Retroarc Research Articles

Control of magmatic halogen composition and redox state on the zonation of metal mineralization across active continental margins: Perspectives from the world-class South China metallogenic province

Active continental margins are the major sites of continental magmatism and associated hydrothermal ore deposits with a broad metal spectrum. Mineralization across active continental margins typically displays spatial zonation, with porphyry copper-(molybdenum‑gold) deposits in volcanic arcs and tin- and tungsten-dominated mineralization occurring further inland in a back-arc setting. Particularly, tin and tungsten commonly form separate deposits in back-arc regions, even though both metals exhibit similar lithophile behavior. The key factors governing this metallogenic zonation remain unclear. The world-class South China metallogenic province hosts over 50 % of the global tungsten resources, along with a significant amount of tin and copper resources distributed in different mineralization belts, making it an ideal location in which to study regional metal zonation. Here, we comprehensively integrate a very large dataset of the halogen volatile composition (F, Cl) and oxygen fugacity of granites related to tin, tungsten, and copper mineralization in the South China continental margin. Our compilation, derived from a substantial collection of zircon, apatite, mica, and whole-rock geochemistry data, suggests that lateral variations in granite magmatism (away from the trench), transitioning from chlorine-rich and oxic to fluorine-rich and reduced conditions, exert the primary control on copper versus tin‑tungsten mineralization in the arc and back-arc regions, respectively. Differences in oxygen fugacity have a minor impact on the decoupling of tin and tungsten mineralization despite tin granites being universally reduced (ΔFMQ = −1.8 to −0.1, where FMQ is the fayalite-magnetite-quartz redox buffer) and tungsten granites having a broader redox range (ΔFMQ = −1.5 to +1.2). Instead, the disparity in fluorine content plays a more crucial role in controlling the spatial separation of tin and tungsten mineralization observed in the back-arc setting. Nd-Hf and He-Ar isotopic modeling calculations suggest that magmas linked to tin mineralization have a more pronounced involvement of F-rich mantle components compared to those associated with tungsten. Elevated fluorine (ca. 650–8000 ppm) in tin-associated magmas allowed an extreme degree of magmatic differentiation and delayed fluid exsolution due to high H2O solubility in F-rich silicate melts, ensuring Sn enrichment in highly evolved melts. In contrast, early fluid exsolution under less F-rich conditions (ca. 100–700 ppm) led to early tin loss from the melts, ultimately resulting in tungsten-dominant mineralization. This work emphasizes the combined influence of halogen composition and redox state on the regional mineralization zonation in world-class metallogenic provinces, providing vectors for global metal exploration in both past and currently active continental margins.

The role of continental heterogeneity on the evolution of continental plate margin topography at subduction zones

The nature of the overriding plate plays a major role for subduction zone processes. In particular, the highly heterogeneous continental lithosphere modulates intra-plate tectonics and the surface evolution of our planet. However, the role of continental heterogeneity is relatively under-explored for the dynamics of subduction models. We investigate the influence of rheological and density variations across the overriding plate on the evolution of continental lithosphere and slab dynamics in the upper mantle. We focus on the effects of variations in continental plate margin and keel properties on deformation, topographic signals, and basin formation. Our results show that the thickness, extent, and strength of the continental plate margin and subcontinental keel play a crucial role for the morphology and topography of the overriding plate, as well as the retreat of the subducting slab. We show that this lateral heterogeneity can directly influence the coupling between the subducting and overriding plate and determine the partitioning of plate velocities across the overriding plate. These findings suggest that back-arc extension and subsidence are not solely controlled by slab dynamics but are also influenced by continental plate margin and keel properties. Large extended back-arc regions, such as the Pannonian and Aegean basins, may result from fast slab rollback combined with a weak continental plate margin and a strong and extended continental keel. Narrow margins, like the Okinawa Trough in NE Japan, may indicate a comparatively stronger continental plate margin and weaker or smaller continental keel. Additionally, continental keel properties may affect the overall topography of the continental lithosphere, leading to uplift of the deformation front and the formation of intermontane basins.

A note on the seismicity of Sumatra, western Sunda Arc, Indonesia, in relation to the potential for back-arc thrusting

The existence of back-arc thrust faults along the eastern part of the Sunda Arc, ranging westwards from Flores to the western tip of Java, has been recognised for decades. In contrast, it is still unknown whether such back-arc thrust faults exist in Sumatra, which is located in the western part of the Sunda Arc. To investigate the possible existence of back-arc thrusts in Sumatra, we examine regional earthquake data reported by the Agency for Meteorology, Climatology and Geophysics of Indonesia, as well as global earthquake data reported by the International Seismological Centre and the United States Geological Survey. It appears that back-arc thrusts in the study area are not extensively developed, unlike in the eastern Sunda Arc, which may be caused by oblique subduction beneath the Sumatran forearc. The stress associated with the trench-parallel component of subduction is largely accommodated by the ~ 1650-km-long dextral strike-slip fault zone of the Great Sumatran Fault. The seismicity data from various sources do, however, show that there is a dipping seismogenic zone in several parts of the back-arc region of Sumatra, in the opposite direction to the NNE subduction of the Indo-Australian plate. This new observation may be related to the presence of spatially intermittent back-arc thrust faults in the study area, which may need to be taken into account when improving Indonesia's national earthquake hazard maps.

Development of major unconformities in the forearc regions: A signal of west Myanmar−Asia assemblage before the late Paleocene

It has long been debated whether the India-Eurasia collision was a single-stage event that began 60-55 million years ago, or whether it was a two-stage process that involved a collision between India and the Trans-Tethyan Arc before the early Paleocene, and the collision of India with Eurasia during the middle Eocene. Here, we report a late Paleogene angular unconformity (ca. 40-28 Ma) in western Myanmar. This angular unconformity developed around the same time as the Assam unconformity (NE India) but is younger than those found in northern Myanmar. Development of these unconformities indicates that an oblique convergence margin in western Myanmar formed before the middle Eocene, with a major dextral strike-slip fault (proto-Sagaing/Shan Scarp Fault) in the backarc. We interpret this oblique convergence margin to be partial continental collision between the West Myanmar Terrane (WMT) and NE India. In backarc regions, syn-rift successions of the Shwebo sub-basin have formed as a consequence of transtensional tectonics along the proto-Sagaing/Shan Scarp Fault since at least the late Paleocene. The syn-rift successions consist of Asian-derived materials that were not identified in the forearc because of the Wuntho-Popa Arc served as a geographical barrier. The presence of the unconformities and tectonic configuration of the Myanmar backarc sub-basins are inconsistent with the scenario inferred from paleomagnetic data, in which the WMT was part of an intra-oceanic arc at near-equatorial latitudes before the late Oligocene. Instead, we propose that the WMT has been part of continental SE Asia since at least the Paleocene (ca. 60-58 Ma). We reconsider the paleomagnetic data and suggest that the Mawgyi Arc, rather than the WMT, is the oceanic fragment that rifted from the northern Gondwana margin during the Late Jurassic. The Mawgyi Arc collided with continental SE Asia (WMT) during the Late Cretaceous, and then with India during the early Eocene (ca. 51-49 Ma). Our results support the collision between India and Eurasia is a multistage event.

Recycled lower oceanic crust signal in early Jurassic A1-type granites, South China: Implications for flat-slab subduction and mantle heterogeneity in the continental back-arc region

Flat-slab subduction is a distinctive phenomenon that leads to the development of a wide orogenic zone (≥ 800 km) along former continental margins, triggering extensive intraplate magmatism and deformation. However, occurrences of flat-slab subduction on ancient convergent margins are exceptionally rare, possibly owing to multiple episodes of structural destruction and/or poor preservation. Furthermore, the impact of flat-slab subduction on mantle heterogeneity remains ambiguous. In this study, we present comprehensive whole-rock geochemical analyses as well as in-situ zircon UPb dating and HfO isotopic data for Early Jurassic A-type granites from the Pitou and Shibixia plutons in the interior of the South China Block to investigate the potential influence of flat-slab subduction on intraplate magmatism. The Pitou granites exhibit characteristics consistent with those of A2-type granite, displaying zircon εHf(t) values ranging from −4.9 to +0.3 and elevated δ18O values of 6.2–7.6‰. These features suggest that the granites formed through the differentiation of a basaltic parental magma, which underwent varying degrees of crustal contamination. In contrast, the Shibixia granites show A1-type geochemical features and were derived from basaltic parental magma without significant crustal contamination. The zircons in these granites exhibit positive εHf(t) values ranging from +6.6 to +10.6 and anomalously low δ18O values of 3.3–4.0‰, suggesting the involvement of recycled altered lower oceanic crust components in their mantle source. This provides strong evidence for the occurrence of flat-slab subduction beneath the interior of the South China Block during the early Mesozoic, which was located at least 800 km away from the trench. Combined with the presence of post-delamination ocean island-like intraplate magmatic rocks in the South China interior, it is suggested that the flat-slab delamination at approximately 190 Ma facilitated late Mesozoic intraplate OIB-like magmatism. This process, associated with flat-slab subduction, is expected to exert a significant impact on mantle isotopic heterogeneity within continental back-arc regions.

Zircon U-Pb and Lu-Hf isotopes reveal the crustal evolution of the SW Angolan Shield (Congo Craton)

The crustal evolution of the Angolan Shield (AS) remains poorly constrained. To address this, we analysed U-Pb and Lu-Hf isotopes in detrital and igneous zircons to investigate the age and provenance of extensive sedimentary strata in southwestern Angola and use it as a proxy to gain insight into the Archean to Mesoproterozoic evolution of the region.Mesoproterozoic maximum depositional ages for the Iona (<1323 ± 13 Ma), Ompupa (<1215 ± 13 Ma), and Cahama (<1184 ± 23 Ma) siliciclastics challenge previous correlations with the Paleoproterozoic Chela Group. Provenance analysis reveals that the Mesoproterozoic strata were derived internally from the AS.Our combined dataset indicates that the widespread Eburnean magmatism (∼2.05–1.93 Ga) resulted from reworking of Archean crust, possibly in collision orogens. A major increase in the εHf(i) and εNd(i) values at ∼ 1.87–1.73 Ga indicates a change in geodynamics, with magmatism of the Epupa–Namibe Metamorphic Complex (ENMC) generated in an extensional accretionary orogen at the southern margin of the Eburnean–Archean crustal block. Magmatism resumed in the Mesoproterozoic (∼1.56–1.50 Ga), with suprachondritic εHf(i) values indicating significant juvenile addition. The Kunene Complex (KC: ∼1.50–1.36 Ga) anorthosite-mangerite-charnockite-granite magmatism displays variable εHf(i) and εNd(i) values, consistent with mixing between reworked ENMC-crust and juvenile melts in a long-lived accretionary orogen back-arc region. Post-KC (∼1.36–1.30 Ga) magmatism shows an increased juvenile contribution, potentially linked to partial melting of ENMC and ∼ 1.56–1.50 Ga juvenile crust during an orogenic event, or alternatively, related to renewed slab retreat and back-arc extension. The Hf isotopic compositions of ∼ 1.29–1.18 Ga zircons are compatible with a renewed input from the depleted mantle and/or reworking of the earlier ∼ 1.56–1.50 Ga juvenile crust. Emplacement of ∼ 1.13–1.10 Ga mafic dikes/sills marks the end of Mesoproterozoic magmatism in the AS. Our new data enhance our understanding of the Archean to Mesoproterozoic crustal evolution of the AS.

Unveiling the transtensional geodynamics of Cenozoic depocenter changes in the Andaman sea: Seismic evidence for the existence of the East Sagaing Fault and the Andaman Basin Central Fault Zone in the Tanintharyi region

The Andaman Sea, situated at the highly oblique convergent boundary of the Indian and the Sunda Plates in the eastern Indian Ocean, has been shaped by transtensional geodynamics throughout the Cenozoic and has given rise to the formation of fault systems in the Andaman back-arc region. Out of these faults, the Sagaing Fault system exists as the active transform boundary between the Burmese and Eurasian Plates. The East Sagaing Fault, which runs through the Gulf of Martaban in the Andaman Sea, is a critical component of this Sagaing Fault system in fostering the transtensional regime and contributing to the opening of the Andaman Sea as a pull-apart basin. In contrast, the Andaman Basin Central Fault Zone, previously misinterpreted as the South Sagaing Fault, is an inactive fault system that experienced deformation until the Middle Miocene and is currently concealed beneath the East Andaman Basin. This temporal disparity in the Andaman Basin Central Fault Zone and East Sagaing Fault activity patterns has led to uncertainty regarding their existence within the Tanintharyi region of the Andaman Sea. To address this uncertainty, 2D and 3D seismic data interpretations were used to explore the distribution of faults, including the presence and extent of these two faults in the Tanintharyi region. Through a comparative analysis of the structural characteristics of the East Sagaing Fault in the northern part, and the Andaman Basin Central Fault Zone in the southern part of the Andaman Sea, this study presents compelling evidence confirming the existence of both faults in the Tanintharyi region with their structural implications. The innovative discoveries in this article contribute to a deeper understanding of Cenozoic depocenter changes in the Tanintharyi region, substantiating the significance of these two faults in transtensional geodynamics and allowing for a comprehensive examination of geodynamic evolution in the Andaman Sea region.

Evidence for probable temporal and thermal trends among Devonian granitic magmas in central Victoria, and the composition of the Selwyn Block

This study provides new constraints on the causes of the chemical and isotopic variability in the central Victorian, Devonian, silicic, magmatic rocks and relates these to crustal architecture in the region. Synthesising present and previous work, it is concluded that the Selwyn Block, which forms the main source region for the Devonian silicic magmas (granitic and silicic volcanic rocks), is heterogeneous in three dimensions, on scales of 1 km and less. The sources of both the I- and S-type magmas were formed in Paleoproterozoic to Mesoproterozoic times, in the distal back-arc region of an Andean-type margin, in a basin that was extending and deepening eastward with time. The prominent rock types are high-grade metamorphosed greywackes, potassic andesites and dacites, with smaller volumes of pelitic rocks. The metasediment-dominated source rocks generally lie at deeper levels and reached higher metamorphic grades than the sources of the I-type magmas. This means that the I-type magmas were generally at lower temperatures and were also more hydrous than the S-type magmas. Heat for the metamorphism and partial melting of the source rocks of the silicic magmas was advected into the crust by mantle-derived magmas. These probably formed an underplate, to drive the regional metamorphism, but numerous, scattered, sill-like mafic bodies caused local contact anatexis and silicic magma production. Along with the emplacement of mafic magma bodies, the ascent of the silicic magmas to the upper crust rendered the deeper parts of the Selwyn Block denser and more mafic.

The Ground-Motion Characterization Model for the 2022 New Zealand National Seismic Hazard Model

ABSTRACT This article summarizes the ground-motion characterization (GMC) model component of the 2022 New Zealand National Seismic Hazard Model (2022 NZ NSHM). The model development process included establishing a NZ-specific context through the creation of a new ground-motion database, and consideration of alternative ground-motion models (GMMs) that have been historically used in NZ or have been recently developed for global application with or without NZ-specific regionalizations. Explicit attention was given to models employing state-of-the-art approaches in terms of their ability to provide robust predictions when extrapolated beyond the predictor variable scenarios that are well constrained by empirical data alone. We adopted a “hybrid” logic tree that combined both a “weights-on-models” approach along with backbone models (i.e., metamodels), the former being the conventional approach to GMC logic tree modeling for NSHM applications using published models, and the latter being increasingly used in research literature and site-specific studies. In this vein, two NZ-specific GMMs were developed employing the backbone model construct. All of the adopted subduction GMMs in the logic tree were further modified from their published versions to include the effects of increased attenuation in the back-arc region; and, all but one model was modified to account for the reduction in ground-motion standard deviations as a result of nonlinear surficial site response. As well as being based on theoretical arguments, these adjustments were implemented as a result of hazard sensitivity analyses using models without these effects, which we consider gave unrealistically high hazard estimates.

Speculations on the Paleozoic legacy of Gondwana amalgamation

Using Gondwana as an example, we show how the geological record can be interrogated to detect significant changes in mantle convection patterns at critical junctures in Earth’s evolution. Evidence of major changes in mantle circulation in the aftermath of late Neoproterozoic-early Paleozoic Gondwana assembly is provided by widespread (i) plume-related magmatism around Gondwana’s periphery, (ii) ironstone deposits related to mantle plume-ocean ridge interaction and enhanced hydrothermal activity, and (iii) super-mature clastic deposits that reflect epeirogenic uplift triggered by mantle upwelling beneath Gondwana combined with deep tropical weathering.In our model, Gondwana assembled above a region of mantle downwelling in which subducted slabs between the converging Gondwanan continents descended to the core-mantle boundary. Renewed subduction along Gondwana’s periphery yielded early arc magmas. But as downwelling beneath Gondwana evolved into upwelling as a result of the ponding of subducted slabs at the base of the mantle, mantle plumes rose from the margins of the nascent upwelling to interact with the edges of Gondwana, where they penetrated the peripheral subduction zones via slab windows, tears and transform faults to generate voluminous calc-alkalic crustal melts in hydrated arc regions and A-type magmas in dry back-arc regions. The plumes also underplated oceanic lithosphere and interacted with adjacent ocean ridges, thereby enhancing hydrothermal activity and the flux of bioessential nutrients, leading to the recurrence of marine iron-rich sedimentary rocks in the geological record. At the same time, upwelling beneath a tropical to equatorial Gondwana led to epeirogenic uplift, deep weathering and erosion, resulting in the production of widespread super-mature clastic deposits. We contend that major changes in mantle convection patterns were encoded into the geological record of Gondwana assembly, influenced global-scale mantle convection patterns, and should be incorporated into geodynamic models for the assembly of Pangea.

The Systematics of Olivine CaO + Cr-Spinel in High-Mg# Arc Volcanic Rocks: Evidence for in-Situ Mantle Wedge Depletion at the Arc Volcanic Front

Abstract We investigated the state of the arc background mantle (i.e. mantle wedge without slab component) by means of olivine CaO and its Cr-spinel inclusions in a series of high-Mg# volcanic rocks from the Quaternary Trans-Mexican Volcanic Belt. Olivine CaO was paired with the Cr# [molar Cr/(Cr + Al) *100] of Cr-spinel inclusions, and 337 olivine+Cr-spinel pairs were obtained from 33 calc-alkaline, high-K and OIB-type arc front volcanic rocks, and three monogenetic rear-arc basalts that lack subduction signatures. Olivine+Cr-spinels display coherent elemental and He–O isotopic systematics that contrast with the compositional diversity of the bulk rocks. All arc front olivines have low CaO (0.135 ± 0.029 wt %) relative to rear-arc olivines which have the higher CaO (0.248 ± 0.028 wt %) of olivines from mid-ocean ridge basalts. Olivine 3He/4He–δ18O isotope systematics confirm that the olivine+Cr-spinels are not, or negligibly, affected by crustal basement contamination, and thus preserve compositional characteristics of primary arc magmas. Variations in melt H2O contents in the arc front series and the decoupling of olivine CaO and Ni are inconsistent with controls on the olivine CaO by melt water and/or secondary mantle pyroxenites. Instead, we propose that low olivine CaO reflects the typical low melt CaO of high-Mg# arc magmas erupting through thick crust. We interpret the inverse correlation of olivine CaO and Cr-spinel Cr# over a broad range of Cr# (~10–70) as co-variations of CaO, Al and Cr of their (near) primary host melts, which derived from a mantle that has been variably depleted by slab-flux driven serial melt extraction. Our results obviate the need for advecting depleted residual mantle from rear- and back-arc region, but do not upset the larger underlying global variations of melt CaO high-Mg# arc magmas worldwide, despite leading to considerable regional variations of melt CaO at the arc front of the Trans-Mexican Volcanic Belt.

Decoding subcontinental lithosphere processes: The key role of fractional crystallization in Central Andes monogenetic volcanism - Insight from El Negrillar volcanic field, Chile

Subduction zones are complex geodynamic settings in which volcanic arcs form due to the interaction between subducting slabs, water, and continental materials. Investigating this extensive Earth recycling system offers valuable insights into the exchange of materials between the crust and mantle. While previous research has primarily focused on analyzing polygenetic volcanoes to explore this interaction, monogenetic volcanic fields (MVF) present an opportunity to delve into the mechanisms operating within the upper and lower mantle. Monogenetic volcanoes are a class of volcanoes that are typically smaller in size but showcase a range of eruptive styles and compositions. These volcanoes can be found in various locations along the arc, including the main Central Volcanic Zone (CVZ) (arc-front volcanoes), and even far from the tectonic plate boundaries in the back-arc region (intraplate volcanism).We carried out a detailed geochemical study of El Negrillar (EN) MVF, located in the Central Andes main-arc, on the Altiplano-Puna Plateau's southwestern border. The volcanic field has 35 eruptive centers and a total of 86 eruptive phases originating from its three clusters: Northern, Central, and Southern. EN magmas range in composition from basaltic andesite to dacite and have produced over 6.8 km3 (DRE) of lava and pyroclastic material, which makes it the most voluminous volcanic field in the Central Andes, and therefore, an excellent example to study MVF in arc-systems. We have conducted an extensive study encompassing major and trace elements, as well as isotopic analysis using whole rock Sr-Nd-Pb. We also present an analysis of our results with available geochemical data from the back-arc and main-arc volcanoes, including other monogenetic volcanoes near EN (<80 km, EN's neighbors). The results revealed that EN magmas and their neighbors are strongly related and exhibit a clear similarity in major and trace element compositions and isotopic ratios, indicating a common source and origin of their melts. Our comparison across the arc surprisingly reveals similarities between EN and back-arc monogenetic volcanism. Both regions indicate geochemical signatures that do not support melting via slab metasomatism, typical of subduction zones. Instead, they show low Th/Ce (<0.1 ppm) and Ba/La (<30 ppm) and high Ce/Pb (>6 ppm), Ba/Th (>100 ppm), high 143Nd/144Nd > 0.51240, and La/Yb ratios at a given La/Sm compared to the main-arc polygenetic volcanoes. Interestingly, EN also exhibits adakite-like signatures with increasing SiO2 content (high Sr > 600 ppm, Sr/Y > 40, and high La/Yb > 20), a term used for igneous rock suites with chemical characteristics that are identical to those of adakites but produced through petrogenetic processes that do not include a slab component. We explore how fractional crystallization and crustal assimilation participate in the development of monogenetic volcanoes, and our findings underscore the significant influence of fractional crystallization on the monogenetic volcanic processes in the Salar de Atacama region. The geochemical changes observed in this area can be attributed to a combination of varying degrees of fractional crystallization within magmas generated through the partial melting of a garnet-enriched environment, which could be primary lower crust or partial melting of a peridotite mantle that has later equilibrated with garnet-bearing crustal melts.

A Long-Lived Accretionary Process during the Amalgamation of the North China Craton: Insights from Neoarchean–Paleoproterozoic Polyphase Magmatism in the Lüliang Complex

Abstract There has been a long debate regarding the timing of the final amalgamation of the North China Craton, which is considered to have occurred either during the Neoarchean or Paleoproterozoic era. One major point of contention is whether there existed a long-lived subduction lasting through the Neoarchean to Paleoproterozoic. The Lüliang Complex contains multiphases of magmatism and thus represents the most viable region to address this controversy. In this study, we carried geochronological and geochemical analysis on the representative granitoids. Secondary ion mass spectrometry U–Pb dating revealed four distinct granitoid groups emplaced at 2531 ± 4, 2189–2173, 2027 ± 25, and 1852 ± 41 Ma, respectively. Notably, the 2531 Ma granitic gneiss was identified for the first time in this region. Based on the geochemical characteristics, the granitoids can be divided into two types. The 2531 and 2027 Ma groups display I-type features, while the 2189–2173 and 1852 Ma groups exhibit A-type geochemical affinities. Both I-type groups exhibit enrichment in Rb, depletion in Nb, Ta, and Ti, moderate fractionated REE patterns, substantial negative Eu anomalies, low Sr/Y ratios, and positive εHf(t) (+3.51 to +5.53 and +5.59 to +7.32, respectively), indicating that they were generated from partial melting of the juvenile mafic crust. In contrast, the 2189–2173 Ma granitoids belong to A2-type and were most likely generated by the partial melting of felsic rocks in the back-arc region, while the 1852 Ma granitoids belong to A1-type and were most possibly the result of partial melting of mafic-intermediate rocks during the post-collisional stage. Based on the records of A-type granitic magmatism and the ~1950 Ma peak metamorphism throughout the Trans-North China Orogen, we propose that a long-lived subduction process (2531–1950 Ma) can mostly explain the existing geological phenomena. It is likely that the subduction between the Eastern and Western Blocks should have commenced at ~2531 Ma, followed by a long-lived subduction. The two blocks ultimately collided with each other to form the North China Craton at ~1950 Ma, which triggered post-collisional exhumation and partial melting at ~1852 Ma.

Systematic Across‐Arc Variations of Molybdenum Isotopes in a Fluid‐Dominated Subduction Zone System

AbstractMass‐dependent Mo isotope variations are a promising new tracer to study magmatic processes in different geological settings. We report the first Mo isotope data for the Kamchatka arc system in the Northwest Pacific, comprising basaltic lavas of a complete Southeast‐Northwest traverse from the volcanic arc front through to the back arc region. The majority of volcanic centers investigated directly override the Hawaii‐Emperor Seamount Chain, which is currently being subducted underneath the arc system. Our Mo isotope data show systematic trends with Ce/Pb, Ce/Mo, Nb/Zr, La/Sm, and 143Nd/144Nd ratios from the volcanic arc front to the back arc. Arc front lavas have higher δ98/95Mo and lower Ce/Pb, Ce/Mo, Nb/Zr, La/Sm compared to back arc lavas. Because the involvement of subducted sediments can be excluded, we attribute the observed variations to a change in the mantle source composition from the arc front to the back arc regions. The isotopic and chemical budget of arc front lavas is dominated by a slab fluid component (high δ98/95Mo, low Ce/Pb, Ce/Mo), whereas mantle‐like Ce/Pb, Ce/Mo, elevated Nb/Zr and La/Sm in the back arc samples suggest an enriched mantle source. Combined δ98/95Mo, Nd, and Pb isotope data in back arc lavas are very similar to those observed for modern ocean island basalts from Hawaii. We thus explore the possibility that the back arc mantle was contaminated by a Hawaii‐type, enriched asthenospheric mantle component from the subducted Hawaii‐Emperor Seamount Chain.

Andean evolution, orogenic deformation and uplift of the Western Cordillera and Altiplano of southern Peru, northern Bolivia and Chile: Eocene-Oligocene lithospheric delamination

Reappraisal of Andean evolution in southern Peru, northern Bolivia and northern Chile provides support for models of orogenic cyclicity in which Andean crustal shortening in the Western Cordillera led to Eocene-Oligocene lithospheric delamination which triggered a renewed pulse of Andean uplift. The sedimentary fill of the Arequipa "rift" back-arc basin (Rhaetian-Neocomian) was controlled by two major graben-bounding fault systems: the Condoroma-Caylloma and Cincha-Lluta-Incapuquio; as well as the Cusco-Lagunillas fault system separating the back-arc region from the Cusco-Puno high. These thinned cortical inherited structures, were the most favourable places to undergo a positive tectonic inversion at the beginning of Andean compression (Albian), and then during Late Cretaceous-Paleocene shortening, they continued to develop as east-vergent crustal decollements which allowed crustal doubling and Western Cordillera thickening. The crustal thickening of >45 km took place along the Condoroma-Caylloma inverted graben where the lower crust, which was probably enriched in basalts, was subjected to high-pressure metamorphism leading to eclogitization, and became denser than the underlying mantle lithosphere and therefore delamination occurred. The subsequent asthenospheric upwelling led to lithospheric thinning, asthenospheric mantle magmatism of the first stage of Andahuaylas-Yauri batholith emplacement (∼48-43 Ma), which crystallized under temperatures of 1200–700 °C and pressures of 1.0–0.5 GPa (>40-30 km deep). This process was followed by the rapid surface uplift of the Western Cordillera that reached 3.1 km and fed the Kayra and Lower Moquegua basins, located on the Altiplano and in the forearc respectively. The delamination process was still active during 43–30 Ma, with second-stage magmatism of the Andahuaylas-Yauri batholith, emplaced continuously from deep mafic intrusions (40-30 km) to superficial porphyries (3 km). This indicates an important uplift of the Western Cordillera to 3.3 km, supported by mantle upwelling and the continuous flat-slab compression that prevented normal faults from developing, and rather produced the shortening of the delaminated crust. The Anta-Soncco foreland basins (43-30 Ma) of the Altiplano and Moquegua basin of the forearc, received the products of Cordilleran denudation at a rate of up to 700 m/My. The alkaline potassic magmatism (29–27 Ma) could represent episodes of post-delamination magmatism and the late stage of flat subduction.

Strain and stress gradients through the backarc regions of Miocene western Japan: A new type of arc-parallel extension

The overriding lithosphere is occasionally stretched parallel to the consuming plate boundary. Such arc-parallel extension manifests itself mostly in forearc regions. Here, we report unique arc-parallel extension, which affected only the backarc regions with arc-parallel strain and stress gradients. The extension took place in the backarc regions of the northern Ryukyu and western SW Japan arcs in the Middle Miocene. The extension was evidenced by NW-SE trending normal faults in the extensional zone and paleostresses that were determined from dike orientations. To determine the age of and spatial extent of this zone, we analyzed fault-slip data from the Amakusa region, northern Ryukyu arc, where early middle Miocene intrusions allowed us to judge the magmatism to be older than the faults that affected Cretaceous and Eocene formations. The slip senses of the faults have been controversial in Amakusa. As a result, the region was found to have belonged to the extensional zone. The extensional zone is 300–400 km long and ∼100 km wide. We also found that the arc-parallel extension resulted in crustal strain with a southwestward increasing trend along the zone. The strain magnitudes showed a negative correlation with distance from the Okinawa Trough. This correlation is explained by the increasing stress magnitude toward the trough. We suggest that the stalled rift propagation of the northern Okinawa Trough caused the unique arc-parallel extension in the Middle Miocene.

Regional-scale resistivity structure of the middle and lower crust and uppermost mantle beneath the southeastern Canadian Cordillera and insights into its causes

SUMMARYSubduction zones are recognized as an important class of plate boundaries and are the location of a number of important geological processes. They are also important because of the mineral and geothermal energy resources formed by plate convergence. While subduction zones around the world have a number of common features, there are also significant differences among them. The Cascadia subduction zone in southern British Columbia is characterized by a relatively hot subducting plate, and a broad backarc region that is believed to exhibit a shallow, convecting asthenosphere. The magnetotelluric (MT) method is a useful tool to study subduction zones and backarc regions because measurements of subsurface resistivity are sensitive to the presence of fluids. A number of previous MT studies have taken place in this region, but they were limited to a 2-D approach to data analysis. As the MT method has developed, it has become clear that there is a significant advantage to using a 3-D approach to data analysis. This paper presents the first regional-scale 3-D resistivity model of the southern Canadian Cordillera and provides new insights into the lithospheric structure and the distribution of fluids. The southeastern Canadian Cordillera has high heat flow and numerous thermal springs, the locations of which are often controlled by faults. However, the deeper thermal structure and origin of the fluids are poorly understood. To develop an improved understanding of the structure of this area, MT data measured at 331 locations were used to create a 3-D model of subsurface electrical resistivity. This study is primarily focused on the Omineca and Foreland morphogeological belts in southeastern British Columbia, which are separated by the southern Rocky Mountain Trench. The resistivity model is presented to a depth of 100 km and a number of conductive features are observed in the crust and uppermost mantle of the southeastern Cordillera. The locations of these conductors broadly matched previously reported conductors, but the 3-D inversion revealed new details of their geometry. The previously reported Canadian Cordilleran Regional conductor was modelled as a number of discrete conductors in the depth range 15–55 km beneath the Omineca belt. Temperatures approximately in the range 400–700 °C are expected at depths of 15–26 km and saline aqueous fluids are likely the cause of the low resistivity. Temperatures approximately in the range 700–1300 °C are expected at depths of 26–55 km and small volumes of partial melt may explain the low resistivity. The Southern Alberta–British Columbia conductor, Red Deer conductor and Loverna conductor were imaged as a single connected conductor, whose low resistivity is likely caused by sulphide mineralization. A group of conductors was imaged near the southern Rocky Mountain Trench in the depth range 10–70 km and their low resistivity is likely caused by interconnected saline fluids and possibly interconnected graphite films. To understand if the distribution of thermal springs was correlated with the 3-D resistivity model, a statistical study was undertaken. This showed no clear correlation between crustal conductance and the distribution of thermal springs.

Tectonic settings influence the geochemical and microbial diversity of Peru hot springs

Tectonic processes control hot spring temperature and geochemistry, yet how this in turn shapes microbial community composition is poorly understood. Here, we present geochemical and 16 S rRNA gene sequencing data from 14 hot springs from contrasting styles of subduction along a convergent margin in the Peruvian Andes. We find that tectonic influence on hot spring temperature and geochemistry shapes microbial community composition. Hot springs in the flat-slab and back-arc regions of the subduction system had similar pH but differed in geochemistry and microbiology, with significant relationships between microbial community composition, geochemistry, and geologic setting. Flat-slab hot springs were chemically heterogeneous, had modest surface temperatures (up to 45 °C), and were dominated by members of the metabolically diverse phylum Proteobacteria. Whereas, back-arc hot springs were geochemically more homogenous, exhibited high concentrations of dissolved metals and gases, had higher surface temperatures (up to 81 °C), and host thermophilic archaeal and bacterial lineages.

Tectonic evolution of the Proto-Qiangtang Ocean and its relationship with the Palaeo-Tethys and Rheic oceans

Abstract An evaluation of the potential geodynamic connections between the evolution of Paleozoic oceans in NW Gondwana and NE Gondwana is challenging. Until recently, most syntheses emphasized only two Paleozoic oceans (the Proto-Tethys and the Palaeo-Tethys) in the east Tethys realm. However, the discovery of early Paleozoic ophiolites along Palaeo-Tethys sutures located south of Proto-Tethys sutures challenges these traditional views. After a comprehensive review of relevant early Paleozoic tectonomagmatic events, we herein recognize and propose a model for the tectonic evolution of a hitherto unrecognized early Paleozoic ocean, which we call the Proto-Qiangtang Ocean. This ocean was short lived; it opened in the late Cambrian, began to subduct in the Middle Ordovician, and closed diachronously westwards between the Late Ordovician and the middle Silurian. Its closure by middle Silurian time indicates that was a spatially and temporally distinct ocean from the Palaeo-Tethys Ocean. The early tectonic evolution of the Proto-Qiangtang Ocean shares many characteristics with that of the Rheic Ocean. Both opened in the late Cambrian in the back-arc region of the Iapetus–Proto-Tethys Ocean, and the Proto-Qiangtang Ocean is considered to represent the eastern extension of the Rheic Ocean. This correlation has important implications for the Paleozoic tectonic evolution and palaeogeography of northern Gondwana.

Tracing mantle components and the effect of subduction processes beneath the northern Antarctic Peninsula

Understanding subduction processes is critical in assessing the long-term evolution of the upper mantle and continental crust. We present new geochemical data on glassy submarine lavas from the Bransfield Strait, the Phoenix and West Scotia ridges, and previously unpublished data of marine sediments from atop the Phoenix Plate. We combined new and published geochemical data from across the northern sector of the Antarctic Peninsula to unravel both large-scale mantle composition and flow across the region. The geochemistry of Phoenix and West Scotia Ridge basalts supports the hypothesis that both ridges are underlain by Pacific MORB mantle, brought into the region through the eastward expansion of the Scotia Plate. Comparisons among lavas from the Phoenix/West Scotia ridges, the South Shetland Islands volcanic arc, and the Bransfield Strait/Prince Gustav Rift back-arc region reveal that the Pacific upper mantle flowed into the mantle wedge and was subsequently modified by subduction processes. The compositions of Bransfield Strait lavas range from strongly subduction-influenced near the strait’s center to akin to Phoenix Ridge MORB toward the strait’s edges. This suggests that the Phoenix MORB mantle has flowed around the slab edges into the mantle wedge, diluting the subduction signal and focusing the subduction-modified mantle towards the center of the strait. Monte Carlo simulations indicate that mixing of Phoenix MORB mantle with 0 to 3 % subduction component having a fluid/sediment ratio of ∼1 can explain the compositional range in the Bransfield Strait. Furthermore, our modeling suggests the presence of a more depleted Phoenix MORB mantle beneath the South Shetland Islands modified by the addition of ∼3 % subduction component with a fluid/sediment ratio ranging from ∼0.18 to 4. The increase in fluid/sediment ratios in the South Shetland Island lavas corresponds to a spatiotemporal progression of volcanism from the southwest (40–140 Ma) to the northeast (30–60 Ma). Finally, we have identified a set of lavas with unique trace element and isotope compositions typical of alkaline volcanism found across other parts of the Antarctic Peninsula. Surprisingly, we find occurrences of these lavas on both sides of the South Shetland Trench, suggesting the presence of a distinct enriched source in the upper mantle throughout the region.