Structure and Tectonic Evolution of the NW Sulu Sea Basin (SE Asia)

We discuss the tectonic structure, seismic stratigraphy and evolution of the NW Sulu Sea using reprocessed 2D reflection profiles. The NW Sulu Sea is located between the Palawan continental shelf and the Cagayan Ridge and represents the northern part of the Sulu Sea, a marginal sea resulting from Paleogene extension and subsequent Neogene contraction due to convergence between the Palawan and the Philippine blocks. The basin consists of six seismo-stratigraphic units overlying crystalline basement. Syn-orogenic depocenters contain calibrated Middle Miocene to, possibly, Lower Miocene units, while rift-related depocenters consist of uncalibrated but tentatively dated Paleogene to Lower Miocene units. Thickness and depth maps of the main units and bounding horizons differentiate the Piedra-Blanca and the Rasa domains, separated by the NW-Sulu-Break major tectonic structure. Fault-bounded rift-related depocenters are strongly segmented. We interpret that NW-SE and NE-SW trending zones accommodate shape and trend variations of these depocenters. We suggest that these zones may link rift segments, recording different extensional deformation. Miocene thrusting and folding in the Piedra-Blanca Domain and mudflow with associated gravitational structures in the Rasa Domain influenced the deposition of syn-orogenic units. Rift structures inherited from rift segmentation may have conditioned the style and distribution of contractional deformation during the subsequent incipient reactivation during contraction. In the context of SE Asia, our results support that the timing of rifting of the NW Sulu Sea overlaps with the opening of the South China Sea and the North Palawan margin, which may indicate a common geodynamic driving force triggering extension. The SE Asia region includes several basins partially floored by oceanic crust, frequently containing or bounded by volcanic island arcs. These basins are known marginal seas and, although it is critical to decipher the geodynamic evolution of the entire region, the structure and tectonic history of most of these marginal seas are still insufficiently known. The Sulu marginal sea lies between the Sulu Archipelago and the Palawan, Mindanao, Negros and Panay islands. The Cagayan Ridge separates the NW and SE Sulu seas, which show a contrasting seafloor morphology, resulting from Paleogene extension and subsequent Neogene contraction. In this work, we study the structure and tectonic evolution of the NW Sulu Sea based on the re-processing and interpretation of seismic reflection profiles acquired in 1982 and 1987. We analyzed and calibrated the seismo-stratigraphy of the sediment infill, determined the main tectonic and gravitational structures and the geometry and spatial distribution of depocenters. Our results show for the first time the basin-wide overall structure and constrain tectonic models, describing the opening and, particularly, the contractional deformation of the NW Sulu Sea. This provides further information to understand the formation of the Sulu Sea and the evolution of convergent systems in SE Asia. A multi-stage tectonic evolution shaped the Piedra-Blanca and Rasa structural domains separated by the gentle NW-Sulu-Break structure NW-SE and NE-SW trending accommodation zones define the geometry and location of the rift-related oldest basin depocenters Thrusts and associated folds deformed the Piedra-Blanca and mudflows affected the Rasa domain during the contractional basin reactivation