We investigate the seismic stratigraphic features offshore northern Taiwan by using newly collected multichannel seismic data. Two significant regional unconformities U1 and U2 have been identified, which further subdivide the sedimentary sequence into three seismic units as SU I, SU II, and SU III. The lowermost seismic unit SU I is a pre-late Miocene sequence, while the middle and upper seismic unit SU II and SU III result from the interactions between the rapid fault-controlled subsidence and the stable thermal-controlled subsidence. We consider that the present-day offshore northern Taiwan is under a post-collisional state and the unconformities U1 and U2 represent a response to the mountain collapse and to the cessation of the regional volcano-tectonic activities. It is not until 1.5 Ma that northern offshore Taiwan became a post-collisional basin and started to receive sediments, with a rapid fault-controlled subsidence. Afterward, the basin became dominated by a stable thermal-controlled subsidence at 0.2 Ma. Although the main volcano-tectonic activities in the northern offshore Taiwan are ceased, modern geophysical and geochemical investigations have suggested that the tectonism and the volcanism are still active and represent potential threatening geohazard.

Using bathymetric and multichannel seismic (MCS) data, we explored the volcanic influence on the bathymetric and stratigraphic features of the Mianhua Terrace. The Mianhua Terrace occupies the marine counterpart of the Northern Taiwan Volcanic Zone (NTVZ) along the collapsed Taiwan orogenic wedge and is dominated by post-collisional magmatism and extensional structures. The bathymetric data showed several semicircular-shaped features near the shelf break. The MCS profiles showed that the Pleistocene unconformity buried beneath the Mianhua Terrace is partly difficult to observe due to seafloor multiples, suggesting that the seafloor is dominated by physically hard lithology, probably volcanic lavas. We interpreted the high-amplitude reflectors and their projected seafloor relief as intrusive sills and associated extrusive edifice. Similarly, we interpreted high-amplitude reflectors in the vicinity of normal faults as intrusive sills emplaced and facilitated by fault structures. A volcanic or hydrothermal mound was also recognized. We propose that the Mianhua Terrace is a breached ramp in a transfer zone between the tips of two successive normal faults along the shelf break. Once the fault tips reactivate and extend toward each other, the Mianhua Terrace may continue to collapse, leading to catastrophic volcanic or associated hydrothermal events.

<p>        Submarine sand waves are known to be induced by tidal currents and their migration has become an important issue since it may affect seafloor installations. In Taiwan Strait, widely spreading sand waves have been recognized on the Changyun Ridge, a tide-dominated giant sand ridge offshore western Taiwan. However, due to lacking of high-resolution and repeated geophysical surveys before, detailed characteristics and migrating features of the sand waves in Taiwan Strait were poorly understood. As new multibeam bathymetric and seismic data were collected repeatedly during 2016 - 2018 for offshore wind farm projects, we can now advance the understanding of sand wave characteristics and migration patterns in the study area. We apply a geostatistical analysis method on bathymetry data to reveal distribution and spatial characteristics of the sand waves, and estimate its migration pattern by using an updated spatial cross-correlation method. Then, sedimentary features, internal structures and thicknesses of sand waves are observed and estimated on high-resolution seismic profiles. Our results show that the study area is mostly superimposed by multi-scaled sandy rhythmic bed forms. However, the geomorphological and migrating characteristics of the sand waves are complicated. Their wavelengths range from 80 to 200 m, heights range from 1.5 to 8 m, and crests are generally oriented in the WNW-ESE direction. Obvious sand wave migration was detected from repeated high-resolution multi-beam data between 2016 and 2018, and migration distances can be up to ~150 m in 15 months. The average elevation change of the seafloor over the whole survey area is ~3.0 m, with a maximum value of 6.9 m. Moreover, the sand waves can migrate over 30 m with ~2.5 m elevation change in 2 months and migrate over 5 m with ~1 m elevation change in 15 days. The results also show that the orientation of wave movement can be reversed even within a small distance. By identifying the base of sand wave on seismic profiles, the thicknesses of sand waves are found ranging from 1 to 10 meters. The base of wave structure become slightly deeper from nearshore to offshore. Our results indicate that the thickness of sand waves increases with degree of asymmetry and migration rate. By bathymetric and reflection seismic data analyses, systematic spatial information of sand waves in the study area are established, and we suggest that not only tidal currents can affect sand wave migration patterns, but also wave structures and thicknesses play important roles in sand wave migrating processes and related geomorphological changes.</p>

Based on multichannel seismic data northern offshore Taiwan, we revisited the postcollisional and volcanic features in the Late Miocene-Pleistocene wedge-top of the early Taiwan orogenic wedge. Seismic data transverse to wedge-top zone show the asymmetric and symmetric folds, along with blind thrusts and back thrusts. Seismic data parallel to the axis of the wedge-top zone show the volcanic features of poor stratified reflectors, forced fold and intrusive step of volcanic sills along and away from dominating normal fault structures. Since the volcanic extrusions and normal faults are probably associated events, we suggest that post-collisional extensional volcanism northern offshore Taiwan was probably formed along inherited primary and secondary fault structures. The wedge-top zone was formed at frontal part of the early Taiwan orogenic wedge in Late Miocene. Subsequent change of regional stress from convergence to extension led to the mountain collapse as well as primary and secondary normal faults. Thus, these post-collisional normal faults are mechanically weak, probably capable of providing pathways for the Pleistocene-present volcanisms northern offshore Taiwan. The facilitation of post-collisional fault structures as transportation routes for deep material to migrate along may be important for the formation of the Quaternary volcanism extrusions northern offshore Taiwan.