Subduction Of Oceanic Lithosphere Research Articles

Late Triassic to Middle Jurassic magmatism in the Eastern Pontides (NE Türkiye): Evidence for divergent double subduction of Paleo-Tethys lithosphere

The Meso-Cenozoic geodynamic evolution of northern Türkiye remains a subject of debate primarily due to a lack of systematic geological, geochemical, geochronological, and geophysical investigations. This paper presents comprehensive geochronological and geochemical data on the Late Triassic andesite porphyry, andesite breccia and quartz diorite porphyry, and the Middle Jurassic diabase, and amphibole-poor and amphibole-rich andesite porphyry from Çevrepınar Igneous Complex (Gümüşhane) in the southern part of the Eastern Pontides Orogenic Belt (EPOB), a well-preserved continental arc in the Alpine-Himalayan orogen. Zircon U–Pb geochronology indicates crystallization ages of ∼208–202 Ma (Rhaetian) for the Late Triassic rocks and ∼175–172 Ma (Aalenian) for the Middle Jurassic rocks. Whole-rock geochemical and Sr-Nd-Pb isotopic data, and zircon εHf(t) data indicate that both the Late Triassic and the Middle Jurassic rocks originated by low-degree melting of a spinel lherzolite lithospheric mantle source modified by subduction-related fluids and/or melts. Based on the new and published data, we suggest that the Late Triassic to Middle Jurassic arc magmatism in EPOB occurred as a result of southward subduction of the Paleo-Tethys oceanic lithosphere beneath the northern margin of Gondwana-Land. Late Triassic to Jurassic arc magmatism and basin evolution occurred synchronously in the northern and southern peripheries of the present-day Eastern Black Sea Basin, indicating divergent double subduction of the Paleo-Tethys oceanic lithosphere beneath the northern margin of Gondwana and southern margin of Laurasia during the Early Mesozoic.

The onset of Paleo-Pacific subduction: Key evidence from the Kaishantun Accretionary Complex, northeastern Eurasia

Subduction of the Paleo-Pacific oceanic lithosphere has dominated the tectonic evolution of northeastern Eurasia since the Mesozoic. We document the time of subduction initiation based on the age, character, and paleogeographic record of the Jilin-Yanji Suture that separates the Jiamusi-Khanka Block (of Northeast China) from the northeastern North China Craton. The suture contains a series of accretionary complexes that provide abundant information for elucidating the evolution of the oceanic plates. Zircon U-Pb and Lu-Hf isotope, as well as zircon trace element data, from nine sedimentary rock samples from the Kaishantun Accretionary Complex in the easternmost segment of the Jilin-Yanji Suture document a volcanic arc setting in the end-Permian to Middle Triassic (255−244 Ma) involving the addition of juvenile crust. Based on our new data and previous studies, we propose that southwestern-directed subduction of the Jilin-Heilongjiang Ocean dominated the evolution of regional tectonics between 260 Ma and 230 Ma, which resulted in the formation of arc-related volcano-sedimentary rocks and the generation of accretionary complexes within the Jilin-Yanji Suture. The Paleo-Pacific Ocean started to subduct beneath northeastern Eurasia at ca. 235 Ma, which accelerated the closure of the Jilin-Heilongjiang Ocean and provided the major driving force for the final amalgamation of the northeastern North China Craton and the Jiamusi-Khanka Block.

Tectonic evolution of the Qiangtang Basin (northern Tibet): Constraints from detrital zircon geochronology

The Mesozoic evolution of the Qiangtang Basin in the Tibetan Plateau, which potentially hosts oil and gas resources, is not fully understood, particularly in the context of the opening and closure of the Tethyan oceans. To better understand the tectonic evolution of the Qiangtang Basin and the history of sedimentation, we obtained 788 new U-Pb detrital zircon ages from 12 sandstone samples. Our results show that the provenance of the Mesozoic strata varies both in space and time. In the central part of the basin (Central Qiangtang), older sedimentary rocks are characterized by unimodal age spectra (300–233 Ma) that indicate a proximal arc source. Trace-element compositions of Permian to Triassic zircons from Lower Triassic strata are characteristic of crystallization in arc and backarc environments, thus supporting previous suggestions that the Qiangtang Basin formed as a backarc basin during subduction of the Paleo-Tethys oceanic lithosphere. Younger (Upper Triassic and Jurassic) strata in the Qiangtang Basin received detrital zircons from diverse sources that included Cambrian to Triassic magmatic rocks and Paleozoic strata in the Central Qiangtang, and Triassic flysch deposits in the Songpan-Garze Terrane. We suggest that this change in provenance — from a unimodal arc source that produced zircons in Lower–Middle Triassic rocks to the diverse provenance recognized in the Upper Triassic strata — marks the closure of the Paleo-Tethys Ocean during the Late Triassic. Jurassic sedimentation in the Qiangtang Basin occurred during the expansion of the Meso-Tethys Ocean (Early–Middle Jurassic) and the subsequent subduction of this ocean (since the Callovian). During the Early Cretaceous, the closure of the Meso-Tethys Ocean led to an uplift and a sedimentary hiatus that marked the end of marine sedimentation in the Qiangtang Basin.

Plate tectonics through Earth’s history: constraints from the thermal evolution of Earth’s upper mantle

ABSTRACT Temporal changes in Earth’s tectonic style play a crucial role in understanding the planet’s evolution. Modern-style tectonics is characterized by the formation of basaltic crust at divergent plate boundaries and its subsequent recycling at subduction zones, accompanied by wedge mantle formation and arc magmatism. It is commonly believed that the secular cooling of the mantle modified the tectonic style from stagnant lid or heat pipe on an early hotter Earth to horizontal tectonics during Meso-Neoarchean. However, various field, petrographic, and geochemical studies suggest that the onset of plate tectonics ranges from Hadean to Neoproterozoic. In this study, we re-evaluate the primary magma temperature (mantle potential temperature, Tp) of the upper ambient mantle, spanning from Eoarchean to Neoproterozoic. We used basalts from several Archean and Proterozoic greenstone belts worldwide, along with Proterozoic ophiolites, to (re)calculate the Tp using the FRACTIONATE-PT method. We observed a Tp range of 1444–1639°C during the Archean and 1414–1611°C during the Proterozoic. These findings indicate a strong correlation with previously estimated Tp values obtained from PRIMELT3 method. This further highlights strong internal consistency among different methods and supports models of a hot ambient mantle during the Archean and Proterozoic. We further reviewed numerical models regarding the effect of mantle temperature on the viability of early Earth plate tectonics. Such model results, composition of Archean continental crust, recent crustal growth models, and field evidence are consistent with the operation of plate tectonics on an early hotter Earth. The transition from a hotter mantle to a colder one from Eo-Neoarchean resulted in thinner oceanic lithosphere and a less depleted lithospheric mantle. Subduction of this thinner oceanic lithosphere led to modern-style tectonics. The intensity and style of plate tectonics on the early Earth differed from modern tectonics, which emerged during the Neoarchean.

The electrical structure of the lithosphere in the southeastern margin of the Amazonian craton: Contribution to West Gondwana tectonics

A high electrical conductivity belt near the southeastern margin of the Amazonian craton, known as Paraguay-Araguaia Conductivity Anomaly (PACA), has been discovered and mapped over a distance of more than 1,000 km by a large array of magnetometer measurements. To precisely determine the location and structure of the conductive anomaly, we conducted a 200-km-long magnetotelluric (MT) profile with typical site spacing of 8 km over the northern part of PACA and crossing the Goiás magmatic arc and Araguaia belt in Central Brazil. Three-dimensional resistivity models derived from this data indicate that the PACA is not a single conductor but comprises at least two contiguous bodies at the upper to middle crust. The enhanced conductivity of the northern part of PACA is likely due to sulfide minerals deposited in an inter-arc or back-arc basin that were deformed and subsequently emplaced tectonically beneath the western Goiás magmatic arc. The PACA is truncated to the west by a prominent sub-vertical resistor, attributed to the Transbrasiliano lineament (TBL), a continental-scale structural feature interpreted here as a mylonitic shear zone developed late in the Neoproterozoic to Cambrian tectonic history involving the collision between Amazonian craton and proto-West Gondwana. We cannot confirm the prevailing view of an east-dip subduction of oceanic lithosphere from the Amazonian plate under the Goiás arc terranes. Instead, our results probably represent the record of an oblique continental collision. Regardless of its origin, the PACA may serve as a proxy for the paleo border of the Amazonian plate. Therefore, additional geophysical mapping, especially in areas covered by thick Phanerozoic sediments, is essential to constrain the tectonic evolution of West Gondwana.

Apatite Fission‐Track Thermochronology in the Tamusu Area, Bayingobi Basin, NW China, and its Geological Significance

AbstractThe Bayingobi basin is located in the middle of Central Asia Orogenic Belt, at the intersection of Paleo‐Asian Ocean and Tethys Ocean, as well as the junction of multiple tectonic plates. This unique tectonic setting underpins the basin's intricate history of tectonic activity. To unravel the multifaceted tectono‐thermal evolution within the southwestern region of the basin and to elucidate the implications of sandstone‐hosted uranium mineralization, granitic and clastic rock samples were collected from the Zongnai Mts. uplift and Yingejing depression, and apatite fission track (AFT) dating and thermal history simulation analysis were performed. AFT dating findings reveal that the apparent ages of all samples fall within the range of 244 Ma to 112 Ma. In particular, the bedrock of the Zongnai Mts. and Jurassic detrital apatite fission tracks have undergone complete annealing, capturing the uplift‐cooling age. Meanwhile, the AFT ages of Cretaceous detrital rocks are either equivalent to or notably exceed the age of sedimentary strata, signifying the cooling age of the provenance. A comprehensive examination of AFT ages and palaeocurrent direction analyses suggests that the Cretaceous source in the Tamusu area predominantly originated from the central and southern sectors of the Zongnai Mts. uplift. However, at a certain juncture during the Late Early Cretaceous, the Cretaceous provenance expanded to include the northern part of the Zongnai Mts. uplift. Based on the results of thermal history simulations and previous studies, it is considered that the Tamusu area has undergone four distinct tectonic uplift events since the Late Paleozoic. The first is the Late Permian to Early Triassic (260–240 Ma), which is associated with the closure of the Paleo‐Asian Ocean and the accretionary orogeny within the Alxa region. The second uplift event took place in the Early Jurassic (190–175 Ma) and corresponded to intraplate orogeny following the closure of the Paleo‐Asian Ocean. The third uplift event is the Late Jurassic to Early Cretaceous (160–120 Ma), which is linked to the East Asia's position as the convergence center of multiple tectonic plates during this period. The fourth uplift event is linked to the Late Early Cretaceous (112–100 Ma), driven either by the westward subduction of the eastern Pacific plate or the mantle upwelling resulting from the Bangong–Nujiang oceanic lithosphere subduction and slab break‐off. The primary stress orientation for the first three tectonic uplift phases approximated a nearly SN direction, while the fourth stage featured a principal stress direction of NW. The fourth tectonic uplift event of the Late Early Cretaceous and basaltic eruption thermal event during this period likely exerted a significant influence on the formation of the Tamusu sandstone‐hosted uranium deposit.

Tectonic evolution of the South Pamir Orogen: Insights from the Permian to cretaceous magmatism

The Pamir orogen originated through the amalgamation of various arc terranes. Despite undergoing numerous studies, the subduction polarity and time regarding the accretion processes remain inadequately constrained. To address these issues, we carried out a detailed study of zircon and apatite U-Pb ages, zircon Hf isotopes, and whole-rock geochemistry for the Permian to Cretaceous gabbro, volcanic rocks and granitoids from the Southern Pamir and the Rushan-Pshart suture. Three stages of magmatism are identified, including 279–266 Ma gabbro and andesite to dacite, 209–208 Ma granitoids, and 108–103 Ma gabbro and granitoids, respectively. The Permian gabbro and basaltic andesite to dacite in Pshart Range are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but are depleted in Ta, Nb, and Ti, suggesting formation from an enriched mantle source in a subduction zone. The Late Triassic I-type granitoids are typical arc calc-alkaline magmatic rocks with εHf(t) values varying from −8.7 to −3.5, which mainly sourced from the partial melting of mixed crustal sources predominately consisting mafic rocks with minor metasedimentary rocks. The Early Cretaceous Pshart granites are I-type granites with relatively low εHf(t) values ranging from −14.6 to −2.9, suggesting that they were crust-derived rocks. The Early Cretaceous Bazardara granites are identified as A2-type granites with the εHf(t) values ranging from −18.3 to +5.3, and were derived from mixing of mantle-derived and crust-derived magmas, or interaction between mantle melts and old crustal rocks. The early Cretaceous gabbro exhibits -MORB-like geochemical characteristics with flat REE pattern (La/YbN = 4.96), moderate enrichment of LILEs and weak depletion of Nb, Ta and Ti (Nb/LaPM = 0.94). Thus, the early Cretaceous A2-type granite, E-MORB-like gabbro and volcanic rocks were formed in an extensional setting. Our new data indicate that the southward subduction of the Rushan-Pshart oceanic slab started in the Early Permian and continued to the Late Triassic. Moreover, combining the previous data, we conclude that the Early Cretaceous granites and gabbro in the Southern Pamir formed in an extensional environment due to the southward rollback after northward near-flat subduction of the Neo-Tethyan oceanic lithosphere.

Cretaceous magmatic arc in Hainan and the peri-South China Sea as evidenced by geochemical fingerprinting of granitoids in the region

Mesozoic magmatic rocks occur widely in the South China Block and are generally interpreted as the manifestations of the subduction of the Paleo-Pacific oceanic lithosphere beneath Asia. Subduction-driven magmatism in southeast (SE) China continued from the Late Permian through the Late Cretaceous with an inferred lull between 125 Ma and 115 Ma that is known in the literature as the Cretaceous “magmatic quiescence”. We report in-situ zircon U–Pb ages, Hf–O and whole-rock Sr–Nd isotopes, and whole-rock geochemistry of Cretaceous granitoids on Hainan Island and discuss their magmatic evolution within the framework of the Late Mesozoic geodynamics of SE China. We recognize two main stages of the emplacement of Cretaceous granitoids on Hainan, first around 120 Ma and then around 100–95 Ma, displaying high-K calc-alkaline, I-type geochemical affinities. Granites in both age groups are enriched in LILE and LREE, but depleted in Nb, Ta, Ba, Sr, and Eu. The 120 Ma granites have zircon εHf(t) values of –2.6 to 2.3 corresponding to Hf crustal model ages, ranging from 0.79 Ga to 1.03 Ga, and δ18O values ranging from 6.9‰ to 7.7‰. Zircons from 100–95 Ma granites have εHf(t) values of –4.2 to 1.1 corresponding to Hf crustal model ages of 1.08 Ga to 1.42 Ga, and δ18O values ranging from 6.7‰ to 8.4‰. Increasing εHf(t) values of the Cretaceous Hainan granites with younger crystallization ages indicate addition of more juvenile components and reworking of crustal material into their melt evolution. The εNd(t) values of the 120 Ma and 100–95 Ma granitoids range between –4.1 to –0.4 and –7.7 to –4.0, respectively. The calculated two–stage model age of the 100–95 Ma granitoids clusters between 1.25 Ga and 1.53 Ga. These isotopic data suggest that magmas of the Cretaceous granitoids were produced by partial melting of Mesoproterozoic metabasaltic rocks, which make up much of the crystalline basement of the southern Cathaysia block. The geochemical and isotopic characteristics of the Cretaceous granitoids on Hainan resemble those of magmatic arcs in the Circum–Pacific orogenic belts and identical to those of nearly coeval granitoid intrusions in the continental fragments within the South China Sea basin. We interpret these Cretaceous granitoids in the Peri–South China Sea region as the remnants of a once contiguous Late Mesozoic magmatic arc system that bounded the southern margin of the entire continental Southeast Asia. Our findings do not support the existence of an episode of magmatic quiescence in the geological record of SE China during the Aptian.

Evolution of Silurian to Devonian magmatism associated with the Acadian orogenic cycle in eastern and southern Newfoundland Appalachians: Evidence for a three-stage evolution characterized by episodic hinterland- and foreland-directed migration of granitoid magmatism

The migration and character of magmatism over time can provide important insights into the tectonic evolution of an orogen. We present evidence for three separate stages of compositionally distinct granitoid magmatism associated with the Acadian orogenic cycle in the eastern and southern Newfoundland Appalachians. The interpretations are based on new zircon U-Pb ages, geochemical data, and Sr-Nd-Hf-O isotopic data for 18 samples from 15 Silurian and Devonian granitoid plutons, combined with previously published data. The three stages outline hinterland- and foreland-directed migration trends and represent subduction (435−420 Ma), syncollision (415−405 Ma), and postcollision (395−370 Ma) settings in the Acadian orogenic cycle. The Silurian plutons (435−420 Ma) of the first stage consist mainly of quartz diorite, tonalite, granodiorite, monzogranite, and syenogranite, with high-K calc-alkaline and enriched Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −5 to −2; εHf[t] = −3 to −1; δ18O = +6‰ to +8‰). They are interpreted to record the subduction of oceanic lithosphere of the Acadian seaway that separated the leading edge of composite Laurentia, represented by the Gander margin, and Avalonia. Early Devonian plutons (415−405 Ma) of the second stage contain more voluminous monzogranite and syenogranite; they have calc-alkaline to high-K calc-alkaline features, adakite-like compositions, and more depleted Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −6 to 0; εHf[t] = +1 to +3; δ18O = +5‰ to +6‰). Plutons of this stage occur mostly to the northwest of the Silurian granitoids, indicating a regional-scale northwestward (hinterland-directed) migration of magmatism with a rate of >9 km/m.y. The migration is interpreted to have been related to the progressive shallow underthrusting of Avalonia beneath the Gander margin (composite Laurentia) at least as far as 90 km inboard. The Middle to Late Devonian plutons of the third stage (395−370 Ma) consist mainly of monzogranite, syenogranite, and alkali-feldspar granite, which are silica- and alkali-rich granites with large negative Eu anomalies. These rocks are concentrated along both sides of the Dover−Hermitage Bay fault zone, which represents the boundary between Avalonia and composite Laurentia, to the southeast of the Silurian and Early Devonian igneous rocks. This stage of magmatism represents a foreland-directed (retreating) migration. The Early Devonian and Middle to Late Devonian episodes of magmatism were separated by a gap between 405 Ma and 395 Ma and recorded an evolution from (high-K) calc-alkaline to alkaline compositions, ascribed to partial delamination of Avalonian lithospheric mantle in a postcollisional setting.

Discovery of Late Permian Adakite in Eastern Central Asian Orogenic Belt: Implications for Tectonic Evolution of Paleo-Asian Ocean

The Permian to Triassic period represents a pivotal phase in the evolution of the Paleo-Asian Ocean, marked by significant tectonic transitions from subduction, collision, and post-orogenic extension. The timing of closure of the Paleo-Asian Ocean in northeastern China has always been controversial. In this contribution, the petrology, zircon U-Pb geochronology and geochemistry are conducted on granite found in well HFD1, Songliao Basin, eastern part of Central Asian orogenic belt. Zircon U-Pb dating indicates that granite crystallized at 258.9 ± 2.2 Ma, as the product of magmatism occurred in the early Late Permian. The rocks have high SiO2, Al2O3, Na2O content, negative Eu anomaly, light enrichment of rare-earth elements, depletion of heavy rare-earth elements, high Sr (448.29–533.11 ppm, average 499.68 ppm), low Yb (0.49–0.59 ppm, average 0.54 ppm), Y (4.23–5.19 ppm, average 4.49 ppm), and high Sr/Y ratios (98–125, average 112) and can be classified as O-type adakite. This is the first discovery of late Paleozoic adakite in the Songliao Basin and the neighboring areas. The geochemistry of adakite indicates derivation by partial melting of MORB-type subducted oceanic crust, indicating that the subduction of the Paleo-Asian Oceanic lithosphere lasted until at least 258.9 Ma.

Metallogeny and Mineral Exploration – Some Personal Reminiscences

The following personalised narrative aims to document the highlights of my involvement in some of the ground breaking developments in Economic Geology and their direct application to mineral exploration and discovery over the past half century. The story begins with my introduction to geology at secondary school and university, followed by doctoral research based on fieldwork in the Andes of South America. Then, as an employee of the Chilean Geological Survey, I got my introduction to porphyry copper deposits before returning to the UK to take up a post-doctoral research fellowship. This formative period concluded with my starting out as an independent geological consultant to the global exploration and mining industry. These early years happened to coincide with the plate tectonics revolution and its radical implications for metallogeny. I realised that porphyry copper and related deposits are integral parts of volcano-plutonic arcs generated during subduction of oceanic lithosphere, and volcanogenic massive sulphide deposits in ophiolite complexes must have formed at oceanic spreading centres. At approximately the same time, application of K-Ar dating to copper deposits led to definition of metallogenic belts and corresponding epochs in the Andes, and then established the timing of their economically important supergene oxidation and enrichment. Subsequently, using more modern and precise U-Pb zircon and Re-Os molybdenite methods, collaborative attempts were made to determine porphyry copper deposit lifespans and ages of various copper belts, deposits and prospects around the world, including the Zambian Copperbelt. The focus on porphyry copper deposits led first to an appreciation of the linkage between them and subaerial volcanism and the importance of potassic alteration as a major host of hypogene copper mineralisation, and then to geological characterisation of the increasing number of gold-rich examples. Appreciation of the importance of hydrothermal breccias in porphyry copper deposits, including recognition of mineralised diatremes, resulted in a classification scheme for breccias that may be extended to related deposit types. Extensive fieldwork showed that zones of advanced argillic alteration, termed lithocaps, constitute the shallow parts of porphyry copper systems. The role of tectonic uplift in both porphyry copper formation and subsequent supergene modification was also charted. The end result of this body of work was a porphyry copper model that can be used as a basic exploration guide. In response to a marked increase in the world gold price in the late 1970s, more effort was devoted to gold concentrations in magmatic arc terranes, commencing with epithermal gold deposits in the shallow lithocaps of porphyry copper systems. This led to an input to classification schemes for epithermal precious metal deposits and, eventually, to assignment of the three main epithermal types to specific tectono-magmatic settings. After years of speculation, porphyry gold deposits were recognised for the first time in northern Chile, followed by definition of a new gold deposit class in association with relatively reduced granitic intrusions. A magmatic-hydrothermal origin for Carlin-type gold deposits was steadfastly supported over many years, notwithstanding its unpopularity until relatively recently, culminating in a proposal for modern analogues. For decades, metallogenic provinces and corresponding epochs have been widely appreciated, but debate concerning their origin(s) persists. The nature of accompanying magmatism could well provide an adequate explanation for at least some provinces (e.g., tin, molybdenum and possibly silver), but precursor metal enrichment in the lowermost crust and/or subcontinental lithospheric mantle may well be required in the case of gold and copper provinces. The story concludes with brief commentary on mineral exploration, which has been my lifelong (pre)occupation. Requirements for success in mineral exploration are discussed, based primarily on familiarity with the circum-Pacific region, followed by analysis of the types of companies and individuals involved and the burgeoning challenges to the exploration process, which, if not remedied, will have dire consequences for future metal production and global plans for the energy transition. My involvement as a lone practitioner in both mineral exploration and metallogenic research probably says something about my independent character traits, but nonetheless has been largely unstructured and certainly unplanned. I have no hesitation in recommending a similar career path for any recent graduate who relishes adventure and is prepared to endure significant work-life imbalance.

A temporal control on the isotopic compositions of the Antarctic Peninsula arc

Radiogenic isotopic compositions of arc magmas are a key tool for studying active margin evolution. They have two isotopic end-members: melts formed mostly from juvenile asthenosphere and melts sourced from evolved continental crust/continental lithospheric mantle. Cordilleran-margins are typically more isotopically juvenile near the trench, and conversely, increasingly evolved landward. However, this model has not been tested on the ~1,500 km long Mesozoic-Cenozoic arc of the Antarctic Peninsula. Here we show that while geochemical compositions remain largely constant, radiogenic isotopes become increasingly juvenile with time. Unlike other continental arcs, there is no association between isotopic composition and spatial distribution. This is attributed to: (i) slow subduction of young oceanic lithosphere, resulting in narrowing of the arc and reduced capacity to incorporate continental crust into melts, and (ii) the Cenozoic decrease in convergence rate, which reduced the friction in the slab-overriding plate interface, allowing the arc melts to increasingly source from young juvenile asthenosphere.

Impact of rift history on the structural style of intracontinental rift-inversion orogens

Abstract Although many collisional orogens form after subduction of oceanic lithosphere between two continents, some orogens result from strain localization within a continent via inversion of structures inherited from continental rifting. Intracontinental rift-inversion orogens exhibit a range of structural styles, but the underlying causes of such variability have not been extensively explored. We use numerical models of intracontinental rift inversion to investigate the impact of parameters including rift structure, rift duration, post-rift cooling, and convergence velocity on orogen structure. Our models reproduce the natural variability of rift-inversion orogens and can be categorized using three endmember styles: asymmetric underthrusting (AU), distributed thickening (DT), and localized polarity flip (PF). Inversion of narrow rifts tends to produce orogens with more localized deformation (styles AU and PF) than those resulting from wide rifts. However, multiple combinations of the parameters we investigated can produce the same structural style. Thus, our models indicate no unique relationship between orogenic structure and the conditions prior to and during inversion. Because the style of rift-inversion orogenesis is highly contingent upon the rift history prior to inversion, knowing the geologic history that preceded rift inversion is essential for translating orogenic structure into the processes that produced that structure.

Early Cretaceous intrusive rocks in the Gangdese arc of southern Tibet do not support shallow subduction of the Neo-Tethyan oceanic lithosphere

The subduction geometry of the Neo-Tethyan oceanic lithosphere during the Early Cretaceous (i.e., shallow vs. steep subduction) remains in dispute due to the limited occurrence of the Early Cretaceous igneous rocks in the Gangdese arc. This study reports petrographical, zircon UPb geochronological, zircon Hf isotopic, whole-rock geochemical, and mineral geochemical data for the ultramafic-mafic and intermediate-felsic rocks in the eastern Gangdese arc, southern Tibet. All these rocks were emplaced at ca. 112–105 Ma (i.e. ca. 110 Ma). The ultramafic-mafic rocks (including hornblendite and hornblende gabbro) display field, petrographic, and geochemical features of cumulates and can be explained by the accumulation of hornblende + plagioclase from an andesitic parental magma. The intermediate-felsic rocks (including quartz diorites and granodiorites) show adakitic features and are most likely related to melts that experienced high-pressure hornblende fractionation followed by plagioclase accumulation. Hornblende thermobarometer reveals that these ultramafic-mafic and intermediate-felsic rocks formed at 837–907 °C and 6.6–11.1 kbar, indicating a rapid decompression condition that is most likely related to an extension setting. The data reported in this study and the literature reveal that the Early Cretaceous magmatism was active in the southern margin of the Gangdese arc, suggesting that the Neo-Tethyan oceanic lithosphere most likely experienced a steep, rather than a shallow subduction beneath the Lhasa Terrane during the Early Cretaceous.

Mississippian olistostromes of Iberia revisited: tectonic drivers of synorogenic carbonate platform/reef destruction

This paper reviews the synorogenic basins formed on the Gondwana side of the Variscan Orogen of Iberia, highlighting the widespread occurrence of calciturbiditic formations and olistostromes containing reef limestone olistoliths in Iberia's Variscan basins. Using key examples from the Variscan Orogen for comparison (the Azrou–Khenifra and Rhenohercynian basins), the significance of these olistostromes and flyschoid deposits is discussed. Our tectonic models of the Variscan belt in Iberia propose possible drivers of synorogenic carbonate platform/reef destruction responsible for the origin of calciturbidites and olistostromes. One model proposes the formation of an orogenic plateau via the lateral flow of partially molten orogenic roots in the context of Laurussia–Gondwana convergence following the destruction of the Rheic Ocean and slab retreat of the Gondwana (lower) plate. An alternative model invokes subduction of Palaeotethys oceanic lithosphere beneath the Gondwana (upper) plate. Both emphasize the Mississippian occurrence of a significant thermal anomaly beneath Gondwana that favoured strong lithosphere thinning, creating ideal conditions for synorogenic carbonate platform/reef destruction and the formation of calciturbidite deposits and olistostromes in Iberia. The Variscan palaeotopography would look similar in both situations. Distinguishing between these models is therefore not straightforward, with differences in the kinematics of regional tectonic transport in the superstructure of Mississippian gneiss domes. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Cretaceous magmatism in the northern Lhasa Terrane: Implications for the tectonic evolution and crustal growth tempos of central Tibet

The subduction and demise of an ocean plate are generally recognized as essential processes that result in the reworking and maturation of the continental crust. The northern Lhasa Terrane in central Tibet represents the forefront of the Lhasa-Qiangtang collision belt following the closure of the Bangong-Nujiang Ocean. Thus, it is a pivotal location to study the transition processes from oceanic lithosphere subduction to continental collision as well as pertinent crustal growth mechanisms. Here, we present zircon U-Pb dating, whole-rock major and trace element and Sr-Nd isotope, and zircon Hf isotope and trace element data of the Mendang igneous complex, Baingoin County, northern Lhasa Terrane. Geochronological results show that the granodiorites, trachydacites, and rhyolites in the Mendang igneous complex formed at ca. 122−116 Ma, 97 Ma, and 73 Ma, respectively. The Early Cretaceous granodiorite samples are peraluminous with high SiO2, Al2O3, and K2O contents, and moderate A/CNK (molar ratio of Al2O3/[CaO + Na2O + K2O]) values, which are similar to those of typical felsic peraluminous I-type granites. The granodiorites are characterized by enrichment in light rare earth elements and large ion lithophile elements (e.g., Rb) and depletion in high field strength elements (e.g., Nb, Ta). They also show the most enriched whole-rock Sr-Nd [(87Sr/86Sr)i = 0.7072−0.7078; εNd(t) = −7.60 to −5.08] and zircon Hf [εHf(t) = −4.46 to +1.02] isotope compositions, indicating that the Early Cretaceous granodiorites were likely derived from an ancient basement under a subduction setting. The trachydacites have uniform SiO2, high Al2O3, Sr contents, and Sr/Y values, and low Y and Yb contents, belonging to adakitic rocks. They show more depleted whole-rock Sr-Nd [(87Sr/86Sr)i = 0.7065−0.7066; εNd(t) = −0.56 to −0.22] and zircon Hf [εHf(t) = 4.36−7.84] isotopes than the granodiorites, suggesting that the trachydacites may have generated from partial melting of the juvenile thickened lower continental crust. The rhyolites have the highest SiO2 and K2O contents in the Mendang igneous complex, and significant depletion of Ba, Sr, Eu, P, and Ti. They have slighter more enriched whole-rock Nd [εNd(t) = −3.71 to −1.16] and zircon Hf [εHf(t) = 1.03−4.31] isotope compositions than the trachydacites. These features suggest that the rhyolites were highly fractionated products of the crustal melts. Whole-rock Sr-Nd and zircon Hf isotopes of the Mendang igneous complex show a kink trend from enrichment to depletion and then transfer to enrichment again, signifying an increased contribution of juvenile materials in the northern Lhasa Terrane toward progressively replacing the ancient lower crust and accumulating to newly formed crust. The estimated crustal thickness beneath the northern Lhasa Terrane shows a sharp increase from the Early to Late Cretaceous and peak at ca. 97 Ma, whereas it largely decreases in the Late Cretaceous. Integrated with previous studies, we propose that the formation of the Mendang igneous complex (122−73 Ma) elaborately documents the regional tectonic transition from oceanic lithosphere subduction to demise as well as continental crustal differentiation and maturation. The rollback and breakoff of the southward subducted Bangong-Nujiang oceanic slab in the Early Cretaceous initiated diverse magmatism in the northern Lhasa Terrane. During the early Late Cretaceous, the widespread adakitic and Mg-rich magmatism was attributed to the delamination of the thickened lithosphere following the final amalgamation of the Lhasa and Qiangtang Terranes. In the late Late Cretaceous, post-collisional extension induced the formation of the rhyolites in the northern Lhasa Terrane. The growth and destruction of the continental crust had certain tempos from the oceanic lithosphere subduction to continental post-collision stage.

Review of temporal trend in Archean granitoid chemistry- implications for Eoarchean (3.7–3.9 Ga) transition in geodynamic regime of crust formation

In this study, we review changes in Archean granitoid chemistry using stacked probability density estimation and statistical change-point analyses. A significant change in Archean sodic granitoid composition occurs in the 3.9–3.75 Ga period, marked by a shift of SiO2, Al2O3, Na2O, V, Cr, Sr, Sr/Y to higher, and TiO2, FeOT, MnO, P2O5, A/NK, A/CNK, Sc, Ba, Ta, Nb, Hf, Y, and rare earth elements to lower values, which reflects different formation P-T regime and petrogenetic processes. Using phase equilibria and trace element modelling, we demonstrate that the pre-3.9 Ga Acasta Tonalite Gneiss (ATG)-like granitoids formed at shallow depths (∼3 km) but are chemically distinct from impactites of all known large impact craters with basaltic/mixed target rocks. Impact melting produces MgO-rich melts at relatively greater “apparent” depths, reflecting target rock chemistry and contribution from the upwelling mantle, and is unlikely to have produced the shallow-seated ATG rocks. The ATG trace element chemistry is similar to many modern-day non-arc settings felsic rocks, which suggests that the melting regime in these extensional settings also existed before 3.9 Ga and ATG formed in a mantle plume-driven extensional tectonic regime involving melting of long-lived mafic proto-crust at shallow crustal depths above mantle upwelling. In contrast, the 3.9–3.75 Ga transitional and post-3.75 Ga TTGs formed by deep-seated (25–50 km) melting in equilibrium with garnet-bearing amphibolite. When considered together with Ti isotopic evidence of shift from tholeiitic to calc-alkaline magmatism in the 4.0–3.75 Ga period, the evidence for a transition from a regime dominated by crustal reworking to one of significant juvenile input at 3.60–3.85 Ga, noted in trace element/Hf-isotope composition of detrital zircon from several cratons and the reported ultra-high pressure metamorphism/interleaving arc-plume sequence, require a greater depth of melting. We interpret this transition to mark a gradual shift from stagnant-lid to intermittent mobile-lid-type plate tectonics involving deeper subduction of oceanic lithosphere. However, from the single occurrence of pre-3.9 Ga ATG rocks and their similarity with some Archean Fe-rich silica-saturated rocks chemistry, it is unknown whether plume magmatism was the only active mechanism or operated in conjunction with mobile-lid tectonics on early Earth, like in Archean.

Insights From Layered Anisotropy Beneath Southern New England: From Ancient Tectonism to Present‐Day Mantle Flow

AbstractSeismic anisotropy beneath eastern North America, as expressed in shear wave splitting observations, has been attributed to plate motion‐parallel shear in the asthenosphere, resulting in fast axes aligned with the plate motion. However, deviations of fast axes from plate motion directions are observed near major tectonic boundaries of the Appalachians, indicating contributions from lithospheric anisotropy associated with past tectonic processes. In this study, we conduct anisotropic receiver function (RF) analysis using data from a dense seismic array traversing the New England Appalachians in Connecticut to examine anisotropic layers in the crust and upper mantle and correlate them with past tectonic processes as well as present‐day mantle flow. We use the harmonic decomposition method to separate directionally‐dependent variations of RFs and focus on features with the same harmonic signals observed across multiple stations. Within the crust, there are multiple features that may be correlated with stratification in the Hartford Basin, faults in the Taconic thrust belt, shear zones formed during Salinic/Acadian terrane accretion events, and orogen‐parallel crustal flow in the Acadian orogenic plateau. We apply a Bayesian inversion method to obtain quantitative constraints on the direction and strength of intra‐crustal anisotropy beneath the Hartford Basin. In the upper mantle, we identify a fossil shear zone possibly formed during oblique subduction of Rheic Ocean lithosphere. We also find evidence for a plate motion‐parallel flow zone in the asthenosphere that is likely disturbed by mantle upwelling near the southern margin of the Northern Appalachian Anomaly in the eastern part of the study area.

Chlorine isotope evidence for Farallon-derived metasomatism of the North American lithospheric mantle

Metasomatism of the sub-continental lithospheric mantle (SCLM) can result in enrichment in volatile elements introduced via subduction of seawater-altered oceanic lithosphere. Subduction-related metasomatism can introduce chlorine isotope heterogeneity to the SCLM inherited from the subducted source, which can be used to characterize the source of metasomatism. This study presents Cl isotope compositions (δ37Cl) of variably metasomatized continental lithospheric mantle xenoliths from the Colorado Plateau and southern Rio Grande Rift (southwestern United States). Fertile mantle xenoliths that represent asthenosphere that has recently been emplaced at the base of the North American lithosphere have δ37Cl values of 0.0 ‰ to +0.1 ‰, suggesting that the convecting mantle has an average δ37Cl value similar to seawater and chondrites. In contrast, metasomatized xenoliths span a range of δ37Cl values from +0.1 ‰ to +1.9 ‰, extending above the average convecting mantle value. The range in δ37Cl values can be explained by mixing between a subduction-related fluid with δ37Cl values between +1.0 ‰ and +1.9 ‰ and a shallow crustal fluid with a δ37Cl value less than +0.3 ‰. The most likely source of the high δ37Cl subduction-related signature is the lower crust in the subducted Farallon plate. Lower crustal Cl likely entered the SCLM either through direct dehydration of lawsonite ± phengite in the deeply subducted Farallon plate or from a mixed serpentinite-oceanic crust source in the subducted Farallon. Regardless of the mechanism, this study provides evidence for subduction of crustal Cl into the mantle, which can introduce isotopic heterogeneity into the SCLM and has implications for the volatile cycling in the HIMU mantle and SCLM.

Record of an oceanic island in the Neo-Tethys Ocean (lowermost Cretaceous Xiazha Formation, Yarlung-Zangbo suture zone, South Tibet)

The Xiazha Formation recently identified in the Yarlung-Zangbo suture zone of southern Tibet has been considered as crucial evidence for early (pre-Cretaceous) subduction of Neo-Tethyan oceanic lithosphere. In this study, we carried out an integrated stratigraphic, petrographic, and geochemical study of the Xiazha Formation, which consists of abyssal chert, siliceous mudrocks, and calcareous turbidites deposited onto basaltic basement. Basalt geochemistry indicates alkali to tholeiitic magmas generated in a within-plate oceanic setting. The siliciclastic fraction contained in calcareous turbidites consists of plagioclase and mafic-rock fragments derived from the underlying basalts, and includes Cr-spinel grains with relatively high TiO2 and moderate Al2O3 concentrations pointing to an OIB source. Based on these data, and virtual lack of detritus of continental origin, we infer that the Xiazha Formation was deposited in oceanic island/plateau within the Neo-Tethyan domain. Its stratigraphy records an evolution from basaltic eruptions to the development of a carbonate platform surrounding the basaltic island, and finally to drowning of the atoll. The Xiazha Formation was not deposited in a subduction-related basin and, therefore, cannot be used as evidence for pre-Cretaceous initiation of the Yarlung subduction zone.