unit according to Guo et al. (2002), Li et al.
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(2004), Li et al. (2005) and Shao et al. (2006). The uncertainty on the estimated
sedimentary environment makes up most of the errors in the paleo-water depth. Two
backstripping methods were used to calculate tectonic subsidence, either 1) Airy
isostasy or 2) flexural backstripping (Allen and Allen, 2013). According to Aitken
(2011), flexural rigidity used for the calculation of flexure backstripping is 1.9�24.
Applying this methodology, we obtained the total subsidence (in red) and the
tectonic subsidence corrected for sediment loading by Airy isostasy (in yellow) or by
flexural backstripping (in black) for the four wells (Fig.7). All wells computations show
that the Cretaceous and Paleogene total and tectonic subsidence rates in the northern
Tarim Basin are very low. The subsidence rates remained approximately stable during
this time, although a slight increase is revealed by the data from well 1 and a slight
decrease is revealed by data from well 4. The total and tectonic subsidence curves from
wells 1?4 show a significant increase in subsidence rate after ~26 Ma (Fig. 7). The
acceleration in subsidence through ~26 Ma?1.7 Ma characterizes the basin associated
with flexure, and it may be indicative of the subsidence history of a pro-foreland basin
(Sinclair and Naylor, 2012). The maximum Cenozoic total and tectonic subsidence are
~4.3 km and ~2.0 km (Fig. 7), respectively, as revealed by the subsidence plot from the
well 2 located in the proximal part of the foredeep zone. The subsidence plots of well 4
in the distal part of foredeep zone show the least Cenozoic subsidence (Fig. 7).
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6 Discussion
The architectures of the southern Tian Shan foreland shown in the seismic profile
(Fig. 4) and its subsidence history (Fig. 7) suggest that the sedimentary sequences
comprise the Cambrian?Silurian, Devonian?Permian, Triassic, Jurassic?Cretaceous,
Paleogene and late Oligocene?Quaternary tectonostratigraphic units from bottom up.
6.1 Implications for Pre-Miocene history
The reverse faults of Fr5 and Fr6 cause the sharp bending of the Cambrian and the
lower Ordovician, but don?t affect the top of Ordovician. These imply that Fr5 and Fr6
moved during the Ordovician. Hence, the Cambrian in the Tarim Basin might be
deposited in a tectonically stable environment, but the basin experienced compressive
stress during the late Ordovician (Fig. 8; Jia et al., 1997; Lin et al., 2011; Lin et al.,
2015). The Silurian only occurs in the middle and southern parts of the seismic profile.
The limited extent may be due to compression and uplift of the northern Tarim Basin
since the end of the Ordovician (Fig. 8b; Lin et al., 2011).
The Devonian?Permian tectonostratigraphic unit is cut by Fr2, Fr3a, Fr3b, Fr4a
and Fr4b, but its thickness is not affected by these faults. The relative constant thickness
of the Devonian and its continuous and conformable seismic facies manifest a relatively
quiescent tectonic environment. The truncation of reflectors in the northward-thinning
Carboniferous?Permian strata indicates erosion in the northern Tarim Basin during the
deposition of the Carboniferous?Permian. It also reveals the southward migration of the
depocenter after the Devonian (Fig. 4), likely associated with the Paleozoic tectonic
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evolution of the Tian Shan range. The closure of the central Tian Shan Ocean and
subsequent closure of the South Tian Shan back-arc basin since the Late Devonian
produced the Central Tian Shan and the South Tian Shan suture zones (Charvet et al.,
2011; Xiao et al., 2013). The northern part of the Tarim Basin, which was positioned in
the south of the South Tian Shan suture zone, formed a foreland basin, and the central
Tarim
Basin
became
an
intracratonic
basin
(Jia
et
al.,
1997).
The
Carboniferous?Permian in the seismic profile were likely deposited in this central
intracratonic basin, so its main depocenter formed in the central area of the Tarim Basin
(Fig. 8c). The erosion in the northern Tarim Basin during this period is also a response
to uplift in the South Tian Shan.
The Triassic tectonostratigraphic unit thins northwards, and reflectors in the unit
terminate northward under the basal Jurassic. The Triassic strata in the profile are also
restricted to the central intracratonic basin, similar to the Permian. The truncation of
reflectors in this unit indicates that denudation still occurred in the northern Tarim
Basin due to uplift of the South Tian Shan during the Triassic (Fig. 8d; Matte et al.,
1996; Jia et al., 1997).
The Jurassic?Cretaceous tectonostratigraphic unit is cut by Fr2. The difference
between the offsets of the Cretaceous and the Triassic?Jurassic along the Fr2 indicates
two phases of fault movements: first phases before deposition of the Cretaceous and
second phase after it. The maximum thickness of the Jurassic occurs in 150 km south of
the Kuqa fold belt (Figs. 4 and 5), and the thickest Cretaceous strata are located ~50 km
north of the Jurassic depocenter. The thickness variations reveal a migration of the
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depocenter towards the South Tian Shan during the Jurassic?-Cretaceous. In the
Mesozoic, the Tian Shan orogen was weakly reactivated and uplifted in response to
accretion at the southern margin of the Asian continent, especially in the Tibet area
(Hendrix et al., 1994; Dumitru et al., 2001; Du and Wang, 2007; Jolivet et al., 2010).
The mild uplift of the Mesozoic Tian Shan drove low-amplitude subsidence of the
foreland area, which increased tectonic subsidence along the northern Tarim Basin.
Hence, the depocenter migrated northward to the Tian Shan area during the
Jurassic?Cretaceous (Fig. 8e).
The Paleogene tectonostratigraphic unit has a relatively constant thickness in the
seismic profile. The R?1?R?4 northward onlaps suggest that a depocenter existed at the
southern end of the profile during the Paleogene. Seismic reflection profiling indicates
that the Paleogene sediments in the Junggar Basin (Wang et al., 2013) and Qaidam
Basin (Wang et al., 2008) are similar to those in the Tarim Basin, showing that the
Paleogene was a tectonically quiet interval responding to dynamic subsidence induced
by subduction of the ancient Indian plate beneath the Asian plate (Wang et al., 2013).
The normal fault of Fn1 in the northern Tarim basin may form during tectonically quiet
interval (Fig. 8f; Zhang et al., 2011; Li et al., 2016).
6.2 Foreland unit and its implications for uplift of the Tian Shan
6.2.1 Nature of the southern Tian Shan Foreland basin
The foreland sedimentary sequences are cut by Ft1, Ft2, and Ft3, accompanied by
the fault-related fold in the Kuqa fold belt. This unit includes the Jidike, Kangcun,
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Kuqa, and Xiyu formations. The strata thin southwards from the proximal part of the
foredeep to the distal part. The seismic facies suggest that the onlaps to the south in the
unit extend ~157 km towards the foreland (Fig. 4). This reflector pattern records the
infill history of the southern Tian Shan foreland basin.
The subsidence rate in this foreland area increased significantly since the late
Oligocene based on well logging data (Fig. 6). Subsidence in the foreland area of the
southern Tian Shan Mountains since the late Oligocene was driven mainly by flexure of
lithosphere induced by the coeval reactivation of the Tian Shan, which is part of the
intracontinental deformation of the Asian continent driven by the India-Eurasia
collision (Tapponnier and Molnar, 1979; Neil and Houseman, 1997; Yin et al., 1998).
Wide-angle seismic reflection/refraction profiling and magnetotelluric probing
demonstrate that the Tarim block underthrusts beneath the Tian Shan (Zhao et al.,
2003). Therefore, the deep structures beneath the southern Tian Shan foreland basin are
characteristic of the plate setting of a pro-foreland basin (Fig. 9). The subsidence plots
from four wells in the southern Tian Shan foreland area also indicate the classic rapidly
accelerating subsidence history of a pro-foreland basin from ~26 Ma onwards (Fig. 7).
In a typical pro-foreland basin, such as the southern Pyrenees Basin and the north
Alpine Basin, the foreland unit onlaps directly onto the subducted cratonic margin
towards the foreland (Naylor and Sinclair, 2008). However, the foreland sediments
already covered the whole northern Tarim Basin at the base of the foreland unit, with
non-sequential southward onlaps within each stratigraphic unit (Fig. 5). The
Jidike?Kuqa formations are present throughout the profile and have a constant
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thickness (~1.5 s; ~2000 m) in the southern part of the profile and in the central area of
the Tarim Basin (Fig. 4; Jia et al., 1997). The Tarim Basin is bounded by the Tian
Shan, the western Kunlun range, and the Altyn ranges (Fig. 1). Thermochronological
and magnetostratigraphic works provide the evidence of exhumations in the three
mountain ranges since the Miocene (Chen et al., 2001; Wang et al., 2006; Cao et al.,
2009; Dumitru et al., 2001; Zhang et al., 2014), which may have induced the
sedimentary overfilling in the Tarim basin since then due to a small amount of outflux
from the basin. This means extra accommodation for sediments due to ponding and a
regional drape, such as the Jidike?Kuqa formations, across the Tarim basin since the
Miocene.
6.2.2 Initiation of the Tian Shan range
The base of the foreland unit is a diachronous unconformable surface, becoming
younger towards the foreland, and a geological record of the oldest clasts in foreland
basin derived from the Tian Shan range. The bottom corresponds to R5, the base of the
Jidike Formation (Fig. 5). The age of R5, in the lowest part of the Jidike Formation, is
constrained at ~26 Ma by magnetostratigraphic study (Huang et al., 2006; Fig. 4 and
Table 1), which is the age of the oldest southward onlap of the foreland unit. This
oldest onlapping sequence, along with widespread onlapping recognized in the foreland
unit, represents more direct evidence of initiation of the modern Tian Shan orogen.
The pro-foreland basin infill records the most recent development of the mountain
belt, and the oldest sediments preserved in it are not older than the initial growth of the
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orogenic wedge (Naylor and Sinclair, 2008). Considering the plate tectonic setting of
the southern Tian Shan foreland area, the Paleozoic Tian Shan orogen could have been
rejuvenated to build the modern southern Tian Shan Mountains no later than the age of
the base of the foreland unit (~26 Ma). The tectonic subsidence rate in the southern
Tian Shan foreland basin increased significantly at ~26 Ma based on well data, which
suggests that subsidence was initiated due to lithosphere flexure driven by uplift of the
southern Tian Shan Mountains (Fig. 9). The Tarim block rotated clockwise relative to
the Junggar and Kazakhstan blocks in the Cenozoic, based on the westward increase in
Cenozoic shortening across the northern Tian Shan thrust belt (Avouc et al., 1993).
GPS measurements in central Asia show that the western part of the Tarim Basin
currently converges with Eurasia at a rate of ~20 mm/yr, but the rate of convergence
between the eastern Tarim and Junggar basins is ~10 mm/yr (Zubovich et al., 2010).
Therefore, the initial uplift of the eastern and western parts of Tian Shan may not have
been synchronous. Further work is required to understand the age of initial uplift in
different parts of the Tian Shan range.
6.2.3 Migration of the forebulge
In this foreland unit, the envelope line of the southward onlap points represents the
trace of the southward migration of the forebulge (Figs. 4 and 5). According to the
migration rate of southward stratal onlaps, the forebulge of the southern Tian Shan
foreland basin migrated from the 25 km to the 50 km south of the southern Tian Shan
orogenic wedge at a rate of 1.6 � 0.1 mm/yr between~26 Ma and ~12 Ma, but from the
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50 km site to the southern end of the profile at a rate of 14.6 � 0.1 mm/yr after ~12 Ma
(R13?71). The migration rate increases by a factor of ~9 at ~12 Ma (Fig. 6).
Migration rate of forelandward stratal onlap in the pro-foreland basin is equal to the
rate of plate convergence plus a component driven by the outward growth of the thrust
wedge, based on numerical modeling (Naylor and Sinclair, 2008). Therefore, the
migration rate of the southward onlaps in the southern Tian Shan foreland basin is the
upper limit of the convergence rate between the Tarim and Junggar blocks.
The acceleration of the southward migration rate at ~12 Ma was possibly due to
accelerated convergence between the Tarim block and the Tian Shan orogen, and an
increase in the outward growth of the thrust wedge driven by a fast subduction rate
(Fig. 9). Increases in exhumation and deformation at ~11 Ma have been identified
throughout the Tian Shan by low-temperature thermochronological works (Bullen et
al., 2001; Sobel et
al., 2006; Macaulay et al., 2013). In addition, a
magnetostratigraphic study on the southern flank of the Central Tian Shan shows an
increase in sedimentation rate at ~11 Ma, suggesting the coeval accelerated uplift and
erosion of the Tian Shan range (Charreau et al., 2006). Evidently, a significant event
occurred in the late Miocene that affected the deformation style in the Tian Shan belt.
The events in the Tian Shan occurring in ~12Ma could have been results of the tectonic
changes to the south in the Himalayan-Tibetan orogen. One hypothesis is that
increasing elevation of the Tibetan plateau during the Miocene led to radially oriented
compressive strain in the area surrounding Tibet, thereby driving deformation into the
Tian Shan (Molnar and Stock, 2009).
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7 Conclusions
1. The sedimentary sequences in the southern Tian Shan foreland area comprise the
Cambrian?Silurian,
Devonian?Permian,
Triassic,
Jurassic?Cretaceous,
and
Paleogene tectonostratigraphic units, overlain by the late Oligocene?Quaternary
foreland unit.
2. The oldest sedimentary sequence in the foreland succession is the base of the Jidike
Formation, which deposited at ~26 Ma based on magnetostratigraphic constraints.
The age of the Jidike Formation suggests that uplift of the modern southern Tian
Shan Mountains and subsidence of the coupled foreland basin started no later than
~26 Ma.
3. The tectonic subsidence rate in the southern Tian Shan foreland basin increased
significantly at ~26 Ma due to lithosphere flexure driven by uplift of the southern
Tian Shan.
4. The forebulge of the southern Tian Shan foreland basin migrated southward at a
rate of 1.6�1 mm/yr between ~26 and ~12 Ma, and at 14.6�1 mm/yr after ~12
Ma. The increase in migration rate at ~12 Ma indicates accelerated convergence
between the Tarim block and the Tian Shan.
Acknowledgments
This work was supported by the National Science Foundation of China [grant
number 41372201] and the National Science and Technology Major Project of China
[grant number 2017ZX05008001]. This study benefited from discussions with Hugh
24 / 53
Sinclair and Mark Naylor. Gratitude is expressed to Gang Xue and Zhongqiang Shan
for helping us with presentation of the seismic profile. Prof. Jacques Charvet and an
anonymous reviewer are warmly thanked for their constructive reviews. We are also
grateful for the help by Prof. Michel Faure (the handling editor) and Miss Diane Chung.
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Figure captions:
Figure 1. (a) Location of the modern Tian Shan intracontinental orogen within the
India-Tibet collision system (modified after Zhang et al., 2014). The movement
direction of the Indian plate is from Wang et al. (2001). (b) Shaded relief map and
thickness contours (unit: m) of the Cenozoic in the Tarim Basin (modified after Jia et
al., 1997). Locations of Figures 2b and 4 are marked with thick lines. Black dots
represent the four drill holes (Wells 1, 2, 3 and 4) in the northern Tarim Basin. (c)
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Geological sketch map of the Tian Shan belt and main structural elements of the study
area (modified after Yu et al., 2014a). (d) The general cross-section across the entire
Tarim Basin (redrawn after Yu et al., 2014b), which illustrates basic structures in the
basin. 1 = Neogene; 2 = Paleogene; 3 = Cretaceous; 4 = Jurassic; 5 = Triassic; 6 =
Permian; 7 = Carboniferous; 8 = Devonian; 9 = Silurian; 10 = Middle and Upper
Ordovician; 11 = Lower Ordovician and Cambrian; 12 = Sinian. See Figure 1a for the
location of the section.
Figure 2. (a) Detailed geological map (Zhang et al., 2014) showing the locations of the
studied seismic profiles in the Kuqa foreland basin (see Fig.1b for location). Colorful
heavy traces indicate the locations of measured magnetostratigraphic sections by
Huang et al. (2006, 2010), Charreau et al. (2006; 2009), Sun et al. (2009), and Zhang et
al. (2014). Black full lines show the locations of seismic profiles. (b) Corresponding
relationship between the magnetostratigraphic ages and seismic reflectors. The
stratigraphic sketch of the seismic profile (Wang et al., 2011) is combined with the
magnetostratigraphic results from several sections along the Kuqa River (Charreau et
al., 2006, 2009; Huang et al., 2006).
Figure 3. Stratigraphic chart of the Kuqa River Section in the Kuqa foreland basin
modified from Li et al. (2005). Timescale is from Charreau et al. (2006) and Huang et
al. (2006). The sedimentary environment of each stratigraphic unit is estimated based
on the references of Guo et al. (2002), Li et al. (2004) and Shao et al. (2006).
37 / 53
Figure 4. Original (a) and interpreted (b) seismic profile across the southern Tian Shan
foreland area. See Figure 1 for location. Vertical exaggeration is ~5?. The formations?
boundaries in the seismic profile are determined based on well logging, outcrop data
near the profile and interpretations of seismic profiles in the Kuqa foreland basin from
Li et al. (2016). Black dots and arrows indicate terminations of seismic reflectors. The
slight anticline near the well 3 is an artifact caused by the direction change of the
profile. (c) & (d) Interpreted zoom of the profile delimited by the dashed square and
showing the characters of Fn1, Fr1, Fr2, Fr3a, Fr3b, Fr4a and Fr4b, without vertical
exaggeration.
Figure 5. (a) Interpreted seismic profile crossing the foredeep of the Kuqa foreland
basin. Reflectors are marked with dark blue lines and named R1?84 and R?1?4. (b)
Tracing lines of seismic reflectors in the Cenozoic and correlation between the
magnetostratigraphic ages and seismic reflectors. The arrows indicate reflector
terminations. The onlap points toward the foreland are marked with a blue dashed line.
The magnetostratigraphic column is from Charreau et al. (2006) and Huang et al.
(2006). See Figure 2 for the correlation between the seismic profiles and the
magnetostratigraphic results. (c) Reflectors in the Paleogene highlight the northward
trend in onlap. (d) Reflectors highlight the oldest onlap southward and the lateral
migration of the sedimentary sequence coupled with the Cenozoic uplift of the southern
Tian Shan Mountains.
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Figure 6. Plot of ages of reflectors of the Kuqa foreland sequences and distances from
their termination points to the southern Tian Shan orogenic wedge. The envelope line of
terminations indicates migration of the forebulge. An increase in the rate of onlap
towards the foreland is identified at ~12 Ma. All age constraints are determined by the
correlation between the seismic profile and magnetostratigraphic results (Charreau et
al., 2006; Huang et al., 2006).
Figure 7. Subsidence curves in wells 1, 2, 3 and 4. The total subsidence and tectonic
subsidence curves are corrected for sediment loading using Airy isostasy
(backstripping), and the tectonic subsidence curve is also corrected for sediment
loading by flexure backstripping using data from the wells in the northern Tarim Basin.
Well positions are shown in Figure 1. The dotted lines mark the age of the base of the
foreland unit. The solid red lines represent total subsidence without any correction for
sediment loading. The black solid lines show the tectonic subsidence corrected for
sediment loading by flexure backstripping, and the solid yellow lines show the tectonic
subsidence corrected for sediment loading based on Airy isostasy. The error bars of
subsidence due to paleobathymetric corrections and eustatic corrections are showed by
the slim black boxes. In the flexure backstripping calculation, the rigidity of the Tarim
plate was estimated at 1.9�24 Nm according to Aitken (2011).
Figure 8. Phanerozoic tectonic evolution in the southern Tian Shan foreland area and
the northern Tarim basin. Cross sections containing the Paleozoic evolution in the Tian
Shan area modified after Li et al. (2004), Charvet et al. (2011) and Xiao et al. (2013).
39 / 53
The history of faulting in the section is based on interpretation of the seismic profile and
previous analysis of gravitational and aeromagnetic data (Lin et al., 2015).
Figure 9. Schematic sketch of the Cenozoic evolution of filling sequence in the
southern Tian Shan foreland basin. The southward onlaps are marked by blue circles.
The length of arrow (V) represents velocity of the southward migration of the forebulge
in the Tarim basin. The numbers of ?1? through ?5? are equidistant markers in the
Tarim blocks.
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Table captions:
Table 1. Locations of seismic reflection terminations, distances between terminations
and the southern Tianshan orogenic wedge and ages of reflectors in the Cenozoic. All
age constraints are determined by the correlation between the seismic profile and
magnetostratigraphical studies (Charreau et al., 2006; Huang et al., 2006).
Table 2. Lithologies and decompaction parameters used in the tectonic subsidence
analysis (Sclater and Christie, 1980).
50 / 53
Tables:
Table 1
Perpendicular distance between
Number of
reflection terminations and the
Orientation of
Formation
Age(Ma)*
Reflector
southern Tianshan orogenic wedge
termination
(km)
R'1
250.7
197.5
Northward
Paleogene
R'2
160.0
133.2
Northward
Paleogene
R'3
188.0
153.1
Northward
Paleogene
R'4
256.2
201.3
Northward
Paleogene
R5
26.5
25.0
Southward
Jidike
26.3�1
R6
36.8
34.8
Southward
Jidike
24.2�1
R9
39.3
37.1
Southward
Jidike
17.9�1
R13
52.6
49.7
Southward
Kangcun
12.2�5
R19
7.7
7.3
Southward
Kangcun
8.9�5
R21
144.9
122.5
Southward
Kangcun
7.6�5
R25
50.6
47.8
Southward
Kangcun
5.3�5
R29
133.3
114.3
Southward
Kuqa
5.0�1
R30
182.4
149.1
Southward
Kuqa
4.9�1
R31
41.6
39.3
Southward
Kuqa
4.8�1
R32
195.0
158.0
Southward
Kuqa
4.7�1
R34
24.4
23.0
Southward
Kuqa
4.6�1
R38
229.2
182.2
Southward
Kuqa
4.3�1
R39
153.2
128.4
Southward
Kuqa
4.2�1
R46
6.1
5.8
Southward
Kuqa
3.7�1
R47
198.1
160.2
Southward
Kuqa
3.6�1
R49
149.6
125.9
Southward
Kuqa
3.4�1
R51
217.0
173.6
Southward
Kuqa
3.3�1
R56
222.6
177.6
Southward
Kuqa
2.9�1
R57
202.7
163.5
Southward
Kuqa
2.8�1
R59
136.6
116.7
Southward
Kuqa
2.6�1
R63
96.8
88.5
Southward
Kuqa
2.3�1
R67
81.3
76.8
Southward
Kuqa
2.0�1
R68
42.8
40.4
Southward
Kuqa
1.9�1
R69
131.1
123.8
Southward
Kuqa
1.9�1
R70
26.4
24.9
Southward
Kuqa
1.8�1
R71
72.8
68.8
Southward
Kuqa
1.7�1
* All age constraints are determined by the correlation between the seismic profile and magnetostratigraphic results
(Charreau et al., 2006; Huang et al.,2006)
Reflection
termination positions
in Fig.5(km)
51 / 53
Table 2
Lithology
Compaction decay length
C*105/cm-1
Sandstone
Shale stone
Limestone
Shaly sand
0.27
0.51
0.71
0.39
Initial porosity
Sediment grain
?0
density ?s/g cm-3
0.49
0.63
0.7
0.56
2.65
2.72
2.71
2.68
52 / 53
80�
100�
90�
Junggar
Junggar Basin
110�
45�
60� North 70�
Kazakhstan
Urumqi
West Kunlu n
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