Marginal basin evolution: the southern South China Sea
Introduction
Understanding of the southern South China Sea plate margins is facilitated by recognition that the regional Mid-Miocene unconformity (MMU; end of rifting ∼19–21 Ma, beginning of post-rift drape ∼16 Ma) is the break-up unconformity marking the transition from active rifting of the Sundaland margin to sea-floor spreading. Eastwards of the West Baram Line, the major Deep Regional unconformity (DRU) separates onland outcrops of the Sabah mélanges from the overlying Tanjong Formation (Hutchison, Bergman, Swauger, & Graves, 2000). Rice-Oxley (1991) summarised the view that the DRU extended offshore only some 40 km from the Sabah coastline. As the oil companies ventured into deeper water, it became clear that this major unconformity can be traced as far as the Northwest Borneo Trough. The similar age MMU can be traced over the whole southern South China Sea. However it may be premature to conclude that the DRU and MMU should be equated since their areas of occurrence are separated by the fundamental Northwest Borneo Trough, on either side of which the tectonic regimes are different.
The vast array of oil company regional seismic lines could, if released into the public domain, result in a well constrained model of the evolution of the southern part of the South China Sea marginal basin. This paper presents my analysis based on the limited available data.
In northern South China Sea the break-up unconformity occurred in the Upper Oligocene (end of rifting ∼28–30 Ma, beginning of post-rift drape ∼24 Ma) (Clift, Lin, & ODP Leg 184 Scientific Party, 2001). In this paper, I explain this timing difference and describe the nature of the Tertiary plate margins of the southern South China Sea pre- and post-unconformity. In addition, the basic structural styles of the southern South China Sea are presented together with my interpretations.
Our present understanding of the South China Sea marginal basin begins with the compilation of Hamilton (1979), significant for insightful understanding of a heretofore little known region. His analysis resulted from unprecedented access to unpublished oil company data. Taylor and Hayes, 1980, Taylor and Hayes, 1983 made this sea their major research interest and their main conclusions have stood the test of time. They identified and documented a sequence of magnetic anomalies 11 through 5d in the zone of sea-floor spreading. A later reassessment by Briais et al., 1989, Briais et al., 1993, supported the earlier interpretation that the pattern extends from 11 to 5c (32–16 Ma) (Lower Oligocene to early Middle Miocene). However, the identification of these anomalies has yet to be confirmed by direct drilling (Hutchison, 1996a). Their identified pattern is illustrated in Fig. 1.
The region, lying between the zone of sea-floor spreading and the Sunda continental shelf, is composed of strongly attenuated continental crust. Huchon, Nguyen, and Chamot-Rooke (2001) have presented an analysis of the propagation of continental break-up in the south-western prong of the South China Sea zone of sea-floor spreading. It has become common to refer to the zone of sea-floor spreading as a marginal basin (Hutchison, 1989) as if it were a separate entity. It is genetically related to the enveloping 170–330 km wide zone of strongly attenuated continental crust, known as Dangerous Grounds. Both these zones comprise the South China Sea marginal basin. Adjacent to the Dangerous Grounds, the end of rifting and onset of sea-floor spreading (∼anomaly 6, see Fig. 1) marks the break-up unconformity. It is known in the Dangerous Grounds as the MMU because there is a hiatus of ∼3–5 Ma and the oldest post-rift draping strata are early Middle Miocene (∼16 Ma). The MMU represents a major change in the tectonics and sedimentation of the southern South China Sea (Fig. 2).
Two distinctly different plate margins are separated by the West Baram Line (Fig. 3), now inactive and largely obscured by a cover of younger strata. Both plate margins ceased activity at the time of the MMU but the sea-floor topography preserves many relict features from their activity. The margin southeast of the Northwest Borneo Trough (Fig. 3) was a compressive margin before becoming a collision zone. The rest of the southern South China Sea was a passive continental rifted margin before the break-up unconformity (MMU).
Section snippets
The passive margin
The geological history of the southern part of the South China Sea marginal basin is divided by the MMU into an earlier active rift and a post-rift episode.
Tectonic interpretation of the Mid-Miocene unconformity
An unconformity is a geological surface that represents a hiatus in the stratigraphic record. The hiatus is unlikely to be universally of the same duration, as in the seismic section of Fig. 11. At the Tatau ‘Horst’ outcrop, the virtual hiatus is of the order of ∼22 Ma whereas only 20 km to the NW it is only ∼5 Ma; the difference resulting from regional tectonic variation prior to the post unconformity drape. Single unconformity outcrops such as Fig. 11 may result in grossly erroneous estimates
The Mid-Miocene unconformity in Sarawak
Prior to the MMU, the sedimentary formations were deposited on the coastal zone of Sarawak with a coastline directed NNW from Bintulu and water deepening towards the ENE towards bathyal conditions beyond what has become known as the West Baram Line (Fig. 10A). Land lay to the west, in what is known as the Penian High. The sediment provenance was Sundaland on the west with fluvial systems directed ENE. The sedimentary sequence offshore, interpreted as lower coastal plain to holomarine inner
The now inactive convergent margin
Hamilton (1979) was the first to suggest that the 2 km deep Northwest Borneo Trough, also known as the Sabah Trough and Palawan Trough, represents a trench that became inactive in the early Miocene when spreading in the marginal basin ceased (Fig. 12). Many geologists (e.g. Hazebroek & Tan, 1993) have criticised this interpretation. The normal progression from abyssal plain, through continental rise, to continental shelf is absent from the region northeast of the West Baram Line. The expected
A tale of two rivers
Only two rivers have built out major deltas into the southern South China Sea, and they are of contrasting character (Fig. 15). Though their measurements are not specific to any one river, Hall and Nichols (2002) recalculated the sediments isopachs of Hamilton (1974) to show that 1,560,300 km3 of Neogene sediment have been deposited offshore Sarawak, Brunei and Sabah. From their study they concluded that at least 6 km of crust has been removed by erosion from Borneo during the Neogene. Present
Conclusions
The South China Sea resulted from rifting and attenuation of the continental crust of Southeast Asia from the Palaeocene to the early Middle Miocene. The northern margin had been occupied by a Jurassic–Cretaceous (Yenshanian) active margin dominated by granites and volcanic rocks. Sea-floor spreading began in the late Lower Oligocene in the north, delayed until the Lower Miocene in the south. The stretching resulted in a passive margin composed of the Sunda Shelf, narrow continental slope, and
Acknowledgements
This paper resulted from an invited talk to Petronas Carigali staff in the Twin-Towers. I am also very grateful to Robert Hall, Christopher Morley and Robert Tate for their help and discussions. I gratefully acknowledge the opportunities to have viewed many seismic records of the South China Sea Institute of Oceanology, Sarawak and Sabah Shell, and Petronas.
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