Geological evolution of the Southern Philippines

The Philippines is a collage of geologic terranes of disparate origin, including at least one continental fragment. It has been characterized as an arc aggregate by Karig (1975). Tectonically, a two-fold division has long been recognised, a NNW-SSE trending sigmoid mobile (seismic) belt being separated from a stable (aseismic) region around the Sulu Sea in the southwest (Gervasio, 1966). The mobile belt is margined, both to east and west, by discontinuous subduction zones and it seems that a similar situation has existed through much of Cenozoic time, the locus of subduction expanding, contracting and "flipping" across the zone. In the southern Philippines an ancient arc crust has been dated as Cretaceous at Cebu and evidently extends at least into Bohol and central Mindanao. By late Eocene times it is thought that an east-dipping subduction zone existed on the west side of the ancestral Philippine islands, with a magmatic arc extending from east Panay through southwest Negros, probably into the Cotabato area of southern Mindanao. Emplacement of an ophiolite in western Panay is thought to have taken place around Late Oligocene times and may have been related to impingement of the Palawan-Busuanga microcontinent on the western side of the Panay-Negros arc. It is considered that in the southern Philippines the focus of subduction then flipped to the east side where westward subduction of oceanic crust occurred beneath the central M\u27indanao-Sangihe arc (Cardwell el al.. 1980). NW-SE trending transcurrent faulting in Negros and Cotabato (southern Mindanao) may be related to the opening of the Sulu and Celebes seas (Oligocene to Miocene). These movements apparently caused the Cotabato block to be displaced southeastwards, along the "Mindanao lineament" (Gervasio, 1966) across the axis of the central Mindanao arc. Recurvature of the Sarangani Peninsula may be part of the same process. Continuing westward subduction beneath the Sangihe-central Mindanao arc in the Pliocene, however, produced a line of volcanoes, with a median rift valley superimposed upon the NW-SE Cotabato trend. East Mindanao is a northward extension of the Talaud arc of Indonesia which lies above a subduction zone opposed and in the process ofcollision with that of Sangihe (Cardwell el al., 1980). It was concluded by these workers that in Mindanao the collision was substantially complete by the Late Miocene. The obliquity of the collision has led to the generation of the Philippine (transcurrent) fault whilst most of the subsequent westward translation of the Philippine Plate has been taken up in the Pliocene-Recent Philippine Trench. The present writer believes (Burton, 1983), however, that part of the continuing post-collision plate convergence has been accommodated by rejuvenation of the Negros Trench and development of the Cotabato Trench on the west side of the mobile zone, evidently with related volcanism in Negros and possibly also in Zamboanga (west Mindanao)