Tertiary tectonic evolution of the NW Sabah continental margin

The NW Sabah continental margin can be sub-divided into 6 tectono-stratigraphic provinces on the basis of differences in structural styles and sedimentation histories: (1) Rajang Group Fold-Thrust Belt, (2) Inboard Belt, (3) Baram Delta, (4) Outboard Belt, (5) NW Sabah Trough and (6) NW Sabah Platform. The Tertiary sedimentary sequence was deposited during two main phases of basin development: (1) A pre-early Middle Miocene phase of generally deep-marine clastic sedimentation. (2) A post-early Middle Miocene phase of clastic shelf/slope deposition, which prograded northwestward over the underlying sediment wedge and is separated from the latter by a major regional unconformity. Evidence for Palaeogene subduction of South China Sea oceanic crust beneath NW Sabah is incomplete. The subduction hypothesis (Haile, 1973Hamilton, 1979) is based on the following main elements: (1) The Rajang Group Fold-Thrust Belt of turbidites and associated ophiolites, interpreted to represent an accretionary prism. (2) The sub-linear NW Sabah Trough, interpreted to represent the sea-bed expression of a subduction trench. A major missing element in this hypothesis is a volcanic arc of the correct age. The NW Sabah Trough can be seen on seismic data to represent the downfaulted SE margin of the NW Sabah Platform. An additional important element in the morphology of the Trough is the Baram Delta toe thrust zone, bounding the Trough to the SE, which was emplaced post Middle Miocene. Furthermore, the Trough terminates abruptly against the Luconia Block. These factors suggest that the present NW Sabah Trough is a relatively young feature, and that if an older, Palaeogene, trench is present, it would occur landward of, and with a different orientation to the NW Sabah Trough. The Palaeogene trench would no longer have a surface expression. Hamilton\u27s (1979) geodynamic interpretation is supported with the proposed modification of a Palaeogene subduction system broadly parallel to the structural strike of the Rajang Group Fold-Thrust Belt. The Inboard Belt wrench tectonics trend parallel to the postulated trench (cf. the Semangko fault of the Sumatra subduction system) as a result of oblique subduction. Counter-clockwise rotation of Borneo from Eocene to Middle Miocene resulted in increasingly oblique subduction. The Baram Delta sediment prism masks the Palaeogene trench. The following three-stage model for the tectonic evolution of the NW Sabah continental margin is proposed: (1) Late Eocene to early Middle Miocene oblique subduction of South China Sea oceanic crust beneath NW Sabah with deposition and subsequent imbrication of deep-marine sediments into an accretionary prism. (2) Diachronous collision of the South China Sea attenuated continental crust (NW Sabah Platform) with Sabah, and associated cessation of ocean-floor spreading in the early Middle Miocene, led to regional uplift and erosion of the accretionary prism, resulting in a major regional unconfornlity (the Deep Regional Unconformity). This was followed by northwestward progradation of clastic sediments over the Inboard Belt from Middle Miocene to early Late Miocene. (3) Resumption of convergent forces between Borneo and the NW Sabah Platform in middle Late Miocene was accompanied by major tectonic activity. The Inboard Belt was subjected to strong compressional deformation associated with major N-S wrench zones, resulting in the formation of a localised to semi-regional unconformity (the Shallow Regional Unconformity). Two new depocentres were formed seaward of the Inboard Belt: the Baram Delta and the Outboard Belt. A thick prograding clastic wedge built out towards NW in both depocentres, whilst the Inboard Belt was continuously eroded. A Late Pliocene/Pleistocene phase of locally developed deformation affected the Inboard Belt, Outboard Belt and Baram Delta