| dc.description.abstract | Taiwan island is located at the junction area of Eurasian Plate and Phillippine Sea Plate. Numerous active faults are distributed all over the Taiwan; some are even extend into the offshore area. Therefore, to understand a complete fault distribution and structural morphology, it is crucial to study both the onshore and the offshore area of the structure. There was a lack of data to study detailed active faults and geological structures in the offshore area. Hence, in this study we used sparker high-resolution reflection seismic, the multibeam bathymetry, sidescan sonar image, sob-bottom profiler, air-gun reflection seismic, and earthquake data, to analyze the distribution of faults and structures in the offshore area and provide tectonic interpretations. The offshore areas of the Hualien, Yilan, and Hisnchu-Miaoli are used as three case studies, as follows. (1) The junction area between northern end of the Longitudinal Valley and the Ryukyu Arc system is the transition from plate collision to plate subduction. Hualien Ridge is one of the obvious features in between. Tectonically it is divided into the active southern part and the inactive northern part. In the southern Hualien Ridge, we find several ~N30◦E trending active faults and some could be linked to the active faults in the onshore Milun Tableland. The structures in the southern Hualien Ridge and the Milun Tableland display a pop-up structure that is subject to the oblique compression from the northwestward motion of the Philippine Sea Plate. The ~N30◦E trending faults are the results of the transpressional system. The Milun Fault is the western boundary of the fault system, which probably terminates northward near 24◦03’N where a pronounced bathymetric depression trending N60◦ W. (2) Yilan Ridge situated at the transition between the Central Range extrusion and Okinawa Trough back-arc basin. Physically it is divided in to the Northern Yilan Range (NYR) and Southern Yilan Ridge (SYR). Based on the seismic profiles, the NYR fold may connect to the Hsuehshan Range. Due to the mountain collapse in NE Taiwan, the N fault series were found on the basement offsets and trending about N45◦E extends to the northern Yilan plain. Nevertheless, the extension between the western flank of the Central Range clockwise lateral extrusion and the NYR Fold, changed a normal fault series trending primarily N60◦E on the NYR Fold, named M fault series. M fault series have propagated eastward along the NYR Fold till the end of the western flank of the Central Range, then connected to the remnant folding structural ridge about N60◦E to N75◦E trending and collapsed to the SYR. The eastern flank of the Central Range extends to the SYR and formed a left-lateral strike-slip fault between the eastern and western flank of the Central Range basement. Because the eastern flank of the Central Range basement is undergoing the clockwise lateral extrusion, and formed the north-dipping normal faults. The north-dipping normal faults extended from the Central Range (Southern Ilan Strcture) and the south-dipping normal faults collapsed from the NYR Fold are a N60◦W fault zone in SYR, called S fault series. Based on the sub-bottom profiles and leveling observations, the most active M fault series in the offshore area could extend to the most active subsidence area in the Ilan plain where is the Choshui Fault and the estuaries of Lanyang River. In other words, the extension of the Ilan plain are due to the Central Range lateral extrusion. (3) In the Hsinchu-Miaoli area, the pre-Tertiary normal faults reactivated after the mountain building, and forming the fault-propagation folds. The trending of the Fold I in Hukoo offshore and Fold II in Hsinchu offshore are N70◦E. Faults F1b and F1a are the transpressional faults with a fault-propagation folds thrust from the north. This could be linked to the Hukou fault directly from the offshore to the onshore, or to the Fengshan river strike-slip structure and cut the Hukou Fault. Fold II and the onshore Chingtsaohu anticline onshore are the continuring folds, and Fault F2a may link to the Hsinchu fault as the same fault series. Fold III in Maioli offshore is really different from the Fold I and Fold II. The trending is N45◦E, and cut by the F3 fault series in W-E trending. Fault F3a is possibly linked to the onshore Longkang Fault. The deformation front is defined as the location where strata has been deformed compressional. The new deformation front has extended to the offshore area from Taichung in the northwest direction and reaches the westernmost at about 120。26’E , finally the trace of the deformation front connects back to the onshore Hukou Fault, foothills in Taoyuan. | en_US |