FORE-ARC-BASIN CLOSURE AND ARC ACCRETION IN THE SUBMARINE SUTURE ZONESOUTH OF TAIWAN

The obliquely propagating Taiwan collision provides an active example of an intraoceanic are being accreted to a young rifted continental ma rgin. The actual accretion of the exotic are is taking place immediate ly south of Taiwan, in a complex area of rapid uplift and shortening b etween the emerging crest of the submarine accretionary prism and the extinct northernmost segment of the Luzon volcanic are. The northern p art of this region accommodates over half of the convergence between t he Philippine Sea and Eurasian plates, based on recent results of tria ngulation and Global Positioning System studies. Assuming that nearly all the plate convergence to the south, in the region of normal intrao ceanic subduction, is concentrated near the active trench, as is true in most subduction zones, this region of are accretion is a zone acros s which approximately 60% of the total plate convergence, amounting to about 50 mm/yr, is being actively transferred. This transfer of slip is presumably caused by the buoyant nature of continental crust that h as been subducting beneath the Taiwan orogen, This arcward transfer of plate convergence has strongly affected development of the suture bet ween the Luzon are and the continental margin, represented by the Taiw an mountain belt. Backthrusting of the accretionary prism in this regi on is accommodated on east-vergent thrust faults, which locally reach the surface and deform the entire forearc sequences, thereby building the Huatung Ridge, a distinct structural and bathymetric ridge east of the main accretionary prism (the Hengchun Ridge). The Huatung Ridge d ams orogenic sediment from the emergent collision in the Southern Long itudinal Trough, a suture basin that projects directly northward to th e Longitudinal Valley of eastern Taiwan, Growth strata in the Southern Longitudinal Trough document progressive uplift of the Huatung Ridge to the east, apparently along east-verging thrust systems. Seismic ref lection and sidescan sonar data south of about 23 degrees N provide no evidence of back-are thrusting along the eastern margin of the Luzon are, as has been hypothesized in order to transfer shortening to the P hilippine Sea plate. Neither do these data show clear evidence of west -vergent thrusting of the Luzon are over adjacent elements of the fore arc, in contrast to the very active thrusting documented onland to the north, along the Longitudinal Valley of eastern Taiwan. The arcward-v ergent structures in the region of are accretion have closed the North Luzon Trough, the major forearc basin. These structures have also bui lt the Huatung Ridge as a compressional ridge of orogenic strata that serves to broaden the accretionary prism toward the are (eastward) and , in so doing, have formed small collisional or suture basins and redi rected orogenic sedimentation patterns throughout this key area. Thus the arcward flank of the collision has evolved in a much more complica ted fashion than the relatively smooth progression followed by the wes tern, frontal slope of the submarine accretionary prism as it evolves northward to the fold-and-thrust belt exposed along strike in western Taiwan. This complexity on the arcward flank of the collision zone is likely a response of the collision complex to continued plate converge nce in the face of increasing resistance, to the north, to the subduct ion of continental crust.