Multibeam Bathymetry of the Håkon Mosby Mud Volcano

The Håkon Mosby Mud Volcano is a natural laboratory to study geological, geochemical, and ecological processes related to deep-water mud volcanism. High resolution bathymetry of the Håkon Mosby Mud Volcano was recorded during RV Polarstern expedition ARK XIX/3 utilizing the multibeam system Hydrosweep DS-2. Dense spacing of the survey lines and slow ship speed (5 knots) provided necessary point density to generate a regular 10 m grid. Generalization was applied to preserve and represent morphological structures appropriately. Contour lines were derived showing detailed topography at the centre of the Håkon Mosby Mud Volcano and generalized contours in the vicinity. We provide a brief introduction to the Håkon Mosby Mud Volcano area and describe in detail data recording and processing methods, as well as the morphology of the area. Accuracy assessment was made to evaluate the reliability of a 10 m resolution terrain model. Multibeam sidescan data were recorded along with depth measurements and show reflectivity variations from light grey values at the centre of the Håkon Mosby Mud Volcano to dark grey values (less reflective) at the surrounding moat.     

Gas hydrate accumulation at the Håkon Mosby Mud Volcano

Gas hydrate (GH) accumulation is characterized and modeled for the Håkon Mosby mud volcano, ca. 1.5?km across, located on the Norway–Barents–Svalbard margin. Pore water chemical and isotopic results based on shallow sediment cores as well as geothermal and geomorphological data suggest that the GH accumulation is of a concentric pattern controlled by and formed essentially from the ascending mud volcano fluid. The gas hydrate content of sediment peaks at 25% by volume, averaging about 1.2% throughout the accumulation. The amount of hydrate methane is estimated at ca. 10⁸?m³ STP, which could account for about 1–10% of the gas that has escaped from the volcano since its origin.     

The ecology and distribution of benthic foraminifera at the Håkon Mosby mud volcano (SW Barents Sea slope)

To investigate a possible influence of submarine methane seepage on benthic foraminiferal communities, Rose Bengal stained (“live”) and empty tests of benthic foraminifera were studied from the sediment surface down to 15 cm sub-bottom depth of 12 sites at the Håkon Mosby mud volcano (HMMV). In addition, one reference site well away from the seep sites, but from similar water depths and the same general hydrographic setting was occupied for comparison. The HMMV is located at 1265 m water depth on the SW Barents Sea continental slope. Distinct living foraminiferal associations at the HMMV are linked to specific sedimentary, microbial, and macrofaunal habitats. In the center of the crater, and in crater areas completely covered by bacterial mats, Cassidulina reniforme is the only, albeit rare, living species. Below the top few millimeters, sediments are anoxic and devoid of living specimens. At the rim of the mud volcano, at sites densely populated by pogonophoran tube worms, three benthic foraminiferal associations are found; (i) a Fontbotia wuellerstorfi–Lobatula lobatula association living attached to the upper parts of pogonophoran tubes, which protrude into oxic water, (ii) a diverse Cassidulina neoteretis association populating dysoxic sediments of the surface centimeter, and (iii) a species-poor Bolivina pseudopunctata association colonizing the subsurface sediments down to four centimeters. Generally, we did not find endemic or seep indicative species or associations at the HMMV. However, the HMMV live faunas dominated by B. pseudopunctata are not found at the reference site nor are they described from comparable water depths and environments without gas seepages from the Norwegian-Greenland Seas.In the center and outer rim of the mud volcano, a C. neoteretis–Reophax guttifer dead association, similar to the one at the reference site, characterizes an assemblage of strongly corroded and partly displaced tests. At bacterial mat sites, a C. reniforme dead association corresponds to the living one. Thus both the living and the dead associations are indicative of a specific bacterial mat environment at the HMMV.     

Extensional rise and fall of a salt diapir in the Sørvestsnaget Basin,SW Barents Sea

Regional extension which initiates and promotes the rise of salt diapirs can also make diapirs fall once the supply of salt from its source is restricted. New observations on the 3D seismic data from a salt diapir in the Sørvestsnaget Basin suggest that salt moves until the end of the Eocene and is subtle to minor readjustments afterwards, revealing a more complex kinematics that previously described. Observations such as salt horns and sags and an antithetic fault linked to the western flank of the diapir suggest that salt syn-kinematics during Middle-Late Eocene included passive rising of the salt, followed by a fall. The salt horns are remnants of a taller salt diapir that, together with the indentation of the Middle-Late Eocene syn-kinematic sediment overburden above the salt, indicate diapiric fall due to restriction of salt supply by extension. Post-kinematic readjustments did not include diapiric reactivation by tectonic compression as previously thought, but minor salt rise by shortening due to gravity gliding after the tilting of the margin during Plio-Pleistocene glacial sediment loading and differential compaction of surrounding sediments. The salt diapir appears to be presently inactive and salt supply may have been restricted from its source already since Late Eocene.     

Changes in seabed morphology, mud temperature and free gas venting at the Håkon Mosby mud volcano, offshore northern Norway, over the time period 2003–2006

The Håkon Mosby mud volcano is a 1.5-km-diameter geological structure located on the Southwest Barents Sea slope at a water depth of 1,270 m. High-definition seabed mapping of the mud volcano has been carried out in 2003 and 2006. A comparative analysis of the bathymetry and backscatter maps produced from the two surveys shows subtle morphological changes over the entire crater of the mud volcano, interpreted to be the consequence of mud eruption events. Mud temperature measurements point to a persistently warm mud at shallow depth in the crater. This is explained by upward fluid advection, rather than conductive cooling of mud flows. The small-scale spatial variability in the temperature distribution may be related to mud outflows or changes in the fluid flow regime. Furthermore, the locations of free gas venting observed in 2006 were found to differ from those of 2003. Our observations of overall similar topographic profiles across the mud volcano in 2003 and 2006 suggest that eruption events would have been modest. Nevertheless, the data bring evidence of significant change in activity even over short time intervals of only 3 years. This may be a characteristic shared by other submarine mud volcanoes, notably those considered to be in a quiescent stage.     

Structural and stratigraphic controls on subsurface fluid flow at the Veslemøy High,SW Barents Sea

Subsurface and seafloor fluid flow anomalies are gaining large interest after the finding of five new hydrocarbon discoveries and observation of large gas flares in the SW Barents Sea. In the present study, we have analysed structural and stratigraphic controls on fluid flow towards the seafloor using gravity cores selected based on subsurface gas anomalies observed on seismic data from the Veslemøy High, SW Barents Sea. The subsurface fluid flow at the Veslemøy High is observed to be controlled by 1) the morphology and orientation of regional faults, structural highs and sedimentary basins, 2) the presence of Paleocene silica ooze deposits that changes microstructure with temperature thereby controlling fluid flow and 3) the location of regional and local open faults formed by glacial loading and unloading. Analysis of extractable organic matter in subsurface Holocene sediments corroborates the active migration pathways inferred from seismic data. Micropalaeontological studies on benthic foraminifera reveal methane seep associated assemblages that confirm the interpretation of subsurface gas anomalies in seismic data. We ultimately link these new results to the geological evolution history of the region to give a comprehensive model for the fluid flow system within the study area.     

Development and post-depositional deformation of a Middle Eocene deep-water sandy depositional system in the Sørvestsnaget Basin,SW Barents Sea

Three-dimensional seismic data from the Sørvestsnaget Basin, SW Barents Sea supported by well data, are used to investigate a Middle Eocene deep-water depositional system. The system forms a NNW-oriented sediment accumulation, characterized by increased seismic amplitudes, and abrupt western termination. The data indicate that post-depositional sand remobilization and injection led to formation of sub-circular sediment blocks up to several km wide to the east of the main accumulation. The deep-water depositional system was deformed by wing-like sandstone intrusions, extending 200–400 m upwards from the margins of the parent sand bodies. The intrusions have polygonal or broadly circular plan view geometries. Deformation is inferred to have been associated with overpressure of the sand bodies as a result of rapid burial, fluid migration into the sealed sand bodies from deeper sources via synsedimentary faults, and fluid drainage from the surrounding mudrock during early compaction. The final triggering mechanism for sand remobilization and injection is inferred to have been fracture propagation due to differential compaction and/or fault-induced earthquakes. The injectite complexes are often associated with folding of overlying strata, which we relate to differential compaction. Intrusion of sand took place during the Middle Eocene. Post-depositional sand remobilization and injection have important implications for hydrocarbon exploration because they cause changes in the reservoir primary architecture, connectivity and structure.     

Deglaciation of the western margin of the Barents Sea Ice Sheet — A swath bathymetric and sub-bottom seismic study from the Kveithola Trough

Kveithola Trough, an E–W trending glacial trough in the NW Barents Sea, was surveyed for the first time during the EGLACOM cruise of R/V OGS-Explora in summer 2008. Swath bathymetry shows that the seafloor is characterized by E–W trending mega-scale glacial lineations (MSGL) that record a fast flowing ice stream draining the Svalbard/Barents Sea Ice Sheet (SBIS) during the Last Glacial Maximum (LGM). MSGL are overprinted by transverse sediment ridges about 15 km apart which give rise to a staircase axial profile of the trough. Such transverse ridges are interpreted to be grounding-zone wedges (GZWs) formed by deposition of subglacial till during episodic ice stream retreat. Sub-bottom (CHIRP) and multi-channel reflection seismic data show that the present-day morphology is largely inherited from the palaeo-seafloor topography at the time of deposition of the transverse ridges, overlain by a draping glaciomarine unit which in places is over 15 m thick. Our data allow the reconstruction of depositional processes which accompanied deglaciation of the Spitsbergen Bank area. The sedimentary drape deposited on top of the GZWs is suggested to have accumulated at a very high rate, (on average in the order of 1–1.5 m ka^?1) and therefore may potentially preserve a high-resolution palaeoclimatic record of deglaciation and post-glacial conditions in this sector of the Barents Sea.     

New insights into the tectono-stratigraphic evolution of the southern Stappen High and its transition to Bjørnøya Basin,SW Barents Sea

Within the context of the southwestern Barents Sea, the southern Stappen High and its transition to the Bjørnøya Basin are still underexplored. Improved quality seismic reflection data are utilised to describe new insights into the Paleozoic to early Cenozoic tectono-stratigraphic evolution of the area, as well as to discuss the structural inheritance and the rift development. Well-defined syn-rift wedges and better resolution images for both the deep Carboniferous and Permian successions are revealed. In particular, both the mid-Carboniferous and Late Permian-earliest Triassic extensional phases are characterized by widespread NE-SW oriented normal faults that are mostly westward dipping. Although Triassic is mostly considered as a tectonically stable period in the Barents Sea, in the southern Stappen High there is clear identification of a localised depocentre (named herein “Intra Stappen Basin”) where syn-tectonic geometries characterize the upper Paleozoic and Triassic deposits. Regional correlation to Middle and Upper Triassic outcrops in southwestern Svalbard reveals possible progradation from a west-northwest Northeast Greenland provenance as a western sediment source area during the Triassic, in addition to the well-known eastern sediment source area. Thin but distinct Jurassic sequences are expected to be present on Stappen High associated with prominent regional NW-SE extension throughout Late Jurassic that culminated during the earliest Cretaceous. Furthermore, structural and stratigraphic relations are observed within the study area that clearly indicate a distinct early Aptian rift phase with increasing evidence for its occurrence in the southwestern Barents Sea. Upper Cretaceous sequences bounded by major low-angle west-dipping detachment faults are observed in southwest Stappen High. During early Cenozoic, the study area was located at the proximity of the paleo-coastline and paleo-shelf edge for both Paleocene and Eocene gravity mass-waste deposits. These are most probably related to a progressively evolving steep bathymetric gradient between the developing margin, mainly towards the west and to the south, and the uplifted Stappen High.     

Hydrocarbon plumbing systems above the Snøhvit gas field: Structural control and implications for thermogenic methane leakage in the Hammerfest Basin,SW Barents Sea

Based on the analysis of the high-resolution 3D seismic data from the SW Barents Sea we study the hydrocarbon plumbing system above the Snøhvit and Albatross gas field to investigate the geo-morphological manifestation and the dynamics of leakage from the reservoir. Fluid and gas escape to the seafloor is manifested in this area as mega-pockmarks 1–2 km-wide, large pockmarks (<100 m wide) and giant pockmarks 100–300 m-wide. The size of the mega pockmarks to the south of the study area may indicate more vigorous venting, whilst the northern fluid flow regime is probably characterised by a widespread fluid and gas release. Buried mega depressions and large-to-giant pockmarks are also identified on the base Quaternary and linked to deep and shallow faults as well as to seismic pipes. A high density of buried and seafloor giant pockmarks occur above a network of faults overlying an interpreted Bottom Simulating Reflector (BSR), whose depth coincides with the estimated base of the hydrate stability zone for a thermogenically derived gas hydrate with around 90 mol% methane. Deep regional faults provide a direct route for the ascending thermogenic fluids from the reservoir, which then leaked through the shallow faults linked to seismic pipes. It is proposed that the last episodic hydrocarbon leakage from the reservoir was responsible for providing a methane source for the formation of gas hydrates. We inferred that at least two temporally and dynamically different fluid and gas venting events took place in the study area: (1) prior to late Weichselian and recorded on the Upper Regional Unconformity (URU) and (2) following the Last Glacial Maximum between ∼17 and 16 cal ka BP and recorded on the present-day seafloor.     

Palynology and depositional environments of the Middle – Late Triassic (Anisian – Rhaetian) Kobbe,Snadd and Fruholmen formations,southern Barents Sea,Arctic Norway

The Middle to Upper Triassic series in the Norwegian Barents Sea represents an important target for hydrocarbon exploration. However, the dating and correlation of these rocks are problematic due to the scarcity of age-diagnostic macrofossils, lateral facies variation and the overall thickness of the succession. Palynology has proven to be a useful dating tool but further work is necessary to improve the zonal resolution, particularly for the Upper Triassic Series. The current paper presents a detailed palynostratigraphy of the Middle – Upper Triassic succession in four exploration wells (7120/12-2, 7228/2-1s, 7-1a and 9-1s) drilled in the Hammerfest and Nordkapp basins. Eleven informal palynological zones are described from the stratigraphic interval spanning the Kobbe, Snadd and Fruholmen formations, which constitute regionally significant hydrocarbon source and reservoir rock facies. The results of this study contribute to a higher resolution palynostratigraphy for the Middle to Upper Triassic strata of the southern Barents Sea (SBS), and an improved correlation with the stratigraphy of the Svalbard Archipelago. Palynofacies data for the succession, presented here for the first time, indicates depositional environments ranging from fluvio-deltaic to offshore marine.     

Near-surface electromagnetic, rock magnetic, and geochemical fingerprinting of submarine freshwater seepage at Eckernf?rde Bay (SW Baltic Sea)

Submarine groundwater discharge in coastal settings can massively modify the hydraulic and geochemical conditions of the seafloor. Resulting local anomalies in the morphology and physical properties of surface sediments are usually explored with seismo-acoustic imaging techniques. Controlled source electromagnetic imaging offers an innovative dual approach to seep characterization by its ability to detect pore-water electrical conductivity, hence salinity, as well as sediment magnetic susceptibility, hence preservation or diagenetic alteration of iron oxides. The newly developed electromagnetic (EM) profiler Neridis II successfully realized this concept for a first time with a high-resolution survey of freshwater seeps in Eckernf?rde Bay (SW Baltic Sea). We demonstrate that EM profiling, complemented and validated by acoustic as well as sample-based rock magnetic and geochemical methods, can create a crisp and revealing fingerprint image of freshwater seepage and related reductive alteration of near-surface sediments. Our findings imply that (1) freshwater penetrates the pore space of Holocene mud sediments by both diffuse and focused advection, (2) pockmarks are marked by focused freshwater seepage, underlying sand highs, reduced mud thickness, higher porosity, fining of grain size, and anoxic conditions, (3) depletion of Fe oxides, especially magnetite, is more pervasive within pockmarks due to higher concentrations of organic and sulfidic reaction partners, and (4) freshwater advection reduces sediment magnetic susceptibility by a combination of pore-water injection (dilution) and magnetite reduction (depletion). The conductivity vs. susceptibility biplot resolves subtle lateral litho- and hydrofacies variations.     

Preliminary analysis of distribution and variation of perennial monthly mean water masses in the Bohai Sea,the Huanghai Sea and the East China Sea

On the basis of perennial monthly mean temperature and salinity data, the classification of monthly water masses at the surface and the bottom in the Bohai Sea, the Huanghai Sea and the East China Sea, has been made by using the method of fuzzy cluster from the modified characteristic of water masses in the shallow water area. In this paper, the basic features, growth and decline patterns of water masses in relation to fishing grounds in the whole shelves of the Bohai Sea, the Huanghai Sea and the East China Sea are discussed with emphasis.     

The structure of zooplankton communities,in the 2 to 2000 μm size range,in the Arabian Sea during and after the SW monsoon, 1994

Zooplankton communities, studied in the surface mixed layer on a 1600 m transect across the Arabian Sea, were found to differ in their temporal and spatial response to seasonal forcing. The transect studied, spanned seasonally eutrophic upwelling, mesotrophic downwelling and aseasonal oligotrophic waters. The nano- and microzooplankton communities constituted a relatively constant compartment in the tropical monsoon ecosystem, whilst the mesozooplankton showed a clear response to both upwelling and season. The heterotrophic nanoflagellates were concentrated in the surface mixed layer, except in the eutrophic upwelling waters of the SW monsoon. They reached maximum cell concentrations of 855 ml^-1 during the SW monsoon and a maximum biomass of 8.4 mg C m^-3 during the intermonsoon. Nanozooplankton standing stocks, in the surface mixed layer, ranged between 7 and 333 mg C m^-2, with highest stocks found during the intermonsoon. The microzooplankton community was dominated by Protozoa, particularly aloricate ciliates and heterotrophic dinoflagellates, which accounted for up to 99% in terms of numbers and up to 71% of the biomass. Sarcodines and metazoan nauplii were recorded in lower numbers (<400 l^-1). The microzooplankton were also concentrated in the surface mixed layer during both periods, except in the eutrophic coastal waters during the SW monsoon, when relatively high biomass values were found below the mixed layer depth. Their standing stocks, in the surface mixed layer, ranged between 50 and 182 mg C m^-2, with the highest concentration found in the mesotrophic offshore waters during the late monsoon period. Total mesozooplankton standing stocks, in the surface 100 m, decreased with distance from the coastal to offshore waters and between seasons, decreasing from 1248 to 238 mg C m^-2 during the late SW monsoon and 656–89 mg C m^-2 during the following intermonsoon. The largest size class, of 1000–2000 μm sized organisms, dominated throughout except at the oligotrophic station during the intermonsoon period, when the smallest class, of 200–500 μm, were more important. The shift in size structure from large to small zooplankton occurred in response to a shift in dominance from large to small phytoplankton cells both spatially, along a eutrophic–oligotrophic gradient, and seasonally. These responses are a result of the physical forcing associated with the monsoon seasons in the Arabian Sea.     

Facies and seismic analysis of the Late Carboniferous–Early Permian Finnmark carbonate platform (southern Norwegian Barents Sea): An assessment of the carbonate factories and depositional geometries

The late Palaeozoic buried Finnmark platform, in the Norwegian Barents Sea, is a depositional system which developed under major modifications of palaeoclimatic and palaeoceanographic conditions, given by the northward drifting of the Arctic depositional domain beyond the tropical belt, the activity of the Gondwanan icecaps, and the gradual closure of the Eastern European seaway. In this study, detailed core/microfacies analysis from the explorations wells 7128/4-1 and 7128/6-1, combined with new 2D seismic sections and available dataset from previous studies in the eastern Finnmark area, have allowed a re-evaluation of the depositional scenarios associated with the Ørn and Isbjørn/Polarrev Formations. The late Asselian–early Sakmarian depositional record of the Ørn Formation, followed by the middle Sakmarian transition to the Isbjørn/Polarrev Formations, shows depth-related variations in sedimentation modes and a change of the platform morphology.In particular, a shift from Chloroforam to Bryonoderm sedimentation mode took place through a stratified water column during the deposition of the Ørn Formation, over what we suggest was a distally steepened ramp. In the late Sakmarian, the deposition of the Isbjørn/Polarrev Formations, characterised by a full Bryonoderm sedimentation mode, took place through a cooler non stratified water column, leading to the gradual instauration of a homoclinal ramp profile.Among the other results, our integrated study suggests the development of subtidal heterozoan mounds associated with the Ørn Formation, predating the formation of the larger, stacked and well-known heterozoan distal complexes of the Polarrev Formation. Given the good potential of the Ørn Formation within the Finnmark depositional domain, our results may help to improve the assessment of future evaluations of reservoir properties in the area, which are closely associated with the depositional style and their post-depositional modifications.     

Marine diatoms of the Xisha Islands, South China Sea Mastogloia Thw, Ex, Wm, Smith: species of Group Ellipticae

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Temporal variability of amino acid, hexosamine, and carbohydrate fluxes on the eastern Brazilian continental margin related to discharge of the São Francisco River, Brazil

Settling particles collected at 1550 m water depth off the São Francisco River, Brazil, between January and May 1995 showed peak fluxes of amino acids, hexosamines, and carbohydrates, which formed the onset of a three-week period of high organic matter (OM) flux, coinciding with the high discharge period of the river. Two phases of OM deposition exist: (1) the fluvial input of nutrients triggering a bloom of non biomineralizing plankton, and (2) suspended sediment mainly derived from shelf erosion increasing the fluxes of refractory OM. This indicates the importance of seasonally varying hydrodynamic conditions and nutrient input from the continent for the production and sedimentation of OM to the continental margin of eastern Brazil.     

Microfossils, carbonate lysocline and compensation depth in surface sediments of the northeastern South China Sea

INTRODUCTIONTwoimportantinterfacespunctuatingtheabyssalverticaltrans sectionarecarbonatelyso clineandcarbonatecompensationdepth (CCD) .Thecarbonatelysoclineislabeledbyabruptincrementofcarbonatedissolutiononcalcitemicrofossilssuchasplanktonicforaminifertests(Berger,1 96 8) ,ordecreasingapparentlyinpercentageofthecarbonateproportion (Panetal ,1 988) .ThoughtherearemanyachievementsonthecarbonatecycleandCCDresearches(Wangetal ,1 995;Rottman ,1 979;Thunelletal ,1 992 ;Zhengetal ,1 993;Mia…     

Sedimentary environment and glacial history during the last 40 ka of the Vøring continental margin,mid-Norway

This study is based on detailed investigation of sediment cores and high resolution seismics. We identified and describe five lithofacies on the Vøring Plateau and eight on the mid-Norwegian continental slope. The various lithofacies are mainly related to the fluctuations of the Fennoscandian Ice Sheet and the varying intensity of bottom currents and inflow of Atlantic water masses. Ocean circulation was highly variable between 40 and 22 ¹⁴C ka BP, being vigorous during interstadials and sluggish during stadials. Between ca 22 and 15 ¹⁴C ka BP the sedimentary environment was significantly influenced by fluctuations of the Fennoscandian Ice Sheet, repeatedly reaching the outermost shelf. These fluctuations are reflected in the sedimentary record as ice-rafted debris (IRD) accumulation peaks, deposition of stratified diamicton, and glacigenic debris flows on the continental slope. During this period the sediment accumulation rate increased, bottom currents influenced the sedimentary pattern, and surface waters were seasonally ice-free, indicating inflow of Atlantic waters. Subsequent to ca 15 ¹⁴C ka BP the glacier influence decreased as the margin of the Fennoscandian Ice Sheet retreated to reach the coast before 12.5 ¹⁴C ka BP. The modern sedimentary environment is characterised by relatively strong bottom current action, causing winnowing or non-deposition down to approximately 1000 m water depth.     

Characteristics of the outer rise seaward of the Manila Trench and implications in Taiwan–Luzon convergent belt, South China Sea

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.