A geophysical study of the Manila Trench, Luzon, Philippines: 2. Fore arc basin structural and stratigraphic evolution

Abstract

The pattern of sediment distribution within the Manila Trench subduction zone west of Luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin. Multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic/hemipelagic sediment layer and oceanic basement in the northern part of the Manila Trench. These turbidites were transported from the uplifted collision zone of Taiwan, southward along the Manila Trench by gravity‐controlled processes. Similarly, thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the North Palawan block and the central Philippines. Sedimentation within the Manila Trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis. In contrast, the fore arc basin system of the Manila Trench receives sediment through submarine canyons incised into the West Luzon continental slope. The rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench. The size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench. A thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region. This generalization may apply to other convergent plate margins as well. The uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins. This variability results in the formation of several different types of fore arc morphology along the Manila Trench. The overall seismic stratigraphy of the West Luzon Trough fore arc basin is remarkably uniform, with seismic sequences continuous for up to 110 km along strike. The unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long‐term (approximately 10 Ma) uplift of the accretionary prism and relatively short‐term (approximately 1–2 Ma) variations in sediment supply to the fore arc basin, modulated by eustatic sea level changes.