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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]

Although the Lichi mélange has been studied intensively over the last decade, its enigmatic nature has generated debates regarding its origin and evolution. Two prominent models have been proposed to explain the Lichi melange: the olistostrome model with slumping sediments and the tectonic collision model with intensive shearing. Neither model can explain what causes the interpreted slumping and complex faulting processes for the sediments. Here, we study the Lichi melange using a time–space equivalence approach, in which the tectonics of the offshore continuation of the Lichi mélange should represent an earlier stage in its evolution. Our study of marine multichannel seismic data suggested that the backthrusts in the accretionary prism propagate arcward above and within the deforming forearc and arc basement to incorporate the sediment and basement materials into the rear of the accretionary prism. Therefore, we proposed the “retrowedge evolution model” whose novel key feature is that the irregular topography of the arc basement affects the taper angle of the retrowedge. The retrowedge has a greater taper than the minimum taper of the prowedge, so the slope can be steeper, and thus favors gravitational failure. Such processes would generate complex faulting and slumping processes in the backthrusted forearc ridge and basin. In sum, this retrowedge model reconciles the mixture of slumping and faulting processes found in the Lichi mélange. [ABSTRACT FROM AUTHOR]

Characterizing properties of marine subsurface sediment helps with siting for offshore infrastructure. Shear‐wave velocity (Vs) provides information on the geotechnical properties of the seabed. We present our initial efforts to obtain a detailed two‐dimensional model of Vs for a large‐offset multi‐channel seismic (MCS) transect collected in shallow waters across the Taiwan Strait using surface waves excited by a large volume airgun. We derived the dispersion curves of the Scholte waves along the 37.5‐km‐long transect using the phase‐shift method and then conducted multimodal inversion to obtain a Vs model down to a depth of 150 m. To estimate the dynamic Poisson's ratio across the transect, we combined the Vs model with a compressional wave velocity model derived from the traditional MCS semblance velocity analysis. Lastly, we approximated the seismic attenuation of the profile. Our results show a large lateral variation in shear‐wave velocity. In the north, a low‐velocity zone with shear‐wave velocities of about 150 m/s was identified, while in the south, the shear‐wave velocity was found to be 300 m/s. With synthetic data, several sensitivity tests were performed to derive optimal parameters for offshore large‐offset streamer data. We particularly focused on the depth of the streamer and source and the water depth in combination with different seabed properties. Our results show that we can robustly derive the shear‐wave velocity, along with the Poisson's ratio, using large‐offset streamer data elsewhere based on the criteria we have tested using field and synthetic data sets. Plain Language Summary: With an underwater air gun and underwater receivers, we can stimulate waves that travel through the water to the seafloor and back. At the seafloor, the wave splits into different types of waves (P‐wave, S‐wave, and Scholte wave). Previous works are mostly focused on P‐waves as S‐waves cannot travel through the water column. The Scholte wave travels along the seafloor and is sensitive to the shear properties of the top 150 m shallow sediments. By using a technique called the phase‐shift method, we can infer the S‐wave velocity from the Scholte wave. Combined with the P‐wave velocity, it can provide critical geotechnical information. We found an area with a relatively low S‐wave velocity of 150 m/s in the north and another area in the south with velocities of 300 m/s. This information can be used to weigh how and where marine constructions have to be built so that they are safe even under different environmental hazards (typhoons, earthquakes, and tsunamis). We are showing that existing data sets in shallow water can be used to estimate geotechnical properties; we further give suggestions in designing surveys to efficiently measure these wave types. Key Points: Scholte wave inferred shear‐wave velocity of shallow seabedLarge‐offset seismic streamer data as a tool to derive S‐wave and P‐wave velocity simultaneouslySensitivity tests for seismic streamer data on Scholte waves using synthetic waveforms [ABSTRACT FROM AUTHOR]

We utilized reflection seismic and bathymetric data to infer the canyon-infilling, fold uplift, and gas hydrate occurrences beneath the frontal fold at the toe of the accretionary wedge, offshore SW Taiwan. The lateral migrating paleo-Penghu canyons has cut across the frontal fold with six distinct canyon/channel incisions marked by channel infills. The longitudinal bathymetric profile along the modern canyon course shows a knickpoint of ~300 m relief at this frontal fold, indicating that the rate of fold uplift is greater than that of canyon incision. The age for the initial thrusting of this fontal fold is around 240 kyr ago, as estimated by using the maximum thickness of growth strata of this fold divided by the sedimentation rate obtained from a nearby giant piston core. Bottom simulating reflector (BSR) on seismic sections indicates the base of gas hydrate stability zone. Beneath the frontal fold, there is a widespread occurrence of BSRs, suggesting the highly probable existence of substantial quantities of gas hydrates. A seismic flat spot and a few push-down reflectors below BSR are found lying beneath the anticlinal axis with bathymetric four-way dip closure. The flat spot, cutting across a series of dipping reflections beneath BSR, may indicate the contact between free gas and its underlying formation water. The push-down reflectors beneath BSRs are interpreted to result from abundant free gas hosted beneath the gas hydrate stability zone. The multiple paleo-canyon infills seen along and beneath the frontal fold and above BSRs may provide thick porous sands to host gas hydrates in the frontal fold. [ABSTRACT FROM AUTHOR]

Fluid migration rates are important parameters for understanding the structural characteristics and evolution of the crustal tectonics and hydrocarbon exploration. However, they are difficult to measure on the seafloor. Densely sampled temperature measurements might shed light on our study of the fluid migration rates. In this study, we first use reflection seismic data to derive geothermal gradient patterns at different sub-seafloor depths, then calculated 1D vertical fluid flow models by analyzing the Péclet numbers in the offshore regions of SW Taiwan. We found Péclet numbers ranging from 2 to 5, implying that vertical fluid flow velocities are between 6 and 44 cm/year. Assumed a homogenous permeable layer and no lateral variation of the temperature fields, we analyzed possible errors caused by depth-dependent thermal conductivity and velocity-depth function. They probably cause less than 2 cm/year of errors in the estimated vertical fluid flow rate. In terms of regional fluid flow patterns, we found higher fluid flow rates near the toe of the trench, and a dramatic increase in fluid flow rate when the continental slope of the Chinese passive margin enters into the trench, and reduced fluid flow rates in the hinterland. We propose that this is a new and potentially useful method to derive regional fluid flow rate models for studying geochemical and biological processes in shallow seafloor sediments and the regional hydrological budget. [ABSTRACT FROM AUTHOR]

Abstract: Bottom simulating reflectors (BSRs) observed on seismic sections are often considered as indicators for the existence of free gas, delineating the base of the gas hydrate stability zone. Abundant BSRs seen on seismic sections acquired off the SW coast of Taiwan indicate the likely and prevalent existence of gas hydrates in the study area. This study aims to characterize the occurrence of BSRs off SW Taiwan and to understand their relationship to topography, tectonic activity, and possible migration paths of gas-bearing fluids in this area. The tectonic setting off SW Taiwan is during the initial stage of arc-continent collision between the Luzon arc and the northeastern continental margin of the South China Sea. A series of west-vergent, imbricated folds and emergent thrusts develop in the accretionary wedge. Each fold-and-thrust sequence corresponds to an elongated submarine ridge if its crest is not buried by flat-lying sediments. By contrast, normal faulting prevails in the northeastern margin of the South China Sea. A correlation between distribution of BSRs, topography, and tectonic features can be observed. Four major occurrences of BSR types of ridge type, basin type, submarine-canyon type, and continental slope type, are recognized on the basis of the relationship of BSRs to topographic and structural features. Main characteristics of BSRs in the study area can be described as: (1) they occur mostly beneath topographic highs; (2) a discordant relationship between surfaces of the seafloor and underlying strata where BSRs are present; (3) BSRs are prevalent especially beneath the crest and flank of the upthrusting, large and inclined slope basins; and (4) in general, a series of high-amplitude dipping reflectors beneath BSRs can be found. These features indicate that gas hydrate may accumulate preferably beneath topographic ridges especially underneath four-way-dip topographic closures. This effect may exist because the buoyancy-driven, gas-bearing fluids tend to migrate upward and laterally toward structural highs and their corresponding topographic ridges. The distribution of BSRs indicates that gas hydrates occur more commonly in the accretionary wedge than in the South China continental margin. We suggest that the more widespread occurrence of gas hydrates in the accretionary wedge is due to the existence of multiple fault zones, which may help to tap more deep-seated gas-bearing fluids, in addition to the shallow biogenic gas, in this region. [Copyright &y& Elsevier]

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]

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. [ABSTRACT FROM AUTHOR]

Radiometric dates of key rock units indicate that a remnant Late Mesozoic ocean of the Huatung Basin is still preserved today east of the South China Sea (SCS). We integrate regional geology with a Cretaceous oceanic basement in the vicinity of the Huatung Basin to reconstruct the Huatung Plate east of the Eurasian continent. Results of geophysical investigations, four expeditions of deep-sea drilling and a renaissance of regional geology allow us to propose a hypothesis that the mechanism responsible for the SCS opening was raised from strike-slip fault on the east. The hypothesis suggests that the SCS opening could highly relate to the strike-slip faults inherited from Late Mesozoic structures onshore–offshore the SE Cathaysia Block to develop rhombic-shaped extensional basins en echelon on the thinned Eurasian continental crust in the Early Cenozoic. It was followed by sinistral strike-slip movements along the boundary between the Eurasian Plate and the Huatung Plate driven by oblique subduction of the Huatung Plate to the northwest coupled with slab-pull force by southward subduction of the Proto-SCS to open up the triangle-shaped oceanic East Sub-basin in the Early Oligocene (33/34 Ma). The spreading ridge then propagated southwestward in the step-over segment between the Zhongnan-Lile and the Red River strike-slip fault systems to open the triangle-shaped oceanic Southwest Sub-basin by 23 Ma. The plate boundary fault was subsequently converted into the Manila Trench when the Eocene Sierra Madre arc of the Huatung Plate had moved from the south to its present latitude by the Middle Miocene. [ABSTRACT FROM AUTHOR]

The South China Sea oceanic lithosphere has been subducting eastward beneath the Huatung Basin/Philippine Sea Plate since the Early Miocene (~18 Ma). The subduction is followed by the oblique collision between the Luzon arc and the subducting Eurasian plate from 6.5 Ma. The North Luzon Trough forearc strata and the Luzon arc are then obducted northwestward as the Coastal Range, eastern Taiwan, in the last 1 Ma. The collision propagates southward and is presently active in the region offshore SE Taiwan. Integrating seismic surveys offshore and a detailed forearc stratigraphy study onshore the Coastal Range, this paper overviews the characteristics of forearc deformation, dynamic sequence stratigraphy, temporal-spatial variations of forearc sedimentation and stratigraphic correlation onshore and offshore forearc sequences in response to the oblique convergent tectonics north of 20°N. Combining onshore and offshore forearc geology together allows us to reconstruct a structural evolution of the North Luzon Trough forearc basin from subduction through collision to obduction, and to discuss the mechanism and processes responsible for developments of the modern-forming forearc Lichi Mélange during the active Taiwan orogeny. Seismic surveys offshore show that forearc deformation in the subduction zone is primarily caused by increase of rear prism slope and west-vergent thrusting of forearc strata along the prism top since the early forearc sedimentation. East-vergent backthrusting occurs during the late forearc sedimentation and propagates arcward when the volcanic arc collides with the accretionary prism in the collision zone. Bivergent thrusting leads to a development of the forearc Huatung Ridge popup as a bathymetric high which further controls the sedimentation of the younger forearc sequence in the collision zone. In response to the syn -sedimentation deformation, forearc depocenter shifts progressively eastward. Forearc stratigraphy thus changes from two mega-sequences bounded by an unconformity in the subduction zone to three mega-sequences juxtaposed from west to east unconformably in the collision zone. As a consequence, the forearc deformation and stratigraphy in the oblique collision zone off SE Taiwan show a characteristic temporal-spatial pattern that the lowest mega-sequence with the most intensive deformation occurs restrictedly in the west, whiles the middle and the upper mega-sequences with mild deformation crop out in the center and the eastern part of the forearc basin, respectively. Detailed biostratigraphy study using planktonic foraminifera and calcareous nannoplanktons indicates that the forearc strata onshore the Coastal Range are also composed of three sequences (lower unit S-1: 6.5–5.8 Ma; middle unit S-2: 5.8–3.0 Ma; and upper unit S-3: <3.0–1 Ma). They were previously mapped as lithostratigraphy units of the Lichi Mélange (6.5–3.0 Ma) in the west and the coherent flysch sequences of the Fanshuliao Formation (5.8–3.0 Ma) and the Paliwan Formation (<3.0–1 Ma) in the east. However, the young coherent forearc sequences in the east thrust westward ubiquitously over the old and highly deformed Lichi Mélange in the west along the listric east-dipping Tuluanshan fault during the obduction. Detailed biostratigraphy study reveals that the lower sequence unit S-1 exposes restrictedly in the Lichi Mélange west of the Tuluanshan fault, whereas the middle sequence unit S-2 are either mapped as part of the Lichi Mélange or the coherent Fanshuliao Formation in both sides of the Tuluanshan fault, respectively. The upper sequence unit S-3 exposes exclusively east of middle sequence unit S-2 in the eastern Coastal Range. Stratigraphy and sedimentology study also reveals a temporal-spatial sedimentation variation of the upper sequence unit S-3 owing to deformation of the sequence units S-1 and S-2 together as a Pliocene Forearc Ridge at ~3 Ma, a scenario analog to development of the modern Huatung Ridge at ~1 Ma in active collision zone offshore. Temporal-spatial pattern of forearc sequences onshore the Coastal Range suggests that these three sequence units are juxtaposed from west to east and are bounded by two unconformities analog to what occur today in the collision zone offshore SE Taiwan. The characteristic deformation and an eastward-youngling trend of strata distribution onshore the Coastal Range all indicate Syn-sedimentation deformation during the oblique collision in 6.5–1 Ma. Furthermore, along the N-S orogenic strike, events of the forearc sedimentation and bivergent thrusting occur earlier onshore the obduction zone in the north than the modern collision zone offshore in the south. Across the orogenic strike forearc strata get older and deformation gets intensive from east to west onshore the Coastal Range, a scenario analog to what observed in the oblique convergent region offshore SE Taiwan. A structure reconstruction reveals that the North Luzon Trough forearc strata have experienced multiple stages of thrust deformation from subduction through collision to obduction. These deformations account for the mechanism and processes to develop the modern-forming highly sheared SSZ-bearing forearc Lichi Mélange tectonically in the western Coastal Range during the last 1 Ma. [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]

[Display omitted] • Acoustic remote sensing can be used to determine different types of sedimentary seabeds using quantitative classification. • Three attributes extracted from acoustic waveforms reveal four types of sedimentary seabeds. • The proposed method differentiates sedimentary seabed types with similar grain size distribution. • Core data from the offshore area of southwestern Taiwan validates the method. • This study provides a sensitive and robust quantitative method and reduces the uncertainty of manual identification between different seabeds. The sedimentary seabed properties are direct indicators of its stability, whose measure has several important implications in understanding and forecasting geological events. This study investigates the classification of sedimentary seabed types in different depositional environments using the three attributes: reflection coefficient (RC), similarity index (SI), and amplitude ratio (AR) values extracted from sub-bottom profiling data. Four sedimentary seabed types were evaluated automatically according to the waveforms in the horizontal and vertical directions. To infer the seabed properties, the attribute SI is used to identify the occurrence of homogeneity of sediment transportation in an area, while RC and AR are used to determine the seabed characteristics in terms of the variation of acoustic energy reflected from the sediment. To validate the results of this quantitative analysis method, sixteen seafloor sediment samples collected at different sites in the offshore area of southwestern Taiwan were analyzed. The automatic classification results suggest that the different sub-bottom profiling images are associated with the grain size distributions and compositions, such as the concentrated distribution, sand, silt, and clay percentages, and the mean bulk density. In addition, an inhomogeneous environment with similar grain size distribution can be identified. The proposed seabed classification is an effective method to study seabed characteristics, especially for identifying chaotic sediments. [ABSTRACT FROM AUTHOR]

Whether the formation of the isolated sand body deposition in the forebulge area of a foreland basin system is structure- or deposition-controlled has puzzled geologists for decades, although sand body deposition is generally believed to be indicative of the position of the flexural forebulge in a foreland basin. The formation of a modern sand body in the forebulge area is thus examined by multi-scale geophysical observations based on combined reflection seismic profiles and compressed high-intensity radar pulse (CHIRP) profiles across the sand deposition along the forebulge of the Western Taiwan Foreland Basin (WTFB), which is a Late Miocene-present foreland basin in the overfilled stage. These profiles suggest that the accumulation of the sand deposits along the forebulge of the WTFB is not directly associated with forebulge faultings. The relief map of the forebulge deposit substratum shows a northwestward tilting slope, and the isopach of the forebulge sand body indicates that a large part of the sand body accumulated along the axis of the Taiwan Strait and the subdued forebulge of the WTFB. The difference between the prevailing directions of tidal currents between the Taiwan Strait and the East China Sea reflects the probable sedimentary influence of the cratonward migrating fold-thrust belt within a foreland shelf. We suggest that the formation and distribution of the sand deposits along the forebulge of the WTFB are generally controlled not only by the transverse downslope sedimentation but also longitudinal hydrodynamic processes at distal parts of the foreland basin. Our explanation provides a plausible tectono-sedimentary cause of the sand body deposition in the forebulge area in an overfilled foreland basin. The sedimentary dynamics of the sand body in the Taiwan Strait may be applicable for understanding the formation of isolated sand bodies in the distal part of the Cretaceous Western Interior Foreland Basin. [ABSTRACT FROM AUTHOR]

What are the geodynamic processes during the transition from the onshore NE Taiwan collision to the offshore southwest Okinawa back-arc basin opening associated to the Ryu-Kyu subduction? What is the local neotectonic scheme of this transition (e.g.: structural sketch map of the outcropping volcanic edifice highlighting major faults and their associated earthquakes)? These are some of the fundamental questions adressed to the Kuei-Shan Tao volcanic island (KST hereafter) which is the unique emerged volcanic outcrops situated within this geodynamic transition area. Several incompatible KST geological mappings had been published without any faults, nor dykes, nor feeders (Ichikawa, 1934 ; Hsu, 1963 ; and Chiu et al., 2010) that needed to be updated and completed on the structural point of view. In order to do so, we acquired a new high resolution UAS-drone topography (Digital Surface Model) through photogrammetric processing, with a ground resolution <10 cm. We analyse and interprete it in detail using morphostructural photo-interpretation methods. Field works on KST is restricted due to technical and administrative reasons, so we compare our morphostructural map to the shoreline outcrops observed from a boat survey. Then, we have updated the Kuei-Shan Tao geological mapping (lava flows and pyroclastic falls), and the structural scheme as well as the major erosionnal landslide processes. Taking into account the re-interpretation of surroundings offshore bathymetry, old and new seismic profiles, the different drillings done in the Kuei-Chia northern flank, previous geophysical works, the existing massive andesite datings), as well as the inferred stress regimes deduced from the earthquake's focal mechanisms, we propose a KST neotectonic map. We propose also a new scenario for the recent KST volcanic evolution. Kuei-Shan Tao geology and geodynamics may have a so great importance for the 0.5 millions citizens of the so close flat lying Ilan Plain in terms of natural hazards (eruptions, tsunamis, earthquakes...). [ABSTRACT FROM AUTHOR]

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 décollement 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. [ABSTRACT FROM AUTHOR]

We interpret seven two-dimensional deep-penetration and long-offset multi-channel seismic profiles in the northernmost South China Sea area, which were collected by R/V Marcus G. Langseth during the TAIwan GEodynamics Research (TAIGER) project in 2009. To constrain the crustal characteristics, magnetic inversion and forward magnetic modeling were also performed. The seismic results clearly show tilted faulting blocks in the upper crust and most of the fault plane connects downward to a quasi-horizontal detachment as its bottom in the south of the Luzon-Ryukyu transform plate boundary. North of the plate boundary, a small-scale failed rifted basin (minimum 5 km in crustal thickness) with negative magnetization probably indicates an extended continental origin. Significant lower crustal material (LCM) was imaged under a crustal fracture area which indicated a continent and ocean transition origin. The thickest LCM (up to 6.5 km) is located at magnetic isochron C15 that is probably caused by the magma supply composite of a Miocene syn-rift volcanic event and Pliocene Dongsha volcanic activity for submarine volcanoes and sills in the surrounding area. The LCM also caused Miocene crustal blocks to be uplifted reversely as 17 km crustal thickness especially in the area of magnetic isochron C15 and C16. In addition, the wide fault blocks and LCM co-existed on the magnetic striped area (i.e. C15-C17) in the south of the Luzon-Ryukyu transform plate boundary. Magnetic forward modeling suggests that the whole thick crustal thickness (>12 km thick) needs to be magnetized in striped way as oceanic crust. However, the result also shows that the misfit between observed and synthetic magnetic anomaly is about 40 nT, north of isochron C16. The interval velocity derived from pre-stack time migration suggests that the crust is composed of basaltic intrusive upper crust and lower crustal material. The crustal nature should refer to a transition between continent and ocean. Thus, the magnetic reversals may be produced in two possible ways: basaltic magma injected along the crustal weak zone across magnetic reversal epoch and because some undiscovered ancient piece of oceanic crust existed. The crustal structure discrimination still needs to be confirmed by future studies. [ABSTRACT FROM AUTHOR]

The outer rise on the distal periphery of a subduction system is caused by emplacement of an accreted load onto the flexed oceanic lithosphere. By examining the bathymetry and free-air gravity anomaly data collected by satellite observations and marine reflection seismic data collected during the TAIGER project, we demonstrate the characteristics of the flexural outer rise seaward of the Manila Trench. The region of the outer rise on the westernmost periphery of the Manila subduction system is characterized by the positive free-air gravity anomaly seaward parallel to the Manila Trench and the morphological rise at the south of the Manila subduction system. A flexure simulation is performed based on the flexural profiles along the southern Manila Trench-outer system and the resulting effective elastic thickness values may provide an alternative aspect for the spreading rates of the South China Sea basin. Since both the western periphery of the Taiwan collision belt and Manila subduction belt are dominated by the strain regime of extension of flexural origin, it appears that the strain regime of flexural extension associated with the flexural forebulge of the Western Taiwan Foreland Basin to the north, and the strain regime of flexural extension associated with the outer rise seaward of the Manila Trench to the south are meridionally interconnected. This revised understanding of the strain regime of flexural extension origin west of the Taiwan-Luzon convergent belt provides an alternative point of view on the strain regime offshore SW Taiwan. [ABSTRACT FROM AUTHOR]

Abstract: Surficial grain-size and down-core clay mineralogical data show that sediment along the inner-most part of the continental shelf in East China Sea is mainly derived from the Yangtze River (Changjiang), spanning from the Yangtze mouth (33°N) ~1000km southward to the southwestern corner of the Taiwan Strait (24°N). High-resolution CHIRP seismic profiles reveal an elongated mud wedge extending along the inner shelf, with a northern depocenter on the modern Yangtze delta and a southern depocenter at 27.5°N. Four distinct acoustic units are delineated within the mud wedge (from bottom up): unit I (late-Pleistocene, mainly valley fills), unit II (formed by transgressions, thin strata), unit III (11–2kyr BP, downlapping strata) and unit IV (2–0kyr BP, flat and opaque strata). Incised valleys, up to 15-m deep, are filled by flat-lying or inclined strata in unit I. The thin (<3m) and acoustically transparent unit II is only seen between 30 and 26°N in water depths between 40 and 90m. Separated by acoustically opaque strata or unconformities, units III and IV are widely distributed. During the past 11kyr Yangtze sediment accumulation has been unsteady, showing two high and one low accumulation-rate periods. The high-accumulation period at 5–8kyr BP may be related to maximum East Asian summer monsoon precipitation in the Yangtze basin; the other high-accumulation period, 0–2kyr BP, probably reflects intensive human activities in the river''s watershed. The low-accumulation-rate period at 2–5kyr BP, which is seen in both northern and southern Yangtze depocenters, is probably related to low river discharge and/or intensified Taiwan Warm Current and China Coastal Current. [Copyright &y& Elsevier]

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]

Horizon velocity analysis and pre-stack depth migration of seismic profiles collected by R/V Maurice Ewing in 1995 across the accretionary prism off SW Taiwan and along the continental slope of the northernmost South China Sea were implemented for identifying gas hydrates. Similarly, a survey of 32 ocean-bottom seismometers (OBS), with a spacing of about 500 m, was conducted for exploring gas hydrates on the accretionary prism off SW Taiwan in April 2006. Travel times of head wave, refraction, reflection and converted shear wave identified from the hydrophone, vertical and horizontal components of these OBS data were applied for imaging P-wave velocity and Poisson's ratio of hydrate-bearing sediments. In the accretionary prism off SW Taiwan, we found hydrate-bearing sediment, with a thickness of about 100-200 m, a relatively high P-wave velocity of 1.87-2.04 km/s and a relatively low Poisson's ratio of 0.445-0.455, below anticlinal ridges near imbricate emergent thrusts in the drainage system of the Penghu and Kaoping Canyons. Free-gas layer, with a thickness of about 30-120 m, a relatively low P-wave velocity of 1.4-1.8 km/s and a relatively high Poisson's ratio (0.47-0.48), was also observed below most of the bottom-simulating reflectors (BSR). Subsequently, based on rock physics of the three-phase effective medium, we evaluated the hydrate saturation of about 12-30% and the free-gas saturation of about 1-4%. The highest saturation (30% and 4%) of gas hydrates is found below anticlines due to N-S trending thrust-bounded folds and NE-SW thrusting and strike-slip ramps in the lower slope of the accretionary prism. We suggest that fluid may have migrated through the relay-fault array due to decollement folding and gas hydrates have been trapped in anticlines formed by the basement rises along the thrust faults. In contrast, in the rifted continental margin of the northernmost South China Sea, P-wave velocities of 1.9-2.2 km/s and 1.3-1.6 km/s, and thicknesses of about 50-200 m and 100-200 m, respectively, for a hydrate layer and a free-gas layer were imaged below the remnant and erosional ridges in the upper continental slope. High P-wave velocity of hydrate-bearing sediment below erosional ridges may also indicate high saturation of hydrates there. Normal faults due to rifting in the South China continental crust may have provided conduits for gas migration below the erosional ridges where P-wave velocity of hydrate-bearing sediment in the passive continental margin of the northernmost South China Sea is greater than that in the active accretionary prism off SW Taiwan. [ABSTRACT FROM AUTHOR]

15 Ma ago, a major plate reorganization occurred in East Asia. Seafloor spreading ceased in the South China Sea, Japan Sea, Taiwan Sea, Sulu Sea, and Shikoku and Parece Vela basins. Simultaneously, shear motions also ceased along the Taiwan–Sinzi zone, the Gagua ridge and the Luzon–Ryukyu transform plate boundary. The complex system of thirteen plates suddenly evolved in a simple three-plate system (EU, PH and PA). Beneath the Manila accretionary prism and in the Huatung basin, we have determined magnetic lineation patterns as well as spreading rates deduced from the identification of magnetic lineations. These two patterns are rotated by 15°. They were formed by seafloor spreading before 15 Ma and belonged to the same ocean named the Taiwan Sea. Half-spreading rate in the Taiwan Sea was 2 cm/year from chron 23 to 20 (51 to 43 Ma) and 1 cm/year from chron 20 (43 Ma) to 5b (15 Ma). Five-plate kinematic reconstructions spanning from 15 Ma to Present show implications concerning the geodynamic evolution of East Asia. Amongst them, the 1000-km-long linear Gagua ridge was a major plate boundary which accommodated the northwestward shear motion of the PH Sea plate; the formation of Taiwan was driven by two simple lithospheric motions: (i) the subduction of the PH Sea plate beneath Eurasia with a relative westward motion of the western end (A) of the Ryukyu subduction zone; (ii) the subduction of Eurasia beneath the Philippine Sea plate with a relative southwestward motion of the northern end (B) of the Manila subduction zone. The Luzon arc only formed south of B. The collision of the Luzon arc with Eurasia occurred between A and B. East of A, the Luzon arc probably accreted against the Ryukyu forearc. [Copyright &y& Elsevier]

The Kenting Plateau is characterized by unusual low relief surfaces that straddle the topographic crest of the northern Manila accretionary prism off southern Taiwan at 400–700 m water depth. Multibeam bathymetric data, reflection seismic data, Acoustic Doppler Current Profiler (ADCP) data, surface grab samples, and sediment cores were collected in and around the Plateau to identify evidence of erosion in the Kenting Plateau and understand how the morphological evolution has been influenced by submarine erosion over geological time scales. The most distinctive feature on the Kenting Plateau is a 3 km × 7 km bean-shaped flat elevated platform (Kuroshio Knoll) revealed by multibeam bathymetry. Seismic data show almost no reflections beneath the seafloor and erosional truncations at the seafloor, especially in the Plateau's eastern half, evidencing widespread erosion. The P-wave velocity of the gravels recovered from the top of the Plateau ranges from 2.2 to 4 km/s. After comparing the velocity with the borehole data from nearby basin the burial depth of the parent rocks was found to be around 2 to 4 km below the seafloor, indicating that the parent rocks have been uplifted and gravels were formed due to erosion of the Plateau. The truncation of the seafloor shown on seismic sections suggests significant erosion on the Plateau. Sand content of the sediment cores decreases away from the Plateau, suggesting that sediment transport is effective in this area with high energy deposition, thereby accumulating coarse sediments on the Plateau and removing fine particles away from it. The presence of a dune field migrating northward of the Plateau, parallel to the Kuroshio Current also evidences active sediment transport in the area. Flow velocity of the Kuroshio Current observed from the ADCP data is very high, reaching up to 1.8 m/s on top of the Kuroshio Knoll (SE domain). We thus interpret that the observed intense erosion is caused by the Kuroshio Current, while the uplift of the Kenting Plateau is partially due to isostatic rebound caused by sediment removal through erosion and compression of the accretionary wedge. The higher sedimentation rate and coarser in grain size during sea level lowstand (20,000–12,000 yrs. BP) suggests that the erosion was more intense during the glacial period compared to that of deglacial period (< 12,000 yrs. BP) as seen from the MD97–2145 core. Submarine erosion is predominant throughout the Plateau, and it controls the geomorphology of the Plateau, especially the Kuroshio Knoll. • Ocean current plays an essential role in shaping ocean floor. • Observed Kuroshio Current in the Kenting Plateau is up to 1.8 m/s. • Intense Kuroshio Current shaped the Kuroshio Knoll into flat topped elevated surface. • The parent rocks of the gravels were buried 2 to 4 km below the seafloor. • Decrease in grain size and sand content away from the Plateau indicates the Plateau acts as source for the sand. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. • Present the original heat flow data for hydrothermal vent investigation. • A geological thermal model is revealed by integrated geophysical analysis. • Propose potential fluid circulation mechanism of the Geolin Mound in Southern Okinawa Trough. [ABSTRACT FROM AUTHOR]

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. • We avoid the 2-step model dimension downsizing in the heat-flow data reduction. • Unclarified trade-offs of sensitivities among different model parameters are revealed. • Drastic heatflow variation in a small area is revealed to imply hydrothermal system. [ABSTRACT FROM AUTHOR]