Back-arc Rifting Research Articles

Age revision of the Neotethyan arc migration into the southeast Urumieh-Dokhtar belt of Iran: Geochemistry and U–Pb zircon geochronology

The Urumieh-Dokhtar magmatic belt of Central Iran runs parallel to the Zagros orogenic belt and has been resulted from Neotethys ocean subduction underneath Eurasia. The Bahr Aseman volcanic-plutonic complex (BAC), covering an area ~2000km2 in the Kerman magmatic belt (KMB) in the southern section of the Urumieh-Dokhtar belt, has long been considered as the earliest manifestation of extensive Cenozoic arc magmatism in KMB. The nature and timing of the magmatism, however, is poorly constrained. An area ~1000km2, in BAC and adjacent Razak volcaniclastic complex and Jebal Barez-type granitoids, was mapped and sampled for geochemistry and geochronology. Andesite and basaltic andesite are the main volcanic components in the study area; plutonic bodies vary from tonalite to quartz diorite, granodiorite and biotite-granite. The rocks in BAC display dominantly normal calc-alkaline character. On spider diagrams, the rocks are characterized by enrichments in LILE relative to HFSE and enrichments in LREE relative to HREE. These features suggest a subduction related setting for the BAC. LaN/YbN ratios for the intrusive and volcanic rocks range from 1.41 to 5.16 and 1.01 to 6.42, respectively. These values are lower than those for other known granitoids in KMB, namely the abyssal, dominantly Oligocene Jebal Barez-type (LaN/YbN=1.66–9.98), and the shallow, dominantly late Miocene Kuh Panj-type (LaN/YbN=12.97–36.04) granitoids. This suggests a less evolved magma source for the BAC igneous rocks. In Y vs. Nb and Th/Yb vs. La/Yb discrimination diagrams, an island-arc setting is defined for the BAC rocks. The rocks further plot in primitive island-arc domain in Nb vs. Rb/Zr and Y/Nb vs. TiO2 diagrams.The BAC volcanic and plutonic rocks yielded zircon U–Pb ages of 78.1 to 82.7Ma and 77.5 to 80.8Ma, respectively. Zircon U–Pb dating of volcanic rocks and granitoids from the adjacent Razak complex and the Jebal Barez-type granitoids indicated 48.2Ma and 26.1Ma ages, respectively, consistent with earlier works on similar rocks elsewhere in KMB.The new data allow a revision of the chronostratigraphy/tectonic history of KMB. In Late Cretaceous, a back arc rift developed extending from Nain to Baft (NB back arc) to the northeast of the Sanandaj-Sirjan magmatic arc. Along with shrinking of the Neotethys Ocean, the dip angle of the subducting slab decreased during the Late Cretaceous, and arc magmatism moved from the Sanandaj-Sirjan zone landward. Meanwhile, Bahr Aseman volcanic-plutonic complex formed as an island-arc in NB back arc rift. Later with arc shift, due to shallowing of subducted slab, magmatism moved toward continent leading to extensive volcanism in Kerman magmatic arc during Eocene and Oligocene, represented by volcanic-sedimentary Razak and Hezar Complexes, respectively.

Origin and model of transform faults in the Okinawa Trough

Transform faults in back-arc basins are the key to revealing the opening and development of marginal seas. The Okinawa Trough (OT) represents an incipient and active back-arc or marginal sea basin oriented in a general NE-SW direction. To determine the strikes and spatial distribution of transform faults in the OT, this paper dissects the NW- and NNE-SN-trending fault patterns on the basis of seismic profiles, gravity anomalies and region geological data. There are three main NW-trending transpressional faults in the OT, which are the seaward propagation of NW-trending faults in the East China Continent. The NNE-SN-trending faults with right-stepping distribution behave as right-lateral shearing. The strike-slip pull-apart process or transtensional faulting triggered the back-arc rifting or extension, and these faults evolved into transform faults with the emergence of oceanic crust. Thus, the transform fault patterns are inherited from pre-existing oblique transtensional faults at the offsets between rifting segments. Therefore, the OT performs the oblique spreading mechanism similar to nascent oceans such as the Red Sea and Gulf of Aden.

Geochronology and geochemistry of the Early Jurassic Yeba Formation volcanic rocks in southern Tibet: Initiation of back-arc rifting and crustal accretion in the southern Lhasa Terrane

Understanding the geological history of the Lhasa Terrane prior to the India–Asia collision (~55±10Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene (~18–10Ma) adakitic magmatism with economically significant porphyry-type mineralization has been interpreted as resulting from partial melting of the Jurassic juvenile crust, but how this juvenile crust was accreted remains poorly known. For this reason, we carried out a detailed study on the volcanic rocks of the Yeba Formation (YF) with the results offering insights into the ways in which the juvenile crust may be accreted in the southern Lhasa Terrane in the Jurassic. The YF volcanic rocks are compositionally bimodal, comprising basalt/basaltic andesite and dacite/rhyolite dated at 183–174Ma. All these rocks have an arc-like signature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba and U) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr–Nd and zircon Hf isotopic compositions, pointing to significant mantle isotopic contributions. Modeling results of trace elements and isotopes are most consistent with the basalts being derived from a mantle source metasomatized by varying enrichment of subduction components. The silicic volcanic rocks show the characteristics of transitional I–S type granites, and are best interpreted as resulting from re-melting of a mixed source of juvenile amphibole-rich lower crust with reworked crustal materials resembling metagraywackes. Importantly, our results indicate northward Neo-Tethyan seafloor subduction beneath the Lhasa Terrane with the YF volcanism being caused by the initiation of back-arc rifting. The back-arc setting is a likely site for juvenile crustal accretion in the southern Lhasa Terrane.

Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases

The plate tectonic setting in which proto-ophiolite ‘oceanic’ lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling. Rapid deep upper-mantle circulation in the models during subduction zone retreat can exhume and emplace material in the forearc proto-ophiolite from as deep as the mantle transition zone, thereby explaining diamonds and other 10–15 GPa UHP phases in Tibetan ophiolites.

P62 伊豆-小笠原弧,スミスリフトから採取された2タイプの玄武岩(ポスターセッション)

P62 伊豆-小笠原弧,スミスリフトから採取された2タイプの玄武岩(ポスターセッション)

A Mediterranean-style model for early Neoproterozoic amalgamation of South China

A Mediterranean-style model is proposed for early Neoproterozoic amalgamation of South China, based on recognition of the Guangxi promontory on the southeastern margin of the Yangtze craton through a synthesis of geologic, geophysical and chronological data. The model suggests that, while the Guangxi promontory collided with the overriding Cathaysia terrane along the Yunkai orogeny during the earliest Neoproterozoic, the convergence between the Yangtze craton and the Cathaysia terrane paused or slowed down, which triggered the upper Rhine Graben-style Kangdian impact rift along the western Yangtze margin and resulted in roll-back of the oceanic slab trapped in the embayments and sequential formation of backarc rifts in the Cathaysia margin during the early Neoproterozoic. This model predicts a complex diachronous Yangtze–Cathaysia collision history, first at the Guangxi promontory at ∼1000Ma and finally at the Guizhou reentrant at ∼830Ma.

Late Cenozoic evolution of the East China continental margin: Insights from seismic, gravity, and magnetic analyses

Seismic reflection profiles reveal the structural characteristics beneath the East China Sea shelf margin and the Okinawa Trough, and provide new constraints on the Late Cenozoic evolution of the East China continental margin. The Frontal Shelf Basin between the Taiwan-Sinzi Uplift and the trough axis occupies the western half of the Northern-Middle Okinawa Trough. In this basin, the Middle-Late Miocene sediments are confined to grabens or half-grabens dominated by listric faults, whereas the overlying Pliocene-Quaternary sequence is characterized by a uniform thickness and dense planar-type faults, suggesting that rifting of the Northern-Middle Okinawa Trough initiated during the Middle Miocene but slowed down during the earliest Pliocene. Since that time, the opening of the Okinawa Trough has been dominated by diffuse rifting. The Southern Okinawa Trough is predominately filled by Quaternary sediments, indicating that its back-arc rifting began during the earliest Pleistocene. Contractional structures identified in the pre-Quaternary sequence beneath the continental slope, along with an erosional Pleistocene-pre-Pleistocene unconformity in the Southern Okinawa Trough, demonstrate the existence of pre-rifting compression and uplifting in this region. We use this evidence and previously published results, to propose an evolutionary model of the East China continental margin during the Late Cenozoic. The Northern-Middle Okinawa Trough began rifting during the Middle Miocene on a paleo-uplift. The Luzon Arc initially impinged upon the Eurasian continental margin during the Late Miocene near the southeastern end of the Miyako Fault Belt and activated the proto-Taiwan Orogeny in today's Southern Okinawa Trough and adjacent regions. During the Late Miocene-Pliocene, the orogeny quickly propagated southwestward along with the west-northwest-moving Philippine Sea Plate. Subsequently, the rifting of the Southern Okinawa Trough was initiated during the earliest Pleistocene and propagated southwestward following the post-collisional collapse of the proto-Taiwan Orogen. This geodynamic scenario resembles that of present-day northern Taiwan.

Late Jurassic–Early Cretaceous episodic development of the Bangong Meso-Tethyan subduction: Evidence from elemental and Sr–Nd isotopic geochemistry of arc magmatic rocks, Gaize region, central Tibet, China

The Bangong Meso-Tethys plays a critical role in the development of the Tethyan realm and the initial elevation of the Tibetan Plateau. However, its precise subduction polarity, and history still remain unclear. In this study, we synthesize a report for the Late Jurassic–Early Cretaceous two-phase magmatic rocks in the Gaize region at the southern margin of the Qiangtang block located in central Tibet. These rocks formed during the Late Jurassic–earliest Cretaceous (161–142Ma) and Early Cretaceous (128–106Ma), peaking at 146Ma and 118Ma, respectively. The presence of inherited zircons indicates that an Archean component exists in sediments in the shallow Qiangtang crust, and has a complex tectonomagmatic history. Geochemical and Sr–Nd isotopic data show that the two-phase magmatic rocks exhibit characteristics of arc magmatism, which are rich in large-ion incompatible elements (LIIEs), but are strongly depleted in high field strength elements (HFSEs). The Late Jurassic–earliest Cretaceous magmatic rocks mixed and mingled among mantle-derived mafic magmas, subduction-related sediments, or crustally-derived felsic melts and fluids, formed by a northward and steep subduction of the Bangong Meso-Tethys ocean crust. The magmatic gap at 142–128Ma marks a flat subduction of the Meso-Tethys. The Early Cretaceous magmatism experienced a magma MASH (melting, assimilation, storage, and homogenization) process among mantle-derived mafic magmas, or crustally-derived felsic melts and fluids, as a result of the Meso-Tethys oceanic slab roll-back, which triggered simultaneous back-arc rifting along the southern Qiangtang block margin.

Subduction-related Late Carboniferous to Early Permian Magmatism in the Eastern Pontides, the Camlik and Casurluk plutons: Insights from geochemistry, whole-rock Sr–Nd and in situ zircon Lu–Hf isotopes, and U–Pb geochronology

Late Carboniferous to early Permian granitoid rocks represent a volumetrically minor component of the Eastern Pontide lithosphere, but they preserve useful information about the region's tectonomagmatic history. The Casurluk and Camlik plutons primarily consist of gabbro, gabbroic diorite, diorite, monzogabbro, monzodiorite and monzonite, which intrude early to middle Carboniferous granitic basement rocks in the region. In this study, we use in situ zircon U–Pb ages and Lu–Hf isotopic values, whole-rock Sr–Nd isotopic values, and mineral chemistry and geochemistry of these plutons to determine petrogenesis and crustal evolution; we also discuss geodynamic implications. LA–ICP-MS zircon U–Pb dating of magmatic zircons from the rocks suggests that the plutons were emplaced during the late Carboniferous to early Permian (302Ma). The metaluminous and I-type intrusive rocks belong to the high-K calc-alkaline series. In addition, they are relatively enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs); they are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), such as Nb and Ti. All of the samples have homogeneous initial ISr values (0.70675 to 0.70792) and low εNd (t) values (−5.1 to −3.3). Zircons from the rocks of both plutons have uniform negative to slightly positive εHf (t) values (−3.5 to 1.4) and old Hf two-stage model ages (1323 to 1548Ma), implying that they have the same source, as well as suggesting the involvement of old enriched lithospheric mantle materials during their magma genesis. These results, combined with the εHf (t) values and two-stage model ages, demonstrate that the primary magmas were derived from partial melting of old lithospheric mantle material metasomatized by subduction-related fluids. Considering other regional geological data from the Sakarya Zone where these plutons formed, we conclude that late Carboniferous to early Permian magmatism in the area involved the partial melting of a phlogopite- and spinel-bearing lherzolite under an extensional regime in a subduction-related setting. Such an extension event would have caused hot asthenospheric upwelling and was likely triggered by back-arc rifting during subduction of the Paleo-Tethyan oceanic slab. Thus, the magmas are likely subduction-related products, implying that southward subduction of Paleo-Tethys oceanic lithosphere in the region began during the late Carboniferous to early Permian.

Geochemical evidence for the Paleoproterozoic arc – Back arc basin association and its importance in understanding the evolution of the Central Indian Tectonic Zone

The Central Indian Tectonic Zone (CITZ) is considered to be one sector in a series of Paleoproterozoic orogens that stitched together the supercontinent Columbia. The Paleoproterozoic Mahakoshal belt forms the foundation of the CITZ. Metabasalts and metaclastic rocks of the Mahakoshal belt have been studied for major elements and trace elements, including the rare earth elements (REE).Metabasalts of the lower Saleemanabad Formation are grouped into two types. However, both types of metabasalt depict a back arc basalt character by showing enrichments in large ion lithophile elements (LILEs like K, Rb, Cs and Ba) and light rare earth elements (LREEs) but show depletions in high field strength elements (HFSEs like Nb, Ta, Y, and Ti) and HREE (heavy rare earth elements) in comparison to the N-MORB. This is further supported by some key elemental ratios like Ba/Nb, Nb/Ta, La/Nb, Eu/Eu*, Nb/Nb*, and REE patterns. The geochemistry of the metaclastic rocks of the lower Saleemanabad, middle Parsoi, and upper Dudhmaniya Formations of the Mahakoshal belt show an arc signature rather than the cratonic signature in the Bundlekhand TTG and N-MORB normalised diagrams. The arc signature (calc-alkaline series) of metaclastic rocks is further supported by various tectonic discrimination diagrams.The overall study indicates that the Mahakoshal belt was formed as a back arc rift basin very close to continental arcs on the hinterland of the Bundelkhand craton at the initial stage of subduction as a result of trench rollback followed by thermal relaxation and basin inversion. The sediment supply started from the continental arcs lying south of the Mahakoshal basin. Furthermore, the present study strengthens the mechanism of subduction-related outgrowth of Columbia along its continental margins and implies that the CITZ is a major Precambrian crustal boundary that began as a Mahakoshal back arc rift by northward subduction.

Structures and active tectonics of compressionally reactivated back-arc failed rift across the Toyama trough in the Sea of Japan, revealed by multiscale seismic profiling

Compressional reactivation of back-arc failed rifts in the Sea of Japan and its mechanism that mobilizes active tectonics based on the crustal structures remains poorly understood. Here, we show new results and interpretations of onshore-offshore deep seismic reflection profiles across the Toyama trough and its southern extension, located in the Sea of Japan back-arc rift, to reveal their deep to shallow structures of the crust and reactivated active faults. We observed (1) approximately 5km thick, syn- and post-rift Neogene sediments underlain by pre-Neogene basement rocks characterized by high velocity anomalies, and (2) crustal thrust wedges adjacent to the rift axis. Shallower high-resolution seismic reflection profiles also suggest Quaternary structural growth of the reactivated thrusts and thrust wedges, consistent with tectonic geomorphology of the middle to late Pleistocene marine and fluvial terraces. These findings demonstrate that the mechanical contrasts between the crustal thrust wedges, primarily composed of the pre-rift continental crust and high velocity lower crust caused by mafic intrusions in the rifting stage, have fundamentally controlled the styles of active deformation during the post-rift Quaternary compressional stress regime. These results also contribute to the construction of the seismic source fault models for tsunamis and seismic hazard estimations of this region. In addition, we emphasize that combining multi-scale seismic reflection profiles with tectonic geomorphology sheds light on complicated structural characteristics and recent activities of reactivated normal faults that are mostly blind because of the dynamic subsidence of failed rift structures.

The influence of Ryukyu subduction on magma genesis in the Northern Taiwan Volcanic Zone and Middle Okinawa Trough — Evidence from boron isotopes

Boron (B) is an excellent geochemical tracer for investigating crustal recycling processes at convergent margins, due to its high fluid mobility under high P–T conditions, distinct elemental abundances and isotopic compositions in the mantle wedge and subducting slabs. The Northern Taiwan Volcanic Zone (NTVZ), wherein the nature of magma genesis has long been a topic of debate, is located at the rear side of the Okinawa Trough (OT), an atypical back-arc rift in the Ryukyu subduction system. In this study, B and B isotopes (δ11B) were measured in 19 volcanic rocks collected from the NTVZ and the middle Okinawa Trough (MOT) to assess the influence of the Ryukyu subduction system on magma genesis.The B concentrations in the MOT and NTVZ volcanic rocks are 5.8 to 13.6mg/L and 2.2 to 48.6mg/L, respectively. The large B abundances variation in the NTVZ was caused mainly by variable degrees of partial melting. The Nb/B and δ11B in the MOT have small ranges of 0.5 to 0.6 and −2.7‰ to 0.2‰, respectively, whereas they range widely from 0.4 to 2.5 and from −8.6‰ to 2.4‰, respectively in the NTVZ. These Nb/B values suggest that the magma contains a smaller subduction component than that normally observed in arcs, although this component is still more substantial than in a typical back-arc setting.The δ11B results indicate insignificant influence of the subducting Philippine Sea Plate at 2.6Ma, but it becomes more substantial later in the NTVZ. The mixing proportions of sediment derived fluids in onshore volcanoes in the NTVZ imply a rather heterogeneous mantle wedge near the plate boundary, most likely due to either a heterogeneous source of slab derived fluids or more complicated mantle flow. A substantial B flux from the subducting slab in the incipient back-arc rifting in the MOT and NTVZ may reflect characteristics of a cold, steep and fast subducting slab, which may be capable of carrying volatiles efficiently into greater depth in subduction zones.The δ11B data also provide insight into the nature of the different subduction components. A three-end-member mixing model suggests that MOT and NTVZ magmas may have incorporated subduction fluids derived from altered oceanic crust or serpentinite, whereas the onshore volcanoes in the NTVZ, and in particular in the Tsaolingshan, may have involved a higher proportion of sediment component.

Off-axis magmatism along a subaerial back-arc rift: Observations from the Taupo Volcanic Zone, New Zealand.

Continental rifting and seafloor spreading play a fundamental role in the generation of new crust. However, the distribution of magma and its relationship with tectonics and volcanism remain poorly understood, particularly in back-arc settings. We show evidence for a large, long-lived, off-axis magmatic intrusion located on the margin of the Taupo Volcanic Zone, New Zealand. Geodetic data acquired since the 1950s show evidence for uplift outside of the region of active extension, consistent with the inflation of a magmatic body at a depth of ~9.5 km. Satellite radar interferometry and Global Positioning System data suggest that there was an increase in the inflation rate from 2003 to 2011, which correlates with intense earthquake activity in the region. Our results suggest that the continued growth of a large magmatic body may represent the birth of a new magma chamber on the margins of a back-arc rift system.

Late Neoproterozoic magmatism in South Qinling, Central China: Geochemistry, zircon U-Pb-Lu-Hf isotopes and tectonic implications

The Neoproterozoic tectonic evolution of the northern margin of the Yangtze Block in South China remains debated. In this study, we present results from LA–ICP–MS zircon U–Pb geochronology on a suite of intermediate-felsic rocks in South Qinling, Central China which show a mean age of ca. 630Ma. The zircon εHf(t) values of these rocks mostly range from +0.44 to +14.78. Geochemically, the granites and syenite show high total alkali contents, with enrichment in LREE, LILE (Rb, Ba, and K), and HFSE (Th, U, Nb, Ta, Zr, and Hf), and depletion in Sr, P, and Ti, similar to the features of A-type granites. The meta-diorite shows high Na2O, with depletion in Eu, Ti, and LILE (Sr, Rb, Ba, and K), and enrichment in HFSE (Th, U, Nb, Ta, Zr, and Hf). The geochemical features are consistent with formation of the intermediate-felsic suite through fractionation from underplated basaltic magma that originated from sub-continental lithospheric mantle metasomatized by asthenosphere-derived oceanic-island-basalt-like (OIB-like) melts, coupled with minor crustal contamination. We correlate the ca. 630Ma magmatism with a back-arc rift setting that probably developed in relation to slab tearing during continued slab rollback.

Transient crustal movement in the northern Izu–Bonin arc starting in 2004: A large slow slip event or a slow back-arc rifting event?

The Izu–Bonin arc lies along the convergent boundary where the Pacific Plate subducts beneath the Philippine Sea Plate. Horizontal velocities of continuous Global Navigation Satellite System stations on the Izu Islands move eastward by up to ~1cm/year relative to the stable part of the Philippine Sea Plate suggesting active back-arc rifting behind the northern part of the arc. Here, we report that such eastward movements transiently accelerated in the middle of 2004 resulting in ~3cm extra movements in 3years. We compare three different mechanisms possibly responsible for this transient movement, i.e. (1) postseismic movement of the 2004 September earthquake sequence off the Kii Peninsula far to the west, (2) a temporary activation of the back-arc rifting to the west dynamically triggered by seismic waves from a nearby earthquake, and (3) a large slow slip event in the Izu–Bonin Trench to the east. By comparing crustal movements in different regions, the first possibility can be shown unlikely. It is difficult to rule out the second possibility, but current evidence support the third possibility, i.e. a large slow slip event with moment magnitude of ~7.5 may have occurred there.

Tectonic focusing of voluminous basaltic eruptions in magma-deficient backarc rifts

The Coriolis Troughs of the New Hebrides subduction zone are among the youngest backarc rifts in the world. They reach depths of >3 km, despite their small size (<100 km in length and only 25–45 km wide) and their proximity to the arc front (∼50 km). The narrow, deep graben morphology is characteristic of magma-deficient arc rifts in the early stages of backarc extension, where the rate of extension and subsidence exceeds the magmatic input. Unexpectedly, the youngest graben, the Vate Trough, contains a centrally-located 1000-m tall and 14-km wide shield volcano with a large, 5×8 km breached summit caldera. The Nifonea axial volcano has a volume of ∼126 km3, reflecting unusually high extrusion rates, given its young age (<3 Ma), and the summit caldera hosts the remnants of a large lava lake, the first described from a submarine backarc setting. Extensive diffuse hydrothermal venting and several clusters of black smoker chimneys, with the highest recorded fluid temperatures (368 °C) in the SW Pacific, occur on the youngest lava flows. Comparison with similar axial volcanoes on the mid-ocean ridges suggests that the 46×106 m3 of sheet flows in the caldera could have been erupted in <30 hours. The focusing of voluminous basaltic eruptions into an otherwise magma-deficient backarc has been linked to strong left-lateral transtension caused by clockwise rotation and segmentation of the southern portion of the arc after collision with d'Entrecasteaux ridge. This study shows that the upper plate stresses can result in dramatic variability in magma supply and hydrothermal activity at the earliest stages of arc rifting and could explain the wide range of melt compositions, volcanic styles and mineral deposit types found in nascent backarc rifts.

Old and juvenile source of Paleozoic and Mesozoic basaltic magmas in the Acatlán and Ayú complexes, Southern Mexico: Nd isotopic constraints

The Neoproterozoic–Paleozoic Acatlán Complex and the Mesozoic Ayú Complex of southern Mexico consist of clastic rocks and rift-related igneous rocks inferred to have originated along a rifted passive margin in southwestern Mexico, either in an inactive (Neoproterozoic–Ordovician) or an active (Devonian–Carboniferous and Triassic–Early Jurassic) tectonic setting. The latter formed on the inner margin of a backarc basin. These passive margin rocks were partly underthrust beneath the Acatlán Complex, into which they were subsequently extruded: extrusion was synchronous with the backarc basin development. Thus, (i) the Neoproterozoic–Ordovician rocks underwent underthrusting, high-pressure metamorphism, and extrusion during the Late Devonian–Carboniferous (365–330Ma); (ii) the Carboniferous rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Permian and Triassic; and (iii) the Triassic–Lower Jurassic rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Jurassic. Nd isotopic data from tholeiitic mafic rocks on either side of the HP extrusion zone reveal that both were underlain by similar peri-Rodinian subcontinental lithospheric mantle in the Neoproterozoic–Ordovician, which was supplemented in the Devonian–Carboniferous and Triassic–Early Jurassic by a juvenile depleted mantle source. The alternation of underthrusting and backarc rifting accompanied by extrusion may be related to flattening and steepening of the Beniof zone, respectively.

Major zircon megacryst suites of the Indo-Pacific lithospheric margin (ZIP) and their petrogenetic and regional implications

Zircon megacrysts (± gem corundum) appear in basalt fields of Indo-Pacific origin over a 12,000 km zone (ZIP) along West Pacific continental margins. Age-dating, trace element, oxygen and hafnium isotope studies on representative zircons (East Australia–Asia) indicate diverse magmatic sources. The U–Pb (249 to 1 Ma) and zircon fission track (ZFT) ages (65 to 1 Ma) suggest thermal annealing during later basalt transport, with < 1 to 203 Ma gaps between the U–Pb and ZFT ages. Magmatic growth zonation and Zr/Hf ratios (0.01–0.02) suggest alkaline magmatic sources, while Ti—in—zircon thermometry suggests that most zircons crystallized within ranges between 550 and 830 °C. Chondrite-normalised multi-element plots show variable enrichment patterns, mostly without marked Eu depletion, indicating little plagioclase fractionation in source melts. Key elements and ratios matched against zircons from magmatic rocks suggest a range of ultramafic to felsic source melts. Zircon O-isotope ratios (δ18O in the range 4 to 11‰) and initial Hf isotope ratios (eHf in the range +2 to +14) encompass ranges for both mantle and crustal melts. Calculated Depleted Mantle (TDM 0.03–0.56 Ga) and Crustal Residence (0.20–1.02 Ga) model ages suggest several mantle events, continental break-ups (Rodinia and Gondwana) and convergent margin collisions left imprints in the zircon source melts. East Australian ZIP sites reflect prolonged intraplate magmatism (~85 Ma), often during times of fast-migrating lithosphere. In contrast, East Asian-Russian ZIP sites reflect later basaltic magmatism (<40 Ma), often linked to episodes of back-arc rifting and spreading, slow-migrating lithosphere and slab subduction.

Non-volcanic crustal movements of the northernmost Philippine Sea plate detected by the GPS-acoustic seafloor positioning

AbstractRepeatedly performing the GPS-acoustic seafloor positioning, we first succeeded in detecting non-volcanic seafloor movements on the Philippine Sea plate (PHS) subducting along the Sagami Trough. At a seafloor geodetic site on the northernmost part of the PHS off the Boso Peninsula, we detected significant eastward motion with respect to the central part of the PHS. This is unaccountable by the coupling between the Pacific plate and the PHS along the Izu-Bonin (Ogasawara) Trench because it would cause the westward elastic deformation at BOSS. It is rather consistent with the rigid motion of the tectonic block in the fore-arc along the Izu-Bonin Trench, associated with the back-arc rift. The other site on the western side of the Sagami Bay had moved toward the north relative to the Izu Peninsula. It suggests that the Izu microplate obviously moves relative to the northern PHS. The difference between the velocities of the Sagami Bay and the Izu Peninsula indicates the coupling on the boundary fault as well.

2.24 Ga mafic dykes from Taihua Complex, southern Trans-North China Orogen, and their tectonic implications

LA-ICP-MS U–Pb zircon geochronological, geochemical and Sr–Nd isotopic data are presented for the mafic dykes in the Zhaiwa Mo deposit, Taihua Complex to characterize the tectonic setting of the southern Trans-North China Orogen (TNCO). These mafic dykes intruded the Taihua Supergroup and yield a concordant 207Pb/206Pb age of 2242±25 Ma. The zircon grains have initial 176Hf/177Hf values of 0.281012–0.281194, with negative ɛHf(t) values ranging from −11.4 to −2.7 and TDM ages ranging 2805–3051Ma. Whole rock geochemistry shows that the dykes are doleritic and have high Fe contents, which usually possess relatively high density. The (87Sr/86Sr)t (t=2242Ma) values fall into two groups, i.e., 0.705605–0.707058 and 0.725237–0.748725. The ɛNd(t) values (−6.67 to −3.27, with corresponding TDM ages of 3020–4063Ma) indicate an enriched mantle source. Geochemical characteristics of the Zhaiwa mafic dykes, including the low TiO2 (0.94–2.00%) and enriched Sr–Nd isotopes, suggest their derivation from a sub-continental lithospheric mantle (SCLM) source modified by recycled sediments. Simulation calculations imply the absence of garnet in the mantle source, indicating the emplacement of these mafic dykes in an extensional setting, e.g., back-arc rifting. Our work implies the existence of a back arc basin at 2.24Ga in the southern TNCO and the final collision between the Eastern and Western Blocks of the North China Craton must have happened after 2.24Ga.