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As an active mountain building, Taiwan Island is deduced from the oblique collision between the Eurasian and the Philippine Sea plate, and its northern part and offshore region are under post-collision collapse. The magmatism induced from the post-collision collapse therefore distributes in the northern Taiwan and its offshore area. A series of submarine volcanoes and igneous rock isles are rooted in the area. For investigation of the volcanic and igneous arrangement, we have collected the magnetic data over the past few decades to combine and compile a map of regional magnetic anomalies. A pronounced magnetic high largely dominates the area of most submarine volcanoes and extends eastward, while the adjacent areas to the north and west are lower. To better understand the magnetic features for the submarine volcanic area, the magnetization for an equivalent magnetic layer thickness was calculated. The result shows that a high magnetization concentrated on the SV7 and extends northwestward that could be a magnetic dipole combining with its northeastern low part. To the southward, the submarine volcanoes SV1, SV3-SV6 locate between this high and another low magnetization. We also applied the enhanced analytic signal technique from the same magnetic data to evaluate the magnetic source strength distribution. Except for the SV2, SV5 and SV6, the 0th degree of enhanced analytical signal shows that most signal high concentrated on the submarine volcanic areas. For higher degree of enhanced analytical signal, the highest magnitude focus on the Ks and SV1, and PV, SV3 and SV4 are slightly minor.

The Mienhua submarine volcano (MHV) is located in the southernmost Okinawa Trough and exhibits vigorous hydrothermal activity. This paper presents a detailed volcanic morphological analysis of the MHV, which has not been fully explored or discussed in terms of its hydrothermal characteristics and their relationship to hydrothermal activities. The MHV is situated at a water depth of 1,370 m and has a width of 2.2 km and relief height of 220 m. The MHV is an asymmetric volcano composed of two summits in the center, rugged mounds in the west, and flat regions in the east. Two hydrothermal vents, Devil Chimney and Witch Mound were discovered through acoustic flares and a high backscatter intensity in the eastern flank. Based on three repeated mappings, no noticeable depth changes were observed at the acoustic flare zone compared to the area of the rugged mounds and two summits. In addition, a sediment core collected in a high backscatter intensity patch displayed low magnetic susceptibility, which could be influenced by the high methane levels in hydrothermal fluid that flows through sediments. An acoustically transparent zone adjoining active flares was observed in the sub-bottom profiles in the southeastern flank of the MHV, suggesting that the morphological and volcanic features are mainly concentrated in the southeast. Based on the seabed characteristics and the distribution of sills and reflectors in the multichannel seismic profiles, we inferred that the MHV is an off-axis vent that has experienced at least two stages of morphological development.

Well-constrained and widely distributed bottom simulating reflectors in various geological settings across both passive and active margins make the area offshore southwestern Taiwan an excellent location to study gas hydrate dynamics and fluid flow systems. Seismic data reveal the presence of subsurface fluid flow systems and ubiquitous free gas and gas hydrates. This article aims to summarize and map the different types of seismic observations related to the gas hydrate system. The spatial distribution of seismic indicators for gas accumulation, fluid migration, and the presence of gas hydrates suggests that topographic highs on both passive and active margins are sites of particularly efficient free gas accumulation and gas hydrate formation. Seismic indicators observed in fold and thrust structures suggest that the fluid system in the active margin is structurally controlled and that the highest gas hydrate saturation levels are found in the cores of thrust anticlines on the lower slope of the active margin area.

Strong ocean current influences a marine seismic survey and forces the streamer off-course from the survey line. The sideway drift of the streamer results in that the reflection data are no longer distributed in common midpoint gathers along the survey line but become swath distribution on one side of the ship track. This effect is known as “streamer feathering” which degrades the profile image of the 2D processed seismic data. However, if we have long streamer or closely spaced parallel 2D seismic survey lines, we may turn this deleterious effect into a good opportunity to generate 3D seismic volumes with swath distributed reflection data. We present two case studies in which 2D seismic data were collected offshore eastern Taiwan where the strong Kuroshio Current heavily influenced the ship speed and caused large streamer feathering. The first case is a large-offset 2D seismic profiling data collected using a 6-km long streamer. We processed the swath part of the reflection data in 3D that not only avoids the inappropriate smearing effect in 2D data processing but also generates a 3D seismic volume to help the seismic interpretation. In the second case, we adjusted our 2D survey strategy when realizing that strong Kuroshio Current was causing significant streamer feathering, and collected a set of closely spaced parallel 2D seismic lines. This multi-swath dataset covers a broad area which enables us to generate a 3D seismic volume. Since our datasets are not real 3D seismic data, we have tailored our processing flows to deal with different data configurations and limitations of each dataset. Our results show that not only we have enhanced 2D seismic images of the originally-interested survey lines, but also provide information on 3D geometry of the geological features imaged. The benefits and limitations of utilizing the streamer feathering effect to generate 3D seismic volumes from 2D seismic profile data are reported. Overall, this approach is a considerable way to handle 2D seismic data with large streamer feathering for both avoiding unreliable 2D seismic images and obtaining information on 3D geometry of the geological features imaged.

The Turtle Island (Kueishantao) is situated in the southernmost Okinawa Trough, a back-arc basin to the north of the Ryukyu Arc. Based on bathymetry around the Turtle Island, obviously hummocky seafloor indicating slope failures have occurred. The Turtle Island could have erupted four times in the last 7000 years. Previous studies have pointed out that volcanic eruptions may be related to island flank collapses. The bathymetric hummocky feature reveals submarine debris avalanches in the north, south and east side. In order to understand the landslide mechanism, we have conducted detailed marine geophysical surveys in the north part of the debris avalanches. The multi-beam bathymetric data, sub-bottom profiler, sidescan sonar, sparker seismic reflection data and remotely operated vehicle (ROV) dives data are used to identify the landslide features, including the depositional and erosional characters. Our results show that the debris avalanche is probably related to the horseshoe scar of the subaerial flank collapsed event. The debris avalanches display a runout distance of up to 4 km northeastward and form a ~4 km2 hummocky terrain with displaced blocks. The biggest block could be ~50 m high and 50 m in diameter. Identified by the sparker seismic data, the chaotic facies of the landslide deposits can be divided into three MTD (Mass Transport Deposit) units (MTD1, MTD2 and MTD3). The main volcanic debris avalanche deposits are identified as MTD3 with a characteristic of broken and discontinuous lateral reflections. Correlated with seismic data and sub-bottom profiles, multiple landslide events are recognized on the basis of different MTD units. The last landslide event is probably related to a faulting and a slope failure of the island and could be highly associated with a climatic event.

Okinawa Trough (OT) is a back-arc basin behind the Ryukyu arc-trench system and located along the eastern margin of the Eurasian continent. It extends from the southwest Kyushu Island to the Ilan Plain of Taiwan (Fig. 1). The width and water depth vary from about 230 km and 200 m in its north to 60 - 100 km and 2300 m in its south (Sibuet et al. 1998). Based on geophysical and geological studies, the Okinawa Trough is considered to be in the rifting stage (Lee et al. 1980; Letouzey and Kimura 1986; Sibuet et al. 1987, 1998; Uyeda 1987; Kimura et al. 1988). Such a tectonic setting is characterized by active normal faulting in its upper crust and numerous intrusions of magma. Hence, the OT is expected to have a favorable environment for the development of a hydrothermal system. Recently, diverse microbial communities and other hydrothermal minerals have been discovered in the middle and northern Okinawa Trough (Uyeda 1987; Kimura et al. 1988; Halbach et al. 1989; Sakai et al. 1990; Glasby and Notsu 2003; Lüders and Niedermann 2010; Ishibashi et al. 2014; Toki et al. 2016; Minami and Ohara 2017). The southern OT is close to Taiwan and topographically has the lowermost trough throughout the Okinawa Trough (Fig. 1), suggesting a pronounced back-arc basin. However, restricted by the limited sampling and sequencing studies, our understanding of its marine environment and hydrothermal potential is poor. Supported by Central Geological Survey (CGS), Ministry of Economic Affairs (MOEA) of Taiwan, a 4-year program entitled “Geological Investigation of Mineral Resource Potential in the Offshore Area of Northeastern Taiwan” was launched in 2016. Significant contributions have been made in understanding the origin, distribution, and characteristics of hydrothermal deposits in the southern Okinawa Trough. Integration of geological, geophysical, geochemical and seafloor observation studies of the background environment is very helpful for understanding the hydrothermal mineral deposits in the southern OT.

This paper aims to understand the hydrothermal sites near the Yonaguni Knoll IV in the Okinawa Trough, and to develop new techniques to study fluid flow patterns for hydrothermal systems and their impact on ore deposits on the seafloor. Hydraulic parameters are important for hydrothermal system studies, but in-situ measurements of fluid migration rates are difficult. Hydrothermal fluids can reach several hundred degrees Celsius, temperatures high enough to perturb hydrothermal fields and pore water migration patterns. Using in-situ temperature data as constraints, we model and synthesize 1-D and 3-D cylindrical hydrothermal models to fit the spatial variations of observed temperature fields. The 1-D modeling uses Péclet number analysis along the conduit. We also construct a 3-D cylindrical model to estimate the temperature and fluid velocity fields using a finite element software. All domains are set to be porous to allow the fluid to flow. The simulation is run until it reaches a semi steadystate solution, allowing both the temperature and velocity fields to stabilize. Results show the dimension of the thermal anomaly zone is likely controlled by advective heat transfer along the vent due to upward fluid flow. We estimate a Péclet number of -1.6, and the vertical fluid flow velocities at these sites are high, approximately 106 m s-1, that is, about 100 m yr-1. This is a spatially averaged estimate over tens to hundreds of meters and does not take into account finer-scale venting, which may be very heterogeneous. The results of this work may help estimate the quantity of metal elements transported through pore fluid migration at modern hydrothermal sites.

Using echo sounders to detect gas plumes in seawater is common, especially in the context of hydrothermal circulation areas or gas hydrate-bearing cold seeps. To understand the distribution of gas plumes in the southernmost Okinawa Trough, we have conducted 13 cruises with a 38 kHz single-beam echo sounder (EK60). A total of 266 gas plumes of acoustic image, associated with the hydrothermal circulation, are detected. To estimate the near-seafloor bottom current speeds, 201 gas plumes are further used. As a result, the gas plumes around the axial depression of the Okinawa Trough generally tilt to the northeast at rising tides and high tides, suggesting a northeastward flow of the bottom current. However, the gas plumes in the Keelung continental slope tilt to the southwest at ebb tides and low tides, suggesting a southwestward flow of the bottom current. Our results significantly show a good estimation of the near-seafloor bottom currents from EK plume images in the case of lacking real observations. The directions of the bottom currents depend on semidiurnal tides. Assuming a quasi-constant speed of upward gas bubbles out of seabed, we have estimated the bottom current speeds in 6 hydrothermal circulation zones near the rifting center of the southernmost Okinawa Trough. The estimated bottom current speeds in submarine volcanic areas vary largely from 2 to 160 cm s-1, but bottom current speeds in relatively flat region are between 20 and 50 cm s-1. The large variation of the bottom current speeds in the submarine volcanic areas could be due to the variable emissions of the gases out of the submarine volcanic areas.

With large topographic gradients, rifted basins serve as efficient traps for particulate matter from adjacent lands and the ocean surface. However, the fate of organic carbon in the sediment, mostly unaltered by the hydrothermal activities known to occur in young rifts, remains poorly understood. In this study, we present an examination of diagenetic activities and carbon remineralization based on the first complete suite of pore-water data of sediment marginally affected by hydrothermal activities in the Southern Okinawa Trough (SOT). The sediment showed an oxygen penetration depth of 1 cm, consumption of NO3- in the top 1 cm, smeared profiles of Mn2+ and Fe2+ with the latter reaching up to 450 μmol L-1, and relatively unchanged SO42- concentrations with depth. Net production rates of dissolved species resolved from pore-water profiles provide an estimate of 1.68 ± 0.21 mmol C m-2 d-1 as the total carbon remineralization rate in the upper 30 cm sediment column, with aerobic carbon oxidation being the major pathway. The rate, one order of magnitude lower than that of the adjacent East China Sea, is attributed to the lower bottom-water temperature and carbon flux in the trough. The high carbon burial efficiency of SOT (68% of carbon reaching the seafloor and processed thereunder) reflects the combined effects of small mountain rivers and rifting-induced particle trapping.

Multi-scale geophysical survey including pseudo-3D seismic, sub-bottom profiling, side-scan sonar, multi-beam and single-beam bathymetry, heat flow investigations as well as bottom-water, core and dredge sampling works have been conducted in a newly discovered hydrothermal field named as Geolin Mounds at about 1,510 depth in the Okinawa Trough. Ship-mounted bathymetric data cannot detect specific morphological features on the seafloor in this field; however, “rock grove” morphological characteristics are observed by using deep-towed side-scan sonar. Moreover, vigorous flare features in water column are detected by multi-beam and single-beam echo sounder, and widely distributed high heat flow anomalies (> 10,000 mW/m2) also exist in the survey area. Due to strong Kuroshio Current during our multi-channel seismic survey, 5-40°streamer feathering effect occurred. To take advantage of swath distributed seismic reflection points caused by streamer feathering, we developed a pseudo-3D technique and produced a 3D seismic cube from this uneven seismic dataset. The 3D seismic imaging and sub-bottom profiling results indicate widely-distributed anomalies such as blanking zone and high-amplitude reflectors around the Geolin Mounds hydrothermal field and could link the specific features above seafloor. The 3D seismic cube also provides better estimation of the areas of blanking zone on selected time slice and better characterizes fault structures in the hydrothermal field. The geochemical analysis results present high Ag, Au, As, Bi, Cd, Cu, Fe, Pb, Sb, and Zn concentrations have been found in our coring and dredging samples. Relatively high concentration of methane, rare earth elements and 3He/4He ratio in near bottom seawater samples are also shown. Based on the geophysical and geochemical works, we propose that the Geolin Mounds hydrothermal field is without underlying submarine volcanos and hydrothermal fluid migration could be related to fault development. This hydrothermal field is in its embryo stage of evolution and constantly supported by active hydrothermal circulation. As a consequence, seafloor massive sulfides deposits and related geomorphological features can keep developing with hydrothermal fluid circulation along migration conduits. The fault structures and volcanic activities due to back-arc spreading process in the Southern Okinawa Trough should dominated creatures of those fluid migrating conduits. We suggest that the Geolin Mounds hydrothermal field could sustainably grow and have high potential of massive sulfides resources in the Southern Okinawa Trough. Furthermore, this hydrothermal field can serve as a good observatory for get better understanding of seafloor edifice development and ore mineralization associated with hydrothermal circulation activities in a back-arc spreading tectonics.

There is a high potential of hydrothermal mineral deposits in the Southern Okinawa Trough offshore northeastern Taiwan. This study aims to integrate bathymetry data, water column images, seabed image, sub-bottom profiles and ROV observations to better understand a hydrothermal field, submarine Mienhua Volcano (MHV). A repeated mapping survey equipped with EM-712 was conducted to see if significant bathymetric changes took place since 50 days in MHV. The volcano of 2 km width and 240-m relief height is present at about 1300-m water depth near Mienhua Canyon. The volcano combines with several small and rugged mounds. From ROV observations, two of the mounds show active hydrothermal vents covered by white tubeworms and a white smoker with some mussels, respectively. In addition, active water column gas flares are observed on the southeastern part of MHV. Based on the identification of sub-bottom profiles, the special distribution of acoustic blanking zone shows the southeastern part is more narrow than the northwestern area. However, comparing the bathymetry mapped in two times, no obvious overall morphological changes are detected, except on the volcano rim. Seabed image also reveals similar backscatter intensity within the rugged mounds, suggested they may be at similar stages of morphological development. We consider that the hydrothermal vents are variable. In addition, the preliminary results indicate the fluid migration is the important process to influence the evolution of the hydrothermal field in MHV. Therefore, we also applied more seafloor images, geochemical and geophysical data which have been reported in MHV. Our results could evaluate the relationship between the flare activities, geomorphological features and the location of the blanking zone in the sedimentary strata.

Six submarine canyons are distributed along the active margin off SW Taiwan. Using multi-beam bathymetric data and reflection seismic profiles to determine canyon head locations with respect to shelf edge, five of these canyons were classified into the following three types: 1. river-connected; 2. shelf-indenting; and 3. slope-confined. The remaining canyon was re-interpreted as a sea valley. Type 1 canyon, the Kaoping Canyon, is directly connected to the Kaoping River and supplied with terrestrial sediment. The canyon head segment tends to generate hyperpycnal flows during flooding, causing intense erosion of seafloor across the shelf and slope and forming a major sediment conduit. The Penghu, Kaohsiung and Fangliao canyons are of type 2 and incise into the shelf, forming a V-shaped head segment with abundant slumping features. Without continuous sediment supply from rivers, the canyon head receives sediments mainly from local slope failures on canyon walls and is unable to generate intense erosive sediment flows, resulting in an indentation into the shelf over a short distance. Oceanographic/hydrodynamic measurements close to the canyons are still not available for the region, despite they could shed light on the on-going sediment dynamics in the area. The Shoushan Canyon is a type 3 canyon characterized by limited sediment input from the Kaoping Shelf and weak canyon erosion, especially during sea-level highstands, resulting in a relatively straight canyon course confined to the upper slope. These submarine canyons share common regional tectonics, sea-level changes and climatic conditions. However, they evolved via local processes mainly controlled by sediment input directly related to head location, resulting in distinct morphologies and sizes. These canyons have remained active during the present-day sea-level highstand. Frequent earthquake and flooding events are the major triggering factors for sediment failures and generation of subsequent turbidity currents, resulting in substantial incision into the canyon. The results of the present study indicate that the SW Taiwan margin is characterized by canyons of type 1 and type 2 (n = 4) with only one type 3 canyon. This represents large percentages of type 1 and type 2 canyons compared to other regions of the world. Narrow shelf and abundant sediment supply are the two main controls for the presence of river-connecting and shelf-incising canyons on the active Taiwan margin.

The depositional history of the paleo-submarine channel plays an instrumental role in controlling the present-day heat flows and geothermal gradients of the Lower Fangliao Basin, a slope basin situated at the upper accretionary wedge, offshore southwestern Taiwan. This wedge is formed from the collision of the Luzon Arc and the Chinese continental margin. The basin has been referred as one of the gas-hydrate prospective areas within the vicinity, where occurrence of mud diapirs and bottom-simulating reflectors (BSRs) is present. Estimated BSRs-derived geothermal gradients infer an average value of 33 °C km−1, while estimated heat flows imply an average value of 41 mW m−2. Closely spaced thermal probes and infrared imaging from piston cores revealed average values for geothermal gradients and heat flows of 55 °C km−1 and 62 mW m−2, respectively. Discrepancies between both measurements are related to the sensitivity of direct thermal measurements over shallow fluid flux, where shallow geothermal gradients increase locally as the fluid migrates upward. Since the BSRs are situated at depths below cut-and-fill channel deposits in the basin depocenter, the channel facies is interpreted as deposition of an active channel prior to being intruded by mud diapirs and abandoned in the Pleistocene. An array of data, including high-resolution seafloor bathymetry, seismic facies, and distribution of thermal anomaly, reveal that the paleo-channel had flowed through the Lower Fangliao Basin following the strike of slope basins and deposited a stacked series of turbidite sands. Submarine ridges in the upper slope of the accretionary wedge developed as thrust-related anticlines, which bordered the slope basins. Rapid deposition and sediment burial in offshore southwestern Taiwan had caused insufficient dewatering process in the paleo-channel sediments, leaving high water saturation within pore spaces and overpressured the sediments. These, together, lead to lower heat flows and thermal gradients (thermal blanketing effect) and contribute to deepen the base of gas hydrate stability zone. Further mud diapiric intrusions and uplifting of seafloors had blocked the course of paleo-channel. The Lower Fangliao Basin was abandoned following the channel course shifted to the south along the present-day Gaoping Canyon course.

Based on seismic reflection profiles and multi-beam bathymetric data, a large submarine landslide named Fangliao Slide is mapped for the first time off SW Taiwan. The Fangliao Slide occurred on the continental slope to the west of the Fangliao Canyon at water depths between 420 and 900 m. The seafloor of the Fangliao Slide has a gentle slope angle (~1 - 2°). The landslide covers an area of ~15 km length and ~10 km width and a volume of ~26 km3. The headwall of the landslide has ~30 m vertical offset at the southern flank of mud diapir MD7-1, and the sidewalls are bounded by fault A in the west and faults C and D in the east. The sliding area is composed of five bathymetric terraces, indicating that the slope failures have occurred several times. The Fangliao Slide can be divided into an upper domain and a lower domain, separated at the water depth of ~600 m where the gas hydrate off SW Taiwan becomes dissociate. The initial slope failure of the Fangliao Slide was probably linked to mud diapirism of MD7-1 and the slope failure in the lower domain was probably augmented by the gas hydrate dissociation. The seafloor morphology in the lower domain is therefore more corrugated than in the upper domain.

Submarine landslides frequently occur in passive continental margins or active margins (Hampton et al. 1996; Wynn et al. 2000; Mienert et al. 2002; Korup et al. 2007; Twichell et al. 2009; Cukur et al. 2016). Submarine landslides have been studied extensively not only for scientific research but also for submarine geohazards. Submarine landslides could jeopardize marine infrastructures, such as offshore drilling platforms or submarine telecommunication cables, and could even trigger disastrous tsunamis (Bondevik et al. 2005; Harbitz et al. 2006; Hornbach et al. 2007, 2008; Hsu et al. 2008; Su et al. 2012; Tappin et al. 2014; Li et al. 2015). For instance, one disastrous tsunami hitting the coastal area of southwestern Taiwan in 1781 or 1782 was reported (Chen 1830; Hsu 1983); the tsunami event was probably generated by submarine landslides in the offshore area of southwestern Taiwan (Li et al. 2015). Moreover, several submarine landslides triggered by the 2006 Pingtung earthquake have induced turbidity currents off southwest Taiwan and destroyed about 14 submarine telecommunication cables off SW Taiwan (Hsu et al. 2008). The area of southwest Taiwan currently has a dense population (more than 3 million people in total), one deep-water Kaohsiung Port, several tanks of liquefied natural gas and a nuclear power plant on the coast (Fig. 1). Numerous submarine telecommunication cables exist off SW Taiwan. If a considerable tsunami event would hit again the costal area of SW Taiwan, the damage could very serious. Likewise, there are two nuclear power plants on the coast of northern Taiwan (Fig. 2), and the population in northern Taiwan has more than 10 million people. Submarine telecommunication cables also exist off northern Taiwan. In any case, it is important to understand the status of seafloor stability in the offshore areas of SW and NE Taiwan. For that, this special issue of submarine geohazard records and potential seafloor instability is aimed to provide some research results, hoping to have a general reconnaissance of submarine landslide potential off Taiwan.

Methane emissions out of the seabed could seriously affect Earthâs climate and are usually associated with the dissociation of gas hydrates stored in marine sediments on the continental margins. Spatially, gas emissions out of the seafloor are not evenly distributed in continental margins. Gas emissions out of the seabed generally occur through submarine mud volcanoes and gas seeps. To understand the seabed gas emissions off SW Taiwan, we investigate the distributions of active submarine mud volcanoes, gas seeps, and gas plumes off SW Taiwan. We examine all of the available sub-bottom profiler and EK echo sounder data. We identified 19 submarine mud volcanoes, 220 gas seeps, and 295 gas plumes. The gas emissions are generally distributed at the crests of mud diapiric ridges. Most of the active mud volcanoes and gas seeps cluster at the KASMVG (Kaoping submarine mud volcanoes group) area. We speculate that the intensive mud volcanism and gas seepage at the KASMVG area are ascribed to submarine channel erosion along the continental slope base. The erosion causes a deep V-shaped channel and a steep BSR (Bottom-Simulating Reflector) slope curve across the continental margin. The upward migration rate of free gas beneath the BSR is thus increased and intensifies mud volcanism and gas seepage at the KASMVG area. The gas seeps can reduce the slope stability and generate small-scale slides. The development of mud volcanoes in an area could effectively disturb the seabed morphology so that large-scale submarine landslides cannot easily happen.

The Longitudinal Valley in eastern Taiwan is generally considered as a collisional suture between the Philippine Sea and the Eurasian plates. Offshore to the northeast of the valley, the Philippine Sea Plate is subducting beneath the Ryukyu Arc. The corner between eastern Taiwan and the Ryukyu Arc system is therefore the transition from plate collision to plate subduction. In consequence, the area has complicated tectonics and frequent earthquakes. In this study, we used marine geophysical data to study the submarine Hualien Ridge that is situated in this plate collision/subduction transition zone. Our results show that the Hualien Ridge is tectonically 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. However, the Milun Fault, the western boundary of the fault system, probably terminates near 24°03′N, where a pronounced bathymetric depression trending N300° cuts across the whole Hualien Ridge. In fact, all the active faults in the southern Hualien Ridge only appear to the south of the bathymetric depression. In contrast, in the northern Hualien Ridge we only find blind normal faults covered by ~100 m thick sediments. The distinct variation of tectonic activity in the Hualien Ridge underlines the transition from the active collision to inactive collision or partial subduction of the Philippine Sea Plate relative to the Eurasian Plate.

The southernmost Okinawa Trough back-arc basin is an active and young basin formed just after the collision of the Philippine Sea Plate against the Eurasian continental margin. The back-arc extension occurs intensively because of the southward or southeastward migration of the southernmost Ryukyu Arc, or the roll-back of the Philippine Sea Plate. To better understand the active tectonics and volcanism of the southernmost Okinawa Trough, we have conducted deep-tow sub-bottom profiler and side-scan sonar surveys across the back-arc basin. Our results show that the volcanism of the southernmost Okinawa Trough is distributed in the southern half of the back-arc basin and occurs along some linear or branched zones roughly parallel to the trough axis. Volcanic seamounts are obviously located along the central depression of the basin and their sizes show lateral variation. On the other hand, the northern half of the southernmost Okinawa Trough back-arc basin has almost no volcanic activity, but contains more brittle normal faults. It is noted that gas plumes out of seafloor are generally associated with hydrothermal mounds or activities, instead of volcanic seamounts. We suggest that the more complete rifting of the southernmost Okinawa Trough back-arc is limited to the east of ~122o30′E. To the west of ~122o30’E, the back-arc extension could be still influenced by the inherited NE-SW trending structures of the continental crust created during the former Taiwan orogeny in this area.

Multi-scale reflection seismic data, from deep-penetration to high-resolution, have been analyzed and integrated with near-surface geophysical and geochemical data to investigate the structures and gas hydrate system of the Formosa Ridge offshore of southwestern Taiwan. In 2007, dense and large chemosynthetic communities were discovered on top of the Formosa Ridge at water depth of 1125 m by the ROV Hyper-Dolphin. A continuous and strong BSR has been observed on seismic profiles from 300 to 500 ms two-way-travel-time below the seafloor of this ridge. Sedimentary strata of the Formosa Ridge are generally flat lying which suggests that this ridge was formed by submarine erosion processes of down-slope canyon development. In addition, some sediment waves and mass wasting features are present on the ridge. Beneath the cold seep site, a vertical blanking zone, or seismic chimney, is clearly observed on seismic profiles, and it is interpreted to be a fluid conduit. A thick low velocity zone beneath BSR suggests the presence of a gas reservoir there. This “gas reservoir” is shallower than the surrounding canyon floors along the ridge; therefore as warm methane-rich fluids inside the ridge migrate upward, sulfate carried by cold sea water can flow into the fluid system from both flanks of the ridge. This process may drive a fluid circulation system and the active cold seep site which emits both hydrogen sulfide and methane to feed the chemosynthetic communities.

This study analyzes both 2D and 3D seismic images around the Palm Ridge area offshore of southwestern Taiwan to understand how the deformation front shifted westward and how tectonic activities interact with submarine canyon paths in the transition area between the active and passive margins. Palm Ridge is a submarine ridge that developed on the passive China continental margin by down-dip erosion of several tributaries of Penghu Canyon; it extends eastward across the deformation front into the submarine Taiwan accretionary wedge. The presence of proto-thrusts that are located west of the frontal thrust implies that the compressional stress field has advanced westward due to the convergence of the Philippine Sea Plate and Eurasian Plate. Since the deformation front is defined as the location of the most frontal contractional structure, no significant contractional structure should appear west of it. We thus suggest moving the location of the previously mapped deformation front farther west to where the westernmost proto-thrust lies. High-resolution seismic and bathymetric data reveal that the directions of the paleo-submarine canyons run transverse to the present slope dip, while the present submarine canyons head down slope in the study area. We propose that this might be the result of the westward migration of the deformation front that changed the paleo-bathymetry and thus the canyon path directions. The interactions of down-slope processes and active tectonics control the canyon paths in our study area.

The architecture and distribution of mud diapirs are shaped by tectonic activity, sediment filling and unbalanced loading characteristics. Mud diapir development also controls spatial variations of intraslope basins in slope areas and can spur upward fluid migration with diapiric intrusion. The offshore area of southwestern Taiwan is an incipient collision zone in which thick sequences of deep marine sediments filled a rapidly subsided foredeep basin during the Pliocene. Large volumes of deposited sediment serve as source materials of diapiric ridges that extend NNE-SSW, and some mud diapirs even extend to on land SW Taiwan with subsurface signatures of gas. This study examines relationships between mud diapir and intraslope basin development in convergent tectonics through seismic and bathymetry data analyses. Four types of mud diapirs are identified: (1) buried symmetrical diapirs; (2) symmetrical diapirs extruded above the seafloor; (3) asymmetric and irregular diapirs; and (4) small individual diapirs manifested as mud intrusions found in local areas. A 3-stage model is proposed as a tool for describing the development and distribution of these types of diapirs. We further examine the relationship between mud diapirs and intraslope basin development patterns by analyzing 2D and 3D seismic images to reveal structural and sedimentary processes occurring in the Lower Fangliao Basin. This basin is characterized by BSR and amplitude anomalies of seismic profiles and is a prospect of the Taiwanese gas hydrate investigation project. An 8-stage development model with six depositional units is proposed as a means to explain the evolution of diapirs, submarine canyons, and fold and fault activities in the Lower Fangliao Basin, in turn revealing the relationship between mud diapir formation and intraslope basin development offshore of southwestern Taiwan.

Voluminous hydrothermal systems are corresponding to the heat flow anomaly discovered from previous geophysical surveys in the Southern Okinawa Trough (SOT), which is an area with a high sedimentation rate supplied by the East China Sea shelf and the island of Taiwan. Our latest fifteen field measurements around the Geolin Mound (GLM) (24° 36.6 ‘N, 122° 53’E) are located at a sedimentary basin west of the Yonaguni Rift, which is nearby the southwestern tip of a NE-SW trending volcano trail in the SOT. The heat flow results illustrate a concentric decrease from 31,477 to 180 mW/m2 in a 1-km radius around the GLM, evidently proving the focus fluid migration along the major fluid conduit. The ultrahigh temperature observations revealed the existence of magmatic components in shallow sediments. Combining the subsurface profiles with the temperature fields around the GLM, a 0.16-km2 acoustic blanking area of the G1 fluid conduit was recognized from the chirp sonar image and numerous interpreted igneous reflections with fluids related features from seismic profiles. As a result, we proposed a conceptual model, which simultaneously illustrates the hydrothermal circulation pattern from the previous general surveys and the enormous thermal anomaly from current detailed measurements. The sill-to-sill feeding sedimentary basin of the GLM is capable of demonstrating the enormous thermal anomaly around the spreading center of the SOT and appropriately represents the potential hydrothermal discharge and recharge by heat flow distribution around the GLM.

This paper presents the seafloor survey results obtained in 2017 using two deep-towed vehicles, FITS and TVG, developed by the Institute of Undersea Technology at National Sun Yat-sen University, for investigating mineral resource potential offshore northeastern Taiwan. The seafloor surveys were conducted at five sites in the Southern Okinawa Trough via three cruises. A total of 39.4 kilometers of seafloor observation was accomplished, and plenty of seafloor characteristics and ecological phenomena were observed through FITS and TVG in the three cruises.

In this study, P- and S-wave velocity models were built based on two pre-stack depth migration (PSDM) profiles of long-offset reflection data and 25 four-component ocean-bottom seismometers (OBS) data at Jiulong Methane Reef off SW Taiwan in the passive margin of the northern South China Sea (SCS). According to the velocity models, the average P-wave velocity and Vp/Vs ratio of the free gas beneath the bottom-simulating reflector (BSR) are 1.52–1.58 km/s and 4.5–5.10, respectively. The depth of the BSR is found at 80–300 m below the sea floor and the sedimentary thickness of the hydrate and the free gas are about 50–100 m and 70–100 m, respectively. P-wave velocity of about 1.75 km/s in the hydrate-bearing sediment is southeastward increased to about 1.85 km/s above the BSR. Similarly, Vp/Vs ratio of about 3.06 in the hydrate-bearing sediment is increased southeastward to about 3.48 above the BSR. Based on the pseudo-3D map of gas-hydrate saturation estimated from the PSDM and OBS models, the average saturations of hydrate and free gas at Jiulong Methane Reef are about 7% and 0.9–2.4%, respectively. The highest hydrate saturation (11%) is located at 5–20 m above the BSR in the SE portion of the Jiulong Methane Reef. On the other hand, the highest gas saturation of about 2% is observed at 10–70 m below the BSR in the NW portion of the Jiulong Methane Reef. We suggested that several normal faults dipping southeastward beneath the continental slope provided conduits for gas migrating northwestward at Jiulong Methane Reef. Therefore, the highest gas saturation is observed below the anticline in the NW portion of the Jiulong Methane Reef and the highest hydrate saturation (high P-wave velocity and high Vp/Vs ratio) is identified above the BSR in the SE portion of the Jiulong Methane Reef.

In order to identify the mud diapirs and mud volcanoes off SW Taiwan, we have examined ∼1500 km long MCS profiles and related marine geophysical data. Our results show ten quasi-linear mud diapirs, oriented NNE–SSW to N–S directions. Thirteen mud volcanoes are identified from the multibeam bathymetric data. These mud volcanoes generally occur on tops of the diapiric structures. Moreover, the active mud flow tracks out of mud volcanoes MV1, MV3 and MV6 are observed through the high backscatter intensity stripes on the sidescan sonar images. The heights of the cone-shaped mud volcanoes range from 65 m to 345 m, and the diameters at base from 680 m to 4100 m. These mud volcanoes have abrupt slopes between 5.3° and 13.6°, implying the mudflow is active and highly viscous. In contrast, the flat crests of mud volcanoes are due to relative lower-viscosity flows. The larger cone-shaped mud volcanoes located at deeper water depths could be related to a longer eruption history. The formation of mud diapirs and volcanoes in the study area are ascribed to the overpressure in sedimentary layers, compressional tectonic forces and gas-bearing fluids. Especially, the gas-bearing fluid plays an important role in enhancing the intrusion after the diapirism as a large amount of gas expulsions is observed. The morphology of the upper Kaoping Slope is mainly controlled by mud diapiric intrusions.

Bottom simulating reflectors (BSRs), high methane flux, shallow sulfide/methane interfaces, fluids venting from the seafloor, authigenic carbonates within sediments, methane reefs, and self-biomes are common seafloor features in the area off southwest Taiwan. The geophysical and geochemical signatures of these features suggest a high potential for gas hydrate (GH) reservoirs in the region. The BSRs are typically interpreted as the boundary between free gases and solid hydrate, whereas the upper reaches of the hydrate stability zone and the distribution of gas hydrate in shallow sediments are not well understood. This study shows the first results of a marine controlled-source electromagnetic survey, conducted in the offshore area of SW Taiwan in 2010. The survey aimed to provide electrical resistivity information of the shallow sediments. Three target areas were surveyed: (1) an area to the southeast of the Xiaoliuchiu Island (gas seep G96), (2) an area in the west of the Yung-An Ridge (YAR) and (3) an area in the northwest of the Good Weather Ridge (GWR). In total, fourteen survey lines with a total length of 72 km were completed. Our preliminary results show that relatively high resistivity anomalies occur within pockmarks and at gas seepage sites. The apparent resistivity is estimated to be about 1 Ohm-m higher than background in G96 and YAR sites, while an anomaly up to 2 Ohm-m is found in the GWR. At gas seep site G96, the high resistivity anomaly may be due to the existence of authigenic carbonates; whereas, the high resistivity anomaly in the NW of the GWR site may also be due to the existence of gas hydrate in the shallow seabed. Based on the resistivity anomaly, the gas hydrate saturation is about 16% in the shallow sediments below the pockmark area in the northwest side of the GWR site.

Yonaguni Knoll IV is an active hydrothermal venting site located on the southern tip of a cross back-arc volcanic trail in the South Okinawa trough. We have compiled 22 heat-flow measurements on the seafloor around the Yonaguni Knoll IV site that reveal drastic heat-flow spatial variation. To obtain the appropriate heat-flow measurements from our recently rebuilt Lister-type marine heat probe, we have updated the processing algorithm in this study to avoid utilizing the sometimes-unstable temperature evolving data from the frictional heating stage. We have carefully calibrated and processed the observed data to yield robust estimates of the seafloor heat-flow emphasizing in particular that the previous implementation of the data reduction scheme by downsizing model parameters into two subsets to be pursued in two separate steps is not theoretically and practically essential. We also examine the trade-offs of sensitivity on model parameters constrained by the thermal decay data and reveal previously unclear knowledge concerning the intertwined mutual dependence that is critical to the performance the data reduction. A contrast of more than 3000 mW/m2 within short distances implies active local hydrothermal circulation. The observed low heat-flow area could be a cold seawater-charging site whereas Yonaguni Knoll IV might stand for a close by discharging output. Although it demands for detailed mapping to establish the 3D geometry of the local hydrothermal circulation, preliminary evidence suggests that the lateral dimension of the circulation cell is around a few kilometres.

Distribution and architecture of slope basins across a continental slope vary as a consequence of accommodation forming, sediment dispersal rates, canyon cutting, sediment filling and different sediment transporting mechanisms. The area offshore Southwestern Taiwan is generally recognized as having active tectonics and high sediment deposition rates. In the Gaoping Slope, slope basins are formed by the developments of folds, faults and diapiric intrusions. Portions of the sediments discharged from the Taiwan mountain belt have been trapped in these basins in the Gaoping Shelf and Gaoping Slope. The rest of the sediments were transported to deep sea areas through submarine canyons. This complex system of folds, faults, diapirs, slope basins, submarine canyons, and sediment deposits has also readjusted the morphology of the Gaoping Slope. This study examines the linkage between accommodation spaces of tectonic and sedimentary processes in the Gaoping Slope through seismic facies analysis. Four seismic facies which include convergent-symmetrical facies, convergent-baselapping facies, chaotic facies, and parallel and drape facies, and different deposition patterns have been recognized in the Gaoping Slope basins. The thick mud layers which are regarded as the source of diapiric intrusions are first observed beneath the basin. Strata records show that the accommodation spaces in various slope basins have increased or decreased during different stages of basin evolution. Because of the competition between regional tectonism (accommodation space variations) and sediment routing distance from provenance to depository (sediment input variations), most under-filled basins lie in the lower slope domain in the Gaoping Slope, but also in the upper slope domain east of the Gaoping Submarine Canyon. This observation suggests that in the inner Gaoping Slope west of the Gaoping Submarine Canyon, sediment deposition rate is higher than the basin subsidence rate, the topography of the upper slope domain there is “healed”, and most sediments are overfilled in the slope basins now. Besides the sequential steps of sedimentary disposal in the filling-and-spilling model, we have also observed evidences which indicate that mass movements and submarine canyons in the area have significantly changed the sediment dispersal patterns in the slope basins of the Gaoping Slope. We suggest that although filling-and-spilling is a key sedimentary process in the Gaoping Slope, tectonic activities, mass wasting events and canyon feeding processes have diversified sediment transporting mechanisms from the inner to outer slopes in the area offshore Southwest Taiwan.

The continental margin off SW Taiwan is in an incipient stage of orogeny and contains numerous active mud diapirs and mud volcanoes. Gas emissions out of the seabed off SW Taiwan are revealed by acoustic images from 38 kHz echo sounders or sub-bottom profilers. However, the mechanism for gas emissions is still poorly understood. In this study, we show that the gas emissions out of the seabed and the associated tremors off SW Taiwan are strongly correlated with the ocean tides, especially with the diurnal and semidiurnal constituents. Particularly, for each tidal day the strong gas emissions (expulsions) and long-duration tremors occur mainly during both the rising periods to the higher high tides and the falling periods to the lower low tides. Both the gas emissions and tremor activities are generally quiescent near the tidal datum. On average, the daily intensity of gas emissions and the magnitude of tremors are positively proportional to the daily tidal range; a larger slope site implies a greater concentration of gas in the shallow sediments. Most of the individual tremors and long-duration tremors observed off SW Taiwan occur as results of the strong gas expulsions out of the gas-bearing seabed. The high-frequency components of the induced tremors quickly decay and are followed by horizontal low-frequency (~7 Hz) harmonic oscillations of the seabed. Long-duration tremors may enhance seafloor instability and increase the potential for submarine landslides.

During TAiwan Integrated GEodynamics Research of 2009, we investigated data from thirty-seven ocean-bottom seismometers (OBS) and three multi-channel seismic (MCS) profiles across the deformation front in the northernmost South China Sea (SCS) off SW Taiwan. Initial velocity-interface models were built from horizon velocity analysis and pre-stack depth migration of MCS data. Subsequently, we used refracted, head-wave and reflected arrivals from OBS data to forward model and then invert the velocity-interface structures layer-by-layer. Based on OBS velocity models west of the deformation front, possible Mesozoic sedimentary rocks, revealed by large variation of the lateral velocity (3.1–4.8 km/s) and the thickness (5.0–10.0 km), below the rift-onset unconformity and above the continental crust extended southward to the NW limit of the continent–ocean boundary (COB). The interpreted Mesozoic sedimentary rocks NW of the COB and the oceanic layer 2 SE of the COB imaged from OBS and gravity data were incorporated into the overriding wedge below the deformation front because the transitional crust subducted beneath the overriding wedge of the southern Taiwan. East of the deformation front, the thickness of the overriding wedge (1.7–5.0 km/s) from the sea floor to the decollement decreases toward the WSW direction from 20.0 km off SW Taiwan to 8.0 km at the deformation front. In particular, near a turn in the orientation of the deformation front, the crustal thickness (7.0–12.0 km) is abruptly thinner and the free-air (−20 to 10 mGal) and Bouguer (30–50 mGal) gravity anomalies are relatively low due to plate warping from an ongoing transition from subduction to collision. West of the deformation front, intra-crustal interfaces dipping landward were observed owing to subduction of the extended continent toward the deformation front. However, the intra-crustal interface near the turn in the orientation of the deformation front dipping seaward caused by the transition from subduction to collision. SE of the COB, the oceanic crust, with a crustal thickness of about 10.0–17.0 km, was thickened due to late magmatic underplating or partially serpentinized mantle after SCS seafloor spreading. The thick oceanic crust may have subducted beneath the overriding wedge observed from the low anomalies of the free-air (−50 to −20 mGal) and Bouguer (40–80 mGal) gravities across the deformation front.

In order to understand gas hydrate related seafloor features in the near shore area off SW Taiwan, a deep-towed sidescan sonar and sub-bottom profiler survey was conducted in 2007. Three profiles of high-resolution sub-bottom profiler reveal the existence of five gas seeps (G96, GS1, GS2, GS3 and GS4) and one pockmark (PM) in the study area. Gas seeps and pockmark PM are shown in lines A and C, while no gas venting feature is observed along line B. This is the first time that a gas-hydrate related pockmark structure has been imaged off SW Taiwan. The relatively high backscatter intensity in our sidescan sonar images indicates the existence of authigenic carbonates or chemosynthetic communities on the seafloor. More than 2,000 seafloor photos obtained by a deep-towed camera (TowCam) system confirm the relatively high backscatter intensity of sidescan sonar images related to bacteria mats and authigenic carbonates formation at gas seep G96 and pockmark PM areas. Water column gas flares are observed in sidescan sonar images along lines A and C. Likewise, EK500 echo sounder images display the gas plumes above gas seep G96, pockmark PM and gas seep GS1; the gas plumes heights reach about 150, 100 and 20 m from seafloor, respectively. Based on multichannel seismic reflection (MCS) profiles, an anticline structure trending NNE-SSW is found beneath gas seep G96, pockmark PM and gas seep GS2. It implies that the gas venting features are related to the anticline structure. A thermal fluid may migrate from the anticline structure to the ridge crest, then rises up to the seafloor along faults or fissures. The seafloor characteristics indicate that the gas seep G96 area may be in a transitional stage from the first to second stage of a gas seep self-sealing process, while the pockmark PM area is from the second to final stage. In the pockmark PM area, gas venting is observed at eastern flank but not at the bottom while authigenic carbonates are present underneath the pockmark. It implies that the fluid migration pathways could have been clogged by carbonates at the bottom and the current pathway has shifted to the eastern flank of the pockmark during the gas seep self-sealing process.

The heavy rainfall accompanying typhoons and summer torrential rains washes organic sediment from the high mountains into the deep sea southwest of Taiwan. The waters depth is over 1000 m just 30 km off the coast. Moreover, Taiwan locates on the collision zone of Eurasian Plate and Philippine Sea Plate. Thick layers of organic sediment, along with activities of tectonic plate movement provides a perfect ground for the formation of gas hydrate. In 2003, Central Geological Survey (CGS), Ministry of Economic Affairs of Taiwan started the preliminary survey for gas hydrate in the southwest waters of Taiwan. The primary acoustic signature for the existence of gas hydrate, Bottom Simulating Reflector (BSR), was found in many regions of the waters. To further confirm and find out the details of the distribution and the deposits, CGS initiated a four-year survey project from 2004 to 2007, and followed by another phase from 2008 to 2011. More than twenty professors formed several teams to take care of issues ranging from BSR seismic survey, deep sea ecology monitoring, in-house gas hydrate formation simulation, gas hydrate triggered geo-hazard assessment. Starting from 2012, Gas Hydrate Project was officially transferred to Taiwan National Energy Program — Phase II as one the themes for finding clean energy for the next generation. This paper briefs on the efforts Taiwan has put on gas hydrate survey in the past decade, the current status, and the future plan. Some of the survey results will also be summarized.

Thirteen mud volcanoes are identified from the multibeam bathymetric data in the gas hydrate potential area of the upper Kaoping Slope, off southwest Taiwan. The heights of the mud volcanoes range from 65 m to 345 m and the size of their bases range from 680 m to 4,100 m in diameter. The slopes of the mud volcanoes are very steep (from 5.3° to 13.6°), suggesting that the mud volcanoes are fed by high-viscosity flows. ROV observations reveal active eruptions of the mud volcanoes MV1, MV5 and MV12, and two modes of eruption (explosive and effusive eruptions) were identified. The MV1 and MV5 are characterized by explosive eruptions and the eruptive cycles are about 3~5 and 5~8 minutes, respectively. The MV12 shows an effusive eruption, characterized by the continuous outpouring of mud together with gas plume. The gas plumes were found on tops of mud volcanoes MV1, MV4, MV5, MV10 and MV12, as indicated by the high-frequency sonar (EK500, EK60 sonars and multibeam echo sounder) images. The mud flows on the flank of six mud volcanoes (MV1, MV3, MV5, MV6, MV9 and MV10), illustrated by the high backscatter intensity bands from the sidescan sonar images. As evidenced by the results of investigation, the mud volcanoes in the upper Kaoping Slope off SW Taiwan are very active.

Bottom simulating reflector (BSR) is a key indicator for the presence of gas hydrate beneath the sea floor. Widely distributed BSRs have been observed in the area offshore of southwestern Taiwan where the active accretionary complex meets with the passive China continental margin. In order to better understand the distribution and characters of the gas hydrate in the region, closely spaced (1.86-km line spacing) multichannel seismic reflection surveys have been conducted in recent years under the support of the Central Geological Survey, ROC. Over 10000 km of multichannel seismic reflection profiles have been collected that cover an area of about 10000 km^2 offshore of southwestern Taiwan. BSRs can be identified along 50% of the seismic profiles that we collected. A newly compiled BSR distribution map suggests that gas hydrates are distributed both in the passive margin of the China continental slope as well as in the submarine Taiwan accretionary wedge, from water depths of 500 to over 3000 m. Gas hydrates are most concentrated underneath anticlinal ridges in the accretionary wedge, and underneath the slope ridges of the passive continental margin that were formed due to sedimentary processes. Active fluid activities are evident from various features observed on seismic reflection and chirp sonar profiles, such as mud volcanoes, gas plumes and gas charged shallow sedimentary layers. Fluid migration model has been established from a set of pseudo 3D seismic reflection data. The predicted locations of high fluid flux correlate well with those interpreted from geochemical analyses that show very high methane concentrations and very shallow sulfate-methane interfaces (SMI). This demonstrates the importance of structural control over gas hydrate emplacement. From the observed gas hydrate distribution and characters, the area offshore of southwestern Taiwan provides an ideal place to study and compare the formation and migration of gas hydrates under different tectonic settings.