Features resulting from the interplay of arc mag- matism, ophiolite accretion, ocean basin closure and other subse- quent tectonic processes are preserved in the Philippine island arc system. Subduction of ocean floor along the trenches surrounding the Philippines is a major factor in shaping the geologic history of this island arc system. Stress-strain relationships, as manifest in both the regional and local setting of the archipelago, are derived from the interaction of at least four major plates: Sundaland, Phil- ippine Mobile Belt, Philippine Sea and, to a certain extent, the Indo-Australian plate. Collision zones in this island arc system are characterized by the involvement of oceanic bathymetric highs (seamounts, spreading ridge, submerged continental fragment). A major strike-slip fault, the Philippine Fault Zone, with compres- sional and extensional components, traverses the whole archipel- ago where all excess stress not accommodated by the surrounding trenches is taken up. Tholeiitic through adakitic to calc-alkaline rock suites characterize the different magmatic arcs. Exposed oce- anic lithospheric fragments exhibit transitional mid-ocean ridge, back arc basin to island arc geochemical characteristics. The observed crustal thickness in the Philippines resulted from com- bined magmatic (volcanism) and amagmatic (ophiolite accretion) processes, with the former being the dominant factor.
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