Fault System, Deformation Style and Development Mechanism ...

Journal of Earth Science, Vol. 23, No. 4, p. 529–541, August 2012 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-012-0273-2

Fault System, Deformation Style and Development Mechanism of the Bachu Uplift, Tarim Basin

Dianjun Tong* (佟殿君)

Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China

Junxia Zhang (张俊霞) School of Economics and Management, China University of Geosciences, Wuhan 430074, China

Huaizhong Yang (阳怀忠) Beijing Research Institute, CNOOC, Beijing 100027, China

Desheng Hu (胡德胜) Zhanjiang Branch, CNOOC, Zhanjiang 524057, China

Jianye Ren (任建业) Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences,

Wuhan 430074, China; Structral Research Center of Oil & Gas Bearing Basin, Ministry of Education, China University of Geosciences, Wuhan 430074, China

ABSTRACT: The Bachu (巴楚) uplift is one of the most active tectonic regions nowadays in the Tarim

basin, which is also a faulted block uplift that was intensively active during the Cenozoic. This study

was based primarily on the geological structure interpretation of seismic profiles, applying the theories

and methods of basin dynamics, structural analysis and tectono-stratigraphic analysis, the geometry

and kinematics features of the fault systems in the Bachu uplift were analyzed in detail. Our study

shows that each fault belt is mainly characterized by compression and overthrusting, most of the faults

initiated and activated during the Mid–Late Himalayan period, and that the general structural styles of

the Bachu uplift were basement-involved pop-up thrust faulted block uplift, of which the southern

margin was covered by the large-scale decollement fault system. The basement-involved structures

widely developed in the higher position of the basement uplift, while decollement fault system devel-

oped mainly at the position with gypsum mudstone. The evolution process of Bachu uplift included

back-bulge slope of the peripheral foreland basin in Mid–Late Caledonian, forebulge in Hercynian–

Yanshanian and the latest compressional faulted block uplift in Mid–Late Himalayan. Meanwhile,

This study was supported by the National Science and Tech-

nology Major Project (No. 2011ZX05009-001) and the Na-

tional Natural Science Foundation of China (No. 41102071).

*Corresponding author: [email protected]

© China University of Geosciences and Springer-Verlag Berlin

Heidelberg 2012

Manuscript received January 15, 2012.

Manuscript accepted April 9, 2012.

the study also suggests that the formation, re-

construction and stabilization of the uplift were

controlled by the development and evolution of

fault systems clearly. In the early forebulge stage,

it was mainly presented as flexural deformation

without the developing of thrust faults in the

Bachu area; to the late stage, under the influence

of violent lateral compression deformation, the

faulted block uplift formed finally.

KEY WORDS: fault system, structural style,

Dianjun Tong, Junxia Zhang, Huaizhong Yang, Desheng Hu and Jianye Ren 530

structural evolution, Bachu uplift, Tarim basin.

INTRODUCTION Uplift is the large-scale positive tectonic unit, and

acts as the important window for us to understand the evolution and structural deformation of sedimentary basins. Moreover, it is also a favorable place for hy-drocarbon accumulation (Zhai and He, 2004), and is an important petroleum exploration domain. Therefore, how to completely reveal the geometrical morphology, geological structure, structural styles and evolution history of the uplift, relationship between deposition and hydrocarbon accumulation is always the study direction of the basin dynamics and petroleum geol-ogy (He et al., 2008; Jia et al., 2003). The structural styles and the evolution processes of western China basins are extremely complicated, with strong super-imposition and reconstruction features. Especially for the Tarim basin, the multi-phase tectonic activities of the large-scale over-thrust fault systems during the periphery plate orogeny not only controlled the intra-basin structures, deformation styles and the uplift-depression framework, but also played an important role in the formation and evolution of the basin and hydrocarbon accumulation (Ren et al., 2011; Wu et al., 2009; Liu et al., 2008; Zhu et al., 2008; Buslov et al., 2007; He D F et al., 2005; He W Y et al., 2000). In the recent decade, the researches on the pe-troliferous basin structures have made significant pro-gresses at home and abroad (Dickinson et al., 1997), and the geometric and kinematics models of fault-related folds have obtained great advances and widely applied in the structural analysis of the com-pressional basins, providing important theories and methods for fault system research of sedimentary ba-sins (Giambiagi et al., 2008; Yang et al., 2007; Andrea et al., 2006; Rowan and Linares, 2000; Hardy and Ford, 1997; Erslev, 1991; Suppe and Medwedeff, 1990; Suppe, 1983; Rich, 1934). In addition, several foreign researchers have investigated and discussed the evolution processes of the uplifts (forebulge and fault block uplift) with different features in compres-sional settings (Mansurbeg et al., 2009; Dávila and Astini, 2007; Keller et al., 2007; Mortimer et al., 2007; DeCelles and Giles, 1996).

Primarily basing on the latest seismic profiles

and wells data, applying the theories and methods of basin dynamics, structural analysis and tectono- stratigraphic analysis, concentrating on the geometric and kinematics features of major fault belts in the Bachu uplift, we studied the assemblage and proper-ties, structural styles and deformation feature of the fault systems. Furthermore, combining with the struc-ture and evolution analysis of the uplift, we attempt to reveal the role and influence of thrusting fault systems in the process of uplift formation and evolution. GEOLOGY SETTING

The Bachu uplift is located at the western central uplift belt of the Tarim basin, covering an area of about 43 000 km2. It is the fault-block uplift that was intensively active during Cenozoic and is also one of the most active tectonic regions nowadays in the Tarim basin (He et al., 2009; Ding et al., 2008). The uplift generally extends NW, of which the east seg-ment trends NWW and the west segment gradually trends NNW. The uplift is separated from other tec-tonic units by boundary faults. The western boundary is the Keping fault demarcating with the Keping faulted uplift; the southern boundary is the Selikbuya- Haimiluosi-Mazhatage fault belt demarcating with the Maigaiti slope; the northern boundary is the Aqia-Piqiakexun fault belt and Tumuxiuke fault belt, transition to the Awati depression; the south-eastern boundary is the Madong fault belt and transition to the Tazhong uplift (Fig. 1). The borehole data and the outcrop sections reveal that the depositional succes-sion of the Bachu uplift is mainly composed of the Paleozoic and Cenozoic with the hiatus of the Meso-zoic. The Middle Ordovician is absent in its eastern area, which indicates the existence of an EW paleo- uplift during the early stage.

Based on the comprehens ive tec tono- stratigraphic interpretation of the seismic profiles across the study area, the basin filling sequences could be divided into three tectonic layers that are separated by the major regional unconformities (Figs. 2 and 3). The lower tectonic layer is composed of Cambrian to Middle Devonian sequences, the middle one is com-posed of Upper Devonian to Triassic sequences, and

Fault System, Deformation Style and Development Mechanism of the Bachu Uplift, Tarim Basin 531

He4

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Guangzhou

Shanghai

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600 km0

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Figure 1. Plane map of the fault system of the Bachu uplift. 1. Thrust fault; 2. fault or fault belt number, F1. Selikbuya fault belt, F2. Bashituo fault belt, F3. Haimiluosi fault, F4. Mazhatage fault belt, F5. Madong fault belt, F6. Tonggang fault belt, F7. Qiaoxiaorgai fault belt, F8. Bachu fault belt, F9. Kangtakumu fault belt, F10. Gudongshan fault belt, F11. Bielitage fault belt, F12. Kalashayi fault belt, F13. Badong fault, F14. Tumuxiuke fault belt, F15. Aqia-Piqiakexun fault belt, F16. Keping fault, F17. Yasongdi decollement fault belt, F18. Selikbuya-Mazhatage shallow decollement fault belt; 3. well location; 4. seismic profiles.

PT

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Figure 2. SN trending tectono-stratigraphic profile across the western Bachu uplift, showing the character-istics of the upper, middle and lower tectonic layers. ∈1–2. Lower to Middle Cambrian; O1+∈3. Upper Cambrian–Lower Ordovician; O2–3. Middle to Upper Ordovician; S+D1–2. Silurian to Lower–Middle Devo-nian; D3+C. Upper Devonian to Carboniferous; P. Permian; T. Triassic; E. Paleogene; N1. Lower Neogene; N2+Q. Upper Neogene to Quaternary (fault belt number and the profile location see Fig. 1). the upper one is composed of Jurassic to Quaternary. The lower tectonic layer has consistent tectonic

deformation with the middle and upper tectonic layers, forming a pop up high-uplift that is controlled by


thrusting faults on both sides. The Ordovician sequence locally deposited in the western area; while at the central and eastern areas the Middle–Upper Ordovician sequences are absent. The middle tectonic layer is thicker in the eastern area than that in the western area, and most of the depositional strata at the central uplift are absent, resulting in that the upper tectonic layer directly overlays on the Carboniferous– Permian (Fig. 2). The upper tectonic layer is mainly the Cenozoic sequences, without the Jurassic to Cre-taceous sequences; the depositional strata gradually overlap on the uplift from south and north sides sepa-rately (Figs. 2 and 3).

The Bachu uplift is obviously separated into sev-eral segments in EW direction and several belts in SN direction. The northern and southern margins of the uplift are controlled by the Mazhatage, Haimiluosi, Selikbuya and Tumuxiuke fault belts, respectively. The anticlinorium structure with the feature of mar-ginal fault-anticline and the central syncline was formed between the boundary fault belts. In addition, the uplift can be remarkably divided into the east and west segments along the wells of Kang2 and Fang1. Most of the fault belts in the west segment extends NW, while in the east segment that changed to nearly EW trending (Fig. 1).

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Figure 3. SN trending tectono-stratigraphic profile across the middle Bachu uplift, showing the characteris-tics of the upper, middle and lower tectonic layers (fault belt number and the profile location see Fig. 1). COMPOSITION AND DEFORMATION FEATURES OF FAULT SYSTEMS IN THE BACHU UPLIFT

The Bachu uplift mainly developed three groups of fault systems striking NW, NWW (nearly EW) and NE, respectively (Fig. 1). The NW striking fault belt (such as the Selikbuya, Kangtakumu, Qiaoxiaoergai, Gudongshan and Aqia-Piqiakexun faults etc.) mainly distributes in western-central Bachu uplift. The NWW striking or nearly EW striking fault belt (such as the Haimiluosi and the Mazhatage fault belts in southern area, the Tumuxiuke and Kalashayi fault belts in northern area, and the Bashituo and Manan fault belts in the Maigaiti slope) is mainly located in the northern and southern margins of eastern uplift and the Mai-gaiti slope. In addition, NE striking faults that are constituted by a series of subparallel faults mainly distribute in the Madong structural belt of the

south-eastern Bachu uplift.

Deformation Styles of Boundary Fault Belts Selikbuya-Mazhatage fault belt

This fault belt consists of Selikbuya, Haimiluosi and Mazhatage fault from west to east, showing an echelon arrangement from NNW to nearly EW striking, forming an arcuate structural zone convex southward and acting as the southern boundary fault of the Bachu uplift (Fig. 1). Selikbuya fault belt

It is located at the west section of southern Bachu uplift. From south to north, the strikes of individual segments vary from NW gradually to NNW in plan view, and dip to NE-NEE, with extending length of about 130 km. In general, the fault belt cuts upward to T2

0 seismic interface (base of Upper Neogene) and


downward to basement through T90 seismic interface

(base of Cambrian). Comprehensive analysis of tectonic deformation, sedimentary characteristics and the unconformity interfaces shows that the Selikbuya fault belt was mainly active in Late Miocene (N1), and it can be divided into three segments from north to south according to the change of structural styles along strike (Figs. 4b, 4c and 4d). Its north segment mainly developed listric thrust faults, with steep upper fault plane and gentle lower part, composing anti-“y” shape structure together with its derivative faults. All of the involved strata during tectonic deformation comprise Pre-Sinian basement to Miocene sequences and formed complicated fault-propagation folds; the faults and associated folds were truncated by T2

0 interface. The sedimentary sequences after the end of Miocene were basically unaffected by fault activities and have not developed tectonic deformation (Fig. 4b). For the middle segment of the Selikbuya fault belt, the master fault plane gradually became steep and the fault-propagation folds resulting from the fault activity tended to be tightly closed. As a result of successive activities of the fault in late stage, all of the strata above the T2

0 interface in the hanging wall deformed and developed a wide and gentle anticline (Fig. 4c). To the south segment, the activity of Selikbuya master fault became weaker and only cut through the deep strata and formed a gentle fault-related fold, as well as showing a ramp-flat thrust fault in the seismic profiles. In addition, the typical characteristics of this segment is the development of two sets of cover-decollement fault systems along the Paleogene gypsum mudstone and Cambrian gypsum bed as the decollement sur-face respectively, that is, the Selikbuya-Mazhatage shallow decollement fault and Yasongdi decollement fault (Fig. 4d). Haimiluosi-Mazhatage fault belt

These two fault belts compose the middle and east boundaries of the Bachu uplift. The Haimiluosi fault belt, which is a thrust-decollement fault along Middle Cambrian gypsum salt bed from Maigaiti slope to Bachu uplift, and formed a fault-propagation fold in its front, trends to NNW and dips to south in profiles. The tectonic deformation including faults and related folds only developed in the sedimentary strata

under T20 interface (Fig. 5a). The NWW striking

Mazhatage fault belt consists of two parallel faults, forming an arcuate structural zone convex northward in plan view. The master fault composes “y” shape back-thrust structure together with its derivative faults in profile view. The fault cuts downward through the basement, and its top is cut by the Selikbuya-Mazhatage shallow decollement fault (Fig. 5b). Aqiakexun-Tumuxiuke fault belts

The Aqiakexun-Tumuxiuke fault belt, which bounds the northern margin of the Bachu uplift, is composed of the Aqia-Piqiakexun fault, Tumuxiuke fault and the Badong fault from west to east (Fig. 1). Aqia-Piqiakexun fault belt

The Aqia-Piqiakexun fault belt is located at the west section of the northern boundary of the Bachu uplift, striking NNW, with length of about 90 km. It cuts upward through Miocene (T2

0) and downward to basement, strongly active in the end of Miocene. In the profiles, it has the flower structural style showing that it was transpressional fault (Fig. 2). Tumuxiuke fault belt

The Tumuxiuke fault belt consists of the Tumuxiuke fault and the Badong fault, acting as the middle-east section of the northern boundary of the Bachu uplift. It is about 166 km long, generally strik-ing NWW and forming an arcuate structural zone convex northeastward in plan view. The Badong fault is located in the southeastern of the east segment of the Tumuxiuke fault and has a length of about 100 km. These two faults are subparallel to each other, show-ing a left lateral en echelon distribution style. Gener-ally speaking, the primary activity period of the Tumuxiuke fault was in the end of the Miocene; its structural styles and deformation characteristics in the profiles have the segmentation feature and can be divided into three segments from west to east. The west segment is the listric thrust fault, cutting upward to T2

0 interface and downward through basement. The strong activities of this fault resulted in complex de-formation of the involved strata and strong erosion, as well as a gentle anticline was formed in hanging wall. The master fault composes a “y” shape structure


Figure 4. (a) Map showing the location of Selibuya fault belt; (b), (c) and (d) the comprehensive tectono-stratigraphic interpretation profiles through north, middle and south section of the fault, respec-tively. T2

0. base of Upper Neogene; T30. base of Lower Neogene; T7

4. base of Middle Ordovician; T90. base of

Cambrian.

Figure 5. The tectono-stratigraphic interpretation profiles through Haimiluosi fault belt (a) and Mazhatage fault belt (b), showing the geometric shape and structural style of the faults (fault belt number see Fig. 1). together with its derivative faults formed during later compression (Fig. 6b). The fault plane of the middle fault segment is steep and cuts upward to T2

0 interface. In this segment, the structural deformation is relatively simple and only an anticline was formed in the hanging wall (Fig. 6c). To the east segment, the fault plane is steep and straight, and only cuts through the deep strata; the associated fault-propagation fold was developed in the hanging wall, the involved strata of tectonic deformation from basement through the

Miocene (N1), and the folding deformation was truncated by T2

0 interface. Another feature of the east segment is that the Badong fault was initiated, and its structural deformation style is similar to that of the Tumuxiuke fault in profiles (Fig. 6d). Tectonic Deformation Features of the Fault Belts Internal the Bachu Uplift

The internal fault belts of the Bachu uplift include the Qiaoxiaorgai fault belt, Kangtakumu fault


belt, Kalashayi fault belt and Gudongshan fault belt, etc.. They control the distribution of the tectonic units of the Bachu uplift (Fig. 1). In plan view, the general strikes of these fault belts are NW or NWW, and their scales are smaller than that of the boundary fault belts and the length of individual fault is no more than 100 km. The tectonic deformation patterns of these faults

are similar to the boundary faults in profiles. The compressional thrust faults and related deformations were mainly formed in the end of the Miocene, re-sulting in the development of complex basement- involved and cover-decollement structures in this up-lift (for example the Gudongshan fault belt, Fig. 7).

Figure 6. (a) Map showing the location of the Tumuxiuke fault belt; (b), (c) and (d) the tectono- stratigraphic interpretation profiles through west, middle and east section of the fault, respectively.

Figure 7. The tectono-stratigraphic interpretation profiles through the northwestern segment (a) and southeastern segment (b) of the Gudongshan fault belt, showing the geometric shape and structural style of the faults (fault number and the location see Fig. 1).

The Shallow Decollement Fault Belt of the Southern Margin in the Bachu Uplift

This study is primarily based on the interpreta-tion of a lot of seismic profiles in the southern Bachu uplift, showing that there developed a large scale

decollement-thrust fault, with Paleogene gypsum mudstone bed acting as the decollement surface and thrusting to the Bachu uplift, and we name it as the Selikbuya-Mazhatage shallow decollement fault belt (Fig. 1). This fault system becomes steep in the vicin-


ity of the Selikbuya-Mazhatage boundary fault, and then extended upward to the surface, controlling the fault-propagation folds in hanging walls (Figs. 3, 4d and 5). The regional seismic sections also indicate that, along the decollement surface, this fault can be traced to the western Kunlun Mountain thrust belt to the south of the Taxinan depression. Meanwhile, it is likely to join with Kashi fault extending westwards, and both of them constitute the front-edge fault of the Taxinan foreland thrust-fault system currently.

The activity of this decollement fault belt is strong in the west segment while weak in the east segment. A large-scale fault-propagation fold devel-oped in the hanging wall in the west segment, which resulted in the strata erosion and shortening signifi-cantly (Figs. 4d and 5). The tectonic deformation became weak eastward, and the fault belt nearly bedding-parallel disappeared to the east segment of the Mazhatage fault. General Deformation Styles of the Fault Systems in the Bachu Uplift

Generally speaking, the tectonic stress field is compressional in the geological evolvement history of the Tarim basin; therefore, it determined the develop-ment mainly of thrusting structural styles in the Bachu uplift. According to the differences of formation mechanism and deformation mode of basement in-volvement, it can be classified to basement-involved structures and cover-decollement structures in detail. Basement-involved compressional structure is the most important deformation style in the Bachu uplift; most of the fault systems (such as the Selikbuya, Tu-muxiuke, Kalashayi and Kangtakumu fault belts, etc.) belong to this type. It generally presents as the master thrusting fault composing the “y” shape structure together with its derivative fault, or two or more back thrusting faults composing the complex pop-up struc-ture (Fig. 2). Another typical structural style, the cover-decollement fault, is mainly distributed in the internal uplift, including fault-propagation folds and fault-bend (fault-slip) folds (such as the south segment of the Gudongshan fault, which belongs to a typical fault-bend fold) (Fig. 7b).

The Bachu uplift is a faulted block uplift formed by the back thrusting of boundary faults. Thus, the

back thrusting fault assemblage is the principal tec-tonic deformation style in the Bachu uplift. The multi-grade back thrusting structures, which were composed of the boundary faults and internal faults, control the distribution of depositional sequences and the basic structural styles of the Bachu uplift. Mean-while, the Selikbuya-Mazhatage shallow decollement fault in southern Bachu uplift overthrusted above the early thrusting systems. As a result, it determined that the general structural style of the Bachu uplift was basement-involved back thrust fault block uplift, of which the southern margin was covered by the large-scale decollement fault system.

THE EVOLUTION PROCESS AND THE CONTROL OF FAULT SYSTEMS OF THE BACHU UPLIFT

Nowadays, the Bachu uplift is a compressional faulted uplift controlled by back thrusting of the boundary faults, of which the north and south sides are the Taxinan depression and the Awati depression (they are synclinal depressions controlled by the op-posite thrusting fault systems). All of these tectonic units have experienced complex evolution processes during their long-term geological history. Figure 9 is a noticeable tectonic evolution section across the Taxi-nan depression-Bachu uplift-Awati depression. Com-bined with the tectonic background of the Tarim basin, the major evolution process of this area are as follows (Fig. 8). Cambrian to Early Ordovician (∈–O1)

This basin was generally in the stage of exten-sional setting, and the normal fault developed in local area. The present Taxinan depression, the Bachu uplift and the Awati depression are a uniform marine sedi-mentary basin, with depositional thickness increasing from south to north (Fig. 8) and the whole terrain was presented as high in the south and low in the north. Middle–Late Ordovician (O2–3)

In this period, as effected by the first episode of the Middle Caledonian period in the latest Early Or-dovician, the regional structural pattern was shown as an uplift in the south and a depression in the north. The present Taxinan depression underwent uplift and


erosion and the Taxinan paleo-uplift formed, which was the forebulge of the peripheral foreland basin. The Bachu uplift was a north-inclination slope zone, which was controlled by the Taxinan pericontinental upwarping, a thinner depositional strata overlapping to the Bachu area from north to south in this stage (Fig. 8). Silurian to Early–Middle Devonian (S–D1–2)

In the end of Late Ordovician (the second epi-sode of the Middle Caledonian period), the structure pattern that an uplift in the south and a depression in the north continuously developed, and the Bachu uplift and adjacent region underwent uplift and ero-sion. The depositional strata overlapped over the Bachu-Taxinan from north to south. In the end of Middle Devonian (related to Late Caledonian orogeny), the Taxinan depression underwent uplift and erosion again by violent tectonic compression. The Bachu uplift was still at a slope zone, but its uplift amplitude was much less than that of the Taxinan area (Fig. 8). Late Devonian to Carboniferous (D3–C)

During this stage, regional subsidence occurred, but the general structural pattern had changed greatly than ever before: the Awati depression continuously subsided, the Taxinan area also began to subside, the depocenter migrated southwards to the Taxinan area, and the sediment thickness decreased northwards. To the end of Carboniferous, the Bachu uplift began to uplift and be eroded, and the rudiments of the Bachu paleo-uplift formed, the structural pattern changed from uplift in the south and a depression in the north in the early stage to uplift was sandwiched by two depressions (Fig. 8). Permian (P)

A rapid subsidence and sedimentation occurred in the Awati depression, responding to collisional orogenesis originated from the northern area of the Tarim basin. The Taxinan depression continuously subsided, but the subsidence amplitude was smaller than that of the Awati region. The Bachu uplift was located at the forebulge of the foreland basin, and then the structural pattern that one uplift sandwiched by

two depressions was much more remarkable. At the end of the Permian, the Bachu uplift suffered a certain uplift. Mesozoic (Mz)

The western Tarim basin underwent intensive compression and uplift which resulted from the colli-sion between the Qiangtang block and the Eurasian plate during the Triassic period, and an extensive up-lift and erosion occurred from the Bachu paleo-uplift to the Taxinan depression; as a result, the Mesozoic sequence was absent. To the end of Mesozoic, the Taxinan depression-Bachu uplift-Awati depression experienced various uplift and erosion (Fig. 8). Paleogene to Quaternary (E–Q)

During Paleogene, after experiencing extensive uplift and erosion during Mesozoic, the tectonic set-ting was relatively stable, both the Taxinan depression and the Awati depression were subsided. As the com-mon forebulge of the north and south depressions, the Bachu uplift was uplifted and eroded, leading to Pa-leogene overlapping the uplift from bilateral depres-sions. During Miocene, the western Kunlun Mountain and the southern Tianshan Mountain orogenic belts were uplifted and thrusted over the Tarim basin again, resulting to the rejuvenated foreland basin. The Bachu uplift acted as the composite forebulge of the Taxinan depression in front of the Kunlun Mountain (foreland basin) and the Awati depression in front of the Tian-shan Mountain (foreland basin). The bilateral depres-sions were rapidly subsided and deposited gigantic Miocene sequence, and the depositional strata sharply overlapped and thinned toward the Bachu uplift until tip-out. Until the end of Miocene, the Bachu uplift became the faulted block uplift because of violently back thrusting of the boundary faults (such as the Tu-muxiuke, Selikbuya and Mazhatage faults etc.). In-fluenced by this, the evolution process of the Bachu uplift changed from a forebulge by the flexural de-formation into a faulted block uplift, which was con-trolled by the thrust faults; its bilateral depressions also changed into the synclinal depressions, which were controlled by facing-thrust of the boundary faults, and the present structural framework was basically formed. Then, controlled by facing-thrust faults, the


Taxinan depression and the Awati depression con-tinuously and quickly subsided, and the subsidence center continuously migrated to the intracontinental area, forming synclinal compressional depressions; the Bachu faulted uplift was continuously active, then the tectonic framework of the present Taxinan depression-Bachu uplift-Awati depression was formed.

During this period, a large-scale shallow decollement fault system slipping along the Paleogene gypsum decollement surface was developed in the southern Bachu uplift. This fault extended westward and con-nected with the Kashi thrust fault system, forming the present front-fringe fault in front of the west Kunlun Mountain thrusting belt (Fig. 8).

∈1-2∈3 1-OO2-3D -C3 S-D1-2PTKEN1N +Q2 Denuded strata

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The deposition of Middle to Upper Ordovician (O )2-3

The deposition of Upper Cambrian-Lower Ordovician ( -O )∈3 1

The deposition of Lower to Middle Cambrian ( )∈1-2

Figure 8. Restored geological tectonic evolution cross-sections of the Taxinan depression, the Bachu uplift and the Awati depression. The locations see Fig. 1.

Fault Activities Controlled the Tectonic Evolution of the Bachu Uplift

Nowadays, the Bachu uplift is a faulted block up-lift, which was controlled by faults. The evolution of the fault system had great effect on the formation, re-construction and stabilization. The early faults back-thrust activity controlled the evolution and tectonic style of the uplift (such as the Selikbuya- Mazhatage fault and the Tumuxiuke fault, etc.). The late secondary faults back-thrust activity controlled the formation of tectonic unit within the uplift. The uplift formation and fault development were not at the same period. As the previous analysis in this paper, during Hercynian period, the Bachu uplift began to develop

(foreland uplift), but the major active period of the faults was in Mid–Late Himalayan. At that time, the Bachu uplift evolved from a forebulge that was con-trolled by bending fold or lithospheric flexure to back-thrust faulted uplift that was controlled by lateral compression. The tectonic feature change of the uplift would result in the difference of the deformation styles and the depositional strata. Therefore, the evolution process analysis of uplift is helpful to thoroughly un-derstand the tectonic evolution history, the tectonic de-formation styles and its controlling on sedimentation. In the following sections, we will discuss in detail the transition process of the Bachu uplift from a forebulge to the compressional faulted block uplift.


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Pz

Pz

Pz

E

N1

N1 N1

EE

N

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N1

EE

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(a) Forebulge stage: flexure derofmation, and non-developmentthrust fault (the deposition of Paleogene-E)

(b) Conversion stage of the forebulge to fault uplift: flexure aggravation, the thrustfaults possible appearance (the deposition of Lower Neogene-N )1

(c) Compressional fault block uplift stage: back thrusting fault block uplift,thrust faults development (before the deposition of Upper Neogene-N )2

(d) The characteristic through flatten T seismic reflection interface20

Strata onlap and thinning

Strata onlap and thinning

Nearly equal thicknesssedimentary

Nearly equal thicknesssedimentary

Maigaiti slope

Bachu uplift

Awati depression

Erosion

High-angle onlap andthinning sharply

Erosion

Figure 9. Schematic diagrams of the evolution process of forebulge transition to fault block uplift (taking the Bachu uplift as example).

From the present seismic profiles (flatten T20

seismic reflection interface), it can be seen that the depositional strata of Miocene onlapped and thinned with a small-angle from the Taxinan depression to southern Bachu uplift. This set of strata sequence should overlap to the central Bachu uplift and then tip-out, according to the sedimentary characteristics and extending trends. Nevertheless, the seismic reflec-tion characteristics of the Miocene are subparallel, and there is no remarkable onlapping and thinning. Like this, according to the tendency of the strata extending outward, there must be a tectonics position (faults) at

the interior Bachu uplift where the Miocene strata are discontinuous (Fig. 9d). Obviously, it is contradictory with the viewpoint that the major faults of the Bachu uplift were active in the end of the Miocene in this article. Therefore, we can conclude that there is a large-scale asymmetrical folding uplift before the thrust fault intense activities (that is before the Bachu uplift formation) (Fig. 9b). The south flank of the up lift is relatively gentle, and the strata overlapped with a small-angle to the uplift; while the north flank of the uplift is steep, so that the strata overlapped to the up-lift with a high angle and thinned sharply. Hereby,


combined with the evolution analysis of the Bachu uplift in this study, we can divide the tectonic evolu-tion of the uplift into two stages.

(1) Forebulge stage (Figs. 9a and 9b): the flexural deformation was the dominant factor, and the thrust faults were not developed and had less uplifting extent, the uppermost strata was subjected to a certain degree of erosion. The foredeep located between the uplift and fold thrust orogenic belt was controlled by flex-ural subsidence, in which the depositional strata over-lapped toward forebulge.

(2) Faulted uplift stage (Fig. 9c): with the front of orogenic belt pushing toward intracraton, the foredeep and forebulge of the foreland basin were involved in violent lateral compressional deformation. The fore-deep area further subsided, which had changed from the early foredeep to synclinal depression, while the forebulge experienced faulted block uplift, and suf-fered intensive erosion. The development of thrust fault is a significant geological structure sign of the transition from forebulge to faulted uplift.

CONCLUSIONS

(1) The typical characteristics of the master boundary faults, internal faults and southern margin shallow decollement faults in the Bachu uplift are mainly compressional and overthrusting; the forming and active periods of most faults were in Mid–Late Himalayan, and that the general structural styles of the Bachu uplift was basement-involved back thrust faulted uplift, of which the southern margin was cov-ered by the large-scale decollement fault system.

(2) There are abundant compressional deformation structural styles in the study area. According to the de-formation degree, the structure styles can be classified to basement-involved type and cover-decollement type. The latter was mainly developed where the gypsum mudstone existed, while the basement-involved struc-ture was widely developed in the higher position of the basement uplift.

(3) The evolution of the Bachu uplift experienced back-bulge slope in Mid–Late Caledonian, forebulge in Hercynian–Yanshanian and the latest compressional faulted block uplift in Mid–Late Himalayan. The for-mation, reconstruction and stabilization of the uplift were greatly controlled by the development and evo-

lution of the fault systems. Combining the analysis of tectonic-sedimentary relationship of the basin, at the early forebulge stage, the flexural deformation was the dominant factor, without the developing of thrust faults. At the late stage, the deformation style changed into faulted block uplift as the result of violent lateral compressional deformation. The development of thrust fault is a significant geological structure sign of the transition from forebulge to faulted block uplift.

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