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Volume 30 Issue 6
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Zhichao Zhou, Lianfu Mei, Hesheng Shi, Yu Shu. Evolution of Low-Angle Normal Faults in the Enping Sag, the Northern South China Sea: Lateral Growth and Vertical Rotation. Journal of Earth Science, 2019, 30(6): 1326-1340. doi: 10.1007/s12583-019-0899-4
Citation: Zhichao Zhou, Lianfu Mei, Hesheng Shi, Yu Shu. Evolution of Low-Angle Normal Faults in the Enping Sag, the Northern South China Sea: Lateral Growth and Vertical Rotation. Journal of Earth Science, 2019, 30(6): 1326-1340. doi: 10.1007/s12583-019-0899-4

Evolution of Low-Angle Normal Faults in the Enping Sag, the Northern South China Sea: Lateral Growth and Vertical Rotation

doi: 10.1007/s12583-019-0899-4
Funds:  The authors wish to thank two anonymous reviewers for their constructive reviews and suggestions, which greatly helped to improve this paper. We thank the editors for the detailed and impartial comments. This study was supported by the Major National Science and Technology Programs, China (Nos. 2016ZX05026-003-001 and 2011ZX05023-001-015)
More Information
  • Low-angle normal faults (dip < 30°,LANFs) are widespread in the northern margin of the South China Sea where the maximum crust thickness is approximately 30.0 km. Based on 3D seismic survey data and drilling wells in the Enping sag,evidences for LANFs that initially formed at high-angles are discussed. After a detailed investigation of extensional fault system and description of 3D fault geometry,the initial fault dips under the model of distributed vertical simple shear are also calculated. The results indicate that the present-day dip angles of the LANFs are in the range of 12° to 29°,and the initial fault dip angles are in the range of 39° to 49°. Deep seismic imaging suggests that the upper crust in the footwall block of the LANFs was tilted at an angle of~14° to 22° due to the isostatic rebound during rifting. Moreover,the temporal and spatial sequences of the lateral growth of the LANFs have been investigated by the seismic interpretation of four isochronous stratigraphic interfaces,which demonstrates that two individual fault segments propagated towards each other and subsequently,were hard-linked during the Early Eocene.
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Evolution of Low-Angle Normal Faults in the Enping Sag, the Northern South China Sea: Lateral Growth and Vertical Rotation

doi: 10.1007/s12583-019-0899-4
Funds:  The authors wish to thank two anonymous reviewers for their constructive reviews and suggestions, which greatly helped to improve this paper. We thank the editors for the detailed and impartial comments. This study was supported by the Major National Science and Technology Programs, China (Nos. 2016ZX05026-003-001 and 2011ZX05023-001-015)
    Corresponding author: Lianfu Mei

Abstract: Low-angle normal faults (dip < 30°,LANFs) are widespread in the northern margin of the South China Sea where the maximum crust thickness is approximately 30.0 km. Based on 3D seismic survey data and drilling wells in the Enping sag,evidences for LANFs that initially formed at high-angles are discussed. After a detailed investigation of extensional fault system and description of 3D fault geometry,the initial fault dips under the model of distributed vertical simple shear are also calculated. The results indicate that the present-day dip angles of the LANFs are in the range of 12° to 29°,and the initial fault dip angles are in the range of 39° to 49°. Deep seismic imaging suggests that the upper crust in the footwall block of the LANFs was tilted at an angle of~14° to 22° due to the isostatic rebound during rifting. Moreover,the temporal and spatial sequences of the lateral growth of the LANFs have been investigated by the seismic interpretation of four isochronous stratigraphic interfaces,which demonstrates that two individual fault segments propagated towards each other and subsequently,were hard-linked during the Early Eocene.

Zhichao Zhou, Lianfu Mei, Hesheng Shi, Yu Shu. Evolution of Low-Angle Normal Faults in the Enping Sag, the Northern South China Sea: Lateral Growth and Vertical Rotation. Journal of Earth Science, 2019, 30(6): 1326-1340. doi: 10.1007/s12583-019-0899-4
Citation: Zhichao Zhou, Lianfu Mei, Hesheng Shi, Yu Shu. Evolution of Low-Angle Normal Faults in the Enping Sag, the Northern South China Sea: Lateral Growth and Vertical Rotation. Journal of Earth Science, 2019, 30(6): 1326-1340. doi: 10.1007/s12583-019-0899-4
  • For the issue of low-angle normal faults (LANFs), long-term studies have been focused on the reconciliation between the observations in the field and predictions made by the classical model (Anderson, 1951). One fundamental recurring contradiction is the original dip angle of the LANFs. They are rotated, initially high-angle faults (e.g., Whitney et al., 2013; Kapp et al., 2008; Buck, 1988; Wernicke and Axen, 1988) or initiated at a low-angle (e.g., Morley, 2014, 2009; Smith et al., 2007; Axen, 2004; Wernicke, 1995; Wernicke et al., 1985). Passive reorientation of initially high-angle normal faults to gentle dips occurred during either domino-rotation of successive normal fault sets (Proffett, 1977) or isostatic adjustments generating footwall flexure and uplift (Wernicke and Axen, 1988). The examples of the LANFs that can be demonstrated to have originated as low angle faults are comparatively few (e.g., Morley, 2014; Collettini, 2011; Morley, 2009). Moreover, it is important to distinguish the evidences for a low-angle dip on a fault at the time it formed (a primary LANF) and evidence for slip while at low angle (LANF slip on a fault that may have formed at a steeper dip) (Axen, 2004). Methods for estimating the original dip angle of the LANFs include (1) the calculation of initial fault dip under the assumption that extension was accompanied by rigid-body rotation or distributed vertical simple shear (Westaway and Kusznir, 1993; Jackson, 1987), (2) the geometric reconstruction under the constrain of unambiguous pre-rift sub-horizontal or high-angle markers in the footwalls (Lecomte et al., 2010; Smith and Faulkner, 2010; Smith et al., 2007; Axen, 2004) or based on the calculation of the uplift and erosion of the footwall area (Morley, 2014; Axen, 2004).

    Laterally, tip growth and segment linkage are two significant mechanisms for fault growth in extensional settings (Taylor et al., 2004; Trudgill and Cartwright, 1994), which allows the development of large faults and the connection of isolated rift basins. In general, traces for the extensional hard linkage involve the complex and asymmetric fault plane (Willemse, 1997; Childs et al., 1995), highly breached relay ramp (Rotevatn et al., 2007; McClay and Khalil, 1998) and highly variable distribution of displacement along fault strike (Walsh et al., 2003; Cartwright and Mansfield, 1998; Dawers and Anders, 1995). The analysis of these traces focuses on the spatial evolution of fault linkage, rather than the temporal sequence.

    Although there are numerous examples known from both outcrop and seismic data, the geometry and kinematics of the LANFs are commonly constrained in 2D space (e.g., Morley, 2014, 2009; Collettini, 2011). The geometries of 2D fault traces are more numerous than those established in 3D space because of the different 2D views of a 3D object displaying complex geometry (Marchal et al., 2003). In cross-sections that are oblique to fault slip direction, 2D studies of fault trace geometry probably indicate the apparent dip angle which is commonly smaller than the true dip angle. Moreover, the along-dip deformation is accompanied by the along-strike growth of the LANFs, but they are usually regarded as two individual research topics (Leyla et al., 2018).

    Geologic record of the LANFs is plentiful in the northern margin of the South China Sea (Figs. 1a and 1b). We put our emphasis firstly on the northern border faults of the Enping sag (Fig. 1c) because these faults show a simple geometry without significant post-tectonic deformation. Datasets from 3D seismic surveys allow us to investigate the extensional fault system and depict the 3D geometry of the LANFs in the Enping sag. The development of the footwall areas and relationship of these faults to the sedimentary basins are well imaged in 3D seismic data. Combined with the variations of fault geometry along strike and at depth, we argue that isostatic rotation of high- angle faults to a low-angle (e.g., Buck, 1988) is applicable to the LANFs in the Enping sag. The geometry of the syn-rift sedimentary deposits supports that the LANFs were still active at extremely low angle before the final breakup of the South China Sea during the Early Oligocene. Moreover, in order to understand the temporal and spatial evolutions of the LANFs in the Enping sag, we investigate the lateral growth of these faults based on a group of well-constrained stratigraphic succession.

    Figure 1.  (a) Bathymetric map of the northern South China Sea showing drilling wells of Mesozoic granitoids; (b) thickness map of the syn-rift sediments in parts of the Pearl River Mouth Basin with traces of syn-rift extensional normal faults (see the drawing extent in Fig. 1a); (c) time structure map of the rift-onset unconformity (Tg) in the Enping sag and adjacent areas (see the drawing extent in Fig. 1b). SCS. South China Sea; QDNB. Qiongdongnan Basin; PRMB. Pearl River Mouth Basin; SWTB. Southwest Taiwan Basin; COB. continent-ocean boundary; EPF. Enping fault; EDF. Endong fault.

  • From 2011 to 2016, the CNOOC conducted two 3D seismic surveys in the Enping sag (see the boundaries in Liu et al., 2016). Now, the area of the 3D seismic data is approximately 10 700 km2, and the maximum two-way travel time (TWT) record is 8 s. Cross lines are NE-SW orientated and in lines are NW-SE orientated. The interpretation of the normal faults and five seismic interfaces (Tg, SB3, SB5, T80 and T70; Fig. 2) was conducted in the 3D seismic data cube at an interval of 500 m. We extracted 11 seismic profiles (L1 to L11; Fig. 3a) from the 3D seismic data to depict the geometry of the normal faults and relationship of the faults to the sedimentary basins in cross section. These seismic profiles were also used to calculate the along-strike variations in displacement (fault heave and throw) and dip angle. The heave and throw are the horizontal and vertical displacements of a normal fault at the level of the rift-onset unconformity (Tg), respectively. The intersection lines between the 3D fault planes and the top surface of the pre-rift basement are projected onto the plane to represent the normal faults (Fig. 1c). In this way, the length of the normal faults is calculated.

    Figure 3.  (a) Major structural elements of the Enping sag with the location of seismic reflection profiles used as illustrations in the present study; (b) depth to the top surface of the pre-rift basement; (c) dip angle of the top surface of the pre-rift basement. See the drawing extent in Fig. 1c.

    The breakup unconformity was penetrated by 7 wells, providing strong constraint for the interpretation of T70 and seismic interfaces above T70 (Fig. 2). The Wenchang Formation was only revealed by Well A (Fig. 2) which is located in the depocenter of the Enping sag (Figs. 1c and 3b). Thus the seismic interface T80 was constrained by Well A (Fig. 2). Depth-time conversion is important for the calculation of fault dip angle. A depth-time relationship was obtained by making synthetic seismogram of Well A based on logging data (Fig. 4), where a quadratic polynomial function (i.e., D=aT2+bT+c) was used. D is the depth below the sea level and is measured in kilometers. T is the TWT below the sea level and is measured in seconds. The error increases with depth, and the percentage of error is no more than 1.9% above the seismic surface T80 (approximately 4 570 m; Fig. 4). An average velocity of the upper crust is 6.2 km/s (Huang et al., 2005).

    Figure 4.  Fitted depth-time relationship of Well A (see the location in Figs. 1c and 3b) based on the application of synthetic seismogram. The error value is also provided.

  • The 3D geometry of the top-basement surface was constructed based on the 3D seismic data, which consisted of the rift-onset unconformity (Tg) and fault planes (Fig. 3b). The maximum and minimum depths of the top-basement surface are approximately 10.0 and 2.0 km, respectively. The water depth is small in the Enping sag (from 85 to 115 m), thus the depth of the top-basement surface is approximately equivalent to the thickness of sediments overlying the pre-rift basement granitoids. The Enping sag is surrounded by two basement highs, including the Hainan uplift to the northwest and the Panyu low uplift to the southeast (Figs. 3a and 3b). The boundary between the Hainan uplift and the Enping sag is defined by the SE-dipping western and eastern Enping faults which strike an overall SW-NE direction and are separated by the fault block B1 (Figs. 3a and 3b). The length of the western and eastern Enping faults is approximately 42.0 and 36.0 km, respectively. Fault block B2 separates the eastern Enping fault and the Endong fault (Figs. 3a and 3b). The S-dipping Endong fault strikes W-E and the length is approximately 30.0 km. The transition from the Enping sag to the Panyu low uplift is a northwestward-tilted slope zone and an ENE-striking and SSE- dipping normal fault (F4) (Fig. 3b). Three S-dipping minor border faults (faults 1-3) developed in the Hainan uplift.

    The dip angle of the top-basement surface was measured (Fig. 3c). The basement highs that surround the Enping sag have low values of dip angle in the range of 0° to 20°, whereas the minor border faults (faults 1-4) in the Hainan uplift and Panyu low uplift show obvious high-angle geometry with dips in the range of 40° to 60°. The Endong fault dips in the range of 30° to 60°. Except for the N-S-striking segment of the western Enping fault which has large value of dip angle in the range of 30° to 70°, the western and eastern Enping faults have low values of dip angle in the range of 10° to 30° (Fig. 3c).

  • The western Enping fault was characterized by an S-dipping, low-angle (~20°), planar high-amplitude reflection and bordered a classical half-graben basin which was filled by wedge-shaped syn-rift units (A1, A2, A3 and B) (Fig. 5). Large values of fault heave (approximately 16.7 km) and throw (approximately 6.0 km) were observed along the western Enping fault at the level of the seismic interface Tg (Table 1). The maximum depth of the syn-rift stratigraphy was approximately 9.8 km. The older syn-rift stratigraphy was tilted to a higher dip, and the youngest syn-rift stratigraphy was nearly horizontal. As a consequence of slip on the western Enping fault, the tilt angle of the hanging wall block decreases southwardly with an average value of ~33° (Fig. 5). An S-dipping listric normal fault (Fault 2) developed in the footwall of the western Enping fault and had low values of fault heave and throw. Only the upper portion of the syn-rift unit B was observed in the hanging wall of the Fault 2. Post-rift normal faults showed high-angle, listric geometry and were widely spread in the middle of the Enping sag (Fig. 5). Low values of fault heave and throw were observed along these normal faults at the level of the seismic interface T70. A deep-seated, N-dipping high-amplitude and continuous reflection was observed in the footwall of the western Enping fault and tilted an average angle of ~20°.

    Figure 5.  Seismic section L3 illustrating the shallow dipping western Enping fault in the Enping sag. See Fig. 3a for the location. The western Enping fault shows an appropriate planar geometry with a dip angle of ~20°.

    Border fault Western Enping fault Eastern Enping fault
    Seismic line L1 L2 L3 L4 L5 L6 L6 L7 L8 L9 L10
    Heave (km) 7.0 11.6 16.7 15.8 14.0 6.6 4.0 5.4 7.2 7.9 6.6
    Throw (km) 3.2 6.3 6.0 5.1 4.8 3.1 1.4 0.7 1.8 2.8 1.8
    Throw : heave 1 : 2.2 1 : 1.8 1 : 2.8 1 : 3.1 1 : 2.9 1 : 2.1 1 : 2.9 1 : 7.7 1 : 4.0 1 : 2.8 1 : 3.7
    δ (°) 28 29 20 15 19 35 14 12 12 19 15
    θ (°) 18 30 33 29 29 14 19 13 37 43 36
    δ0 (°) 41 49 45 39 42 * * * * * *
    Φ (°) 46 59 53 44 48 * * * * * *

    Table 1.  Along-strike variations in fault heave (km), fault throw (km), fault throw: heave ratio, present-day fault dip (δ), tilt of the oldest sediments (θ), original fault dip (δ0) and cut-off angle (Φ) of the western and eastern Enping faults, Enping sag

  • The western Enping fault soled into and did not cut the low-angle (~14°), planar eastern Enping fault which was characterized by an S-dipping, high-amplitude reflection (Fig. 6). These two border faults delaminated two fault blocks from the Hainan uplift and were separated by a northward-tilted block (B1). The eastern Enping fault had a fault heave of approximately 4.0 km and a fault throw of approximately 1.4 km (Table 1). Moreover, the maximum depth of the syn-rift stratigraphy was approximately 5.2 km in the hanging wall which was tilted at an average angle of ~19° and overlain by the syn-rift units A3 and B (Fig. 6). The western Enping fault had a fault heave of approximately 6.6 km and a fault throw of approximately 3.1 km (Table 1), and dipped at ~35° toward the south. Moreover, the maximum depth of the syn-rift stratigraphy was approximately 7.1 km in the hanging wall which was tilted at an average angle of ~14° and overlain by the syn-rift units (A2, A3 and B) (Fig. 6). A deep-seated, dome-shaped high- amplitude and continuous reflection was observed in the footwall of the eastern Enping fault. The northern and southern flanks of this reflection dipped at ~14° and ~17°, respectively.

    Figure 6.  Seismic section L6 illustrating the overlap of the western and eastern Enping faults. See Fig. 3a for the location.

  • The eastern Enping fault was characterized by an S-dipping, convex-upward high-amplitude reflection which dipped at ~12° above 4.4 s and at ~18° below 4.4 s (approximately 9.0 km) (Fig. 7). The fault heave and throw of the eastern Enping fault were approximately 5.4 and 0.7 km, respectively. Moreover, the maximum depth of the syn-rift stratigraphy was approximately 5.1 km in the hanging wall which was tilted at an average angle of ~13° and overlain by the syn-rift units (A2, A3 and B) (Fig. 7). During the deposition of the syn-rift unit B, the eastern Enping fault was inactive, and the activity of the Fault 1 began. The Endong fault soled into and did not cut the eastern Enping fault. These two border faults were separated by a northward-tilted block (B2). The fault heave and throw of the Endong fault were approximately 1.4 and 1.4 km, respectively. The Endong fault dipped at ~45° toward the south. Moreover, the maximum depth of the syn-rift stratigraphy was approximately 6.0 km in the hanging wall which was tilted at an average angle of ~12° and overlain by the syn-rift units (A3 and B) (Fig. 7). A deep-seated, dome-shaped high-amplitude and continuous reflection was observed in the footwall of the eastern Enping fault (Fig. 7). The northern and southern flanks of this reflection dipped at ~16° and ~13°, respectively.

    Figure 7.  Seismic section L7 illustrating the overlap of the eastern Enping fault and Endong fault. See Fig. 3a for the location.

    In conclusion, the Enping sag displayed a half-graben geometry which was bounded by three major border faults in the north (Figs. 5, 6 and 7). The western and eastern Enping faults showed low-angle geometry, whereas the Endong fault showed high-angle geometry (Fig. 3c). In the overlapping segment, the western Enping fault detached a fault block from the hanging wall of the eastern Enping fault and merged downward with the low- angle eastern Enping fault (Figs. 3b and 6). Similarly, the Endong fault detached a fault block (B2) from the hanging wall of the eastern Enping fault and merged downward with the low-angle eastern Enping fault (Figs. 3b and 7).

  • An isolated normal fault is defined as a fault that propagates in the way of tip growth and has no interaction with surrounding faults (Soliva and Benedicto, 2004). The 3D geometry of an isolated fault displays a simple fault plane with a single ramp. This type of fault exhibits nearly symmetric displacement (heave or throw) distribution which shows largest displacement at or near the center of the fault scarp and decreasing displacement toward the fault tips (Fig. 8a). The displacement is zero where the fault scarp disappears. When two isolated normal faults have similar dip direction, and they propagate towards each other and overlap, a relay ramp will form between the overlapping fault tips (Soliva and Benedicto, 2004; Fig. 8b). In this case, two principal fault planes are observed with overlapping parallel segments, and the displacement profile exhibits two symmetric segments with two peaks. With increasing extension, the two principal planes connect up via a relay fault or a tip fault, which forms a single longer fault plane with a hanging wall or/and a footwall splay (Marchal et al., 2003; Fig. 8c). The displacement profile of the connected fault plane exhibits a curve with three peaks.

    Figure 8.  Diagrams showing map view and displacement profile of (a) an isolated fault; (b) a fault array containing two overlapping segments; (c) a hard-linked fault containing a principal fault plane and a footwall or hanging wall splay (modified from Soliva and Benedicto, 2004).

    The 2D seismic reflection profiles have demonstrated that the western and eastern Enping faults dip in the same direction (toward the south) and are overlapped by a northward-tilted fault block (B1) (Fig. 6). The detailed 3D geometry of the western and eastern Enping faults is described and analyzed using dataset from 3D seismic survey. The results suggest that these two border faults have a principal fault plane with a hanging wall splay (Fig. 9a) which is the eastern tip of the western Enping fault and a characteristic connected secondary structure. Fault displacements, including heave and throw at the rift-onset unconformity (Tg), have been calculated along the strike of the western and eastern Enping faults in the Enping sag (Fig. 10). The fault displacement patterns are similar to the example of a segmented normal fault which has experienced a process of hard-linkage (Fig. 8c). The displacement profiles of the principal fault show three peaks of fault displacement (heave and throw). Maximum heaves vary from 16.7 km for the western Enping fault, to 14.4 km for the overlapping segment and 7.9 km for the eastern Enping fault (Fig. 10). Similarly, maximum throws vary from 6.3 km for the western Enping fault, to 3.8 km for the overlapping segment and 2.8 km for the eastern Enping fault. Maximum heave and throw of the hanging wall splay are 10.0 and 3.1 km, respectively.

    Figure 9.  (a) 3D geometry of the western and eastern Enping faults showing the arrangement of fault troughs and ridges; (b) dip direction of the western and eastern Enping fault surfaces.

    Figure 10.  (a) Along-strike variations in fault heave and throw; (b) along-strike variations in present-day fault dip (δ), tilt of the oldest sediments (θ), cut-off angle (Φ), original fault dip (δ0) of the western and eastern Enping faults. Data are calculated based on seismic profiles L1-L10 (see Fig. 3a for the location).

    The overlap and linkage of individual fault segments will result in the connection of the isolated rift basins within the continent. Therefore, the along-strike propagation of the syn-tectonic stratigraphy records the history of fault growth. We interpreted four seismic interfaces (SB3, SB5, T80, T70) in a 3D space (Fig. 11), which are the top surface of the four syn-rift units (A1, A2, A3 and B; Fig. 2). At the early stage of rifting, three isolated rift basins developed in the Enping sag and were bounded by three border faults (Fig. 11a). During the deposition of the syn-rift unit A2, the connection occurred between the rift basins that were bounded by the western Enping fault and Endong fault (Fig. 11b). The three isolated rift basins were finally connected during the deposition of the syn-rift unit A3 (Fig. 11c). Thereafter, the rift basin extended northward due to the development of high-angle normal faults in the Hainan uplift (Fig. 11d). From the 3D seismic data, a 2D seismic profile was extracted along the fault trace of the western and eastern Enping faults at the level of the rift-onset unconformity (Tg) (Fig. 12). This profile presents the along-strike variations in the syn-rift deposition. The syn-rift units A1 and A2 were deposited in two isolated rift basins which were connected during the deposition of the syn-rift unit A3 (Fig. 12).

    Figure 11.  The results of seismic interpretation showing the depth of four isochronous sequence stratigraphic interfaces (SB3, SB5, T80 and T70) in the Enping sag.

    Figure 12.  Seismic section A-B illustrating the along-strike variations in the syn-rift deposition along the western and eastern Enping faults. See Fig. 3a for the location.

    Moreover, fault traces of the three border faults were traced at the level of the five seismic interfaces (Tg, SB3, SB5, T80, T70) (Fig. 13). The Endong fault propagated in the way of tip growth during rifting. The western and eastern Enping fault propagated towards each other in the way of tip growth during the deposition of the syn-rift units A1 and A2. These two border faults were overlapped by a relay ramp (fault block B1) and then hard-linked by a relay fault during the deposition of the syn-rift unit A3. The eastern Enping fault was inactive during the deposition of the syn-rift unit B (Figs. 6, 7 and 13).

    Figure 13.  Depth to the top surface of the pre-rift basement with the superposition of fault traces and basin boundaries at four different periods of time. The drawing extent is the same with Fig. 3a.

  • The fault plane of the western and eastern Enping faults are presented in a 3D view with contour lines, which shows complex geometry with an arrangement of fault troughs and ridges (Fig. 9a). The trend of these roughs and ridges indicates an SSE direction of fault slip which is approximately parallel to the N-S- striking segment of the western Enping fault (strike-slip fault segment) and oblique to the NE-striking segment of the western Enping fault and the NE-striking eastern Enping fault (oblique extensional fault segment). The dip direction of the western and eastern Enping faults is very complex and ranges from 90°E to 270°W (Fig. 9b). Two flanks of the fault troughs show opposite direction of fault dip. The western flank dips toward the east, whereas the eastern flank dips toward the west. However, the trend of the axis of the toughs is in the range of 166°SSE to 173°SSE. Therefore, seismic profiles were constructed in parallel to the trend of the fault troughs, which is important for the calculation of true fault dips.

    It is important to demonstrate whether the western and eastern Enping faults are rotated, initially high-angle normal faults or formed at a low-angle. We firstly exclude the possibility of domino-style rotation induced by the underlying faults, because there are no significant normal faults in the footwall areas of the western and eastern Enping faults (Figs. 3b and 5). A deep-seated, dome-shaped, high-amplitude seismic reflection is observed in the footwall of the western and eastern Enping faults (Figs. 5, 6, 7 and 14). The minimum depths of the axis are in the range of 11.2 to 13.1 km. The maximum depths of this reflection are in the range of 16.5 to 18.4 km. The differences between the maximum and minimum depths are in the range of 4.9 to 5.5 km. The crust thickness of the Hainan uplift is approximately 30.0 km (Nissen et al., 1995). We argue that this high-amplitude reflection is the surface separating the brittle upper crust and ductile lower crust. Below this reflection, the low-angle fault plane is characterized by a thick ductile shear zone (Fig. 5). Therefore, the brittle upper crust in the footwall of the LANFs was tilted at an angle of ~14° to 22° due to the isostatic footwall rebound.

    Distributed vertical simple shear model was generally used to describe the deformation style in the surroundings of normal faults and estimate the initial fault dips (δ0) for particular present-day dips of faults (δ) and oldest beds (θ) (Westaway, 2005; Westaway and Kusznir, 1993; Fig. 15). However, the bedding-fault cutoff angle (Φ) was also used to indicate the initial dips of normal faults if the extension was accompanied by rigid-body rotation (Morley, 2014, 2009; Axen, 2004; Westaway and Kusznir, 1993; Fig. 15). In our study, we calculated both the initial dip angle (δ0) and cutoff angle (Φ) based on the seismic profiles from L1 to L5 (Table 1) in which the western Enping fault shows simple planar geometry (Fig. 5). The results of calculation indicate that the initial dip angle ranges from 39° to 49° (Fig. 10). By comparison, the cut-off angle shows a higher value between 44° and 59° (Fig. 10). The dip distribution for typical normal faults extends over the range from 30° to 75°, with a peak at 45° (Collettini, 2011; Fig. 16). Therefore, we argue that the western Enping fault formed initially at a high angle and rotated to the present-day low-angle geometry as the result of isostatic footwall rotation during rifting (Fig. 17).

    Figure 14.  Seismic section L11 illustrating the shallow dipping eastern Enping fault in the Enping sag (see Fig. 3a for the location). The eastern Enping fault shows convex-up geometry.

    Figure 15.  Schematic summary of tilting around an isolated normal fault by distributed vertical simple shear (Westaway and Kusznir, 1993).

    Figure 16.  Graph of fault throw: heave ratio as a function of fault dip, illustrating the significant difference in throw : heave ratio and fault dip for a typical normal fault (1 : 2.0-3.7 : 1; 30°-75°) compared with the western Enping fault (1 : 3.1-1 : 1.8; 15°-29°).

    Figure 17.  Restoration of the seismic section L3 (Fig. 5) illustrating how the Enping sag evolved during progressive extension. (a) Pre-rift crustal condition; (b) after 10 km of extension; (c) at the time of continental breakup in the south (~33 Ma); (d) the present-day geometry.

  • It is commonly difficult to prove that the LANFs were initially low-angle or high-angle (Morley, 2009; Axen, 2004), which represents two distinct mechanisms for the formation of the LANFs. Several methods were established by former researchers to distinguish between them, such as the calculation of initial fault dip or the geometric reconstruction (e.g., Smith et al., 2007; Axen, 2004; Westaway and Kusznir, 1993). These methods are also available for the cases of the LANFs in the northern South China Sea. In addition, we provide high-quality seismic data to investigate the deep crustal deformation below the low-angle fault plane (e.g., Figs. 5 and 6). The results suggest that the footwall block has been subject to significant rotation (~14° to 22°) due to isostatic rebound. The planar geometry and initial fault dip indicate that the western Enping fault formed at a high angle (typically between about 39° and 49°; Figs. 10 and 16) which was supposed to be associated with the pre-Cenozoic basement structure (Ye et al., 2018). We exclude the possibility of passive rotation induced by the underlying faults, because there are no significant normal faults in the footwall areas of the western and eastern Enping faults (Figs. 1c and 5). In contrast, significant isostatic rotation of the eastern Enping fault was contributed by the overlying high-angle normal faults (Fig. 14).

    The activity of a growth fault was recorded by contemporaneous sedimentary fillings. The seismic interpretation indicates that the low-angle western Enping fault (~20°) was still active before the onset of seafloor spreading in the south (Fig. 5), which supports the existence of low-angle normal faulting and is conflict with the rolling hinge model (Wernicke and Axen, 1988). To understand the crustal conditions necessary for low-angle normal faulting, it is important to investigate the lithology of pre-rift basement rocks which were penetrated by eight wells in the Enping sag and the areas around (Fig. 1). The samples are granitoids that formed as the product of an east-facing Andean-type volcanic arc during the Late Mesozoic (Xu et al., 2016; Yan et al., 2014, 2011). The granitoids have less cohesion than other basement rocks within the continental crust (Westaway, 1999). Normal faults will continue to slip if the dip lessens to 30°.

    The isostatic rotation of a normal fault is accompanied by the lateral growth in the way of tip propagation or segment linkage (Trudgill and Cartwright, 1994). It is important to demonstrate both the temporal and spatial sequences of fault growth. The syn-rift sediments in a half-graben basin are constrained by the fault plane and tilted hanging wall block. We constructed a set of isochronous seismic interfaces in the syn-rift unit (Fig. 2). The interpretation of these interfaces in the 3D seismic data provides information of fault distribution and length at different periods of time (Figs. 12 and 13). In this way, the temporal and spatial evolutions of fault growth are investigated.

  • The Enping sag is located in the continental shelf of northern South China Sea and displays a half-graben geometry that is bounded by the LANFs in the north. A large amount of 3D seismic survey data and drilling wells enables us to investigate the evolution of the LANFs in the Enping sag, especially for the initial fault dip and lateral fault growth. The major conclusions of this study are as follows.

    (1) The NE-striking western and eastern Enping faults are the northern border fault of the Enping sag. They are low-angle (12°-29°), high-displacement (~16.7 km heave), SE-dipping normal faults and have not associated metamorphic core complexes.

    (2) The arrangement of fault troughs and ridges indicates that the slip of the western and eastern Enping faults was in SSE direction which was approximately parallel to the N-S-striking segment of the western Enping fault and oblique to the NE-striking segment of the western Enping fault and the NE-striking eastern Enping fault.

    (3) During the Early Eocene, the western and eastern Enping faults propagated towards each other in the way of tip growth, and then they were overlapped by a relay ramp and finally hard-linked by a relay fault. The eastern Enping fault was inactive during the deposition of the Enping Formation.

    (4) The calculation of initial fault dip and the tilt of the footwall block indicate that the western and eastern Enping faults originated as high-angle faults (39°-49°) and were subsequently rotated to a low angle (12°-29°) due to isostatic rebound during rifting.

  • The authors wish to thank two anonymous reviewers for their constructive reviews and suggestions, which greatly helped to improve this paper. We thank the editors for the detailed and impartial comments. This study was supported by the Major National Science and Technology Programs, China (Nos. 2016ZX05026-003-001 and 2011ZX05023-001-015). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0899-4.

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