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The Sulu Sea is a marginal sea which is bordered in the northwest by the island of Palawan and in the southeast by the Sulu Archipelago. The northeast trending volcanic Cagayan Ridge divides the Sulu Sea into the Northwest Sulu basin and the Southeast Sulu basin.

A seismic line across the Northwest Sulu basin is shown. Between the Palawan shelf and an acoustic basement high of unknown nature lies a northwest-trending sedimentary basin. The thickness of the sediments is about 8,000 m (26,247 ft) in the center of the basin according to refraction seismic measurements and gravity modeling. A middle Miocene age is assumed for the distinct unconformity between 2.5 and 3 secs (two-way traveltime).A chaotic-diapiric zone about 25 km (15.5 mi) wide exists in the center of the basin which can be followed over a length of about 150 km (93 mi). The configuration and the internal structural style of the chaotic-diapiric zone have some similarities with diapiric structures of the oil-bearing province of West Sabah. The observed chaotic-diapiric zone within the Northwest Sulu basin is interpreted to represent a melange, formed in pre-middle Miocene time and which was reactivated by strike-slip motions of the North Palawan block along the Ulugan Bay fault in the late Neogene to Recent.

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