Active Back-arc Basin Research Articles

The crustal structure inferred from gravity data and seismic profiles beneath the Okinawa Trough in the western Pacific

• The partitioned inversion is employed to calculate the depth of the Moho interface. • The depth of the Moho interface varies from 16 km to 26 km. • The stretching factor ranges from 1.4 to 2.5, increasing along the direction from north to south. • The inversion results indicate the crustal thinning in the Okinawa Trough. The Okinawa Trough is an active back-arc basin of the Ryukyu Island arc-trench system caused by the subduction of the Philippine Sea plate. The different controlling effects of plate subduction result in the unique characteristic of the crustal structure, fault development and magmatic activities. Studies on the crustal structure and properties are better to understand the origin and evolution of back-arc basins. To overcome the non-uniqueness of the gravity interface inversion and spatial limitation of seismic profiles, 19 published seismic results are selected for the soft constrained strategy. Furthermore, the partitioned interface inversion with the exponential density contrast model is utilized to obtain more detailed undulations of the Moho interface. The crustal thickness and the stretching factors of the Okinawa Trough are obtained as well. The results are reliable after the evaluation of the fitting degree of observed data, and the evaluation of recovered data and selected constraint points respectively. The results show that the overall variation trend of the crustal thickness in the study area gradually decreases from the west to east. The crustal thickness of the Okinawa Trough is larger than that of the East China Sea shelf basin, ranging from 12 to 22 km. The Okinawa Trough is a transitional crust formed during the transformation from continental crust to oceanic crust, with the overall characteristics of extensional and thinning continental crust. However, a new oceanic crust may have accreted in the central graben of the southern sections of the trough.

Lithostratigraphy, Lithogeochemistry, and Tectono-Magmatic Framework of the ABM Replacement-Style Volcanogenic Massive Sulfide (VMS) Deposit, Finlayson Lake District, Yukon, Canada

Abstract The Upper Devonian ABM deposit is a bimodal-felsic, replacement-style volcanogenic massive sulfide (VMS) deposit within the Finlayson Lake district in Yukon, Canada. The deposit is hosted by predominantly felsic volcanic rocks of the upper Kudz Ze Kayah formation that were deposited in an active back-arc basin in three sequences consisting of interbedded felsic volcaniclastic rocks and argillites, and felsic lava flows and domes, and felsic and mafic sills. The felsic rocks fall into two groups, Felsic A and Felsic B (FA and FB), based on immobile elements and their ratios. Relative to the FB group, the FA group has high Zr concentrations (>550 ppm) and generally higher contents of high field strength elements. The FA/FB chemostratigraphy roughly coincides with the lithostratigraphic sequences. Sequence 2 hosting the mineralization consists of FB felsic rocks; the hanging-wall Sequence 3 and footwall Sequence 1 felsic rocks have FA signatures. An argillite lens, recording a period of volcanic quiescence, occurs at the upper contact of Sequence 2. From reconstruction of the basin architecture, two sets of synvolcanic faults are inferred. The synvolcanic faults were interpreted based on thickness changes of volcanosedimentary units and the distribution of coherent rocks. During breaks in volcanism, synvolcanic faults acted as conduits for upwelling hydrothermal fluids, which were diverted laterally into unconsolidated volcaniclastic rocks and formed the replacement-style VMS mineralization. Although the mineralized lenses are hosted by FB felsic rocks, their replacement-style nature implies that the mineralizing processes occurred during the break in volcanism and were genetically associated with the overlying FA felsic volcanic rocks.

Spatial variations of effective elastic thickness of the lithosphere in the Okinawa Trough

The Okinawa Trough is an active back-arc basin of the Ryukyu island arc-trench system caused by the Phillippine Sea plate subduction. Its unique location makes it attractive to study the lithosphere of continental margins, and it is of great significance to understand the dynamic mechanism of the trench-arc-basin convergent plate boundary. In this paper, we developed the spectral correlation method to calculate the effective elastic thickness of the lithosphere in the study area. The results show that the effective elastic thickness of the lithosphere is distributed in the range of 9–30 km and the lithosphere strength gradually decreases accordingly. Comparing the spatial variations of effective elastic thickness with the data sets of tectonic activities, we conclude that: The spatial variations of the effective elastic thickness have the negative correlation with that of the heat flow. The thermal tectonic activities of the trough gradually become intense from north to south. There are close links between the spatial variations of the effective elastic thickness and the distribution earthquakes. Most of earthquakes are distributed in the relatively lower effective elastic thickness areas or steep effective elastic thickness gradient zones, which indicate that the weak lithosphere and areas with steep change of effective elastic thickness are prone to accumulate and then release tectonic stresses causing earthquakes. The total characteristics of tectonic activities of the Okinawa Trough indicate a strong trend from north to south in the deep region, and the opposite in the shallow region.

Neogene subsidence pattern in the multi-episodic extension systems: Insights from backstripping modelling of the Okinawa Trough

Abstract Extensional systems of back-arc basins have been widely recognized and studied in SE Asia, however the interactions between extensional tectonics and spatial-temporal distributions of tectonic subsidence in an active back-arc basin are poorly known. Using a backstripping technique, we analysed 33 multichannel seismic profiles to calculate the Neogene tectonic subsidence across the entire Okinawa Trough (OT) for the first time. Reconstruction of the tectonic subsidence shows that the OT can be roughly divided into four subsidence stages with variable rate. By comparing the temporal-spatial distribution of the intensive subsidence area, we confirmed that rifting events first occurred in the northern segment (NOT) and middle segment (MOT) from the Miocene to Late Pliocene, and then propagated to the southern segment (SOT) after the Late Pliocene. The factors controlling the rapid tectonic subsidence over the entire OT can be classified into two categories: 1) NE-to NNE-trending normal faulting, related to extension of the OT, and NW- to NWW-trending strike-slip faulting; i.e., the Tokara Fault and the Miyako Fault, related to subduction of the Philippine Sea Plate under the Eurasian Plate; 2) The anomalous rapid subsidence occurred in the southernmost part of the SOT is likely related to the subducted Gagua Ridge which caused magma intrusion resulting from partial melting. We suggest that magmatic bodies produced by partial melting of subducted materials intruded into the crust and created a dense lithosphere, causing this rapid tectonic subsidence.

Seismic structure of rifting in the Okinawa Trough, an active backarc basin of the Ryukyu (Nansei-Shoto) island arc\u2013trench system

The Okinawa Trough, to the southwest of Kyusyu, Japan, is an active backarc basin of the Ryukyu (Nansei-Shoto) island arc–trench system caused by the Philippine Sea plate subduction. Unlike other backarc basins around Japan, the Okinawa Trough, ~ 1000 km in length, is unique because the crustal thinning due to backarc rifting is currently in progress in the entire trough. We conducted extensive seismic reflection and refraction surveys to detect detailed variation in seismic structures associated with the rifting tectonics. Seventeen seismic lines were shot, including ten across-trough and seven along-trough lines in the Okinawa Trough. Moho depths estimated mainly from PmP travel times indicate that the crust beneath the trough is thinner than that below the East China Sea shelf and the Ryukyu Island Arc. The shallowest Moho of the across-trough lines was not necessarily detected at the center of the trough, defined as the deepest water depth, but was located beneath the western margin of the northern and middle Okinawa Trough. An M7.1 earthquake occurred in the area on November 14, 2015, and intense aftershock activity was observed along the western margin of the trough. These earthquakes with extension axes of northwest–southeast direction demonstrated that the area is undergoing tectonic rifting. The Moho depth is over 25 km in the northern region of the Okinawa Trough and decreases down to ~ 13 km as it goes to the south, and the thinnest crust of ~ 7 km occurs beneath the Yaeyama Rift in the southern Okinawa Trough. Despite the crustal thinning by the backarc rifting, the P wave velocity models across the Okinawa Trough show that the continental/island arc crust composed of an upper, middle, and lower crusts is continuous between the East China Sea shelf and the Ryukyu Arc. The multichannel seismic profiles along and across the rifts in the southern Okinawa Trough show more volcanic intrusions in the southern side than in the north, which may be related to the position of the volcanic front, which is undetermined in this region.

Evolution of Cambrian and Early Ordovician arcs in the Kyrgyz North Tianshan: Insights from U-Pb zircon ages and geochemical data

Geochronological, geochemical, and structural studies of magmatic and metamorphic complexes within the Kyrgyz North Tianshan (NTS) revealed an extensive area of early Palaeozoic magmatism with an age range of 540–475 Ma. During the first episode at 540–510 Ma, magmatism likely occurred in an intraplate setting within the NTS microcontinent and in an oceanic arc setting within the Kyrgyz-Terskey zone in the south. During the second episode at 500–475 Ma, the entire NTS represented an arc system. These two phases of magmatism were separated by an episode of accretionary tectonics of uncertain nature, which led to obduction of ophiolites from the Kyrgyz-Terskey zone onto the microcontinent. The occurrence of zircon xenocrysts and predominantly negative whole-rock ɛNd(t) values and ɛHf(t) values of magmatic zircons suggest a continental setting and melting of Precambrian continental sources with minor contributions of Palaeozoic juvenile melts in the generation of the magmatic rocks. The late Cambrian to Early Ordovician 500–475 Ma arc evolved mainly on Mesoproterozoic continental crust in the north and partly on oceanic crust in the south. Arc magmatism was accompanied by spreading in a back-arc basin in the south, where supra-subduction ophiolitic gabbros yielded ages of 496 to 479 Ma. The relative position of the arc and active back-arc basin implies that the subduction zone was located north of the arc, dipping to the south. Variably intense metamorphism and deformation in the NTS reflect an Early Ordovician orogenic event at 480–475 Ma, resulting from closure of the Djalair-Naiman ophiolite trough and collision of the Djel'tau microcontinent with the northern margin of NTS. Comparison of geological patterns and episodes of arc magmatism in the NTS and Chinese Central Tianshan indicate that these crustal units constituted a single early Palaeozoic arc and were separated from the Tarim Craton by an oceanic basin since the Neoproterozoic.

Magma plumbing and hybrid magma formation at an active back-arc basin volcano: North Su, eastern Manus basin

North Su is the central volcanic edifice of the SuSu Knolls hydrothermal district, located in an extensional transform zone of the eastern Manus back-arc basin. The submarine volcano hosts a vigorous hydrothermal system with black and white smokers, diffuse fluid discharge, and accumulation of native sulfur. Among its dacitic to andesitic lavas are remarkable hybrid rocks, containing highly magnesian olivine (Fo81–94) phenocrysts with thin orthopyroxene reaction rims in a dacitic matrix. We investigated these rocks in order to understand their formation, to constrain temperature and depth of magma storage, and to identify links between magmatic processes and hydrothermal activity. The compositions, zonations and reaction rims of phenocrysts suggest that the hybrid lavas formed by mixing when dacitic to rhyodacitic magma was invaded by Mg-rich andesite to basaltic andesite with boninite affinities. The resulting melts show strong enrichment of large ion lithophile elements, moderate enrichment of light rare earth elements and depletion of Nb, Ta and Ti relative to N-MORB, characterizing them as back-arc basin magmas. Compositional zonations at the outermost rims of olivine phenocrysts suggest a period of no more than a few days between the inferred mixing event and eruption. Mineral-melt thermometers indicate a temperature of ∼1010 °C for the hybrid magma. Mineral-melt barometers give a wide range of pressures with a mean of 19 ± 217 MPa, reflecting equilibration at shallow depths as well as limited barometer performance at low pressures. In addition, many clinopyroxene phenocrysts lack equilibrium between their strongly zoned rims and the host melt. The H2O (0.9–5.3 wt%) and CO2 (70–4730 ppm) contents of melt inclusions in clinopyroxene and plagioclase phenocrysts provide better constraints on the pressure range for pre-eruptive magma storage. The data suggest that dacite phenocryst crystallization and magma mixing occurred at 40–140 MPa pressure, about 1–5 km beneath the summit of North Su. The sulfur–H2O relationship in melt inclusions reflect a degassing trend that is consistent with formation of an aqueous-sulfurous magmatic fluid, inferred to produce highly acidic vent fluids at North Su (Seewald et al., 2015). The melt inclusion data also indicate significant fractionation between F, Cl and H2O after magma degassing, possibly reflecting formation of a high-salinity brine at depth and fluorine fixation in minerals formed in the subsurface. Our results suggest that the dynamic magmatic system, with magma mixing occurring shortly before ascent and eruption, causes rapid changes in composition and quantity of exsolved fluids. Due to the short distance to the seafloor, such fluid fluctuations are rapidly transferred to the hydrothermal systems. This may be the main cause for the observed temporal, spatial and compositional variations in hydrothermal activity at North Su, and possibly at other vent fields of the Manus basin.

Geochemical and Sr-Nd-Pb isotopic compositions of volcanic rocks from the Iheya Ridge, the middle Okinawa Trough: implications for petrogenesis and a mantle source

As an active back-arc basin, the Okinawa Trough is located in the southeastern region of the East China Sea shelf and is strongly influenced by the subduction of the Philippine Sea Plate. Major element, trace element and Sr-Nd- Pb isotopic composition data are presented for volcanic rocks from the Iheya Ridge (IR), the middle Okinawa Trough. The IR rocks record large variations in major elements and range from basalts to rhyolites. Similar trace element distribution characteristics together with small variations in 87Sr/86Sr (0.703 862–0.704 884), 144Nd/143Nd (0.512 763–0.512 880) and Pb isotopic ratios, demonstrate that the IR rocks are derived from a similar magma source. The fractional crystallization of olivine, clinopyroxene, plagioclase, and amphibole, as well as accessory minerals, can reasonably explain the compositional variations of these IR rocks. The simulations suggest that approximately 60% and 75% fractionation of an evolved basaltic magma can produce trace element compositions similar to those of the intermediate rocks and acid rocks, respectively. The analysis of their Sr-Nd-Pb isotopic content ratios suggest that the source of the rocks from the IR is close to the depleted mantle (DM) but extends to the enriched mantle (EMII), indicating that the mantle source of these rocks is a mixture between the DM and EMII end members. The simulations show that the source of the IR volcanic rocks can be best interpreted as the result of the mixing of approximately 0.8%–2.0% subduction sediment components and 98.0%–99.2% mantlederived melts.

The influence of a subduction component on magmatism in the Okinawa Trough: Evidence from thorium and related trace element ratios

The Okinawa Trough (OT) is a back-arc, initial continental marginal sea basin located behind the Ryukyu Arc-Trench System. Formation and evolution of the OT have been intimately related to subduction of the Philippine Sea Plate (PSP) since the late Miocene; thus, the magma source of the trough has been affected by subduction components, as in the case of other active back-arc basins, including the Lau Basin (LB) and Mariana Trough (MT). We review all the available geochemical data relating to basaltic lavas from the OT and the middle Ryukyu Arc (RA) in this paper in order to determine the influence of the subduction components on the formation of arc and back-arc magmas within this subduction system. The results of this study reveal that the abundances of Th in OT basalts (OTBs) are higher than that in LB (LBBs) and MT basalts (MTBs) due to the mixing of subducted sediments and EMI-like enriched materials. The geochemical characteristics of Th and other trace element ratios indicate that the OTB originated from a more enriched mantle source (compared to N-mid-ocean ridge basalt, N-MORB) and was augmented by subducted sediments. Data show that the magma sources of the south OT (SOT) and middle Ryukyu Arc (MRA) basalts were principally influenced by subducted aqueous fluids and bulk sediments, which were potentially added into magma sources by accretion and underplating. At the same time, the magma sources of the middle OT (MOT) and Kobi-syo and Sekibi-Syo (KBS+SBS) basalts were impacted by subducted aqueous fluids from both altered oceanic crust (AOC) and sediment. The variable geochemical characteristics of these basalts are due to different Wadati-Benioff depths and tectonic environments of formation, while the addition of subducted bulk sediment to SOT and MRA basalts may be due to accretion and underplating, and subsequent to form mélange formation, which would occur partial melting after aqueous fluids are added. The addition of AOC and sediment aqueous fluid to MOT and KBS+SBS basalts is therefore the result of cold subducted slab dehydration combined with a rapid subduction rate (82mm/a), leading to the migration of fluids into the mantle wedge. The presence of these attributes is likely because the OT was a back-arc, initial continental marginal sea basin.

Geomorphological development of the Goto Submarine Canyon, northeastern East China Sea

The geomorphological development of submarine canyons is controlled by multiple factors, such as sea-level fluctuations, tectonic activity, and ocean currents, but the relative contributions of each factor vary among these canyons. The development of submarine canyons along active rift margins is not fully understood because there are relatively few documented examples of such phenomena. The Goto Submarine Canyon offers a good example of this type of structure because it is located on the flank of the Okinawa Trough, which is an active back-arc basin. Through multi-beam bathymetry, multi-channel seismic reflection profiling, and remotely operated vehicle surveys, we reveal that this canyon is more than four times wider than other submarine canyons and has a gradient that is half as deep and a remarkably flat canyon floor. A series of upstream-facing cliffs with reliefs up to 110 m and longitudinal lineaments are also associated with this canyon. These morphological features can mainly be attributed to direct sediment inputs from the Yellow River during glacial stages, vertical incision prevented by erosion-resistant beds, and faulting and tectonic tilting of resistant strata. The possibility of tidal current-induced erosion is also suggested. These processes have been influenced primarily by regional factors, such as the wide continental shelf, carbonate cementation, and active rifting of the Okinawa Trough. The contribution of rifting, including faults and tectonic tilting, to the canyon's morphology is expected to be a general characteristic of submarine canyons along active rifting margins.

Barite geochemistry from hydrothermal chimneys of the Okinawa Trough: insight into chimney formation and fluid/sediment interaction

We present 210Pb/226Ra and 87Sr/86Sr data of barite separates from mineralized hydrothermal sulfate-sulfide chimneys collected from two young and active back-arc basins (i.e., Okinawa Trough and southern Mariana Trough), to examine the age of chimney formation and the relationship between the fluid chemistry and the field occurrence. Precipitation ages of purified barites in the mineralized chimneys are 24.8 (±1.3)-73.9 (±13.6) years BP2005 for chimney #220 from the Hakurei site in the Izena Cauldron (middle Okinawa Trough), 14.1 (±0.6)-53.4 (±2.8) years BP2005 for chimney #562-R13 from the Yaeyama graben (southern Okinawa Trough), and 29.8 (±1.4)-39.2 (±2.9) years BP2005 for chimney #903-R7 from the Archaean site in the southern Mariana Trough. Therefore, the time required for hydrothermal fluid venting to form these chimneys is of the order of decades. 87Sr/86Sr values for barites from chimney #220 are ∼ 0.7094; these values are higher than those of the JADE site (0.7091) at the slope of the Izena Cauldron and ambient seawater (0.70918). Given the relatively high Ag and Sb contents in chimney #220 bulk samples, we suggest that interactions have occurred between the hydrothermal fluid and the sediment cover. In contrast, barites from the southern Mariana Trough have relatively low 87Sr/86Sr values (∼ 0.7046), implying that magmatic fluids play a significant role in mineralization in this sediment-starved setting.

Bottom Simulating Reflector and Gas Seepage in Okinawa Trough: Evidence of Gas Hydrate in an Active Back-Arc Basin

To look for gas hydrate, 22 multi-channel and 3 single-channel seismic lines on the East China Sea (ECS) shelf slope and at the bottom of the Okinawa Trough were examined. It was found that there was indeed bottom simulating reflector (BSR) occurrence, but it is very rare. Besides several BSRs, a gas seepage was also found. As shown by the data, both the BSR and gas seepage are all related with local geological structures, such as mud diapir, anticline, and fault-controlled graben-like structure. However, similar structural “anomalies” are quite common in the tectonically very active Okinawa Trough region, but very few of them have developed BSR or gas seepage. The article points out that the main reason is probably the low concentration of organic carbon of the sediment in this area. It was speculated that the rare occurrence of gas hydrates in this region is governed by structure-controlled fluid flow. Numerous faults and fractures form a network of high-permeability channels in the sediment and highly fractured igneous basement to allow fluid circulation and ventilation. Fluid flow in this tectonic environment is driven primarily by thermal buoyancy and takes place on a wide range of spatial scales. The fluid flow may play two roles to facilitate hydrate formation: to help gather enough methane into a small area and to modulate the thermal regime.

Lithospheric structure of an active backarc basin: the Taupo Volcanic Zone, New Zealand

Seismic data from both explosive and earthquake sources have been used to model the crustal and upper-mantle velocity structure beneath the Taupo Volcanic Zone (TVZ), an active backarc basin in central North Island, New Zealand. Volcanic sediments with P-wave velocities of 2.0–3.5 km s−1 reach a maximum thickness of 3 km beneath the central TVZ. Underlying these sediments to 16 km depth is material with velocities of 5.0–6.5 km s−1, interpreted as quartzo-feldspathic crust. East and west of the TVZ, crust with similar velocities is found to depths of 30 and 25 km, respectively. Beneath the TVZ, material with P-wave velocities of 6.9–7.3 km s−1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust. The base of the crust at 30 km depth under the TVZ is marked by a strong seismic reflector, interpreted as the Moho. Modelling of arrivals from deep (>40 km) earthquakes near the top of the underlying subducting Pacific Plate reveals a region with low mantle velocities of 7.4–7.8 km s−1 beneath the crust of the TVZ. This region of low mantle velocities is best explained by the presence of partially hydrated upper mantle, resulting from dehydration of hydrous minerals (e.g. serpentinite) carried down by the underlying subducting plate. Within the lower crust beneath the TVZ, a region of high (0.34) Poisson's ratio is observed, indicating the presence of at least 1 per cent partial melt. This melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust, where it provides a source for the voluminous rhyolitic magmas of the TVZ.

Tectonics and history of the Andaman Sea region

The Andaman Sea is an active backarc basin lying above and behind the Sunda subduction zone where convergence between the overriding Southeast Asian plate and the subducting Australian plate is highly oblique. The effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right-lateral fault system. The late Paleocene collision of Greater India and Asia with approximately normal convergence started clockwise rotation and bending of the northern and western Sunda Arc. The initial sliver fault, which probably started in the Eocene, extended through the outer arc ridge offshore from Sumatra, through the present region of the Andaman Sea into the Sagaing Fault. With more oblique convergence due to the rotation, the rate of strike-slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike-slip motion. These basins in sequence are the Mergui Basin starting at ∼32 Ma, the conjoined Alcock and Sewell Rises starting at ∼23 Ma, East Basin separating the rises from the foot of the continental slope starting at ∼15 Ma; and finally at ∼4 Ma, the present plate edge was formed, Alcock and Sewell Rises were separated by formation of the Central Andaman Basin, and the faulting moved onshore from the Mentawai Fault to the Sumatra Fault System bisecting Sumatra.

Continental growth at convergent margins facing large ocean basins: a case study from Mesozoic convergent-margin basins of Baja California, Mexico

Mesozoic rocks of the Baja California Peninsula form one of the most areally extensive, best-exposed, longest-lived (160 my), least-tectonized and least-metamorphosed convergent-margin basin complexes in the world. This convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins: a progression from highly extensional (phase 1) through mildly extensional (phase 2) to compressional (phase 3) strain regimes. This trend is largely due to the progressively decreasing age of lithosphere that is subducted, which causes a gradual decrease in slab dip angle (and concomitant increase in coupling between lower and upper plates), as well as progressive inboard migration of the arc axis. This paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin. Phase 1 consists of Late Triassic to Late Jurassic oceanic intra-arc to backarc basins that were isolated from continental sediment sources. New, progressively widening basins were created by arc rifting and sea floor spreading, and these were largely filled with progradational backarc arc-apron deposits that record the growth of adjacent volcanoes up to and above sea level. Inboard migration of the backarc spreading center ultimately results in renewed arc rifting, producing an influx of silicic pyroclastics to the backarc basin. Rifting succeeds in conversion of the active backarc basin into a remnant backarc basin, which is blanketed by epiclastic sands. Phase 1 oceanic arc–backarc terranes were amalgamated by Late Jurassic sinistral strike slip faults. They form the forearc substrate for phase 2, indicating inboard migration of the arc axis due to decrease in slab dip. Phase 2 consists of Early Cretaceous extensional fringing arc basins adjacent to a continent. Phase 2 forearc basins consist of grabens that stepped downward toward the trench, filled with coarse-grained slope apron deposits. Phase 2 intra-arc basins show a cycle of (1) arc extension, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions, followed by (2) arc rifting, characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites. This extensional-rifting cycle was followed by mid-Cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent, caused by a decrease in slab dip as well as a possible increase in convergence rate. Phase 2 fringing arc terranes form the substrate for phase 3, which consists of a Late Cretaceous high-standing, compressional continental arc that migrated inboard with time. Strongly coupled subduction resulted in accretion of blueschist metamorphic rocks, with development of a broad residual forearc basin behind the growing accretionary wedge, and development of extensional forearc (trench–slope) basins atop the gravitationally collapsing accretionary wedge. Inboard of this, ongoing phase 3 strongly coupled subduction, together with oblique convergence, resulted in development of forearc strike-slip basins upon arc basement. The modern Earth is strongly biased toward long-lived arc–trench systems, which are compressional; therefore, evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California. This convergent margin is typical of many others, where the early to middle stages of convergence (phases 1 and 2) create nonsubductable arc–ophiolite terranes (and their basin fills) in the upper plate. These become accreted to the continental margin in the late stage of convergence (phase 3), resulting in significant continental growth.

Geochemistry of late Paleozoic mafic igneous rocks from the Kuerti area, Xinjiang, northwest China: implications for backarc mantle evolution

The composition of Kuerti mafic rocks in the Altay Mountains in northwest China ranges from highly geochemically depleted, with very low La, Ta and Nb and high εNd(t) values, to slightly enriched, arc lava-like composition. They display flat to light rare earth element (REE)-depleted patterns and have variable depletions in high field-strength elements (HFSE). These mafic rocks were most probably derived from a variably depleted mantle source containing a subduction component beneath an ancient intra-oceanic backarc basin. Together with the slightly older arc volcanic rocks in the Altay region, the Kuerti mafic rocks display generally positive correlations of their key elemental ratios (e.g., Th/Nb, La/Yb and Th/Yb). These indicate that the more mid-ocean ridge basalt (MORB) component was contained in these magmas, the less arc component was present in their mantle source. Therefore, we propose a two-stage melting evolution model to interpret the compositional evolution of the Kuerti mafic rocks and associated arc volcanic rocks. First, arc basaltic melts were extracted from the hydrated arc mantle wedge beneath Kuerti, leaving behind a mantle source that is variably depleted in incompatible trace elements. Then, mafic rocks were erupted during seafloor spreading in the Kuerti backarc basin from the upwelling asthenospheric mantle. The variably depleted mantle source produced mafic rocks with composition ranging from arc lava-like to more geochemically depleted than MORB. The recognition of Kuerti mafic rocks as backarc basin basalts (BABB) is consistent with the proposed tectonic model that an active backarc basin–island arc system along the paleo-Asian ocean margin was formed in the Altay region during Devonian–Early Carboniferous. New data further indicate that the final orogenic event in the Altay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian.

Peridotites from the Mariana Trough: first look at the mantle beneath an active back-arc basin

Two dives of the DSV Shinkai 6500 in the Mariana Trough back-arc basin in the western Pacific sampled back-arc basin mantle exposures. Reports of peridotite exposures in back-arc basin setting are very limited and the lack of samples has hindered our understanding of this important aspect of lithospheric evolution. The Mariana Trough is a slow-spreading ridge, and ultramafic exposures with associated gabbro dykes or sills are located within a segment boundary. Petrological data suggest that the Mariana Trough peridotites are moderately depleted residues after partial melting of the upper mantle. Although some peridotite samples are affected by small-scale metasomatism, there is no evidence of pervasive post-melting metasomatism or melt-mantle interaction. Spinel compositions plot in the field for abyssal peridotites. Clinopyroxenes show depletions in Ti, Zr, and REE that are intermediate between those documented for peridotites from the Vulcan and Bouvet fracture zones (the American- Antarctic and Southwest Indian ridges, respectively). The open-system melting model indicates that the Ma- riana Trough peridotite compositions roughly corre- spond to theoretical residual compositions after 7% near-fractional melting of a depleted MORB-type upper mantle with only little melt or fluid/mantle interactions. The low degree of melting is consistent with alow magma budget, resulting in ultramafic exposure. We infer that the mantle flow beneath the Mariana Trough Central Graben is episodic, resulting in varying magma supply rate at spreading segments.

Bathymetry and magnetic anomalies in the Havre Trough and southern Lau Basin: from rifting to spreading in back-arc basins

The Havre Trough and Lau Basin are active back-arc basins related to Pacific-Australian plate convergence. These back-arc basins provide insight into the evolution of rifting through various stages, including a final stage of oceanic spreading. We carried out a swath bathymetry and magnetic anomaly survey focused on the less-studied Havre Trough and southern Lau Basin. The western portion of the whole Havre Trough is deeper and more thickly sedimented whereas shallow bathymetry and volcanic edifices are dominant in the east. We suggest that crustal rifting progresses asymmetrically with respect to the trough axis. The active tectono-magmatic zone migrates trenchward during ongoing widening. Rifting along normal faults trending in the longitudinal trough strike occurs in the southern Havre Trough. Volcanism is restricted to a line along the normal faults. Circular or elongated knolls in the northern Havre Trough indicate that en echelon type segmented rifting of short strike length and successive volcanic intrusion takes place. Volcanism in this area is dominated by diapir-like magma intrusion into an extended and thinned arc crust. The southernmost tip of the spreading center of Lau Basin is defined by the deep graben, striking at N20°E, located at 24°00′S, 177°10′W.

琉球列島久米島の鮮新統宇江城岳層からピクライト玄武岩の発見

Picrite basalts containing MgO up to 16 wt.% were found in the lowest part of the Pliocene Uegusukudake Formation on the north coast of Kumejima island ; in the close proximity northwest of the island lies an active back-arc basin, Okinawa Trough. The high Fo (92.2) of olivine and Cr2O3 (56.5%) of spinel in the picrite basalts indicate the existence of a magma with >13% MgO and >0.15% Cr2O3. The Kumejima picrite basalts are tholeiitic arc picrites having relatively high SiO2 and low incompatible element abundances.They may have formed accompanying the opening of the Okinawa Trough.

Upper mantle structure of the southwest Pacific from regional waveform inversion

We determine the upper mantle seismological structure of the southwest Pacific by inverting complete regional seismograms recorded by the Southwest Pacific Seismic Experiment, a deployment of 12 broadband seismographs in the Fiji, Tonga, and Niue islands. We first present a method to invert entire seismic waveforms for a laterally homogeneous velocity structure. Partial derivatives are computed using an efficient reflectivity code for differential seismograms [Randall, 1994], and a conjugate gradient method is used to perform the nonlinear inversion. Band pass‐filtered (10–55 mHz) vertical and radial component seismograms, extending from the P arrival to the surface waves, are used to constrain the structure. Waveforms are selected to provide regional distance (400–1500 km) propagation paths predominantly within one of the tectonic regions of the southwest Pacific. Source depths ranging from 10 to 240 km allow the deeper regions of the model to be resolved with relatively short path lengths. Results show extremely large lateral variability in the upper mantle structure of the southwest Pacific. The inactive South Fiji Basin shows upper mantle velocities much lower than those previously observed for the Pacific lithosphere of a similar age. The North Fiji and Lau Basins, containing active back arc spreading systems, show upper mantle shear velocities as slow as 3.8 km s−1. The exceptionally low seismic velocities in the active back arc regions may represent the slowest upper mantle velocities in the world. The velocity difference between the active back arc basins and the old Pacific lithosphere immediately east of the Tonga Trench is maximum at 30–90 km depth, with a variation of ∼18%, indicating partial melting in this depth range. Velocity heterogeneity of >4% extends to a depth of 170 km. Velocity structure below 220 km depth is poorly resolved, but there is some evidence of a 2% difference extending to depths of 350 km.