Remnant Arc Research Articles

Local earthquake seismic tomography of the Southernmost Mariana subduction zone

We employed seismic tomography to examine the velocity structure of the upper mantle in the Southernmost Mariana subduction zone. Our study focuses on data collected during a six-month experiment from 15 December 2016 to 12 June 2017, using 11 ocean bottom seismometers. By examining over 3700 local arrival times, we are able to determine the three-dimensional Vp and Vs structure. The subducting slab in this region displays a P- and S-wave velocity 2~6% higher than normal mantle and a lower Vp/Vs, with an average dip of 45° at depths ranging from 50 to 100 km. Additionally, our velocity images also shed new lights to the velocity anomalies of the mantle wedge region on top of the subducting slab, from the trench to the remnant arc. We observed slower velocity anomalies in the mantle wedge beneath the Southwest Mariana Rift, the West Mariana Ridge, and the forearc. In the outer forearc, a low-velocity anomaly is observed at depths shallower than 50 km, indicating mantle serpentinization and the presence of water. Additionally, a melt production region is observed beneath the central part of the forearc block at a depth of 40–60 km suggesting the possibility of melting processes in this region.

Records of the accretionary, collisional and post-collisional evolution of western Gondwana in the high-grade core of the Araçuaí-Ribeira orogenic system, SE Brazil

High-grade metamorphism, extensive partial melting, and polyphase deformation often obliterate records of pre-collisional stages in ancient orogens, making it hard to unravel Precambrian accretionary events. In western Gondwana, the high-grade core of the Araçuaí-Ribeira Orogenic System (AROS) comprises pre-collisional rock-assemblages largely disrupted, overprinted, and remelted by collisional and post-collisional tectono-metamorphic and igneous events, hampering reconstructions of its accretionary history. Detailed studies reveled remnants of an Early Tonian (860–840 Ma) island arc, forming km-size roof-pendants and mega-xenoliths within the composite Caxixe batholith, in the boundary zone between the Araçuaí and Ribeira orogens. We further characterize the Caxixe batholith and surrounding plutons, showing that the main stage of magmatism is composed of pre-collisional orthogneisses, partially migmatized and crystallized between ca. 607–580 Ma, thus, much later than the Tonian island arc records. Those Ediacaran granitic rocks represent intrusions of calc-alkaline magmas with elemental geochemistry and isotopic (εHf(t): −6 to −10, whole-rock εNd(t): −4.9 to −9.5, and 87Sr/86Sr(i): 0.706–0.711) data correlated with the continental-margin Rio Doce arc. This contrasts with signatures indicative of a mantle source εHf(t) = +10 to + 14, εNd(t) = +0.9 to + 6.4, and 87Sr/86Sr(i) = 0.698–0.704) of the Early Tonian arc remnants. Intruding the pre-collisional granitic plutons, garnet-bearing leucogranites dated at ca. 575 Ma and a mafic intrusion of ca. 560 Ma mark the transition from the syn-collisional to late-collisional stages. Post-collisional, non-deformed, A-type granites with the most evolved isotopic signatures (εHf(t): −22; εNd(t): −5.6 to −13.2) intruded at ca. 500 Ma, ending the long-lasting orogenic history documented in the region. Our results demonstrate that a basement composed of an early Tonian juvenile arc docked with a continental margin and hosted several intrusions of an early Ediacaran continental-margin magmatic arc that, in turn, were intruded by collisional and post-collisional magmas from the Late Ediacaran to Cambrian, finally forming the main plutonic masses of the Araçuaí-Ribeira Orogenic System (AROS) core, during the complex history of western Gondwana assembly.

Spatiotemporal distribution of seamount volume along the Kyushu-Palau Ridge: Implications for rejuvenated volcanism

• Accurate quantification of seamount volcanism reveals multiphase volcanism along the Kyushu–Palau Ridge. • The proximity of Pre-Oligocene lithospheric weak zones to high concentrations of seamount volume indicate tectonic inheritance facilitated magma migration. • Rejuvenated volcanism related to the propagated back-arc spreading during Oligocene contributed to the development of the Kyushu-Palau Ridge. The Kyushu-Palau Ridge (KPR) is a remnant arc that initially formed with the subduction of the Pacific plate beneath the West Philippine Sea. The KPR split from the proto-Izu-Bonin-Mariana (IBM) arc with the back-arc spreading at ∼ 30 Ma. Even though most of the seamounts have been dated, the factors that drive the volcanic arc evolution along the KPR remain unknown. In this study, we quantify the spatiotemporal distribution of the seamount volume along the KPR, representing a quantitative indicator of erupted volcanism. A spatial filtering method based on the White Top-Hat Transform is used to identify seamounts from the bathymetry. Combining seismic data interpretation and gravity data inversion, we estimate the seamount volume distribution along the KPR. Results show that high concentrations of the seamount volume are in proximity to pre-Oligocene lithospheric weak zones, indicating that tectonic inheritance has facilitated magma migration during the early-stage volcanic arc generation. Subsequently, the younger than 30 Ma age of the majority of the seamounts suggests a late-stage rejuvenated volcanism in the Oligocene. In addition, the spatiotemporal variations of the seamount volume in the northern and central segments are synchronous with the propagated spreading of the Perce Vela and Shikoku basins, further indicating that the rejuvenated volcanism was driven by the propagated back-arc spreading. Rejuvenated volcanism can thus be an important contributive factor to remnant volcanic arc formation.

Enhanced and Asymmetric Melting Beneath the Southern Mariana Back‐Arc Spreading Center Under the Influence of Pacific Plate Subduction

AbstractThe back‐arc spreading at the southern Mariana is categorized as slow, but surface morphological and geophysical features of the spreading centers suggest that the spreading process is associated with enhanced melting in the upper mantle, due to water being derived from the subducted Pacific slab. A marine magnetotelluric experiment was performed along a transect across a segment at 13°N to reveal the key processes of melting, dehydration, and dynamics in the upper mantle, and their relationships to the surface characteristics. Our inversion model of electrical resistivity shows (a) a conductive body at 10–20 km depth beneath the spreading axis, and (b) another conductive area expanding asymmetrically under and around the conductive body. Away from the spreading center, there is (c) a resistive area thickening up to ∼40 km on the remnant arc side, and (d) another resistive area with a constant thickness of ∼150 km on the trench side. Implications of these model features are (a) a melt body beneath the spreading axis; (b) a hydrous mantle above the subducted slab and asymmetric passive decompression melting in the mantle wedge; (c) a residual mantle off from the spreading axis; and (d) a cold mantle wedge tip and the subducted Pacific mantle. The structure markedly contrasts with that in the central Mariana Trough at 18°N, suggesting that the horizontal distance between the location of the spreading center and the root of the buoyant upwelling above the subducted slab is a key parameter that controls the mantle dynamics beneath the back‐arc spreading.

A Single Dras‐Kohistan‐Ladakh Arc Revealed by Volcaniclastic Records

AbstractTectonic interpretations of arc remnants in the Himalayan orogen remain uncertain, despite their important implications for the overall convergence history between India and Eurasia. Provenance results from deep‐water volcaniclastic rocks of the Indus Suture Zone in Ladakh provide new constraints on the Mesozoic tectonic evolution of the Dras and Kohistan‐Ladakh arcs. Detrital zircon (DZ) U‐Pb ages and whole‐rock geochemistry of the fault‐bounded Upper Cretaceous Nindam and Paleocene Jurutze formations present age patterns and compositions that are consistent with those of the Dras and Kohistan‐Ladakh arcs, respectively. The combination of DZs of the Nindam and Jurutze formations with the igneous zircons of the Dras and Kohistan‐Ladakh arcs shows similar age distributions that support a Late Jurassic to Paleocene tectonic connection between all these units. We argue that the secular trends in geochemical composition of DZs and volcaniclastic material are consistent with the magmatic evolution of one convergent margin, which shifted from a primitive to a mature stage during the Late Cretaceous. The recognition of a single Dras‐Kohistan‐Ladakh arc sets the stage for reevaluating competing scenarios of the Mesozoic evolution of the India–Eurasia convergent system. We find that the most likely scenario is that of a Jurassic arc formed above a south‐dipping intraoceanic subduction zone and accreted to Eurasia during the Early Cretaceous, after which it evolved above a north‐dipping subduction zone.

Provenance of the Early–Middle Devonian Shugouzi Formation in the Quruqtagh, Northeastern Xinjiang: Constraints from Sandstone Geochemistry and Heavy Mineral Analysis

AbstractThe Early–Middle Devonian Shugouzi Formation in the Quruqtagh block consists mainly of clastic rocks. However, their provenance has been scarcely studied since it was named. Geochemistry of clastic rocks was commonly used to interpret the provenance. Detrital heavy mineral analyses help frame the U‐Pb age from zircon grains, integrated with geochemical data from detrital tourmaline and spinels. These techniques were used to characterize components of the sediment flux and define erosion areas in the Qurugtagh block, further providing evidence about the tectonic evolution of the South Tianshan and Tarim plate. The maximum depositional age constrained by detrital zircon dating was Early–Middle Devonian. Multiple diagrams for sedimentary provenance using major and trace elements indicate that continental island arc‐related felsic rocks were the major source rocks for the Shugouzi Formation. Detrital tourmalines are dravite and schorl. The results of detrital tourmaline electron probe microanalysis (EPMA) show that the source rocks are mainly metasedimentary rocks and granitoids. The detrital chromian spinels within the sediments are characterized by high chroumium (Cr#) and varying magnesium (Mg#). The discrimination plots reveal that these spinels were sourced from island arc magmatic rocks. The laser ablation inductively‐coupled plasma mass spectrometry (LA‐ICP‐MS) U‐Pb chronology of detrital zircons suggests that the sediments were derived mainly from 414–491 Ma and 744–996 Ma magmatic rocks. Paleocurrent restoration, sandstone geochemistry, EPMA, and detrital zircon geochronology indicate that the source rocks were predominantly derived from Late Ordovician and Devonian magmatic rocks and subordinately from recycled Neoproterozoic magmatic rocks. Comprehensive analyses of the source areas suggest that a remnant arc still existed in the Early Devonian and the Shugouzi Formation was deposited in a passive continental margin.

Intra-oceanic submarine arc evolution recorded in an ~1-km-thick rear-arc succession of distal volcaniclastic lobe deposits

Abstract International Ocean Discovery Program (IODP) Expedition 351 drilled a rear-arc sedimentary succession ~50 km west of the Kyushu-Palau Ridge, an arc remnant formed by rifting during formation of the Shikoku Basin and the Izu-Bonin-Mariana arc. The ~1-km-thick Eocene to Oligocene deep-marine volcaniclastic succession recovered at Site U1438 provides a unique opportunity to study a nearly complete record of intra-oceanic arc development, from a rear-arc perspective on crust created during subduction initiation rather than supra-subduction seafloor spreading. Detailed facies analysis and definition of depositional units allow for broader stratigraphic analysis and definition of lobe elements. Patterns in gravity-flow deposit types and subunits appear to define a series of stacked lobe systems that accumulated in a rear-arc basin. The lobe subdivisions, in many cases, are a combination of a turbidite-dominated subunit and an overlying debris-flow subunit. Debris flow–rich lobe-channel sequences are grouped into four, 1.6–2 m.y. episodes, each roughly the age range of an arc volcano. Three of the episodes contain overlapping lobe facies that may have resulted from minor channel switching or input from a different source. The progressive up-section coarsening of episodes and the increasing channel-facies thicknesses within each episode suggest progressively prograding facies from a maturing magmatic arc. Submarine geomorphology of the modern Mariana arc and West Mariana Ridge provide present-day examples that can be used to interpret the morphology and evolution of the channel (or channels) that fed sediment to Site U1438, forming the sequences interpreted as depositional lobes. The abrupt change from very thick and massive debris flows to fine-grained turbidites at the unit III to unit II boundary reflects arc rifting and progressive waning of turbidity current and ash inputs. This interpretation is consistent with the geochemical record from melt inclusions and detrital zircons. Thus, Site U1438 provides a unique record of the life span of an intra-oceanic arc, from inception through maturation to its demise by intra-arc rifting and stranding of the remnant arc ridge.

Itina Trough and Other SW Pacific Examples of Rifting Across Former Subduction/Collision Zones

AbstractThe Itina Trough is a late Miocene rift graben formed at the beginning of Woodlark Basin opening where the northern Louisiade Plateau micro‐continent had collided with the eastern Pocklington Rise remnant arc. We infer that it was the collisional suture of this Plateau subducted at the Pocklington Trough that either thickened the crust and weakened the lithosphere and/or adjoined the apparently thicker Louisiade micro‐continent to the thinner crust of the eastern Pocklington Rise, and thereby localized the rifting. Other examples of rifting across a remnant arc‐trench system in the SW Pacific include the South Rennell Trough and the Norfolk Basin.

Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBM’s oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.

Characteristics of gravity and magnetic fields and deep structural responses in the southern part of the Kyushu-Palau Ridge

The southern part of the Kyushu-Palau Ridge (KPR) is located at the conjunction of the West Philippine Basin, the Parece Vela Basin, the Palau Basin, and the Caroline Basin. This area has extremely complex structures and is critical for the research on the tectonic evolution of marginal seas in the Western Pacific Ocean. However, only few studies have been completed on the southern part, and the geophysical fields and deep structures in this part are not well understood. Given this, this study finely depicts the characteristics of the gravity and magnetic anomalies and extracts information on deep structures in the southern part of the KPR based on the gravity and magnetic data obtained from the 11th expedition of the deep-sea geological survey of the Western Pacific Ocean conducted by the Guangzhou Marine Geological Survey, China Geological Survey using the R/V Haiyangdizhi 6. Furthermore, with the data collected on the water depth, sediment thickness, and multichannel seismic transects as constraints, a 3D density model and Moho depths of the study area were obtained using 3D density inversion. The results are as follows. (1) The gravity and magnetic anomalies in the study area show distinct zoning and segmentation. In detail, the gravity and magnetic anomalies to the south of 11°N of the KPR transition from high-amplitude continuous linear positive anomalies into low-amplitude intermittent linear positive anomalies. In contrast, the gravity and magnetic anomalies to the north of 11°N of the KPR are discontinuous and show alternating positive and negative anomalies. These anomalies can be divided into four sections, of which the separation points correspond well to the locations of deep faults, thus, revealing different field-source attributes and tectonic genesis of the KPR. (2) The Moho depth in the basins in the study area is 6–12 km. The Moho depth in the southern part of KPR show segmentation. Specifically, the depth is 10–12 km to the north of 11°N, 12–14 km from 9.5°N to 11°N, 14–16 km from 8.5°N to 9.5°N, and 16–25 km in the Palau Islands. (3) The KPR is a remnant intra-oceanic arc with the oceanic-crust basement.which shows noticeably discontinuous from north to south in geological structure and is intersected by NEE-trending lithospheric-scale deep faults. With large and deep faults F3 and F1 (the Mindanao fault) as boundaries overall, the southern part of the KPR can be divided into three zones. In detail, the portion to the south of 8.5°N (F3) is a tectonically active zone, the KPR portion between 8.5°N and 11°N is a tectonically active transition zone, and the portion to the north of 11°N is a tectonically inactive zone. (4) The oceanic crust in the KPR is slightly thicker than that in the basins on both sides of the ridge, and it is inferred that the KPR formed from the thickening of the oceanic crust induced by the upwelling of deep magma in the process of rifting of remnant arcs during the Middle Oligocene. In addition, it is inferred that the thick oceanic crust under the Palau Islands is related to the constant upwelling of deep magma induced by the continuous northwestward subduction of the Caroline Plate toward the Palau Trench since the Late Oligocene. This study provides a scientific basis for systematically understanding the crustal attributes, deep structures, and evolution of the KPR.©2021 China Geology Editorial Office.

Accretionary models for the Neoproterozoic evolution of the Borborema Province: advances and open questions

The current knowledge on the accretionary evolution of the Borborema Province is evaluated considering recently published data and interpretations. Early to late Neoproterozoic eclogite, ophiolite, and magmatic arc remnants have been documented and point to oceanic crust consumption. Isotopic contrasts as well as geophysical anomalies across the major domain boundaries are suggestive of collisional sutures and a speculative model of terrane accretion is presented here. On the other hand, the lack of concise evidence for some of the proposed deep-seated structures as well as putative lithospheric continuity in both sides of it suggest reworking of previously continuous lithosphere through intracontinental deformation in localized sectors of the province interior. It is patent that one model does not preclude the other and available evidence so far has triggered the emergence of conciliatory proposals for the Neoporterozoic history the Borborema Province.

Hikurangi Plateau subduction a trigger for Vitiaz arc splitting and Havre Trough opening (southwestern Pacific)

AbstractSplitting of the Vitiaz arc formed the Tonga-Kermadec and Lau-Colville Ridges (southwestern Pacific Ocean), separated by the Lau Basin in the north and Havre Trough in the south. We present new trace element and Sr-Nd-Hf-Pb isotope geochemistry for the Kermadec and Colville Ridges extending ∼900 km north of New Zealand (36°S–28°S) in order to (1) compare the composition of the arc remnants with Quaternary Kermadec arc volcanism, (2) constrain spatial geochemical variations in the arc remnants, (3) evaluate the effect of Hikurangi igneous plateau subduction on the geochemistry of the older arc lavas, and (4) elucidate what may have caused arc splitting. Compared to the Kermadec Ridge, the Colville Ridge has higher more-incompatible to less-incompatible immobile element ratios and largely overlapping isotope ratios, consistent with an origin through lower degrees of melting of more enriched upper mantle in the Vitiaz rear arc. Between ca. 8 and 3 Ma, both halves of the arc (∼36°S–29°S) included a more enriched (EM1-type) composition (with lower 206Pb/204Pb and 207Pb/204Pb and higher Δ8/4 Pb [deviation of the measured 208Pb/204Pb ratio from a Northern Hemisphere basalt regression line] and 87Sr/86Sr) compared to older and younger arc lavas. High-Ti basalts from the Manihiki Plateau, once joined to the Hikurangi Plateau, could serve as the enriched Vitiaz arc end member. We propose that the enriched plateau signature, seen only in the isotope ratios of mobile elements, was transported by hydrous fluids from the western margin of the subducting Hikurangi Plateau or a Hikurangi Plateau fragment into the overlying mantle wedge. Our results are consistent with plateau subduction triggering arc splitting and backarc opening.

Computations on the Thermal Characteristics of SF6 and CO₂ Switching Arcs on a Microscopic Scale.

Pure sulfur hexafluoride is a colorless, odorless, non-toxic, inflammable, chemically inert and thermally stable gas and has proven its worth as an excellent interruption and dielectric medium. SF6 has been successfully used for interruption and insulation purposes as interrupters and circuit-breakers in gas-insulated substations. Due to its long lifetime and high global warming potential, this gas was put on the list of fluorinated greenhouse gases in the Kyoto Protocol aimed at controlling the emission of man-made greenhouse gases. This factor makes the search for an environmentally friendly alternative to SF6 all the more urgent. In this paper, we conducted computations on the thermal and aerodynamic behaviors of SF6 and an alternative CO₂ switching arcs in a self-blast chamber in order to compare the switching phenomena and the thermal reignition from an engineering point of view. Through the complete work, the 3,000 K isotherm of the remnant arc column within microseconds after a current zero was used to evaluate the thermal reignition of SF6 and CO₂ switching arcs with the slope of the tangential line of the transient recovery voltage on a microscopic scale.

Geological characteristics of the Nankai Trough subduction zone and their tectonic significances

The Nankai Trough subduction zone is a typical subduction system characterized by subduction of multiple geological units of the Philippine Sea Plate (the Kyushu-Palau Ridge, the Shikoku Basin, the Kinan Seamount Chain, and the Izu-Bonin Arc) beneath the Eurasian Plate in the southwest of Japan. This study presents a geophysical and geochemical analysis of the Nankai Trough subduction zone in order to determine the features and subduction effects of each geological unit. The results show that the Nankai Trough is characterized by low-gravity anomalies (−20 mGal to −40 mGal) and high heat flow (60–200 mW/m2) in the middle part and low heat flow (20–80 mW/m2) in the western and eastern parts. The crust of the subducting plate is 5–20 km thick. The mantle composition of the subducting plate is progressively depleted from west to east. Subduction of aseismic ridges (e.g., the Kyushu-Palau Ridge, the Kinan Seamount Chain, and the Zenisu Ridge) is a common process that leads to a series of subduction effects at the Nankai Trough. Firstly, aseismic ridge or seamount chain subduction may deform the overriding plate, resulting in irregular concave topography along the front edge of the accretionary wedge. Secondly, it may have served as a seismic barrier inhibiting rupture propagation in the 1944 Mw 8.1 and 1946 Mw 8.3 earthquakes. In addition, subduction of the Kyushu-Palau Ridge and hot and young Shikoku Basin lithosphere may induce slab melting, resulting in adakitic magmatism and the provision of ore-forming metals for the formation of porphyry copper and gold deposits in the overriding Japan Arc. Based on comparisons of their geophysical and geochemical characteristics, we suggest that, although the Izu-Bonin Arc has already collided with the Japan Arc, the Kyushu-Palau Ridge, which represents a remnant arc of the Izu-Bonin Arc, is still at the subduction stage characterized by a single-vergence system and a topographic boundary with the Japan Arc.

Tonian island arc remnants in the northern Ribeira orogen of Western Gondwana: The Caxixe batholith (Espírito Santo, SE Brazil)

The Tonian Serra da Prata magmatic arc is part of an association of several magmatic arcs of the Ribeira orogenic system in Southeastern Brazil. Along with the Cryogenian Rio Negro arc, the Serra da Prata arc represents an intra-oceanic arc system while the Ediacaran Rio Doce arc represents an active continental margin arc. Here, we characterize the northernmost segment of the Serra da Prata arc, based on lithochemical and isotopic (whole-rock Sm-Nd and Rb-Sr, and zircon U-Pb and Lu-Hf) data from the Caxixe batholith, located in Espírito Santo State. The Caxixe batholith mostly consists of granodioritic to granitic and minor tonalitic to gabbroic rocks, with calc-alkaline, metaluminous to slightly peraluminous, magnesian, I-type signature and magmatic arc affinity. The rocks display LREE enrichment, positive to slightly negative Eu and negative Nb-Ta anomalies. Zircon U-Pb (LA-ICP-MS and SHRIMP) data reveal magmatic crystallization ages from 859 ± 7 Ma to 847 ± 8 Ma (for zircon cores) and a probably metamorphic age of 834 ± 9 Ma (for zircon rims). Hf Isotopic data yield positive εHf(t) values between +10 and +14, and TDM Hf model ages from 1.01 Ga to 0.84 Ga, in agreement with whole-rockNd and Sr data with εNd(t) values from +0.9 to +6.4 and TDM Nd model ages between 1.2 Ga and 0.8 Ga, and 87Sr/86Sr(i) of 0.6979–0.7035. Our data characterize a juvenile magmatic arc with striking mantle contribution, formed on a supra-subduction intra-oceanic setting similar to modern island arcs. As a whole, the Serra da Prata juvenile arc points to a large ocean that underwent intra-oceanic subduction in the Early Tonian. As a corollary, the Angola, Congo – São Francisco and Paranapanema paleocontinental blocks should be far from each other during the Early Tonian, separated by that large ocean. In a broad scenario, involving ophiolites and juvenile arcs of southern and central Brazil, several cratons of Western Gondwana once had been parts of paleocontinental pieces separated by very extensive Tonian-Cryogenian oceanic realms, including the Neoproterozoic Goiás–Pharusian and Adamastor oceans.

Remnants of a Middle Triassic island arc on western margin of South China Block: Evidence for bipolar subduction of the Paleotethyan Ailaoshan Ocean

Identifying ancient magmatic arcs in collisional orogens is important for regional tectonic reconstruction. Here, we first identified Middle Triassic (ca. 235–237 Ma) arc-type hornblende diorite and granodiorite in the Ailaoshan high-grade metamorphic belt in SW China, east of the Paleotethyan Ailaoshan-Song Ma suture. These rocks show arc-type geochemical affinities, such as large-ion lithophile element (LILE; e.g., Rb and Ba) enrichments and high-field-strength element (HFSE; e.g., Nb, Ta and Ti) depletions. They are characterized by positive εNd(t) (+2.90 to +4.63), εHf(t) (+9.9 to +14.1), and higher δ18O (mean 6.35–6.65‰) than typical mantle-derived magmas. Whole-rock geochemical and Sr-Nd-Hf-O isotope data demonstrate that the hornblende diorite was derived from depleted MORB-like mantle metasomatized by subducted oceanic sediments, and the granodiorite was formed by fractional crystallization from mafic magmas. These two rock types likely represent remnants of a continental arc, whose upper arc architecture may have been mostly-/completely-eroded by the subsequent Indosinian (Triassic) and Himalayan (Cenozoic) orogenic events. Tectonically, the presence of Triassic arc remnants east of the Paleotethyan Ailaoshan-Song Ma suture suggests the occurrence of east-dipping (present orientation) subduction of the Paleotethys beneath the South China Block, a hypothesis discarded by most previous studies due to the lack of arc magmatic evidence. Combined with previous studies in the Ailaoshan tectonic zone and its southern extension (Song Ma tectonic zone), we propose that the Paleotethyan Ailaoshan Ocean may have been bipolar-subducted westward and eastward beneath the Indochina and South China blocks, respectively.

Quasi-Three-Dimensional Computations of the Microscopic Thermal and Dielectric Characteristics of an SF6 Rotating Switching Arc.

Pure sulfur hexafluoride (SF6) is chemically inert, non-flammable, non-toxic and thermally stable, and it has excellent dielectric strength and arc-quenching and control properties. The switching-off process of SF6 arc discharges occurs at the region between the contacts during the opening sequence to interrupt the flow of excessive current in a faulty network. The arc is tolerated in a controlled manner until a natural current zero when the arc discharge is rapidly quenched to restrict the thermal and dielectric reignition to the interruption. An SF6 self-blast switching chamber combines two advantages of blowing by heat expansion of the SF6 and arc rotation by electromagnetic effect of coil to improve the switching performance on thermal and dielectric reignition. The thermal and aerodynamic behaviors of an SF6 rotating switching arc in the chamber physically are complex and difficult to understand only by measurement due to their three-dimensional effects. Since the late nineteen-eighties, significant progress has been made in the method of computational fluid dynamics describing the physical processes occurring in the switching arc. The final goal of computer simulation technology on the arc switching is to predict the switching phenomena on the thermal and dielectric reignition from an engineering point of view. In this paper, we have conducted quasi-three dimensional computations to predict the thermal and dielectric reignition of SF6 rotating arcs occurring after a current zero in the self-blast switching chamber. Through the complete work, the microscopic thermal and aerodynamic behaviors of the remnant arc column after a current zero should be good criteria to predict the thermal and dielectric reignition of the rotating switching arc in the chamber.

New Age and Geochemical Data from the Southern Colville and Kermadec Ridges, SW Pacific: Insights into the recent geological history and petrogenesis of the Proto-Kermadec (Vitiaz) Arc

The intra-oceanic Kermadec arc system extends ~1300 km between New Zealand and Fiji and comprises at least 30 arc front volcanoes, the Havre Trough back-arc and the remnant Colville and Kermadec Ridges. To date, most research has focussed on the Kermadec arc front volcanoes leaving the Colville and Kermadec Ridges virtually unexplored. Here, we present seven 40Ar/39Ar ages together with a comprehensive major and trace element and Sr-, Nd-, and Pb-isotope dataset from the Colville and Kermadec Ridges to better understand the evolution, petrogenesis and splitting of the former proto-Kermadec (Vitiaz) Arc to form these two remnant arc ridges. Our 40Ar/39Ar ages range from ~7.5–2.6 Ma, which suggests that arc volcanism at the Colville Ridge occurred continuously and longer than previously thought. Recovered Colville and Kermadec Ridge lavas range from mafic picro-basalts (MgO = ~8 wt%) to dacites. The lavas have arc-type normalised incompatible element patterns and Sr and Pb isotopic compositions intermediate between Pacific MORB and subducted lithosphere (including sediments, altered oceanic crust and serpentinised uppermost mantle). Geochemically diverse lavas, including ocean island basalt-like and potassic lavas with high Ce/Yb, Th/Zr, intermediate 206Pb/204Pb and low 143Nd/144Nd ratios were recovered from the Oligocene South Fiji Basin (and Eocene Three Kings Ridge) located west of the Colville Ridge. If largely trench-perpendicular mantle flow was operating during the Miocene, this geochemical heterogeneity was likely preserved in the Colville and Kermadec sub arc mantle. Between 4.41 ± 0.35 and 3.40 ± 0.24 Ma some Kermadec Ridge lavas record a shift from Colville Ridge- to Kermadec arc front-like, suggesting the proto-Kermadec (Vitiaz-) arc split post 4.41 ± 0.35 Ma. The Colville and Kermadec Ridge data therefore place new constraints on the regional tectonic evolution and highlight the complex interplay between pre-existing mantle heterogeneities and material fluxes from the subducting Pacific Plate. The new data allow us to present a holistic (yet simplified) picture of the tectonic evolution of the late Vitiaz Arc and northern Zealandia since the Miocene and how this tectonism influences volcanic activity along the Kermadec arc at the present.

In search of Gondwana heritage in the Outer Melanesian Arc: no pre-upper Eocene detrital zircons in Viti Levu river sands (Fiji Islands)

Volcanic arcs of the Southwest Pacific, collectively referred to as the Outer Melanesian Arc, are generally thought to result from subduction of the Pacific Plate since the Late Cretaceous. Meanwhile, it is largely accepted that eastward roll-back of the old and dense oceanic plate allowed opening of marginal basins, which isolated large blocks of the former Gondwana margin. Incidentally, some ‘intra-oceanic’ volcanic arcs may have been nucleated on small continental fragments. Detrital zircons collected from sand banks in the mid-reaches of rivers from Viti Levu Island have been analysed for U–Pb geochronology and geochemistry, in order to search for a possible ancient continental arc basement, remnants of a Late Cretaceous arc, and determine the timing and evolution of Fiji arc magmatism. In contrast with some other places of the Outer Melanesian Arc (Solomon, Vanuatu), no pre-upper Eocene zircons have been found. Thus, Gondwana-derived fragments or Late Cretaceous–Paleocene arc remnants are unlikely to form the basement of Viti Levu. Zircon geochemistry confirms the purely intra-oceanic character of volcanic-arc magmatism as well. Variations in some trace-element ratios closely reflect the evolution of Viti Levu Arc from upper Eocene inception to upper Miocene climax and finally Pliocene intra-arc rifting and abandonment.

Crustal and lithospheric structure of inactive volcanic arc terrains in Fiji

The crustal and lithospheric velocity structure of the Fiji area may place important constraints on the evolution of island arcs in the region. Here, we obtain receiver functions from one permanent seismic station and three temporary networks on the Fiji Platform, the Lau Ridge, and other small islands to develop point estimates of crustal thickness under the networks. We obtain preliminary estimates of crustal thickness and Poisson's ratio from H-κ stacking of the receiver functions. We then perform an inversion for shear-wave velocity structure using a joint inversion of receiver functions and Rayleigh wave dispersion curves from ambient seismic noise and teleseismic events. The joint inversion results show an average crustal thickness of ~30 km beneath Viti Levu, the largest Fiji island, suggesting an extensive period of arc crustal formation and thickening. In contrast, we find shallower Moho depths averaging ~22 km beneath the smaller Fiji islands and the Lau Ridge. The limited thickness of the high velocity lid obtained by inversion indicates that the lithosphere is thinned beneath the marginal Fiji islands, which may be due to the early rifting event forming the Lau Basin together with the crustal thinning of this area as well, and to thermal lithospheric erosion processes associated with adjacent back-arc spreading. A relatively slow velocity middle crustal layer, roughly of 11–16 km thickness, exists throughout Fiji, suggesting that remnant arcs contain significant volumes of felsic crustal rocks necessary for continental crust formation.