The Manila Trench and West Luzon Trough—I. Bathymetry and sediment distribution

Abstract

Data from bathymetric (PDR) and seismic-reflection (profiler) traverses made across the island margin west of Luzon, Philippines, are here reported. A new contour chart of sea floor topography is presented for the region.A shallow arcuate depression approximately 2700 fms (4940 m) deep, herein called the Manila Trench, extends as a narrow, nearly continuous furrow sub-paralleling the west coast of Luzon from 16° 40′N south to about 13°N. The Manila Trench is divided into two deeps at about 15° 50′N, 119° 15′E by a submarine ridge trending southwest from the coast near the Lingayen Gulf. The north end of the trench is bounded by a submarine ridge trending east-west from the coast along latitude 16° 50′N. The south end of the trench extends southeastward as a major re-entrant into Mindoro Straits, west of Mindoro Island.A sediment-filled trough, herein called the West Luzon Trough, forms a terrace and trough approximately midway between the floor of the Manila Trench and the continental shelf. The trough is bounded on the north by the southwest-trending ridge from the Lingayen Gulf vicinity and on the south by a ridge trending northwest-to-north from Lubang Island. The latter ridge forms the seaward border of the West Luzon Trough.A group of seismic-reflection profiles illustrates the variation in sea floor topography and sub-bottom structure along the length of the trench and the trough. With the exception of the West Luzon Trough, no detectable amount of sediment is present eastward of the trench floor. A fairly uniform thickness of acoustically transparent sediment overlies a rough oceanic basement layer in the adjacent South China Sea basin. These sediments are continuous along the west wall of the trench and end abruptly as a perched ledge just west of the trench floor. The steep slopes of the perched ledges and evidence of fairly recent step-faulting along the west wall of the trench indicate post-depositional subsidence or extension of the sea floor near the axis of the trench.