Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea

Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea

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Journal of Natural Gas Geoscience xx (2017) 1e10 http://www.keaipublishing.com/jnggs

Original research paper

Q2

Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea Caili Lu¨ a,*, Gongcheng Zhang a, Dongsheng Yang a, Huijun Gao b

Q1

b

a CNOOC Research Institute, Beijing, 100027, China China National Offshore Oil Corporation, Beijing, 100027, China

Received 12 April 2017; revised 31 May 2017 Available online ▪ ▪ ▪

Abstract Based on the flexural-cantilever model and flexural isostasy model, three independent quantitative methods have been used to calculate the stretching factors of the upper crust, whole crust, and whole lithosphere in the deep-water area of the Pearl River Mouth Basin, northern South China Sea. These results demonstrate that depth-dependent stretching has occurred within the lithosphere of the study area. The lithospheric extension shows lateral differences between the Baiyun Sag and the KaipingeShunde Sags. The broad forearc pre-rifting basement and hot thinned lithosphere tend to generate a structural style of wide half-graben in the Baiyun Sag, while the volcanic arc basement and normal or thickened lithosphere form a structural style of narrow half-graben in the KaipingeShunde Sags. In line with the lithospheric deformational features and tectonic evolution stages, we propose three various dynamic mechanisms at different tectonic stages, and they are probably uniform, composite, and depth-dependent extension models, respectively. Copyright © 2017, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Pearl River Mouth Basin; Deep-water area; Differential extension; Stretching factor; Dynamic model

1. Introduction The petroleum geological conditions of a sedimentary basin are generally controlled by their geologic features and tectonic evolution processes. The formation and evolution of a basin are structurally determined by the dynamic changes in the deep crust and lithosphere [1]. The oil-gas exploration in the deep-water area has higher risks and economic threshold compared to the onshore and shallow water areas, thus, the basic geological research plays a leading role in the early exploration stage, especially for the dynamic deformation mechanism of the deep lithosphere. Pure shear and simple shear models are two simplified end members to account for * Corresponding author. E-mail address: [email protected] (C. Lu¨). Peer review under responsibility of Editorial office of Journal of Natural Gas Geoscience.

the extensional rifted margins [2‒4]. Lister et al. [5] proposed and then further improved five detachment models for the formation of passive margin incorporating the effects of distributed pure shear of the crust and mantle beneath the detachment zone [6]. In recent years, various depth-dependent stretching models have been used to account for different deformation mechanisms above and beneath the detachment faults of the lithosphere [7‒9]. A single mechanism cannot explain the lithospheric extension in the northern margin of the South China Sea, but the combination of a variety of mechanisms is favored as a more reasonable explanation methodology [10,11]. The research on lithospheric deformation is mainly carried out in the Baiyun Sag of the deepwater area, PRMB. Sun et al. [12] studied the tectonic evolution history of the Baiyun Sag and proposed that the strong ductile thinning and necking deformation of the lower crust display a hot and thinned lithospheric extension. The Moho surface identified in the

http://dx.doi.org/10.1016/j.jnggs.2017.07.001 2468-256X/Copyright © 2017, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: C. Lu¨, et al., Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.07.001

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deep reflection seismic profile by Huang et al. [13] reveals a mirror image relationship between Moho surface and sedimentary basement, as well as a stepwise crustal thinning from the continental shelf, and the continental slope to the deep-sea basin. The simulated stretching factor of the crust in the Baiyun Sag shows it is much greater in the lower crust than the upper crust, and it is probable that the initial crustal is a hot and thinned crust [14,15]. However, the deep lithospheric deformation characteristics and dynamic mechanism controlling the tectonic evolution of the deep-water area, PRMB, are not yet clear. We study the dynamic model of lithospheric extension contributing to clarify the tectonic dynamic mechanism and further understand the petroleum geological conditions in the deep-water area, northern South China Sea. 2. Geologic setting The South China Sea is located in a special tectonic position where two tectonic regions (paleo-Pacific and Paleotethys) are mixed and stacked [16]. The ocean basin had experienced complex evolution processes in the Cenozoic. The Cenozoic continental margin of northern South China Sea is deemed to be a quasi-passive type that transformed from an active type in the late Mesozoic [17,18]. The PRMB is an NEeSW trending rift basin, it has undergone complicated tectonic movement due to the conjunct influence of the Eurasian, Pacific, and Indian plates during the Cenozoic [19,20]. Generally, the deep-water area of PRMB mainly refers to the Zhu II Depression, it consists of the Baiyun, Liwan, Kaiping, and Shunde Sags, and the Yunkai Low Uplift. It has a total area of about 3  104 km2 (Fig. 1) [21]. In a tectonic setting, the deep-water area of PRMB lies on the oceanecontinent transition zone during the Cenozoic and is

bounded by the Panyu Low Uplift in the north, Southern Uplift in the south, Dongsha Uplift in the east, and Shenhu Uplift in the west. Among the sags, the Baiyun Sag is the largest and thickest one with the greatest sediment thickness of over 11 km [15]. The Baiyun and Kaiping Sags are separated by the Yunkai Low Uplift. The sag-controlling faults, most of which showing detachment features (Fig. 2), are developed from the Cenozoic basement in an NEeSW or near EeW strike direction [22]. The deep-water area of PRMB developed eight stratigraphic formations, namely Quaternary, Wanshan, Yuehai, Hanjiang, Zhujiang, Zhuhai, Enping, and Wenchang Formations from top to bottom, they divided by seismic interfaces T20, T30, T32, T40, T60, T70, T80, and T100, respectively (Figs. 2 and 3). However, the definition of interface T80 as the formation interface between Wenchang and Enping is still under dispute. Based on the latest 3D reflection seismic data together with the two wells drilling into the Wenchang Formation, we have revised the previous stratigraphic division plan at odds with actual conditions. 3. Methods and results As for a rifted continental margin, the stretching factor is an important indicator describing lithospheric extension [23]. Three independent methods have been used to estimate the stretching of the lithosphere at different depths. These involve three corresponding stretching factors: upper-crustal, wholecrustal, and whole-lithospheric stretching factors. In this study, three NWeSE-directed seismic lines through three sags (Baiyun, Kaiping, and Shunde Sags, see Fig. 1 for location) were selected to be able to calculate the stretching factors of the lithosphere. The stretching factors in the seismic lines represent the lithospheric extension of the three

Fig. 1. Geotectonic map of the Pearl River Mouth Basin (Modified from Refs. [19]). Please cite this article in press as: C. Lu¨, et al., Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.07.001

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Fig. 2. An interpreted well-tie seismic profile showing the stratigraphic divisions in the study area.

Fig. 3. Chronostratigraphic Chart of the deep-water area in the Pearl River Mouth Basin (Modified from Refs. [14,15]).

aforementioned sags respectively. Based on the flexuralcantilever model and flexural isostasy model, quantitative simulation of the lithospheric extension has been carried out on the three seismic lines with the software Flex-Decomp™ for inversion and Stretch™ for forward modeling.

3.1. Upper-crustal stretching factors In general, the upper crust deforms in a brittle mode showing fault-block sliding and/or rotating. Furthermore, several experiments have demonstrated that faults are more

Please cite this article in press as: C. Lu¨, et al., Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.07.001

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likely to deform by vertical shear than block rotate [24]. By making this assumption, we ignore the block rotation component, which may slightly overestimate fault-related extension. To capture the error and uncertainty inherent in deriving stretching factors from faults, we adjusted the initial fault dip angle. It then ranged from 25 to 70 for each fault. The relationship of lithospheric deformation to thinning factor 3 and stretching factor b is defined by the following Eq. 2: 1 ð1Þ b In this case, the upper crustal extension E is derived from the stretching factor b by:  Z Z  1 E ¼ εdx ¼ 1 dx ð2Þ bðxÞ

ε¼1

Here, x stands as the distance from the reference point along the seismic line, and x ¼ 0 is established at the location of the continental shelf. To get a simplified model, the upper-crustal stretching factor b is defined by the equation:

3.2. Whole-crustal stretching factors The variation in crustal thickness of a rifted basin is assumed to be the consequence of crustal extension and thinning. The stretching factor and its related crustal thinning are a positive correlation in the whole crust. The key issue to the whole-crustal stretching factor is to select the initial crust thickness of the lithosphere. The northern continental margin of the South China Sea is deemed as the southward extension of the South China continental block [25,27]. The crustal thickness of the South China continent is ~30 km from the acoustic velocity [28]. The initial crustal thickness (t0) is then set to 30 km in our model. The present-day crustal thickness (tc) equals to the depth of the Moho surface that can be obtained by gravity inversion [30]. Assuming a constant initial crustal thickness (t0), the whole-crustal-derived stretching factor is b ¼ t0 =tc

ð5Þ

In this case, 1 þ C is the maximal stretching factor, and W is the length of lithosphere before stretching. The upper crustal extension due to faulting is generally levelled by a pure shear of the deep lithosphere, thus, when W >> E, we can get C ¼ 2E/(W-E ). The total stretching factor bt (x) associated with all faults is the product of stretchingfactor profiles associated with each fault:

In the calculated profiles of a whole-crustal stretching factor, a peak occurs at the deepest position of each sag (Fig. 4‒6). The highest values obtained are 1.28, 1.32, and 4.10 in lines L01, L02, and L03, respectively. In lines L01 and L02, the average value of b is about 1.10 in the northwestern peaks and ~1.20 in the southeastern ones. As for line L03, b is approximately ~1.30 in the northwestern continental shelf area but it increases rapidly from ~1.80 to ~4.10 at the deepest position of the Baiyun Sag on the continental slope. Later on, it then becomes steady to be ~2.05 near the oceanic crust of the South China Sea. It is indicated that the crust becomes gradually thinner from the continent to ocean.

bt ðxÞ¼ b1 ðxÞb2 ðxÞb3 ðxÞ…bn ðxÞ

3.3. Whole-lithospheric stretching factors

b ¼ 1 þ Csin2 ðpx=WÞ

ð3Þ

ð4Þ

The upper-crustal stretching factor b has been calculated by summing the related and measured parameters, comprising the location of the upper end, horizontal extensional amount, and initial dip angle, and direction on seismically imaged faults. The property parameters of modeling lithosphere are determined by previous research results [25‒28] (Table 1). It is notable that as much as 35%e40% of the brittle faulting may not be present on the seismic profile due to insufficient resolution [29]. Thus, the calculated stretching factor is artificially increased by 40% to be closer to reality. The stretching factors in the selected three lines are between 1.02 and 1.40, with a very small fluctuation range (Fig. 4‒6). Table 1 Property parameters for restoring post-rift thermal subsidence strata. Parameters

Values

Original lithospheric thickness Original crustal thickness Upper-crustal thickness Mantle density Crustal density Sediment skeleton density Water density Temperature at the top of asthenosphere Coefficient of thermal expansion

125 km 30 km 15 km 3300 kg/m3 2800 kg/m3 2680 kg/m3 1000 kg/m3 1333  C 3.28  105

The re-equilibration of the temperature from the lithospheric extension generates post-rift “thermal” subsidence [2]. The post-rift subsidence history of a rifted margin recorded in stratigraphic data contains information that allows the magnitude of lithospheric stretching to be determined. Twodimensional flexural backstripping and decompaction have been applied to restore the post-rift thermal subsidence strata [31] and define the whole-lithospheric stretching factor. The sediment load L(x) changes with the horizontal distance x, and the vertical flexural isostatic offset u(x) is obtained by the differential equation: v4 u þ ðrm  ri Þgu ¼ LðxÞ ð6Þ vx4 where rm is the mantle density at the bottom of the lithosphere, ri is the density of the filling materials in the sedimentary basin, g is the gravitational acceleration, and D is the flexural strength of the lithosphere that is given by the Eq. (7). D



YT3e 12ð1  g2 Þ

ð7Þ

where Y is Young's modulus, g is Poisson's ratio, Te is the effective elastic thickness of the lithosphere.

Please cite this article in press as: C. Lu¨, et al., Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.07.001

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Fig. 4. Interpreted seismic profile L01 and stretching factors of the lithosphere at different depths for the Shunde Sag (see Fig. 1 for location).

Fig. 5. Interpreted seismic profile L02 and stretching factors of the lithosphere at different depth for the Kaiping Sag (see Fig. 1 for location).

Moreover, the vertical offset u(x) can be expressed by the load L(x) and flexural isostatic response equation R(x): 2 ∞ 3 Z∞ Z 1 uðxÞ ¼ RðkÞ4 LðxÞeikx dx5  eikx dx ð8Þ 2p ∞



where RðkÞ ¼ 1=½ðrm  ri Þg þ Dk4  and k is the wave number. In addition, the decompaction correction of the strata has been conducted by the porosity-depth method [32]: 4 ¼ 40 ecy

ð9Þ

where 4 is the porosity at the depth y, 40 is the surface porosity, c is the compaction factor. In a similar way, the formation thickness H is related to the depths extending from the bottom to the top by the equation:

Zy1 H¼

40 ecy dy

ð10Þ

y2

where y1 is the distance at the top, and y2 is the depth at the bottom. The simulated results reveal that the whole-lithospheric stretching factors for b are respectively 1.05e1.46 in line L01, 1.20e1.40 in line L02, and 1.15e6.05 in line L03. Similar with the variation trend of a whole-crustal factor, the whole-lithospheric stretching factor has the highest value in the deepest place of each sag. The whole-lithospheric b curves in the Kaiping and the Shunde Sags have a gentle variation with the distance away from the peak, while the b value in the Baiyun Sag varies a wide range, especially a steep curve section in the continental slope area.

Please cite this article in press as: C. Lu¨, et al., Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.07.001

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Fig. 6. Interpreted seismic profile L03 and stretching factors of the lithosphere at different depths for the Baiyun Sag (see Fig. 1 for location).

4. Dynamic models of lithospheric extension The Zhu II Depression of PRMB was an intracontinental rift during the deposition of the Wenchang Formation, and in a continental margin environment since the period of deposition of the Enping Formation [15]. The tectonic evolution of the Zhu II Depression can be divided into three stages: intracontinental rift (Wenchang Formation), riftedrift transition (Enping Formation), and post-rift thermal subsidence (Zhuhai Formation and the aforementioned stages) (Fig. 3). In the Zhu II Depression, it is probable that there are three various dynamic mechanisms at different tectonic stages: (1) uniform extension model, (2) composite extension model, and (3) depth-dependent extension model.

structures in the deep-water area of the Pearl River Mouth Basin can bring about different brittle deformation between the KaipingeShunde and the Baiyun sags (Fig. 7a). As for the KaipingeShunde Sags seated in the volcanic arc area, they have a thicker brittle upper crust but thinner ductile lower crust and mantle lithosphere, and their brittle lithosphere have been strongly ruptured under the tensile stress. A structural style of narrow half-graben is formed in the KaipingeShunde Sags. Conversely, the Baiyun Sag lies on the broad forearc area. Its pre-rift lithosphere has a thinner brittle crust but a thicker ductile. Under the same tensile stress field, the Baiyun Sag was cracked more weakly than the KaipingeShunde sags, hence, showing a structural style of wide half-graben. The Wenchang Formation deposition in the Baiyun Sag is thinner than that of the KaipingeShunde Sags.

4.1. Uniform extension model 4.2. Composite extension model In the Early-Middle Eocene, the Zhu II Depression began rift within the continent when a number of faults were created. The Wenchang Formation was deposited in the faulted sags of the downthrown side. The strong block-faulting activity has built the basic structural framework in the deep-water area of PRMB. Similar with the continental rift basin, the lithospheric extension chiefly displays a brittle rupture in the upper crust. In this intracontinental rift stage, the whole lithosphere deform uniformly and their stretching factors vary within a small range (1
In the late stage of deposition of the Wenchang Formation, as rifting progressed, Zhu II Depression entered into the riftedrift transition stage. The subsidence and deposition center became concentrated in the Baiyun Sag where the Enping Formation was developing to be thicker. The tectonic activity in this stage inherited the former to a certain degree but with reduced tectonic subsidence. A lot of detachment faults can be found in the interpreted seismic profile (Figs. 8 and 9). The sag-controlling faults of the Kaiping Sag have a listric feature, gentle in the upper part and steep in lower part, which is rooted in the large-scale low-angle basement fault (detachment surface) (Fig. 8). In the Baiyun Sag, five listric normal faults (F1eF5) appear to extend downward and merge into a unified basal detachment surface (Fig. 9). In addition, the similar north-dipping detachment faults also developed in

Please cite this article in press as: C. Lu¨, et al., Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.07.001

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Fig. 7. The uniform extension model within the continental lithosphere.

the Lile Basin, the southern continental margin of the South China Sea [11]. Thus, we infer that these detachment faults are formed prior to the seafloor spreading of the South China Sea when Nansha micro-block has not yet separated from the Cathaysia block. However, it is still controversial whether the low-angle detachment faults are formed originally at an initial low angle or transformed from the early rotated high-angle normal faults. Reston and McDermott [35] interpreted the multistage faulting of West Galicia margin. They proposed that the low-angle detachment fault is generally accompanied by mantle-derived magma activity. The mantle-derived basaltandesitic lavas, revealed by the drilling well BY7-1-1 in the Baiyun Sag, were mainly formed during the deposition period of the Enping Formation [36]. Furthermore, many authors confirmed that the Cenozoic magmatic rocks were mainly generated in the late rift stage [12,37]. Therefore, we suggest that the low-angle detachment faults have been developed since the late stage of deposition of the Wenchang Formation, and the obvious detachment occurred in the Enping Formation in the central Baiyun Sag. The thickened upper crust in the volcanic arc area has become normal due to the early adjustment of intracontinental rifting. The Lherzolitic mantle xenoliths collected from the deep-water area of PRMB proved asthenosphere upwelling and related lithospheric thinning [37]. The maximum value reached for the upward emplacement of the asthenosphere was ~12 km [10,38]. The asthenosphere upwelling can enhance magma intrusion and create regional plastic flows in the lower crust or between the crust-mantle boundary. It is possible that the outcome of the shear stress made the maximum principal stress axis deflect the vertical direction [39]. As a result, an early high-angle normal fault is transformed into a low-angle detachment fault due to simple shearing. As stretching progressed, the upper crust became highly fractured and the

lithospheric deformation transformed to some degree into a ductile mode. In the KaipingeShunde Sags, only a minimal amount of volcanic rocks were generated, thus, indicating a weak magma activity and a relatively cool or normal lithosphere. The brittle upper crustal deformation shows numerous fault blocks sliding and rolling along their low-angle detachment fault surfaces. In addition, a ductile pure shear deformation emerged for the most part in the lower crust and mantle lithosphere (Fig. 10). On the other hand, some intense magmatic activity happened in the Baiyun Sag, indicating its hot thinned lithosphere [12]. The lithospheric necking is superimposed on the kinematic mode of flexural deformation, this led to a gentle rotation of fault block, therefore, showing wide detachment half-graben pattern. 4.3. Depth-dependent extension model The South China Sea opened at ~32 Ma corresponding to the regional breakup unconformity T70 and the tectonic event of the Nanhai movement (Fig. 3). In and after the depositional stage of the Zhuhai Formation, post-rift thermal subsidence prevails in the entire region entering the period of unified depression. The activity of deep fluids from asthenosphere is strengthened due to the spreading seafloor of the South China Sea. The discrete upwelling of deep materials gradually extends upward from the bottom of the lithosphere to generate a plastic flow. This process occurs mainly in the plastic layers, comprising of lower crust and mantle lithosphere, which have high temperatures but low viscosities. As a consequence, the lower crust and mantle lithosphere are intensely and rapidly thinned. Nevertheless, the brittle upper-crust nearly stopped the tectonic activity and proceeds to a flexural isostatic compensation. In short, the lithospheric deformation is non-

Fig. 8. An interpreted seismic profile showing fault features in the Kaiping Sag. Please cite this article in press as: C. Lu¨, et al., Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.07.001

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Fig. 9. An interpreted seismic profile showing listric faults in the Baiyun Sag.

Fig. 10. The composite extension model in the riftedrift transition stage.

Fig. 11. The depth-dependent extension model related to an active efflux of lower crust.

uniform and its extension is depth-dependent. Our calculated results show that the maximal stretching factor is evident in the whole lithosphere, followed by the whole crust, and the upper crust being the minimum (Figs. 4‒6). It is proven that the lithospheric extension increases with increasing depth. During the thermal subsidence stage, previous researchers have also explained an accelerated subsidence of the Baiyun Sag. Some authors emphasize that the intense thinning crust causes inhomogeneous extension [4043], and others regard this phenomenon as a depth-dependent extension of lithosphere [23,26,44,45]. Most researchers have agreed on the South China Sea being formed from mantle upwelling [46]. Along with the initial seafloor spreading of South China Sea, some possible active effluxes have arisen in the lower crust. In line with the plastic flow above, we propose that the lower crust is most likely to have evacuated the underlying Baiyun

Sag (Fig. 11). The lacking in the materials underplating the upper crust can create a sudden increase in the subsidence rate of the sedimentary basement, which is consistent with the deep lithospheric structure of the Baiyun Sag [9,47]. 5. Conclusions The differences in the stretching factors of the upper crust, whole crust, and whole lithosphere demonstrate a depthdependent extension in the deep-water area of PRMB. Moreover, the characteristic lithospheric structures before the preCenozoic rifts and mantle upwelling underlying the Baiyun Sag have brought about differences in stretching degree, structural style, and related formation thickness from the KaipingeShunde sags. The broad forearc basement and hot thinned lithosphere tend to generate a structural style of wide

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half-graben in the Baiyun Sag, while the volcanic-arc basement and normal or thickened lithosphere create a structural style of narrow half-graben in the KaipingeShunde Sags. Based on the deformational features of the lithosphere and tectonic evolution stages, three various dynamic mechanisms have probably existed in the study area, and they are uniform, composite, and depth-dependent extension models, respectively. The whole lithosphere extends uniformly and displays primarily brittle rupture in the upper crust during the intracontinental rift stage. Numerous listric faults are developed and rooted in the large-scale basal detachment surface in the riftedrift transition stage, and the lithospheric necking is superimposed on the flexure deformation in the Baiyun Sag. In terms of the depth-dependent extension model, it is likely for the lower crust underlying the Baiyun Sag to outflow actively explaining an accelerated subsidence during the post-rift thermal subsidence stage. Funding Supported by National Science and Technology Major Project of the Ministry of Science and Technology of China (2016ZX05026-007).

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