Sedimentary sequences offshore northeastern Taiwan and the offshore projection of the Shanjiao Fault zone

The offshore area of northern Taiwan is currently in an extensional regime, known for a post-collision area of the Taiwan mountain belt that has formed due to the oblique collision of the Philippine Sea Plate against the Eurasian continental margin since the late Miocene. The active collision has undergone a progressive migration of the Taiwan mountain belt from the northeast to the southwest direction. After the relaxation of the compressional stress due to the westward subduction of the Philippine Sea Plate, the northern Taiwan orogen has collapsed together with the opening of the southern Okinawa Trough. Here, we show the transitional process from compressional to extensional regime in the area off northern Taiwan and reconstruct the tectono-sedimentary evolution by using multi-channel reflection seismic data. We process and interpret multi-channel seismic profiles and use borehole data to determine the ages of sedimentary units and stratal surfaces/unconformities. Based on the seismic facies, we have recognized two distinctive domains separated by the Offshore Shanjiao Fault (OSF): a shelf basin to the northwest and a collapsed zone to the southeast. The tectono-sedimentary evolution off northern Taiwan since the late Miocene can be described in 4 stages. (1) The compressional structures off northern Taiwan were formed during the collision period starting around 6 Ma, a series of folds-and-thrusts aligned in the NE-SW direction. (2) Due to the oblique collision, some NW-SE-trending strike-slip faults were developed across the collision belt to accommodate the differential displacements between two adjacent segments of the folded belt. (3) During the transition from compressional to extensional regime at about 2.7 Ma, a post-collisional erosion and subsidence occurred in the collapsed zone to form the basal unconformity. This post-collisional collapse is associated with the opening of the southern Okinawa Trough, a back-arc basin, due to the westward migration of the Philippine Sea Plate subduction beneath the southern Ryukyu arc. (4) The offshore area of northern Taiwan has subsided continuously with sediment accumulation since the early Pleistocene (~2.0 Ma), accompanying the rifting of the southern Okinawa Trough.