Accepted Manuscript Growth and linkage of a complex oblique-slip fault zone in the Pearl River Mouth Basin, northern South China Sea Ke Huang, Guangfa Zhong, Min He, Lihua Liu, Zhe Wu, Xuefeng Liu PII:
To appear in:
Journal of Structural Geology
Received Date: 29 November 2017 Revised Date:
1 September 2018
Accepted Date: 4 September 2018
Please cite this article as: Huang, K., Zhong, G., He, M., Liu, L., Wu, Z., Liu, X., Growth and linkage of a complex oblique-slip fault zone in the Pearl River Mouth Basin, northern South China Sea, Journal of Structural Geology (2018), doi: 10.1016/j.jsg.2018.09.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
Growth and linkage of a complex oblique-slip fault zone in the Pearl River
Mouth Basin, northern South China Sea
Ke Huanga, Guangfa Zhonga, *, Min Hea, b, Lihua Liub, Zhe Wub, Xuefeng Liuc
4 5 6
Shanghai 200092, China
Road, Shenzhen 518054, China
State Key Laboratory of Marine Geology, Tongji University, 1239 Siping Road,
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Research Institute of Shenzhen Branch, CNOOC China Limited, 3168 Houhaibin
Shangda Road, Shanghai 200444, China.
School of Communication and Information Engineering, Shanghai University, 99
* Corresponding author. Tel.: +86 21 65982784. E-mail address: [email protected]
E-mail addresses: [email protected]
(K. Huang), [email protected]
(Z. Wu), [email protected]
Key words: Oblique-slip normal fault, fault growth pattern, strike and dip linkage,
Three-dimensional seismic, Pearl River Mouth Basin, South China Sea
ACCEPTED MANUSCRIPT 23
Oblique-slip faults are ubiquitous in sedimentary basins, but have been largely
neglected relative to their dip-slip counterpart. Case studies of oblique slip faults are
very limited. In this study, we used 3D seismic data, tied to well control, to investigate
the geometry, kinematic characteristics, growth and linkage, and tempo-spatial
evolution of a complex oblique-slip growth fault zone in the Pearl River Mouth Basin
(PRMB) of the northern continental margin, South China Sea (SCS). The
E-W-oriented 30.5-km-long fault zone consists of two superimposed oblique-slip fault
systems. The lower fault system is an integrated zig-zag fault consisting of three
ENE-WSW- to E-W-striking, right-stepping, sinistral en-echelon fault segments,
which were hard-linked by two NW-SE-trending fault segments; while the upper fault
system is composed of nine WNW-ESE-striking, left-stepping, dextral en-echelon
component faults, which had overlapping fault tips and were partly hard-linked by
breaching of the relay zones. The lower and upper fault systems were activated in the
Eocene and the Oligocene to Pliocene, which respectively correspond to the syn-rift
and post-rift stages in the PRMB. The fault throws (up to 288 m) and activity rates
(up to 34.5 m/Ma) of the upper fault system are overall much less than those of its
lower counterpart. Reactivation of the lower syn-rift fault system shows clear
influence on the development of post-rift en-echelon fault segments. A four-stage
evolution model of the oblique-slip fault zone was presented.
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ACCEPTED MANUSCRIPT 1. Introduction
Accumulated evidence indicates that most of the faults in rifted basins are difficult to
describe by using a simple dip-slip fault model (Crider, 2001; Morley et al., 2004,
2007; Morley, 2007; Whipp et al., 2014; Duffy et al., 2015; Henstra et al., 2015;
Reeve et al., 2015; Morley, 2016). In rift basins, oblique slip faults should be
ubiquitous, but have been largely ignored in literature relative to their dip-slip
counterpart. It is suggested that the formation of oblique-slip faults in rifted basins
could be associated with reactivation of pre-existing faults or crustal weakness zones
trending oblique to the regional extension direction (Bellahsen and Daniel, 2005;
Henza et al., 2010, 2011; Chattopadhyay and Chakra, 2013). Variations in the
direction of regional extension may provide favorable conditions for the development
of oblique-slip fault systems in rifted basins (Bonini et al., 1997; Clifton et al., 2000;
Morley et al., 2004, 2007; Morley, 2007; Tingay et al., 2010; Paredes et al., 2013;
Whipp et al., 2014; Morley, 2016).
Numerous sand-box modeling experiments have been carried out to investigate the
origin of oblique-slip faults in extensional settings (Withjack and Jamison, 1986; Tron
and Brun, 1991; McClay and White, 1995; Bonini et al., 1997; Higgins and Harris,
1997; Keep and McClay, 1997; Clifton et al., 2000; McClay et al., 2002; Schlische et
al., 2002; Henza et al., 2010, 2011). Results indicate that formation of an oblique-slip
fault is controlled simultaneously by both the extension displacement perpendicular to
and the shear displacement parallel to the trend of the host rift. The relative amounts
of extension and shear components depend on the acute angle, α, between the rift
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ACCEPTED MANUSCRIPT trend and the relative displacement direction on opposite sides of the rift (Withjack
and Jamison, 1986). Depending on the angle (α), oblique-slip faults may be dip-slip or
strike-slip dominated, that is, fault segments that lie at a relatively low angle to the
extensional direction will display smaller dip-slip and larger strike-slip displacement
components (Morley et al., 2004).
Research on fault growth and linkage is a hot topic in structural geology during the
past decades (Cartwright et al., 1995; Morley, 1999; Crider, 2001; Peacock, 2002;
Walsh et al., 2003; Kim et al., 2005; Baudon and Cartwright, 2008a, 2008b, 2008c;
Frankowicz and McClay, 2010; Giba et al., 2012; Jackson and Rotevatn, 2013;
Paredes et al., 2013; Tvedt et al., 2013; Wilson et al., 2013; Fazli Khani and Back,
2015; Robson et al., 2016, 2017). Available references, however, have been mostly
focused on dip-slip rather than oblique-slip normal faults (Cartwright et al., 1995;
Morley, 1999; Peacock, 2002; Walsh et al., 2003; Kim et al., 2005; Baudon and
Cartwright, 2008a, 2008b, 2008c; Paredes et al., 2013; Tvedt et al., 2013; Wilson et
al., 2013; Fazli Khani and Back, 2015; Robson et al., 2016, 2017). It has been
confirmed that normal faults are mostly segmented in both map view and section view
(Cartwright et al., 1995; Childs et al., 1995; Peacock, 2002; Walsh et al., 2003;
Frankowicz and McClay, 2010). Growth of a segmented normal fault can be
explained by two models, the isolated fault model (Walsh and Watterson, 1988;
Cartwright et al., 1995; Walsh et al., 2003; Giba et al., 2012; Jackson and Rotevatn,
2013; Jackson et al., 2016) and the constant length model (Morley, 1999; Walsh et al.,
2002, 2003; Giba et al., 2012; Jackson and Rotevatn, 2013; Jackson et al., 2016; Nicol
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ACCEPTED MANUSCRIPT et al., 2016). According to the isolated fault model, fault segments in a fault zone tend
to behave independently, and grow by a simultaneous increase in both displacement
and length (Walsh and Watterson, 1988; Cartwright et al., 1995; Walsh et al., 2003;
Giba et al., 2012; Jackson and Rotevatn, 2013; Jackson et al., 2016). A isolated fault is
characterized by decrease or deficit of summed displacement in the overlap of two
adjacent fault segments (Walsh et al., 2002). In contrast, fault segments comprising a
constant length fault zone tend to rapidly attain their near-final lengths in the early of
their slip history, with fault displacement mostly built after their fault length has been
established (Morley, 1999; Walsh et al., 2002). Hence a constant length fault tends to
be under-displaced for its length during much of its life. Low or no displacement
deficits can be recognized in the areas of overlapping faults (Morley, 1999; Walsh et
al., 2002, 2003; Giba et al., 2012; Jackson and Rotevatn, 2013; Jackson et al., 2016;
Nicol et al., 2016). In addition, fault linkage can occur through soft or hard linkage
(Peacock, 2002; Kim et al., 2005). Soft linkage describes faults that are only
kinetically linked, while hard linkage describes fault segments that are physically
linked by e.g. breaching of the relay zones (Peacock, 2002; Kim et al., 2005).
In this study, we applied 3D seismic data to investigate the characteristics, growth and
linkage, and tempo-spatial evolution of a complex oblique-slip fault zone in the Pearl
River Mouth Basin (PRMB), northern South China Sea (SCS) margin. The fault zone
consists of two superimposed en-echelon oblique-slip fault systems, which developed
in the syn-rift and post-rift stages, respectively. Both the syn-rift and post-rift
oblique-slip fault systems contain several fault segments showing different step and
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ACCEPTED MANUSCRIPT slip senses, suggesting that the fault systems formed under different tectonic stress
environments. Moreover, the syn-rift oblique-slip fault system formed earlier shows
significant control on the development of the post-rift fault system. Our research has
implications for understanding the complexity and diversity of faults in rifted basins,
and further for understanding the geological evolution of the basins, in view of the
pervasiveness of oblique-slip faults in rifted basins and the scarcity of related case
studies in literature. Application of oblique-slip fault model into the exploration
practice will undoubtedly broaden the horizon of searching for petroleum traps and
reservoirs and improve the success rate of petroleum exploration in the rifted basins.
In addition, as far as our knowledge, no literature has addressed the issue of
oblique-slip faults in the PRMB, therefore our research is of significance for more
accurately addressing the regional tectonic evolution of the basin.
2. Geologic background
The PRMB is located in the middle of the northern SCS continental margin and
comprises the largest Cenozoic hydrocarbon-bearing sedimentary basin in the margin.
The basin trends in a ENE-WSW direction parallel to its host continental margin and
is subdivided into five secondary tectonic units of the same strike, which from
north-northwest to south-southeast are the northern uplift, northern depression
(including the Zhu-I and Zhu-III depressions), central uplift (consisting of the Shenhu,
Panyu, and Dongsha uplifts), southern depression (consisting of the Zhu-II and
Chaoshan depressions), and southern uplift zones (Fig. 1) (Li, 1993; Chen et al., 2003;
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ACCEPTED MANUSCRIPT Shi et al., 2014).
The Zhu-I depression occupies the middle to east part of the northern depression zone.
It consists of five sub-tectonic units, named ‘sags’ by local geologists, which from
southwest to northeast are the Enping, Xijiang, Huizhou, Lufeng and Hanjiang sags
(Fig. 1A). Each sag contains several grabens or half grabens separated by low-relief
The fault zone investigated here, named the EP18 fault zone, lies in the northeastern
Enping sag and comprises the northern boundary of a Paleogene half graben to the
south (EP 18 half graben in Fig. 1B). The Enping sag, covering an area of
approximately 5000 km2, comprises one of the major hydrocarbon-producing sags in
the Zhu-I depression (Xu et al., 2014; Fig. 1A). Similar to the other sags or
sub-tectonic units in the PRMB, the Enping sag has experienced two stages of
geological evolution, which are the Eocene syn-rift and the Oligocene to Recent
post-rift stages (Li, 1993; Chen et al., 2003). Horizon T70 is interpreted as the
breakup unconformity (corresponding to regional Nanhai tectonic event, Fig. 2),
which separates the underlain syn-rift from the overlying post-rift successions. Other
important syn-rift and post-rift unconformities include horizons T80, T60, and T32
and T30, which correspond to Zhuqiong-II, Baiyun, and Dongsha tectonic events,
respectively (Fig. 2). The stratigraphy of the Enping sag consists, from bottom to top,
of semi-deep to deep lacustrine shales with thin sandstone interbeds in the lower
Eocene Wenchang formations; fluvial-lacustrine, swamp and deltaic sandy shales with
thin coal interbeds in the upper Eocene Enping Formation; littoral sandy shales in the
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ACCEPTED MANUSCRIPT Oligocene Zhuhai Formation; and marine sandstones, siltstones, and shales in the
lower Miocene Zhujiang Formation, the middle Miocene Hanjiang Formation, the
upper Miocene Yuehai Formation, and the Pliocene Wanshan Formation, as well as
the Quaternary layers (Fig. 2; Chen et al., 2003).
Several sets of faults, trending in the WNW to NW, E-W to ENE-WSW, and NE-SW
directions, have been identified in the PRMB (Li, 1993; Chen et al., 2003). The
NE-SW-striking faults primarily formed during the syn-rift stage (Eocene, Tg-T70) in
response to NW-SE-orientated extension (Lüdmann and Wong, 1999; Wang et al.,
2011; Xu et al., 2014; Leyla et al., 2015; Wu et al., 2015; Hu et al., 2016a, 2016b); the
E-W- to ENE-WSW-trending faults were mostly developed during the early post-rift
stage (Oligocene to early Early Miocene, T70-T50) in response to roughly
N-S-orientated extension (Lüdmann and Wong, 1999; Chan et al., 2010; Wang et al.,
2011; Wu et al., 2015; Hu et al., 2016a, 2016b); and the WNW- to NW-trending faults
predominantly formed during the middle to late post-rift stage (late Early Miocene to
Pliocene, T50-T20) associated with NNE-SSW-orientated extension (Chan et al.,
2010; Wu et al., 2015; Hu et al., 2016a, 2016b).
3. Data and methods
A 1300 km² three-dimensional prestack time-migrated seismic data volume in the
Enping sag was available, covering the fault zone studied here (Fig. 1B). Our target
interval includes the whole Cenozoic sedimentary succession in the basin, with depths
in our study area ranging from seafloor at approximately 110 m deep to the
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ACCEPTED MANUSCRIPT pre-Cenozoic basement of about 7000 m. Corresponding interval velocity increases
from 1700 m/s near seafloor to about 4500 m/s near the basement, as revealed by
drilling data. The dominant frequency of our seismic data varies from 45 Hz in the
shallower interval to about 25 Hz in the deeper interval close to the basement.
Therefore, the vertical resolution of our seismic data in terms of tuning thickness is
variable, ranging from approximately 9 m in shallower interval to 45 m in deeper
interval. In addition, drilling data from one nearby exploration well (well A in Fig. 1B)
were used to make a synthetic seismogram for horizon calibration (Fig. 3).
The fault zone was characterized by integrating profile- and horizon-slice-based
interpretation (Figs. 4-5). Through interpreting the seismic profiles, fault elements
including morphology and occurrence of the fault zone were determined, and fault
throws were quantitatively measured (Fig. 4). Horizon slices with seismic coherence
attribute were used to describe the plane-view characteristics of the fault zone (Fig. 5).
In this study, the coherence attribute was built on the amplitude variance algorithm
(Bahorich and Farmer, 1995).
In order to quantitatively describe the fault zone, we measured fault throws through a
total of 151 seismic profiles oriented perpendicular to the studied fault zone at an
average spacing of 200 m. Here, a fault throw in time difference (ms TWT) was
measured between the corresponding footwall and hanging wall cut-offs of a horizon
on the seismic profiles, which was subsequently converted from time (ms TWT) to
depth (m) using a velocity model derived from the time-depth relationships from
well-logging data. These fault throws were further used to calculate parameters
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ACCEPTED MANUSCRIPT depicting the activity and growth history of the fault zone, including expansion index
(EI) and fault activity rate (FAR). EI, defined as the ratio of hanging-wall to footwall
thicknesses of a given layer, is a traditional and widely-used parameter for kinematic
analysis of growth faults (Thorsen, 1963; Jackson and Rotevatn, 2013; Tvedt et al.,
2013; Jackson et al., 2016). FAR is calculated by dividing thickness difference
between the hanging-wall and footwall for a given layer by the corresponding
deposition time. In making these measurements and calculations, ten age-dated
regional seismic horizons were used, which from bottom to top are Tg (base of of the
Cenozoic basin, ~66 Ma), T80 (base of the Upper Eocene, ~38 Ma), T70 (base of the
Oligocene, ~33.9 Ma), T60 (base of the Lower Miocene, ~23.03 Ma), T50 (boundary
between the lower and upper portions of the Lower Miocene, ~19.1Ma), T40 (base of
the Middle Miocene, ~15.97 Ma), T35 (boundary between the lower and upper
portions of the Middle Miocene, ~13.82 Ma), T32 (base of the Late Miocene, ~10
Ma), T30 (base of the Pliocene, ~5.33 Ma) and T20 (base of the Quaternary, ~2.59 Ma)
Several types of plots, including T-x plots (Figs. 6-7), T-z plots (Fig. 8), EI and FAR
plots (Fig. 8), and throw-strike projections (Fig. 9), as well as time thickness maps
(Fig. 10), were compiled to analyze the temporal and spatial distribution
characteristics of the fault throws, from which the growth and linkage history of the
fault segments were inferred. A T-x plot in which fault throws are plotted against the
distance along the strike of the fault depicts the lateral variations of fault throws along
the strike of a fault. It is used to analyze the segmentation and lateral linkage of faults
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ACCEPTED MANUSCRIPT in map view. Throw minima on a T-x plot represent the position of fault segment
interconnections, and throw maxima represent the center of each fault segment
(Cartwright et. al., 1995, 1996; Jackson and Rotevatn, 2013; Tvedt et al., 2013). A T-z
plot in which fault throws are plotted against the depth to the midpoint between the
respective hanging wall and footwall cut-offs displays vertical throw variations along
a fault plane. It is used to analyze the vertical segmentation and dip-linkage of a fault
and to reconstruct its growth history (Mansfield and Cartwright, 1996; Nicol et al.,
1996; Meyer et al., 2002; Childs et al., 1996, 2003; Ge and Anderson, 2007; Baudon
and Cartwright, 2008a, 2008b, 2008c; Jackson and Rotevatn, 2013; Robson et al.,
2016, 2017). A throw-strike projection in which throw contours are plotted on a
vertical fault plane projection is used to illustrate throw distribution across fault
surfaces and provide insights into the growth and linkage history of segmented faults
in three-dimensional fault surfaces (Chapman and Meneilly, 1991; Walsh and
Watterson, 1991; Childs et al., 1993; Tvedt et al., 2013). The EI and FAR plots in
which EI or FAR values are plotted against the corresponding strata depth or
deposition time quantitatively reveal the variations in fault activity intensity with time.
Time thickness maps of major stratigraphic intervals within the growth sequence
reveal the variation of fault-controlled depocentres with time, therefore indirectly
reflect the growth history of the bounding faults (Jackson et al., 2016).
It should be pointed out that potential uncertainties may exist in our methodology,
including those associated with data resolution, fault and horizon picking, treatment
of fault dragging, and compaction effect. Firstly, as decreasing of data resolution with
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ACCEPTED MANUSCRIPT depth, uncertainty in fault throw measurements will increase. Secondly, errors in
manual fault and horizon picking will inevitably cause uncertainty in measurement of
fault throws. Fortunately, this uncertainty was minimized by our reliable fault and
horizon picking on the high-quality 3D seismic data volume. Thirdly, improper
treatment of drag folds may cause certain uncertainty in throw measurements. We
followed the traditional method in structural geology to exclude the drag-folding
effect by defining the cut-offs using an extrapolated line that follows the regional
trend of the horizon prior to folding (e.g. Chapman and Meneilly, 1991; Mansfield
and Cartwright, 1996; Wilson et al., 2013; Duffy et al., 2015). Finally, we did not
make a decompaction correction on the measured fault throws being lack of reliable
local lithology information, which may result in potential uncertainty of throw
measurements. This uncertainty increases with burial depth (Taylor et al., 2008).
According to the porosity-depth equations in the PRMB (Equations 4 and 5 in He et
al., 2017), for an assumed sand-mud interbedded unit consisting of 50% mudstone
and 50% sandstone, compaction-caused thickness reduction will be up to 20% at 1000
m burial depth, 41% at 3000 m, 49% at 5000 m. This compaction-caused thickness
reduction will give rise to underestimating the fault throws. The deeper the burial
depth, the lager the degree of this underestimation.
The EP18 fault zone is approximately 30.5 km long and has an approximately E-W
strike (Figs. 1 and 5). Coherence horizon slices indicate that the fault zone consists of
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ACCEPTED MANUSCRIPT 5 to 9 fault segments at different horizons (or depths). This fault zone is evidently
composed of two tiers of fault systems separated by horizon T70, which are named
the lower and upper fault systems, respectively (Fig. 5). This vertical zonation is also
reflected in the fault parameter plots including the fault T-z plots (Fig. 8), EI and FAR
plots (Fig. 8), and throw-strike projections (Fig. 9).
4.1. The lower fault system
As shown in the coherence horizon slices at horizons T70 and T80, the lower fault
system consists of three right-stepping, sinistral ENE-WSW- to E-W-striking fault
segments, named F1 to F3 from west to east, which are linked by two NW-SE-striking
short fault segments, named A and B (Figs. 5I-5L). These fault segments were
connected end-to-end to form a zigzag E-W-striking fault system. Noted that there is a
WNW-ESE-striking fault segment, named C, to the south of fault segment F3,
approaching westwards to but not linked with the E-W-striking fault system (Figs.
In cross sections, the lower fault system offset downward into the basement (below
horizon Tg) and upward to horizon T70 at the Eocene-Oligocene boundary; and it
shows a listric fault surface geometry, with dips ranging from 35° to 48° and
decreasing downwards (Fig. 4). The fault system displays obvious growth
characteristics (Fig. 4), with a maximum thickness of syn-rift growth strata in the
hanging wall up to 4500 m (Figs. 4 and 10D).
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ACCEPTED MANUSCRIPT 4.1.2. Throw distribution
As shown in the T-x plot (Fig. 6B), throws of the lower fault system are highly
variable, with the maximum found at horizon Tg in fault segment F2. Overall, fault
throws are larger at the center of the fault system, and decrease toward both the east
and west ends. Although the lower fault system consists of five fault segments as
shown in the horizon slices (Figs. 5I-5L), no obvious segmentation characteristics can
be identified in the T-x plots (Fig. 6B). Nevertheless, an abnormal throw decrease at a
distance of ~24 km is noted in the T-x plot (Fig. 6B).
The lower fault system shows an asymmetric pattern of throw distribution in the T-z
plots (Figs. 8A-E) in which throws decrease from horizon Tg upward to horizon T70.
Neglecting possible erosion (especially in the footwall) and compaction effects, the
maximum cumulative throw of the lower fault system is up to 5000 m (Figs. 6B and
8A-E). The fault system is characterized by tight horizontal contours on the
throw-strike projections (Fig. 9).
4.1.3. Fault activity
The lower fault system offset horizons between Tg and T70 during the Eocene (Figs.
4-6, 8-10). EI values may be up to 9.5, and are normally larger in the center and
decreases toward both ends (Figs. 8A-E). FAR values varied between 10.1 and 70.1
m/Ma during the Early Eocene (Tg-T80) and between 53.6 and 509.3 m/Ma during
the Late Eocene (T80-T70) (Figs. 8A-E). It should be noted that no EI values
available for the Early Eocene interval between horizons Tg and T80 in most portions
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ACCEPTED MANUSCRIPT of the lower fault system due to no net depositional thickness on the footwall, leading
to the EI values to be infinite (Figs. 4 and 8B-E).
4.2. The upper fault system
4.2.1. General characteristics
The upper fault system developed over the lower one, and shows a similar
E-W-trending strike and approximately equal trace lengths to its lower counterpart
(Fig. 5). The upper fault system contains nine separate to partially connected fault
segments, which are named fault segments f1 to f9 from west to east, respectively
(Figs. 5A-H). Each fault segment is from 1.2 to 8.5 km in length. These fault
segments trend in a WNW-ESE direction and are arranged in a left-stepping, dextral,
en-echelon pattern (Figs. 5A-H).
All the fault segments comprising the upper fault system are normal growth faults
characterized by decreasing-upward fault throws (Figs. 4 and 8). Compared to their
lower counterparts, these fault segments are more or less planar and show obviously
steeper dips (between 59° and 65°) in cross-section view (Fig. 4). These fault
segments were physically linked in their central portions to the underlain syn-rift fault
system (Fig 4). At the linked portions, the fault planes are of a listric geometry,
consisting of a steeper-dipping upper portion (the upper fault system) and a
gently-dipping lower portion (the lower fault system) (Figs. 4A, E-I). As away from
their central portions, the upper fault segments were linked through a gently dipping
connecting segment with the underlain lower fault system, making the linked fault
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ACCEPTED MANUSCRIPT 325
plane in a sigmoid shape (Figs.
segments were mostly not physically linked with the underlain lower fault system
(Figs. 4C, H).
Overall, the upper fault segments show much smaller throws and much lower EI and
FAR values than their lower counterparts (Figs. 4, 8-9).
4.2.2. Fault segment f1
Fault segment f1 lies at the western end of the upper fault system and approximately
over the western portion of fault segment F1 (Fig. 5). The fault segment strikes in a
WNW-ESE direction, dips SSW, and is up to 6.7 km in length, with fault length
gradually decreasing upward (Figs. 5A-H). In cross-sections, fault segment f1 shows
evident growth characteristics, with stratal thickness in its hanging wall obviously
greater than that in its footwall (Fig. 4). The fault segment was physically linked with
the underlain fault segment F1 in its center; but toward fault tips it failed to offset
downward the horizons below T60, and therefore was not physically linked with the
lower fault system (Fig. 4C).
The T-z plots (Figs. 8A, F) taken at the central portion of fault segment f1 show an
asymmetric pattern of throw distribution, with throws decreasing from horizon T70
upward to horizon T20. On the throw-strike projection (Fig. 9A), fault segment f1 is
characterized by semielliptical throw contours, with a maximum throw at horizon T70
in its central portion, from which throws decrease upward and toward tip-lines. The
fault segment shows much smaller throws (up to 230 m) than its underlain counterpart,
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4B-D). Further toward their tips, the upper fault
ACCEPTED MANUSCRIPT fault segment F1 (up to 1500 m, Fig. 9A).
Fault segment f1 offset horizons downward to T70 and upward to T20 (Figs. 4-5). It
achieves a maximum throw of up to 235 m at horizon T70, (Fig. 7B), from which
fault throw decreases upward. Laterally, the fault segment shows larger throws at its
center, from which throws decrease toward both ends (Fig. 7B). Throws of the fault
segment decrease upwards from horizons T70 to T20 (Fig. 8F). EI and FAR values are
variable, which are relatively higher in the T60-T50 (early Early Miocene) and
T35-T20 (late Middle Miocene to Pliocene) intervals (Fig. 8F).
4.2.3. Fault segments f2 to f6
Fault segments f2 to f6 occupy the middle portion of the upper fault system. They are
arranged in an E-W-orientated en-echelon pattern in map view, tracing the
corresponding portions of the underlain lower fault system, including fault segments
eastern F1, A, F2 and western F3. Each of the fault segments has a WNW-ESE strike.
These fault segments laterally overlapped and were connected each other (Figs.
5A-H). In addition, fault segments f2 and f3 are linked by an additional small
E-W-striking fault (fault s in Fig. 7A). The f2 to f6 fault segments show their greatest
lengths at horizon T50 (Fig. 5M), from which the trace length of each fault segment
decreases both downward to horizon T70 and upward to horizon T20.
Fault segment f2 occurs directly over and follows the traces of the eastern F1, A, and
F2 fault segments of the lower fault system (Fig. 5). It is approximately 8.5 km,
comprising the longest fault segments in the upper fault system.
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ACCEPTED MANUSCRIPT Fault segment f3 occupies a horizontal position similar to fault segment B, but the
strike of the fault segment shows a minor counterclockwise rotation from NW-SE to
WNW-ESE (Fig. 5). The fault segment is approximately 5.5 km long, which is
evidently longer than its underlying counterpart, fault segment B (about 3 km).
Fault segments f4 to f6 in general follow fault segment F3, but each of the fault
segments has a WNW-ESE strike, which is obviously different from the underlain
ENE-WSW-extending fault segment (Fig. 5).
As shown in the T-z plots (Figs. 8B-E) taken at the center of each fault segment, fault
segments f2 to f5 show a throw maxima zone at horizon T50, which is separated by a
throw minima zone near horizon T70 from the underlain lower fault system. From the
throw-strike projections (Figs. 9B-D), fault segments f2 to f5 are characterized by an
approximately symmetric distribution of throws and elongated, elliptical throw
contours, which are centered in the throw maxima zone (near horizon T50). The
throws observed between the T70-T50 strata decrease from horizon T50 downward to
T70 (Figs. 9B-D).
Fault segments f2 to f6 offset horizons from T50 upward to T20 (Figs. 4-5 and 8G-J),
and achieve their maximum fault throws at horizon T50, from which the throws in
general decrease downward to T70 and upward to T20 (Figs. 4-5, 7B and 8G-J).
Laterally, throws are highly variable along these fault segments, with a maximum of
288 m at the center of fault segment f3. For each fault segment, throws are normally
greatest at its center and decrease toward fault tips (Figs. 7B and 9B-D). EI and FAR
values are relatively higher in the T40-T35 (early Middle Miocene) and T32-T30
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ACCEPTED MANUSCRIPT (Late Miocene) intervals (Figs. 8G-J).
4.2.4. Fault segments f7 to f9
Fault segments f7 to f9 lie in the east of the upper fault system, over the eastern
portion of fault segment F3 (Figs. 5E-H). Trending in a WNW-ESE orientation, these
fault segments arrange in an en-echelon pattern in which each fault segment is
separated by an unbroken relay zone (Figs. 5E-H; Peacock, 2002; Kim et al., 2005).
These fault segments are evidently shorter in length (less than 2 km) compared to the
other fault segments (f1 to f6, 2-8.5 km) in the upper fault system (Figs. 5E-H).
Similar to fault segments f2 to f6, fault segments f7 to f9 are physically linked in their
central portions with the underlain syn-rift fault segment F3 (Fig. 4I).
The fault segments offset the interval between horizons T70 to T50, with fault throws
gradually decreasing upward (Fig. 4I). Trace length of each fault segment shows a
similar upward-decreasing pattern (Figs. 5E-H).
5.1. Oblique-slip nature of the fault zone
As described above, the fault segments comprising both the lower and upper fault
systems are all normal growth faults (Fig. 4). The lower fault system consists of two
sets of fault segments of different strikes, i.e. the ENE-WSW- to E-W-striking F1 to
F3 and the NW-SE-striking A and B fault segments. These fault segments were
physically linked in an end-to-end pattern to form a zigzag E-W-striking fault system
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ACCEPTED MANUSCRIPT (Figs. 5-6). The upper fault system consists of nine WNW-ESE-striking, left-stepping,
en-echelon arranged fault segments (Figs. 5 and 7). All these indicate that both the
lower and upper fault systems were formed in an oblique-slip extension stress
environment. This argument is also supported by the findings that the E-W-oriented
fault zone was oblique to the simultaneous regional stress field direction. It is
suggested that the regional stress field orientation in the PRMB experienced clear
clockwise rotation from NW-SE extension during the syn-rift stage (Eocene, Tg to
T70) to N-S extension during the early to middle post-rift stage (Oligocene to early
Early Miocene, T70 to T50), and further to NNE-SSW extension during the middle to
late post-rift stage (late Early Miocene to Pliocene, T50 to T20) (Lüdmann and Wong,
1999; Chan et al., 2010; Wang et al., 2011; Wu et al., 2015; Hu et al., 2016a, 2016b).
The regional NW-SE-orientated extension stress field acting on the E-W-trending
fault zone might cause the formation of the right-stepping, sinistral en-echelon lower
fault system during the Eocene ages. Similarly, regional N-S to NNE-SSW orientated
extension acting on the E-W-trending fault zone might give rise to the development of
the left-stepping, dextral en-echelon upper fault system during the Oligocene to
Pliocene ages. It is suggested that step sense is directly related to the slip sense of an
en-echelon fault zone, that is, left-stepping fault segments are indicative of dextral
displacement, whereas right-stepping segments indicate sinistral displacement
(Tchalenko, 1970; Sylvester, 1988; Richard, 1991; Smith and Durney, 1992; Crider,
2001; Cembrano et al., 2005; Swanson, 2006; Ghosh and Chattopadhyay, 2008;
Dooley and Schreurs, 2012).
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ACCEPTED MANUSCRIPT 5.2. Time and intensity of fault activity
As previously described, the lower fault system was active during the Eocene
(Tg-T70) with major activity during the late Eocene (T80-T70) (Fig. 8). In general,
the central portion of the lower fault system has greater throws and activity rates than
those at both ends (Figs. 6 and 8), indicating that the lower fault system was more
intensely active in its central portion.
The upper fault system was overall active during the Oligocene to Pliocene ages (Figs.
5 and 8). Nevertheless, certain difference in fault-activity time of individual
component fault segments is noted: fault segment f1 was active during the Oligocene
to Pliocene (T70-T20), fault segments f7 to f9 during the Oligocene to early Early
Miocene (T70-T50), and fault segments f2 to f6 during the late Early Miocene to
Pliocene (T50-T20). Also, different fault segments achieved their maximum throws
and trace lengths at different horizons: fault segments f1 and f7 to f9 at horizon T70
4M, 7B-E and 8B-D). This indicates that different segments were nucleated at
different times: fault segments f1 and f7 to f9 at the western and eastern ends of the
upper fault system nucleated earlier (during the beginning of the Oligocene), while
fault segments f2 to f6 in the middle nucleated later (during the late Early Miocene).
In addition, in terms of fault activity, fault segments f2 to f4 in the center of the upper
fault system normally show greater throws and higher EI and FAR values than those
of the other fault segments on both sides (Figs. 7-9), which indicates that the central
portion of the upper fault system had more intense activity than that at both ends.
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2D, 2H, 4M, 7A and 8A), and fault segments f2 to f6 at horizon T50 (Figs.
ACCEPTED MANUSCRIPT In short, the lower and upper fault systems were active during the Eocene and the
Oligocene to Pliocene ages, which rightly correspond to the syn-rift and post-rift
stages, respectively (Fig. 2). Therefore, we interpreted the lower and fault systems as
syn-rift and post-rift fault systems, respectively. In addition, the lower fault system
shows much larger fault throws and higher EI and FAR values than those in the upper
fault system, which indicates that activity of the EP18 fault system is much stronger
in the syn-rift stage than that in the post-rift stage.
5.3. Growth pattern and strike linkage of the fault systems
The lower fault system is an integrated zig-zig fault, which consist of five fault
segments with two sets of strike: E-W to ENE-WSW (fault segments F1 to F3) and
the NW-SE (fault segments A and B) (Figs. 5I-L). These fault segments connected
end by end, and physically linked (hard-linked) each other. However, no evident
throw decreases in the linkage zones of adjacent fault segments can be identified on
the T-x plots (Fig. 6B), which could indicate that these fault segments might be linked
very early, and rapidly grow together as an integrated fault. Moreover, as shown on
horizon slices, the length of the lower fault system is almost unchanged throughout its
growth history (Figs. 5I-L). Therefore, we speculate that the lower fault system might
grow in accordance with the constant-length fault model (Morley, 1999; Walsh et al.,
2002, 2003; Giba et al., 2012; Jackson and Rotevatn, 2013; Jackson et al., 2016; Nicol
et al., 2016). It means that the fault segments comprising the lower fault system might
attain their near-final lengths very early, and be quickly linked each other to form an
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ACCEPTED MANUSCRIPT integrated fault, and then get accumulation in throws without further significant lateral
propagation. Similar examples of oblique-slip faults have been reported in rifted
basins like the Taranaki Basin in New Zealand (Giba et al., 2012), the Suez Rift in
Egypt (Jackson and Rotevatn, 2013), and the Egersund Basin offshore Norway (Tvedt
et al., 2013).
The upper fault system consists of nine en-echelon aligned growth fault segments
(Figs. 5A-H). The fault segments in the center, f2 to f6, were partially hard-linked,
especially in the H50-H35 interval (Figs. 5C-F); while those on both ends of the fault
system, including fault segments f1 in the west end and fault segments f7 to f9 in the
east end, were not physically linked with adjacent fault segments as indicated by the
presence of the unbroken relay zones between the fault segments (Figs. 5A-H). This
could indicate that activity in the middle portion of the upper fault system was overall
more intense than in the west and east ends of the same fault system.
Throws of the fault segments in the upper fault system are normally greater at their
center and decrease toward their lateral tips (Fig. 7B). Evident throw decreases are
noted in the overlapping areas of adjacent fault segments (Fig. 7B). We suggest that
the growth of single fault segments in the upper fault system might be consistent with
the isolated fault model (Barnett et al., 1987), in which individual fault segments
could be nucleated in isolation from other fault segments, grow with a simultaneous
increase in both displacement and fault length by radial propagation of tip-lines
(Walsh and Watterson, 1988; Cartwright et al., 1995; Walsh et al., 2003; Giba et al.,
2012; Jackson and Rotevatn, 2013; Jackson et al., 2016). Similar literature examples
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ACCEPTED MANUSCRIPT come from the Canyonlands Grabens of SE Utah (Cartwright et al., 1995), the Levant
Basin in the eastern Mediterranean (Baudon and Cartwright, 2008a), and the
Egersund Basin offshore Norway (Tvedt et al., 2013).
5.4. Dip linkage of both fault systems
Dip linkage of the upper fault segments with the underlain lower fault system shows
obvious segmentation characteristics.
Fault segment f1 at the western end of the post-rift fault system show similar
WNW-ESE strike with the underlying syn-rift fault segment F1 (Fig. 5M). Both fault
segments (f1 and western F1) are almost completely hard-linked (Fig. 9A). Throws of
fault segment f1 gradually decrease from horizon T70 upward. We infer that fault
segment f1 could be resulted from the upward propagation of the lower fault segment
(F1). Nevertheless, both ends of fault segment f1 were not linked with the underlying
F1 fault segment (Fig. 4C), which could represent a lateral propagation of this fault
Fault segments f7 to f9 at the eastern end of the post-rift fault system were only
physically linked in their middle portions with the underlain syn-rift fault system.
Similar to fault segment f1, throws of these fault segments (f7 to f9) decrease from
horizon T70 upward, which could be the result of upward propagation of
corresponding syn-rift fault segments (eastern F3). However, fault segments f7 to f9
ENE-WSW-extending fault segment (eastern F3). The latter might be associated with
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ACCEPTED MANUSCRIPT the lateral propagation of these fault segments themselves.
Fault segments f2 to f6 in the central portion of the post-rift fault system show a
different fault-throw distribution pattern from the f1 and f7 to f9 fault segments at
both ends of the same fault system. These fault segments (f2-f6) achieve a throw
maximum at horizon T50, from which fault throws decrease downward to T70 (Figs.
8B-E, 9B-D). There is a throw minimum zone near horizon T70, which is separated
the upper fault segments from the underlain syn-rift fault system (Figs. 8B-E and
9B-D). Fault segments f2 to f6 were physically linked with the lower fault system
only at their central portions (Fig. 4). We infer that these linkages could be associated
with downward propagation of the throws of the fault segments. Toward both ends,
these fault segments are deviated from and not linked to the underlying syn-rift fault
system (Fig. 4H). This deviation in the strike of the fault segments may be associated
with a contemporaneous tectonic stress field different from that during the formation
of the previous lower fault system.
In summary, there are two dip linkage models between the post-rift fault segments and
the underlain lower fault system. One is the upward propagation of the lower fault
segments, and another is the downward propagation of the upper fault segments. Both
linkage models can be distinguished by their different patterns of throw distribution.
Throw distribution in the upward propagation model is characterized by a gradual
upward decrease in fault throws from the lower to the upper fault systems, whereas
throw distribution in the downward propagation model is featured by a throw
minimum zone between the lower and upper fault systems (Baudon and Cartwright,
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ACCEPTED MANUSCRIPT 2008b; Jackson and Rotevatn, 2013; Tvedt et al., 2013). The upward propagation
model occurs in the west and east ends of the fault zone, between fault segments f1
and western F1 in the west end, and fault segments f7 to f9 and eastern F3 in the east
end. The downward propagation model is observed in the central portion of the fault
zone, between fault segments f2 to f6 and the underlying syn-rift fault system.
5.5. Influence of the pre-existing fault system
The pre-existing lower fault system might exert an important influence on the
development of the upper fault system. The major evidence is as follows: (1) Most of
the fault segments in the upper fault system were physically linked in their central
parts with the underlying lower fault system. In these linked portions, the upper fault
segments normally achieved their greatest throws. We infer that the upper fault
segments could have nucleated at the linked central portions over the lower fault
system. In other words, the pre-existing lower fault system controlled the nucleation
sites of the post-rift fault segments (Henza et al., 2010, 2011). (2) the upper fault
system is overall superimposed on the lower fault system. It shows a similar
E-W-trending strike and has an approximately equal extending length to the lower
fault system. This indicates that the pre-existing lower fault system could control the
position, strike, and scale of the upper fault system. Similar phenomena have been
observed in the Tertiary rift basins of Thailand, where late-stage faults followed
pre-existing faults, showing evident control from the pre-existing faults (Kornsawan
and Morley, 2002; Morley et al., 2004).
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ACCEPTED MANUSCRIPT Difference in nucleation horizons (and therefore geological time) between the central
fault segments (f2 to f6) and the end fault segments (f1 and f7 to f9) in the upper
system is noted: the central fault segments (f2 to f6) at horizon T50, and the end fault
segments (f1 and f7 to f9) at horizon T70. It means that only the west and east ends of
pre-existing lower fault system were active during the early post-rift stage (T70-T50).
This could indicate that regional tectonic stress was relatively weak in the early
post-rift stage, and therefore only the tips of the pre-existing lower syn-rift fault
system reactivated, possibly due to the effect of stress concentration, which led to the
nucleation of fault segments f1 and f7 to f9 at the west and east ends of the fault zone.
Nucleation at horizon T50 of fault segments f2 to f6 at the central portion of the upper
fault system could be associated with local increase of tectonic stress. Similar
selective or segmented reactivation of pre-existing faults have been reported by
Baudon and Cartwright (2008b).
Reactivation of pre-existing faults resulting in the formation of en-echelon
oblique-slip fault segments has been reported from several rifted basins that
experienced multiple phases and different directions of extension (Morley et al., 2004;
Frankowicz and McClay, 2010; Giba et al., 2012; Jackson and Rotevatn, 2013). It
may represent an important formation mechanism for the formation of late-stage
en-echelon fault segments. In previous case studies, pre-existing faults are normally
dip-slip normal faults, and only the late-stage faults are en-echelon oblique-slip fault
segments (Morley et al., 2004; Frankowicz and McClay, 2010; Giba et al., 2012;
Jackson and Rotevatn, 2013). In our study, both the pre-existing lower and the
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ACCEPTED MANUSCRIPT resulting upper fault systems are all composed of several en-echelon oblique-slip fault
segments (Fig. 5), and the lower and upper fault segments show different strikes and
opposite step senses: the lower fault system consists of ENE-WSW- to E-W-striking,
right-stepping en-echelon fault segments, while the upper fault system has
WNW-ESE-striking, left-stepping en-echelon fault segments. Formation of both the
lower and upper oblique-slip fault systems could be associated with the variations in
extension direction. Regional geological studies indicate that the stress field
orientation in the PRMB experienced clear clockwise rotation from NW-SE extension
during the syn-rift stage (Eocene, Tg-T70) to N-S extension during the early post-rift
stage (Oligocene to early Early Miocene, T70 to T50), and further to NNE-SSW
extension during the middle to late post-rift stage (late Early Miocene to Pliocene,
T50 to T20) (Lüdmann and Wong, 1999; Chan et al., 2010; Wang et al., 2011; Wu et
al., 2015; Hu et al., 2016a, 2016b).
In addition, the syn-rift fault system studied here strikes in an overall E-W direction,
which is different from the simultaneous NE-SW-orientated faults formed during the
syn-rift stage in the PRMB (Wang et al., 2011; Wu et al., 2015; Hu et al., 2016a,
2016b). We speculate that an E-W-trending crustal weakness zone could exist before
the syn-rift extension, which controlled the location and strike of the lower fault
system formed later. Along this weakness zone, the right-stepping en-echelon syn-rift
fault system developed under a local sinistral shear stress field caused by the
NW-SE-orientated extension acting on the E-W-trending crustal weakness zone. This
hypothesis is also supported by the following observations: (1) Although the lower
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ACCEPTED MANUSCRIPT fault system shows evident segmentation characteristics in the horizon slices (Figs.
5I-L), its throw distribution (greater in the center, and decreasing towards lateral tips,
Fig. 6B) is more similar to a coherent fault system, indicating possible influence of a
pre-existing fault; (2) As discussed above, the length of the fault system was almost
unchanged during its life (Figs. 5I-L); therefore, the lower fault system could grow in
accordance with the so-called constant-length model (Morley, 1999; Walsh et al.,
2002, 2003; Giba et al., 2012; Jackson and Rotevatn, 2013; Jackson et al., 2016; Nicol
et al., 2016). Several studies indicate that under the influence of a pre-existing fault,
growth of later formed fault tends to be consistent with the constant-length model
(Morley, 1999; Giba et al., 2012; Jackson and Rotevatn, 2013).
5.6. The evolution model of the fault zone
Based on the growth and linkage characteristics of the syn-rift and post-rift fault
systems and regional stress field background, the evolution of the fault zone studied
here can be summarized in four stages as follows.
(1) Syn-rift stage: the growth of the E-W-striking lower oblique-slip normal fault
system during the Eocene (Tg-T70) in response to a regional NW-SE-orientated
extension (Fig. 11A). As previously discussed, the lower fault system could be built
on a pre-existing E-W-trending crustal weakness zone, which was oblique to the
regional NW-SE extension during the syn-rift stage, causing a local sinistral shear
stress field. The latter resulted in the formation of the syn-rift oblique-slip fault
system. The syn-rift fault system consists of three right-stepping, sinistral,
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ACCEPTED MANUSCRIPT ENE-WSW- to E-W-striking fault segments (fault segments F1, F2 and F3) that were
hard-linked by two NW-SE-striking fault segments (fault segments A and B, Figs.
5I-L). These syn-rift fault segments could have attained their near-final lengths very
early and quickly linked to each other to form a continuous integrated fault. Then the
integrated fault accumulated throw without further significant lateral propagation.
Therefore, the growth pattern of the syn-rift fault system is overall in accordance with
the so-called constant length model (Childs et al., 1995; Walsh et al., 2002, 2003;
Giba et al., 2012; Jackson and Rotevatn, 2013; Jackson et al., 2016; Nicol et al., 2016).
Serving as the boundary fault of the EP18 half graben, the syn-rift fault system
controlled the sedimentary evolution of the half graben, leading to the formation of
thick syn-rift growth strata consisting of the Wenchang and Enping formations (Fig.
(2) Early to middle post-rift stage: a relatively dormant stage of fault activity during
the Oligocene to early Early Miocene (T70-T50) (Fig. 11B). Only the f1 and f7 to f9
fault segments at the western and eastern ends of the fault zone were active during
this early post-rift stage (Figs. 8 and 10C), which could have been associated with
stress concentration on both ends of the pre-existing lower fault system. Under the
action of a regional N-S-orientated extension, the E-W-trending western end of the
fault zone expressed dip-slip activity, resulting in the formation of the post-rift f1 fault
segment, while the ENE-WSW-orientated eastern portion of the fault zone displayed
an oblique-slip activity, causing the development of the en-echelon aligned post-rift
fault segments, f7 to f9.
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ACCEPTED MANUSCRIPT (3) Middle post-rift stage: nucleation and growth of the central post-rift oblique slip
fault segments (f2 to f6) during the late Early Miocene to late Middle Miocene
(T50-T32), in response to a regional NNE-SSW-orientated extension (Fig. 11C).
During this stage, fault segment f1 persistently developed, and growth of fault
segments f7 to f9 terminated. Fault segments f2 to f6 nucleated and grew in their
central portions (Fig. 10B). These fault segments grew by radial propagation,
overlapping and interacting with each other (Fig. 11C) during this stage. The growth
pattern of the post-rift fault segments (f2 to f6) is in accordance with the isolated fault
model (Walsh and Watterson, 1988; Cartwright et al., 1995; Walsh et al., 2003; Giba
et al., 2012; Jackson and Rotevatn, 2013). Both ends of fault segments f2 to f6 are
characterized by elliptical throw contours on the throw-strike projections in the
T50-T32 (late Early Miocene to late Middle Miocene) interval (Figs. 10B-D), which
could indicate that these fault segments were still increasing their lengths and not
linked at that time.
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(4) Late post-rift stage: continued growth of the post-rift fault segments, f1 to f6,
during the Late Miocene to Pliocene (T32-T20, Figs. 10A and 11D). Both the west
and east ends of fault segments f2 to f6 are characterized by horizontal throw contours
in this stage in the throw-strike projections (Figs. 10B-D), which may suggest that
these fault segments were linked together. The growth of the fault zone overall
terminated at the end of the Pliocene (Fig. 4).
ACCEPTED MANUSCRIPT 6. Conclusions
(1) The EP18 fault zone, constituting the northern boundary of a half graben in the
Enping sag of the northern PRMB, is a complex oblique-slip fault zone consisting of
two tiers of oblique-slip fault systems: a lower (syn-rift) and an upper (post-rift)
oblique slip fault systems, which were active during the Eocene (Tg-T70) and the late
Early Miocene to Pliocene (T50-T20) ages, respectively.
(2) The lower (syn-rift) fault system is an integrated zig-zig fault consisting of three
right-stepping, sinistral, ENE-WSW- to E-W-striking fault segments (F1, F2 and F3)
that are hard-linked by two NW-SE-striking fault segments (A and B). These fault
segments might have rapidly attained their near-final lengths very early and quickly
linked together as the kinematically related components of a single coherent fault
system. Then, the throws of the fault system accumulated without significant further
lateral propagation, and therefore, the growth pattern of the fault system is in
accordance with the constant length model.
(3) The upper (post-rift) fault system is composed of nine WNW-ESE-oriented,
left-stepping, dextral, en-echelon fault segments (f1 to f9), which show much smaller
throws and less intense activity than that of their lower syn-rift counterparts. Fault
segments f1 and f7 to f9 at the western and eastern ends of the fault system formed by
upward propagation of the underlain lower fault system, while fault segments f2 to f6
in the middle nucleated at horizon T50 over the pre-exiting lower fault system and
then propagated downward and linked with the underlain lower fault system. Growth
of the upper fault segments is overall consistent with the isolated fault model in which
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ACCEPTED MANUSCRIPT individual fault segments nucleated in isolation and grew by radial propagation of
tip-lines. Most of the fault segments laterally overlapped and interacted with each
other, and were soft-linked (fault segments f1 to f2 and f7 to f9) or hard-linked (fault
segments f2 to f6).
(4) The pre-existing lower (syn-rift) fault system shows clear influence on the
formation of the upper (post-rift) fault segments by controlling the position, strike,
and scale of the post-rift fault system and the nucleation sites of the post-rift fault
segments. (5) A conceptual model for the evolution of the EP18 fault zone is
presented. Four evolution stages are inferred, which are the formation of the
E-W-striking syn-rift oblique-slip normal fault system during the Eocene (Tg-T70) in
response to a regional NW-SE-orientated extension, a relatively dormant stage of fault
activity during the Oligocene to early Early Miocene (T70-T50), the nucleation and
growth of the post-rift oblique-slip normal fault system during the late Early Miocene
to late Middle Miocene (T50-T32) in response to a regional NNE-SSW-orientated
extension, and the continued growth of the post-rift fault segments during the Late
Miocene to Pliocene (T32-T20).
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We would like to thank the Shenzhen Branch of CNOOC Ltd. for providing the data
set and permitting us to publish the results. We are grateful to Christopher Morley, an
anonymous reviewer, and the editor Ian Alsop, for their thorough and constructive
reviews, which have substantially improved the manuscript. This work was partly
ACCEPTED MANUSCRIPT 712
funded by the National Science Foundation of China [grant numbers 91428039,
41676029, and 91028003].
715 716 717
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Figure 1. (A) Sketch map showing the location of the PRMB in the northern SCS
margin and subdivisions of structural elements in the basin. Image is from Google
Earth (accessed on 27 Nov 2017). EP, Enping sag; XJ, Xijiang sag; HZ, Huizhou sag;
LF, Lufeng sag; HJ, Hanjiang sag; SH, Shenhu uplift; PY, Panyu uplift; SD, Shunde
sag; BY, Baiyun sag. (B) Enlarged map showing the major half grabens and
associated boundary faults (red line segments) in the Enping sag (light blue-circled
area). Also shown are the locations of the 3D seismic horizon slices shown in Figs.
5-7 (black solid pentagon), and drill well A for synthetic seismogram in Fig. 3. NUZ,
Northern Uplift Zone; EX, Enxi low- relief tectonic high; PY, Panyu uplift.
Figure 2. Simplified tectono-stratigraphic column in the PRMB (modified from He et
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al., 2017), with key horizons and seismic stratal units used in this study.
Figure 3. Synthetic seismogram of well A in the Enping Sag for horizon calibration.
See Fig. 1B for location of the well.
Figure 4. Seismic profiles showing along-strike variations of cross-sectional
characteristics of the lower or syn-rift (bold red lines) and upper or post-rift (thin red
lines) fault segments. Index map at the bottom shows the locations of the seismic
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Figure 5. Coherence horizon slices at (A) horizon T30, (C) horizon T35, (E) horizon
T50, (G) horizon T60, (I) horizon T70, (K) horizon T80 and their corresponding line
drawings (B, D, F, H, J, and L) showing the spatial distribution and temporal
evolution of the EP18 fault zone. URZs, unbroken relay zones. Sub-figure (M)
assembles all of the fault traces in sub-figures B, D, F, H, J, and L together to show
the tempo-spatial distribution of the fault zone.
Figure 6. (A) The coherence horizon slice at horizon T80 showing the characteristics
of the lower (syn-rift) fault system. The dark dots along the fault traces indicate the
intersections of the selected seismic profiles for throw measurements within the fault
zone. (B) The throw-distance (T-x) plots at horizons Tg, T80 and T70 showing the
lateral variations of fault throws of the lower (syn-rift) fault system.
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Figure 7. (A) The coherence horizon slice at horizon T50 showing the characteristics
of the upper (post-rift) fault segments (f1 to f5). The dark dots along the fault traces
indicate the intersections of the selected seismic profiles for throw measurements with
the fault zone. (B) The throw-distance (T-x) plots at horizons T70 (triangles), T60
(circles) and T50 (stars) showing the lateral variations of fault throws of the upper
(post-rift) fault segments. Also shown are the locations of T-z plots for fault segments
f1~f5 in Fig. 8.
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Figure 8. Representative throw-depth (T-z) (black broken lines), EI and FAR (color
histograms) plots in the centers of the post-rift fault segments, showing the
throw-depth, EI and FAR variations of the post-rift fault segments: (A) and (F) fault
segment f1; (B) and (G) fault segment f2; (C) and (H) fault segment f3; (D) and (I)
fault segment f4; and (E) and (J) fault segment f5. Sub-figures (F) to (J) in the lower
panel are the enlargements of sub-figures (A) to (E) in the upper panel. EI, expansion
index. FAR, fault activity rate. See Fig. 7B for locations of these plots.
Figure 9. Throw-strike projections of upper (post-rift) fault segments f1 (A), f2 (B),
f3 (C), and f4 (D). The cross marks denote the projected locations on the
corresponding fault surfaces of the throw data points used for the contouring. Notice
that the upper (post-rift) fault segments are linked downward with the underlain lower
(syn-rift) fault system in their middle portions. The latter is indicated by the tightly
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show horizontally elongated, elliptical throw contours centered on the throw maxima
zone at horizon T50, suggesting that these upper (post-rift) fault segments nucleated
at horizon T50 and grew by radial propagation of their tips. In contrast, fault segment
f1 shows an asymmetric throw distribution with throws decreasing from horizon T70
upward to horizon T20, indicating it was activated during T70-T20.
Figure 10. Time thickness maps of major stratigraphic intervals adjacent to the EP18
fault zone. (A) the Late Miocene to Pliocene (T32-T20), (B) the late Early Miocene to
late Middle Miocene (T50-T32), (C) the Oligocene to early Early Miocene (T70-T50),
and (D) the Eocene (Tg-T70).
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Figure 11. A conceptual model for the evolution of the EP18 fault zone: (A) the
growth of the lower (syn-rift) oblique-slip normal fault segments during the Eocene
(Tg-T70) in response to the regional NW-SE extension; (B) a relatively dormant stage
of fault activity during the Oligocene to early Early Miocene (T70-T50), with fault
activity only seen at the western and eastern ends of the fault zone, where the upper
(post-rift) fault segments, f1 and f7 to f9, developed in response to the regional N-S
extension; (C) fault segment f1 continuously growing upward, and fault segments f2
to f6 nucleated and propagated downward and upward during the late Early Miocene
to late Middle Miocene (T50-T32) in response to the regional NNE-SSW extension;
(D) continued growth of the upper (post-rift) fault system during the Late Miocene to
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A complex oblique-slip fault zone consisting of superimposed syn-rift and post-rift fault systems is investigated. The pre-existing syn-rift fault system shows a clear influence on the post-rift
Both fault systems show complicated growth and linkage characteristics.
A four-stage evolution model of the oblique-slip fault zone is presented.
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