The Manila Trench and West Luzon Trough—II. Gravity and magnetics measurements

Abstract

The Manila Trench and West Luzon Trough are two long submarine features which lie on the continental side of the Philippine Island Arc, west of central Luzon. A free-air gravity anomaly of − 145 mgal is centered over the West Luzon Trough. A smaller anomaly of − 80 mgal is centered over the Manila Trench. Two crustal models of the trench and trough based on gravimetric, bathymetric, and seismic profiler measurements are presented. These two models represent the supposed extremes of many hypothetical models. Both models require an abrupt change in crustal thickness of about 15 km just shoreward of the axis of the trench. The sediments and structure of the trough as inferred from seismic profiler measurements cannot alone account for the observed gravity anomaly over the trough.Magnetic anomalies cannot be correlated over appreciable distances and the existence of any magnetic anomaly trends is questionable. An abrupt change in the character of the magnetic anomalies occurs near the trench axis. The anomalies are much smaller east of the trench than west of it. There is no magnetic expression of the ridge that serves as the seaward boundary of the West Luzon Trough and this suggests this ridge is not of basaltic composition. The Manila Trench and West Luzon Trough and their associated gravity and magnetic anomalies do not fit into any simple picture of ocean-floor spreading.