Ching-Hui Tsai
1
, Shu-Kun Hsu
1
, Yi-Ching Yeh
1
and Chao-Shing Lee
2
1 Institute of Geophysics, National Central University, Chung-Li, Taiwan 32054
2 Institute of Applied Geophysics, National Taiwan Ocean University, Keelung, Taiwan 20224
New magnetic evidence indicates that the oceanic crust may extend to 21°30’N in the northern South China Sea (SCS). To examine the characters of the possible continent-ocean boundary, we have studied seismic reflection profiles across the SCS continental margin. We have also used gravity anomalies and a simple four-layer crustal model to calculate the depth distribution of the Moho discontinuity beneath the northern SCS continental-ocean boundary (COB). According to the analysis of the seismic data and gravity modelling, we can distinguish the crustal structures across the continental margin into three zones, including continental crust zone (CC), thinning continental crust zone (TCC) and oceanic crust zone (OC). Some profiles contain a volcanic intrusion/extrusion zone (VZ). Our results agree with the existence of oceanic crust in the northernmost SCS as observed by magnetic data.
Accordingly, we have moved the continent-ocean boundary (COB) from the southeast of the Dongsha Island to the northeast. Morphologically, the COB is along the base of the continental slope. However, we could not recognize the characteristics of the continent-ocean transition zone (COT) as those in the western Iberian non-volcanic margin or the Lofoten volcanic margin. Between the under formed continental crusts and the oceanic crusts, the continental thinning zone are prominent.
In our study area, the Moho depths vary from ca. 28 km to ca. 10 km and the crustal thicknesses vary from ca. 24 km to ca. 6 km. Our modellings also show the oceanic crust in the northern SCS is slightly thicken than normal oceanic crust that could be caused by intrusive magma or underplating after the formation of the SCS oceanic crust.

Fig1. The inset map shows the major tectonic elements and the magnetic lineations of the South China Sea (up-left). The black dotted line shows the continent-ocean boundary (COB) by Briais et al., 1993. The black box points out our study area in the inset map. The detail topography of study area is displayed by color map. The black lines are locations of the refraction/reflection profiles used in this study. The white dotted line displays the position of the COB that is redefined by this study. PRMB: the Pearl River
Mouth Basin. SCS: the South China Sea.

Fig.2 (a) profile FF’ and (b) interpretation (see the profile location in Fig.1). The profile displays numbers of intrusive and extrusive igneous bodies. We find this kind of igneous zone is distributed more than 150 km long (from distance
160 km to 310 km). This profile shows that the basements are separated by normal faults and form many graben systems. Many younger fault systems are occurred because of older fault systems below them in the continental slop. We call the area ‘Disturbed zone’ (box A) which seems the uplift forces below here and causes a sequence of fault systems between these sub-basin surrounding by these igneous bodies.

Fig.3 (a) profile GG’ and (b) interpretation (see the profile location in Fig.1). We can find the distinct oceanic crustal basement between distance 140 km to distance 230 km. Besides the sediment bending caused by sediment subsidence in the boxes A and
B, the strata are smooth and continuous. We still can find the ‘Disturbed zone’ in the end of this profile (box C). The fault systems are distributed widely in the basement below the continental slop.

Fig.4 The results of crustal modelling are combined with gravity and magnetic anomaly data. We can distinguish the crust into three types: continental crust
(CC), thinning continental crust (TCC) and oceanic crust (OC). The magnetic anomalies are always relatively high in the OC zone and igneous zone. The continental shelf edges also observe these high magnetic anomalies. The crustal thickness is ca. 6-8 km in the OC zone. The TCC zone has greater depth variations from ca. 26 km to 12 km.

Fig.5 The Moho depth and crustal thickness are superposed on the bathymetric map
(a and c) and free-air anomaly map (b and d). According to the results of crustal modeling, we can distinguish five crustal zones in the northern continental margin of the SCS. They are the CC, TCC, OC, VZ and the strength zone framed by box A, respectively. The TCC zone should be the continent-ocean transition zone (COT) in the northern margin of the SCS. So, we could define the COB position is along the boundary between the TCC and
OC. The results show there is an existence of the OC which is separated from the main SCS oceanic crust by the VZ in the northeast SCS. We suggest the age of the SCS is older than present. The crustal thickness shows very thin
(Fig.4d) in the area framed by box A, however, the magnetic anomaly describe the lower value in this area. So, we suppose the thin crust is made by continental strength, and no oceanic crust exists here.