Southern Okinawa Trough Research Articles

A synproportionation-controlled hybrid hydrothermal mineralization system in the Okinawa Trough

Hybrid hydrothermal systems, in which both water-rock reactions and magmatic fluids contribute to sulfide mineralization, have recently been reported in some submarine arcs and mature back-arc basins. Owing to their high mineralization efficiency, hybrid hydrothermal systems have attracted significant attention from geologists. This study provides first clues for the identification of a novel hybrid hydrothermal system (synproportionation-controlled instead of the more common disproportionation-controlled) in the Tangyin field, Southern Okinawa Trough. The studied samples are native sulfur-type precipitates, primarily composed of elemental sulfur with minor sulfide disseminations. The distinct crystallization conditions and S isotope compositions of elemental sulfur and sulfide disseminations indicate their different genesis. The S in sulfide disseminations (δ34S = −2.7 to 1.6 ‰) is derived from water-rock reaction and subsequently influenced by fluid phase separation, which is further demonstrated by the trace element contents of pyrite (low Tl/Pb, Sb/Pb, Te/As, Te/Sb ratios). In contrast, the elemental sulfur (δ34S = 3.9 to 4.3 ‰) is formed by the synproportionation of magmatic SO2 and hydrothermal H2S from the boiled liquid. A simple two-component mixing calculation indicated that the contributions of the water-rock reactions and magmatic fluids to the Tangyin hydrothermal system were approximately equal. The higher temperature (> 350 °C) and oxygen fugacity conditions may be the key factors for the development of a synproportionation-controlled hybrid hydrothermal system in the Tangyin field. Hydrothermal systems in back-arc basins exhibit higher sulfide accumulation rates than those in mid-ocean ridges; this may be attributed to the prevalence of hybrid hydrothermal vents in back-arc settings.

Elemental Compositions of Clinopyroxenes from Southern Okinawa Trough Basalt: Implications for Magmatism and Structural Environment in an Initial Back-Arc Basin

Elemental Compositions of Clinopyroxenes from Southern Okinawa Trough Basalt: Implications for Magmatism and Structural Environment in an Initial Back-Arc Basin

Contrasted changes of sea-surface salinity in the southern and northern Okinawa Trough since the mid-Holocene

A reconstruction of sea-surface salinity (SSS) in the southern Okinawa Trough (OT) since 6800 cal. yr BP has been presented in this study. We found that the SSS record increased gradually from the middle to late Holocene, indicating reduced summer precipitation in Taiwan. Our results also suggest regional patterns of SSS with SSS increasing in the southern OT and decreasing in the northern OT since the mid-Holocene. Observational data indicate that the SSS in the southern and northern OT are strongly affected by fluvial discharge from Taiwan and the Changjiang River, linked to summer precipitation in South and Central Eastern China, respectively. The contrasting changes in SSS from the OT since the mid-Holocene suggest different spatial patterns of precipitation, with decreasing summer precipitation in South China and increasing summer precipitation in central-eastern China due to the weakened East Asian Summer Monsoon (EASM) and solar insolation. The delayed seasonal transitions of the EASM, with a longer Meiyu stage and a shorter midsummer stage from the mid-Holocene to late Holocene, are consistent with the delayed northward migration of the westerlies in the late Holocene.

Experimental Hydrothermal Alteration of Rhyolite and Andesite at 325 °C and 300 Bar: Implications for a Potential Role of Volcanic Glass in the Fluid Composition in the Okinawa Trough

The experimental study of water–rock reactions under high-temperature and -pressure conditions is a useful approach to constrain controlling factors of the fluid composition in a natural hydrothermal system. Previous studies have focused mainly on the mid-ocean ridge fields, and the hydrothermal alteration of intermediate-to-felsic rocks has been less emphasized despite its potential importance in the fluid chemistry in an arc/back-arc basin setting. We examined the alteration processes of fresh rhyolite and andesite rocks collected from the middle and southern Okinawa Trough, respectively, at 325 °C and 300 bar (the estimated condition at the reaction zone in the fields), especially focusing on the behavior of silica between the solid and liquid phases. The experimental fluids are characterized by the high Si concentration up to 30 mM, indicating the substantial dissolution of volcanic glass in the analyzed rocks. The high Si concentration in the fluids was presumably buffered by amorphous silica, precipitated from the fluids as a precursor of hydrothermal quartz, during the experiments. Our results emphasize a previously overlooked role of volcanic glass/amorphous silica in the fluid composition in the Okinawa Trough and are consistent with the previous model of pumice replacement mineralization for the SMS deposit formation in the trough.

Genetic divergence and migration patterns of a galatheoid squat lobster highlight the need for deep-sea conservation.

Information on genetic divergence and migration patterns of vent- and seep-endemic macrobenthos can help delimit biogeographical provinces and provide scientific guidelines for deep-sea conservation under the growing threats of anthropogenic disturbances. Nevertheless, related studies are still scarce, impeding the informed conservation of these hotspots of deep-sea biodiversity. To bridge this knowledge gap, we conducted a population connectivity study on the galatheoid squat lobster Shinkaia crosnieri - a deep-sea foundation species widely distributed in vent and seep ecosystems in the Northwest Pacific. With the application of an interdisciplinary methodology involving population genomics and oceanographic approaches, we unveiled two semi-isolated lineages of S. crosnieri with limited and asymmetrical gene flow potentially shaped by the geographic settings, habitat types, and ocean currents - one comprising vent populations in the Okinawa Trough, with those inhabiting the southern trough area likely serving as the source; the other being the Jiaolong (JR) seep population in the South China Sea. The latter might have recently experienced a pronounced demographic contraction and exhibited genetic introgression from the Okinawa Trough lineage, potentially mediated by the intrusion of the North Pacific Intermediate Water. We then compared the biogeographic patterns between S. crosnieri and two other representative and co-occurring vent- and seep-endemic species using published data. Based on their biogeographical subdivisions and source-sink dynamics, we highlighted the southern Okinawa Trough vents and the JR seep warrant imperative conservation efforts to sustain the deep-sea biodiversity in the Northwest Pacific.

Connecting link between a sediment source and its deep-sea sink: Ilan Shelf offshore northeastern Taiwan

The northeast Taiwan margin provides an excellent example of the route of sediment flux from a small mountainous river, the Lanyang River, to a deep-sea basin, the Southern Okinawa Trough (SOT), via a very narrow（8–14 km in width）Ilan Shelf at different time scales. Based on seismic and sub-bottom chirp profiles and onshore borehole and offshore core data, we identify three echo types on the Ilan Shelf that delineate sheets of sediment, mass movement deposits, and echoes indicative of igneous activity. By recognizing lithofacies and seismic facies changes, the Last Glacial Maximum Unconformity and maximum flooding surface are correlated in onshore cores and offshore reflection seismic profiles. With the aids of sequence stratigraphic analysis, the postglacial sedimentation rates on the Ilan Shelf are estimated to range from 0.04 to 1.53 cm/yr. Considering its sediment budget in a source-to-sink system, the Ilan Shelf can be regarded primarily as a temporary trap for fluvial sediment discharged from the Lanyang River. Overall, it has trapped at least 50% of the discharged sediment during the Holocene, and the bulk of sediment was transported, primarily via mass movement to SOT. We deduce that earthquake-induced faulting, axial incision in the channel, and retrogressive failures appear to be triggers for mass wasting, which then combined with channel transport, can play leading roles in rapid downslope transportation to the trough. This inference is indicated by high deposition rates in the western end of SOT and widely distributed mass transport deposits observed in the shallow strata of SOT. We found that the deep strata of SOT are deformed by normal faults, indicating that the sediment accommodation space is structurally controlled by the active back-arc extension. This study documents that frequent typhoons and flooding events, together with strong tidal currents, are significant agents of sediment input over the short term. In contrast, earthquakes and volcanic events occasionally affect sediment dispersal from the Ilan Shelf to SOT.

Origins of magmatism in the Okinawa Trough: Fe–Sr–Nd–Pb–Hf isotopic constraints

ABSTRACT The Okinawa Trough is an incipient back-arc basin formed by the subduction of the Philippine Sea Plate beneath the Eurasian Plate and is a natural laboratory for studying basin evolution, magmatism, and crust-mantle interactions during the early stages of back-arc spreading. Back-arc spreading and recent magmatism are occurring mainly in the Middle Okinawa Trough (MOT) and Southern Okinawa Trough (SOT). In order to investigate the magmatism in the Okinawa Trough, volcanic rock samples from the MOT and SOT were analysed for Fe – Sr – Nd – Pb – Hf isotopic compositions. The Fe isotopic compositions of basaltic andesite samples from the SOT are similar to or lower than MORBs. The fluid/melt derived from the subducted slab, especially the fluid dehydrated from slab serpentinites and sediment-derived melts, entered the overlying depleted mantle wedge and participated in magma generation, which resulted in the low δ56Fe values of the SOT basaltic magmas. The δ56Fe values of volcanic rocks from the MOT increase with increasing SiO2 contents, suggesting a gradual enrichment of heavy Fe isotopes during the evolution of the magma as it ascended to the surface. While the Sr – Nd – Pb – Hf isotopic compositions do not vary significantly, suggesting that the different samples are the products of different degrees of crystal fractionation of comagma. The fractional crystallization of olivine, pyroxene, and ilmenite minerals, which enrich in light Fe isotopes, resulted in the gradual increase in Fe isotopic values of magma during its evolution. The dacites and rhyolites from the southernmost Okinawa Trough have the similar δ56Fe values with the basaltic andesite and the basalt from the SOT and MOT, accompanied by significantly lower Nd and Hf isotope ratios relative to basalts, reflecting that the contamination of crustal materials would not change the magma in Fe isotope compositions evidently. Compared with the MOT, the volcanic rocks from the SOT was more affected by subduction components on their underlying mantle, and those from the southernmost experienced more contamination by crustal materials in the evolving magma.

Deep genesis of the high heat flow anomaly in the Okinawa Trough

The Okinawa Trough (OT) is characterized by high heat flow anomalies up to 10,000 mW/m2 (especially in the middle area), which is rarely seen in heat flow research all over the world. The genesis of the high heat flow anomaly is probably related to mantle upwelling. However, the deep genesis is yet to be studied. We used heat flow values collected from the Global Heat Flow Database to calibrate the locations of high heat flow anomalies. Gravity data were used to calculate the residual geoid and interlayer geoid anomalies, which can help in characterizing the morphology of mantle upwelling beneath the OT. Earthquake hypocenter distribution maps were presented to show the position of the Philippine Sea Plate beneath the OT. According to P-wave tomography profile and previous research, we discussed the deep genesis of the high heat flow anomaly and proposed a preliminary model to explain the process. We suggest that the deep mantle upwelling, probably from a depth of −1200 km, enters the upper mantle through the front of the subducted Pacific plate. The deep mantle upwelling converges with the molten mantle flow caused by the fluid from the dehydration of the Pacific Plate and the Philippine Sea Plate. Two mantle upwelling branches were revealed to feed the southern OT (SOT) and Changbai Volcano, and the mantle upwelling under the middle OT (MOT) was the second branch from the mantle upwelling branch under the SOT. The MOT should be more influenced by mantle upwelling from deep sources with extremely high temperatures.

Muricauda okinawensis sp. Nov. and Muricauda yonaguniensis sp. Nov., Two Marine Bacteria Isolated from the Sediment Core near Hydrothermal Fields of Southern Okinawa Trough.

Two strains, 81s02T and 334s03T, were isolated from the sediment core near the hydrothermal field of southern Okinawa Trough. The cells of both strains were observed to be rod-shaped, non-gliding, Gram-staining negative, yellow-pigmented, facultatively anaerobic, catalase and oxidase positive, and showing optimum growth at 30 °C and pH 7.5. The strains 81s02T and 334s03T were able to tolerate up to 10% and 9% (w/v) NaCl concentration, respectively. Based on phylogenomic analysis, the average nucleotide identity (ANI) and the digital DNA-DNA hybridization (dDDH) values between the two strains and the nearest phylogenetic neighbors of the genus Muricauda were in range of 78.0-86.3% and 21.5-33.9%, respectively. The strains 81s02T and 334s03T shared 98.1% 16S rRNA gene sequence similarity to each other but were identified as two distinct species based on 81.4-81.5% ANIb, 85.5-85.6% ANIm and 25.4% dDDH values calculated using whole genome sequences. The strains 81s02T and 334s03T shared the highest 16S rRNA gene sequence similarity to M. lutimaris SMK-108T (98.7%) and M. aurea BC31-1-A7T (98.8%), respectively. The major fatty acid of strains 81s02T and 334s03T were identified similarly as iso-C15:0, iso-C17:0 3-OH and iso-C15:1 G, and the major polar lipids of the both strains consisted of phosphatidylethanolamine and two unidentified lipids. The strains contained MK-6 as their predominant menaquinone. The genomic G+C contents of strains 81s02T and 334s03T were determined to be 41.6 and 41.9 mol%, respectively. Based on the phylogenetic and phenotypic characteristics, both strains are considered to represent two novel species of the genus Muricauda, and the names Muricauda okinawensis sp. nov. and Muricauda yonaguniensis sp. nov. are proposed for strains 81s02T (=KCTC 92889T = MCCC 1K08502T) and 334s03T (=KCTC 92890T = MCCC 1K08503T).

Pre-eruptive water content and volatile degassing processes in the southern Okinawa Trough magma: Implications for subduction zone water recycling and magmatic contributions to hydrothermal systems

Pre-eruptive water content and volatile degassing processes in the southern Okinawa Trough magma: Implications for subduction zone water recycling and magmatic contributions to hydrothermal systems

S, Pb, and Fe isotope compositions of sulfides in middle and southern Okinawa Trough: Implying the complicated hydrothermal systems in back-arc spreading centers

Sulfur, lead, and iron isotopic systematics of sulfides from the Iheya North hydrothermal field in middle Okinawa Trough and the Yonaguni Knoll IV hydrothermal field in southern Okinawa Trough were investigated to understand the origin of ore-forming material and isotope fractionation during sulfide precipitation in back-arc spreading centers. The δ34S values (from 8.60‰ to 9.68‰) of sulfide minerals in Iheya North field suggest sulfur was derived from the leaching of basement rocks and the reduction of seawater sulfate. But an additional light sulfur source from the dissolution of biogenic sulfides in sediments is indispensable for the observed low δ34S values (between 4.78‰ and 4.97‰) of Yonaguni Knoll IV sulfides. Lead isotope compositions of all sulfides vary in a relatively wide range (206Pb/204Pb = 18.432 to 18.563, 207Pb/204Pb = 15.579 to 15.671, and 208Pb/204Pb = 38.539 to 38.979, respectively) which involves the mixing of mantle-derived and sediment-derived lead in varying proportions. Unusually high radiogenic lead isotope ratios of Yonaguni Knoll IV sulfides (207Pb/204Pb = 15.628 to 15.671, and 208Pb/204Pb = 38.854 to 38.979) imply the presence of old crustal fragments beneath southern OT. The hydrothermal fluids in OT are characterized by extremely low calculated δ56Fe values (−1.00‰ to −1.32‰) which is attributed to reducing sediments. The paired Fe-isotope compositions of pyrite and chalcopyrite revealed significantly different iron isotope compositions of hydrothermal fluids in study areas and extremely complicated ore-forming processes. Rapid precipitation of pyrite from FeS precursors accompanying iron isotopic disequilibrium moves from FeS-fluid equilibrium to pyrite-fluid equilibrium during pyrite recrystallization. The combination of sulfur, lead, and iron isotopic data suggests that hydrothermal systems in back-arc spreading centers related to plate subduction are generally affected by mantle, overlying sediments as well as potential old continental crust fragments, which results in various hydrothermal fluids and complicated mineralization processes.

Genomic insights into phage-host interaction in the deep-sea chemolithoautotrophic Campylobacterota, Nitratiruptor

The genus Nitratiruptor represents one of the most numerically abundant chemolithoautotrophic Campylobacterota populations in the mixing zones of habitats between hydrothermal fluids and ambient seawater in deep-sea hydrothermal environments. We isolated and characterized four novel temperate phages (NrS-2, NrS-3, NrS-4, and NrS-5) having a siphoviral morphology, infecting Nitratiruptor strains from the Hatoma Knoll hydrothermal field in the southern-Okinawa Trough, Japan, and conducted comparative genomic analyses among Nitratiruptor strains and their phages. The Nitratiruptor temperate phages shared many potential core genes (e.g., integrase, Cro, two structural proteins, lysozyme, and MazG) with each other despite their diverse morphological and genetic features. Some homologs of coding sequences (CDSs) of the temperate phages were dispersed throughout the non-prophage regions of the Nitratiruptor genomes. In addition, several regions of the phage genome sequences matched to spacer sequences within clustered regularly interspaced short palindromic repeats (CRISPR) in Nitratiruptor genomes. Moreover, a restriction-modification system found in a temperate phage affected an epigenetic feature of its host. These results strongly suggested a coevolution of temperate phages and their host genomes via the acquisition of temperate phages, the CRISPR systems, the nucleotide substitution, and the epigenetic regulation during multiple phage infections in the deep-sea environments.

Planktic-benthic foraminifera ratio (%P) as a tool for the reconstruction of paleobathymetry and geohazard: A case study from Taiwan

The calcite tests of foraminifera are an important biogenic component of marine sediments. The abundance of foraminiferal tests in marine sediments broadly varies with bathymetry, thus has been used to reconstruct paleobathymetry. It is also promising as a tracer for downslope transport triggered by earthquakes and typhoons, especially if the displaced material from shallow locality contrasts strongly with the background autochthonous sediments in terms of foraminiferal abundance, such as the ratio of benthic and planktic foraminifera termed %P. However, its applicability in sediments off Taiwan has not been assessed. Taiwan is located in the path of typhoons and at tectonic plate margins, where typhoons and earthquakes may trigger submarine geohazards. This, combined with the fact that its seafloor spans a large bathymetric range, render this region an ideal natural laboratory to evaluate the applicability of %P as a proxy for tracing submarine geohazards and bathymetry. Here we report foraminiferal abundance, %P, grain size and elemental data from 148 surface sediment samples off 6 sectors off Taiwan, namely Southern Okinawa Trough, Hoping-Nanao-Hateruma Basins, Taitung-Hualien, Hengchun Ridge, Gaoping, and Changyun Sand Ridge. Of all the hydrographic and sedimentological parameters assessed, seafloor bathymetry is the major driver of foraminiferal abundance and %P in these regions. Notably, several data points deviate from the regional %P-water depth relationship. Based on sedimentological parameters and previous studies, we posit that these outliers may have to do with local sedimentation setting. These processes include earthquake-induced sediment transport via submarine canyon in the Southern Okinawa Trough, typhoon-triggered sediment flushing in Gaoping Canyon, cross-shelf and northward advection of planktic foraminifera on the Gaoping shelf, and carbonate dissolution in the deep Hateruma Basin. Off Taiwan, the %P value in sediments increases exponentially with bathymetry (R2 = 0.72, n = 81), and agrees well with the global calibration obtained by combining data from several regions of the global ocean (R2 = 0.86, n = 1004). The regional %P-water depth relationship may be useful for reconstructing paleobathymetry here, albeit with an uncertainty in the range of 14–1600 m. The uncertainty increases with water depth. Our results also highlight the potential of the %P index as a tracer for downslope transport and lateral advection in the water column. In conclusion, the downcore application of %P has the potential to reconstruct past geohazard events while also identifying autochthonous sediment sequences that are suitable for paleoceanographic reconstruction.

Magnesium isotopic composition of back-arc basin lavas and its implication for the recycling of serpentinite-derived fluids

Dehydrated fluids expelled from the serpentinised mantle in subducted slabs contribute significantly to arc magmatism and element cycling on Earth. However, the recycling mechanism of serpentinite-derived fluids at various subduction zone depths remains unclear, as does their contribution to the genesis of back-arc basin lavas. In this study, we investigated magnesium (Mg) isotopic compositions of lavas from the Okinawa Trough (OT) and the Lau Basin, because Mg isotopes have shown great potential in tracing the dehydration of slab serpentinites in recent years. After excluding dubious samples affected by alteration, lavas from the OT and the Lau Basin have heavier Mg isotopic compositions than mid-ocean ridge basalt (MORB) mantle, which could be attributed to the involvement of slab serpentinite-derived fluids rather than crustal assimilation or subducted sediment inputs, as indicated by the isotopic modelling results. The δ26Mg values of the southern OT (SOT) and the southern Lau Basin lavas are generally higher than those in the middle OT (MOT) and the northern Lau Basin, respectively, with an average of −0.09 ± 0.02‰ (2SD, n = 6) for the SOT, −0.18 ± 0.10 ‰ (2SD, n = 9) for the MOT, −0.13 ± 0.08 ‰ for the southern Lau Basin (2SD, n = 5), and − 0.19 ± 0.06 ‰ (2SD, n = 10) for the northern Lau Basin. The binary modelling results show that various quantities of serpentinite-derived fluids could explain the variations in Mg isotopic compositions observed in the OT and the Lau Basin. We suggest that the thermal structure of the inter-subduction zone controls the signal of Mg-rich serpentinite-derived fluids, based on both reported δ26Mg values in subduction zones and our findings. In contrast, the contributions of these fluids to different segments in a specific subduction zone may depend on the slab depth beneath sites of magmatic activity.

Heavy Copper Isotopes in Arc‐Related Lavas From Cold Subduction Zones Uncover a Sub‐Arc Mantle Metasomatized by Serpentinite‐Derived Sulfate‐Rich Fluids

AbstractSerpentinite dehydration during subduction releases fluids into the sub‐arc mantle, yet the oxidizing capability of these fluids remains controversial. Here, redox‐sensitive Cu isotopes and in situ S isotopes of magmatic sulfides in fresh lavas from three subduction zones in the western Pacific were analyzed to determine the speciation of multi‐valence elements in the serpentinite‐derived fluids. Notably, the basaltic lavas from the Mariana Arc, eastern Manus Basin, and southern Okinawa Trough have isotopically heavy Cu (δ65Cu = +0.26 to +0.67‰) compared to the mantle‐like δ65Cu values of the middle Okinawa Trough and Mariana Trough basalts (+0.11 to +0.14‰). In addition, positive correlations between δ65Cu and slab fluid indicators (e.g., B/Nb, Ba/Th, and Ba/La) combined with positive δ34S values (∼+4‰) of magmatic sulfides in the Mariana Arc lavas, suggest that fluids with isotopically heavy Cu and S metasomatized their mantle sources. Theoretically, the bearing fluids released by serpentinite dehydration have extremely heavy Cu and S isotope compositions. Consequently, the sulfate‐rich fluids infiltration into the sub‐arc mantle produces heavy Cu and S isotopes in arc‐related magmas. Comparatively, sulfate‐rich fluids are absent at back‐arc depths due to the complete breakdown of serpentine minerals during the early stages of subduction, which results in the mid‐ocean ridge basalts‐like δ65Cu and δ34S values (∼−1‰) observed in the middle Okinawa Trough and Mariana Trough basalts. Therefore, heavy Cu isotopes in arc‐related magmas uncover that the sub‐arc mantle was metasomatized by serpentinite‐derived sulfate‐rich (oxidizing) fluids, which accounts for the oxidized nature of arc magmas in cold subduction zones.

Heterotrophic Sulfur Oxidation of Halomonas titanicae SOB56 and Its Habitat Adaptation to the Hydrothermal Environment.

Halomonas bacteria are ubiquitous in global marine environments, however, their sulfur-oxidizing abilities and survival adaptations in hydrothermal environments are not well understood. In this study, we characterized the sulfur oxidation ability and metabolic mechanisms of Halomonas titanicae SOB56, which was isolated from the sediment of the Tangyin hydrothermal field in the Southern Okinawa Trough. Physiological characterizations showed that it is a heterotrophic sulfur-oxidizing bacterium that can oxidize thiosulfate to tetrathionate, with the Na2S2O3 degradation reaching 94.86%. Two potential thiosulfate dehydrogenase-related genes, tsdA and tsdB, were identified as encoding key catalytic enzymes, and their expression levels in strain SOB56 were significantly upregulated. Nine of fifteen examined Halomonas genomes possess TsdA- and TsdB-homologous proteins, whose amino acid sequences have two typical Cys-X2-Cys-His heme-binding regions. Moreover, the thiosulfate oxidation process in H. titanicae SOB56 might be regulated by quorum sensing, and autoinducer-2 synthesis protein LuxS was identified in its genome. Regarding the mechanisms underlying adaptation to hydrothermal environment, strain SOB56 was capable of forming biofilms and producing EPS. In addition, genes related to complete flagellum assembly system, various signal transduction histidine kinases, heavy metal transporters, anaerobic respiration, and variable osmotic stress regulation were also identified. Our results shed light on the potential functions of heterotrophic Halomonas bacteria in hydrothermal sulfur cycle and revealed possible adaptations for living at deep-sea hydrothermal fields by H. titanicae SOB56.

Sedimentary sequences offshore northeastern Taiwan and the offshore projection of the Shanjiao Fault zone

The offshore area of northern Taiwan is currently in an extensional regime, known for a post-collision area of the Taiwan mountain belt that has formed due to the oblique collision of the Philippine Sea Plate against the Eurasian continental margin since the late Miocene. The active collision has undergone a progressive migration of the Taiwan mountain belt from the northeast to the southwest direction. After the relaxation of the compressional stress due to the westward subduction of the Philippine Sea Plate, the northern Taiwan orogen has collapsed together with the opening of the southern Okinawa Trough. Here, we show the transitional process from compressional to extensional regime in the area off northern Taiwan and reconstruct the tectono-sedimentary evolution by using multi-channel reflection seismic data. We process and interpret multi-channel seismic profiles and use borehole data to determine the ages of sedimentary units and stratal surfaces/unconformities. Based on the seismic facies, we have recognized two distinctive domains separated by the Offshore Shanjiao Fault (OSF): a shelf basin to the northwest and a collapsed zone to the southeast. The tectono-sedimentary evolution off northern Taiwan since the late Miocene can be described in 4 stages. (1) The compressional structures off northern Taiwan were formed during the collision period starting around 6 Ma, a series of folds-and-thrusts aligned in the NE-SW direction. (2) Due to the oblique collision, some NW-SE-trending strike-slip faults were developed across the collision belt to accommodate the differential displacements between two adjacent segments of the folded belt. (3) During the transition from compressional to extensional regime at about 2.7 Ma, a post-collisional erosion and subsidence occurred in the collapsed zone to form the basal unconformity. This post-collisional collapse is associated with the opening of the southern Okinawa Trough, a back-arc basin, due to the westward migration of the Philippine Sea Plate subduction beneath the southern Ryukyu arc. (4) The offshore area of northern Taiwan has subsided continuously with sediment accumulation since the early Pleistocene (~2.0 Ma), accompanying the rifting of the southern Okinawa Trough.

The Huapinghsu Channel/Mienhua Canyon System as a Sediment Conduit Transporting Sediments From Offshore North Taiwan to the Southern Okinawa Trough

By examining bathymetric and seismic reflection data in the shelf-slope region offshore northeast of Taiwan, the morphology of the Huapinghsu Channel/Mienhua Canyon System was refined and the occurrence of axial incision in the major erosional trough of the Mienhua Canyon was identified. The Huapinghsu Channel extends from its head at a water depth of 120 m seaward across the shelf-slope region and merges with the Mienhua Canyon before finally emptying into the Southern Okinawa Trough. This review with a new idea illustrates this channel/canyon system as a sediment conduit transporting sediments from western Taiwan rivers to the Southern Okinawa Trough. Within the proximal reach of the Mienhua Canyon, in situ suspended particles were found associated with bottom nepheloid layers, which transport resuspended particles along the canyon course, and eventually into the Southern Okinawa Trough even during sea-level highstand. The shelf area around the channel head is subject to the influence of frequent occurrences of large typhoons and seasonal migrations of oceanic currents. Numerical examinations indicate that the flow velocities related with large waves or currents satisfy the critical values to the threshold of the motion of fine sand, implying gravity flows occur around the channel head, sweep unconsolidated sediments in the channel head, or even incise the channel floor at the present day. In this paper, sea level changes from the Last Glacial Maximum (LGM, ∼21 ka BP) to the present and sediment input to the head of the Huapinghsu Channel head are considered the major factors in the control of the Huapinghsu Channel/Mienhua Canyon system, which functions as a sediment conduit in the study area. Tectonics and climate are the minor factors that contribute to shelf sediment transport to the Southern Okinawa Trough via this channel/canyon system.

Detrital Zircon Provenance in the Sediments in the Southern Okinawa Trough

The provenance of sediments in the Southern Okinawa Trough since the late Holocene has been a controversial scientific issue during the past 20 years. Previous studies based on isotope proxies generally indicated Taiwanese rivers as the primary source in the Southern Okinawa Trough since the late Holocene. Based on the zircon U-Pb geochronology, this study identified how sediments from the Yangtze River/East China Sea shelf had contributed significantly to the Southern Okinawa Trough in the past 624 a BP. Notably, this study found two Paleoarchean zircon grains, which indicated they originated from older orogenic belts. These data shed new light on the provenance of sediments, and a partial supply from the mainland of China cannot be excluded.

Lithium, Oxygen and Magnesium Isotope Systematics of Volcanic Rocks in the Okinawa Trough: Implications for Plate Subduction Studies

Determining the influence of subduction input on back-arc basin magmatism is important for understanding material transfer and circulation in subduction zones. Although the mantle source of Okinawa Trough (OT) magmas is widely accepted to be modified by subducted components, the role of slab-derived fluids is poorly defined. Here, major element, trace element, and Li, O and Mg isotopic compositions of volcanic lavas from the middle OT (MOT) and southern OT (SOT) were analyzed. Compared with the MOT volcanic lavas, the T9-1 basaltic andesite from the SOT exhibited positive Pb anomalies, significantly lower Nd/Pb and Ce/Pb ratios, and higher Ba/La ratios, indicating that subducted sedimentary components affected SOT magma compositions. The δ7Li, δ18O, and δ26Mg values of the SOT basaltic andesite (−5.05‰ to 4.98‰, 4.83‰ to 5.80‰ and −0.16‰ to −0.09‰, respectively) differed from those of MOT volcanic lavas. Hence, the effect of the Philippine Sea Plate subduction component, (low δ7Li and δ18O and high δ26Mg) on magmas in the SOT was clearer than that in the MOT. This contrast likely appears because the amounts of fluids and/or melts derived from altered oceanic crust (AOC, lower δ18O) and/or subducted sediment (lower δ7Li, higher δ18O and δ26Mg) injected into magmas in the SOT are larger than those in the MOT and because the injection ratio between subducted AOC and sediment is always >1 in the OT. The distance between the subducting slab and overlying magma may play a significant role in controlling the differences in subduction components injected into magmas between the MOT and SOT.