Science in China Ser. D Earth Sciences 2004 Vol.47 Supp. II 53ü65
53
Neogene deformation of the Kuqa-Tianshan
Basin-range System
WANG Qingchen1, ZHANG Zhongpei1, LIN Wei1, SONG Wenjie2 & GUO Hong2
1. Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029,
China;
2. Tarim Petroleum Company, CNPC, Korla 841000, China
Correspondence should be addressed to Wang Qingchen (email: [email protected])
Received October 17, 2003
Abstract The Neogene deformation of the Kuqa-Tianshan Basin-range System is characterized by discrepancy in paleostress patterns on the basin-range boundary and in the basin interior,
and by discrepancy in deformation styles of the basement and the cover. Measurement and paleostress reconstruction of the brittle faults display a stress pattern with NW-SE and/or NNW-SSE
extension on the basin margin and NW-SE compression in the basin interior. The basement was
cut into blocks separated by boundary faults and upthrusts that were recognized in the seismic
reflection profile. The block-faulting could have caused vertical uplift of the basement and gravitational sliding in the overlying sedimentary cover. Theoretical calculation indicates two generations of potential decollement folds within the basin, with one being mudstone in the Triassic
Huangshanjie formation and the other the Cenozoic salt-gypsum layers.
Keywords: Kuqa basin, Tianshan Mountains, Neogene, paleostress, gravitational slide.
DOI: 10.1360/04zd0024
The Kuqa basin is located in the north of the
Tarim superimposed basin[1], that is bounded on the
north by the southern Tianshan and on the south by the
Tabei upwarp. Thick continental sediments of Triassic
to Quaternary in age have filled the Kuqa basin with
area of 16000 km2. The structural coupling between
the Kuqa basin and the Tianshan Mountains has become one of the hotspots in study of continental dynamics. Most of scholars have considered the Kuqa
basin as a foreland basin or a rejuvenated foreland
ü
basin[2 6], which resulted from the southward thrust of
the Tianshan orogenic belt. On the other hand, discovery of abundant natural gas in recent years has made
the Kuqa basin an important energy base for the WestEast Gas Pipeline project. Deformation mechanics of
a petroliferous basin control the oil-gas migration and
accumulation in the basin. Therefore, such a study has
not only academic significance but also potential economic significance.
The source bed of hydrocarbon in the Kuqa basin
developed in the Triassic and Jurassic strata, and the
reservoirs are composed of Cretaceous sandstones
with the Tertiary gypsum-salt layers as their seal beds.
The oil and gas formed and accumulated in the basin
during Neogene[7]. Therefore, the present paper will
mainly discuss the Neogene deformation and its
mechanism.
1 Architecture of the Kuqa-Tianshan Basin-range
System
The Kuqa basin has been subdivided into four
Copyright by Science in China Press 2004
54
zones, namely from north to south, the North Monocline Zone, the Kelasu-Yiqikelike Anticline Zone, the
Baicheng-Yengisar Sag, and the Qiulitag Anticline
Zone[6]. These four zones, together with the southern
margin of the Tianshan and the Tabei Upwarp, compose a basin-range system, which we name the KuqaTianshan Basin-range (KTBR) System.
1.1 The Basement and cover of the Kuqa basin
The basement of the Kuqa basin is mainly composed of the Paleozoic sedimentary rocks. Some
magmatic and metamorphic rocks occur locally, such
as Permian granite along the Kuqa river and
pre-Mesozoic metamorphic rocks in the Yengisar area.
The sedimentary cover of the Kuqa basin is composed
of the Mesozoic-Cenozoic strata. Except the southern
margin of the Tianshan where occur mainly the
pre-Mesozoic rocks, the other five zones differ from
each other in thickness of the Mesozoic-Cenozoic
strata (table 1). The thickness of the Mesozoic strata
reaches the maximum (about 5000 m) in the North
Monocline Zone, and thins dramatically from the Kelasu-Yiqikelike Anticline Zone toward south. The
thickness of the Cenozoic strata reaches the maximum
(>5000 m) in the Baicheng-Yengisar Sag, and thins
toward both north and south. Several unconformities
and disconformities developed in the Kuqa basin.
They occur separately beneath the Triassic, Cretaceous,
Paleogene, Neogene and Quaternary strata. Although
the unconformity beneath the Triassic can hardly be
recognized in seismic reflection profiles, it can be seen
in many outcrops. For example, the Triassic conglomerate overlies unconformably on the Permian sedimentary or volcanic rocks in the west, such as in Yushugou and Kapushaliang River. The borehole in the
east, such as YN2 and MN1, revealed that the Triassic
overlies unconformably on the Paleozoic limestone or
metamorphic rocks. The Cretaceous strata overly the
pre-Cretaceous rocks with a low-angle unconformity
in the Tabei Upwarp and the southern part of the Kuqa
basin, but with a disconformity in the middle and
northern parts of the Kuqa basin. A low-angle unconformity developed also beneath the Tertiary. The
low-angle unconformity can be recognized in outcrops
only when thick strata between the Paleogene and
Science in China Ser. D Earth Sciences
Cretaceous have been omitted, such as those in the
Kelasu River and the Kuqa River. On the other hand,
in seismic reflection profiles the unconformity is
marked by a reflecting layer (T8), which cuts the older
strata and is overlapped by younger strata. A disconformity developed at the bottom of the Neogene Jidike
Formation. Some old strata have been omitted from
beneath the Jidike Formation in the Kuqa River. The
whole Cretaceous and upper Jurassic have disappeared
even in the Yengisar, where the Jidike Formation sits
directly on the Middle Jurassic coal series. Such a
strata gap can be observed only in the Kumgeliemu
anticline, Bashijiqike anticline, Yiqikelike anticline,
and Tougermin anticline. All of the anticlines belong
to the Kelasu-Yiqikelike Anticline Zone. An unconformity developed between the Quaternary Xiyu Formation and its underlying strata.
1.2
Structural geometry of the KTBR System
The basin-range boundary is located on the
southern margin of the Tianshan. However, the
boundary we see today is but a tectonically reworked
boundary, rather than the original deposition boundary.
Our observation indicated that the present boundary
can be subdivided into two sections, with the Yanbulak
area serving as the dividing point[8]. To the west of
Yanbulak, the Southern Margin of the Tianshan contacts the North Monocline Zone. The Tertiary strata sit
unconformably on the Mesozoic strata, which in turn
overlies unconformably on the Paleozoic rocks. Brittle
faults dipping toward the basin interior have been observed both above and below the unconformity. To the
east of Yanbulak, the Southern Margin of the Tianshan
directly contacts the Kelasu-Yiqikelike Anticline Zone.
The Tertiary strata occur against the Paleozoic with a
fault in between. Although the fault is covered by
Quaternary talus or alluvial fans in most of outcrops, it
is exposed in the Qedir ravine. The shear sense of the
fault indicates that it is a normal fault with the hinging
wall slipping down to the basin. The occurrence of the
Tertiary beddings changes from steep-dipping in the
lower horizons to gentle-dipping in the upper horizons,
implying a syn-tectonic deposition[8,9]. Relic saltgypsum layers can be seen in the Qedir and Yeyungou
area.
Neogene deformation of the Kuqa-Tianshan Basin-range System
55
Table 1 Preserved thickness of the Mesozoic-Cenozoic strata in the Kuqa Basin (m)
Formations (F.)
Quaternary
TU
Xiyu F.(Qx)
BYS
KYAZ
NMZ
01360
Kuqa F.((N2-Q)k)
Neogene
QAZ
2002770
7701290
Kangcun F.(N1k)
400800
8001500
10001300
6001000
300500
Jidike F.(N1j)
600800
10001300
8002500
8001550
300600
0400
400600
10001400
800
300700
~450
~600
~800
1000800
400
0200
0200
5001000
12001600 0
500
600800
8001600
Suweiyi F.(E2-3s)
Paleogene
Kumgeliemu
group(E1-2k)
Bashijiqike F.(K1bs)
Cretaceous
Baxigai F.(K1b)
Shushanhe F.(K1s)
Yageliemu F.(K1y)
Kelazha F.(J3k)
Qigu F.(J3q)
Qiakemake F.(J2q)
Jurassic
Kizilnur F.(J2k)
Yengisar F.(J1y)
Ahe F.(J1a)
Taliqike F.(T3t)
Huangshanjie F.(T3h)
Triassic
600
Karamay F.(T2-3k)
Ehuoblak F.(T1e)
Paleozoic
Underlying strata
TU, Tabei Upwarp, QAZ, Qiulitag Anticline Zone, BYS, Baicheng-Yengisar Sag; KYAZ-Kelasu-Yiqikelike Anticline Zone; NMZ, North
Monocline Zone.
, Unconformity;
,
On the south of the North Monocline Zone, an
up-down-up warp structure is composed of the Kelasu-Yiqikelike Anticline Zone, Baicheng-Yengisar
Sag and Qiulitag Anticline Zone. The structure is
characterized by wide-flat syncline and narrow-steep
disconformity;
,
conformity.
anticline. For example, the Kumgeliemu anticline
along the Kelasu River has a 4 km-wide core while the
width of its north limb is larger than 12 km. The anticlines in the Qiulitag Anticline Zone have cores with
width ranging between 3ü5 km, and those in the Ke-
56
Science in China Ser. D Earth Sciences
lasu-Yiqikelike Anticline Zone, such as the Kasangtuokai anticline, the Kanyaken anticline, and the
Yiqikelike anticline, have cores of about 2 km in width.
On the contrary, the Baicheng-Yengisar Sag between
the Kelasu-Yiqikelike Anticline Zone and the Qiulitag
Anticline Zone is as wide as 14 km. All those synclines and anticlines have been considered as the
ü
fault-related folds[3 5].
We have demonstrated that the boundary fault of
the KTBR system has shear sense of normal fault[8].
On the other hand, many high-angle thrusts occur in
the anticlinal core. Some of them throw to south,
whereas others to north. Those high-angle thrusts all
flatten and converge to a basal decollement, which is
located not only in the Cenozoic salt-gypsum layers,
but also in thick mudstones. We have observed many
small faults in the anticlinal cores and their limbs.
Those faults throw either parallel to or oblique to the
bedding. Striation of cm- to m-scale developed on the
fault plane. We consider those faults as the fold-related
faults[10].
2
2.1
Deformation of the KTBR System
Brittle fault slip and paleostress
We have studied the brittle faultss along the basin-range boundary of the KTBR system. Here we
provide more fault slip data measured from the Paleozoic rocks in the Southern Margin of the Tianshan,
from the Mesozoic rocks along the basin-range
boundary, and from the Cenozoic rocks in the Kelasu-Yiqikelike Anticline Zone and the Qiulitag Anticline Zone (fig. 1). The measured faults belong either
to regional fault or fold-related fault, and have recorded the paleostress during the formation of the
faults and folds. In paleostress reconstruction we use
the rotational optimization method[11,12] of the TENSOR computer program developed by Delvaux.
Among the obtained parameters, R (= (σ2−σ3)/(σ1−σ3))
is the ratio of principal stress differences, α the average angular deviation, and nt the measurements of the
faults (fig. 2).
The complementary observation shows that the
recorded paleostress varies according to its position
(figs. 2 and 3). The brittle faults measured from the
Paleozoic rocks in the Southern Margin of the Tianshan recorded NW or NNW extension (see Sites 1, 2,
4, and 7), as well as strike-slip (such as Site 3). Along
the basin-range boundary, faults in both Paleozoic and
Mesozoic recorded near N-S extension (see Sites 5ü
8), as is similar to the previous observation[8]. The
faults measured from the Mesozoic and Tertiary rocks
in the Kelasu-Yiqikelike Anticline Zone and the
Qiulitag Anticline Zone recorded NW-SE compression
(such as those at Sites 9 and 10). In addition, NNE
sinistral shearing was measured at Yanshuigou (Site 12)
to the northwest of Kuqa, where the Qiulitag Anticline
Zone was curved.
As to the faulting ages, there have no geochronological dating published yet. At present, however,
the faulting ages can be inferred according to the
truncating relationship of those faults, unconformity in
the strata, ages of the growth fold in which faults developed. At least two generations of faulting have
been recognized. The early generation is parallel to the
bedding, while the late one cuts the bedding and the
early one (such as those seen at Sites 6, 9, and 10).
Almost all of the late faults formed in a radial
extension state, while the early faults vary according
to their positions, with compression inside the basin
and extension along the basin margin (figs. 2 and 3).
Study of the growth folds in the Kuqa basin by Lu et
al. showed that the folds developed earlier in north
than in south[4]. The Jiesidelike anticline in the North
Monocline Zone developed during the deposition of
the Jidike Formation; the Kalabahe anticline and Kelasu anticline in the Kelasu-Yiqikelike Anticline Zone
developed during the deposition of the Kangcun Formation; the East Qiulitag anticline in the Qiulitag Anticline Zone started to form since deposition of the
Kuqa Formation. Accordingly, the early faults we
measured might develop contemporarily with, or
slightly later than those folds. Therefore, the faults
along the basin-range boundary might develop since
the deposition of the Jidike Formation (about 23 Ma),
while the faults inside the basin might develop earlier
in north than in south. If such is the case, the faults in
the Kelasu-Yiqikelike Anticline Zone developed since
Neogene deformation of the Kuqa-Tianshan Basin-range System
57
Fig. 1. Simplified geological map of the Kuqa area. The strata abbreviations are the same as those in table 1. The circled numbers 113 mark the
observation sites where faults were measured, whereas the black dots mark the observation sites in ref. [8]. Basin units in the upper left map: , the
Southern Margin of the Tianshan; , the North Monocline Zone; , the Kelasu-Yiqikelike Anticline Zone; , the Baicheng-Yengisar Sag; , the
Qiulitag Anticline Zone; , the Tabei Upwarp. KC-Z90 indicates the seismic reflection profile in fig. 6.
17 Ma, and those in the Qiulitag Anticline Zone developed since 5 Ma. The radial extension should develop in the latest stage. However, we have no evidence to judge whether the radial extension are also
earlier in north than in south or they all developed
about 3 Ma when the Xiyu Formation deposited.
Plotting all measurements in a profile, we get an
areal paleostress pattern in which the basin-range
boundary was overwhelmed by extension with a
strike-slip component, while the basin interior by
compression (figs. 3 and 4). The faulting, together
with folding, might have propagated from the north
58
Science in China Ser. D Earth Sciences
Fig. 2. Fault slip and paleostress. In each diagram of (a)(h), the right great circle shows fault slip data. Curves along the big circle mark the fault
plane, and the black dots indicate the striations with tails being the slipping direction of the hinging wall. Double tails stand for the strike-slip. Arrows
outside the circles indicate compression (black ones) and extension (white ones). σ 1 is represented by the circle with dot, σ 2 by the triangle with dot,
and σ 3 by the square with dot. All stereoplots are Schmit’s projection of the lower hemisphere. The upper left circle shows paleostress. The black
arrows stand for compression, while the white ones for extension. The lower left histogram shows the angular deviation (α) and fault counting frequency. The faults of the early stage are plotted and marked with a, while those of the late stage with b.
Fig. 3. Regional paleostress in the Kuqa area. Stereoplots marked with numbers 1ü13 are measured from sites 1ü13 in fig. 1, while those
unnumbered ones are cited from ref. [8]. Other legends are the same as those in figs. 1 and 2.
Neogene deformation of the Kuqa-Tianshan Basin-range System
59
Fig. 4. Fault slip and paleostress plotted in the Kuqa profile. The inferred ages of the first generation of fault are shown in rectangles. The position of
the profile is shown in fig. 1. Other legends are the same as those in figs. 1ü3.
60
Science in China Ser. D Earth Sciences
Neogene deformation of the Kuqa-Tianshan Basin-range System
margin to the interior of the Kuqa basin.
2.2
Kinematics of the Kuziweng slab
The above study has shown that the paleostress
along the basin-range boundary was overwhelmed by
extension with a strike-slip component, while that in
the basin interior by compression (figs. 3 and 5). Then,
what is the relationship between the coexisting marginal extension and interior compression? To answer
the question, we investigated the outcrops in the Kuqa
area.
The observations along the Yushugou ravine and
the East Yushugou ravine display a rock slab composed of strata vertically from the Triassic Huangshanjie Formation up to the Jurassic Ahe Formation
(fig. 5). We name the slab the Kuziweng Slab, whose
length is about 11 km. The north margin of the Kuziweng Slab is located near the basin-range boundary
and bounded by a south-dipping fault with dip angle
larger than 60°. The fault has its root in the thick mudstone of the Triassic Huangshanjie Formation. Small
drag folds and fragmentation have developed above
and below the fault planes. To the south of the fault,
the strata occurrence changes from steep to flat. The
61
Slab itself has been folded slightly. At the turning
point from steep to flat, developed low-angle thrusts
with a south vergence (see the diagram marked by the
circled A in fig. 5). The south end of the Kuziweng
Slab crops out near the mouth of the Yushugou ravine
(marked by the circled B in fig. 5). The flat-lying
sandstone of the Upper Triassic Taliqike formation
turns to warp up and thrust onto the sandstone belonging also to the Taliqike formation. The thrust is
rooted in the thick mudstone of the Huangshanjie formation and has developed in the core of an anticline.
Such kind of a fault belongs to the detachment
fold[13,14]. Another anticline, the Kanyaken anticline,
occurs to the south of the Kuziweng Slab. A
high-angle thrust developed also in its core (Site 10).
The Dushanzi-Kuqa Road cuts the anticlinal core that
is composed of the Tertiary and Cretaceous strata. The
anticline tilts up eastwards and exposes the Jurassic
strata in its core. Therefore, the Kanyaken anticline
could be another detachment fold with its decollement
also in the thick mudstone of the Triassic Huangshanjie Formation.
The investigation along the Yushugou profile indicates that the extension at the basin-range boundary
Fig. 5. Profile showing the Kuziweng Slab. For strata abbreviation see table 1. Site numbers are the same as those in fig. 1. The strata and fault
occurrences in the inserted diagrams above the profile are shown by strike/dip and dip angle.
62
transfers to the compression inside the basin through a
slab sliding. Regionally, the Kuziweng Slab can be
traced westwards to the Kuqa Steel Factory, and eastwards to Yanbulak. To the west of Yanbulak, the
boundary between the North Monocline Zone and the
Kelasu-Yiqikelike Anticline Zone is located on the
south of the Kuziweng Slab (figs. 1 and 5). To the east
of Yanbulak, continuing uplift of the Southern Margin
of the Tianshan has caused severe erosion that eroded
away the equivalent slabs in the North Monocline
Zone. As a result, the Kelasu-Yiqikelike Anticline
Zone appears directly against the Southern Margin of
the Tianshan.
2.3 Deformation revealed in seismic reflection profile
A seismic reflection profile, KC-Z90, across the
Kuqa area displays the following deep structures
(fig. 6):
(1) A group of high-angle faults and thrusts have
developed along the basin-range boundary. We call
those high-angle faults as boundary normal faults (BF)
and those high-angle thrusts as boundary upthrusts
(BUT). Some of them have been exposed to the surface, while others not. Those BF and BUT cut the
basement into blocks of various size. It is the movement of the blocks that caused the folds in the overlying sedimentary cover.
(2) Faults and thrusts developed also in the
Mesozoic and Cenozoic strata that accumulated in the
Science in China Ser. D Earth Sciences
Kuqa basin. However, essentially they are of low-angle and slipped parallel to the bedding. They can warp
up and transfer to high-angle thrusts at anticlinal cores.
Sliding slab, such as the Kuziweng Slab, can form in
this way. We call those bedding-parallel low-angle
faults as detachment faults. The detachment faults develop always in an incompetent layer, such as
salt-gypsum layer and thick mudstone. The movement
of the detachment faults can produce detachment folds
and other hybrid fault-bend or fault-propagate/ detachment folds, and form the above mentioned structure pattern of wide-flat syncline vs. narrow-steep anticline. Most of the fold-related faults develop in core
of the narrow anticline, as implying that strains tend to
concentrate in the core.
(3) The unconformity marks the top surface of
the basin basement. The elevation the top surface increases from the Tabei Upwarp to the Southern Margin
of the Tianshan. The elevation contrast reaches the
maximum on the opposite sides of the BF and BUT,
above which detachment folds have developed.
3
Discussion on deformation mechanism
Many published papers explained the deformation of the Kuqa basin by using an orogenic wedge
pushing model in which southward push of the Tianshan wedge results in the thrusts and folds in the Kuqa
Basin[2,4,15,16]. However, our observation indicated that
the extension regime prevailed alone the basin-range
boundary, and suggested a block uplifting model[8].
Fig. 6. Explained structure of the KC-Z90 seismic reflection profile. The profile was provided by the Tarim Petroleum Company, CNPC. See fig. 1
for the profile position. The strata abbreviations are the same as those in table 1. YK3 and DQ5 are borehole number. BF, Boundary normal fault; BUT,
boundary upthrust.
Neogene deformation of the Kuqa-Tianshan Basin-range System
63
Then, could the vertical uplift of the Tianshan Mountains cause the compression inside the Kuqa Basin?
failure could be modified as
We have noticed that the basement and sedimentary cover of the KTBR system have deformed discrepantly. The deformation of the basement proceeded
mainly along the basin-range boundary. The basement
there was cut into blocks by the boundary normal
faults and upthrusts, and the blocks rose stairlikely
toward the mountain range. If not taking into account
of the boundary normal faults, the boundary upthrusts
themselves might easily be considered as products of
southward wedge pushing. The discovery of the
boundary normal faults forces us to consider them together with the upthrusts as an integrated boundary
kinematic system. In fact, not all thrusts are the records of nappe structures. The low-angle thrusts
clearly imply a horizontal σ1, while the high-angle
upthrusts could develop when σ1 is vertical. Theoretical analysis and analogue experiments all showed that
uplift of the block cutting basement could produce a
bundle of upthrusts and normal faults along the block
boundary [17,18].
where τo is the critical shear stress at failure and φ the
internal friction angle. Assuming λ is the pore fluid
pressure ratio (λ = p / S), the critical state when a rock
slab just begins to slide is (fig. 7):
Could those detachment folds form without the
wedge pushing? Or, could the sedimentary layers slide
down under their own weight? Accept the concept of
effective normal stress[19] (effective normal stress σ =
S − p, where S is the normal stress and p the pressure
of the pore fluid), the Mohr-Coulomb criterion for
τ = τo + (S − p) tan φ, 
ρgz sin θ = τo + (1 – λ) ρgz cos θ tan φ.
(1)
(2)
where θ is the critical slope of a decollement. Eq. (2)
indicates that whether or not a rock slab could slide
under its own weight (fig. 7) depends on weight of the
slab (ρgz), slope of the decollement (θ), cohesive
strength (τo) of the rocks containing the potential decollement, internal friction angle (φ), as well as pore
fluid pressure ratio (λ). In the case of the Kuqa basin,
the key points are how big the weight of the sedimentary layers is, how steep the slope is, and whether or
not a critical slope can be produced by the Tianshan
uplift. If we consider the deposition of the Kangcun
formation (17ü5 Ma) as the period when the detachment folds formed, the total thickness of the strata of
the Triassic to the Jidike Formation should be greater
than 7000 m in the North Monocline Zone and Kelasu-Yiqikelike Anticline Zone. Since the strata pile is
composed of sandstone with intercalation of conglomerate and mudstone, their average density is about
2300 kg/m3. Then, the strata weight will produce a
normal pressure: ρgz=135 MPa. When slope is small
Fig. 7. Critical state for gravitational slide. S is the normal stress that the strata weight exerts to decollement, while T is the shear stress. θ is the
critical slope of decollement. See the text for detailed discussion.
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Science in China Ser. D Earth Sciences
enough, we have cos θ ≅ 1. Then eq. (2) could be
written as
tan θ ≅ τo /ρgz + (1 – λ) tan φ .
(3)
When θ <25°, the introduced error in eq. (3) is smaller
than 10%[20]. Accordingly, for sandstone, φ =30° and
τo=10 MPa; for mudstone, φ = 27°and τo=2 MPa. Using the empirical values λ= 0.7[22]and λ = 0.9[19], eq.
(3) could give the following results. When λ = 0.7, a
slope of 13°can produce a decollement in the basal
sandstone of Triassic, while that of 9.4° can produce a
decollement in mudstone of Triassic. When λ = 0.9, a
slope of 7° can produce a decollement in the Triassic
basal sandstone, while that of 3.6° can produce a decollement in mudstone of Triassic. Obviously, during
uplift of the Tianshan Mountains, the Triassic mudstone has a priority to form decollement on which the
overlying strata can slide down.
The above calculation shows that a slop of 3.6°ü
9.4° are necessary for a decollement to form in the
Triassic mudstone. The horizontal distance from the
Kelasu-Yiqikelike Anticline Zone to the Southern
Margin of the Tianshan is about 30 km. To realize the
critical slope, height difference of the basement below
the Triassic strata must reach 1.9 km when slop is 3.6°,
or 4.9 km when slop is 9.4°. Yin et al. (1998)[23] estimated that the south Tianshan had uplifted a magnitude of 1.1ü3.7 km by the time of 24 Ma when the
Cenozoic strata in the Kuqa basin began to deform,
and has uplifted a magnitude of 0.9ü1.3 km since
Miocene. Therefore, if the strata in the Kuqa basin are
flat-lying before deformation, the height difference
between the Kelasu-Yiqikelike Anticline Zone and the
Southern Margin of the Tianshan had reached 5 km
when the Jidike formation had finished its deposition.
Therefore, it is implied that during the deposition of
the Kangcun Formation (17ü5 Ma), the sedimentary
cover could slide down under its own weight along a
decollement developed in mudstone of the Triassic
Huangshanjie Formation even with a small pore fluid
pressure ratio (λ=0.7).
On the other hand, the strata from the top of the
Paleogene salt-gypsum layers to the bottom of the
Kangcun Formation have a maximum thickness of
3500 m. The cohesive strength (τo) and the internal
friction angle (φ) of the salt-gypsum layers are close to
those of mudstone because that those salt-gypsum layers in fact are interlayered salt/gypsum and mudstone.
Calculation by using eq. (3) shows that at least a slope
of 10° when λ = 0.7, or of 4.4° when λ = 0.9 should be
reached in order to develop a detachment layer in the
salt-gypsum layers when the Kangcun Formation began to deposit. Such a slope is larger than that for the
decollement to form in the Triassic mudstone. Therefore, it is impossible to form another decollement by
gravity in the Paleogene salt-gypsum layers when the
Kangcun Formation began to deposit. In order to develop gravitational slide on a smaller slope in the Paleogene salt-gypsum layers, it is necessary to increase
the weight of the overlying strata. Obviously, this can
be realized only when the overlying Kuqa Formation
deposited or even later. The above calculation theoretically explains why the decollement has developed
in the thick mudstone of the Triassic Huangshanjie
Formation. It is also implied that the decollement in
the Paleogene salt-gypsum layers might develop when
the Kuqa Formation was depositing (< 5 Ma).
4
Conclusion
The KTBR system could be subdivided into 6
units. They are, from north to south, the Southern
Margin of the Tianshan, the North Monocline Zone,
the Kelasu-Yiqikelike Anticline Zone, the BaichengYengisar Sag, the Qiulitag Anticline Zone, and the
Tabei Upwarp. The basin-range boundary on the south
of the Southern Margin of the Tianshan is a tectonic
deformed boundary, where developed the boundary
normal faults and upthrusts.
The Neogene deformation of the KTBR system
has produced many brittle faults that belong to
fold-related fault and secondary fault. The field investigation and paleostress reconstruction show that brittle faults in the Paleozoic rocks of the Southern Margin of the Tianshan recorded the NW and/or NNW
extension, those along the basin-range boundary recorded the near N-S extension, and those in the Kelasu-Yiqikelike Anticline Zone and the Qiulitag Anticline Zone recorded the NW-SE compression.
Neogene deformation of the Kuqa-Tianshan Basin-range System
Study of outcrops and seismic reflection profile,
as well as theoretical calculation, has shown that a
group of boundary faults, including normal faults and
upthrusts, developed due to the uplift of the Tianshan
Mountains. The boundary faults and upthrusts cut the
basement into blocks, whose movement resulted in
both stair-like uplift of the basement along the basin
margin and gravitational slide in the sedimentary cover.
Many rock slabs with mudstone of the Triassic
Huangshanjie Formation as their decollement might
form in the Southern Margin of the Tianshan and the
North Monocline Zone during deposition of the
Kangcun and Kuqa Formations. It is the southward
sliding of those rock slabs that caused the structure
pattern of extension along the margin and compression
inside the basin. As a result, two generations of detachment folds developed, with the early generation
formed on the decollement in the Triassic mudstone
and the late generation on that in the Cenozoic
salt-gypsum layers.
It should be noticed that the role of gravitational
potential energy playing in the orogenesis and the continental crust deformation is still in a hot debating[24].
The present paper shows only an infant result, which
should be verified in future. It is necessary to carry out
more detailed study of both the surface deformation
and the deep structures of the lithosphere.
Acknowledgements This work was supported by the National Key
Basic Research and Development Program (Grant No. G1999043303)
and the Project of Tarim Petroleum Company, CNPC (Grant No.
11102040428). We are grateful to Prof. J. Charvet from the Orleans
University, for giving an instruction in brittle deformation analysis. We
also thank Prof. Jia Chengzao, Wang Qinghua, Cai Zhenzhong, and
Peng Gengxin from the CNPC, with whom we have had fruitful discussions. Peng Shoutao took part in the field work.
References
1. Wang, Q., Jin, Z., Superimposed basin and oil-gas formation and
accumulation, China Basic Science (in Chinese), 2002, 6: 4ü7
2. Liu, H., Liang, H., Cai, L. et al., Foreland thrust system and basin
evolution on both sides of the Tianshan Mountains, Earth Science
(in Chinese), 1994, 19(6): 727ü741.
3. Lu, H., Howell, D. G., Jia, D. et al., Rejuvenation of the Kuqa
foreland basin, northern flank of the Tarim basin, northwest China,
International Geology Review, 1994, 36: 1151ü1158.
4. Lu, H., Jia, D., Chen, C. et al., Nature and timing of the Kuqz
Cenozoic structures. Earth Science Frontiers (in Chinese), 1999,
6(4): 215ü221.
5. Jia, C., Structures and Oil-gas of the Tarim Basin in China (in
65
Chinese), Beijing: Petroleum Industry Press, 1997, 348ü364
6. Jia, C., He, D., Lei Z. et al., Oil-gas Perspective in Foreland Trust
Belts (in Chinese), Beijing: Petroleum Industry Press, 2000, 79ü
83.
7. Liang, D., Zhang, S., Zhao, M. et al., Hydrocarbon sources and
stages of reservoir formation in Kuqa depression, Tarim Basin,
Chinese Science Bulletin, 2002, 47(supp.): 62ü70.
[Abstract] [PDF]
8. Wang, Q., Zhang, Z. Lin, W., Late Tertiary faults and their paleostress along the boundary between the Kuqa basin and the Tianshan Mountains, Chinese Science Bulletin, 2004, 49(4): 374ü381.
[Abstract] [PDF]
9. Li, Z., Wang, Q., Wang, D. et al., Depositional record constraints
on Late Cenozoic uplift of Tianshan and tectonic transformation
in Kuqa depression, West China, Acta Sedimentologica Sinica (in
Chinese), 2003, 21(1): 38ü45.
10. Mitra, S., Fold-accommodation faults. AAPG Bulletin, 2002, 86:
671ü693.
11. Delvaux, D., Moeys, R., Stapel, G. et al., Paleostress reconstructions and geodynamics of the Baikal region, Central Asia, Part 1.
Paleozoic and Mesozoic pre-rift evolution, Tectonophysics, 1995,
252: 61—101.[DOI]
12. Delvaux, D., Moeys, R., Stapel, G. et al., Paleostress reconstructions and geodynamics of the Baikal region, Central Asia, Part 2.
Cenozoic rifting. Tectonophysics, 1997, 282: 1ü38. [DOI]
13. Suppe, J., Principles of Structural Geology. Englewood Cliffs:
Prentice-Hall, 1985, 309ü369.
14. Mitra, S., A unified kinematic model for the evolution of detachment folds., Jour. Structural Geology, 2003, 25: 1659ü1673. [DOI]
15. Burchfiel, B. C., Brown, E. T., Deng, Q. et al., Crustal shortening
on the margins of the Tien Shan, Xinjiang, China, International
Geology Review, 1999, 41: 665ü700.
16. Deng, Q., Feng, X., Zhang, P. et al., Active Structures in the Tianshan Mountians (in Chinese), Beijing: Seismology Press, 2000,
188ü385
17. Sanford, A. R., Analytical and experimental sydy of simple geologic structures, Geol. Soc. America Bull., 1959, 70: 19ü52.
18. Lowell, J. D., Antithetic faults in upthrusting, AAPG Bull., 1970,
54: 1946.
19. Hubbert, M. K., Rubey, W. W., Role of fluid pressure in mechanics of overthrust faulting, Geological Society of America Bulletin,
1959, 70: 115ü166.
20. Hsu, K. J., Role of cohesive strength in the mechanics of overthrust faulting and of landsliding, Geological Society of America
Bulletin, 1969, 80: 927ü952.
21. Touloukian, Y. S., Judd, W. R., Roy, P. F., Physical Properties of
Rodcks and Minerals, New York: McGraw-Hill Book Company,
1981, 63ü90.
22. Davis, D., Suppe, J., Dahlen, F. A., Mechanics of fold-and-thrust
belts and accretionary wedges, J. Geophys. Res., 1983, 88: 1153
ü1172.
23. Yin, A., Nie, S., Harrison, T. M. et al., Late Cenozoic tectonic
evolution of the southern Chinese Tian Shan, Tectonics, 1998,
17(1): 1ü27. [DOI]
24. Ray, P., Vanderhaeghe, O., Teyssier, C., Gravitational collapse of
the continental crust: definition, regimes and modes, Tectonophysics, 2001, 342: 435ü449. [DOI]