Forearc-basin closure and arc accretion in the submarine suture zone south of Taiwan

Abstract

The obliquely propagating Taiwan collision provides an active example of an intraoceanic arc being accreted to a young rifted continental margin. The actual accretion of the exotic arc is taking place immediately south of Taiwan, in a complex area of rapid uplift and shortening between the emerging crest of the submarine accretionary prism and the extinct northernmost segment of the Luzon volcanic arc. The northern part of this region accommodates over half of the convergence between the Philippine Sea and Eurasian plates, based on recent results of triangulation and Global Positioning System studies. Assuming that nearly all the plate convergence to the south, in the region of normal intraoceanic subduction, is concentrated near the active trench, as is true in most subduction zones, this region of arc accretion is a zone across 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 has been subducting beneath the Taiwan orogen. This arcward transfer of plate convergence has strongly affected development of the suture between the Luzon arc and the continental margin, represented by the Taiwan mountain belt. Backthrusting of the accretionary prism in this region 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 dams orogenic sediment from the emergent collision in the Southern Longitudinal Trough, a suture basin that projects directly northward to the 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 reflection and sidescan sonar data south of about 23°N provide no evidence of back-arc thrusting along the eastern margin of the Luzon arc, as has been hypothesized in order to transfer shortening to the Philippine Sea plate. Neither do these data show clear evidence of west-vergent thrusting of the Luzon arc over adjacent elements of the forearc, in contrast to the very active thrusting documented onland to the north, along the Longitudinal Valley of eastern Taiwan. The arcward-vergent structures in the region of arc accretion have closed the North Luzon Trough, the major forearc basin. These structures have also built the Huatung Ridge as a compressional ridge of orogenic strata that serves to broaden the accretionary prism toward the arc (eastward) and, in so doing, have formed small collisional or suture basins and redirected orogenic sedimentation patterns throughout this key area. Thus the arcward flank of the collision has evolved in a much more complicated fashion than the relatively smooth progression followed by the western, 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 convergence in the face of increasing resistance, to the north, to the subduction of continental crust.