Formation and contractional reactivation of the NW Sulu Sea (SE Asia)

Located in SE Asia in between the Palawan and the Philippine islands, the lozenge-shaped Sulu Sea corresponds to a marginal sea that displays a complex seafloor morphology. The NE-SW trending Cagayan Ridge separates a southeastern deep-water domain, which is bounded by the Sulu Trench towards the east, from a shallower and narrower northwestern domain. Interpretations of low-resolution 2D streamer datasets, ODP Leg 124 drilling results, magnetic, geochemical, and geochronological studies, and gravity inversion results led to distinctive tectonic models, with contrasting basin formation mechanisms, and ages of opening and subsequent contractional reactivation. The debates remain because the structure of most of the Sulu Sea and its along-strike structural variability remain underexplored to date.

We focus on this work on the first detailed analysis of the structure and seismo-stratigraphy of the NW Sulu Sea. Based on the reprocessing, calibration of the Silangan-1 exploration borehole, and interpretation of > 5384 km of 2D seismic data along 19 regional profiles of an irregular grid that covers the whole NW Sulu Sea, we identify, map and interpret the seismo-stratigraphic horizons and units, major structures, and rift-related and syn-orogenic depocenters and structural domains. We define six seismo-stratigraphic units in the NW Sulu Sea, consisting of Quaternary to Paleogene sediments, which developed during an early phase of Paleogene to early Miocene extension, a following early to Middle Miocene phase of contraction, and a late Miocene to Quaternary stage of relative tectonic quiescence. While transpressional faults core uplifted basement areas, strike-slip, high-angle and low-angle oblique extensional faults crosscut continental crystalline basement of variable thickness and bound pull-apart basins, half-grabens and sags respectively. The distribution and trend of rift-related depocenters describe a strong structural segmentation and vary along NW-SE and NE-SW oriented zones. Thrust-cored anticlines, inverted transtensional and transpressional faults and mud diapirs deform the sediment pile and control the geometry of syn-orogenic depocenters distinctively across the NW Sulu Sea.

Normal and oblique trending sets of faults controlled the extension and compartmentalized the NW Sulu Sea. Subsequent contractional reactivation differentiated NE and SW basement and sedimentary domains, separated by the NW Sulu Break Elevation. These domains show a contrasting overall architecture, basement thickness, contractional structures and distribution of rift-related and syn-orogenic depocenters. Rift segmentation, and particularly, basement thickness variations, may have conditioned the type and distribution of contractional deformation.