Three primary-arc chains of peridotite all convex toward the Pacific constitute the framework of the Philippines. These include a Mid-Mesozoic western arc and a late Cretaceous central and eastern arcs. The first two arcs outline the Sulu Sea Basin while the second and third embrace all the major basins and troughs including the Celebes Sea Basin. Except for the eastern Luzon segment on the north side of the great Philippine Fault, most of the eastern arc is submerged, presumably along the axis of the Philippine Deep, although traceable southward into eastern Celebes. Its presence is indicated, however, by geophysical data and by reverse secondary arcuate structures along the eastern rim of the Archipelago.

Within the main geosynclinal belt, the principal depressions are rimmed by hook-like Quaternary volcanic arcs which are mostly convex southward or southwestward. Several of these arcs exhibit a concentric outer belt of Tertiary acid to intermediate intrusive rocks. A prominent eastern volcanic arc roughly coincides with the Philippine Fault zone.

Contrary to earlier views, the eastern arc is not sigmoidal in nature. The supposed equivalent of its northern segment, the Taiwan arc, has the characteristics of a secondary arc often confused with that of a primary arc.




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