Gravity anomalies and deep structural controls at the Sabah-Palawan margin, South China Sea

Abstract

The hydrocarbon-rich southeastern margin of the South China Sea is divided by NW-SE lineaments into a series of sharply contrasting segments distinguished by, among other things, abrupt changes in gravity patterns. The Sabah segment is bounded to the SW by the Tinjar or West Baram line and to the NE by the Balabac line at the southwest margin of the Sulu Sea. The most prominent gravitational feature of this segment is the strong freeair gravity low associated with the Sabah Trough which lies about 150 km offshore. Seismic reflection data suggest that loading by prograding sediments and gravity driven thrust sheets has depressed the extended continental crust of the South China Sea below its level of local isostatic compensation, producing the trough as a foreland basin in which sedimentation has failed to keep pace with subsidence. The load masses themselves, supported in part by the rigidity of the underlying crust and lithosphere, are above their levels of local compensation and deep Neogene sedimentary basins lie on the flanks of Bouguer and free-air gravity highs. Gravity values decrease across the Sabah coast so that the Crocker Ranges, including Mt Kinabalu, rest in rough isostatic equilibrium on presumably weaker lithosphere. The free-air gravity anomaly associated with the Sabah Trough is smaller than would have been predicted from the thickness of the water column, suggesting crustal thinning beneath the trough axis. This is not a characteristic of normal foreland basins and can therefore be assumed to predate basin formation. It can be concluded that the NE-SW trending belts of parallel gravity anomaly and geomorphology, of which the Sabah Trough is the most obvious, have been controlled by the pre-existing fabric of the crust and lithosphere of the South China Sea since they are discordant to the Palaeogene geological trends in Sabah. Reconstructions of the Tertiary history of the Sabah segment can be based on this assumption, which also suggests that sediments deposited in rift basins formed during the Palaeogene break-up of the South China margin were the source for much, if not all, of the hydrocarbon reserves of the area.

Seismic images of a collision zone offshore NW Sabah/Borneo

“…The shallow level of the Moho of about 22 km or even less implies that the crust is likely to have been already thin before subduction/collision took place as suggested by Milsom et al (1997). However, especially the gravity data require an additional thinning from beneath the Dangerous Grounds to beneath the Compressed Thrust Zone (Fig.…”

“…Moreover, Hinz et al (1989) suggested that the NW Borneo Trough is not underlain by a southward subducting oceanic crust but that the lower plate is comprised of continental crust similar in structural style to that of the Dangerous Grounds continental terrane. Under the assumption that the NW Borneo Trough is not a subduction trench, it may thus be either interpreted as a sediment-starved foreland trough (Hazebroek and Tan, 1993;Milsom et al, 1997) downfaulted by thrust front loading of the Dangerous Grounds continental terrane or as an asymmetric sedimentary sag basin that developed by uniform-sense simple shearing on the proto-South China Sea continental margin (Schlu¨ter et al, 1996).…”

The deep-water fold-and-thrust belt offshore NW Borneo: Gravity-driven versus basement-driven shortening

“…The principal features of the geological evolution of northern Borneo are summarized in Hinz et al (1989), Hutchison (1996aHutchison ( , 1996b, Milsom et al (1997), Petronas (1999), Hutchison et al (2000), Hall and Wilson (2000), Morley et al (2003), Morley andHall et al (2008). Northern and Central Borneo were built from the Mesozoic to Holocene, and they record a complex plate tectonic history involving oceanic and continental crust.…”

“…Following complete subduction of the proto-South China Sea oceanic crust, continental crust of the Dangerous Grounds region was partially subducted beneath the Crocker Formation basin of NW Borneo in the latest early Miocene before its buoyancy locked the system (James, 1984;Levell, 1987;Hazebroek and Tan, 1993;Hutchison, 1996aHutchison, , 1996bSandal, 1996;Hall, 1996;Milsom et al, 1997). Subsequently, northern Borneo experienced signifi cant compressional deformation, as documented onshore in folded sedimentary units of late early Miocene to middle Miocene ages (e.g., Sandal, 1996;Morley et al, 2003;Back et al, 2001Back et al, , 2005Back et al, , 2008, as well as offshore in folded and thrusted middle Miocene to present-day shelf and slope sequences (e.g., Levell, 1987;Hinz et al, 1989;Hazebroek and Tan, 1993;Morley et al, 2003;Ingram et al, 2004).…”

The structural evolution of folds in a deepwater fold and thrust belt – a case study from the Sabah continental margin offshore NW Borneo, SE Asia

“…With the ongoing spreading of the South China Sea, an older region of oceanic crust, the Proto-South China Sea (Hinz et al, 1989;Hall, 1996;Morley, 2002;Hall et al, 2008) was subducted beneath NW Borneo. In Early Miocene times, the continental crust of the Dangerous Grounds entered the subduction zone, before its buoyancy locked the system in the latest Early Miocene (James, 1984;Levell, 1987;Hinz et al, 1989;Hazebroek and Tan, 1993;Hutchison, 1996a, b;Sandal, 1996;Hall, 1996;Milsom et al, 1997). This collision history is reflected in significant compressional deformation documented onshore northern Borneo in folded sedimentary units of late Early Miocene age (e.g.…”