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The Formosa Ridge cold seep is among the first documented active seeps on the northern South China Sea passive margin slope. Although this system has been the focus of scientific studies for decades, the geological factors controlling gas release are not well understood due to a lack of constraints of the subsurface structure and seepage history. Here, we use high‐resolution 3D seismic data to image stratigraphic and structural relationships associated with fluid expulsion, which provide spatio‐temporal constraints on the gas hydrate system at depth and methane seepage at modern and paleo seafloors. Gas has accumulated beneath the base of gas hydrate stability to a critical thickness, causing hydraulic fracturing, propagation of a vertical gas conduit, and morphological features (mounds) at paleo‐seafloor horizons. These mounds record multiple distinct gas migration episodes between 300,000 and 127,000 years ago, separated by periods of dormancy. Episodic seepage still seems to occur at the present day, as evidenced by two separate fronts of ascending gas imaged within the conduit. We propose that episodic seepage is associated with enhanced seafloor sedimentation. The increasing overburden leads to an increase in effective horizontal stress that exceeds the gas pressure at the top of the gas reservoir. As a result, the conduit closes off until the gas reservoir is replenished to a new (greater) critical thickness to reopen hydraulic fractures. Our results provide intricate detail of long‐term methane flux through sub‐seabed seep systems, which is important for assessing its impact on seafloor and ocean biogeochemistry. Plain Language Summary: Gas hydrates are ice‐like compounds that form in marine sediments. They can reduce the permeability of the sediments by clogging up the pore spaces, and influence how methane gas flows through sediments and then seeps out of the seafloor. Seepage of methane into the water column plays an important role in seafloor biology and ocean chemistry. In this study, we use 3D seismic imaging to investigate the subseafloor sediments of a ridge in the South China Sea where gas is currently seeping into the ocean. Our data show, in high detail, how gas migrates upward through the sediments due to the buoyancy of gas. Our data also reveal mound structures at certain depths beneath the seafloor. We interpret that these mounds represent distinct phases in the geological past where gas was seeping out of the seafloor. This indicates that gas seepage at this ridge has switched on and off (episodically) throughout geological time. We speculate that the episodic seepage is associated with rapid seafloor sedimentation, which changes pressure conditions beneath the seafloor. Our work improves the understanding of how gas seepage processes can change on geological timescales. Key Points: Gas has accumulated beneath the base of gas hydrate stability, causing vertical gas conduit formation and seabed moundsMounds imaged within the conduit record episodic seepage between 300 and 127 kyrs agoQuiescence may be associated with enhanced seafloor sedimentation that increases effective stress at the top of the gas reservoir [ABSTRACT FROM AUTHOR]

Estimates of the sub‐seabed fluid flow rates are important for understanding hydrological budgets, biogeochemical cycles, and physical properties of the sediments. Fluid flow rates and directions, however, are difficult to measure, particularly beneath the seafloor. We developed a rapid method to estimate regional fluid migration rates using an extensive database of seismic reflection profiles taken offshore SW Taiwan. We observe bottom‐simulating reflector (BSR) that deflects toward the seafloor near thrust faults that indicate localized heat flow variations. At these sites, advecting warm pore fluids transport heat to shallower depths and force the BSR shallower. Our 2D steady‐state numerical method quantifies the fluid flow rates required to cause such thermal anomalies. We found that fluid flow rates near the trench of the accretionary wedge range between 0.1 and 16 m3 yr−1 m−1, with slower and faster rates generally associated with slope basin discontinuities and faults, respectively. To evaluate the fluid pattern evolution from subduction to collision, we studied three transects: one along the Manila subduction zone in the south and two in Taiwan's initial collision zone in the north. We quantified the fluid budget and partitioning of fluid flow between focused discharge through faults and diffusive flow through the wedge. Faults in Taiwan's accretionary wedge capture on average 25% of the total dewatering flux in the younger subduction zone and 38.5% in the tectonically mature collision zone. Our method provides estimates of fluid migration rates along convergent plate boundaries, and contributes to our understanding of focused fluid flow processes in many other regions. Plain Language Summary: Fluids play a key role in many subduction zone processes. However, quantitative constraints on flow expulsion rates and directions are limited. Efficient upward fluid migration through subbottom conduits can be generated tectonically, such as faults. Faults are ubiquitous along convergent margins; yet, a quantitative understanding of their impact on regional fluid budgets, flow rates, and distribution at vent sites remains unclear. We developed a rapid numerical method to constrain 2D focused fluid flow rates using seismically derived thermal structure and applied it to the subduction‐collision zone system off SW Taiwan. To study the influence of long‐term tectonic processes on the fluid budget, we remotely mapped the distribution and amount of focused fluid flow across the convergent margin, using a widespread shallow subbottom temperature field derived from a spatially dense seismic data set covering an area of more than 25,000 km2. We combined the results with other previously published geophysical data sets to calculate the margin fluid budget. We found stronger fluid advection from depth along the collision zone, where thicker sediments are deformed more intensively. Our approach to quantify fluid fluxes is applicable to a range of tectonic regimes and can provide critical insight into local, regional, or even global fluid budget estimates. Key Points: Developed a method to quantify fluid flow rates from BSR‐based temperature dataIdentified fluid partitioning patterns off SW Taiwan from subduction to collisionFocused fault‐related flow controls the local depth of the hydrate stability zone [ABSTRACT FROM AUTHOR]

Large amounts of methane, a potent greenhouse gas, are stored in hydrates beneath the seafloor. Sea level changes can trigger massive methane release into the ocean. It is not clear, however, whether surficial seafloor processes can cause comparable discharge. Previously, fluid migration was difficult to study due to a lack of spatially dense seismic and thermal observations. Here we examine a gas hydrate site at Four‐Way‐Closure Ridge off SW Taiwan using a high‐resolution 3‐D seismic cube, together with bottom‐simulating reflections (BSRs) mapped in the cube, a thermal probe data set, and 3‐D thermal modeling results. We document, on a scale of tens of meters, the interaction between surficial sedimentary processes, fluid flow, and a dynamic gas hydrate system. Fluid migrates upward through dipping permeable strata in the limb, the slope basin, and along thrust faults and ridge‐top normal faults. The seismic data also reveal several double BSRs that underlie seabed sedimentary sliding and depositional features. Abrupt changes in subsurface pressure and temperature due to the rapid seabed sedimentary processes can cause a rapid shift of the base of the gas hydrate stability zone. This shift may be either downward or upward and would result in the accumulation or dissociation of hydrate in sediments sandwiched by the double BSRs, respectively. We propose that dynamic surficial processes on the seafloor together with shallow focused fluid flow affect hydrate distribution and saturation at depth and may even result in methane expulsion into the ocean if such localized features are common along convergent plate boundaries. Plain Language Summary: Gas hydrates are ice‐like compounds in marine sediments. Shallow surface dynamic processes may affect the hydrate saturation beneath the seabed. We combine 3‐D seismic and thermal probe data, with numerical geothermal modeling to investigate the geological processes controlling the distribution and formation of gas hydrates beneath thrust ridge anticlines. We also study fluid flow patterns under the seabed and found that localized fluid flow and rapid surficial erosional processes have significantly altered the temperature and pressure conditions of hydrate bearing sediment strata at depth, ultimately influencing gas hydrate formation and dissociation. We propose to conduct hydrate exploration close to thrust anticlines, where such active processes might enrich the saturation of gas hydrates or even influence fluid emission into the ocean if similar processes are widespread along continental margins. Key Points: Mass wasting and rapid sedimentation can generate double BSRs and enhance hydrate saturationRapid seabed processes might trigger hydrate dissociation and active venting of methane and other fluids in shallow sedimentary sectionFocused fluid flow updip along the slope basin strata can locally enhance the hydrate saturation [ABSTRACT FROM AUTHOR]

Temperature measurements carried out on 9 hydrocarbon exploration boreholes together with Bottom Simulating Reflectors (BSRs) from reflection seismic images are used in this study to derive geothermal gradients and heat flows in the northern margin of the South China Sea near Taiwan. The method of Horner plot is applied to obtain true formation temperatures from measured borehole temperatures, which are disturbed by drilling processes. Sub-seafloor depths of BSRs are used to calculate sub-bottom temperatures using theoretical pressure/temperature phase boundary that marks the base of gas hydrate stability zone. Our results show that the geothermal gradients and heat flows in the study area range from 28 to 128°C/km and 40 to 159mW/m2, respectively. There is a marked difference in geothermal gradients and heat flow beneath the shelf and slope regions. It is cooler beneath the shelf with an average geothermal gradient of 34.5°C/km, and 62.7mW/m2 heat flow. The continental slope shows a higher average geothermal gradient of 56.4°C/km, and 70.9mW/m2 heat flow. Lower heat flow on the shelf is most likely caused by thicker sediments that have accumulated there compared to the sediment thickness beneath the slope. In addition, the continental crust is highly extended beneath the continental slope, yielding higher heat flow in this region. A half graben exists beneath the continental slope with a north-dipping graben-bounding fault. A high heat-flow anomaly coincides at the location of this graben-bounding fault at the Jiulong Ridge, indicating vigorous vertical fluid convection which may take place along this fault. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to identify the mud diapirs and mud volcanoes off SW Taiwan, we have examined ~1500km 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 65m to 345m, and the diameters at base from 680m to 4100m. 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. [ABSTRACT FROM AUTHOR]

Abstract: The accretionary wedge of offshore southwestern Taiwan contains abundant deposits of gas hydrate beneath the sea floor. High concentrations of methane in pore waters are observed at several locations with little data concerning historical methane venting available. To understand temporal variation of methane venting in sediments over geologic time, a 23-m-long Calypso piston core (MD05-2911) was collected on the flank of the Yung-An Ridge. Pore water sulfate, dissolved sulfide, dissolved iron, methane, sedimentary pyrite, acid volatile sulfide, reactive iron, organic carbon and nitrogen as well as carbonate δ13C were analyzed. Three zones with markedly different pyrite concentration were found at the study site. Unit I sediments (>20 mbsf) were characterized with a high amount of pyrite (251–380 μmol/g) and a δ13C-depleted carbonate, Unit II sediments (15–20 mbsf) with a low pyrite (15–43 μmol/g) and a high content of iron oxide mineral and Unit III sediments (<10 mbsf) by a present-day sulfate–methane interface (SMI) at 5 m with a high amount of pyrite (84–221 μmol/g) and a high concentration of dissolved sulfide. The oscillation records of pyrite concentrations are controlled by temporal variations of methane flux. With an abundant supply of methane to Unit I and III, anaerobic methane oxidation and associated sulfate reduction favor diagenetic conditions conducive for significant pyrite formation. No AOM signal was found in Unit II, characterized by typical organically-limited normal marine sediments with little pyrite formation. The AOM induced pyrite formation near the SMI generates a marked pyrite signature, rendering such formation of pyrite as a useful proxy in identifying methane flux oscillation in a methane flux fluctuate environment. [Copyright &y& Elsevier]