Download Tectonic Evolution and Dynamics of Deepwater Area of Pearl River

Journal of Earth Science, Vol. 20, No. 1, p. 147–159, February 2009
Printed in China
DOI: 10.1007/s12583-009-0016-1
ISSN 1674-487X
Tectonic Evolution and Dynamics of
Deepwater Area of Pearl River Mouth Basin,
Northern South China Sea
Dong Dongdong (董冬冬)
Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences,
Qingdao 266071, China; CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology,
Chinese Academy of Sciences, Guangzhou 510301, China
Zhang Gongcheng (张功成), Zhong Kai (钟锴)
CNOOC Research Center, Beijng 100027, China
Yuan Shengqiang (袁圣强)
Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences,
Qingdao 266071, China; Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Wu Shiguo* (吴时国)
Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences,
Qingdao 266071, China; College of Georesource and Information, China University of Petroleum,
Qingdao 266555, China
ABSTRACT: Quantitative studies on the evolution and dynamics of the deepwater area of Pearl River
Mouth basin (PRMB) were carried out based on the latest geological and seismic data. The study area
is generally in an extensional state during the Cenozoic. The major extension happened in the earlier
syn-rift stages before 23 Ma and the extension after 23 Ma is negligible. Two rapid subsidence periods,
32–23 Ma and 5.3–2.6 Ma, are identified, which are related to the abrupt heat decay during margin
breakup and the collision between the Philippine Sea plate and the Eurasian plate, respectively. The
strongest crustal thinning in the Baiyun (白云) sag may trigger the syn-rift volcanism along the weak
faulted belt around the sag. The Cenozoic tectonic evolution of the study area could be divided into five
stages: rifting (~50–40 Ma), rift-drift transition (~40–32 Ma), early post-breakup (~32–23 Ma), thermal
subsidence (~23–5.3 Ma) and neotectonic moveThis study was supported jointly by the CAS Knowledge In-
ment (~5.3–0 Ma).
novation Program (No. KZCX2-YW-203), the National Basic
KEY WORDS: Pearl River Mouth basin, South
Research Program of China (No. 2007CB411703), Key Labo-
China Sea, tectonic evolution, dynamic mecha-
ratory of Marginal Sea Geology, Chinese Academy of Sciences
nism, stretching factor.
(No. MSGL08-22), the MLR National Petroleum Resource
Strategic Target Survey and Evaluation Program, and the Taishan Scholarship Program of Shandong Province.
*Corresponding author: [email protected]
Manuscript received October 28, 2008.
Manuscript accepted December 1, 2008.
INTRODUCTION
Deepwater basins have increasingly become the
most important field of Chinese hydrocarbon exploration in recent years. There are widespread deepwater
basins distributed on the northern South China Sea
(SCS) (Wu et al., 2009; Zhang et al., 2007). The
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Dong Dongdong, Zhang Gongcheng, Zhong Kai, Yuan Shengqiang and Wu Shiguo
LW3-1 gas field with hundreds of billions of cubic
meters of gas reserves was found in the deepwater
area of the Pearl River Mouth basin (PRMB) in June
2006, showing the enormous hydrocarbon potential of
the deepwater area.
The tectonic evolution controls the petroleum
geological condition of hydrocarbon-bearing basins.
Earlier studies involved the tectonic evolution of the
PRMB (Xu, 1999; Li, 1993; Ru, 1988), but mostly
concentrated on the shallow water depressions. In recent years, with the acceleration of deepwater hydrocarbon exploration, tectonic evolution studies on the
deepwater area of PRMB were carried out (Pang et al.,
2009; Peng et al., 2005; Sun et al., 2005; Liu and He,
2001). Nevertheless, many issues about the tectonic
features and the evolution process of this area still remain controversial, for example, how the evolutionary
process is divided, what the deep dynamic mechanism
and evolutionary model are. In addition, the related
earlier work was based on the unconformities and
fault identification in the seismic profiles, lack of detailed quantitative study, and the low precision of the
basic seismic data is also an adverse condition.
In this article, we use the high-quality seismic
data and the latest geological documents as the basic
data, carry out a quantitative study about the tectonic
evolution of the deepwater area of the PRMB, and
discuss further the dynamic mechanism and the evolutionary model of the northern SCS.
GEOLOGICAL BACKGROUND
The PRMB is an NE-elongated Cenozoic marginal rift basin developed on the pre-Tertiary basement, which has undergone complicated tectonic
movement due to the conjunct influence of the Eurasian, Pacific and Indian plates. Generally, its deepwater area is located mainly in the Zhu II depression,
consisting of the Baiyun, Liwan, Kaiping and Shunde
sags, with a total area of > 40 000 km2 and water
depth of 200–2 000 m (Fig. 1). The deepwater area of
PRMB lies on the ocean-continent transition belt and
the upper slope, bounded by the Panyu low uplift in
the north, Dongsha uplift in the east, southern uplift in
the south and Shenhu uplift in the west. Among the
sags, the Baiyun sag is the largest and thickest one.
The Baiyun and Kaiping sags are separated by the
Yunkai low uplift, and there is the Baiyun low uplift
between the Baiyun and Liwan sags.
Figure 1. Bathymetry and structural units in and around Zhu II depression of the Pearl River Mouth basin.
Five seismic lines and two wells used in the study are shown.
In the deepwater area of the PRMB developed 8
stratigraphic formations, namely Quaternary, Wanshan,
Yuehai, Hanjiang, Zhujiang, Zhuhai, Enping, and
Wenchang formations from the top to the bottom, di-
vided by seismic interfaces T0, T1, T2, T4, T6, T7, T8
and Tg, respectively (Fig. 2).
The proven source rocks are the Early Oligocene
mid-deep lacustrine mudstone, transitional mudstone,
Tectonic Evolution and Dynamics of Deepwater Area of Pearl River Mouth Basin, Northern South China Sea
marine mudstone and coal system. The probable
source rock is the Eocene mid-deep lacustrine mudstone, and the possible one is the Upper Oligocene and
Miocene marine mudstone. There are at least three reservoir cases in deepwater area, including Oligocene
transitional sandstones, Neogene–Quaternary deepwater sandstone and reef, and Eocene terrestrial sandstone. The Neogene marine mudstone is the regional
seal (Zhang et al., 2007).
Figure 2. Chronostratigraphic chart and sea level
curve of the deepwater area of the Pearl River
Mouth basin.
DATA AND METHOD
In 2004, high-quality multichannel seismic reflection data were acquired in the deepwater area of
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the PRMB by China National Offshore Oil Corporation (CNOOC). The acquisition system consists of a
3 000 m long 120-channel streamer, with hydrophone
group interval of 25 m. The sampling interval is 2 ms
and the maximum record is 11 s. This study selects
five of these seismic lines as the basic data, which are
named Lines 01 to 05 from west to east (Fig. 1). These
lines have clear basement reflection and relatively integrated stratigraphic sequences, and cross all the major structural units of the study area. Quantitative
studies are carried out based on these lines as well as
some drilling wells in or around the deepwater area.
The stratigraphic development history is rebuilt on the
basis of the balanced cross-section restoration with the
software 2DMOVE. Tectonic subsidence of the selective seismic lines is calculated with the software
THERMODEL based on the backstripping method,
and then the tectonic subsidence history is built. To
discuss the deep dynamic mechanism of the tectonic
evolution, the stretching factors of the lithospheric
mantle and crust are computed. On the basis of the
quantitative work, the tectonic evolutionary features
and the dynamic mechanism of the deepwater area of
PRMB could be studied in detail.
Time-depth conversion is the base of the study,
so the VSP data of two deepwater drilling wells, W1
and W2, are used to fit the time-depth curve in order
to improve the study accuracy. The location of the two
wells is shown in Fig. 1. Well W1, with the total depth
of 3 527 m, penetrated the basement Tg, but well W2,
with the total depth of 3 843 m, only penetrated the
Zhuhai Formation. The fitted time-depth relation is in
the form of a 2nd-order polynomial function. The
time-depth conversion below the well penetration is
carried out by extrapolation. The converted depths of
deeper layers might differ from the real depth and are
considered as approximations.
The lithology of stratigraphic units is an important influential factor in porosity restoration for decompaction. The systematic analysis on 41 wells of
PRMB, including 4 deepwater wells, is used to calculate the lithologic thickness percentage of different
stratigraphic units and the generalized formation lithology (Dong et al., 2008). Exponential form is
adopted to express the porosity-depth relation. In situ
logging data from ODP Site 1148 adjacent to the
Dong Dongdong, Zhang Gongcheng, Zhong Kai, Yuan Shengqiang and Wu Shiguo
150
working area (see Fig. 1 for site location) show that
the compaction history of the muddy sediments in the
well follows closely that predicted by Sclater and
Christie (1980) for the North Sea basin (Clift and Lin,
2001). Therefore, the compaction parameters for the
study area are selected after Sclater and Christie
(1980), as shown in Table 1.
Table 1
Petrophysical parameters of variable lithology
used in this study
Lithology
Shale Sand stone Limestone Coal
Initial porosity
0.6
0.52
0.6
0.9
Compaction
0.515
0.217
0.22
0.7
2.402
2.8
2.72
1.8
-1
factor (km )
Density (g/cm3)
As for deepwater basins, the estimation of paleowater depth is generally an important source of uncertainty in subsidence analysis. The paleo-water depth in
this article is inferred according to the latest study of
the paleo-geography environment of the area (Dong et
al., 2008). Generally, paleo-water depth is negligible
for the alluvial-fluvial sediments, 20–200 m for the
shore-neritic sediments, 200–500 m for the upper
slope and 500–2 000 m for the turbidite on the lower
slope and abyssal plain. Besides, referring to the latest
study, the shelf break zone lay on the southern side of
the Baiyun sag before 23 Ma and then switched to the
current location (Pang et al., 2009). All the information is used to estimate the paleo-water depth.
Relative sea level change is also an influential
factor in the tectonic subsidence calculation. The second order sea level curve of the PRMB based on Peng
et al. (2005) is applied in this article.
RESTORATION
OF
STRATIGRAPHIC
DEVELOPMENT HISTORY
Method
The technique of balancing cross-sections is a
method for the research of extensional structures, by
which the stratigraphic development history could be
restored and used to analyze the tectonic evolution
(Wu et al., 2005; Woodward et al., 1989). 2DMOVE is
an established structural analysis and modeling program that allows line-length and area balancing of
cross-sections, decompaction, airy isostatic and paleo-
water depth corrections are carried out in balancing
cross-sections. Simple shear unfolding and inclined
shear are two algorithms used in the workflow. The
shear vector is designed to be 90° and the hanging
wall is slipped along the fault to join with the foot
wall in order to remove the effect of the fault throw.
The total stretching feature is the cumulative
stretching effect during the rifting process, which
could be expressed by the stretching factor β, defined
by β=L1/L0, where L0 and L1 are the lengths of the section before and after the stretching, respectively. The
stretching degree of every tectonic stage is expressed
by the stretching rate ri = ( Lit − Lib ) ∆Ti , where Lit and
Lib are the lengths of the section when the top and the
bottom of layer i formed, respectively, and ∆Ti is the
time interval of the layer i.
Results
The balanced cross-sections of the selective lines
are restored. Figure 3 shows the restoration results of
Line01 and Line03. Line01 has the total length of
about 56 km, crossing the largest sag in the west of the
study area, the Shunde sag, which can be divided into
northern and southern sags by the inter low uplift (Fig.
3a). The northern sag is composed of two parallel half
grabens with fault in the north and onlap in the south,
while the southern sag is a classic half graben with
fault in the south and onlap in the north. Late Eocene
to Early Oligocene Enping Formation developed well
with thickness up to 1 500 m in the center of the
southern sag. Line03 has a total length of > 130 km,
crossing the Baiyun sag, the west of Liwan sag, and
part of the southern uplift (Fig. 3b). The Baiyun sag
exhibits an SN-direction width of > 100 km and a
butterfly depression shape in Line03. There are just a
few faults developed in its northern slope, while many
faults developed in its southern slope. The faults are
mainly developed before 23 Ma (corresponding to the
seismic interface T6), while just few with small throws
developed after that.
Figure 3 shows that the deepwater area of PRMB
is generally in an extensional state during the Cenozoic. The extension was strong till 23 Ma, and the later extension was slight. The (cumulative) stretching
factors and the stretching rates in every tectonic stage
of all the five selected lines are calculated. It is nota-
Tectonic Evolution and Dynamics of Deepwater Area of Pearl River Mouth Basin, Northern South China Sea
ble that as much as 40% of the brittle faulting may not
be imaged on the seismic profiles due to insufficient
resolution (Clift et al., 2002; Walsh et al., 1991). Thus
the calculated stretching factors are artificially increased by 40% to be closer to reality. Resulting cumulative stretching factors of the selected lines during
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Cenozoic are between 1.15 and 1.35 (Fig. 4a). The
calculated stretching rates suggest that the major extensional periods are the first three tectonic stages
during the earlier rifting, corresponding to the deposition of the Wenchang, Enping and Zhuhai formations
before 23 Ma (Fig. 4b).
Figure 3. Balanced cross-section restoration of Line01 (a) and Line03 (b).
Figure 4. Stretching factor and stretching rate curves of the selected lines.
During the first three tectonic stages, the strongest extension happens during 50–40 Ma (Wenchang
Formation) with the maximum stretching rate close to
1.8 mm/a. The extension gradually become weaker at
the following stages, which indicates that the deep
tectonic activity was relatively intense and the lithosphere thinned greatly during the deposition stage of
the Wenchang Formation. The extension after 23 Ma
Dong Dongdong, Zhang Gongcheng, Zhong Kai, Yuan Shengqiang and Wu Shiguo
152
is very weak with the stretching rate smaller than 0.2
mm/a. The stretching factor and rate curves also reflect that the present extensional state had been established generally by 23 Ma. In the balanced
cross-sections, most faults developed before 23 Ma
and have large throws. Meanwhile, the stratigraphic
development was not uniform. There are thick deposits in the sags but thin or even no deposits on the uplifts, which exhibits block faulting during the rifting
stage. Few faults developed after 23 Ma, and most of
them are bedding faults or the remobilized earlier
basement faults with small throws, which reflects
weak faulting activity during the later stages. It is also
noted that there are widespread deposits in the whole
area including the uplifts after 23 Ma (from Zhujiang
Formation to the present), which exhibits the features
of the post-rift thermal subsidence.
RESTORATION OF TECTONIC SUBSIDENCE
HISTORY
Method
The total subsidence of the basin consists of two
parts, one driven by tectonic activity (tectonic subsidence) and the other driven by sediment load (load
subsidence). Tectonic subsidence plays an important
role in the study on the basin formation mechanism.
We separated the tectonic subsidence from the total
subsidence
using
the
software
package
“THERMODEL” based on backstripping. Correction
about the compaction, paleo-water depth and paleosea level change was implemented. The tectonic subsidence after the complete correction could be expressed as follow (Allen and Allen, 1990; Bond and
Kominz, 1984)
⎡ ⎛ ρ − ρs
Y = Φ ⎢S ⎜ m
⎢⎣ ⎝ ρ m − ρ w
⎞
⎛ ρw
⎟ − ∆SL ⎜
⎠
⎝ ρm − ρw
⎞⎤
⎟ ⎥ + (Wd − ∆SL )
⎠ ⎥⎦
(1)
where Y is tectonic subsidence value; Φ is compensation degree (weighing the degree to airy equilibrium);
S is the sediment thickness corrected by decompaction;
ρm, ρs, ρw are the densities of the mantle, sediment
column and water, respectively; Wd is the paleo-water
depth; and ∆SL is the paleo-sea level height relative to
the current. The lithosphere of the PRMB can be
thought to reach the total equilibrium for its weakness
(Clift et al., 2002; Clift and Lin, 2001), so Φ could be
taken as 1. Pseudo-well data are extracted from the
interpreted seismic lines for 8 seismic sequences of T0,
T1, T2, T4, T6, T7, T8 and Tg (see Fig. 2 for the chronologic ages) at 2.5 km interval, distributing in all the
structural units. These data were used to calculate tectonic subsidence curves.
Results
Tectonic subsidence curves show that Cenozoic
tectonic subsidence varies dramatically in the different
structural units in the deepwater area of the PRMB
(Fig. 5). The subsidence is small in structural highs,
while great in the centre of sags. For example, there is
only 2 km tectonic subsidence in the Panyu low uplift
and 2–4 km in the Baiyun low uplift, but up to 10 km
in the center of the Baiyun sag. The intensive variability of the tectonic subsidence among the structural
units greatly reflects the block rifting during the
syn-rift stage, when the subsidence centers were separated by structural highs. Tectonic subsidence rates
can reflect the tectonic intensity during the different
stages to a high degree, so the tectonic subsidence rate
and average tectonic subsidence rate are calculated.
Two rapid subsidence periods, 32–23 and 5.3–2.6 Ma
respectively, are identified (Fig. 6).
Rapid subsidence period during 32–23 Ma
The slope of tectonic subsidence curve is the
subsidence rate (Fig. 6). There was an important subsidence period between 50 and 40 Ma, with large subsidence and high subsidence rate. The subsidence in
the Baiyun sag is up to 4.4 km as shown on the Line04.
The basic basin framework formed in this period. The
tectonic subsidence and its rate became smaller after
40 Ma, reflecting a relatively weak tectonic activity.
The seafloor spreading of SCS started at about 32 Ma,
which generated the break-up unconformity T7. But,
unlike typical passive marginal basins, the tectonic
subsidence rates during this period do not decrease but
increase to 40–32 Ma for most calculation points and
for the average tectonic subsidence rate (Fig. 6). Especially on Line03 and Line05, the tectonic subsidence rates in 32–23 Ma are larger than that in 40–32
Ma by 77%. Meanwhile, the results also manifest that
the tectonic subsidence rate decreased obviously since
23 Ma (T6) (Figs. 5 and 6).
Tectonic Evolution and Dynamics of Deepwater Area of Pearl River Mouth Basin, Northern South China Sea
Rapid subsidence period during 5.3–2.6 Ma
Although the whole PRMB entered the relatively
steady tectonic subsidence stage with decreased tectonic subsidence rate after 23 Ma, an obvious rapid
subsidence period can be distinguished between 5.3
and 2.6 Ma on all the selective seismic lines (Fig. 6).
The average tectonic subsidence rates are generally
153
>50 m/Ma during this stage, and higher than those
during the previous stage by 50 %, which indicates a
relatively intense tectonic activity. Nevertheless, this
rapid subsidence period lasted short, no longer than 3
Ma, so it did not bring a large tectonic subsidence,
usually smaller than 0.5 km just as shown on the average rate curves (Fig. 6).
Figure 5. Tectonic subsidence curves of Line03 and Line04. BS. Baiyun sag; BLU. Baiyun low uplift; PLU.
Panyu low uplift.
Figure 6. Two rapid subsidence periods (shadowed
areas) shown on the avg. tectonic subsidence rate
curves.
Discussion on rapid subsidence periods
The break-up unconformity T7 (corresponding to
the onset of seafloor spreading of the SCS at 32 Ma) is
commonly regarded as the boundary of syn-rift and
post-rift stages for the PRMB (Peng et al., 2005; Sun
et al., 2005; Guo et al., 2001; Liu and He, 2001).
Unlike the ‘normal’ Atlantic passive continental margin, subsidence in the deepwater area of the PRMB
after the seafloor spreading underwent an anomalous
acceleration event, maybe indicating the lasting rifting
activity during ca. 32–23 Ma. The inaccuracy due to
the uncertainty of the earlier geological age may not
be totally eliminated, but, anyway, it is a noteworthy
phenomenon. Dong et al. (2008) discussed this with
the geological and geophysical evidence and interpreted the formation mechanism. It was thought that
the low strength of the lithosphere (Shi et al., 2005;
Clift and Lin, 2001; Zhang and Wang, 2001; Zhang et
al., 2001) retarded the strain attenuation caused by
seafloor spreading and the deepwater area of PRMB
was still in the syn-rift stage during 32–23 Ma.
Spreading ridge jumped southwards during 24–21 Ma,
which aroused the acceleration of seafloor spreading
(Zhong et al., 2004; Li et al., 2002). This made the basins of the northern SCS far away from the spreading
ridge, and more lava exposed due to the spreading acceleration. It sped up the lithospheric cooling and further triggered the entire post-rift thermal subsidence.
A rapid subsidence after 5.3 Ma was identified in
the middle and the east of the Qiongdongnan basin
(Yuan et al., 2008), the west of the PRMB, which was
thought to be controlled by the Red River fault (RRF)
transition from sinistral to dextral strike-slip at ~5 Ma.
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Dong Dongdong, Zhang Gongcheng, Zhong Kai, Yuan Shengqiang and Wu Shiguo
It is also suggested that this rapid subsidence period
faded gradually from west to east and vanished totally
in the PRMB. This study identified a rapid subsidence
period during 5.3–2.6 Ma. During this time, the seafloor spreading had ended long before and the PRMB
was far away from the RRF, so the triggering mechanism should be in the east of the PRMB. The collision
of the Philippine Sea plate and the Eurasian plate at
the present Taiwan region at about 5 Ma was a significant event impacting the regional tectonics (Yao et
al., 2004; Li et al., 1998; Pelletier and Stephan, 1986).
Therefore, the rapid subsidence of 5.2–2.6 Ma found
in the PRMB is most likely to be related to this collision event, which remobilized faulting and the accelerated tectonic subsidence.
STRETCHING FACTORS OF LITHOSPHERIC
MANTLE AND CRUST
Model and Method
The stretching factor of the lithosphere is an important parameter reflecting the tectonic evolution and
deep dynamics of the sedimentary basin. This study
applies the depth-dependent discontinuous stretching
model (Hellinger and Sclater, 1983; Beaumont et al.,
1982; Royden and Keen, 1980) to calculate the
stretching factors of the lithospheric mantle and crust.
Unlike the uniform stretching model developed by
McKenzie (1978), this model postulates that the crust
and lithospheric mantle have different stretching factors, but each stretches independently and uniformly
like that in the McKenzie model (Fig. 7). Therefore,
the calculation about thermal subsidence is similar to
the expression derived by McKenzie (1978). Referring
to the analysis of uniform stretching model in Allen
and Allen (1990), we derive the initial subsidence expression of the depth-dependent discontinuous
stretching model. Several assumptions are made: the
stretching is instantaneous, the lateral temperature
gradients are much smaller than the vertical gradients,
the internal heat production from radioisotopes is ignored, the geothermal gradient is constant in the lithosphere, and lithostatic equilibrium is maintained
before and after stretching. It is hoped that this may
provide the 1st order approximation for a relatively
undeveloped study of the deepwater area of the
PRMB.
Figure 7. Depth-dependent discontinuous stretching model before and after stretching.
Lithostatic pressure before and after stretching
should be equal, that is
y
y
yc ρ c + ysc ρ sc = ys ρ s + c ρ c + sc ρ sc +
βc
β sc
(2)
yc ysc
−
( yc + ysc − ys −
)ρ
β c β sc m
ρ m = ρ m* (1 − αV Tm ) , ρsc = ρsc* (1 − αV Tsc ) ,
y
T
ρ c = ρ c* (1 − αV Tc ) , Tsc = m (1 + c ) , Tc = Tm yc , in that
2
yl
2 yl
where
subscripts c and sc denote the crust and lithospheric
mantle respectively; β, y, ρ*, ρ and T are the stretching
factor, initial thickness, density at 0 ℃ temperature,
average density, and average temperature if the surface temperature is 0 ℃, respectively; ys is the initial
subsidence in the syn-rift stage; Tm is the temperature
of aesthenosphere; ρm is the density of the lithospheric
mantle at Tm temperature; αV is the thermal expansion
coefficient.
In the McKenzie (1978) model, simplified thermal subsidence at post-rift stage can be derived by
Tectonic Evolution and Dynamics of Deepwater Area of Pearl River Mouth Basin, Northern South China Sea
solving the 1D unsteady heat flux equation (Allen
and Allen, 1990)
β
π
S (t ) ≈ E0 sin (1 − e−t /τ )
(3)
π
β
where E0 = 4 yl ρ m* αV Tm / π 2 ( ρm* − ρs ) , τ = yl 2 / π 2κ , in
that yl is the initial thickness of the lithosphere; ρs is
the average deposit density filled in the basin; t is the
time from the cessation of syn-rift stage; τ is the thermal evolution constant; κ is the coefficient of thermal
diffusion; β is the stretching factor of the lithosphere;
and the other parameters are the same as those menTable 2
155
tioned above.
As for the discontinuous and instantaneous stretching model, Allen and Allen (1990) suggested that
the post-rift thermal subsidence is determined by the
stretching of the lithospheric mantle, thus βsc can be
obtained by solving the nonlinear equation (3) with β
replaced by βsc. Then the stretching factor of crust βc
can be obtained from equation (2). The parameters
used in the calculation are shown in Table 2.
Parameters used in the thermal and subsidence model
y1
yc
ρ*c
ρ*m
ρs
Tm
κ
αV
125 km
30 km
2 800 kg/m3
3 330 kg/m3
2 500 kg/m3
1 330 ℃
10-6 m2/s
3.28×10-5/℃
Results and Discussion
Although three syn-rift stages existed in the
deepwater area of PRMB, they have the continuity
and could be taken as one stage to a certain degree. It
is reasonable to consider later thermal subsidence as
one complete stage in spite of the late slight acceleration of subsidence. Therefore, by 23 Ma the rifting
process could be simply divided into two super-stages
of syn-rift and post-rift for calculation. The distribution curves of stretching factors in the different structural units of the selected 5 lines are acquired. The β
curves of the lithospheric mantle and crust basically
mirror the Cenozoic sediment basement (Fig. 8).
The stretching factors of the crust amount to 3.1
in the Baiyun sag, showing strong crustal thinning
with residual thickness less than 10 km. There would
be volcanism during syn-rift stage if this factor is larger than 2.5 (Gong and Li, 1997). Actually, there is
proven to be middle-late syn-rift magmatic activity in
the fringe of the Baiyun sag (Sun et al., 2005), and
obvious volcanoes can be distinguished on the Baiyun
low uplift (Dong, 2008). This volcanic activity may
come into being by the mantle upwelling along the
weak faulted belt around the Baiyun sag due to the
dramatic thinning of the lithosphere under the sag.
The stretching factors of the lithospheric mantle
and crust reflect the magnitude of thermal subsidence
and syn-rift subsidence, respectively. The stretching
factors of the lithospheric mantle and crust reach
Figure 8. Curves of the stretching factors along
Line01 (a), Line04 (b), and Line05 (c).
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Dong Dongdong, Zhang Gongcheng, Zhong Kai, Yuan Shengqiang and Wu Shiguo
maximum and closest in the center of the Baiyun sag
in Line04 (Fig. 8b), reflecting its everlasting subsidence all through the Cenozoic. As for Line05, the
stretching factor of the crust varies slightly, but that of
the lithospheric mantle changes greatly in different
structural units and increases southward (Fig. 8c).
This indicates that the whole deepwater area lay on
the similar structural location and had similar tectonic
subsidence, but the southern Liwan sag underwent
large-scale thermal subsidence in the Late Cenozoic.
The stretching factors of the lithospheric mantle
are larger than those of the crust in all the lines (Fig.
8a). The southern slope of the southern Shunde sag
has larger stretching factors of the lithospheric mantle,
indicating a strong uplift during the syn-rift. In fact, in
the southern Shunde sag the syn-rift sequences consist
of only a thin Zhuhai Formation and without the
Wenchang and Enping formations, showing a continual uplift before 23 Ma. The preferential lithospheric
mantle extension relative to the crust in the deepwater
area of the SCS coincides with the findings in the
shallow water area (Clift and Lin, 2001). The stronger
extension of the lithospheric mantle relative to the
crust may retard the post-rift thermal relaxation, leading to the lag of the large-scale thermal subsidence
(Dong et al., 2008).
DISCUSSION ON EVOLUTION MODEL
The deepwater area of the PRMB is located on
the slope of the northern South China Sea. Its formation and evolution are related closely with the formation of the SCS, and also concerned with the collision
of the Philippine Sea plate and the Eurasian plate (Ru
and Pigott, 1986).
The studies on the deepwater area of the PRMB
are relatively undeveloped. The quantitative studies in
this article will help to understand the evolutionary
process in this region. The restoration of stratigraphic
development history and calculation of stretching factors of the crust and lithospheric mantle might
contribute to not only clarifying the extensional process of the basin but also finding the deeper dynamic
mechanism for the evolution. Based on this study and
the previous researches (Zhang et al., 2007; Shi et al.,
2005; Lin et al., 2003), the tectonic evolutionary
model of the northern SCS in the PRMB region can be
divided into five stages in detail as follows.
Rifting (~50–40 Ma)
The deepwater area of the PRMB entered the
earlier stage of rifting from 50 to 40 Ma, triggered by
the first episode of the Zhuqiong movement during
Early–Middle Eocene. The lithospheric mantle and
crust stretched and thinned strongly. The tectonic subsidence varies from the different units. The Shunde
and Kaiping sags had small tectonic subsidence as a
result of the relatively higher location, while the tectonic subsidence of the Baiyun sag was up to ~5 km
during this period, with a high tectonic subsidence rate
of > 100 m/Ma. The variation reflects the strong
blocked-rifting activity, which formed the basic structural framework of the deepwater area of the PRMB.
Rift-Drift Transition (~40–32 Ma)
The second episode of the Zhuqiong movement
during Late Eocene and Early Oligocene initiated the
second rifting stage and rift-drift transition. As rifting
progressed, the focus of rifting became concentrated at
the present-day continent-ocean boundary (COB). The
tectonic activity of this stage inherited the former to a
certain degree, but with reduced tectonic subsidence
and tectonic subsidence rate. Transient, small-scale
mantle convection was induced by the intense rifting
and led to the crustal uplift (Oligocene) in the northern
SCS in Taiwan region (Lin et al., 2003). As for the
deepwater area of the PRMB, we infer that the similar
phenomenon most likely occurred below the Baiyun
sag, because the crust below this sag thinned strongly
to less than 10 km, closer to the oceanic crust, and
there was extrusive volcanism in the southern flank of
the Baiyun sag (Dong, 2008). The crustal uplift induced by mantle upwelling may decrease the tectonic
subsidence rate during this period. Later on, seafloor
spreading occurred and the breakup unconformity
formed then.
Early Post-Breakup (~32–23 Ma)
The South China Sea opened at ~32 Ma (or 30
Ma according to the time scale of Cande and Kent
(1995)) and generated the regional breakup unconformity T7, called the Nanhai movement. Unlike the
normal Atlantic-type passive margin (e.g., Steckler
Tectonic Evolution and Dynamics of Deepwater Area of Pearl River Mouth Basin, Northern South China Sea
and Watts, 1978), the tectonic subsidence rate of this
stage increased compared with the rate before. This
suggests that the rifting activity was still going on and
the syn-rift stage was prolonged. This stage of rapid
subsidence is possibly induced by the abrupt heat decay following the heat addition due to the stronger
mantle extension when final continental breakup became concentrated at the present COB (Lin et al.,
2003). However, in view of the tectonic complexity in
the SCS, this mechanism for increased subsidence is
worth conducting further research on. The region
transited into the post-rift stage gradually.
The stretching state of the margin was similar to
the present at the latter part of this stage. The (cumulative) stretching factor of the brittle upper crust
comes up to 1.35. The stretching factor β of the lithospheric mantle and crust amounts to 3.5 and 3.1, respectively at the maximum in the Baiyun sag, showing
intensive thinning and strong tectonic activity. The
shelf break of the PRMB region laid on the southern
fringe of the Baiyun sag before 23 Ma, and developed
large-scale delta deposits during ~32–23 Ma (Pang et
al., 2009).
Thermal Subsidence (~23–5.3 Ma)
The shelf break in this region jumped onto the
northern slope of the Baiyun sag at 23 Ma and kept
the location till now. Then the slope sedimentary system started to develop (Pang et al., 2009) (Fig. 3), resulting in several superimposed deepwater fans. Some
authors suggested that the spreading ridge jumped
southwards during ~24–21 Ma (Zhong et al., 2004;
Lin et al., 2003) and aroused the acceleration of seafloor spreading (Li et al., 2002). This event made the
basins in the northern SCS far from the spreading
ridge, and considerable melted lava exploded out of
the seafloor to generate more oceanic crust, which
sped up the lithospheric cooling in the northern SCS.
Therefore, normal thermal subsidence prevailed in the
entire region and the region entered into the sustained
integral depression. Much evidence shows that there
was a significant tectonic movement or an important
tectonic transition at ~23 Ma, which was named ‘Baiyun movement’ (Pang et al., 2009).
The seafloor spreading of the South China Sea
ceased at ~16 Ma under the confines of the adjacent
157
plates. Shi et al. (2005) identified a rapid subsidence
period at ~17 Ma in the Baiyun sag according to the
backstripping calculation of well data. Unfortunately,
this phenomenon is not identified in this study, and the
difference in the precision between well and seismic
data may be one reason.
Neotectonic Movement (~5.3–0 Ma)
The Philippine Sea plate subducted westwards
and then collided with the Eurasian plate, leading to
E-W compression, S-N extension, and EW-running
faults. This event is called the neotectonic movement,
which triggered another rapid subsidence. This tectonic subsidence is small (not exceeding 0.5 km) and
has limited impact on the post-rift thermal subsidence.
During this stage, the study area had already lain in
the deepwater environment, with semi-pelagic to pelagic deposits.
CONCLUSIONS
This article carries out quantitative studies on the
evolution and dynamics of the deepwater area of the
PRMB, on the basis of the latest geological and seismic data. Stratigraphic development history and tectonic subsidence history are restored with balanced
cross-section and backstripping methods respectively.
Stretching factors of the lithospheric mantle and crust
are calculated based on the depth-dependent discontinuous stretching model. The studies reveal the following features.
(1) The deepwater area of PRMB is generally in
an extensional state during the Cenozoic. The extension had been strong till 23 Ma, and the later extension was slight. The stretching factors obtained from
the balanced cross-sections are between 1.15 and 1.35,
which may reflect the stretching of the upper crust.
The major extensional period is the first three tectonic
stages from 50 to 23 Ma, with the maximum stretching rate close to 1.8 mm/a during 50–40 Ma. The extensional process coincided with the development and
remobilization of the regional faults.
(2) Two anomalous rapid subsidence periods,
32–23 and 5.3–2.6 Ma, are identified during the Cenozoic evolution of the deepwater area of PRMB. The
first period may be induced by the abrupt heat decay
following the heat addition due to the stronger mantle
Dong Dongdong, Zhang Gongcheng, Zhong Kai, Yuan Shengqiang and Wu Shiguo
158
extension when final continental breakup became
concentrated at the present COB. In view of the tectonic complexity in the SCS, more accurate mechanism should depend on further research. Unlike the
normal passive continental margin, the rifting activity
between 32 and 23 Ma was still going on and the
syn-rift stage is prolonged in the study area. The second period is related with the collision between the
Philippine Sea plate and Eurasian plate.
(3) The crustal stretching factor is the largest in
the centre of the Baiyun sag, reaching 3.1, showing
the strongest crustal thinning, which triggered the
volcanism of syn-rift stage along the weak faulted belt
around the Baiyun sag. The stretching factors of the
lithospheric mantle are larger than those of the crust in
different structural units, which may be one mechanism for the delay of large-scale post-rift thermal subsidence.
(4) Based on this and earlier studies, the tectonic
evolution of the deepwater area of the PRMB could be
divided into five stages in detail, which are rifting
(~50–40 Ma), rift-drift transition (~40–32 Ma), early
post-breakup (~32–23 Ma), thermal subsidence
(~23–5.3 Ma) and neotectonic movement (~5.3–0 Ma).
The Cenozoic tectonic evolutionary model is established in conclusion.
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