During the middle and late Miocene the northern continuation of the Luzon volcanic arc approached the Asian continental margin through subduction of oceanic crust on its western side. Collision deformation began in late Miocene time because middle Miocene marine fossils are present in the slate belt of Taiwan. South Taiwan, the crust-crust contact between the Philippine and Asia plates occurs at the large negative free-air gravity anomaly over the deformed accretionary wedge of the Manila Trench. The deformed sediment wedge has been uplifted to shallower depths toward Taiwan and forms part of the southernmost peninsula of Taiwan. The pattern of gravity anomalies suggests that the Luzon arc has underthrust the Central Range of Taiwan, to at least 40 km west of the longitud nal valley. Thus the west edge of the Philippine plate at Taiwan appears to be chisel shaped in cross section. Recent transcurrent faulting along the longitudinal valley is being superimposed on this collisional suture. The Ilan plain is a landward extension of the Okinawa Trough and forms the southwestern end of that extensional back-arc structure of the Ryukyu Island arc. The crust-crust contact between the Ryukyu arc and the Philippine plate occurs at the negative free-air anomaly of the accretionary wedge. On the west this contact abuts the northward end of the longitudinal valley fault. We speculate that the Luzon arc convergence zone in the region of Taiwan has been sealing southward and that concomitantly a new plate boundary may be developing on the east side of Luzon and progres ing northward.

Magnetic anomalies trending N70°E in the China basin west of Luzon are associated with the crest of an extinct spreading ridge which formed the deep part of the China basin by separating older but shallower crust. Magnetic anomalies in the westernmost Philippine basin suggest the existence of north-trending anomalies. The consistency of the skewness, the small amplitude factor, and to a lesser extent the bathymetric cross sections in the westernmost Philippine basin compared with those of the Central Basin Ridge region on the southeast support the view that the Gagua Ridge may be an extinct spreading center and that the adjacent topography was formed by seafloor-spreading.