The Marinduque intra-arc basin, Philippines: basin genesis and in situ ophiolite development in a strike-slip setting

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 N-NW. A conspicuous volcanic ridge trends E-NE 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 N-S 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. -from Authors