Age and nature of orogenesis of the philippines

Abstract

From the study of regional geological data, it can be summarized that the orogenesis of the Philippines belongs to two geotectonic cycles which are more or less synchronous with the “Variscan” and “Alpine” geotectonic cycles. An Early Mesozoic sedimentation cycle terminated by an orogenic phase at the close of the Jurassic, about contemporaneous to the “Nevadan Phase” of North America, is regarded as a transitional phase. The Paleozoic geotectonic cycle, represented by the basement complex of the region was essentially characterized by: 1. (1) Initial development of a nearly north—south geosyncline, deposition of ophiolitic rocks, thence, tectonization of the basin with synorogenic emplacement of gabbro and possibly ultramafic rocks before or during Carboniferous Period. 2. (2) Transgression and deposition of flysch with limestone possibly from the Carboniferous to the Permian, followed by probable regression and local eruptive volcanism, then tectonization of the geosyncline into a platform with synorogenic intrusion of quartz diorite and granitic stocks, regional metamorphism of the ophiolites into basic amphibolite schist and partly into greenschists and of the lower part of the flysch sequence into mica-chlorite-amphibole schist to slate and other low grade greenschist facies, possibly at the close of the Paleozoic. 3. (3) Series of post-platform diastrophism accompanied by emplacement of post orogenic granitic stocks, granite rhyolite dikes, and later dacite and andesitic dikes. A period of non-deposition and truncation of the Paleozoic fold belts apparently marked the Early Triassic, and the “transitional phase” essentially started during Late Triassic with deposition of flysch frequented by conglomerate, thence, with intercalated spilitic lavas, chert and/or pyroclastics during Late Jurassic followed by severe folding of the geosynclinal pile. Magmatic intrusion is practically absent except for a quartz diorite stock emplaced in northern Palawan and dynamic and/or load-metamorphism effected generally oblique foliation on the closely folded Paleozoic flysch and concordant schistosity on the lower parts of the Triassic Sequence. The Late Mesozoic to Cenozoic geotectonic cycle essentially commenced during the Cretaceous Period through a series of sedimentation cycles with volcanic activity. An orogenic phase with accompanying intrusions, gradually transformed the “Mobile Belt” into a platform during Middle Miocene to Pliocene, while correspondingly, the “aseismic” southwestern region was by and large, reduced through series of concentric fracturing and gradual subsidence, although some of its portion has grown or developed series of fold belts by differential uplifts of the basement blocks and periodic shiftings of the Tertiary miogeosynclinal zones.