| Citation: | Dengke Hu, Di Zhou, Xiangjie Wu, Min He, Xiong Pang, Yuwei Wang. Crustal Structure and Extension from Slope to Deepsea Basin in the Northern South China Sea. Journal of Earth Science, 2009, 20(1): 27-37. doi: 10.1007/s12583-009-0003-6
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The newly acquired long-cable multi-channel seismic (MCS) lines were used to study the crustal structure and extension in an NW-SE elongated 150 km by 260 km strip from the slope to the deepsea basin in the northern South China Sea (SCS). These profiles are of good penetration that Moho is recognizable in ~70% length of the lines. Seismostratigraphic interpretation and time-depth conversion were conducted. A power function
The South China Sea (SCS) is one of the largest marginal seas in the rim of the western Pacific Ocean (Tamaki and Honza, 1991). Its evolution and hydrocarbon potential have been the focus of attention for geoscientists all over the world. Inspired by the worldwide success of deep water hydrocarbon exploration in recent decades, the exploration and related scientific researches have been advanced to the slope area of the northern SCS, starting from the largest slope basin, the Baiyun sag (BYS), in the southern Pearl River Mouth basin (PRMB). Preliminary studies have revealed the distinct features of the sag compared with the sedimentary basins in the adjacent shelf area (Pang et al., 2007), and the first well at water depth of 1 500 m encountered 56 m natural gas and demonstrated its good hydrocarbon potential.
In order to image the basement and deep structures of the BYS and to understand the deep control on the basin formation and hydrocarbon accumulation, long-cable multi-channel seismic (MCS) lines with a total length of nearly 1 500 km were deployed in 2002 and 2004 by the China National Offshore Oil Corporation (CNOOC). These are the first set of deep-penetrating MCS lines in the SCS, also the first set of Chinese industrial MCS lines across the slope and reached the deepsea basin. These lines controlled an NW-SE elongated strip of approximately 150 km wide and 260 km long, and provided an unprecedented opportunity for studying the crustal structure across the continent ocean boundary (COB) in the northern SCS.
Huang et al. (2005) analyzed the earliest one of these long-cable MCS lines and named it as line DSRP2002, which imaged the basement and Moho at the center of the BYS for the first time. This line, however, was a broken line composed of 4 segments in order to connect the ODP 1148 well. It strikes from SSE gradually to SE and does not reach the deepsea basin (Fig. 1). This article analyzed 3 dip lines across the slope to the deepsea basin, and 5 strike lines connecting them, covering an NW-SE-elongated rectangular area across the continental slope and the COB to the deepsea basin (Fig. 1). Time-depth conversion was carefully designed to increase the quality of the depth estimates. The basement and Moho surface were imaged, the thickness and stretching factor of the crust were calculated, and the stretching factor of the lithosphere was estimated. Tectonic implications of the results were expounded.
The SCS lies near the junction of three plates, Eurasian, Pacific, and India-Australia, also the junction of the two super-convergent zones, CircumPacific and Tethys. It was formed by stretching and breakup of the continental margin of the South China block. The opening of the east (central) subbasin of the SCS occurred during 30–16 Ma (Briais et al., 1993) (ages of magnetic anomalies here and in subsequent sections have been modified according to the revised geomagnetic polarity timescale of Cande and Kent (1995)). The age of the SW subbasin is still controversial, e.g., 23–16 Ma according to Briais et al. (1993), but 42–37 Ma according to Yao et al. (1994). The northern margin of the SCS is a passive margin with several large sedimentary basins. To the east the SCS is subducting towards the Philippine archipelago along the Manila trench. To the south the Nansha block, a formal piece of the South China block tiered off by the SCS opening, subducted beneath the Kalimantan Island in Early Cenozoic. To the west the SCS was separated from the Indochina peninsula by an NS-running strike-slip fault.
The Pearl River Mouth basin (PRMB) is the largest sedimentary basin in the passive margin of the northern South China Sea (Fig. 1). It consists of two depression zones sandwiched by three uplift zones. The northern depression zone is composed of Zhu 1 and Zhu 3 depressions and located in the shelf area, while the southern depression zone is composed of Zhu 2 and Chaoshan depressions and located mainly along the slope. The PRMB is filled with Cenozoic sediments, except the Chaoshan depression which contains mainly Mesozoic strata. In general the evolution of the PRMB (except the Chaoshan depression) is divided into two stages by the breakup unconformity at ~30 Ma, the rifting stage before and post-rifting stage after. In the rifting stage sedimentation was concentrated in discrete intermountain sags, while in the post-rifting stage regional thermal subsidence occurred, and marine sedimentation dominated the entire basin (Gong and Li, 1997).
The Baiyun sag (BYS), south of the Panyu low uplift, is the largest sag (~20 000 km2) and the first target for deepwater exploration in the PRMB. The Liwan sag (LWS) discovered recently lies southeast of the BYS and is separated from the BYS by the South Baiyun uplift. An NS-striking uplift of ~50 km width separates the BYS and LWS to the west from the Chaoshan depression to the east. We named it as the East Baiyun uplift (Fig. 1)
Basement of the PRMB has been encountered in nearly 80 wells at depth of 1 000–4 000 m and composed of Mesozoic granites mainly and Paleozoic quartzite in the west (Pang et al., 2007). The depth of the basement at depression centers, as imaged by MCS profiling, is up to ~6 km in the shelf area (Gong and Li, 1997) and exceeds 11 km in the slope area (Huang et al., 2005).
Before the CNOOC long-cable MCS profiling, the crustal structure of the PRMB and adjacent areas was studied by gravity modeling (Su et al., 2004; Wang and Dang, 2000; Liu et al., 1983), expanded spread seismic profiling (Nissen et al., 1995; Yao et al., 1994), and ocean bottom seismic measurements (Qiu et al., 2001; Yan et al., 2001). In comparison with the results from the deep crustal soundings inland of South China, it is generally agreed that the shelf and slope of the northern South China Sea are floored by an extended crust, typical for a passive continental margin. The Moho surface shoals from ~30 km in depth along the coast to < 14 km in the deepsea basin, and rises locally at basin centers mirroring the basement surface. The crustal thickness (excluding sediment layers) varies from 30–28 km near the coast to 7–8 km in the deepsea basin, and thins significantly in basin centers.
The lithosphere structure in the northern SCS is poorly known. S-wave tomographic study revealed a 70–80 km thick lithosphere in the northern SCS, and 64 km in the central subbasin (Cai et al., 2001).
The long-cable MCS profiles were acquired by CNOOC in 2002 and 2004 using 576 channels with a shot point spacing of 37.5 m and common midpoint spacing of 12.5 m. The cable length varied from 200 to 7 400 m, and the sampling rate was 2 ms. The data processing followed a standard industrial pre-stack procedure. Post-stack deconvolution, band-pass, and coherency filtering were then applied to the stacked data, followed by the finite-difference migration. The total length of the lines is nearly 1 500 km. In addition, a long profile (Line 10 in Fig. 1), which is composed of several segments and subjected to gravity-magnetic inversion for deep crustal structure (Hu et al., 2008), was also used to provide constraints to the eastern boundary of the study area. The values of basement depth revealed by the wells near the northern boundary of the BYS were used to provide additional constraints to the Tg contours.
Seismostratigraphic interpretation of the basin fillings was conducted in CNOOC. Interfaces Tg to T2 were identified, among which Tg is the top of the basement with the most likely age in the range of Late Paleogene to the earliest Eocene, T7 approximates the breakup unconformity at 30 Ma, and T6 represents the Paleogene/Neogene boundary at 23.8 Ma.
The Moho surface was observed in ~70% length of these MCS lines as a set of high amplitude, semicontinuous reflectors at TWT 8–10 s. The Moho reflectors become obscured where reflectors of volcanic type are observed in shallower depths. In these cases the Moho surface was interpolated or extrapolated.
A special effort was placed on the time-depth conversion of the interfaces because many of them are deep-seated. The cubic or quadratic polynomial function is commonly used in fitting time-depth relation based on well data. However, it could not be used in our applications because the velocity of a cubic polynomial function is a quadratic polynomial function, which will increase to a maximum at certain depth and then decrease toward deeper depth. Thus, for a basin with great sediment thickness such as the BYS, a cubic polynomial time-depth function would introduce artificial velocity inversion at depth, no matter what is the reality. For example, a cubic polynomial function was obtained by fitting the time-depth data from the adjacent well PY33-1-1, then a velocity inversion occurred at depth equal and greater than TWT = 4.15 s (depth = ~5 km) (Zhou et al., 2008). If a quadratic polynomial function is used to fit the time-depth relation, then the velocity will increase linearly with depth without inversion, but in reality we would expect a monotonously increasing velocity with reduced gradient. Thus for the time-depth conversion of deep-seated strata a cubic polynomial function would tend to underestimate, while a quadratic polynomial function would tend to overestimate the depth. Neither could be used to represent the time-depth relation of deep-seated strata, such as the strata deeper than 5 km in the BYS.
Huang et al. (2005) used the interval velocity for time-depth conversion of the line DSRP2002, but the profile was divided into two segments using different sets of velocity values. This is because for a certain sediment type the velocity is dependent not only on depth alone, but also on its age, and along the profile in the deep-water segment sedimentation is thin and thus older strata appear at much shallower than those in the shallow-water segment. By using interval velocity the dependence of velocity on age was considered, but the dependence of velocity on depth was subdued. In addition, the division of profiles into segments and the selection of interval velocities for each segment are in fact rather subjective and difficult to apply in a large area.
We used power function D = atb + c to fit the time-depth data of the well PY33-1-1, where D is depth; t is the TWT from the seabed; and a, b, and c are constant, with 1 < b < 2 and c = 0.745. This function can achieve the same goodness of fit at shallow depth as those of a cubic polynomial function, but give much more reasonable estimates at greater depth. The velocity predicted by a power function is also a power function, which increases monotonously towards depths at reduced gradient (Zhou et al., 2008). This power function, however, is applicable only to sediment strata and not to the underlying crust. The earth's crust is assumed un-compactable, here an interval velocity of 6.4 km/s was used to estimate the crustal thickness. It should be pointed out that the advantage of power function for time-depth conversion is only to guarantee the converted depths from being under- or over-estimated by cubic or quadratic polynomial time-depth functions. It could not, however, take into account the velocity variation of the strata at the same depth but with different ages, which is the common disadvantage of using any time-depth function for the time-depth conversion in a large area.
After data processing, interpretation, and time-depth conversion, geological sections along the long-cable MCS lines were constructed, showing the variation of crustal structure along the lines. Figures 2–4 present two NW-SE lines and one connection line.
The Line 6 (Fig. 2) provides a complete image of the crustal structure across the center of the BYS, the eastern portion of the LWS to the south, the COB, and the northern portion of the deepsea basin. The northern segment of this line is the same one as the segment A of the line DSRP2002 in Huang et al. (2005), but the interpretation has been modified slightly. This northern segment was jointed with the southern segment collected in 2004 so that it becomes a straight line. It shows that the basement Tg dips down from < 5 km depth at the edge to ~14 km at the center of the BYS, and then shoals up again to < 7 km at the southern edge of the sag. The total thickness of the sediments south of the BYS was < 5 km and decreases to < 3 km near the COB. In the deepsea basin south of the COB, the Neogene sediments above the oceanic basement are characterized by undeformed horizontal reflectors and have a total thickness of 1–3 km.
The crust north of the BYS is nearly nonreflectable, but in and adjacently south of the BYS discontinuous reflections of medium amplitude are seen below and sub-parallel to Tg in the upper crust, perhaps indicating metamorphosed pre-Cenozoic basement.
The Moho depth decreases stepwisely from ~28 km in the northern edge of the BYS southwards to 19, 15, and 12 km with ramps at the shelf break (the northern flank of the BYS), lower slope, and the COB, respectively. Accordingly the crustal thickness decreased southwards from 25 to 15, 13, and 7 km respectively. Under the center of the BYS the crust thins significantly to < 7 km. A high velocity layer of 0.5–3 km appeared in the lower crust adjacently above Moho (Huang et al., 2005).
The Line 8 (Fig. 3) crosses the eastern BYS, the South Baiyun uplift, the LWS, and the COB. In the profile under the center of the BYS, Tg is at 9 km depth, shallower than that in the Line 6. Under the southern flank of the BYS again, there are volcanic reflectors in the upper crust, but in the South Baiyun uplift the basement under Tg looks like granitic or metamorphic. The LWS has multiple depocenters forming W shape in the profile. A thick layer of north-dipping strong to weak reflectors is observed under Tg in the northern depocenter, perhaps indicating Mesozoic strata as those inferred in the Chaoshan depression. Southwards to the COB there are no volcanic seamounts observed, different from the situation in the Line 1530.
The Moho reflectors are clear under the northern and central BYS, part of the South Baiyun uplift, the northern LWS, and the deepsea basin, but obscured under the southern flank of the BYS and the southern South Baiyun uplift, and undulated under the southern LWS and the COB. The depth of Moho varies also stepwisely from 25 km under the BYS to 19 km under the LWS, and 14 km in the deepsea basin, with ramps at the southern South Baiyun uplift and near the COB. The crustal thickness is 22 km in the northern end of the profile and under the South Baiyun uplift, 14 and 13 km under the BYS and LWS respectively, and 9 km in the deepsea basin. The high velocity layer above Moho perhaps exists under the BYS but was unable to be delineated.
COB is observed in Lines 5–8 and represented by coincident sharp decrease of crustal thickness and sharp change of topography from rugged lower slope to flat deepsea basin at ~3 500 m isobath (Figs. 2 and 3).
Magmatic activities are indicated by two types of seismic reflectors (Figs. 2–4). One consists of concave, convex, and chaotic reflectors in the upper crust, with disturbed, obscured, or blanked reflections in the overlying sediments up to T3 (~13.8 Ma). These are interpreted as indicating the Tertiary volcanic activities, including the oceanic crust in the deepsea basin of the SCS. Another type consists of obscured or convex reflections from sub-Moho upwards even to the seafloor, erased completely the Moho reflectors at depth. These are interpreted as Quaternary basaltic volcanoes. The magmatic activities were more intensive in the South Baiyun uplift and in the lower slope. The Quaternary basaltic intrusion in the middle segment of the Line 4 separated the depocenters in the LWS (Fig. 4).
The study area is a rectangular strip of 260 km long and 140 km wide, elongated in an NW-SE direction across the slope to the deepsea basin in the central portion of the northern South China Sea (Fig. 1). Contour maps of Tg depth, Moho depth, and crustal thickness were constructed based on long-cable MCS lines plus the Line 10 and adjacent wells (Fig. 5) to show their areal variations.
In the map of Tg (top of the basement) depth (Fig. 5a), the BYS and LWS are characterized by downwards in the BYS and LWS, and by upwellings in the Panyu low uplift, South Baiyun uplift, and East Baiyun uplift. The Tg depth in the centers of the BYS and LWS is 15 and 9 km respectively. The Tg interface in the lower slope and deepsea basin south of the LWS varies gently at intermediate depth abound 5 km.
The depth variation of the Moho interface revealed by the long-cable profiles (Fig. 5b) agrees with the general pattern of the Moho isobath estimated by gravity inversion (Wang and Dang, 2000), but provided more details. The COB delineated by the 15 km contour of the Moho depth is also similar to the COB estimated previously (Fig. 6 inset). The Moho is deeper than 25 km in the Panyu low uplift and the East Baiyun uplift, and shoals to 24–20 km in the BYS, 20–17 km in the LWS, and < 16 km in the deepsea basin. The most striking feature is an NNW-running nose of Moho high from the COB towards the LWS and the BYS about 170 km long and 50–120 km wide, which is named here as the "Baiyun Moho Nose". In the nose area Moho upwells to the depth of < 22 km. Along the east border of this nose the Moho deepens at high gradient, which may be a manifestation of a deep-seated NW-striking crustal or even lithospheric fault, which coincides with the SE extension of the F2 delineated by Sun et al. (2008). The northern border of this nose is represented by a high Moho gradient, which is the leftmost Moho ramp as seen in Fig. 2. It coincides with the northern boundary of the BYS also of the Zhu 2 depression, and might be a deep-seated NEE-striking crustal fault. The western border of the nose is the F1 fault, which is the western border of the Yunkai low uplift west of the BYS. The doming of Moho in the nose area must have affected the lithosphere rheology, which perhaps was the cause of the W-E segmentation of the crustal and geological structures along the slope of the northern South China Sea. The PRMB strikes NE west of the nose, EW within the nose, and NE again east of the nose. The strongest subsidence along the slope occurred in the BYS where the Baiyun Moho Nose exists (Fig. 6). There is no any topographic expression of the Baiyun Moho Nose and its boundary faults.
Previous studies on the crustal and lithosphere extension in the northern SCS were focused on the shelf and upper slope due to the limited penetration of the MCS data available by the time (Clift et al., 2002; Clift and Lin, 2001). The newly acquired long-cable MCS profiles provided an unprecedented opportunity for studying the details of crustal extension in the area of transition from the slope to the deepsea basin in the northern SCS.
The crustal thickness was obtained by subtracting Tg depth from the Moho depth (Fig. 5c), and the crustal stretching factor βc was obtained by dividing the thickness of the crust before stretching by that after stretching (Fig. 5d). The thickness of the original crust is assumed as 32 km, the crust thickness of the South China near coast area (Jin and Sun, 1997). The contours in Figs. 5c and 5d show a general correspondence with the distribution of the basins. The crustal thickness in the BYS is mostly < 20 km and reduced to < 7 km at the sag center, corresponding to the crustal stretching factor of 1.6 to 5. It should be noted that the easternmost portion of the BYS extended to the East Baiyun uplift and was less stretched. The crustal stretching matches better the shape of the LWS, with crustal thickness of 16 to < 8 km. South of the COB in the deepsea basin the crust thins rapidly from 10 to < 7 km, corresponding to the stretching factor of 3.2 to > 5, respectively.
The area west of the LWS has a crustal stretching factor βc > 2, lower than the sags but higher than other areas. This indicates that crustal thinning occurred along the Baiyun Moho Nose, suggesting a good correlation of the crustal stretching with the mantle upwelling.
The newly acquired ~1 500 km long-cable MCS profiles controlled an NW-SE elongated strip of approximately 150 km wide and 260 km long across the slope to the deepsea basin in the northern SCS. These profiles are of good penetration that the Moho interface is recognizable in ~70% length of the profiles. This provides an exceptional opportunity for studying the details of crustal structure and extension in the area of deepwater sedimentary basins and continentocean transition in a passive margin.
Seismostratigraphic interpretation and time-depth conversion were conducted for these lines. Because the lines cross the Baiyun sag where the sediment thickness is very large, the cubic or quadratic polynomial function that commonly used in industry for the time-depth conversion would introduce a false velocity inversion or an overestimation of velocity at depth greater than 4 or 5 km. Through experiments, a power function D = atb + c was used in the time-depth conversion, which can achieve the same goodness of fit at shallow depth as those of a cubic or quadratic polynomial function, but give much more reasonable estimate at greater depths. In this way the depths of deep-seated interfaces presented in this article are more reliable and guaranteed from being under- or over-estimated by cubic or quadratic polynomial time-depth functions.
Results of this study confirmed some conclusions from previous studies of the same area, for example, the Moho depth shoals and the crustal thickness decreases stepwisely towards the deepsea basin, and the Moho upwells beneath the centers of the BYS and LWS mirroring the Tg variation. The Moho depth defined in this study confirmed the general pattern estimated by previous gravity inversion. The COB defined by a sharp thinning of the crust coincides well with the sharp change of topography from lower slope to deepsea plain, which is typical for a deep sea generated by seafloor spreading.
An important finding of this study is the along-strike variation of the crustal structure in the slope area of the northern South China Sea. The Baiyun Moho Nose was recognized that extends from the COB NW-ward until the northern edge of the BYS, about 170 km long and 100–50 km wide, with Moho depth < 20 km. This nose is bounded to the east, west, and north by belts of high Moho gradients indicative of crustal or even lithospheric faults. Both the BYS and LWS were developed along this nose, with a sharp decrease of the crustal thickness under the sags to 7–8 km, equal to the thickness of oceanic crust. The doming of Moho in the nose area must have affected the lithosphere rheology, which perhaps was the cause of the W-E segmentation of the crustal and geological structures along the slope of the northern South China Sea, and the cause of the strong crustal stretching in the BYS and LWS.
Another finding is the generally increasing volcanism towards the COB. Tertiary syn-tectonic volcanic activities widely spread from the southern flank of the BYS to the deepsea basin, while Quaternary volcanism ascended from the upper mantle straight up to the overlying sediments or even the seafloor.
The crustal stretching factor βc was calculated for the study area assuming the original crust thickness of 30 km. In the centers of the BYS and LWS, βc exceeds 5 and 4, respectively. In the deepsea basin of the SCS, βc exceeds 3.5.
The northern margin of the South China Sea is an excellent natural laboratory for studying passive margin kinematics and dynamics. Further studies are needed.
ACKNOWLEDGMENTS: This study was kindly funded by the National Natural Science Foundation of China (Nos. 40576027, 40876026), the National Basic Research Program of China (Nos. 2007CB41170405, 2009CB219401), the Knowledge Innovation Program of the South China Sea Institute of Oceanology, Chinese Academy of Sciences (No. LYQY200704) and technically supported by the CNOOC.| Briais, A., Patriat, P., Tapponnier, P., 1993. Updated Interpretation of Magnetic Anomalies and Seafloor Spreading Stages in the South China Sea: Implications for the Tertiary Tectonics of Southeast Asia. J. Geophys. Res., 98(B4): 6299–6328 doi: 10.1029/92JB02280 |
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Dengke Hu, Di Zhou, Xiangjie Wu, Min He, Xiong Pang, Yuwei Wang. Crustal Structure and Extension from Slope to Deepsea Basin in the Northern South China Sea. Journal of Earth Science, 2009, 20(1): 27-37. doi: 10.1007/s12583-009-0003-6
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