Intra-oceanic Subduction Initiation Research Articles

Detachment Fault‐Hosted Subduction Re‐Initiation of the (Ultra)Slow‐Spreading Western Neo‐Tethys in the Jurassic

AbstractSubduction initiation in oceans is key to understanding regional and global plate tectonics and ocean basin dynamics; however, its genetic mechanism is still enigmatic. The most famous model that predicts intraoceanic subduction initiation along transform faults or fracture zones (i.e., the Subduction Initiation Rule) has been widely used to explain arc‐like geochemical signatures within the Neo‐Tethyan ophiolites, but fails to account for the speculation and/or calculation of a sub‐parallel relationship between the Jurassic subduction zones and paleo ridges of the western Neo‐Tethys. Here, we propose a ridge‐parallel detachment fault‐hosted subduction re‐initiation model for the Jurassic western Neo‐Tethys. Based on field geological and geochemical evidence, we show that the Refahiye ophiolite in the İzmir–Ankara–Erzincan suture of northern Turkey has diagnostic characteristics indicative of (ultra)slow seafloor spreading. An (ultra)slow‐spreading nature also characterizes many Neo‐Tethyan ophiolites from the Alps to Southeast Asia, either at mid‐ocean ridges or in suprasubduction zones. Due to its extremely weak nature, detachment faults were probably a key candidate for the Jurassic intraoceanic subduction re‐initiation of the western Neo‐Tethys in response to far‐field compression after its long‐lived northward subduction. This model is applicable to both the Jurassic and Early Cretaceous ophiolites, but is questionable for the Late Cretaceous ophiolites. Its utility deponds on the kinematics of ancient plate boundaries and compositions of ophiolites. The detachment fault‐hosted subduction re‐initiation model can explain the arc‐like geochemical features of the Jurassic western Neo‐Tethyan ophiolites, but its effect on the structure and component of ophiolites is diverse.

Origin of the Mussau Trench in the Western Pacific: Geochemical and mineralogical constraints from basalts and serpentinized peridotites

The Mussau Trench is apparently an incipient subduction zone. However, petrologic and geochemical evidence for the nature and origin of the Mussau Trench is lacking. We present the first mineralogical and geochemical analyses of basalts and serpentinized peridotites from the Mussau Trench. All sampled Mussau Trench basalts yield MORB-like trace-element patterns and lack the geochemical characteristics of typical arc lavas. The Dupal-type isotopic compositions of the MORB-like lavas confirm that the overriding plate shows more affiliation to the Caroline Plate than to the Pacific Plate. The Mussau Trench could have formed along a pre-existing transform fault in the Caroline Plate. Peridotites from the Mussau Trench are highly serpentinized and can be divided into two groups. Group I peridotites have low MgO/SiO2 ratios and contain talc and quartz, suggesting that they were metasomatized by SiO2-rich fluids. Group II peridotites have higher Al2O3/SiO2 ratios at a given MgO/SiO2 ratio and rarely contain talc or quartz. The peridotites in the Mussau Trench differ from typical fore-arc peridotites in the Mariana Trench; for example, the Mussau peridotites are less depleted in trace elements and have lower spinel Cr# (<0.6) than those from the Mariana Trench, suggesting that they originated in the lithosphere of the Caroline plate. The temperatures (<688 °C) and pressures (<7.3 kbar) estimated using amphibole compositions (dominantly edenite and pargasite) suggest that the peridotites were serpentinized by high-temperature slab-derived fluids. Although the limited plate convergence at the Mussau Trench was insufficient to cause arc magmatism, subducted slab-derived fluids caused serpentinization of the overriding lithospheric mantle in an infant subduction channel. Therefore, the serpentinized peridotites and basalts in the Mussau Trench provide important information on the marks and processes of intra-oceanic subduction initiation.

Formation of the Xigaze Metamorphic Sole under Tibetan continental lithosphere reveals generic characteristics of subduction initiation

Metamorphic soles found under allochthonous oceanic lithosphere, or ophiolites, are interpreted as derived from lower plate oceanic crust material accreted to upper plate mantle during intraoceanic subduction initiation. Their metamorphic evolution is inferred to reflect the thermal structure at the site of subduction nucleation, with granulite-bearing soles linked to initiation at hot spreading centers. Here we present garnet Lu-Hf geochronology for the granulite-bearing sole of the Xigaze ophiolite in South Tibet, whose oceanic crust formed ∼130 Ma through continental forearc extension. Our study shows that sole metamorphism was ongoing by 144 Ma, implying that north-directed subduction began at least 14 million years before oceanic forearc spreading. The upper plate at the time of subduction initiation was thus continental, not oceanic. Our results demonstrate that metamorphic characteristics of soles are independent of the specific tectonic setting at the subduction nucleation site and rather provide generic constraints on the subduction initiation process.

Tectonic evolution of the Meso-Tethys Ocean in the Jurassic: Birth, growth, and demise of a missing intra-oceanic arc

Reconstructing ophiolite ages and tectonic attributes can elucidate ancient ocean tectonic evolution. We present petrological, chronological, geochemical, and isotopic data for Ritu and Dongco ophiolites (Bangong–Nujiang suture zone, Tibetan Plateau). The ∼171 Ma Ritu and ∼169 Ma Dongco diabase are derived from high-temperature, highly depleted mantle metasomatized by subduction fluids; they are similar to fore-arc basalts. The ∼165 Ma Ritu andesites, from enriched mantle metasomatized by subduction fluids and a small proportion of subduction sediment melts, have affinity with the oceanic plateau. The ∼161 Ma Ritu dacites and Dongco granodiorites arose via interaction between subduction sediment melts and the mantle wedge, and ∼158 Ma Dongco granites were derived from subduction oceanic plate metasomatized by subduction sediment melts; they are all similar to arc magmatic rocks. Their high Na2O and low K2O and Th contents, and depleted zircon εHf(t) values, differ from those of southern Qiangtang terrane continental arc magmatic rocks, indicating intra-oceanic origin. The Middle Jurassic diabase and regional contemporaneous fore-arc basalts and boninites likely formed during intra-oceanic subduction initiation. The Middle–Late Jurassic dacites, granodiorites, granites, and regional contemporaneous high-Mg andesites likely formed in a mature intra-oceanic arc. Therefore, the Meso-Tethys Ocean may contain a ∼1000 km intra-oceanic subduction zone with Middle–Late Jurassic mature intra-oceanic arc. Subduction initiation of the intra-oceanic subduction zone likely arose via subduction transference caused by collage of the oceanic plateau and southern Qiangtang terrane at ∼182–177 Ma. At ∼171–155 Ma, the intra-oceanic arc gradually formed fore-arc basalts, boninites, high-Mg andesites, and tholeiitic/calc-alkaline arc rocks, indicating final maturity. The intra-oceanic arc west section potentially collided with the southern Qiangtang terrane ∼155–145 Ma, while the middle section potentially lasted until Early Cretaceous. Our study constrains Meso-Tethys Ocean evolution, providing an important basis for the study of intra-oceanic arcs in orogenic belts.

Mantle-wedge alteration facilitates intra-oceanic subduction initiation along a pre-existing fault zone

Intra-oceanic subduction initiation may occur where a transform fault or a fracture zone is located between the younger and older lithospheres. However, the mechanisms and conditions controlling the initiation of subduction remain debated. Here, we present two-dimensional visco–elasto–plastic numerical models of vertically-forced (or spontaneous) subduction initiation along such a pre-existing fault zone where slab-derived fluids enter the overlying mantle wedge. The models show that just after sinking of the older lithosphere, hydrothermal alteration to form serpentine (antigorite) or talc occurs within a juvenile, thin mantle wedge, resulting in the development of a low-viscosity shear zone (1019–1020 Pa s) at its base. The low-viscosity shear zone acts to decouple the mantle wedge from the slab, thereby facilitating slab subduction. Continued slab subduction generates shallow (<50 km depth) flux melting in the inner part of the mantle wedge at 3–5 Myr after the start of the model run, consistent with the timing of boninitic magmatism after the onset of subduction initiation in the Izu–Bonin–Mariana system. Our models that incorporate water release from the slab require average fracture zone (or transform fault) shear strengths of ≤7 MPa to attain a self-sustaining subduction zone. The fault strengths might be achieved in nature where seawater infiltrates deep into the fault zone.

Geochemistry of Waziristan Ophiolite Complex, Pakistan: Implications for Petrogenesis and Tectonic Setting

The Waziristan ophiolite complex (WOC), a segment of paleo-suture zone between the Indo-Pakistan plate to the south-east and the Afghan microplate to the north-west, is primarily composed of serpentinized dunites and serpentinites after harzburgites, crosscut by pyroxenites. The crustal portion dominantly comprises basalts and dolerites, with less abundant plagiogranites. Whole-rock XRF and trace element ICP-MS analyses were undertaken to determine the degree of melt extraction and subsequent geochemical depletion or enrichment of peridotite protoliths. The investigated peridotites are extremely refractory and show high Mg# values (0.88–0.92), low Al2O3 (0.19–0.96 wt%), total alkali values (0.02 &lt; Na2O + K2O ≤ 0.20 wt%), CaO (0.04–0.73 wt%), and TiO2 (0.001–0.017 wt%), but are rich in Cr (up to 3550 ppm) and Ni (up to 2340 ppm). Bulk REE modelling suggests that Waziristan peridotites underwent a high degree (15%–25% melting) of melt extraction in the closed system of spinel-bearing peridotite facies at shallow depths above a subduction zone and, in their chemistry (Sc, REE, Al2O3, and MgO), resemble Izu-Bonin-Mariana (IBM) forearc peridotites. Their U-shaped REE patterns and trace elements resemble peridotites derived from supra-subduction zone (SSZ) settings, especially a forearc setting, and also reflect metasomatism by slab-derived fluids. The ultramafic rocks preserve depletion in REE content (0.03 &lt; ΣREECN &lt; 0.60), indicating the highly depleted nature of refractory peridotites. The basalts (Mg#, 0.48-0.68) are similar in composition to typical boninite and show low to moderate MgO (6.2–13.0 wt%), low total alkali content (0.01–4.45 wt%) and TiO2 (0.13–0.17 wt%), but are higher in Al2O3 (≈11.9 wt%). They show a sub-alkaline affinity and possess geochemical signatures that are transitional from island arc basalt (IAB) magmas to boninitic magmas due to the changes from an extensional to a compressional regime during the initiation of subduction. These geochemical signatures suggest the formation of basalts from melts, resulting from the re-melting of the depleted mantle during intra-oceanic subduction initiation. The higher Th/Nb and V/Ti ratios of the studied basalts further confirm their generation in an SSZ setting characterized by subduction-derived fluids under higher oxidizing conditions. The mafic-ultramafic rocks of the WOC were, therefore, likely formed during intra-oceanic subduction initiation (forearc spreading) in the SSZ setting; they were exhumed along a thrust fault, and obducted onto the forearc region as result of the collision between the Indian plate and the Afghan microplate.

Effects of Elasticity on Subduction Initiation: Insight From 2‐D Thermomechanical Models

AbstractDespite extensive modeling efforts, the dynamics of subduction initiation (SI), including the role of elasticity, are not fully understood. Using two‐dimensional thermomechanical models with visco‐plastic (VP) and visco‐elasto‐plastic (VEP) rheologies, we systematically investigate the role of elasticity in intraoceanic SI using two model setups: spontaneous initiation without imposed convergence and induced initiation with imposed convergence. In spontaneous models, the overriding plate age of &lt;20 Ma and the subducting plate age of &gt;50 Ma generally lead to vertically driven SI with either rheology, but for a given age contrast, SI is easier to occur with the VEP rheology. In induced model with either rheology, when the two plates are young and have a small age contrast, the resulting SI is horizontally driven, and elasticity does not affect SI significantly, regardless of the convergence rate. However, when the thermal age contrast is large and a convergence rate is relatively low, the SI in induced models is vertically driven and similar to that in the spontaneous models, and the VEP rheology leads to faster SI than the VP rheology. This effect of elasticity becomes smaller with increasing initial horizontal compressional stress but does not become fully negated by the initial stress of &lt;∼50 MPa. Therefore, inclusion of elasticity with reasonable shear modulus and initial stress values results in a weaker slab, making it easier for vertically driven SI to occur when the age contrast is relatively large.

Early Jurassic intra-oceanic subduction initiation along the Bangong Meso-Tethys: Geochemical and geochronological evidence from the Shiquanhe ophiolitic complex, western Tibet

Due to the unclear tectonic setting of the Jurassic ophiolites within the Bangong-Nujiang suture, how and when intra-oceanic subduction initiated within the Bangong Meso-Tethys remain controversial. Based on the new petrological, geochemical, and geochronological data reported in this paper, ∼190–180 Ma MORB-like ophiolites and OIB-like mafic rocks were identified from the Shiquanhe ophiolitic complex in the western Bangong-Nujiang suture. The MORB-like gabbros exhibit strong light rare earth elements (LREEs) and heavy rare earth elements (HFSEs) depletion and were derived from partial melting of a highly refractory depleted mantle source with limited slab-derived fluids. The OIB-like mafic rocks are enriched in LREEs and HFSEs and were originated from the melting of upwelling asthenospheric mantle without subduction additions. The data for the MORB-like ophiolites reported in this paper and previously published data demonstrate the formation of Early Jurassic fore-arc ophiolites in response to the Early Jurassic intra-oceanic subduction initiation within the Bangong-Nujiang suture. The Early Jurassic OIB-like rocks formed in the extensional tectonic setting, together with regional geological evidence, suggest that the Early Jurassic initial subduction event within the Bangong Meso-Tethys resulted in broad upper plate extension. Our study not only reveals the intra-oceanic subduction initiation age of the Bangong Meso-Tethys but also implies that subduction initiation can cause strong extension in a wider region of the upper plate in the Tethys oceanic system, not just in the fore-arc region.

Boninitic blueschists record subduction initiation and subsequent accretion of an arc–forearc in the northeast Proto-Tethys Ocean

Abstract Subduction of oceanic lithosphere is a diagnostic characteristic of plate tectonics. However, the geodynamic processes from initiation to termination of subduction zones remain enigmatic mainly due to the scarcity of appropriate rock records. We report the first discovery of early Paleozoic boninitic blueschists and associated greenschists from the eastern Proto-Tethyan North Qilian orogenic belt, northeastern Tibet, which have geochemical affinities that are typical of forearc boninites and island arc basalts, respectively. The boninitic protoliths of the blueschists record intra-oceanic subduction initiation at ca. 492–488 Ma in the eastern North Qilian arc/forearc–backarc system, whereas peak blueschist facies metamorphism reflects subsequent subduction of the arc/forearc complex to high pressure at ca. 455 Ma. These relations therefore record the life circle of an intra-oceanic subduction zone within the northeastern Proto-Tethys Ocean. The geodynamic evolution provides an early Paleozoic analogue of the early development of the Izu–Bonin–Mariana arc and its later subduction beneath the extant Japanese arc margin. This finding highlights the important role of subduction of former upper plate island arc/forearcs in reducing the likelihood of preservation of initial subduction-related rock records in ancient orogenic belts.

From subduction initiation to hot subduction: Life of a Neoarchean subduction zone from the Dengfeng Greenstone Belt, North China Craton

Abstract We report a spatially and temporally linked arc magmatic association of 2.55–2.50 Ga mid-ocean ridge basalt (MORB)-affinity gabbros, arc-affinity basalts, high-Mg basalts/basaltic andesites (HMBA), Nb-enriched basalts (NEB), and a tonalite-trondhjemite-granodiorite (TTG)-like granodiorite from the Dengfeng greenstone belt (DFGB), North China Craton. These are identical to arc magmatic rock suites from hot subduction in the present plate mosaic, and resemble suites formed during subduction initiation. Laser ablation–inductively coupled plasma–mass spectrometry zircon U-Pb dating indicates that gabbros and granodiorites intruded at ca. 2.55 Ga and the HMBAs formed at ca. 2.50 Ga, indicating a long duration of subduction zone magmatic activity of at least 50 m.y. in the DFGB. The gabbros are characterized by flat light-rare earth element (LREE) patterns without negative Nb and Zr anomalies, showing a MORB-like geochemical affinity. Combined with previously reported normal-MORB-type basalts, we suggest that they may represent magmatic products during intraoceanic subduction initiation where their mantle source was slightly metasomatized by subduction-derived fluids during the initiation stage. The arc-affinity basalts are characterized by slightly enriched LREE patterns with negative Nb and Zr anomalies. The HMBAs have higher contents of MgO, Ni, and Cr and display more fractionated rare earth element (REE) patterns and large negative Nb and Zr anomalies than the arc-affinity basalts. The NEBs are characterized by high absolute contents of Nb (9.9–14 ppm) and high ratios of Nb/Laprimitive mantle (pm) (0.40–0.46) and Nb/Thpm (0.44–0.54), and moderately fractionated REE and minor negative Nb anomalies with no Zr anomalies. One granodiorite sample is characterized by a highly fractionated REE pattern and negative Nb and Zr anomalies. Geochemical modeling suggests that the association of the normal arc-affinity basalt (NAB)-HMBA-NEB can be interpreted to be generated by hybridization of mantle wedge peridotites by slab-derived TTG-like melts. Positive values of whole-rock εNd (t) and zircon εHf (t) of the 2.55–2.50 Ga magmatic associations are consistent with a long-term depleted mantle source for the mafic assemblage. The 2.55–2.50 Ga magmatic association from MORB-affinity gabbros and basalts to arc-affinity NAB-HMBA-NEB may record a continuous Neoarchean geodynamic process from intraoceanic subduction initiation to mature arc magmatism. The NAB-HMBA-NEB association with regional sanukitoids and high-Al TTGs indicates that hot subduction may have played an important role in the production of arc-related magmatism during the Archean.

Refertilization of Mantle Peridotites from the Central Indian Ridge: Response to a Geodynamic Transition

AbstractThe phenomena of reactive percolation of enriched asthenospheric melts and pervasive melt-rock interactions at mid oceanic ridge-rift systems are the principal proponents for mantle refertilization and compositional heterogeneity. This study presents new mineralogical and geochemical data for the abyssal peridotites exposed along the Vema and Vityaz fracture zones of the Central Indian Ridge (CIR) to address factors contributing to the chemical heterogeneity of CIR mantle. Cr-spinel (Cr#: 0.37-0.59) chemistry classifies these rocks as alpine-type peridotites and corroborates a transitional depleted MORB type to enriched, SSZ-related arc-type magma composition. HFSE and REE geochemistry further attests to an enriched intraoceanic forearc mantle affinity. The distinct boninitic signature of these rocks reflected by LREE&amp;gt;MREE&amp;lt;HREE and PGE compositions substantiates refertilization of the CIR mantle harzburgites by boninitic melt percolation concomitant to initiation of oceanic subduction. The mineral chemistry, trace, and PGE signatures of the CIR peridotites envisage (i) replenishment of depleted sub-ridge upper mantle by impregnation of subduction-derived boninitic melts, (ii) tectonic transition from mid oceanic ridge-rift to an embryonic suprasubduction zone, and (iii) initiation of spontaneous intraoceanic subduction along submarine transform faults and fracture zones of slow-spreading CIR owing to the weakness and mechanical instability of older, denser, and negatively buoyant Indian Ocean lithosphere.

The Late Triassic-Jurassic magmatic belt and its implications for the double subduction of the Neo-Tethys Ocean in the southern Lhasa subterrane, Tibet

The southern Lhasa subterrane (SL), located between the Yarlung-Tsangpo Suture Zone and the Luobadui-Milashan Fault, preserves a complete Late Triassic-Jurassic magmatic record related to Neo-Tethys Ocean evolution. Therefore, the SL is key to understanding the geodynamic processes occurring during the controversial early-stage evolution of this ocean. Here, we present new zircon U–Pb data and a detailed synthesis of geochronological, geochemical, and Sr-Nd-Pb-Hf isotope data from Late Triassic-Jurassic igneous rocks in the SL to constrain the early-stage evolution of the Neo-Tethys Ocean. The data suggest that these igneous rocks can be divided into two groups. Group 1 magmatism lasted from ca. 243 Ma to 145 Ma, with three stages at ca. 243–200 Ma, 199–165 Ma, and 164–145 Ma, and is characterized by high positive εHf(t) and εNd(t) values and low (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values. Group 2 magmatism lasted from ca. 225 Ma to 160 Ma, with two stages at ca. 225–190 Ma and 189–160 Ma, and is characterized by lower εHf(t) and εNd(t) values and higher (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values than Group 1. We suggest that the Triassic-Jurassic magmatic belt in the SL was controlled by double subduction; the Group 1 igneous rocks formed in an intraoceanic arc setting, while the Group 2 igneous rocks formed in a continental margin arc setting. An integrated interpretation of magmatism, ophiolites, sedimentary rocks, and metallogenesis within the SL constrains the geodynamic processes that occurred during the early-stage tectonic evolution of the Neo-Tethys Ocean as follows. (1) During the Middle-Late Triassic, the initiation of intraoceanic subduction between Gondwana and the SL produced the ~243–200 Ma magmatism of Group 1. Subsequently, a new continental margin subduction system formed on the southern margin of the SL, which induced the ~225–190 Ma magmatism of Group 2. (2) With ongoing subduction, slab roll-back of the intraoceanic subduction system resulted in the ~199–165 Ma magmatic flare-up of Group 1. Additionally, slab roll-back of the continental margin subduction system resulted in extension of the overriding plate and formed the ~189–160 Ma magmatic flare-up of Group 2. (3) Finally, the slab roll-back associated with intraoceanic subduction transitioned to flat subduction and produced the 164–145 Ma magmatism of Group 1, and porphyry Cu-Au mineralization developed during the stress transition period. In summary, the double subduction system in the early-stage evolution of the Neo-Tethys Ocean controlled the tectonic, magmatic, and metallogenic events in the SL.

Sm-Nd geochronology and petrologic investigation of sub-ophiolite metamorphic sole from the Dinarides (Krivaja-Konjuh, Ophiolite Complex, Bosnia and Herzegovina)

The Dinaridic segment of Neotethys was affected by a widespread shortening and related subduction-accretion-obduction processes that commenced in the middle Jurassic. In the Dinarides, the Krivaja-Konjuh Ophiolite Complex (KKOC) stands as the largest ophiolite complex with a well-exposed metamorphic sole which is the key to understanding the dynamics of intraoceanic subduction initiation in this part of Neotethys. In this contribution we present Sm-Nd geochronology on a granulite facies amphibolite from the KKOC, as well as a detailed petrological description. A five-point isochrone age calculated from clinopyroxene, plagioclase, garnet, amphibole and whole rock is 162 ± 14 Ma (MSWD = 6.2), whereas garnet and whole rock yield 160 ± 7 Ma. Ages calculated from all data points except clinopyroxene are 162 ± 5 Ma (MSWD = 1.09). Petrographic investigations suggest that these ages date granulite facies metamorphic conditions (i.e. peak metamorphism of Grt-Cpx amphibolite) rather than post-peak exhumation or obduction processes. Phase textural relationships are in line with previous research, which indicated a peak metamorphism equilibration pressure and temperature of ~1 GPa and ~800 °C, respectively. Granulite facies conditions are elucidated for an igneous precursor, which underwent a multi-stage metamorphism that gave rise to recrystallization of igneous clinopyroxene and plagioclase, epitaxial growth of amphibole, and garnet blastosis. Taking into account the age of gabbronorite from the youngest segment of the KKOC oceanic crust (Taorcian to Bathonian) and ages of radiolarian assemblages from the KKOC melange (Bajocian to Bathonian), it may be inferred that within ~25 Ma the Dinaridic segment of Neotethys evolved rapidly from active ridge spreading through a stage of intraoceanic subduction and arc magmatism toward sub-ophioliteexhumation and further obduction along the Adria passive margin at the end of the Jurassic era.

Infant intra-oceanic arc magmatism due to initial subduction induced by oceanic plateau accretion: A case study of the Bangong Meso-Tethys, central Tibet, western China

The Meso-Tethyan oceanic plateaus are becoming conspicuous as giant units on the oceanic floor and have played important roles in both continental marginal orogenesis and Tethys oceanic evolution. In this study, we present mineralogical, geochronological, geochemical and Sm–Nd isotopic data for basaltic lavas from the Namco ophiolite and a high-Mg pillow lava–dyke–gabbro association from the Pengco ophiolite in central Tibet. Zircon U–Pb and Ar–Ar dating reveals that the Namco lavas erupted at ∼181 Ma while the Pengco boninitic association formed at ∼164 Ma. The Namco lavas display nearly flat rare-earth element (REE) patterns with no Nb–Ta depletions as well as high εNd values, characteristic of oceanic plateau lava. In contrast, the Pengco high-Mg rocks exhibit low REE concentrations below the normal mid-ocean ridge basalt (N-MORB), ubiquitous Nb–Ta depletions and low εNd values, and the dykes and gabbros are characterized by U-shape REE patterns, indicating that they could have derived from a depleted mantle source that was contaminated by sedimentary flux and marking a mid-Jurassic initial intra-oceanic arc magmatism erupted on the Early Jurassic Meso-Tethyan oceanic plateau represented by the Namco ophiolite. Our Pengco boninitic rocks, along with the literature data, indicate a 167–160 Ma boninitic-like initial intra-oceanic arc within the Bangong Meso-Tethys, running from the Shiquanhe area to the Naqu area with a length of ∼1000 km, which was uniformly built on the Early Jurassic Meso-Tethyan oceanic plateau. Our literature investigation also indicates a ∼175 Ma accretionary orogeny with distinct signature of the oceanic plateau involvements along the southern Qiangtang continental margin, which is manifested by regional metamorphic, magmatic and depositional records. We thus suggest that the accretion of the Early Jurassic Meso-Tethyan oceanic plateau onto the southern Qiangtang continental margin resulted in the extensive orogeny along the continental margin, jammed the subduction zone at ∼175 Ma and induced intra-oceanic subduction initiation as well as the intra-oceanic infant arc magmatism in the Meso-Tethys at ∼164 Ma.

Where does subduction initiate and cease? A global scale perspective

The thermo-mechanical evolution of the Earth's mantle is largely controlled by the dynamics of subduction zones, which connect the surface tectonic plates with the interior. However, little is known about the systematics of where subduction initiates and ceases within the framework of global plate motions and evolving continental configurations. Here, we investigate where new subduction zones preferentially form, and where they endure and cease using statistical analysis of large-scale simulations of mantle convection that feature self-consistent plate–like lithospheric behaviour and continental drift in the spherical annulus geometry. We juxtapose the results of numerical modelling with subduction histories retrieved from plate tectonic reconstruction models and from seismic tomography. Numerical models show that subduction initiation is largely controlled by the strength of the lithosphere and by the length of continental margins (for 2D models, the number of continental margins). Strong lithosphere favours subduction inception in the vicinity of the continents while for weak lithosphere the distribution of subduction initiation follows a random process distribution. Reconstructions suggest that subduction initiation and cessation on Earth is also not randomly distributed within the oceans, and more subduction zones cease in the vicinity of continental margins compared to subduction initiation. Our model results also suggest that intra-oceanic subduction initiation is more prevalent during times of supercontinent assembly (e.g. Pangea) compared to more recent continental dispersal, consistent with recent interpretations of relict slabs in seismic tomography.

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.

Near-ridge initiation of intraoceanic subduction: Effects of inheritance in 3D numerical models of the Wilson Cycle

Despite its global significance, much of the Wilson Cycle, which infers kinematic inversion from plate divergence to convergence, remains conjectural, in particular for what concerns subduction initiation. We present a high-resolution 3D thermomechanical numerical model of the Wilson Cycle, evolving from continental rifting through breakup and oceanic spreading to convergence and self-consistent subduction initiation. In the models, intra-oceanic subduction initiates off-ridge and is an intrinsically three-dimensional process. Its location is controlled by convergence-induced ridge swelling, and the subduction geometry is controlled by three main factors: (1) the inherited compositional and thermal heterogeneity of the ridge and oceanic lithosphere from rifting and seafloor spreading; (2) the curvature/obliquity of the ridge with respect to the convergence direction and (3) the duration of transition from plate divergence to convergence. The modelled mechanisms are consistent with geological records, for example in the Oman subduction-obduction system, and are undervalued elements of the Wilson Cycle.

Timing and mechanism of Bangong-Nujiang ophiolite emplacement in the Gerze area of central Tibet

The Bangong-Nujiang suture zone (BNSZ) separates the Lhasa terrane from the Qiangtang terrane and contains remnants of the Bangong-Nujiang oceanic lithosphere (ophiolites). Despite decades of research, when and how the Bangong-Nujiang ophiolites were emplaced remains enigmatic. In the Gerze area (western segment of the BNSZ), the geochemistry and provenance discrimination of chromian spinels (Cr-spinels) from the pre-collisional subduction complex (Mugagangri Group) and syn-collisional peripheral foreland basin succession (Wuga Formation) can help us solve this fundamental problem in the BNSZ evolution. This study compares the geochemistry of Cr-spinels from the Mugagangri Group and Wuga Formation with those from the Bangong-Nujiang ophiolites. Cr-spinels in the Bangong-Nujiang ophiolites have either low TiO2 (0.01–0.15%) and low Al2O3 (11.74–26.76%), indicating an SSZ peridotite origin, or high Al2O3 (45.28–49.15%), indicating a MORB peridotite origin. Cr-spinels from the ultramafic fragments within the Mugagangri Group have extremely low TiO2 (<0.06%) and geochemically overlap with those from the Dong Co ophiolite, suggesting that these ultramafic fragments were sourced from the Dong Co ophiolite above the subduction zone rather than off-scrapped remnants from the subducting oceanic lithosphere. Compositional fingerprints of detrital Cr-spinels from the Wuga Formation indicate provenance either derived from the Bangong-Nujiang ophiolites or recycled from the Mugagangri Group in the north, with minor input possibly from the Lhasa terrane in the south, consistent with the depositional pattern of a peripheral foreland basin. Provenance data reveals that the Bangong-Nujiang ophiolites in the Gerze area had been emplaced and exposed to erosion during northward oceanic subduction prior to the Lhasa-Qiangtang collision. Contrasting the Tethyan-type Yarlung-Zangbo ophiolites in southern Tibet, the Bangong-Nujiang ophiolites in central Tibet are Cordilleran-type in terms of emplacement mechanism, which were uplifted above sea-level by progressive growth of the subduction complex structurally beneath ophiolite. The emplacement of the Cordilleran-type ophiolites in the western segment of the BNSZ is divided into two stages: (1) intra-oceanic subduction initiation at ~177–179 Ma based mainly on zircon U-Pb dating of plagiogranite from the SSZ-type Laguo Co ophiolite; (2) accretionary emplacement of the ophiolites at ~151–168 Ma constrained by the depositional age of the Mugagangri subduction complex. Final closure of the Bangong-Nujiang Tethyan Ocean may convert the ophiolite emplacement mechanism from “accretionary” to “collisional” at ~150–152 Ma, evidenced by the first development of a peripheral foreland basin.

On the formation of oceanic detachment faults and their influence on intra-oceanic subduction initiation: 3D thermomechanical modeling

Extensional detachment faults, which have been widely documented in slow-spreading and ultraslow-spreading ridges on Earth, can effectively localize deformation due to their weakness. After the onset of oceanic closure, these weak oceanic detachments may directly control the nucleation of a subduction zone parallel to the former mid-ocean ridge, as is suggested for the Neotethys in Middle Jurassic times. So far, this hypothesis has only been tested by 2D numerical models, whereas the geometry of detachment faults is intrinsically three-dimensional. Here, we conducted a series of 3D numerical thermomechanical experiments in order to investigate the formation of detachment faults in slow oceanic spreading systems and their subsequent response upon inversion from oceanic spreading to convergence.Numerical results show that during the oceanic spreading stage, the formation of detachment faults strongly depends on the magnitude of the healing rate of faulted rocks in the oceanic lithosphere, that reflects the stability of hydrated minerals along fractured rocks. The detachment faults formed in our 3D numerical models deviate from the “rolling hinge model” of oceanic detachment faulting where fault footwalls are rotated and oceanic core complexes are thereby formed. Our results accentuate that the controlling physical parameters for the development of oceanic core complexes and detachment faults can differ, and that their coupled development in nature remains a key target for future research.Upon modeled transition to compression, previously formed asymmetric spreading patterns are prone to asymmetric inversion, where one oceanic plate thrusts under the other. Our results suggest that detachment faults accommodate significant amounts of shortening during the initiation of oceanic closure, but, in contrast to the previously proposed simple conceptual model, no direct inversion of a single detachment fault into an incipient subduction zone is observed. Instead, a widespread interaction of multiple detachment faults occurs after the onset of convergence. Ultimately, the nascent subduction zone cuts through the base of several pre-existing detachment faults, thereby forming an initial accretionary wedge in the incipient fore-arc.

Middle Jurassic limestone megabreccia from the southern margin of the Slovenian Basin

The Slovenian Basin is a Mesozoic deep-water paleogeographic unit, located along the border between the eastern Southern Alps and the Dinarides, that records geodynamic signals from the opening of both the Piedmont-Liguria and the Neotethys oceanic domains. In the Middle Jurassic, it was bordered by the Dinaric (Adriatic) Carbonate Platform to the south and the Julian High submarine plateau to the north. The southern margin of the basin is characterized by a several tens of meters thick sedimentary sequence of Bajocian-Bathonian (Callovian?) age that is dominated by limestone megabreccia shed from the Dinaric Carbonate Platform, sedimented by debris-flows in a toe-of-slope sedimentary environment. It is accompanied by rud/grain/packstone beds sedimented via (high-density) turbidity-flows. This megabreccia unit represents the proximal equivalent of the lower resedimented limestones of the Tolmin Formation. The matrix within lithoclasts indicates resedimentation from ooidal shoals and the erosion of basinal and slope sediments. Lithoclasts are of Norian to Lower Jurassic age, and originated from (A) platform margin carbonates, i.e. Triassic marginal reef and Lower Jurassic sand-shoal limestones, (B) deep open-shelf or slope coarse bioclastic limestones, and (C) older basinal rocks. The lithoclast analysis enables the reconstruction of the platform-basin transitional zone that is not preserved (exposed) due to overthrusting. The limestone megabreccia indicates enhanced tectonic activity causing repeated collapse of the platform margin, probably connected to the initiation of intraoceanic subduction within Neotethys followed by ophiolite obduction onto the eastern distal margin of the Adria.