Spatial Characteristics and Controlling Factors of the Strike-Slip Fault Zones in the Northern Slope of Tazhong Uplift, Tarim Ba

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Spatial Characteristics and Controlling Factors of the Strike-slip Fault Zones in the Northern Slope of Tazhong Uplift, Tarim Basin: Insight from 3D Seismic Data Xiaoying Han 1, 2, 3, Liangjie Tang 1, 2 *, Shang Deng 4, Zicheng Cao 5

1. State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China 2. Basin ＆ Reservoir Research Center, China University of Petroleum, Beijing 102249, China 3. College of Mining Engineering, North China University of Science and Technology, Tangshan, 063009, China 4. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China 5. Exploration and Production Research Institute of Northwest Oilfield Branch Company, SINOPEC, Urumqi 830000, China

*Corresponding author: Liangjie Tang E-mail address: [email protected] Telephone: +86 13701288636 Postal address: No18 Fuxue Road, Changping District, Beijing China University of Petroleum-Beijing, Beijing 102249, China

Abstract: The detailed characteristics of the Paleozoic strike-slip fault zones developed in the northern slope of Tazhong uplift are closely related to hydrocarbon explorations. In this study, five major strike-slip fault zones that cut through the Cambrian-Middle Devonian units are identified, by using 3D seismic data. Each of the strike-slip fault zones is characterized by two styles of deformation, namely deeper strike-slip faults and shallower en-echelon faults. By counting the reverse separation of the horizon along the deeper faults, activity intensity on the deeper strike-slip faults in the south is stronger than that on the northern ones. The angle between the strike of the shallower en-echelon normal faults and the principal displacement zone (PDZ) below them is likely to have a tendency to decrease slightly from the south to the north, which may indicate that activity intensity on the shallower southern en-echelon faults is stronger than that on the northern ones. Comparing the reverse separation along the deeper faults and the fault throw of the shallower faults, activity intensity of the Fault zone S1 is similar across different layers, while the activity intensity of the southern faults is larger than that of the northern ones. It is obvious that both the activity intensity of the same layer in different fault zones and different layers in the same fault zone have a macro characteristic in that the southern faults show stronger activity intensity than the northern ones. The Late Ordovician décollement layer developed in the Tazhong area and the peripheral tectonic events of the Tarim Basin have been considered two main factors in the differential deformation characteristics of the strike-slip fault zones in the northern slope of Tazhong uplift. They controlled the differences in the multi-level and multi-stage deformations of the strike-slip faults, respectively. In particular, peripheral tectonic events of the Tarim Basin were the dynamic source of the formatting and evolution of the strike-slip fault zones, and good candidates to accommodate the differential activity intensity of these faults. Key words: Strike-slip faults, Flower structure, En-echelon faults, Activity intensity, Evolution

1 Introduction

The northern slope of Tazhong uplift is a promising area for hydrocarbon explorations in the Tarim Basin (Lin et al., 2012; Tian et al., 2012; Huang 2014). It is located in the middle part of the Tarim Basin adjacent to the Awati sag in the northwest and the Manjiar sag in the northeast. The Paleozoic strike-slip fault systems developed in this area have received much attention because their detailed characteristics are closely linked to the migration and accumulation of hydrocarbon resources (Wu et al., 2011，2012; Ma et al., 2012; Yang et al., 2013; Zhou et al., 2013; Lan et al., 2015). A majority of the previous studies werefocused on the structural characteristics and distribution of the strike-slip fault systems (Zhang et al., 2008; Wu et al., 2011, 2012; Ma et al., 2012; Li et al., 2013; Yang et al., 2013; Zhou et al., 2013; Zhen et al., 2015; Li et al., 2016). Most authors believed that the strike-slip fault zones were characterized by a simple geometric feature that may appear as a positive or negative flower structure on the cross section (Zhang et al., 2008; Wu et al., 2011，2012; Ma et al., 2012; Li et al., 2013; Yang et al., 2013; Zhou et al., 2013; Lan et al., 2015). In this study, based on the interpretations of high-quality 3D seismic data, the strike-slip fault zones are described to have distinct geometric and structural features in deep and shallow layers. Only limited studies exist on the vertical segmentation of the strike-slip fault zones (Huang et al., 2014; Li et al., 2016), and little attention has been paid to the differential structural characteristics and their controlling factors of the strike-slip fault zones in previous studies. Furthermore, quantitative study of activity intensity of the strike-slip fault zones have been conducted in the area of interest. In this study, the focus of the work is the five strike-slip fault zones that were developed in the 3D seismic

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/1755-6724. 14333.

1 volumes of S1 and SN. The main aim of this study is (1) to unravel the distinct geometric characteristics of the strike-slip fault zones in different layers; (2) to determine the differences in the activity intensity of the strike-slip fault zones; (3) to investigate the genetic relation between the deeper faults and shallower faults and clarify the controlling factors of their formation and evolution.

2 Geological Settings

The Tarim Basin, with an area of approximately 56×104 km2, is among the largest petroleum stores in China (Fig. 1a). It is surrounded by three mountains: the Tianshan fold mountains belt to the north, the western Kunlun Mountains to the southwest, and the Altyn Mountains to the southeast (Fig. 1b).The Tazhong area is located in the central uplift belt of the Tarim Basin (Fig. 1b). Taking the Tazhong-Ⅰ thrust fault as boundary, the hanging-wall block is Tazhong uplift and the footwall block is the northern slope of Tazhong uplift (Fig. 1c) (Huang, 2014). The northern slope of Tazhong uplift, in which tectonic framework is the east part is higher than the west part (Fig. 2), extends over 350km in the NW-SE direction and approximately 100 km in the SW-NE direction, with a total area of approximately 35000 km2 (Fig. 1c).

Fig.1. (a) Location of the Tarim Basin in China (b) Location of the Tazhong area in the Tarim Basin and (c) Enlarged view of the central part of figure. 1b shows the distribution of strike-slip faults at the top of the Cambrian (T80 horizon) in the northern slope of Tazhong uplift. The locations of the 3D seismic volumes used in this study are labeled. S1 3D seismic volume covers an area of about 1200 km2. The areas of SN 3D seismic volume are approximately 4600 km2. The 3D seismic data derived from Exploration and Production Research Institute of Northwest Oilfield Branch Company, SINOPEC. (China basemap after China National Bureau of Surveying and Mapping Geographical Information) At present, more than 50 exploratory and production wells have been drilled in the Tazhong area. The Paleozoic Erathem encountered by the wells include the Upper Cambrian, Ordovician, Silurian, the Upper Devonian, Carboniferous and Permian. The Paleozoic sedimentary succession of the Tazhong area, from the bottom to the top, has been divided into twenty-two Formations. Marine carbonate beds characterized by dolomites were developed in the Cambrian-Middle Ordovician. Of these, the Wusonger and Awatage Formation mainly contained thickness evaporates. There was an unconformity (T74, Table. 1) between the Middle and the Upper Ordovician, where the Middle Ordovician Yingshan Formation be overlain by the Upper Ordovician Lianglitage Formation (Lin et al., 2012). The Upper Ordovician strata consisted of the Lianglitage Formation and Sangtamu Formation. The Lianglitage Formation is mainly composed of limestone and pure limestone is believed to be the dominant oil and gas reservoirs in the Tazhong area. There was a widespread angular unconformity (T70, Table. 1) between the Silurian and Ordovician systems. The Silurian-Middle Devonian

2 This article is protected by copyright. All rights reserved. strata were composed of mudstones, sandstones, and some argillaceous siltstone. The Donghetang Formation, which overlaid the Middle Devonian strata above an angular unconformity (T60, Table. 1), mainly consisted of a set of gray, and dark gray fine sandstone. The Carboniferous sedimentary succession was a set of deposits, which mainly contained mudstones, argillaceous siltstone and sandstones. During the Permian, volcanic rocks, which might be related to the evolution of Paleo-Tethys Ocean, were widespread in the Tazhong area (Li et al., 2011). A simplified stratigraphic chart that includes both the main seismic reflectors and litho-stratigraphic units of the Tazhong area is given in Table. 1. Table. 1 Paleozoic stratigraphy of the Tazhong area

3 Differential structural styles of the strike-slip fault zones

Based on comprehensive interpretations of the seismic cross sections and coherence slices, five major strike-slip fault zones have been mapped in the 3D seismic volumes (Figs. 2-3). From the north to south, they are fault zones S1, SN3, SN1, SN4 and SN2, which are named by drilling wells, e.g., Well #S1, #SN3. The strike-slip fault zones are nearly vertical and crosscut the Cambrian-Middle Devonian units. Notably, significant differences in structural styles exist between the deeper and shallower layers in a vertical direction (Fig. 2). The deeper faults on the sectional view, interpreted as positive flower structures or vertical faults with reverse separation, are steeply dipping and propagate upwards from the basement and terminate at the Late Ordovician Sangtamu Formation mudstone (Fig. 2). On the shallower seismic cross sections, the shallower faults, which cut through the Late Ordovician Sangtamu Formation mudstone and straight up to the bottom of the Late Devonian Donghetang Formation, converge at a depth into a steep fault zone to form negative flower structures (Fig. 2). On the map view, every major deeper fault is segmented, the segments are all NNE-trending faults and are developed within the principal displacement zone (PDZ) (Fig. 3 a＇-a" and Fig. 3 c＇-c"). Each of the shallower fault zones is composed of a series of NW-trending faults, which are regularly spaced and present right-stepping en-echelon arrangements (Fig. 3 b＇-b"、Fig. 3d and Fig. 3e).

Fig.2. AA′ is NW-SE-trending cross-section in the S1 3D seismic volu.me and BB′ is NW-SE-trending cross-section in the 3D seismic volumes of SN1 and SN2. Zoomed-in images (middle) showing the distinct flower structure. The blue layer represents the Late Ordovician décollement layer developed in the Tazhong area. The locations of the seismic sections are shown in Figure 1c.

Fig.3. (a) The coherence slice of T74 horizon from the S1 3D seismic volume. (a′) The interpretation of the S1 3D seismic volume showing the distribution and orientation of the strike-slip faults. (b) The coherence attribute slice of T62 horizon of the S1 3D seismic volume. (b′) The interpretation of the SN 3D seismic volume showing the distribution and orientation of the strike-slip faults. (c) The coherence slice of T74 horizon from the 3D seismic volumes of SN1 and SN2. (c′) The interpretation of the fault traces of the en-echelon faults in the S1 3D seismic volume. (d) The coherence slice of T63 horizon of the 3D seismic volumes of SN1 and SN2. (e) The interpretation of the fault traces of the en-echelon faults in the SN 3D seismic volume. The locations of the 3D seismic data volumes are shown in Figure 1b.

5 This article is protected by copyright. All rights reserved. In three dimensions, the strike-slip fault zone displays a complex geometric feature that consists of a lower positive flower structure and an upper negative flower structure (Fig. 4). Based on the geometric characteristics described above, the deeper faults can be determined unambiguously as strike-slip faults and the shallower faults are believed to be en-echelon normal faults.

Fig.4. 3D model of the differential structural styles of the strike-slip fault zone in the deeper and shallower layers. The blue layer represents Late Ordovician Sangtamu Formation mudstone.

4 Differential activity intensity of the strike-slip fault zones

A common method for estimating the activity intensity is to consider the change in fault throw. In this study, the fault throw along the shallower faults can be seen clearly on the cross sections (Fig. 2). The fault throw of the deeper faults is not very obvious, but local uplifts are clearly observed along the Cambrian-Late Ordovician units in both walls of the deeper faults (Fig. 2). Hence, we use the reverse separation of the horizon along the deeper faults as an indicator to analyze the change in fault throw, which is used to determine the activity intensity of the deeper faults. The larger reverse separation of the horizon usually suggests stronger activity of the faults. 4.1 Methodology and description In this paper, there are two schematic drawings (Figs. 5a and 5b) that summarize quantitatively the reverse separation of the horizon, which are applicable to horizontal and inclined formation, respectively. In general, when sediments deposit horizontally, it suitable to use the schematic drawing in Fig. 5a to calculate the reverse separation of the horizon along the deeper faults. As mentioned above, T60 horizon is an angular unconformity between the Late Devonian and Middle Devonian (Fig. 2 and Table. 1). In the south part of the northern slope of Tazhong uplift, the stratigraphic units under the T60 horizon were uplifted, and the Devonian units and even certain parts of the top of the Silurian units were eroded and truncated (Fig. 2). The total reverse separation of the inclined horizon along the deeper faults is the sum of the regional reverse separation and local reverse separation. However, only the local reverse separation is related to fault activity. Fig. 5b shows a calculational method for quantitatively calculating the local reverse separation of the inclined horizon along the deeper faults, where the stratigraphic units are tilted. As pointed out by Han et al. (2017) the prominent reverse separations of the horizon along each deeper fault are recorded in the T74 and T72 horizon, this study chooses the T74 horizon as the indicator to calculate the reverse separation along the deeper faults. In addition, Fault SN3 is unrepresentative because of its short extension in the SN 3D seismic volume, and so, this study did not involve this calculation. The results of other faults in the 3D seismic volumes are shown in Fig. 6. For shallower faults, the procedure for fault throw determination and the corresponding calculating equation are shown in Fig. 5c. In the S1 3D seismic volume, the maximum growth index of each fault is recorded in the Yimugantawu Formation (Han et al., 2017), which indicates that the fault throw of the T62 horizon of each shallower fault is the maximum. Thus, the T62 horizon was chosen as the indicator to calculate the fault throw of the shallower faults. It is worth mentioning here that the units of the Silurian-Devonian were eroded and truncated to different degrees from the north to the south (the T62 horizon were absent) in the SN 3D seismic volume (Fig. 2), which makes it difficult to analyze the activity intensity of the shallower faults. In the S1 3D

6 This article is protected by copyright. All rights reserved. seismic volume, the strata associated with shallow faults were widespread. This paper selected en-echelon normal faults in the S1 3D seismic volume (Fig. 3c) to calculate the fault throw of the T62 horizon.

Fig.5. (a) Schematic sketch diagrams of the reverse separation of the horizontal horizon. (b) Schematic sketch diagrams of the reverse separation of the inclined horizon. (c) Schematic sketch diagrams of the fault throw.

Fig.6. Histogram showing the reverse separation of the T74 horizon of the major fault zones. The locations of the seismic sections used to calculate are shown in Figure 3a′ and Figure 3c′.

4.2 Results and interpretation From the calculation results shown in Fig.6, we can observe that the reverse separation at different parts of the same fault zone are inconsistent. The difference among them is relatively large. The maximum reverse separation along the Fault zone S1 is 140 m and the minimum is 30 m. The reverse separation along the Fault zone S1 toward the southward tip is larger than that toward the northward tip. The maximum reverse separations along Fault zone SN1 is 175 m and the minimum is 48 m. The difference among the reverse separation along Fault zone SN4 considerable and reaches 160 m between the maximum and minimum. The range of the reverse

7 This article is protected by copyright. All rights reserved. separation along Fault zone SN2 toward the northward tip is 30-120 m and on the southward part is 110-360 m. According to the above data analysis, the activity intensity of the Fault zone S1 toward the south tip of the fault is stronger than that toward the north tip. The variation of fault activity of Fault zone SN1 in the adjacent position is larger. The variation of activity of Fault zone SN4 is also very significant in different locations. The activity of Fault zone SN2 on the southern part is stronger than that toward the northward tip. Furthermore, Fig. 7 represents a line chart for the maximum and average fault throw of these four fault zones. From the result we can find that the activity intensity of the southern fault is larger than that of the northern ones.

Fig.7. Maximum and average reverse separation of the T74 horizon of the deeper faults. The locations of the faults are shown in Figure 3a′ and Figure 3c′. It is well known that a general relationship between the maximum displacement (Dmax) and fault length (L) over the normal faults may be of many orders of magnitude (see Kim and Sanderson (2005) for more details). The range of the extension length between the en-echelon normal faults belonging to Fault zone S1 is 103-104 m and the Dmax /L ratios of these normal faults almost distribute within the fitting line for normal faults (Fig. 8). From Fig. 9 we can see that the different parts of the same normal fault have different fault throws and the maximum fault throw (maximum displacement) along the en-echelon normal faults is more than 160 m. For comparison between the maximum and average fault throw of the normal faults with a similar extended scale (Fig. 10), we conclude that activity intensity of the en-echelon normal faults in the northern part is weaker than that in the southern part.

Fig.8. Plots of maximum displacement (Dmax) against normal fault length (L). SS, sandstone; LS, limestone; SH, shale (modified from Kim and Sanderson, 2005).

Fig.9. Histogram showing the fault throw of the en-echelon normal faults belong to Fault zone S1. The locations of the seismic sections used to calculate are shown in Figure 3b′.

Fig.10. Line chart showing the maximum and average fault throw of the faults with similar extension lengths. The locations of the faults are shown in Figure 3c′. Accordingly, differential activity intensity of the faults in the northern slope of Tazhong uplift has the

9 This article is protected by copyright. All rights reserved. following characteristics: (i) the reverse separations of each deeper fault are inconsistent, but we can conclude that the activity intensity on the southern deeper faults is stronger than that on the northern ones; (ii) the fault throw on the southern en-echelon normal faults belonging to the Fault zone S1 is bigger than on the northern ones in the S1 3D seismic volume, and it may attest that the activity intensity in the south is greater than that in the north; (iii) the comparison between the reverse separation along the deeper faults and the fault throw of the shallower faults suggests that activity intensity of the Fault zone S1 in the different layers have similar performance that the activity intensity of the southern faults is larger than that of the northern ones.

5 Discussions

In the past years, knowledge regarding the formation and growth of deep faults and shallow faults and their genetic mechanism have been poorly investigated. In this section, the genetic relations between the deeper and shallower faults, and the control factors of the differential deformation of the strike-slip fault zones will be discussed 5.1 Genetic relations of the deeper and shallower faults As stated above, the deeper and shallower faults display different geometric characteristics. However, the existence of genetic relations between the deeper and shallower faults are to be further verified. Since these normal faults were active during the Middle Silurian to the Middle Devonian (Han et al., 2017), while no NE-SW extension is documented in the northern slope of Tazhong uplift at that time, it is very likely that the formation of these faults were related to activity of the deeper strike-slip faults. As presented above, each of the shallower fault zones is composed of a series of NW-trending normal faults. In general normal faults show NNE-trending arrangements, which is in accordance with the t principal displacement zone direction of basement (Figs. 3). The angle between the strike of each en-echelon normal fault and their principal displacement zone is approximately 48° on an average, indicating that the en-echelon normal faults might be tensional fractures that were formed during the progressive sinistral displacement above an NNE-striking basement strike-slip fault (Wilcox et al. 1973; Stefanov and Bakeev, 2015; Akira et al., 2016). Furthermore, the en-echelon normal faults of different fault zones have different angle distribution with their principal displacement zones. The main distribution of the angles between the strike of the en-echelon normal faults of the fault zones S1, SN3 and SN1 and their principal displacement zones are 40°～44°, 48°～52°, and 54°～56°, respectively (Figs. 11). Such a variation indicates that the inclination between the strike of these en-echelon normal faults and their principal displacement zones is likely to have a tendency to decrease slightly from the south to the north. This may prove that the activity intensity on the southern shallower en-echelon normal faults is stronger than that on the northern ones.

Fig.11. Distribution of the angles between the strike of each en-echelon normal fault and the principal displacement zone (PDZ) in different fault zones.

5.2 Controlling factors of differential deformation of the strike-slip fault zones 5.2.1 Influence of the ductile layer developed in the northern slope of Tazhong uplift Most authors believe that some ductile layers (e.g. evaporites, mudstone, etc.) have separation and decoupling effect (Tang, 1992; Zouaghi et al., 2005; Konstantinovskaya and Malavieille, 2011; Tang et al., 2012; Li et al., 2016), which has a significant influence on the multi-layer differential deformation of the fault. The well-correlated section across the northern slope of Tazhong uplift shows that the Sangtamu Formation is composed of gray to gray-dark thick mudstone and silty mudstone, intercalated with pale grey siltstone and argillaceous siltstone (Fig. 12). The thickness isopach map illustrated that the Sangtamu Formation was widespread in the northern slope of Tazhong uplift and it has a thickness of more than 2000 m (Fig. 13). In terms of mechanical properties, the mudstone is a ductile layer, and in terms of tectonics properties it performs as the weak layer and has certain fluidity. Each of the strike-slip fault zones presented above has a deeper positive flower structure (or a vertical fault with reverse separation) and a shallower negative flower structure, which were divided by the Late Ordovician Sangtamu Formation (Fig. 2). Moreover, the strike-slip faults are weakly active in the Sangtamu Formation as displayed on the cross sections (Fig. 2). It may suggest that their activity were masked by the decoupling effect of the thick mudstone. The Upper Ordovician Sangtamu

10 This article is protected by copyright. All rights reserved. Formation, hence, is believed to be the dominant décollement layer, which a provided favorable material basis for the differential deformation of the strike-slip fault zones in deeper and shallower layers.

Fig.12. Well-correlated section across the northern slope of Tazhong uplift showing the lithology, depth, and logging data of the wells. For well locations, see Figure 1c.

Fig.13. Mudstone thickness chart of the Upper Ordovician Sangtamu Formation in (a) the S1 3D seismic volume and (b) the SN1 and SN2 3D seismic volumes. The location of the 3D seismic data volumes is shown in Figure 1c.

5.2.1 Influence of the ductile layer developed in the northern slope of Tazhong uplift The Tarim Basin experienced a complicated Paleozoic accretionary and collisional history (Kang and Kang,

12 This article is protected by copyright. All rights reserved. 1996; Zhao et al., 2006; Ren et al., 2011). The Rodinia supercontinent broke up and the Tarim block was separated from it during the Precambrian to the Early Ordovician (Jia, 1997) (Fig. 14a). Meanwhile, the north Tarim block stretched and gradually formed the South Tianshan Ocean, while the Altyn Ocean developed between the Tarim and Altyn blocks (Gao et al., 2009) (Figs. 14a and 14b). During this period, under extension, the early normal faults were developed in the Tarim Basin (Gao and Fan, 2012; Li et al., 2013) (Fig. 2; faults labeled in green). Although the subduction between the ancient Kunlun Ocean and Tarim block commenced at the end of the early Ordovician, their activity intensity is not strong (Xiao et al., 2000; Wang, 2004; Ren et al., 2011). With the progressive closure of the ancient Kunlun Ocean, subduction of the ancient Kunlun Ocean reached its highest level at the early period of the Middle-Late Ordovician period (Ren et al., 2011; Li et al., 2012). In the southwest, the collision of and suturing between the Tarim block and Middle Kunlun block at the end of the Late Ordovician, which was associated with the closure of the Ancient Kunlun Ocean (Zhang et al., 2002; Li et al., 2009) (Fig. 14a). It is very likely that the formation of the strike-slip faulting was appropriate to accommodate the shear stress caused by the compressional stress from the southwest. In particular, where the NE-striking preexisting structures were in existence prior to the compressional stress from the southwest, these weak basement zones will obviously be reactivated into the strike-slip faults. After the ending of the Kunlun Caledonian collision orogenyl intensive folding orogeny occurred in the Arkin tectonic domain from the Silurian to Devonia (Wu et al., 2011; Huang, 2014) (Fig. 14b). The intense thrusting from the southeast, which caused shortening and uplifting of the southern part of the Tazhong area, is believed to be the most important tectonic movement that allows the deeper strike-slip fault system reactivated into a sinistral strike-slip regime. The formation of the en-echelon normal faults was triggered by the reactivation of a deeper strike-slip faults. From the Late Devonian to Carboniferous, the Paleo-Tethys Ocean was formed (Mattern and Schneider, 2000; Yang et al., 2005) (Fig. 14a). After the Carboniferous, the Paleo-Ocean of the surrounding of the Tarim Basin closed gradually.

Fig.14. (a) Schematic map showing the tectonic setting of the Kunlun Mountains and Tianshan Mountains fold belts and (b) Schematic map showing the tectonic setting of the Altyn orogenic zone. MKB: Middle Kunlun block; AKO: Ancient Kunlun Ocean; SwU: Southwest uplift; STRV: South Tianshan rift valley; KP: Kazakhstan plate; PSTO:

Prototype phase of South Tianshan Ocean; STO: South Tianshan Ocean; FTB: Fold-and-thrust belts; TzU: Tazhong uplift; TbU: Tabei uplift; MTB:

13 This article is protected by copyright. All rights reserved. Middle Tianshan block; PTO: Paleo-Tethys Ocean; HtU: Hetian uplift; ND: North Depression; MTIA: Middle Tianshan island arc; AB: Altyn block; AAO: Ancient Altyn Ocean; FOB: Fold orogenic belt Considering the analysis of the peripheral tectonic events of the Tarim Basin, and the angle between the strike of each shallower en-echelon normal fault and the deeper faults below them, the deeper strike-slip faults may experience a two-phase evolution and the shallower faults were genetically linked with the second phase of the movement along the deeper faults. Collision of and suturing between the Tarim block and Middle Kunlun block are considered to be the dynamic factors that cause the formation of the deeper strike-slip faults. The occurrence of the en-echelon normal faults as contemporaneous with the reactivation of the deeper strike-slip faults is caused by the intensive folding orogeny in the Arkin tectonic domain. Furthermore, we concluded that the activities both on the southern deeper strike-slip faults and southern shallower en-echelon normal faults are stronger than that on the northern ones based on the discussion in preceding sections (Sections 4.2 and 5), which may experimentally verify that the dynamic source was transmitted approximately from the south to the north when the strike-slip faults developed during two periods.

6 Conclusions

Based on the analysis of the 3D seismic data pertaining to the northern slope of Tazhong uplift, five main strike-slip fault zones were investigated and the conclusions are as follows. (1) The strike-slip fault zones which affected all the units from the Cambrian to the Middle Devonian are nearly vertical and thoroughgoing, but they have two different styles of deformation corresponding to the deeper and shallower layers. The deeper faults rooted in Cambrian- Upper Ordovician layers are determined as strike-slip faults and the shallower ones restricted to the Late Ordovician-Middle Devonian units are believed to be en-echelon normal faults. (2) Activity intensity both on the deeper southern strike-slip faults and shallower southern en-echelon normal faults are stronger than that on the northern ones. The deeper strike-slip faults and shallower en-echelon normal faults belonging to the Fault zone S1 have similar activity intensities, and the activity intensity of the southern faults is larger than that of the northern ones. That is to say, both the activity intensities of different fault zones in the same layer and different parts of the same fault zone in the different layers have a macro tendency to decrease slightly from the south to the north. (3) The pre-existing weak zones, the Late Ordovician décollement layer developed in the Tazhong area and the peripheral tectonic events of the Tarim Basin have been thought to be the three main factors for the differential deformation of the strike-slip fault zones: (i) the pre-existing weak zones controlled the distribution locations of the deeper strike-slip faults; (ii) the Sangtamu Formation thick mudstone, which is believed to be the Late Ordovician dominant décollement layer, provided favorable material basis for vertical segmentation of the strike-slip fault zones; (iii) peripheral tectonic events of the Tarim Basin were the dynamic source of the development and evolution of the strike-slip fault zones and correlated well with the differential activity intensity of these faults.

Acknowledgements

This study is financially supported by National Natural Science Foundation of China (Grant No. 41572105, 41172125) and the National Key Basic Research Program of China (973 Program) (No. 2011CB214804). Gratitude is expressed to Northwest Oilfield Company, SINOPEC, for providing us with valuable seismic data.

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