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It is becoming increasingly clear that the West Philippine Basin displays abnormally complex tectonism, resulting from the effects of several types and ages of deformation. Reconstruction of the basin history requires that these deformations and their effects be separated and understood individually. One of the logical ways to approach this problem is to investigate those areas where the deformational zones intersect or overlap. The northwest corner of the basin is one such area, where the Central Basin Fault appears to intersect the Luzon arc and is in turn covered by a large sediment apron which appears to postdate a period of subduction occurring along the eastern flank of the arc (Karig, 1973). Although no deformation of the apron sediments is observed, the insular slope to the south is the site of a recent subduction zone which may be extending itself northward (Fitch, 1972; Karig, 1973). The recognition of the region's tectonic importance led to a rather intensive survey program during the Western Pacific geotraverse of the NOAA Ship Oceanographer. Seismic reflection, magnetic, and gravity profiles were collected along satellite-controlled track lines (Figure 1). Also available since the earlier reconnaisance study (Karig, 1973) are satellite-navigated profiles from V-21 and V-24 cruises of LDGO, Tasaday 5 of SIO and the DSDP Leg 31. The previously available data were incorporated and reanalyzed after being adjusted to the satellite-controlled tracklines. The study began as an attempt to locate the northwestern end of the Central Basin Fault and to investigate its behavior at the basin margin. A major obstacle confronting such an endeavor is that the delineation of basement ridges in marginal basins is nearly impossible without detailed surveys, because the usual track spacing is several times the topographic wavelength. However, in the turbidite-fed apron adjacent to Luzon, the sediments were ponded and dammed between ridges, and because these ponds lie at different depths, the ridge correlation becomes relatively easy. Therefore, rather reliable contours were drawn at small intervals over the apron, but no new contouring was attempted elsewhere. These new data also permitted the better delineation of channels feeding the sediment apron, which in turn had significant implications for the late Tertiary tectonism of northern Luzon.

Structure and Sediment Distribution in the Northwest Corner of the West Philippine Basin