The tectonics of northward propagating subduction along eastern Luzon, Philippine Islands

Marine multichannel seismic reflection and other geophysical data acquired along the eastern margin of Luzon indicate that subduction of the Philippine Sea plate is occurring south of approximately 18°N. The thick sediments of the East Luzon Trough are deformed by folding and thrust faulting near the base of the continental slope, and the acoustic basement underlying the sediment section can be traced as it dips landward beneath the east Luzon margin. Earthquake activity in this region indicates that underthrusting is presently occurring. The intensity of observed deformation decreases to the north between 16°N and 18°N, and north of approximately 18°N no recent compressive deformation can be recognized. These observations indicate that the locus of subduction is propagating northward along the east Luzon margin. A narrow linear negative free air gravity anomaly is associated with the zone of recent deformation and subduction. A zone of closely spaced faults disrupts the entire sediment section near 15.25°N and is coincident with a linear east-west trending belt of shallow earthquake epicenters. This zone may represent the surface expression of an east trending transform fault that connects the Philippine Trench to the newly developing east Luzon subduction zone. From approximately 17°N to 19°N, the Sierra Madre Basin, which contains up to 4.5 km of sediment, lies between the East Luzon Trough and the Luzon coast. NNE striking basement ridges can be traced from beneath the Sierra Madre Basin into the deep ocean environment of the adjacent West Philippine Basin. The East Luzon Trough, the Sierra Madre Basin, and the intervening bathymetric ridge, the Isabella Ridge, may represent the trench, forearc basin, and subduction complex, respectively, of a relict subduction system which was active during Oligocene time along eastern Luzon. Arcuate NNE to NNW trending bathymetric troughs divide the North Luzon Ridge into a series of discrete blocks. Regional seismicity and focal mechanism data may indicate that these troughs represent the traces of presently active strike slip and thrust faults. This faulting within the North Luzon Ridge may absorb the relative convergence between Luzon and the Philippine Sea plate which, to the south, is taken up along the active Philippine Trench and eastern Luzon subduction zones.