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The Mayodia ophiolitic complex of Arunachal Pradesh, India is a highly dismembered body located close to the Namche Barwa syntaxis. It structurally represents a klippe and consists dominantly of serpentinite and dunite within which amphibolite and hornblendite occur. Detailed field observations indicate that amphibolites are concordant bodies within serpentinite, whereas hornblendite occurs as dykes. Geochemically, serpentinite samples indicate their abyssal/forearc origin showing refertilisation due to melt-mantle interactions. Massive dunite bodies, occurring as dykes within serpentinites, are replacive in origin and represent fossilized melt channels. The geochemistry of amphibolites is akin to gabbros formed at back-arc basin. Later, serpentinites were severely intruded by hornblendite dykes during the waning stage of back-arc spreading. Finally, the entire litho-package was obducted onto the passive margin of Indian plate as the Mayodia ophiolitic complex which subsequently got folded, deformed and metamorphosed during orogenesis. During this process, the crustal section of the complex was entirely eroded, preserving only the mantle section of this ophiolite. [ABSTRACT FROM AUTHOR]

The Mount Grace metamorphosed carbonatites (Late Devonian) outcrop as thin (0.5–4 m), laterally discontinuous, strata-bound mappable lenses within the Monashee complex of the southeastern Canadian Cordillera. The host stratigraphic sequence (Monashee cover gneiss) was metamorphosed and deformed in the Late Cretaceous to early Eocene followed immediately by exhumation of the Frenchman Cap and Thor Odin domes. We present seven stratigraphic logs for Mount Grace carbonatites including new and previously described outcroppings spanning ∼30 km. The Mount Grace carbonatite units were deposited regionally within or near the top of a shallow marine sedimentary sequence within miogeoclinal strata of the western margin of paleo-North America (Laurentia). The distribution of the Mount Grace carbonatite lithofacies and the preserved depositional structures and textures suggest that these are pyroclastic deposits resulting from phreatomagmatic eruptions. Our new data enhance the volcanological story with an eruption scenario involving phreatomagmatic reactions and deposition from pyroclastic density currents, sourced from multiple centers within a field of monogenetic maar volcanoes. The distribution of the Mount Grace carbonatites parallel to the western margin of the paleo-North American continent correlates well with regional Late Devonian alkaline magmatism associated with development of an extensional back-arc basin. [ABSTRACT FROM AUTHOR]

Inherited rift topography controls the sediment routing, timing of sand supply, and sedimentary linkage of early post‐rift depocentres. Exhumed examples of early post‐rift turbidite systems are rare and previous studies have examined the evolution of individual depocentres; in contrast, the detailed evolution of early post‐rift turbidite systems across multiple depocentres has never been documented. Current fill‐and‐spill models do not detail the stratigraphic architecture and evolution of sedimentological characteristics of multiple intraslope fans developed across topography, including bed type distributions. Here, the evolution of three intraslope fans that developed across two early post‐rift depocentres is documented along an 18 km long transect in the southern Neuquén Basin, Argentina. The relative chronology of sand supply in depocentres is constrained with new U–Pb ages, and sediment source areas with provenance analysis. The early post‐rift intraslope fans record progradation of the system and progressive sedimentary linkage of post‐rift depocentres, transverse to local syn‐rift structures, with sediment routing subparallel to the cratonic basin margin. The large‐scale stratigraphic architecture of intraslope fans indicates an evolution as a fill‐and‐spill system, with initial confinement through flow stripping and overspill to spillover with erosion and bypass across a transverse topographic high separating the depocentres. Changes in early post‐rift intraslope fan characteristics, including thickness, sandstone content, lobe complex stacking patterns, stratal termination patterns and bed type distribution, record changing confinement through time within a depocentre, and spatially across depocentres. The strong spatial and vertical stratigraphic variability of transitional flow deposits and hybrid event beds reflects enhanced erosion, sediment bypass and flow transformation across transverse relief between the two depocentres during the spillover phase. These findings advance current understanding of early post‐rift turbidite systems and refine fill‐and‐spill models, which will help the prediction of spatial and vertical changes in rock quality and connectivity in subsurface hydrocarbon reservoirs and CO2 storage sites. [ABSTRACT FROM AUTHOR]

The Meseta del Lago Buenos Aires (MLBA) in southern Patagonia, a volcanic plateau formed from ~12 Ma to present, provides an opportunity to investigate the temporal evolution in volcanism as this region transitions from the subduction of the Nazca plate to the formation of the slab window produced by the collision of the Chile Ridge and the Andean subduction zone. Here, we report new major, minor, and trace element contents, as well as Sr, Nd, Pb, and Hf isotopes of the MLBA lavas. Three distinct geochemical endmembers can be distinguished in the MLBA basalts: a subduction-influenced endmember, a transitional component similar to the South Atlantic enriched mid-ocean ridge basalts, and an enriched component akin to the EM1 mantle composition. Lavas older than ~1.5 Ma define a compositional continuum between the subduction-influenced and transitional endmembers; this trend is also present in many other southern Patagonian plateaus regardless of their distance to the trench, eruption age, and the composition of the continental blocks where they are located. In contrast, MLBA basalts younger than ~1.5 Ma uniquely define a transition into the EM1 mantle component at the time when this region was affected by the slab window. The estimated pressures and temperatures of mantle-melt equilibration for the MLBA basalts indicates an increase in both parameters after the formation of the slab window that roughly correlate with the changes in lava composition. The basalts' composition from all southern Patagonia plateaus points to the presence of the South Atlantic mid-ocean ridge basalt mantle influenced by the Discovery, Shona, and Bouvet hotspots rather than the sub-slab mantle, as represented by the Chile Ridge basalts. This observation challenges the hypothesis that the sub-slab mantle within the slab window has had an important role in the composition of the erupted lavas. Instead, it suggests the presence of a South Atlantic mantle beneath southern Patagonia either within the mantle wedge, consistent with a long-lasting South Atlantic convection cell beneath South America, or in the subcontinental lithospheric mantle metasomatized before or just after the opening of the South Atlantic basin, as demonstrated by the composition of southern Patagonia mantle xenoliths. Although it is difficult to precisely distinguish the contributions of the asthenosphere from that of the metasomatized subcontinental lithospheric mantle beneath this region, our work suggests significant contributions from the latter in the composition of the MLBA lavas. [ABSTRACT FROM AUTHOR]

The widespread occurrence of Carboniferous Wuluate Formation volcanic rocks in the West Kunlun is closely related to the evolution of the Paleo-Tethys but has received little attention. In this study, we conducted detailed fieldwork on the Wuluate Formation in West Kunlun and performed zircon U‒Pb dating, whole-rock major and trace element analysis, whole-rock Sr-Nd isotope analysis, and in situ zircon Hf isotope analyses. The Wuluate Formation is characterized by a bimodal volcanic suite. Zircon U‒Pb dating indicates that the rhyolites erupted at ca. 348–353 Ma, which is coeval with the zircon U‒Pb dating result of 354 Ma for the basalt. The basalts show a tholeiitic magmatic evolution trend with flattened REE patterns and depleted whole-rock Sr-Nd and zircon Hf isotopes, indicating geochemical affinity to back-arc basin basalt (BABB). Trace element modelling results indicate that the primitive basaltic compositions fit well with the products of the DMM + sediments + H2O source under melting conditions of asthenospheric temperature and thin lithosphere thickness, and the REE modelling results show that the basalts underwent 40% to 95% crystallographic differentiation of olivine, monazite, hornblende, plagioclase, and magnetite. The silicic rocks are calcic to calcic-alkaline and meta/peraluminous. They exhibit flattened REE patterns and slightly enriched zircon Hf isotopes, which could originate from Precambrian metaigneous sources of West Kunlun. The Kungai bimodal suite is likely to have formed in a back-arc setting corresponding to the northward subduction of Paleo-Tethys. The subduction of Paleo-Tethys had already occurred, and a back-arc basin had developed before the Early Carboniferous. [ABSTRACT FROM AUTHOR]

[Display omitted] • A regional geology systematic review based on cross-sections and space–time diagrams. • The Dom Feliciano Belt orogenic cycle records pre-, syn- and post-collisional stages. • PT-paths are characteristic of hinterland and foreland tectonic domains. • Northern DFB foreland rifting contrasts with central DFB active margin in the Tonian. • The symmetric schismogenesis of West Gondwana Orogen models can be conciliated. The Dom Feliciano Belt evolution is reviewed based on cross-sections, space–time diagrams, P-T paths, and Sr–Nd isotopic data of pre-collisional metaigneous rocks. The belt is divided into northern, central and southern sectors, subdivided into tectonic domains, developed at Neoproterozoic pre-, syn- and post-collisional stages. The northern sector foreland pre-collisional setting represents a rift, with tholeiitic (meta)volcanic rocks (∼800 Ma) chronocorrelated to hinterland intermediate and acidic orthogneisses of high-K calc-alkaline arc signature. In contrast, the central sector records a complete section from the forearc towards the back-arc region during pre-collisional times. In the western domain, ophiolites (∼920 Ma) are associated with arc-related orthogneisses and metavolcanic rocks (880–830 Ma; 760–730 Ma). At back-arc position, continental arc-related magmatism (800–780 Ma) is registered by hinterland orthogneisses and central foreland metavolcanic rocks. Ophiolites on the hinterland opposite side comprise two compositional groups, with N-MORB and supra subduction signature, interpreted as a back-arc basin record (∼750 Ma). The pre-Neoproterozoic basement of the whole belt is correlated with the Nico Perez Terrane and Luis Alves Block (Archean to Mesoproterozoic, with Congo Craton affinity). This contrasts with the Piedra Alta Terrane (Rio de La Plata Craton, only Paleoproterozoic), westernmost Uruguay. The suture between the Piedra Alta and Nico Perez terranes is correlated with the suture zone in the westernmost central sector. Transpression affected both foreland and hinterland during collision (660–640 Ma), with high-T/low-P hinterland progressive exhumation, whilst foreland low- to medium-grade correlated sequences record underthrusting. Post-collisional processes included magmatism throughout the belt (640–580 Ma), strain partitioning along strike-slip shear zones, and foreland basin fill. Late tectono-metamorphic and magmatic processes (560–540 Ma) were attributed to the Kalahari Craton collision. Arc magmatism migration due to subduction angle variations suggests modern-style plate tectonics during Gondwana amalgamation. Diachronism and kinematic inversion are characteristic of an oblique convergent multi-plate orogenic system. [ABSTRACT FROM AUTHOR]

Vast and geochemically diverse volcanic rocks from the western Alborz Magmatic Assemblage (AMA) represent the back-arc of the central Iran Neotethyan arc (Urumieh–Dokhtar Magmatic Assemblage; UDMA). Volcanic rocks of the west AMA record valuable information on the timing, source region(s) and geodynamic setting of magmatism. Over 30 days of field study and sampling, investigation of 170 thin sections, 30 whole-rock geochemical analyses, 13 whole-rock Sr–Nd isotopic ratios and U–Pb age dating of zircon separates from 7 samples furnished the present study data. Eocene (38.5 Ma) OIB-type volcanic rocks from South Kaleybar indicate an anorogenic (extensional) setting. This lithospheric-scale extensional event induced influx of asthenospheric mantle into the sub-arc wedge, of which the partial melts differentiated to produce OIB-type melts. The OIB-type melts incorporated some inherited zircons in their ascent through the Cadomian crust. A continued extensional regime led to asthenospheric upwelling and produced mafic melts that produced 27.5-Ma-old subalkaline series volcanics. The LILE-depleted signature of the South Kaleybar subalkaline volcanic rocks implies that their mantle source region experienced previous partial melting event(s), probably during OIB-type magmatism in the Eocene. Alkaline volcanism (24.4 Ma) and concurrent high-silica adakitic volcanism (24.3 Ma–23.4 Ma) followed subalkaline magmatism. The alkaline rock signature in the study area range from 'Nb–Ta depleted' to 'plume-type'. This is consistent with lithosphere–asthenosphere interaction in an arc-related setting. Simultaneous partial melts of delaminated lower crustal rocks reacted with the asthenosphere and produced adakitic melts. Asthenospheric, lithospheric and crustal contribution to the magmatism in South Kaleybar express the back-arc signature of magmatism in Eocene to Late Oligocene times. [ABSTRACT FROM AUTHOR]

The Fuchuan ophiolite is located in the northeasternmost segment of the Neoproterozoic Jiangnan orogen and consists mainly of harzburgites, with minor dunites, pyroxenite and gabbro veins and dykes. In order to investigate the genesis and tectonic setting of the Fuchuan ophiolite and chromitites, in situ analyses of unaltered chromites and silicates were carried out. Trace element analyses of unaltered chromites from the Fuchuan chromitites indicate the parental magma is of mid‐ocean ridge basalt (MORB)‐like origin, with the Ti/Fe3+#–Ga/Fe3+# diagram of chromites showing that the chromitites are a result of melt/rock interaction of MORB melts with mantle peridotites, and that the Fuchuan harzburgites present the dual features of MORB and supra‐subduction zone peridotites (SSZP). Trace and rare earth element (REE) analyses of olivines and orthopyroxenes from the Fuchuan harzburgites hint at the possibility of mantle metasomatism influenced by SSZ‐subducted fluids. Finally, integrating with previous study, the Fuchuan ophiolite and chromitites might have been formed in a back‐arc spreading ridge between the Yangtze and Cathaysia blocks during the Neoproterozoic. [ABSTRACT FROM AUTHOR]

Located on the Hellenic Arc, the Christiana‐Santorini‐Kolumbo (CSK) rift zone represents one of the most active and hazardous volcano‐tectonic systems in the Mediterranean. Although this rift zone has been intensively studied, its tectonic evolution and the interplay of volcanism and tectonism are still poorly understood. In this study, we use high‐resolution reflection seismic imagery to reconstruct the opening of the rift basins. For the first time, we relate the activity of individual faults with the activity of specific volcanic centers in space and time. Our analysis shows a pre‐volcanic NNE‐SSW‐oriented paleo basin underneath the CSK volcanoes, representing a transfer zone between Pliocene ESE‐WNW‐oriented basins, which was overprinted by a NE‐SW‐oriented tectonic regime hosting Late Pliocene volcanism that initiated at the Christiana Volcano. All subsequent volcanoes evolved parallel to this trend. Two major Pleistocene tectonic pulses preceded fundamental changes in the volcanism of the CSK rift including the occurrence of widespread small‐scale volcanic centers followed by focusing of activity at Santorini with increasing explosivity. The observed correlation between changes in the tectonic system and the magmatism of the CSK volcanoes suggests a deep‐seated tectonic control of the volcanic plumbing system. In turn, our analysis reveals the absence of large‐scale faults in basin segments affected by volcanism indicating a secondary feedback mechanism on the tectonic system. A comparison with the evolution of the neighboring Kos‐Nisyros‐Yali volcanic field zone and Rhodos highlights concurrent regional volcano‐tectonic changes, suggesting a potential arc‐wide scale of the observed volcano‐tectonic interplay. Plain Language Summary: How do regional tectonic movements and large volcanoes interact? Seismological studies indicate complex volcano‐tectonic feedback links, but, so far, information on the long‐term interactions between tectonics and volcanism is rarely available. The Christiana‐Santorini‐Kolumbo (CSK) rift zone lies in the Aegean Sea and is notorious for its devastating volcanic eruptions, earthquakes, and tsunamis. This region offers the opportunity to study volcano‐tectonic interactions over several million years. In this study, we use high‐resolution seismic imagery to reconstruct the evolution of the rift basins and the CSK volcanoes. We find that all volcanoes lie in a Pliocene transfer zone connecting extensional basins. Volcanism initiated as this older tectonic regime was intersected by a NE‐SW‐directed fault system. Subsequently, all volcanoes evolved parallel to this trend. Several distinct tectonic reorganizations occurred in the Pleistocene, which had a pronounced influence on the CSK volcanoes. In turn, our analysis indicates that the emergence of volcanism also impacted the tectonic evolution of the rift system hindering the evolution of large‐scale normal faults in the volcanic basins. The observed tectonic reorganizations seem to reflect major changes in the stress regime of the Hellenic Arc, potentially also affecting adjacent volcanic centers whose volcano‐tectonic evolution is only poorly constrained so far. Key Points: We reconstruct the volcano‐tectonic evolution of the Christiana‐Santorini‐Kolumbo rift zone using multichannel seismic dataThe overprint of a Pleistocene NE‐SW striking fault system on a Pliocene E‐W oriented system initiated the emergence of volcanismRegional tectonics had a primary control on the volcanic plumbing system, while magmatism had a secondary influence on the tectonic system [ABSTRACT FROM AUTHOR]

The continents across Earth have grown through accretion and amalgamation of plume-generated oceanic plateaux and subduction-originated magmatic arcs. The evidence of such evolutionary processes is well preserved in the greenstone belts of Canada, South Africa, China, Australia and India. Unlike accretion that leads to lateral growth of the continents, flood basalt volcanism results in the superposition of large volumes of dense mafic material above relatively less dense sialic upper continental crust. The resulting density inversion is liable to generate tectonic stresses resulting in faulting of under burden along weak planes. The nature of the crust obscured beneath flood basalt provinces that have accumulated over the past 500 My in Large Igneous Provinces (LIPs) has remained enigmatic, largely because of the voluminous lava pile that deters access to the concealed basement, unless otherwise retrieved through deep continental drilling. Here, we present in situ zircon U Pb geochronology and bulk-rock geochemistry of a volcanic sequence discovered at −400 m m.s.l in the deep bore-hole well-core KBH-5 of the southwestern Deccan Traps, India. The ∼300 m thick metavolcanic succession includes a basalt – Nb-enriched basalt – andesite – Mg-andesite – dacite – adakite suite. The zircons yielded a 2.58 Ga U-Pb age for the suite, which also incorporates entrained zircon xenocrysts of 2.7 Ga age from a preexisting juvenile material during its emplacement. The bore-hole KBH-5 is located on an NNW-SSE trending normal fault, paralleling a series of major faults west of the Western Ghat Escarpment. Although the crustal block plummeted ∼1000 m downward along the fault plane, the geochemical attributes of the metavolcanic rocks are thoroughly comparable to those reported in the Medur Formation of the Shimoga greenstone belt, ∼300 km south of KBH-5 in the western Dharwar craton. Our study provides the first physical evidence of Dharwar greenstone belt(s) extending further beneath the Deccan Traps, which was previously only speculative based on short-wavelength gravity highs. The metavolcanic sequence evolved in a subduction-related back-arc tectonic regime and accreted to the active continental margin of India in the Neoarchean. Unveiling of potentially similar scenarios in cratons elsewhere entrapped beneath the vast canopy of continental flood basalts, would reveal new crustal growth events during the Archean -Proterozoic transition across the Earth. • We report the discovery of Neoarchean arc-volcanics beneath the Deccan Traps. • Basalt – Nb-enriched basalt – Mg-andesite – Adakite – Dacite suite of rocks. • 2.58 Ga contributes to peak of crustal growth activity in the Dharwar Craton. • Shimoga greenstone belt extends ∼300 km further north beneath the Deccan Traps. • It is a subduction accreted orogen at the paleo active continental margin of India. [ABSTRACT FROM AUTHOR]

This research presents the first whole-rock Pb isotope data for Tonian rocks from the Brazilian sector of the Punta del Este Terrane (amphibolites, magnesian schists and quartz-syenitic leucosomes of the Arroio Grande Ophiolite, and amphibolite and metagabbro xenoliths within granitoids), Dom Feliciano Belt. These rocks represent metamorphic remnants of the South Adamastor paleo-ocean. The new Pb isotope data suggest that these rocks are derived from a crustally-contaminated, depleted MORB mantle source, corroborating previously published hypotheses based on elemental geochemistry and Sr–Nd isotope data. The comparison with their Uruguayan (amphibolites of the Paso del Dragón Complex, Punta del Este Terrane) and Namibian (metagabbros and metabasalts of the Chameis Subterrane, Marmora Terrane) counterparts suggests that these rocks represent: (i) distinct regions (e.g. mid-ocean ridge, seamounts, trench) of the South Adamastor back-arc paleo-oceanic lithosphere, which were juxtaposed together during the closure of the basin; (ii) and/or, along the back-arc ridge, the magmatism was chemically heterogeneous. [Display omitted] • First Pb isotopic study on the Brazilian Punta del Este Terrane's oceanic rocks. • Pb Isotope data reveal crustal-contaminated, Depleted MORB Mantle Source. • Heterogeneous magmatism in the South Adamastor paleo-oceanic lithosphere. [ABSTRACT FROM AUTHOR]

Arc volcanism is a key process influencing Earth’s climate, continental growth, and the formation of mineral deposits. Therefore in this contribution, we have compiled whole-rock geochemistry of frontal arc and rear-/back-arc basalts, crustal thickness, and slab parameters (e.g., slab age, slab dip, and convergence velocity) from global convergent margins to investigate the factors controlling along-arc and across-arc variations. Crustal thickness or lithosphere thickness plays a dominant role in elements immobile in aqueous fluids (e.g., Zr/Yb and Nb/Yb). The effect is imposed through mantle partial melting for both frontal arc and rear-/back-arc rocks. Slab thermal structure also affects these immobile elements and gives rise to along-arc variations. Both slab sediment and altered oceanic crust can melt especially in hot subduction zones which yield across-arc variations (e.g., Nd isotope). Aqueous fluids (represented by element ratios such as Ba/Nb and Sr/Nd) also show across-arc variations as they decrease toward rear-/back-arc. This meanwhile decreases mantle wedge melting as rear-/back-arcs show higher Zr/Yb and Nb/Yb. However, no correlations between aqueous fluids and slab parameters suggest aqueous fluids in arc rocks are controlled by complex processes. We summarize factors such as slab alteration, slab dehydration, and mantle metasomatism might impose an effect on the content of fluid mobile elements in arc rocks.

We present here the first regional set of Sr-Nd-Pb isotopic compositions, major and trace element compositions and K Ar ages for a representative suite of back-arc samples of the Sredinny Range (SR) of Kamchatka. Together with previously published analyses, this unique dataset allowed us to trace the source heterogeneity and invoke new mechanisms to explain the variably enriched geochemical signatures. Our results indicate that the Sr isotopic ratios and the LILE content of the studied rocks were mainly influenced by the subduction fluid. Neodymium isotopes, as well as the HFSE and REE distributions in the rocks, require a more complex explanation, depending on the geographical location of the studied samples and the age of their eruption. Melts representing Miocene rocks of the northern part of the SR and most of the rocks from the eastern and central flanks of the SR (from Miocene until present) were produced from a depleted MORB-like metasomatized mantle, and were then exposed to varying degrees of crustal assimilation and fractional crystallization. Higher HFSE contents and lower 143Nd/144Nd ratios in the western flank lavas, as well as high HFSE contents in the Quaternary lavas of the northern part of the SR and some of the lavas from the eastern flank of the southern part of the SR, require an alternative source for their enrichment. Delamination of the lithosphere explains the unusual, enriched signature of these rocks, whereas their variably enriched neodymium isotope ratios identify the various ages of the separation of the lithosphere from the mantle. The Nd isotopic composition of the rocks together with their HFSE content, therefore, serves as an unusual tracer for the enriched mantle domain, showing the presence of the older lithospheric blocks and indicating the timeframes for this source involvement in magma generation. • The first regional-scale compositional dataset for rocks from the Kamchatka back-arc. • 143Nd/144Nd and HFSE indicate the presence of enriched mantle under the region. • Delamination of the metasomatized lithosphere explains the enrichment. • 143Nd/144Nd is the best indicator of the variable age of the delaminating lithosphere. [ABSTRACT FROM AUTHOR]

• A new record of 560 Ma banded iron formation in southwestern Asia. • BIF in association with volcanic rocks and glaciogenic metasediments suggest active-magmatic basin at ca. ∼560 Ma. • All evidence point to the suboxic conditions in Ediacaran seawater during deposition of iron formation. The late Neoproterozoic Takab banded iron formation (BIF) occurs in one of the most important part of the Iranian plateau containing Precambrian crustal segments, now embedded in the Alpine-Himalayan orogenic system. The Takab BIF is hosted by low to medium grade metamorphic sequence including schist, quartzite and marble and its structure is dominated by bands or streaks. The ore body is mostly composed of quartz and iron oxides (magnetite, hematite, maghemite, goethite in different proportions), occurring as alternating iron- and silica-rich laminates. LA-ICP-MS U–Pb dating on zircons in schists from both the lower and upper layers of the orebody indicate a depositional age of ca. ∼560 Ma for iron deposits. Major, trace and rare earth elements of the ore body support the mixing of seawater and hydrothermal fluids through iron precipitation and up to ca. 20 % incorporation of terrigenous materials into the chemical precipitate of the original BIF. The provenance indicators as well Nd isotopic data, point towards the Neoproterozoic felsic-to-intermediate crystalline basement of the Takab area as the main sediment source. Fe isotope data together with Ce anomalies suggest that suboxic conditions prevailed in Ediacaran seawater at the time of deposition of the ore body. Combined with geochronological and geochemical data from other Neoproterozoic formations and crystalline rocks on the Iranian plateau, as well as supportive evidence for the occurrence of glacial sediments and bimodal volcanism in the studied area, we propose that the formation of the Takab BIF occurred in a back-arc basin environment, hence corresponding to a Rapitan-type iron ore. The co-occurrence of late Ediacaran BIFs and glaciogenic metasedimentary formations suggest the presence of active-magmatic marine anoxic basins at ca. ∼560 Ma, equivalent to the Ediacaran post-Gaskiers glaciation time. [ABSTRACT FROM AUTHOR]

• The Sargur and Bababudan Groups are characterized as distinct intracratonic rifts. • The Chitradurga and Hiriyur Groups are different stages of back-arc rifting. • Most of the rifting episodes are failed rifts. • Structural relationships show back-arc fold-and-thrust belt in the CSB. • Present structural architecture of the WDC is of a hinterland-fold-and-thrust belt. The Chitradurga Schist Belt in the Western Dharwar Craton provides a unique opportunity to investigate regional-scale tectonic processes not only in the Dharwar Craton but also in the Meso to Neoarchean periods. This study focuses on analyzing the field and structural relationships among different geological units in the Chitradurga Schist Belt, combined with zircon U–Pb geochronological and whole-rock geochemical analysis. Geological investigations reveal the presence of five main units in the study area: the Basement Gneiss, Sargur, Bababudan, Chitradurga, and Hiriyur Groups. Unconformities or fault contacts separate each group. Our findings on stratigraphic relations, geochronology, and geochemistry indicate that the Sargur and Bababudan Groups originated from distinct rifting events associated with mantle plumes. Zircon U–Pb dating suggests the initiation of Sargur Group rifting around 3300 Ma, while the Bababudan basin rifting began < 3000 Ma and ended ∼2800 Ma due to convergence associated with westward subduction in the eastern region. Back-arc rifting occurred during convergence and subduction, leading to the formation of the Chitradurga and Hiriyur Groups around 2700 Ma. The Chitradurga basin rifting commenced at approximately 2600 Ma, evidenced by the age of felsic volcanic rocks, while the Hiriyur basin rifting began around 2560 Ma, based on the youngest cluster age in the conglomerate. Geochemical analysis of metavolcanic rocks from different groups supports plume-related rifting in the Bababudan and Sargur basins, and subduction-related back-arc rifting in the Chitradurga and Hiriyur basin. Despite the rifting events, none resulted in the formation of an oceanic plate; all appear to be failed rifts. D 0 /D 1 events represents sedimentary structures. The D 2 structures (NNW-SSE upright fold and west-verging reverse shear zone) indicate the formation of a back-arc fold-thrust belt accompanied with a shallowing subduction angle. The subsequent D 3 structures, characterized by strike-slip sinistral shear, likely associated with collision and the transformation of the back-arc fold-thrust belt into a hinterland thrust belt, accompanied by probable transpression. Consequently, the present structural architecture represents a hinterland fold-thrust belt. [ABSTRACT FROM AUTHOR]

New field and petrographic observations, whole-rock major- and trace-element geochemical data, Nd and Sr isotope systematics and U-Pb SHRIMP zircon ages of the Peltetec metaophiolite and related rocks (Ecuador) allow the characterization of the igneous and sedimentary protoliths and the evaluation of the geodynamic evolution of the active western margin of Gondwana during the Mesozoic. The Peltetec ophiolitic belt, exposed along the western margin of the Cordillera Real (Ecuador) and previously considered of Cretaceous age, comprises slices of metamorphosed (greenschist facies) and variably deformed peridotites, gabbros (locally cumulate), basaltic lavas and dykes, hydrothermal breccias and sedimentary rocks. The geochemical characteristics of the tholeiitic basaltic rocks indicate a Sr-Nd isotope depleted mantle source with variable imprint of subduction zone-derived or continental crust-assimilated components, as indicated by enrichment in Th, negative anomaly of Nb and relative depletion in high field strength elements (HFSE). Geochemical diversity allows defining different groups of noncumulate igneous rocks with oceanic island arc and NMORB-like back-arc affinities. U-Pb zircon ages from a metabasite yielded 228.1 ± 1.7 Ma, interpreted as the crystallization age of the magma. These data allow proposing the generation of the Peltetec ophiolite in a back-arc setting as a consequence of late Triassic extension of the western continental margin of Gondwana. Granites located at the western margin of the Peltetec Unit with S-type affinity and equal Triassic age (U-Pb age of 228.2 ± 0.8 Ma) document coeval anatexis of the continental crust as a consequence of heating associated with the same extensional event that originated the ophiolite. Geologic correlation allows enlarging such tectonic scenario to include ophiolitic units of similar age in Colombia. [ABSTRACT FROM AUTHOR]

The transition from subduction to transform motion along horizontal terminations of trenches is associated with tearing of the subducting slab and strike-slip tectonics in the overriding plate. One prominent example is the northern Tonga subduction zone, where abundant strike-slip faulting in the NE Lau back-arc basin is associated with transform motion along the northern plate boundary and asymmetric slab rollback. Here, we address the fundamental question: how does this subduction-transform motion influence the structural and magmatic evolution of the back-arc region? To answer this, we undertake the first comprehensive study of the geology and geodynamics of this region through analyses of morphotectonics (remote-predictive geologic mapping) and fault kinematics interpreted from ship-based multibeam bathymetry and Centroid-Moment Tensor data. Our results highlight two notable features of the NE Lau Basin: 1) the occurrence of widely distributed off-axis volcanism, in contrast to typical ridge-centered back-arc volcanism, and 2) fault kinematics dominated by shallow-crustal strike slip-faulting (rather than normal faulting) extending over ∼120 km from the transform boundary. The orientations of these strike-slip faults are consistent with reactivation of earlier-formed normal faults in a sinistral megashear zone. Notably, two distinct sets of Riedel megashears are identified, indicating a recent counter-clockwise rotation of part of the stress field in the back-arc region closest to the arc. Importantly, the Riedel structures identified in this study directly control the development of complex volcanic-compositional provinces, which are characterized by variably-oriented spreading centers, off-axis volcanic ridges, extensive lava flows, and point-source rear-arc volcanoes. This study adds to our understanding of the geologic and structural evolution of modern backarc systems, including the association between subduction-transform motions and the siting and style of seafloor volcanism.

The Cascades back-arc in northern California is dominated by monogenetic tholeiitic basalts that erupted throughout the Pleistocene. Elucidating their eruptive history and processes is important for understanding potential future eruptions here. We focus on the well-exposed monogenetic volcano that emplaced the Brushy Butte flow field, which constructed a ∼150 m tall edifice, has flow lobes up to >10 km long, and in total covers ∼150 km2 with an eruptive volume of 3.5 km3. We use a multidisciplinary approach of field mapping, petrography, geochemistry, paleomagnetism, geochronology, and lidar imagery to unravel the eruptive history and processes that emplaced this flow field. Tholeiitic basalts in northern California have diverse surface morphology and vegetation cover but similar petrographic appearances, which makes them hard to distinguish in the field. Geochemistry and paleomagnetism offer an independent means of distinguishing tholeiitic basalts. Brushy Butte flow field lavas are similar in major-oxide and trace-element abundances but differ from adjacent tholeiitic basalts. This is also apparent in remanent magnetic directions. Additionally, paleomagnetism indicates that the flow field was emplaced during a geologically brief time interval (10–20 years), which 36Cl cosmogenic dating puts at 35.7 ± 1.7 ka. Lidar imagery shows that these flows erupted from at least 28 vents encompassing multiple scoria cones, spatter cones, and craters. Flows can be grouped into four pulses using stratigraphic position and volume. Pulse 1 is the most voluminous, comprising eight eruptions and ∼2.3 km3. Each subsequent pulse started rapidly but decayed quickly, and each successive pulse erupted less lava (i.e., 2.3 km3 for pulse 1, 0.6 km3 for pulse 2, 0.3 km3 for pulse 3, and 0.2 km3 for pulse 4). Many of these flows host well-established lava channels and levees (with channel breakouts) that lead to lava fans, with some flows hosting lava ponds. Similar flow features from tholeiitic eruptions elsewhere demonstrate that these morphologies generally occur over weeks, months, or longer (e.g., Puʻu ʻŌʻō eruption at K–llauea, Hawaiʻi). This multidisciplinary study shows the range of eruptive styles and durations of a Cascades back-arc eruption and illustrates how potential future tholeiitic eruptive activity in the western United States might progress.

Variations in the stable isotopic composition of seawater Sr (δ88Sr) is a new tool for estimating the rates of global carbonate sedimentation over geologic time, yet the isotope compositions of the major sources and sinks of Sr to the world oceans are still in need of further constraint. We report δ88Sr values of vent fluids from arc/back-arc seafloor hydrothermal systems in the western Pacific. In the sediment-starved hydrothermal fields of the Manus Basin, Izu-Bonin Arc, and Mariana Trough, the δ88Sr values of end-member fluids for each site showed little variation (0.29–0.30‰) and were close to the average value of oceanic volcanic rocks, reflecting dissolved Sr sourced from host rocks. Chlorine-depleted fluids from phase-separated hydrothermal systems in the North Fiji Basin had the end-member δ88Sr values of 0.26, 0.28, and 0.29‰. Thus, both sediment-starved and phase-separated vent fluids had the end-member δ88Sr values indistinguishable from or very close to the range of oceanic volcanic rocks. Therefore, the δ88Sr compositions in these hydrothermal sites are controlled predominantly by Sr sourced from host rock with a small influence from secondary mineral precipitation/re-dissolution. Fluids from the sediment-hosted hydrothermal fields of the Okinawa Trough, however, were characterized by low δ88Sr values of approximately 0.22‰ and high 87Sr/86Sr ratios, indicating interactions with sedimentary carbonates. As for the modern oceanic δ88Sr budget, the sediment-hosted sites lower the global hydrothermal δ88Sr. Since both sediment-starved and -hosted hydrothermal systems provide a long-term control on the global Sr cycle, the end-member δ88Sr value is an important constraint on the evolution of Sr cycling in past oceans.

Recent studies suggest the existence of two ophiolites in Palawan Island: the Central Palawan Ophiolite (CPO) and the Southern Palawan Ophiolite (SPO). New geochemical data in this study from the volcanic rocks of the Palawan ophiolites further support this interpretation. The CPO lavas have back-arc basin basalt (BABB) affinities with LILE enrichment and depleted LREE. In contrast, volcanic rocks of the SPO are geochemically diverse, including low-TiO2 boninitic andesites, MORB-like lavas, and high TiO2, high Nb lavas. Sr, Nd and Pb isotopic ratios of the CPO and SPO samples suggests that a mantle source was enriched due to the interaction of depleted MORB mantle with EMI component and ancient Mariana Trench-like sediment. Utilizing available paleomagnetic data and regional plate reconstruction models, we propose that the CPO represents the Late Eocene back-arc basin fragments formed within the proto-South China Sea. The SPO has a subduction initiation origin and formed in the Sundaland-Eurasian margin in Early Cretaceous. Both ophiolites were emplaced on the Palawan Continental Terrane (PCT) due to the collision of the PCT with Borneo and Cagayan Ridge.

The bathymetry, petrology, marine magnetic, and seismic-SBP data have identified the northwest-southeast direction submarine ridge that shows hydrothermal activity. This activity occurred through Mount Baruna Komba, Abang Komba, and Ibu Komba. The volcanic rocks are andesite basaltic lava flows, tuff, and pumice. The andesite basaltic lava shows porphyritic, intergranular, intersertal to glomeroporphyritic textures. The rock composes anhedral minerals of k-feldspar, plagioclase, and pyroxene. These minerals present in small-sized, short prismatic dispersed in very fine groundmass minerals or glasses. Most of the volcanic rocks have experienced various degrees of alteration. The k-feldspar and plagioclase are most dominantly transformed into sericite, clay mineral, carbonate, epidote and oxide mineral, opaque mineral, and secondary plagioclase through the albitization process, while pyroxene replaced by chlorite. Other minerals are biotite and quartz, and base metals are present Cu, Zn, Ag, As, Pb, and gold. Mineralization categorizes as the phyllic zone, sub-prophylithic zone, and phyllic-potassic zone that formed at a temperature range of 250-400oC. The submarine hydrothermal alteration in the Komba Ridge is associated with a volcanogenic sulphide deposit controlled by crust thinning due to the crust rifts in the back-arc tectonic setting.

Volcaniclastic and coherent volcanic rocks of the Permian island arc Brook Street Terrane crop out along the Southland coast, New Zealand, near Riverton. Extensive along-strike exposures reveal lateral variations in these submarine deposits and provide a rare opportunity to examine deposits of an arc environment within a thin slice of time. We describe pillow lavas, dikes, hydroclastic breccias, tuffs, lapilli tuffs and argillites, all hydrothermally altered and metamorphosed to prehnite-pumpellyite facies. Trace element geochemical data indicate multiple primitive island arc to MORB-like magma sources. Two separate volcanic centres apparently tapped different mantle domains in a source region transitional between the arc and back-arc. The centres' proximity and inferred small sizes suggest they were satellite vents of a larger arc volcano. Tuff and lapilli tuff were formed as hyaloclastite during eruption of pillow lava, and redeposited by subaqueous mass flows. Peperitic contacts indicate that the lavas intruded unconsolidated volcaniclastic deposits. We infer that hydroclastic fragmentation associated with pillow emplacement produced debris that was then intruded by later pillows, with local peperite formation, as eruption persisted. The hydroclastic debris from these satellite vents was resedimented in turbidite fans with channels, and periodic turbidity currents delivered fine-grained material from the volcano's main vent. [ABSTRACT FROM AUTHOR]

The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al 2 O 3 : 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44–0.81) and low to intermediate Mg# (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite. Unlabelled Image • Magmatic age of the Bou Azzer ophiolite (Moroccan Anti-Atlas) is 759 ± 2 Ma. • Sirwa and Bou Azzer ophiolites originate from a same intra-oceanic back-arc system. • Both ophiolites recorded multiple arc-back-arc evolution stages. [ABSTRACT FROM AUTHOR]

Active and remnant back-arc regions do not follow a typical conductive lithosphere cooling model, but instead have an apparent two-stage cooling, defined by a high heat flow back-arc region during subduction and a second post-subduction heating event that extends elevated heat flow for several 10s million years. Numerical one-stage cooling models have not reproduced observed heat flow anomalies in active subduction zones using physically realistic parameters and require a secondary post-subduction heating mechanism. Here, an extension driven-volcanism model is developed to examine extension driven heating and volcanism as a mechanism to produce a prolonged thermal anomaly within back-arc lithosphere. This model is tested using the recorded thermal evolution of the Northern Cordillera Volcanic Province (NCVP), a Neogene-Quaternary alkaline volcanic province located in the remnant back-arc region of the Pacific-North American subduction zone in British Columbia, Canada. A single steady-state lithosphere geotherm does not intersect all previously published temperature estimates, suggesting previous data record the thermal evolution of the NCVP. Calculated geotherms at equilibrium with the minimum and maximum MELTS temperatures predict an increase in reduced mantle heat flow (Qm) from 43 to 72 mW/m2 and lithosphere thinning from a depth of 87 to 48 km. The newly developed extension-volcanism model reproduced the calculated pre- and post-volcanism thermal regimes for the NCVP and supports that extension within the remnant back-arc could produce the present heat flow anomaly and volcanism. The model most readily produces volcanism when Qm is ~45–65 mW/m2, a typical range for back-arcs. Back-arc regions are prime locations for limited volcanism because their warmer thermal regime minimizes tectonic stress requirements to produce volcanism. Additionally, two-stage cooling of back-arcs can be explained with a time-dependent extension-volcanism thermal feedback mechanism that is possible because of the subduction driven pre-heating of back-arc regions. [ABSTRACT FROM AUTHOR]

The Manus Basin hosts a broad range of vent fluid compositions typical for arc and back‐arc settings, ranging from black smoker to acid‐sulfate styles of fluid venting, as well as novel intermediate temperature and composition "hybrid" smokers. We investigated B, Li, Mg, and Sr concentrations and isotopic compositions of these different fluid types as well as of fresh and altered rocks from the same study area to understand what controls their compositional variability. In particular, the formation of acid‐sulfate and hybrid smoker fluids is still poorly understood, and their high Mg concentrations are explained either by dissolution of Mg‐bearing minerals in the basement or by mixing between unmodified seawater and magmatic fluids. Mg isotope ratios of the acid‐sulfate fluids from the Manus Basin are seawater‐like, which supports the idea that acid‐sulfate fluids in this study area predominantly form by mixing between unmodified seawater and a Mg‐free magmatic fluid. Changes in the B, Li, and Sr isotope ratios relative to seawater indicate water‐rock interaction in all acid‐sulfate fluids. Further, the combination of δ7Li with B concentrations of the same fluids links changes in δ7Li to changes in (1) basement alteration, (2) water‐to‐rock ratios during water‐rock interaction, and/or (3) the reaction temperature. These isotope systems, thus, allow tracing of basement composition and acid‐sulfate‐driven alteration of the back‐arc crust and help increase our understanding of hydrothermal fluid‐rock interactions and the behavior of fluid‐mobile elements. Key Points: Mg in acid‐sulfate fluids is seawater derivedLi isotopes in acid‐sulfate fluids trace basement alteration [ABSTRACT FROM AUTHOR]

DOI: 10.17014/ijog.4.3.169-179A shear-wave splitting analysis was determined to observe anisotropic structures of an upper mantle layer beneath Sumatra. The data were collected from 35 BMKG stations with the magnitude of more than 6.25 Mw and the epicentre of 85o - 140o. A shear-wave splitting measurement was calculated by using Splitlab based on three methods simultaneously. The result of the shear-wave splitting measurement in the Sumatra Forearc and Fault Zone shows that there are two anisotropic layers. The first layer has a dominant-fast-polarization direction that is parallel with a trench, and has the delay time duration of 0.5 - 0.9 s-. It is presumed that it is caused by a shear-strain as a result of the existences of Mentawai and Sumatra Fault Zones. The second layer has a dominant-fast-polarization direction that is perpendicular to the trench with the delay time duration of about 1.1 - 1.9 s-. It is presumed that it is caused by a movement of a subduction plate on a mantle wedge. The measurement in the backarc shows that there is only one anisotropic layer that is a subduction plate. It is also found that there is a transition of an orientation change on the subduction plate between Sumatra and Java. The change of the polarization direction is probably related to the age difference and the direction velocity of the absolute plate movement (APM) from Sumatra to Java.

Volcanic rocks in the Xiemisitai Mountains, in Northwest Junggar were recently considered to form in the Silurian rather than the Devonian period. However, the characteristic and tectonic setting of Middle Silurian volcanic rocks in this area are poorly constrained. In this study, major and trace elements and Sr-Nd-Os isotopic composition analyses were synthetically conducted on the Middle Silurian basalt and basaltic andesite in northwest of the Xiemisitai Mountains, West Junggar. These volcanic samples are characterized by high MgO (5.15%-7.81%) with Mg of 56.0-60.7, low CaO (4.56%-5.09%) and FeOT/MgO (1.15-1.40), similar to those of high-Mg volcanic rocks. Basalts have εNd(t) of 2.65, (87Sr/86Sr)i of 0.70439, and (187Os/188Os)i of 0.4901. The slightly enriched isotopic signatures, high Th/Yb (2.2-2.7) and low Ba/La (17.4-33.4) indicate that they originated from depleted mantle metasomatized by melts from subducted sediments and suffered from crustal contamination. The volcanic rocks are calc-alkaline and exhibit arc-like geochemistry with the enrichment of light REE and large ion lithophile elements. It is suggested that these rocks were probably formed in a back-arc setting due to northward subduction of the Junggar Oceanic slab.

Mantle flow is a key feature that can affect melt production and composition in and around subduction zones. In this study, I use three-dimensional numerical models to investigate the role of mantle flow on subduction-related volcanism during back-arc extension. Results show that for a time interval of about 10-15 Myr during back-arc basin formation and spreading a wide convection cell brings mantle that has already partially depleted at the back-arc to the mantle wedge. Before and after this phase, the mantle reaching the sub-arc melting region is fertile. Thus, changes in back-arc activity and mantle flow pattern can be responsible for changes in magmatic composition and the amount of magmatism at the arc. These results are consistent with many examples of present-day subduction zones, in which phases of actively spreading back-arc correspond to a gap or a decrease of the arc volcanic activity. • Numerical models show that mantle flow affects melt production in subduction zones. • Mantle flow brings depleted mantle from the back-arc to the sub-arc melting region. • Active back-arc spreading can result in a phase of less or no activity at the arc. [ABSTRACT FROM AUTHOR]

The Okinawa Trough (OT) is a back‐arc, initial marginal sea basin that is located behind the Ryukyu arc–trench system. Pumice, including both white and black pumice, is the most widely distributed magmatic rock in the OT. Here, the major and trace element concentrations and Sr–Nd–Pb isotopic compositions of the white and black pumice from the middle OT (MOT) are presented and combined with previously published elemental data for volcanic rocks from the MOT. Their different magma sources and magmatic evolutions are discussed, and their different magma chamber structures are provided in this paper. These results demonstrate that the white pumice has two magma sources: One was produced by the crystallization‐based differentiation of basaltic magma, and the other was produced by the assimilation‐fractional crystallization (AFC; with less than 5% contamination) of basaltic magma. The black pumice also includes two magma sources: One is similar to that of the first white pumice, whereas the other was produced by basaltic magma that experienced more than 20% upper crustal contamination. There are two different structure magma chambers for white and black pumice in the MOT crust: The deeper one generated no contaminated pumice, whereas the shallow one generated contaminated black pumice. The contamination of the white pumice (<5%) may have occurred during the eruption of magma on the seafloor, rather than in the magma chamber. Different magma sources and evolutions of white and black pumice imply the complexity of magmatism in the Okinawa Trough. [ABSTRACT FROM AUTHOR]

Abstract Based on the geochemical characteristics of the Miocene-Quaternary volcanic rocks of the Sredinny Range of Kamchatka, we divide it into northern and southern provinces; the latter comprises the "eastern", "western", and "central" flanks. We present new data on the composition of Neogene-Quaternary volcanic rocks in the southern part of the Sredinny Range of Kamchatka: Khangar and Icha volcanic massifs and Mt. Yurtinaya on the "western" flank, Bystrinsky and Kozyrevsky Ridges on the "eastern" flank, and Anaunsky Dol and Uksichan massif located in between. We show systematic differences in the composition of rocks from the "western" and "eastern" flanks. During the Neogene, a typical island-arc volcanism took place within the "eastern" flank. Quaternary volcanic rocks of this area have both island-arc and within-plate geochemical features. We propose to call rocks of this type hybrid rocks. Within the "western" flank, hybrid volcanism has been manifested since the Neogene, while typical island-arc rocks are not found. Magma generation processes on the "western" flank of the Sredinny Ridge are influenced by an enriched mantle source; the effect of fluid is less pronounced here as compared to the rocks of the "eastern" flank, where it is clearly traced. [ABSTRACT FROM AUTHOR]

Slab tearing induces localized deformations in the overriding plates of subduction zones and transfer zones accommodating differential retreat in back-arc regions. Because the space available for retreating slabs is limited in the Mediterranean realm, slab tearing during retreat has been a major ingredient of the evolution of this region since the end of the Eocene. The association of detailed seismic tomographic models and extensive field observations makes the Mediterranean an ideal natural laboratory to study these transfer zones. We review in this paper the various structures in back-arc regions differential retreat from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models to better understand the partitioning of deformation between high-angle and low-angle faults, as well as the 3-D kinematics of deformation in the middle and lower crusts. Simple, archetypal, crustal-scale strike-slip faults are in fact rare in these contexts above slab tears. Transfer zones are in general instead wide deformation zones, from several tens to several hundred kilometers. A partitioning of deformation is observed between the upper and the lower crust with low-angle extensional shear zones at depth and complex association of transtensional basins at the surface. In the Western Mediterranean, between the Gulf of Lion and the Valencia basin, transtensional strike-slip faults are associated with syn-rift basins and lower crustal domes elongated in the direction of retreat (a-type domes), associated with massive magmatic intrusions in the lower crust and volcanism at the surface. On the northern side of the Alboran Sea, wide E-W trending strike-slip zones in the brittle field show partitioned thrusting and strike-slip faulting in the external zones of the Betics, and E-W trending metamorphic core complexes in the internal zones, parallel to the main retreat direction with a transition in time from ductile to brittle deformation. On the opposite, the southern margin of the Alboran Sea shows short en-échelon strike-slip faults. Deep structures are not known there. In the Aegean-Anatolian region, two main tear faults with different degrees of maturity are observed. Western Anatolia (Menderes Massif) and the Eastern Aegean Sea evolved above a major left-lateral tear in the Hellenic slab. In the crust, the differential retreat was accommodated mostly by low-angle shear zones with a constant direction of stretching and the formation of a-type high-temperature domes exhumed from the middle and lower crust. These low-angle shear zones evolve through time from ductile to brittle. On the opposite side of the Aegean region, the Corinth and Volos Rift as well as the Kephalonia fault offshore, accommodate the formation of a dextral tear fault. Here, only the brittle crust can be observed, but seismological data suggest low-angle shear zones at depth below the rifts. We discuss the rare occurrence of pure strike-slip faults in these contexts and propose that the high heat flow above the retreating slabs and more especially above slab tears favors a ductile behavior with distributed deformation of the crust and the formation of low-angle shear zones and high-temperature domes. While retreat proceeds, aided by tears, true strike-slip fault system may localize and propagate toward the retreating trench, ultimately leading to the formation of new plate boundary, as shown by the example of the North Anatolian Fault.

Identifcation of subduction to back-arc tectonic transitions is critical to the study of accretionary orogens. Some new geochronological and geochemical data for the Late Devonian (380–361 Ma) Kezier mafic–felsic complex and Tuwu granitic pluton in this study have well recorded such transition feature in the Chinese Eastern Tianshan orogen. The Kezier gabbro is classified as high-Al basalt given their high Al contents (A1 2 O 3 > 17 wt%). They were originated from a depleted mantle fluxed by fertile mantle components at shallow depths under hydrous condition. Both the Kezier and Tuwu granites show geochemical characteristics of high total alkali, Ga/Al, HFSE, low CaO, and strongly negative Ba, Sr, and Eu anomalies, demonstrating a close affinity with A-type granites. The granites were probably generated by high temperature melting (zircon saturation temperature up to 916 °C) of crustal rocks under a low-pressure (<0.8–1.0 GPa) condition. These rocks can help identify the Late Devonian back-arc extensional event in the Eastern Tianshan. This extensional event contemporaneous with the onset of the earliest arc magmatic records southward rejuvenating. The back-arc extensional with crustal high-temperature melting event can be reconciled with the Kangguer Ocean rollback, which forced the arc rifting occur on the Dananhu fore-arc side during the Late Devonian. • Late Devonian gabbros and granites were identified in the Chinese Eastern Tianshan; • The high-Al gabbros were formed by hydrous pressure-release partial melting; • The A-type granites were derived from high-T and low-P melting of juvenile crust; • The Eastern Tianshan orogen was back-arc extension during the Late Devonian. [ABSTRACT FROM AUTHOR]

The origin of ancient continental blocks in the Tibetan plateau and their paleogeographic locations in the Rodinia supercontinent remain enigmatic. Here, we report the early Cryogenian metamorphic magmatic rocks (including amphibolites and granitic gneisses) from the North Lhasa terrane of the central Tibetan Plateau. The amphibolites (ca. 822 Ma) are tholeiitic and exhibit both MORB- (e.g., flat patterns of rare-earth and high field strength elements) and arc-like (e.g., elevated Th/Yb tatios) geochemical affinities. In combination with high positive zircon ε Hf (t) (+6.9 to +12.4) and whole-rock ε Nd (t) (+4.4 to +10.4) and low zircon δ 18 O (4.87 to 5.81‰) values, their geochemical data indicate a depleted mantle source affected by subduction components. The granitic gneisses (ca. 810 and 806 Ma) are A 2 -type granitoids and have relatively lower zircon ε Hf (t) (+4.7 to +6.9) and whole-rock ε Nd (t) (+3.5) and higher zircon δ 18 O (5.44–8.08‰) values. Their protoliths were probably generated by partial melting of Mesoproterozoic crustal rocks. The amphibolites and granitic gneisses are geochemically distinct from the Cryogenian rift-related magmatic rocks in the interior of the Rodinia supercontinent but similar to the coeval back-arc magmatic rocks at the northwestern edge of the Rodinia supercontinent, thus providing new constraints on the early Cryogenian paleogeographic location of the North Lhasa terrane. [ABSTRACT FROM AUTHOR]

The study reports and synthesizes the available geological and geochemical data on the East Vardar ophiolites comprising most known occurrences from the South Apuseni Mountains in Romania to the tip of the Chalkidiki Peninsula in Greece. The summarized geological data suggest that the East Vardar ophiolites are mostly composed of the magmatic sequences, whereas the mantle rocks are very subordinate. The members of the magmatic sequences are characterized by the presence of abundant acid and intermediate volcanic and intrusive rocks. The age of these ophiolites is paleontologically and radiometrically constrained and these data suggest that the East Vardar ophiolite formed as a short-lived oceanic realm that was emplaced before the uppermost Kimmeridgian. A relatively weak adakitic affinity is shown by intra-ophiolitic acid and intermediate rocks in many East Vardar provinces. It can be taken as evidence that the subduction of the young and hot slab, most likely along the earlier spreading ridge has occurred. A paleo-tectonic reconstruction consisting of four stages is proposed. It involves: a) an early/mid-Jurassic northnortheastward subduction of the West Vardar oceanic plate; b) the formation of a mid-Jurassic volcanic arc and a narrow back-arc oceanic stripe of East Vardar behind it; c) the mid-/Upper Jurassic initiation of East Vardar subduction along the ridge axis, and d) complex and heterogeneous emplacement of the East Vardar ophiolites. So far available data allow for having relatively clear ideas about the origin and evolution of the East Vardar ophiolites. However, in order to provide better understanding of all aspects of its evolution we need to answer additional questions related to the true structural position of the East Vardar ophiolites slices in Serbia, the exact nature of subduction that caused back-arc spreading (intraoceanic vs subduction under continent?) and the full significance of the adakitic signature shown by rocks in the East Vardar provinces other than Demir Kapija. [ABSTRACT FROM AUTHOR]

A shear-wave splitting analysis was determined to observe anisotropic structures of an upper mantle layer beneath Sumatra. The data were collected from 35 BMKG stations with the magnitude of more than 6.25 Mw and the epicentre of 85o - 140o. A shear-wave splitting measurement was calculated by using Splitlab based on three methods simultaneously. The result of the shear-wave splitting measurement in the Sumatra Forearc and Fault Zone shows that there are two anisotropic layers. The first layer has a dominant-fast-polarization direction that is parallel with a trench, and has the delay time duration of 0.5 - 0.9 s-. It is presumed that it is caused by a shear-strain as a result of the existences of Mentawai and Sumatra Fault Zones. The second layer has a dominant-fast-polarization direction that is perpendicular to the trench with the delay time duration of about 1.1 - 1.9 s-. It is presumed that it is caused by a movement of a subduction plate on a mantle wedge. The measurement in the backarc shows that there is only one anisotropic layer that is a subduction plate. It is also found that there is a transition of an orientation change on the subduction plate between Sumatra and Java. The change of the polarization direction is probably related to the age difference and the direction velocity of the absolute plate movement (APM) from Sumatra to Java. [ABSTRACT FROM AUTHOR]

Seismic anisotropy in the East Japan arc has been extensively investigated by conducting shear-wave splitting measurements, receiver-function analyses, and tomographic inversions of body-wave travel times and surface-wave dispersion data, which have provided a wealth of information on dynamic processes associated with active subduction of the Pacific plate. Measuring shear-wave splitting is popular and effective to detect seismic anisotropy, but it has poor depth resolution. This problem has been overcome by conducting 3-D anisotropic tomography, which has high-resolution in both lateral and vertical directions. Both P and S wave anisotropies are revealed in the crust, which are caused by alignment or preferred orientation of crustal minerals and stress-induced microcracks related to active faults. Trench-normal fast-velocity directions (FVDs) of azimuthal anisotropy are revealed in the back-arc mantle wedge, reflecting subduction-driven convection there. Trench-parallel FVDs appear in the forearc mantle wedge under the land area, which may reflect deformation that results in B-type olivine fabric. The forearc mantle wedge offshore may lack anisotropy, suggesting that it is stagnant and decoupled from the subducting slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. The most significant findings of the body-wave anisotropic tomography are its constraints on the slab anisotropy. The subducting Pacific slab exhibits mainly trench-parallel FVDs, which reflect shape-preferred orientation of crystals and cracks related to normal faults produced in the outer-rise area before the plate subduction, overprinting the fossil anisotropy that the Pacific plate gained when it was produced at the mid-ocean ridge. Trench-parallel intraslab fast velocity planes of anisotropy intersect the slab upper surface at high angles (∼45–90°), reflecting aligned hydrated faults in the slab. Ruptures of the hydrated faults in the upper part of the slab may cause large intraslab earthquakes (M ≥7.0) that take place frequently beneath the forearc area. Trench-normal FVDs also appear in the subslab mantle, which may reflect asthenospheric shear deformation associated with the overlying slab subduction. [Display omitted] • Seismic anisotropy occurs in all depth domains of the East Japan subduction zone. • The greatest findings are robust constraints on anisotropy in the subducting slab. • Slab deformation plays an important role in both mantle flow and intraslab fabrics. • Subslab upwelling flow may affect megathrust earthquakes on the slab interface. [ABSTRACT FROM AUTHOR]

This paper presents an ancient example of asymmetric back-arc extension found in the Ulleung Basin, the southwestern East Sea (Sea of Japan). The paper focuses on the formative processes of variable asymmetric features of the basin and on their relationship to the style of lithospheric stretching. In the Ulleung Basin, features resulting from asymmetric back-arc extension include: 1) the syn-extensional volcanism concentrated to the northern flank of the basin, 2) the overwhelming number of north-dipping syn-extensional normal faults, 3) the style of boundary faults different between the northern and southern conjugate margins, and 4) the thickness of the crust and the depth to the Moho discontinuity that together decrease toward north. We argue that the fundamental control of such asymmetric features was the simple-shear-style lithospheric extension, which presumably involved a crustal detachment, the mid-crustal extension of north-dipping syn-extensional listric normal faults. By the simple-shear-style extension, syn-extensional faults evolved to dip toward north, and at the same time volcanic activities that had been initially widespread throughout the basin became to concentrate to the north of the basin. The northward deviantion of volcanic centers and the zone underlain by maximally attenuated crust (possibly the extinct spreading center) might be the result of separation between the loci of basin subsidence and asthenospheric upwelling that is common in simple-shear cases. [ABSTRACT FROM AUTHOR]

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 5 km 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. [ABSTRACT FROM AUTHOR]

Volcanic rocks in the Xiemisitai Mountains，in Northwest Junggar were recently considered to form in the Silurian rather than the Devonian period. However，the characteristic and tectonic setting of Middle Silurian volcanic rocks in this area are poorly constrained. In this study， major and trace elements and Sr-Nd-Os isotopic composition analyses were synthetically conducted on the Middle Silurian basalt and basaltic andesite in northwest of the Xiemisitai Mountains，West Junggar. These volcanic samples are characterized by high MgO (5. 15% - 7. 81 %) with Mg of 56. 0-60. 7，low CaO (4. 56% - 5. 09%) and FeOT/MgO (1. 15 - 1. 40)，similar to those of high-Mg volcanic rocks. Basalts have εNd(t) of 2. 65，(87Sr/86Sr) i of 0. 70439， and (187Os/188Os) i of 0. 4901. The slightly enriched isotopic signatures，high Th/Y b (2 .2 - 2. 7) and low Ba/La (17. 4 - 33. 4) indicate that they originated from depleted mantle metasomatized by melts from subducted sediments and suffered from crustal contamination. The volcanic rocks are calc-alkaline and exhibit arc-like geochemistry with the enrichment of light REE and large ion lithophile elements. It is suggested that these rocks were probably formed in a back-arc setting due to northward subduction of the Junggar Oceanic slab. [ABSTRACT FROM AUTHOR]

Understanding the geological history of the Lhasa Terrane prior to the India–Asia collision (~ 55 ± 10 Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene (~ 18–10 Ma) 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 183–174 Ma. 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. [ABSTRACT FROM AUTHOR]

The eastern Blacksea magmatic arc (EBMA) in the eastern Sakarya Zone (ESZ) provides an excellent opportunity to investigate birth of an extensional intra-arc and back-arc settings in the Late Cretaceous over the Early Cretaceous northern passive margin of the Neotethys Ocean. Volcano-stratigraphy clearly shows that the Late Cretaceous volcanic activity of the EBMA occurred in two major phases. Bimodality, characterized by mafic/basaltic rocks at the base and felsic/silicic types on top of it, is a typical feature of the lower (LVS) and upper (UVS) volcanic successions in the Giresun region of the ESZ. U Pb and Ar Ar ages support the volcanic succession as two-stage (LVS: ca. 92–85 and UVS: ca. 83–67 Ma) bimodal volcanism. Both the volcanic successions are represented by similar rock types consisting of tholeiitic to calc-alkaline basalt-basaltic andesites and calc-alkaline to shoshonitic dacite-rhyolites. Basaltic (M1- and M2-series) and felsic/silicic (F1- and F2-series) samples of the LVS and UVS have an arc-like signature with enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs). Also, the felsic/silicic samples of the F1- and F2-series show prominent negative Sr and Eu anomalies (Eu/Eu* = 0.4 to 0.9), suggesting that plagioclase fractionation played a key role on the evolution of both felsic series. Bimodal rock series in two phases have a wide range of 87Sr/86Sr (i) (0.7048–0.7075) and 143Nd/144Nd (i) (0.5123–0.5127) ratios with variable ɛNd (i) values of −3.8 to +3.0. 206Pb/204Pb (i) , 207Pb/204Pb (i) and 208Pb/204Pb (i) isotope ratios of the Giresun volcanic rocks vary in the range of 17.97–18.52, 15.55–15.65 and 37.53–38.56, respectively. Geochemical and isotopic data suggest that the parental magma of the M1-basaltic rocks were probably derived from a shallow (spinel-bearing) mantle metasomatized by slab/sediment-derived fluids. In contrast, the M2-basalts seem to have been originated from a deeper mantle source (spinel-garnet transition zone) enriched by slab/sediment-derived fluids and hydrous melts (bulk sediment) metasomatism with some contributions of lower/upper crustal materials. The least evolved basaltic samples in two phases are consistent with moderate (∼10–15%) to high degree (∼20–30%) partial melting of the metasomatized mantle. The silicic melts of the F1- and F2-rocks series, on the other hand, were likely derived from melting of lower crustal materials consisting of meta-basalts/andesites and lesser amount of meta-sediments. Subsequently, these melts experienced FC ± AFC and mixing processes during their ascent and emplacement to generate high-silica (rhyolitic) melts. Our data, combined with previous studies, suggest that two-stage bimodal volcanic rocks of the Late Cretaceous in the ESZ were formed in the transition from an extensional continental intra-arc to a back-arc setting during the northward subduction of the northern branch of Neotethys Ocean. [Display omitted] • Late Cretaceous two-stage (92–85 and 83–67 Ma) bimodal volcanism in the ESZ. • Bimodality is characterized by basaltic and dacitic-rhyolitic rocks in both stages. • Basaltic rocks were derived from mantle metasomatized by slab/sediment fluids±melts. • Dacite-rhyolites were originated from the melting of a heterogeneous crustal source. • Two-stage bimodal volcanism formed in an extensional arc/back-arc setting. [ABSTRACT FROM AUTHOR]