Far-field and gravity tectonics in Miocene basins of Sabah, Malaysia

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Abstract
At the end of the early Miocene, at c. 17 Ma, oceanic spreading ceased in the South China Sea Basin, as a series of collisions were initiated between continental blocks derived from the Asian mainland and the northwestern margins of Borneo and Palawan. In the NW Sabah Basin this tectonic event caused a major period of uplift and erosion which produced the so-called ‘Deep Regional Unconformity’. While these mainly compressional events were occurring at the southern margin of the South China Sea, the Sulu Sea was undergoing extension, with rifting in the northwest and oceanic spreading in the southeast. Recent seismic surveys in the Sabah offshore area show that the Deep Regional Unconformity can be traced through from the South China Sea into the Sulu Sea as a boundary between two contrasting depositional systems that are both represented in sedimentary sections onshore eastern Sabah. Eastern Sabah changed from an environment of deep marine clastic deposition in the Oligocene and early Miocene, to shallow marine and terrestrial sedimentation in the mid to late Miocene, with a major period of sedimentary mélange formation occurring at the time of the Deep Regional Unconformity. Inversion of the Miocene sequence in eastern Sabah appears to be limited to the edges of basement blocks, which were moved by far-field tectonic stresses. Post Middle-Miocene basin evolution in the onshore (Central Sabah Basin) and adjacent parts of the Sulu Sea offshore (Sandakan Basin) has been strongly influenced by mud diapirism and gravitational sagging of progradational sand-rich sediments into underlying muds and mélange units. These sequences appear to have been decoupled from later tectonic events by the great thickness of underlying muds and mélanges.