Oligocene magmatic accretion and transtensional tectonics in the Central Basin Fault rift of the West Philippine back-arc basin: New insights from high-resolution multichannel seismic data

High-resolution multichannel seismic transects that span from oceanic crust domains, and the Central Basin Fault rift to the Kyushu-Palau Ridge are utilized to investigate the dynamic mechanism of the Oligocene tectonic evolution in the West Philippine Basin based on an analysis of sismostratigraphic sequences and the geometries of acoustic substrates. In the oceanic domains, the physiography is characterized by ridges and troughs, and the pelagic stratigraphic succession with transparent facies covers the irregular top of the basement. Relatively homogeneous intracrustal reflections probably correspond to an area of abundant magma supply during the major spreading period. However, a widespread discrete, layered and high-amplitude intracrustal reflector is present in the Central Basin Fault rift that can be interpreted as an area of weaker and unstable magma supply after the major spreading period. The first group of reflectors shows typical low-angle fault shapes; the second and third groups of layered intracrustal reflectors are characterized by spatially limited high-amplitude seismic reflections. Inclined and mounded features of the intracrustal reflectors are interpreted as the result of tectonic processes, suggesting the presence of later magmatic accretion and transtensional tectonics within the acoustic substrate. Additionally, in the Kyushu-Palau Ridge, the anticline that appears within the stratigraphic infills of the basins serves as evidence of transtensional faulting. Overall, we propose that the Oligocene tectonic evolution of the West Philippine Basin occurred during a later transcurrent period due to Oligocene magmatic accretion followed by transtensional tectonics. The changes in the magmatic supply rate and tectonic processes of the Central Basin Fault rift may have originated from the compressive process within the Kyushu-Palau Ridge.