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Neotectonic Interpretation of the different marine surveys (ACT, …) swath bathymetry and different onshore-offshore sismic profiles combined to classical structural fieldwork, geodetic, seismological and interferometric studies lead us to propose a global structural scheme and confirm the regional escape tectonics affecting both onshore-offshore of SW Taiwan. First, it is highlighted here the difficulty to only interprete the swath bathymetry even in the northern tip of the Manila accretionnary prism which is a rather simple geological context but affected by both (1) a strong amount of sedimentation due to the Taiwan mountain belt erosion, and (2) to the submarine erosion of the giant Penghu canyon. Second point, is the importance of the seismic interpretation in order to get the offshore bedding and structural data combined with the swath bathymetry and to the photointerpretation of the digital Terrain Model combined to the accurate geological maps to precisely delineate the blocks that is inferred to be submitted to a classical escape tectonic. Third, the precise study of the two new major structural boundaries Fangliao and Young-An structures which guides the SW Taiwan extrusion. Combined with onshore studies (e.g. interferometry (DINSAR), geodetic, seismology and field work) which gives (1) locations, characterization and quantification on the interseismic displacements and (2) lead us to modify our view of the global tectonic structures of the SW Taiwan. To conclude, it is highly recommended to combine both approach on and offshore geology in order to better understand geology and active structures in that part of the world.

Gas hydrates are ice-like crystalline solids composed of a hydrogen bonded water lattice entrapping low-molecular weighted gas molecules commonly of methane. These form under conditions of relative high pressure and low temperature, when the gas concentration exceeds those which can be held in solution, both in marine and on-land permafrost sediments. Simulating the mechanisms leading to natural gas hydrate emplacement in geological environments requires the modeling of the temperature, the pressure, the chemical reactions, and the convective/diffusive flow of the reactive species. In this study, we take into account the distribution of dissolved methane, methane gas, methane hydrate, and seawater, while ice and water vapor are neglected. The starting equations are those of the conservation of the transport of momentum (Darcy’s law), energy (heat balance of the passive sediments and active reactive species), and mass. These constitutive equations are then integrated into a 2-dimentional finite element in space, finite-difference in time scheme. In this study, we are able to examine the formation and distribution of methane hydrate and free gas in a simple geologic framework, with respect to geothermal gradient, dewatering and fluid flow, the methane in-situ production and basal flux. The temperature and pressure fields are mildly affected by the hydrate emplacement. The most critical parameter in the model appears to be the methane (L+G) and hydrate (L+G+H) solubility: the decrease in methane solubility beneath the base of the hydrate stability zone (BHSZ) critically impacts on the presence of free gas at the base of the BHSZ (thus the presence of a BSR), while the sharp decrease of hydrate solubility above the BHSZ up to the sea bottom critically impact on the amount of methane available for hydrate emplacement and methane seep into the water column.

A high-resolution seismic reflection survey was conducted during the NT07-05 cruise over the Formosa Ridge offshore southwestern Taiwan where strong and continuous bottom simulating reflections (BSR) have been observed. Previous seafloor pictures taken from a deep-towed camera indicate that there are some chemosynthetic colonies. During the NT07-05 cruise, not only large and dense chemosynthetic communities were confirmed at the plume site, ROV Hyper-Dolphin has also discovered that both deep-sea mussel Bathymodiolus platifrons, and galatheid crab Shinkaia crosnieri are vigorously populated at this site. By integrating swatch bathymetry, multichannel seismic and high-resolution seismic reflection data, we now have a better understanding on the structural characters of the cold seep site. The cold seep is situated at the summit of the Formosa Ridge southern peak. Submarine canyons that incised continental slope on both sides of the ridge are the controlling factors of the ridge formation. The sedimentary strata are generally flat lying but have been deformed by mass wasting processes. Strong BSR is observed 400 to 500 ms below the seafloor of the ridge, with many bright reflections beneath it. There is a narrow vertical blanking zone raising from BSR to the crest of the ridge. This narrow zone is interpreted to be the fluid conduit of the seep site. BSR may form a good cap to trap gas below, and this “gas reservoir” is shallower than the canyon floors on either side of the ridge. We suggest that this “ridge type” gas reservoir configuration enables the cold sea water to get into the fluid system, and forms a special kind of “hydrothermal” circulation that feeds the unusual chemosynthetic communities observed at the Formosa Ridge cold seep site.

International audience; Detailed seafloor mapping in the area east of Taiwan revealed trench-parallel stretching and folding of the Ryukyu forearc and lateral motion of the accretionary wedge under oblique convergence. East of 122ø40'E, a steep accretionary wedge is elongated in an E-W direction. A major transcurrent right-lateral strike-slip fault accommodates the strain partitioning caused by an oblique convergence of 40 ø. A spectacular out-of-sequence thrust may be related to the subduction of a structural high lying in the axis of the N-S trending Gagua Ridge. This asperity is likely responsible for the uplift of the accretionary wedge and forearc basement and may have augmented strain partitioning by increasing the coupling between the two plates. West of 122ø40'E, the low-taper accretionary wedge is sheared in a direction subparallel to the convergence vector with respect to the Ryukyu Arc. The bayonet shape of the southern Ryukyu Arc slope partly results from the recent (re)opening of the southern Okinawa Trough at a rate of about 2 to 4 cm/yr. Right-lateral shearing of the sedimentary forearc with respect to the nonlinear Ryukyu backstop generates trench-parallel extension in the forearc sediment sequence at dilational jogs and trench-parallel folding at compressive jogs. The Hoping Basin lies above a diffuse trench/trench/fault (TTF) or TFF unstable triple junction moving toward the south along a N-S transform zone which accommodates the southward drift of the Ryukyu Arc with respect to Eurasia.

A joint French-Taiwanese cruise in May and June 1996 on the French research vessel L'Atalante has provided detailed structural images of the sea floor east and southwest of Taiwan. High quality swath bathymetry (see Figure 1) and seismic reflection profiles collected in southeastern and southwestern offshore areas near Taiwan will increase our understanding of how the Luzon Arc is deformed by its collision with the Chinese passive margin and how the Manila Trench connects with the deformation front on Taiwan. The data collected off the eastern coast of Taiwan will elucidate the nature of the plate boundary between the Ryukyu subduction zone and the Taiwan mountain belt and reveal the mode of back arc opening in the Southern Okinawa Trough. The investigations will contribute to understanding the processes of accretion of an arc to a continent prior to the ultimate collision between continents.