The crustal thinning of the northern continental margin in
the South China Sea
Ching-Hui Tsai1, Shu-Kun Hsu1, Yi-Ching Yeh1 & Chao-Shing Lee2
1
Institute of Geophysics, National Central University, No. 300, Jungda Rd, Jhongli
City, Taoyuan, Taiwan 32054, R.O.C.
2
Institute of Applied Geophysics, National Taiwan Ocean University, No. 2, Pei-Ning
Rd, Keelung City, Taiwan 20224, R.O.C.
Key words: continental margin, crustal structure, gravity modeling, thinning
continental crust
Abstract
New magnetic evidence indicates that the oceanic crust may extend to 21°30’N.
However, due to lack the deep crustal refractor and reflector data between eastern
Dongsha Island and southwestern Taiwan offshore to evidence this inference. A series
of seismic cruises are executed and many seismic data are acquired by us. We carried
out a synthesis of the existing seismic reflection profiles across the continental margin.
In addition, we used the corresponding gravity anomalies and a simple four-layers
crustal model to simulate the depth variation of the Moho discontinuity across the
northern continental margin of the South China Sea.
According to the results of modeling and seismic data, we can separate the
crustal structures into five types including continental crust (CC), thinning continental
crust (TCC), oceanic crust (OC), volcanism zone (VZ) and Tension/Convergent Area
(TCA). The modeling results display that an existence of oceanic crust observed by
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new magnetic data is confirmed. But it is separated with main oceanic crust of the
SCS by VZ. Based on our models, we agree the position of the continent-ocean
boundary (COB) with previous studies at western part of the Dongsha Island, but we
suggest the position of the COB in the eastern part of the Dongsha Island should be
moved landward to near the end of continental slop. By our result, we could not
recognize the characteristic of the continent-ocean transition zone (COT) as same as
the western Iberian non-volcanic margin or the Lofoten volcanic margin. However,
there are maybe to exist another type of COT in the northern margin of the SCS. Our
magnetic inversion model perhaps can provide this kind of idea.
1. Introduction
The South China Sea (SCS) was surrounded by four main tectonic structures
including Taiwan orogen, Luzon arc, Indo-China peninsula and Mainland China
(Fig.1). It is the biggest marginal sea of West Pacific. Basically, it can be divided into
three portions including the northern continental margin, the southern continental
margin and the oceanic basin. The oceanic crust of the SCS was formed by the
seafloor spreading between 30 to 15 Ma (Briais et al., 1993). Except the northeast and
southeast corners, the continent-ocean Boundary (COB) of the SCS was suggested to
be almost along the 3000m-isobath near the continental slope (Fig.1). In this paper,
our study area is focused on the continental margin of the northern SCS (Fig.1). Based
on the bathymetry map, Dongsha Island is located at the center of our study area
(Fig.1). The topography appears different on both sides of Dongsha Island and some
main features are existed. The east side of Dongsha Island is marked by the presence
of the Formosa Canyon (FC). This canyon has developed along the northwest part of
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the LRTF (Luzon-Ryukyu Transform Fault; Hsu et al., 1998 and Hsu et al., 2004). In
the west side, Xisha Trough is a distinct bathymetric low which was interpreted as a
locus of extension (Fig.1; Taylor and Hayes, 1983). Crustal modeling shows that the
thin crust overlain by a thick sedimentary basin is below the area of Xisha Trough
(Nissen et al., 1995a) and to be formed a part of the Yinggehai Basin (YGHB; Fig.1).
The PRMB (Pearl River Mouth Basin) is distributed in most region of the northern
continental shelf of the SCS (Fig.1) and the maximum thickness of the deposits is ca.
10 km in deep depressions (Jin, 1989). There are many submarine volcanoes in the
south part of Dongsha Island. The evidences of seismic data show intrusive and
extrusive igneous bodies are existed widely in Dongsha region. Thomas and Wong
(1999) suggested these magma intrusion and uplift event are correlated with the
collision between Taiwan and East China.
The continental rifting resulted in a series of extensional basins in the northern
margin of the SCS. Because the oil advantages of continental shelf in the northern
SCS, a large number of seismic lines have been collected and hundreds of exploration
wells have been drilled in these basins. However, surveys on deep crust structures and
deeper ocean were relative less in early period. Following the geophysics are
respected, there were more deep crustal co-operate research to be carried out. As
results of these data, they provide us to understand comparatively the crustal
structures in northern margin of the SCS. These data showed the conditions of
continent and ocean crust clearly and indicated the position of continent and ocean
boundary (COB; Briais et al., 1993, Nissen et al., 1995a, Yan et al., 2001 and so on).
To the northernmost position between Dongsha Island and Taiwan Island, due to lack
of good correlated magnetic data and deep seismic profiles, most previous studies do
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not suggest the existence of the oceanic crust. However, new magnetic evidence
indicates that the oceanic crust may extend to 21°30’N (Hsu et al., this issue).
The main objective of this work is to provide for understanding the crustal
structures of continental margin in the northern SCS overall. For this purpose, we
compared new seismic data collected between Dongsha Island and Taiwan Island with
previous studies. Most of these data are transections across continental shelf, slop and
ocean basin. We carried out a synthesis of these existing seismic reflection profiles.
We also used the corresponding gravity anomalies and a simple four-layer crustal
model to simulate the depth variation of the Moho discontinuity across the SCS
northern margin.
2. Data acquisition and process
2.1 seismic data
In early August 2001, a wide angle refracted survey in the continental margin of
the northern South China Sea (SCS) (Fig.1), MLTW-OBS cruise, was conducted
between Taiwan and China geophysical scientists. The R/V Ocean Research I of
Institute of Oceanography, National Taiwan University deployed and recovered the
OBSs and the R/V Shiyan II of South China Sea Institute of Oceanology, Chinese
Academy of Sciences provided source and received seismic data. During this survey,
we acquired eleven OBS’s data and three multi-channel seismic (MCS) lines. We only
present the seismic data in this study. Further, we had a seismic survey, MCS689
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cruise, in the northern SCS during late July 2003. In this paper, more than 1000 km
data including 97304Aa (stacked section; Fig.4), mltw (stacked section; Fig.5), and
mcs689-5 (migrated section; Fig.6) and 97304Ab (stacked section; Fig.7) MCS
profiles were presented. The software ProMax (developed by Landmark) was used to
process these seismic data. Finally for applying the strata sections to do gravity
inversion simulation, our assumptions are the sea water velocity 1.45 km/s and
sedimentary velocity 2.2 km/s. Base on the assumptions, we convert the seismic data
from two-way travel time section to depth section.
2.2 previous crustal and seismic profiles
We cite six previous seismic profiles and crustal models for comparing with our
data (table 1) and taking an entire view in the northern margin of the SCS. Two of
them are MCS profiles including f13 (modified from Lüdmann et al., 2001; Fig.3) and
ew45 (modified from Sibuet et al., 2002). The others are crustal models including
espW and espE (Eastern and Western transects by Nissen et al., 1995a), OBS93 (Yan
et al., 2001) and ab (Ludwig et al., 1979; Taylor and Hayes, 1983). These transects
almost pass through continental shelf and slope into ocean basin. Compile all of these
data, we try to re-identify the crustal structures including continental crust (CC),
thinning continental crust (TCC), oceanic crust (OC) and COB etc., in this study. We
suppose this study will help us to understand clearly in the entire northern margin of
the SCS.
2.3 gravity data
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Shipboard free-air gravity (Fig.11c) is the initial source of our crust model along
the eight profiles (Table 1). For developing our model (Fig.2), we consult the Nissen’s
results (1995b) to assume the densities of sea water, sediment, crust and mantle with
ρw=1.03 g/cm3, ρs=2.65 g/cm3, ρc=2.84 g/cm3 and ρm=3.28 g/cm3 respectively. We use
a Fortran program mstr2d.f provided by Dr. Shu-Kun Hsu, National Central
University, Taiwan, to calculate the Mantle Bouguer Gravity Anomaly (MBGA) along
every-transect. The sampling interval is 0.2 km. First, we use crust density as basis to
correct the sea water and sediment effects and then to calculate the bouguer anomaly.
Here, the anomaly curve should take a response coming from the depth variation of
the Moho discontinuity. Second, we assume a synthetic Moho depth and then
calculate the gravity effect provided by it (synthetic). In this part, we make better
fitting between these two curves. The simulation results are presented by Fig.8, Fig.9
and Fig.10. The final results will help us to define the crustal structures along
every-transect.
2.4 magnetic data
We have two magnetic data sources, one is from Coordinating Committee for
Geoscience Programmes in East and Southeast Asia (CCOP) and the other is provided
by Yeh (this issue). For comparing with magnetic data, we track along the eight
profiles (Fig.8, Fig.9 by CCOP and Fig.10 by Yeh). The magnetic anomaly curves
provide us good contrast with crustal models.
3. Crustal types and characteristics in study area
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There are many models to explain the evolution of continental margin. The
differences between these models are different extension mechanical and igneous
activity. The previous studies suggest the north and south margins of the SCS was an
asymmetric conjugated pair and the north part can be classified as non-volcanic
margin (Yan et al., 2001). No matter which model, the favors evolutionary sequences
are pre-rift, syn-rift and post-rift. The crustal structures crossed the continental margin
of the northern SCS are described by CC, TCC, continent-ocean transition zone (COT)
and OC. In this study, our purpose is to re-define the crustal structures in the northern
margin of the SCS which area is covered between Dongsha Island and Taiwan Island.
For this hypothesis, we define four situations (Gong et al., 1997). First, the continental
crust (CC): the Moho discontinuity is gentle, crust thickness is thickest, gravity
anomaly value is relative lower and magnetic anomaly changes gently. The crustal
thickness is about 30 km. Second, the thinning continental crust (TCC): the Moho
discontinuity shows greater slope, crust thickness becomes thinner gradually into
seaward direction, gravity anomaly curve also has greater slope and magnetic
anomaly value is still relative lower but rough. The crustal thickness (including
anomaly mantle uplift zone) is between 16 km to 27 km. Third, the oceanic crust (OC):
the Moho discontinuity is relative shallower, crust thickness is the thinnest and gentle,
and relative greatest anomaly values of gravity and magnetic. The crustal thickness
(including residual old oceanic crust) is between 6 km to 9 km. Fourth,
continent-ocean transition zone (COT): previous studies could not indicate this kind
of crustal structure in the SCS. Most of researches are focused on the Atlantic. For
example, in the Western Iberia margin, a very thin oceanic crust (typically 2-4 km)
underlain by a high velocity material (Vp= 7.3-7.6 km/s; Whitmarsh et al., 1993)
which was believed to be serpentinized peridotite layer. In the northern margin of the
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SCS, the high velocity layer (HVL) was found which was below the continent/ocean
lower crust and the HVL displayed a velocity of 7.1-7.4 km/s (Nissen et al., 1995a
and Yan et al., 2001). The velocity is not so high as the case in the Western Iberia
margin. Based on the characteristics described above, we try to separate crustal
structures of northern margin of the SCS into different types from our models.
4. Data interpretation
The topography shows the northern margin of the SCS appears NE-SW direction.
The area near Dongsha Island is a plateau and previous study (Briais et al., 1993)
shows that the position of COB is changed to along deeper ocean basin at this region
(Fig.1). In this paragraph, we divide study area into two parts to interpret. One
position is in the West side of Dongsha Island, including four transects espW, ab,
OBS93 and f13 (Fig.1). The other position is in East side of Dongsha Island,
including four transects espE, mltw, mcs689-5 and ew45 (Fig.1).
4.1 West position
4.1.1 ESPW Transect
In this transect, we use the Moho depth which is defined by Nissen et al. (1995b)
to simulate the gravity anomaly (synthetic curve in Fig.8a). The result shows a nice fit
between calculated and synthetic curves except 270 to 370 km (Xisha Trough; frame
C’). We find the processes of mantle uplift were occurred and to make crustal thinning
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(frame A, Frame B and frame C). The anomalously low free-air anomaly (frame C’)
should be caused by subsidence which is located in the region of mantle uplift. The
regional higher magnetization materials are closed to surface to bring out regional
relative high magnetic anomaly in frame C’’. In gravity, we also can find regional
high bouguer anomaly value in these three positions (frame A’, frame B’ and frame
C’). That means the lateral variations of density are happened in these areas. We
believe these effects are from a sequence of subsidence caused by rifting in
continental shelf of the northern SCS. In the area (frame A and B), anomalously thin
crust is overlain by a relatively thick sedimentary section which comprises a part of
the Pearl River Mouth Basin (PRMB; Nissen et al., 1995a). To compare with bouguer
anomaly, we suppose the Xisha Trough is an uncompleted extension as same as Taylor
and Hayes (1993). In the seaward direction of the end of shelf (black triangular;
Fig.8a), the free-air and magnetic (frame D) anomalies show relative high value. We
define the CC is from 0-280 km and the TCC zone is from 280-370 km in this
transect.
4.1.2 Ab Transect
In this profile (Fig.8b), we find quite abrupt changeover from more than 22 km
to about 10 km in Moho depth between 290 and 350 km (TCC). We also find that the
data of bouguer anomaly and magnetic anomaly show relatively high values (frame
C’) and (Frame C”) behind the TCC zone. The crustal thickness is ca. 6 km (frame C)
and the magnetic lineations are displayed in magnetic anomaly curve (frame C’’). The
other places we should pay attention are in the positions ca 100 km (frame A) and 200
km (frame B). Here, we see two thinning crustal zones. Both of them, we believe they
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should be caused by short period rifted force as same as espW transect and also are a
part of the PRMB. As same as espW profiles, we also find the regional anomalous
value in magnetic curve (frame D), but do not show in free-air gravity curve in the
seaward direction of the end of shelf (black triangular; Fig.8b). According to these
appearances, we define that the CC is between 0-290 km and the TCC zone is from
290-350 km. The OC is behind 350 km.
4.1.3 OBS93 Transect
We get better fitting in Moho discontinuity with Yan’s model (Fig.8c). In the
seaward direction of the end of shelf (black triangular; Fig.8c), the magnetic data also
displays relatively high anomaly curve (frame A) and the value is higher than the
values in Fig.8a and Fig.8b. In this profile, the crustal thickness is thinning from ca.
28 km to about 10 km between 140 and 350 km in frame B. The bouguer anomaly
curve also shows large slope in this region. The magnetic data (frame B’) describes
relatively high and rough curve. It is similar magnetic reversal, but we believe they
are caused by intruded igneous rock by comparing with seismic data (Yan et al., 2001).
We find Behind 350 km (frame C), the relatively higher anomaly values in bouguer
and magnetic (frame C’ and frame C”) should be brought by oceanic crust which has
high density and high magnetization. In this transect, we can define that TCC is from
140 to 350 km and the wide volcanism were occurred from fracture of faults. The CC
is between 0-140 km and the OC is behind 350 km.
4.1.4 F13 Transect
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This transect is that we modify seismic profile from Lüdmann et al., 2001 (Fig.3
and Fig.9a). It has better control in strata sequences because of well log data drilled by
ODP (point out the positions in Fig.1; site 1146 and site 1148). We see seismic
basement separate by normal faults and form grabens filled by sediment between
them. We can’t find any intruded or extruded igneous bodies along this transect. This
feature is really different with OBS93 (Fig.8c). But when we see the 97304Aa profile
(Fig.4) located upstream of profile f13, the profile shows us many normal faults
occurred between sedimentary strata (below T1). And we detect an intruded igneous
at about 80 km and causes topography lifted. We can see sedimentary truncation
(frame T), so we think there are many uplift forces around Dongsha Island by tectonic
action or volcanism intruded bodies. The crustal modeling (Fig.9a) shows that crustal
thickness becomes thinner from 0 to 190 km gradually (frame B) and changes gently
behind 190 km (frame A). We also see large slope in bouguer anomaly curve (frame
B’), and lower/rough curve in magnetic data (frame B”). By magnetic anomaly curve
(frame A”), it shows a relatively high anomaly behind 190 km. The gravity anomaly
has a variation at same position (frame A’). To combine with seismic profile, magnetic
anomaly and gravity anomaly results, we define that the position of COB should be
located at 190 km and the TCC should be located between 0 and 190 km. The oceanic
crustal should be behind 190 km.
4.2 East position
4.2.1 ESPE Transect
We use the Moho depth built by Nissen (1995b) to calculate synthetic gravity
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anomaly curve (Fig.9b), and get a perfect fitting curve. The result points out the
oceanic crust should be behind ca 380 km because the thin crustal thickness (frame B;
ca 6-8 km), the relatively high values both on bouguer anomaly (frame B’) and
magnetic anomaly (frame B”). Another place we should notice is the position
surrounded by frame A. A relatively high magnetic anomaly takes place here in the
seaward direction of the end of shelf (black triangular; Fig.9b). And we detect that the
crustal thickness becomes thinner seaward at ca 140 km to 250 km. We also find the
zone from 250 to 380 km shows that crustal thickness is thin and gentle but seismic
basement is rough (frame C). The bouguer anomaly jumps to relatively high value
(frame C’) and magnetic anomaly also shows high value (frame C”) than surrounding
area. We guest that there are high density materials below this region and they could
be intruded igneous bodies. We call this region as volcanism zone (VZ). In this
transect, we define that the CC is between 0 and 140 km. The TCC is between 140
and 250 km or 140 km and 380. The result causes two possible locations of the COB
that one is located at 250 km and the other one is located at 380 km along this transect
(Fig.9b). We will discuss lately. The present location of the OC is behind 380 km and
the VZ is existed between TCC and OC.
4.2.2 Mltw Transect
The seismic interpretation shows that the seismic basements are separated by
normal faults and form grabens (Fig.5). We also find there are many fault systems to
be occurred because of older fault systems below them. We believe that is why so
many canyon systems appear frequently along this profile. Another characteristic of
this profile is that many intruded and extruded igneous bodies are found. By seismic
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profile, these igneous bodies are passed through the fractures of basement, and space
between them is filled by sediment to form basins. The strata of basins are always
smooth except the area between 210 km and 290 km (Fig.5). We call the area
‘Disturbed zone’ (frame A). It seems that there are uplift forces below this area and
cause a sequence of fault system between these volcanism bodies. Compare with
topography, we can find here is a plateau and many seafloor volcanoes are extruded
(Fig.1). The previous studies describes the igneous bodies occurred over Dongsha
Island area maybe relate to the local Dongsha Event starting from the end of middle
Miocene (Yao et al., 1994; Thomas and Wong, 1999). The crustal modeling (Fig.9c)
shows us that the crustal thickness between 0 and 160 km is thinning gradually and
large slope is showed in bouguer anomaly curve. The Moho discontinuity below the
area from 160 to 310 km is smooth and shallow (frame C). The gravity data show
gentle curve (frame C’). The relatively high magnetic anomaly value is found in this
area (frame C”). We guest the higher magnetic anomaly value maybe is affected by
high density intrusion of igneous bodies at this area. We still can find anomalously
high magnetic value in this transect (frame A) and it is also located behind the
seaward direction of the end of shelf (black triangular; Fig.9c). We define the OC is
behind 310 km (frame B) because of relatively high magnetic anomaly value (frame
B”) and curve changes to high value on bouguer anomaly (frame B’). The modeling
shows that the TCC should be from 0 to 150 km or 0 to 310 km. This result causes
two possible locations of the COB as same as transect espE. We will discuss lately,
too. The VZ is also located between TCC to OC, and is from 150 km to 310 km.
4.2.3 Mcs689-5 Transect
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The seismic profile of mcs689-5 shows us oceanic crust between ca 140 to 230
km (Fig.6), but the basement from 140 to 160 km is not very sure. Except the
subsidence effect causes sedimentary bending (frame A and frame B of Fig.6), the
strata are smooth and continuous. An extruded volcano is located at ca 240 km, and
we believe that it is produced by the same way like ‘Disturbed zone’ (Fig.5) at this
region (frame C of Fig.6). The seaward basement behind the volcano is interrupted by
these igneous actions, and is caused regional lifting by thrust faults. The result shows
that seismic basement are separated by large number of faults in the area between 30
and 120 km. The segment of basement is shorter than mltw profile (Fig.5). Maybe the
reason is that spreading rate is faster than other places of the SCS around this area.
The anomalously high magnetic data which is located in the seaward direction of the
end of shelf (black triangular; Fig.10a) is still clearly at this profile (frame A of
Fig.10a), and we also find relative high free-air anomaly at this area (frame C). The
modeling result shows the region from 90 to 140 has the same thinning crust with
oceanic crust defined by seismic interpretation between 140 and 230 km (frame D),
but the magnetic anomaly value of this area is lower (frame D’). So, we guess this
zone maybe is still belonged to TCC, but not sure. The TCC zone shows the crust is
thinning rapidly at seaward direction and bouguer anomaly curve (frame C’) also
shows large slope. We define the TCC should be between 0 and 90 km and maybe the
region from 90 to 140 is also belong the TCC. Compare the seismic data with
magnetic anomaly (frame B”) and bouguer anomaly (frame B’), the OC should be
located 140 km to 230 km (frame B). The Disturbed zone is behind OC.
4.2.4 EW45 Transect
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By modeling result, we can find the crustal thickness behind 70 km is quite thin
(Fig.10b). But previous study (Yeh, 2001) represented here were lower magnetic
anomaly value and could not find magnetic lineation between SW offshore of Taiwan
and Formosa Canyon. The only high pick of magnetic anomaly (frame A’) is caused
by a volcanism body (frame A). We think this situation of thinner crust should be
caused by faster rifting during extension of the South China Sea. The crust thins
abruptly is agreed our suggestion. We believe that high density material is really
closed to surface because of crust thinning. Therefore, we can observe higher bouguer
gravity value in this area. In fact, the 97304Ab profile (Fig.7) shows us the basements
are different between both sides of the Formosa Canyon (FC). We can look for
reflection of basement clearly at WSW side of the FC, but is not at ENE side. The
reflection of basement seems to distribute deeply at this ENE side of the FC. The
thicker sediment in the ESE side of the FC is also the evidence. To compare with
mltw (Fig.5), mcs689-5 (Fig.6) and 97304ab (Fig.7), we support that Formosa
Canyon is a transform fault (Yeh, 2001). We find an interesting situation that the
free-air anomaly is really different in frame B and frame C (Fig.10b). The higher
value in frame B may be caused by sharp change of basement (60 km to 70 km) or by
the residual arc/forearc body below this area (Sibuet et al., 2002). The lower value in
frame C may be caused by subduction near the Manila Trench.
5. Discussion and conclusion
Due to lack the gravity data and deep crustal structure data acquired by ourselves,
we can not to compute the more reliable density for simulating our own seismic
profiles. Another problem is the quality of seismic data our own is not good. We only
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use a common sedimentary velocity (2.2 km/s; whole sedimentary sections) and water
velocity (1.03 km/s) to compute the depth of basement because of the velocity
analysis is uncertain. The crustal modeling presented in this study is based on the
simple four-layer density parameters of Eastern Transect by Nissen et al. (1995b).
However, to compare the modeling results of these transects with magnetic data, we
believe the primary frameworks should be believable.
We combine the results of crustal structures to bathymetry map (Fig.11a and
Fig.11b), free-air anomaly map (Fig.11c and Fig.11d). We also superpose the Moho
depth (Fig.11a and Fig.11c) and crustal thickness (Fig.11b and Fig.11d) on
bathymetry map and free-air anomaly map. Structurely, we can separate the northern
margin of the SCS to five types, CC, TCC, OC, VZ, and TCA (Tension and
Convergent Area; frame A of Fig.11a). The CC is covered whole continental shelf
approximately, the Moho average depth is ca 30-24 km (the deepest is ca 32 km at
transect espE and shallowest is ca 24 km at transect OBS93). The TCC is distributed
over continental slop and widest place is in the SW Dongsha Island. Based on free-air
data (Fig.11c), we found the anomalously low gravity values are existed at this area
until Xisha Trough. The evidences from seismic data show these places are filled
thick sediment. We suggest the period of continental rift caused the crust thinning and
subsidence effects, and then a large number of sediment fill between these grabens to
form parts of PRMB and YGHB (Yinggehai Basin; Yan et al., 2001). The Moho depth
below TCC is ca from 26 km to 12 km (Fig.11a).
The Moho depth of OC is ca from 12 km to 8 km and crust thickness is about 6-9
km (Fig.11b). The results of seismic data and crustal modeling (Fig. 6 and Fig.10a)
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show there is an existence of OC in SE Dongsha Island and be separated by VZ. The
new magnetic data can support this observation (Hsu et al., this issue). If those results
are correct, the age of the SCS should be older than the present age. According to
previous studies and seismic data in this study, we can find there are many intruded
and extruded igneous bodies around this area of VZ. We believe they are related to a
series of volcanism/magmatism from ocean basin pass through continental slop into
Dongsha Island (Yao et al., 1994 and Thomas and Wong, 1999). Figures 9b and 9c
display the thickness of VZ is about 6-14 km and Moho discontinuity is flat. We
suggest that existences of thinning crust and faults are over VZ. After that, the
regional magmatism or underplating are occurred here, and be sources for these
igneous bodies. Based on these results, we re-define the position of COB. In SW of
Dongsha Island, we have position as same as Briais et al. (1993). But we suggest the
position of COB should move landward (Fig.11).
TCA is a peculiar zone in northern margin of the SCS. Sibuet et al. (2002)
mentioned the thick sediment is underlain by oceanic crust and large igneous body. He
also suggested a part of oceanic crust of the northern SCS subducts beneath another
piece of oceanic crust and the magnetic effect of the subducting plate is not recorded
in surface magnetic data. The modeling result show very thin crust begins to appear
after 70 km and the magnetic anomaly value is quite low (Fig.10b). The crust
thickness is thin from 16 km to 2 km abruptly near 70 km. We doubt the existence of
oceanic crust of the northern SCS in TCA zone by Sibuet et al because of
anomalously low magnetic value. We suggest the abruptly thinning crust should
caused by rapidly extensional force because of old oceanic crust subducted along
Manila Trench.
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The crustal model show that the seismic velocities decrease gradually from
oceanic above the SDRs (seaward dipping reflectors) to continental on the land side.
Thus the zone is transitional in nature in Lofoten volcanic margin, north Atlantic
(Mjelde et al., 1996). A thin crust is found to be underlain by upper mantle material of
7.2-7.9 km/s velocity at shallow depths (1-2 km). And a serpentinized ridge was found
which could correspond to a layer of serpentinized peridotite in the west Iberia
non-volcanic margin. The zone that seismic velocities are intermediate between those
generally observed in the lower continental or oceanic crust and the uppermost mantle
is called continent-ocean transition zone (COT; Pinheiro et al., 1996). Those situations
could not be found in northern margin of the SCS. Nevertheless, we find the region
from 120 km to160 km in mcs689-5 profile is dubious. The seismic reflection of
basement (140-160 km; OC?) is rough but not clear as oceanic crust. Another
basement (120-140 km; SB?) is flat and a series of parallel reflectors. We also suspect
wider region from 90 km to 160 km in crustal model of transect mcs689-5 (question
mark in Fig.10a). The crustal thickness is only ca 6 km and magnetic anomaly display
a pick. According to these doubtful points above, we do a simple magnetic simulation
to test our idea (Fig.12). The result shows the magnetization values are higher than
common values along this profile. Benes at al. (1997) had observed a large variations
in the magnetization from 4 A/m to 22 A/m indicate inhomogeneity neovolcanism
activity of the magnetic layer when the new oceanic crust formed during the Bruhnes
in the western Woodlark Basin. A metasedimentary crust is formed in direct contact
with an abnormal mantle produced by partial serpentinization (Nicolas, A., 1985). He
cited three models to produce transitional zone in his paper. One of them seemed to
conform the situation in northern margin of the SCS: a brittle behaviour of continental
18
crust which would be progressively invaded by mafic dykes, its extension being that
of the accumulated thickness of the dykes, and would finally yield without stretching
along the mechanically weakened zone of dyke intrusion, passing more abruptly to
oceanic crust (Turcotte, D. L. et al., 1977 and Mohr, P., 1983). We suggest Box (1)
and Box (2) (Fig.12b) could be an extension continental crust and normal oceanic
crust, Box (3) is a transitional zone invaded by dykes and Box (4) is a pure oceanic
crust that was not disturbed by sediment. Box (5) looks like a large igneous body that
has strong magnetization as same as oceanic crust. It could be a part of arc and forearc
body that exist near continental slop by Sibuet et al. (2002).
There is an anomalously high free-air anomaly zone which is separated on
continental slop extended to eastern part of the FC offshore SW Taiwan (Fig.11c). But
the data only display relatively high anomaly value at transects mcs689-5 (Fig.10a;
frame C) and ew45 (Fig.10b; frame B), not including the other transects in the area of
continental slop. If there is an existence of arc and forearc body by Sibuet et al. (2002),
we guest is only extended to the landward area of mcs689-5.
The modeling results also show a series of anomalously high magnetic anomaly
in the seaward of continental shelf (Fig.8, Fig.9 and Fig.10) except transect ew45
(Fig.10b). This phenomenon could be caused by material difference below continental
slop. Previous studies displayed the existence of high velocity layer e.g. underplating
layer underlay lower continental in seaward direction. Partial melting would cause
magma intruded lower continental, particularly in the TCC zone. That may be a
reason to interpret this anomalously high magnetic anomaly zone.
19
Finally, we take a sample idea to computed the stretching factors “α” (Table 2a;
the positions are located in Fig.11b). The Results show the values of stretching factors
“α” are during 1.14-1.50. We conclude the rifting rates are not same in northern
continental margin of the SCS. The result causes to differ in wide of the TCC zone.
Due to different rifting rate, the crustal thicknesses of the northern SCS margin are
also un-uniform. We find the crustal thickness is persisted thicker in the center part
than the position in east and west parts (Fig.11b). Maybe this is the reason why the
volcanism zone is not separated on whole margin.
Acknowledgements
We thank the staffs of Ocean-Research I to help us collecting MCS data during
ORI-620 and MCS689 cruses. We also thank Mr. Shao-Yung Liu and Mr. S.D. Chiu,
National Center of Oceanic Research (NCOR), to pre-processes the MCS data. During
the period of writing paper, we thank Dr. Chao-Shing Lee, Institute of Applied
Geophysics, National Taiwan Ocean University, Dr. Shu-Kun Hsu, Institute of
Geophysics, National Central University, and Yi-Ching Yeh, Institute of Geophysics,
National Central University, to provide discussion with author. We also thank the
National Science Council, Taiwan, R.O.C., to provide support to let us survey cruise
successfully.
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Figure captions:
Fig1. The map of the major tectonic elements and the magnetic lineations of the South
China Sea. The black dotted line shows the continent-ocean boundary by
Briais et al., 1993. The black frame is our study area. The detail topography of
study area. The black lines are locations of previous crustal profiles and
seismic profiles in this study. The data source are described in Table 1. PRMB:
Pearl River Mouth Basin. YGHB: Yinggehai Basin.
Fig.2 A simple four-layer crust model. We separate the simulated method into two
stages. The first stage, we use crust density as basis to correct the sea water
and sediment effects and calculate the bouguer anomaly (calculated).The
second stage, we assume a synthetic Moho depth and then calculate the gravity
effect provided by it (synthetic) and make better fitting with curve produced
by first satge. The result are showed in Fig.5.
Fig.3 (a) f13 seismic profile and (b) interpretation. This profile is modified from
Lüdmann et al., 2001. We can see many graben filled by thick sediment
between a lot of continental blocks. There are not observed of intruded and
extruded bodies along this profile.
Fig.4 (a) 97304Aa seismic profile and (b) interpretation. This profile shows us many
normal faults occurred between sedimentary strata. And we detect an intruded
igneous at about 80 km and causes topography lifted. We can see sediment
truncation at frame T. The uplift force is existed closed to Dongsha Island.
24
Fig.5 (a) mltw seismic profile and (b) interpretation. There are many intruded and
extruded igneous bodies along this profile. We find the igneous zone is more
than 150 km long. The data shows that the basements are separated by normal
faults and form many graben. We also find there are many younger fault
systems to be occurred because of older fault systems below them. We call the
area ‘Disturbed zone’ that is surrounded by frame A. It seems that there are
uplift forces below this area and cause a sequence of fault system between
these igneous bodies.
Fig.6 (a) mcs689-5 seismic profile and (b) interpretation. There are also many fault
systems separating the basement. We find distinct oceanic crustal basement
between 160 to 230 km. Besides some subsidence effect to cause sediment
bending (frame A and B), the strata are smooth and continuous. We also find
the ‘Disturbed zone’ in the end of this profile. It should be had same reason
like mltw-profile.
Fig.7 (a) 97304Ab seismic profile and (b) interpretation. We still can find out the
uplift force near Dongsha Island. The basement is progressively downward
from WSW to ENE. The basement is overlain by thicker sediment in the
eastern part of the FC.
Fig.8 The gravity modeling results with gravity and magnetic anomaly curves. (a)
Transect espW, (b) Transect ab and (c) Transect OBS93. We find the relative
high magnetic anomaly in the area from shelf into slop and from continental
25
crust into oceanic crust. There are always relative lower magnetic anomaly in
the thinning continental crust (TCC) zones besides some igneous zones. The
results show that there are wider TCC zones around the Dongsha Island and
become narrower in the two ends of margin in the northern South China Sea.
Fig.9 The gravity modeling results with gravity and magnetic anomaly curves. (a)
Transect f13, (b) Transect espE and (c) Transect mltw. Explanation of analyses
and symbols are in the text and in Fig.8.
Fig.10 The gravity modeling results with gravity and magnetic anomaly curves. (a)
Transect mcs689-5 and (b) Transect ew45. Explanation of analyses and
symbols are in the text and in Fig.8.
Fig.11 The zone of the thinning continental crust (TCC) defined in this study is
bounded by black dotted lines and is superposed on the bathymetric map (a),
free-air anomaly map (c), respectively. The Moho depth and crustal thickness
are also superposed on. The volcanic intrusion area is marked by VG
(volcanism zone). The OC indicate the existence of oceanic crust. The CC
means continental crust. The TCA means tension/convergent area.
Fig.12 The magnetic model of mcs689-5 profile. Model (I) is an initial model. We
separate two parts, oceanic (Box (2)) and continental crust (Box (1)), in this
model. We assume the magnetization of oceanic crust is 4 A/m and lesser ten
times in continental crust. Model (II) is an final model. In this model, we find
very high magnetization between 140 to 160 km (Box (4)). Besides, there
26
should be a large igneous body (Box (5)) close to edge of continental shelf.
Table1. The data sources applied for the gravity model in this study.
Table2. (a) The simple diagram shows how to compute stretching factor “α”. The
positions of profile are located in Fig.11b. (b) The results.
27