Extended Abstract

Subduction zones are sites of plate consumption which exist in response to the formation of new oceanic crusts in spreading ridges. This whole cycle of formation and destruction of oceanic crusts, known as the Wilson Cycle, has made it possible for the earth to retain its size in its 4½ billion years of existence. Interestingly, a look in the immediate vicinity of the Philippine island arc system shows that there are no active spreading centers in its surrounding. Nevertheless, at present, the process and effects of subduction are very much apparent in the archipelago. This shows that the subduction occurring around the Philippines is a result of a more regional geologic process rather than a simple response to a nearby accretion of oceanic crust (e.g. Simons et al. 1999; Yu et al. 1999; Rangin et al. 1999a; 1999b). A possible consequence of this is the collision and subduction of oceanic bathymetric highs (e.g. extinct spreading centers, seamounts, rifted microcontinental block) which actually characterize Philippine geology (e.g. Rangin 1991; Pubellier et al. 1999). This is more apparent in the Northern and Central Philippine region where evidence of cusping, microblock rotation, ophiolite emplacement and arc polarity reversal are present (e.g. McCabe et al., 1985; Yumul et al., 2000).

The subduction polarity reversal from the west-dipping proto-East Luzon Trough to the east-dipping Manila Trench has been attributed to the arrival of the Benham Rise, an oceanic bathymetric high, into the proto-East Luzon Trough. This supposedly terminated the subduction of the West Philippine Sea plate along the proto-East Luzon Trough and resulted to arc flipping. We present here a model that shows that available geological data suggest that the Early Miocene indentation and subsequent collision of the Palawan microcontinental block with the Philippine mobile belt led to the counterclockwise rotation of Luzon, onramping it towards the South China Sea crust and initiating arc flipping (see Figure 1 and Figure 2). This was made possible by the conversion of a strike-slip fault, bounding the western side of Luzon, into a thrust fault. The thrust fault ultimately evolved into a subduction zone - the Manila Trench, where the South China Sea plate subducted. The model postulates a late Early Miocene subduction polarity reversal from east to west in Northern Philippines (Bellon and Yumul, 2000). This would also correspond to the age of the Manila Trench initiation. Available GPS data shows that the continued subduction of the South China Sea crust along the Manila Trench is brought about by the NNW motion of the Philippine archipelago synchronous with the motion of the West Philippine Sea plate. Available information does not support the notion that the Manila Trench is a dying subduction zone.