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The initiation of subduction remains an enigmatic process and a variety of conceptual models has been proposed to explain such initiation. Conceptual models have been tested with geodynamic models and have been applied to various subduction settings around the globe. None of these tested models, however, are applicable to the Scotia subduction zone in the Southern Atlantic (also referred to as South Sandwich subduction zone), where subduction started in the Late Cretaceous/Early Cenozoic in a pristine ocean basin setting devoid of other subduction/collision zones. How this subduction zone initiated remains intensely debated, as exemplified by the variability of published plate tectonic reconstructions. We present new tectonic reconstructions of the Scotia region involving a relatively simple middle-Late Cretaceous plate boundary configuration that involves a new mechanism of subduction initiation, Subduction Invasion Polarity Switch (SIPS). SIPS involves a long-lived, wide and deep subduction zone (South American-Antarctic subduction zone) that imposes major horizontal trench-normal compressive deviatoric stresses on the overriding plate. The overriding plate consists of a narrow continental lithospheric (land) bridge at the trench (Cretaceous-Early Cenozoic Antarctica-South America land bridge) with oceanic lithosphere behind it (Weddell Sea-Atlantic Ocean). The stresses cause shortening and thrusting at the continent-ocean boundary in the backarc region of the overriding plate, forcing oceanic lithosphere under continental lithosphere, starting the subduction initiation process, and eventually leading to a new, self-sustaining, subduction zone (Scotia subduction zone) with an opposite polarity (dipping westward) compared to the long-lived subduction zone (dipping eastward). The model thus involves invasion of a new subduction zone into a pristine ocean basin (Atlantic Ocean), with the primary driver being a long-lived subduction zone in another ocean basin (Pacific Ocean). To test the physical viability of the SIPS model, we have conducted numerical geodynamic simulations of buoyancy-driven subduction. Numerical results demonstrate that the SIPS model is viable, with compressive stresses in the overriding plate resulting from strong trenchward basal drag induced by subduction-driven whole-mantle poloidal return flow and compression at the subduction zone plate boundary due to the high resistance of the subduction zone hinge of the long-lived subduction zone to retreat westward. Subduction initiation starts in the overriding plate after ~100 Myr of long-lived subduction, eventually resulting in the formation of a new, opposite-dipping, subduction zone. Notably, this new subduction zone develops at the continent-ocean boundary for models without and with a pre-imposed weak zone. Apart from the Scotia Sea region, the SIPS model might also be applicable to subduction initiation that has occurred elsewhere in the geological past (e.g. the New Caledonia, Lesser Antilles-Puerto Rico, Rocas Verdes and Arperos subduction zones), and that is presently in a very early stage of development in the Japan Sea.

The Oceanographer Transform Fault is an oceanic transform fault that offsets a segment of the Atlantic Mid-Oceanic Ridge (MOR), southwest of the Azores. We investigate how deformation is accommodated along an active transform through the interpretation of fault patterns and geomorphological features on high resolution bathymetry and a petrological and kinematic analysis of thin sections.The bathymetric interpretation yielded six different domains which consisted of 1) the main transform zone with E-W running strike-slip faults, 2) the NNE-SSW oriented MOR valley, 3) the abyssal domain hosting NNE-SSW oriented normal faults that bound the abyssal hills, 4) the abyssal domain hosting NE-SW oriented faults, oriented obliquely to the mid-oceanic ridge and the main transform valley, 5) a volcano- and lava flow rich domain and 6) a shallow domain with corrugations oriented perpendicular to the MOR with little volcanic cover.The thin section analysis reveals a complete ophiolitic sequence, including serpentinized peridotite, gabbro and basalt with varying degrees of alteration. Samples retrieved from depths >3500 m show that deformation occurs mainly in the ductile domain through bulging and sub-grain rotation of plagioclase, lamellar feldspar formation (in gabbro), shearing and recrystallisation of gabbro and serpentinization of peridotite. Brittle deformation manifests itself through fracturing of crystals, displacement of plagioclase sub-crystal domains and veining. Especially gabbroic samples show a decrease in serpentinized veins with decreasing depth. Basalts are found only at shallow depth, seemingly covering gabbro, appearing not to be affected by deformation at all, only occasional cracks filled with pristine calcite are observed.The combination of geomorphological features identified on high-resolution bathymetry maps and the petrological and kinematic analysis of thin sections showed that deformation along the transform fault differs from the deformation that happens at the MOR. Deformation at the MOR is characterized by 1) axis-parallel normal faulting, pulses of volcanism, resulting in elongated ridges and volcanic cones on the ocean floor and the formation of dykes under magma-rich circumstances, and core complex exhumation during magma-starved periods that occurred between 1.8 – 4.2 Ma and around 7.5 Ma along the southwestern MOR segment of the OTF and 2) heavily sheared zones that extend obliquely from the MOR-transform intersection into the adjacent older plate. Deformation at the transform fault is accommodated through serpentinization at depths deeper than 3000 m, leading to pop-up structures in the main transform zone and causing fracturing in the overlying gabbro, allowing hydrothermal fluids to heavily alter deeper rocks and migrate to shallower depths with decreasing alteration of the oceanic crust with decreasing depth.We hypothesize that the transform fault itself at depth accommodates stresses to a large extent via serpentinization processes in response to strike-slip tectonic activity in a very narrow band in the active, deepest part of the main transform zone. Deformation patterns other than serpentinization and serpentinite veining that are observed in rock samples along the transform fault are the result of earlier tectonic activity that took place during or shortly after the formation of the rock at the MOR.

Although the Geosciences remain one of the least diverse scientific communities, we need more quantitative data to capture how homogeneous or diverse the community actually is. It is also unclear how this non-diverseness translates into workplace safety. Unsafe working conditions are frequently reported in mainstream media, but it remains difficult to develop targeted and effective solutions without knowing who is affected. To obtain data on how different members of the geoscience community experience workplace environments, we released an anonymous survey which can be accessed via:[https://qfreeaccountssjc1.az1.qualtrics.com/jfe/form/SV_6LLqSaXRyLZ3yZg]. The survey interrogates topics affecting workplace safety, such as sexual harassment, discrimination, (un)equal gender treatment. It also includes recommendations and strategies to improve overall workplace safety. Initial findings show that around 40-50% of respondents (n=78) have sometimes experienced a) disrespectful comments or actions, b) people questioning the respondents’ professional expertise, and c) sexist or racist language in their workplace. Such experiences predominantly caused about 40% of total respondents to consider leaving their institutions or changing careers. Our survey also showed that, only around 18% of respondents feel supported by their institutional administrations to report an incident, or trust the reporting system to be fair and unbiased. This preliminary outcome means that there is a major task at hand at the institutional level to transform current working environments into a safe space where geoscientists can thrive. The updated results and insights from this survey will be presented at the EGU General Assembly in 2023.

Recent studies on oceanic transform faults, one of the three fundamental types of plate boundaries, has suggested that they may not be purely conservative features and that the crust formed adjacent to them (on the "inside corners" of the ridge-transform intersection) may differ in structure and composition significantly from outside-corner crust. Here we present a geological map of the Oceanographer Transform (Atlantic Ocean, southwest of the Azores) created by combining an interpretation of multibeam bathymetry, rock sampling and seafloor visual observations. We find that outside- and inside-corner crust at the ridge transform intersection have distinctive morphologies and petrography: the outside corner shows rough seafloor, from which only pillow basalts are recovered, extending all the way to the fracture zone. The inside corners, in contrast, are characterized by both rough, basaltic seafloor and regions that are much smoother, from which serpentinized peridotite are often recovered. The width of the inside-corner region showing this variable morphology, bathymetry and petrography seems to vary over time from 10 to 25 km. In two places, oceanic core complex crust is recognized close to the transform in this inside-corner region. We emphasize that plate production at the inside corner appears to occur via a variety of magmatic and amagmatic processes.

Subduction zones and their associated slabs are the main drivers of plate tectonics and mantle flow, but how these zones initiate remains enigmatic. In the Scotia Sea region, subduction started in the Late Cretaceous/Early Cenozoic in a pristine ocean basin setting devoid of other subduction/collision zones. How this subduction zone initiated remains intensely debated, as exemplified by the variability of published plate tectonic reconstructions. Despite such variability, several works argue for a subduction initiation mechanism, in which a South America-Antarctica relative plate motion change, in combination with a particular plate boundary geometry in the western Weddell Sea, caused convergence across a transform plate boundary segment that subsequently evolved into a subduction zone. Here we discuss this kinematic model of subduction initiation, and, following geometric and kinematic arguments, highlight several unsolved issues that call for alternative explanations. Furthermore, we present new tectonic reconstructions of the Scotia region involving a simpler middle-Late Cretaceous plate boundary configuration, which avoid the geometric and kinematic problems of earlier reconstructions and that call for a new mechanism of subduction initiation. We refer to this mechanism as Subduction Invasion Polarity Switch (SIPS), which involves a long-lived and wide subduction zone (South American-Antarctic subduction zone) with lower mantle slab penetration, which imposes major horizontal trench-normal compressive deviatoric stresses on the overriding plate. This plate consists of a narrow continental lithospheric (land) bridge at the trench (Cretaceous-Early Cenozoic Antarctica-South America land bridge) with oceanic lithosphere behind it (Weddell Sea-Atlantic Ocean). The stresses cause shortening and thrusting at the continent-ocean boundary in the backarc region of the overriding plate, forcing oceanic lithosphere under continental lithosphere, starting the subduction initiation process, and eventually leading to a new, self-sustaining, subduction zone (Scotia subduction zone) with an opposite polarity compared to the long-lived subduction zone. The model thus involves invasion of a new subduction zone into a pristine ocean basin (Atlantic Ocean), with the primary driver being a long-lived subduction zone in another ocean basin. To test the physical viability of the SIPS model, we have conducted numerical geodynamic simulations of buoyancy-driven subduction. Numerical results demonstrate that the SIPS model is viable, with compressive stresses in the overriding plate resulting from strong trenchward basal drag induced by subduction-driven whole-mantle poloidal return flow and compression at the subduction zone plate boundary. Subduction initiation starts in the overriding plate after ∼100 Myr of long-lived subduction, eventually resulting in the formation of a new, opposite-dipping, subduction zone. This new subduction zone develops at the continent-ocean boundary for models without and with a pre-imposed weak zone. We further propose that the SIPS model might explain subduction initiation elsewhere, including the New Caledonia subduction zone in the Southwest Pacific, the Lesser Antilles-Puerto Rico subduction zone in the Caribbean region, and the subduction zones that consumed the Rocas Verdes and Arperos backarc basins in South America and Central America, respectively. We further postulate that active backarc shortening in the Japan Sea, with eastward under-thrusting of Japan Sea oceanic lithosphere below the Japan arc, represents an early stage of SIPS.

The MagellanPlus workshop “BlackGate” addressed fundamental questions concerning the dynamic evolution of the Mediterranean–Black Sea (MBS) gateway and its palaeoenvironmental consequences. This gateway drives the Miocene–Quaternary circulation patterns in the Black Sea and governs its present status as the world's largest example of marine anoxia. The exchange history of the MBS gateway is poorly constrained because continuous Pliocene–Quaternary deposits are not exposed on land adjacent to the Black Sea or northern Aegean. Gateway exchange is controlled by climatic (glacio-eustatic-driven sea-level fluctuations) and tectonic processes in the catchment as well as tectonic propagation of the North Anatolian Fault Zone (NAFZ) in the gateway area itself. Changes in connectivity trigger dramatic palaeoenvironmental and biotic turnovers in both the Black Sea and Mediterranean domains. Drilling a Messinian to Holocene transect across the MBS gateway will recover high-amplitude records of continent-scale hydrological changes during glacial–interglacial cycles and allow us to reconstruct marine and freshwater fluxes, biological turnover events, deep biospheric processes, subsurface gradients in primary sedimentary properties, patterns and processes controlling anoxia, chemical perturbations and carbon cycling, growth and propagation of the NAFZ, the timing of land bridges for Africa and/or Asia–Europe mammal migration, and the presence or absence of water exchange during the Messinian salinity crisis. During thorough discussions at the workshop, three key sites were selected for potential drilling using a mission-specific platform (MSP): one on the Turkish margin of the Black Sea (Arkhangelsky Ridge, 400 m b.s.f., metres below the seafloor), one on the southern margin of the Sea of Marmara (North İmrali Basin, 750 m b.s.f.), and one in the Aegean (North Aegean Trough, 650 m b.s.f.). All sites target Quaternary oxic–anoxic marl–sapropel cycles. Plans include recovery of Pliocene lacustrine sediments and mixed marine–brackish Miocene sediments from the Black Sea and the Aegean. MSP drilling is required because the JOIDES Resolution cannot pass under the Bosporus bridges. The wider goals are in line with the aims and scope of the International Ocean Discovery Program (IODP) “2050 Science Framework: Exploring Earth by Scientific Ocean Drilling” and relate specifically to the strategic objectives “Earth's climate system”, “Tipping points in Earth's history”, and “Natural hazards impacting society”.

The MagellanPlus workshop "BlackGate"addressed fundamental questions concerning the dynamic evolution of the Mediterranean-Black Sea (MBS) gateway and its palaeoenvironmental consequences. This gateway drives the Miocene-Quaternary circulation patterns in the Black Sea and governs its present status as the world's largest example of marine anoxia. The exchange history of the MBS gateway is poorly constrained because continuous Pliocene-Quaternary deposits are not exposed on land adjacent to the Black Sea or northern Aegean. Gateway exchange is controlled by climatic (glacio-eustatic-driven sea-level fluctuations) and tectonic processes in the catchment as well as tectonic propagation of the North Anatolian Fault Zone (NAFZ) in the gateway area itself. Changes in connectivity trigger dramatic palaeoenvironmental and biotic turnovers in both the Black Sea and Mediterranean domains. Drilling a Messinian to Holocene transect across the MBS gateway will recover high-amplitude records of continent-scale hydrological changes during glacial-interglacial cycles and allow us to reconstruct marine and freshwater fluxes, biological turnover events, deep biospheric processes, subsurface gradients in primary sedimentary properties, patterns and processes controlling anoxia, chemical perturbations and carbon cycling, growth and propagation of the NAFZ, the timing of land bridges for Africa and/or Asia-Europe mammal migration, and the presence or absence of water exchange during the Messinian salinity crisis. During thorough discussions at the workshop, three key sites were selected for potential drilling using a mission-specific platform (MSP): one on the Turkish margin of the Black Sea (Arkhangelsky Ridge, 400mb.s.f., metres below the seafloor), one on the southern margin of the Sea of Marmara (North Imrali Basin, 750mb.s.f.), and one in the Aegean (North Aegean Trough, 650mb.s.f.). All sites target Quaternary oxic-anoxic marl-sapropel cycles. Plans include recovery of Pliocene lacustrine sediments and mixed marine-brackish Miocene sediments from the Black Sea and the Aegean. MSP drilling is required because the JOIDES Resolution cannot pass under the Bosporus bridges. The wider goals are in line with the aims and scope of the International Ocean Discovery Program (IODP) "2050 Science Framework: Exploring Earth by Scientific Ocean Drilling"and relate specifically to the strategic objectives "Earth's climate system", "Tipping points in Earth's history", and "Natural hazards impacting society".

The Aravalli-Delhi Mobile Belt (ADMB) in the northwestern part of the Indian Shield represents the final stage of a complex tectonic evolution witnessed by the recognition of three distinct orogenies that have resulted in northwestward accretion of the terranes belonging to Archaean to Neoproterozoic ages. In this contribution, a review of the deformation structures of different terranes is discussed with their tectonic implications and global correlation with other supercontinent assemblies. In the west, the NE-SW trending Neoproterozoic South Delhi terrane is marked by coaxial folding between DF1 and DF2 along the NE-SW axis and cross folded by DF3 folds in the NW-SE axis. Several meso- to large-scale DF2 thrusts and DF4 fractures occur in the belt, that acted as channels for the exhumation of granulite and basement gneisses. Excess shortening led to orogen parallel extension and lateral escape of the material that reactivated the DF2 thrusts as strike-slip faults. Based on the ages of syn-DF1 granite gneisses, DF4 fractures, the South Delhi orogeny has been constrained between 0.87 and 0.6 Ga. The Paleoproterozoic North Delhi Terrane is marked by a coaxial folding between NF1 and NF2 folds and later cross-folded by NW-SE trending NF3 folds, producing dome- and basin-structures. Age of syntectonic granite and late-stage metamorphism constrain the north Delhi orogeny between 1.8 and 0.96 Ga. The Paleoproterozoic Aravalli Terrane is divided into a shallow-marine eastern and deeper marine western part by the Rakhabdev suture zone. The entire assemblage of terranes was folded by NE-SW isoclinal and recumbent AF1 folds which, with progressive deformation, were reoriented with a E-W axial trend. The AF2 is upright and NE-SW trending. The AF3 folds are E-W to NW-SE trending and have produced type 1 and type 2 interference patterns, with AF2 and AF1 respectively. Age of syn-AF1 migmatisation in the northern part and syn-AF3 granites in the south constrain the Aravalli orogeny between 1.7 and 0.96 Ga, coeval with the North Delhi orogeny. The granulite and charnockite were tectonically emplaced within the Sandmata Complex during the Aravalli orogeny. The Archean Bhilwara terrane, produced from the Bhilwara orogeny, marks the stabilisation of the crust in NW India by the intrusion of Berach and equivalent granites at 2.6 Ga. The terrane is divided into the Sandmata and Mangalwar complexes that consist of migmatite gneisses with slivers of greenstone. Several Neoarchean to Paleoproterozoic volcano-sedimentary schist belts were tectonically interlaced within the Mangalwar Complex. The migmatitic rocks of the terrane show flow folding in various directions while the schist belts are characterized by extremely appressed NE-SW trending reclined folds (BF1 and BF2), inverted BF2 folds, E-W open BF3 folds, and multiple strike-slip shear zones and thrusts. The ADMB exhibits a syntaxial bend in the eastern part attributed to indentation tectonics by Berach granite during syn-South Delhi orogeny. The Aravalli orogeny can be correlatable with Nuna orogeny, whereas the South Delhi orogeny can be correlated with the Pan-African orogeny that gave rise to Columbia and Gondwanaland Supercontinent assembly. The Grenville orogeny has experienced thermal rejuvenation in the Aravalli and Bhilwara terranes.

Although many different mechanisms for subduction initiation have been proposed, only few of them are viable in terms of consistency with observations and reproducibility in numerical experiments. In particular, it has recently been demonstrated that intra‐oceanic subduction triggered by an upwelling mantle plume could greatly contribute to the onset and operation of plate tectonics in the early and, to a lesser degree, modern Earth. On the contrary, the initiation of intra‐continental subduction still remains underappreciated. Here we provide an overview of 1) observational evidence for upwelling of hot mantle material flanked by downgoing proto‐slabs of sinking continental mantle lithosphere, and 2) previously published and new numerical models of plume‐induced subduction initiation. Numerical modeling shows that under the condition of a sufficiently thick (>100 km) continental plate, incipient downthrusting at the level of the lowermost lithospheric mantle can be triggered by plume anomalies of moderate temperatures and without significant strain‐ and/or melt‐related weakening of overlying rocks. This finding is in contrast with the requirements for plume‐induced subduction initiation within oceanic or thinner continental lithosphere. As a result, plume‐lithosphere interactions within continental interiors of Paleozoic‐Proterozoic‐(Archean) platforms are the least demanding (and thus potentially very common) mechanism for initiation of subduction‐like foundering in the Phanerozoic Earth. Our findings are supported by a growing body of new geophysical data collected in various intra‐continental areas. A better understanding of the role of intra‐continental mantle downthrusting and foundering in global plate tectonics and, particularly, in the initiation of “classic” ocean‐continent subduction will benefit from more detailed follow‐up investigations. ISSN:1525-2027

The exhumation of the deep crustal rocks through brittle crust by extensional tectonics is recognized in orogens of all ages. Paleostress analysis are generally used to understand the brittle exhumation process. In this study, we reconstructed the paleostress of faults of the Neoproterozoic Ambaji Granulite, South Delhi Terrane of the Aravalli-Delhi Mobile Belt, NW India by analyzing the fault slip data in Win Tensor program. Several NE-SW and WNW-ESE faults have been mapped in the area and found to be normal faults with a few strike-slip faults. The strike-slip faults are pre-kinematic to normal faults. Tensor solutions for 237 fault slip data points estimate WNW-ESE extension for strike slip faults and NW-SE direction extension for normal faults. From these results, we interpret that the NE-SW striking, orogen-parallel normal faults were produced from a NW-SE directed extensional stress and are primarily responsible for brittle exhumation of the granulite through crustal extension and thinning at 764–650 Ma. This is comparable to earlier studies on brittle exhumation along the Southern Tibet detachment in the Higher Himalayas. On a more regional scale, our results are in agreement with the extensional tectonics that affected the entire Aravalli-Delhi Mobile Belt and adjoining continents of the Neoproterozoic-Cambrian Gondwanaland Supercontinent.

The exhumation of the Neoproterozoic Ambaji granulite in the Aravalli-Delhi mobile belt, NW India, took place along NNW-SSE trending D2-shear zones. The shear zones evolved from a high temperature (>700 °C) thrust-slip shearing event in the lower-middle crust to a low temperature (450 °C) retrograde sinistral top-to-NW shearing event at the brittle-ductile-transition (BDT). The vorticity of flow (Wm) along the shear zones is estimated with the Rigid Grain Net and strain ratio/orientation techniques. The Wm estimates of 0.32–0.40 and 0.60 coincide with the high temperature event and suggests pure shear dominated deformation. The low temperature phase coincides with Wm estimates of 0.64–0.87 and ~1.0 implying two flow regimes. The shear zone was first affected by general non-coaxial deformation and gradually became dominated by simple shearing. We interpreted that the high temperature event happened in a compressive tectonic regime which led to horizontal shortening and vertical displacement of the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate a non-steady strain during exhumation of granulite. This tectonic evolution is comparable with that of the Himalayas.