The Manila Trench and West Luzon Trough—III. Seismic-refraction measurements

Abstract

Data from 13 seismic-refraction profiles recorded west of central and northern Luzon in the South China Sea are presented in two schematic structure sections. The eastern edge of the South China Sea Basin has a velocity-structure similar to that of normal oceanic crust, except that the average thickness of the main crustal layer (layer 3) is only about half normal thickness, and the M-discontinuity is shallower than normal by about 2–3 km. The intermediate level West Luzon Trough off central Luzon contains approximately 4 km of sediment with velocities of 2·1 and 2·6 km/sec overlaying a 4·4 km/sec basement layer which presumably constitutes the outer ridge of the trough. Measurements made west of northern Luzon indicate that in physiography and velocity-structure it is similar to the region off central Luzon, except there is no topographic trench along the foot of the island margin. Instead there is a subtle depression in the sea floor with sediments much thicker than are found off central Luzon. This indicates that if a trench is forming in the area, it is being filled contemporaneously with sediments transported from the north. There is an intermediate level sediment-filled depression off North Luzon that is not as well developed as its southern counterpart, but which nevertheless constitutes a major linear feature. The outer ridge consists mainly of material with velocity about 3·6 km/sec and is interpreted as sedimentary. The outer ridge follows the western convex side of the north and central Luzon arc and is broken only by a promontory complex off the coast near the Lingayen Gulf. The ridge may represent the outer non-volcanic component of a double island-arc system, similar to the outer non-volcanic ranges situated along the eastern convex side of the Mindanao Arc.