Abstract
Analyses of the compiled magnetic, bathymetric and seismic reflection data show that the West Philippine Basin was formed by seafloor spreading from the Central Basin Spreading Center in two distinctly different spreading phases, before and after subduction was initiated along the Palau-Kyushu trend at about 45 Ma B.P. These episodes are recognized by differences in the strike of magnetic lineations, spreading rates, density and strike of fracture zones, and basement relief. Relative to the present orientation of magnetic and fracture zone trends, it is found that from 60-45 Ma B.P. the West Philippine Basin crust was created by NE-SW symmetric spreading about the proto-Central Basin Spreading Center at a half-rate of 44 mm/a. This episode can be identified in the regions just south of the Daito Ridge Complex, east of the Philippine Trench and in a small basin east of the Gagua Ridge. Shortly after 45 Ma B.P., the spreading direction changed to a N-S orientation with a reconfiguration of the Central Basin Spreading Center into numerous short E-W segments offset by closely-spaced N-S transform faults. Spreading slowed to a half-rate of 18 mm/a and ceased at 35 Ma B.P. This later phase is recognized across the entire E-W extent of the central West Philippine Basin and in a small area between the Gagua Ridge and Taiwan. Considering the distance between surrounding trench axes and the Central Basin Spreading Center, the duration of spreading, the spreading rates and the regional tectonic evolution, a subduction-related backarc spreading origin of the West Philippine Basin is unlikely. Thus, it is concluded that the West Philippine Basin originated at 45 Ma B.P. by trapping of normal ocean crust west of the initial subduction along the Palau-Kyushu trend. The 45-35 Ma B.P. period represents the dying phase of spreading on the Central Basin Spreading Center following isolation of the West Philippine Basin from the plate driving forces of the Pacific.
Lee, Chao-Shin (1983). Origin and evolution of the West Philippine basin. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -569456.