Abstract

The Marinduque basin is a marine intra-arc basin in the north-central Philippine volcanic arc system. A suite of marine geophysical observations show the basin as a whole to be rhombic in shape, with its long axis trending north-northwest. A conspicuous volcanic ridge trends east-northeast across the basin, dividing it into two smaller active depocenters. Symmetric linear magnetic anomalies, striking parallel to, and centered over, the central volcanic ridge indicate that it formed by extension in a north-south direction by a process analogous to sea-floor spreading. Geometric and temporal relations between the central volcanic ridge and several en echelon sets of north-trending, left-stepping faults indicate that sea-floor spreading occurred in the extensional stepover zone of a left-lateral, strike-slip fault system, and that the Marinduque basin as a whole is a composite pull-apart basin whose floor is in part composed of oceanic-type crust. The evolution of the central volcanic ridge presents an actualistic model for in situ development and emplacement of ophiolites in an island-arc setting.

The presently active, left-lateral Philippine fault zone truncated the eastern flank of the basin as well as the central volcanic ridge, following their initial formation. Shortening associated with a probable restraining bend in the Philippine fault has resulted in uplift of basin flank sediments, which have been thrust back toward the basin interior. The Philippine fault has also displaced fragments of the ridge and basin floor toward the north.

The overall history of the Marinduque basin further suggests that strike-slip processes may play an important role in the origin and growth of intra-arc basins. Such basins may ultimately become isolated in a forearc or backarc setting.

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