Mid-ocean Ridge Research Articles

Complex Martinique Intermediate‐Depth Earthquake Reactivates Early Atlantic Break‐Up Structures

AbstractEarthquakes that rupture several faults occur frequently within the shallow lithosphere but are rarely observed for intermediate‐depth events (70–300 km). On 29 November 2007, the Mw7.4 Martinique earthquake struck the Lesser Antilles Island Arc near the deep end of the Wadati‐Benioff‐Zone. The sparse regional seismic network of 2007 previously hampered a detailed examination of this unusually complex event. Here, we combine seismic data from different studies with regional moment tensor inversion results and 3D full‐waveform modeling. We show that the earthquake is a doublet consisting of dip‐slip and strike‐slip motion along two oblique structures, both activated under extensional stress along the strike of the slab. Comparison with tectonic reconstructions suggests that the earthquake ruptured along a re‐activated ridge‐transform segment of the subducted Proto‐Caribbean spreading ridge. The unprecedented resolution of the source process highlights the influence of pre‐existing structures on localizing slab deformation also at intermediate‐depth.

On-land manifestation of along-strike transitioning of the Red Sea from continental rifting to sea floor spreading

This study uses Landsat Thematic Mapper (TM), Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM), and aeromagnetic data to examine possible reactivation of the Ediacaran, NE-trending Ad Damm Fault Zone (AFZ) and the Fatima Fault Zone (FFZ) in the western Arabian Shield. Reactivation is by latest Paleogene to Neogene and Quaternary structures of the Red Sea. The AFZ is the tectonic boundary between the Tonian Jeddah terrane in the northwest and the Asir terrane to the southeast of the Arabian Shield. The AFZ and FFZ coincide with possible onshore continuation of a transform fault that separates the sea floor spreading segment of the Red Sea in the south from its transitional segment to the north. It has been suggested that the AFZ is a candidate for the formation of a future onshore transform fault that would result in an eastward jump of the Red Sea sea floor spreading center onto the Arabian Shield. The future spreading center will follow the latest Neogene to Quaternary volcanic rocks (Harrat) that are exposed on the eastern side of the Red Sea. The Landsat TM image and the SRTM DEM show that the AFZ dissects the Red Sea topographic escarpment, and both the AFZ and FFZ control paleo-drainage system (wadis). The aeromagnetic data show that there is a ∼100 km wide zone between the AFZ and FFZ dominated by NE-trending short wavelength magnetic anomalies, possibly related to regional tectonic fabric. Additionally, the aeromagnetic data show that both fault zones dissect NW-trending short wavelength magnetic anomalies, interpreted as latest Paleogene to earliest Neogene dike swarms associated with the opening of the Red Sea. This dissection also includes possible Red Sea related narrow marginal grabens and horsts that were formed onshore and offshore parallel to the Red Sea shoreline. Red Sea-related structure might have reactivated the AFZ and FFZ.

Earthquake Seismicity Reveals the Location and Significance of the Shona Mantle Plume in the South Atlantic Ocean

AbstractThe South Atlantic Ocean hosts several well‐studied volcanic ridges and seamount chains, but the origin of their associated mantle plumes is debated. Reduced seismicity on the southern Mid‐Atlantic Ridge (MAR) suggests anomalously ductile thermomechanical conditions at 52°S and 47.5°S. These low seismicity patches extend 120–560 km along‐axis, and correspond with axial high spreading ridge morphology, geochemical anomalies, and mantle wave speed patterns likely associated with the Shona and Discovery plumes. Bathymetric data show that the northern extent of the Shona swell is associated with increased volcanism, elevated axial bathymetry, and a series of northward‐propagating rifts, with the overall swell geometry suggesting a buoyancy flux of 0.4–0.5 Mg s−1. The nearby Bouvet Island may be a product of a branch of the larger Shona plume swell, which has influenced crustal accretion on the southern MAR for the past 24 million years.

Geochemistry and magmatic petrology of meta-ophiolites from the Bajgan Complex (Makran Accretionary Prism, SE Iran): new insights on the nature of the Early Cretaceous Middle East Neotethys

The Bajgan Complex in the North Makran Domain (Makran Accretionary Prism) comprises disrupted meta-ophiolitic sequences originating from oceanic crust protoliths. They include ultramafic and mafic cumulates, isotropic gabbros, plagiogranites and basalts. Ultramafic–mafic cumulates and plagiogranites exhibit compositions akin to rocks formed in mid-ocean ridge settings. Isotropic gabbro and basalt protoliths can be subdivided into three distinct geochemical types. Type-1 rocks are sub-alkaline (Nb/Y < 0.1) with low Th, Nb and Ta contents and La N /Yb N ratios <1, resembling those of normal mid-ocean ridge basalts (N-MORB). Type-2 rocks display slight enrichment in Th, Ta, Nb (Nb/Y = 0.36–0.45) and La N /Yb N = 2.12–3.20, resembling the chemistry of enriched (E-) MORB. Type-3 basalts show an alkaline nature (Nb/Y = 0.88–1.82), significant Th, Ta and Nb enrichment, and high La N /Yb N ratios (7.01–20.08), resembling the chemistry of alkaline basalts (ocean island basalts, OIB). Petrogenetic modelling indicates that N-MORB protoliths originated from a depleted MORB mantle source, whereas E-MORB and OIB protoliths were generated from partial melting of sub-oceanic depleted sources that underwent varying degrees of OIB-type enrichment. The Bajgan meta-ophiolitic protoliths were formed within a Late Jurassic to Cretaceous oceanic basin influenced by mantle plume activity and plume–ridge interaction. Supplementary material : Field photographs, micrographs, rocks sampled, analytical methods and results, and supplementary tables are available at https://doi.org/10.6084/m9.figshare.c.7193937 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

Geochemical constraints on ridge-plume interaction in the West Philippine Basin

The West Philippine Basin is not only a back-arc basin resulting from the subduction of the paleo-plates, but it has also undergone modification due to intraplate processes. However, the interaction between intraplate processes and back-arc spreading processes remains unclear. In this study, we present geochemical compositions of basaltic lavas from various tectonic sites in the West Philippine Basin. Lavas from the Benham Rise and its adjacent seamount, as well as basement crust samples from the West Philippine Basin, are similar to oceanic island basalts and mid-oceanic ridge basalts, respectively. The lavas from the Central Basin Fault lie between the two aforementioned types. Based on the mixing modeling of geochemical data, morphological structure of the West Philippine Basin, and age data for lavas, we developed a conceptual model for the three-stage tectonic evolution, with the simultaneous influence of back-arc spreading and mantle plume processes.

Melt-Rock Reaction in the Lower Oceanic Crust and the Influence on the Evolution of Mid-Ocean Ridge Basalts at the Central Indian Ridge (7°50′–8°30′S)

Abstract Various crustal processes shape both the lower oceanic crust and mid-ocean ridge basalts (MORBs). To better understand how these crustal processes influence MORB compositions, we conducted comprehensive petrographic and geochemical investigations on gabbroic rocks and erupted lavas dredged from a segment of the Central Indian Ridge (CIR) spanning from 7°50′S to 8°30′S. The petrographic and geochemical analyses of the gabbroic rocks revealed evidence of melt-rock reaction through reactive porous flow in olivine gabbro and gabbro. This process resulted in distinctive features in clinopyroxene, including disequilibrium textures with a troctolite/anorthosite matrix, complex variations in Mg#-Cr-Ti [Mg# = molar Mg/(Mg + Fe2+)] relationships, and considerable enrichment and fractionation of incompatible trace elements. A significant finding of our study is the close resemblance of trace element ratios in MORB and olivine-hosted melt inclusions to those of melts in equilibrium with clinopyroxene from olivine gabbro and gabbro with Sr anomaly (Sr/Sr* = SrN/sqrt[PrN*NdN]; N refers to chondrite-normalized values) greater than ~0.7. This observation strongly indicates that the composition of MORB is influenced by the melt-rock reaction taking place in the lower oceanic crust. Furthermore, our findings suggest that evolved melts in equilibrium with clinopyroxene having Sr/Sr* values lower than ~0.7 are less likely to erupt onto the seafloor and are instead trapped within the lower oceanic crust. Oxide gabbronorite is characterized by coarse-granular, pegmatitic textures and exhibits mineralogically and chemically more evolved characteristics compared to olivine gabbro and gabbro. It is inferred that the oxide gabbronorite formed through the in situ freezing of highly evolved melts within a melt-rich layer. Finally, we present a comprehensive model for melt evolution in the lower oceanic crust at the 7°50′S–8°30′S CIR by integrating all petrological and geochemical data obtained from gabbroic rocks, MORB, and olivine-hosted melt inclusions. This holistic model contributes to a better understanding of the intricate processes governing MORB composition in the context of the lower oceanic crust dynamics at slow-spreading ridges.

Two episodes of Early Palaeozoic high-pressure metamorphism in North Balkhash ophiolite zone (Central Kazakhstan, western Central Asian Orogenic Belt): Evidence for tectonic evolution of Junggar–Balkhash Ocean

The structure of serpentinite mélange of the North Balkhash ophiolite zone (Central Kazakhstan; west Central Asian Orogenic Belt) is ascertained to be high-grade formations assigned to epidote-glaucophane eclogites and garnet blueschists, and their varieties. The rocks follow a clockwise ‘subduction-type’ evolution with the estimated near–peak metamorphic conditions of 1.6–1.9 GPa and 500–560 °C. 40Ar39Ar phengite ages of ∼491 Ma and ∼ 465 Ma obtained for the eclogites and garnet blueschists, respectively, are interpreted to reflect the near-peak to shortly retrograde stages of rock evolution and to record the late Cambrian and Middle Ordovician episodes of high-pressure re-equilibration in the North Balkhash zone. Protoliths of the eclogites were N-MORB-like mafic rocks, which comprised structurally different (from the lower gabbroic to the upper dolerite/basalt) parts of pre-late Cambrian oceanic crust and were formed in the spreading centre setting at the expense of depleted mantle source. Protoliths of the garnet blueschists and associated rocks were represented by volcanogenic–sedimentary, predominantly mafic, complexes (tuffaceous sandstones, greywackes), an accumulation of which and subsequent involvement into subduction occurred at ∼478–465 Ma in the intra-oceanic (fore-arc) setting. Zircon core ages indicate formation of the source of the protoliths of the garnet blueschists occurred in the 509–478 Ma range. A comprehensive correlation of the metamorphic and igneous formations of the North Balkhash zone with those assigned to the adjacent complexes of the southern part of the West Junggar area (NW China) suggested their mutual Early Palaeozoic tectonic evolution within the Junggar–Balkhash Ocean.

Reinterpretation of Marine Magnetic Anomalies in the Bay of Bengal: Implications for Plate Tectonics During the Mid‐Cretaceous

AbstractThe breakup of India from Antarctica in the Early Cretaceous and subsequent seafloor spreading in the NW‐SE direction initiated the formation of the Bay of Bengal and its conjugate Enderby Basin. Two hypotheses are proposed to explain the evolutionary tectonics between India and Antarctica. One involves a single breakup, while the second invokes another breakup episode. Further, a major reorganization of the plate boundaries in the mid‐Cretaceous caused the spreading center to align E‐W by Late Cretaceous. The tectonics involved in this spreading direction change is unknown and difficult to understand as it occurred in the mid‐Cretaceous, coincident with the Cretaceous Normal Superchron. In the present study, we have generated a revised magnetic isochron map of the Bay of Bengal and used the modulus of its analytic signal to understand the evolutionary tectonics between the two plates, India and Antarctica. Mesozoic magnetic anomalies M12n to M0, and mid‐Cretaceous incursion Q2 (108 Ma) are identified in the western basin of the Bay of Bengal, while Q2, Q1 (92 Ma) and Late Cretaceous isochron A34 are inferred in its eastern basin. Gravity grid reconstructions demonstrate asymmetrical crustal accretion in the conjugate corridors. Fan‐shaped spreading and a southward ridge jump between 108 and 92 Ma in the eastern basin, aided the ∼40° clockwise rotation of the spreading center. The presence of volcanic basement beneath the additional crust suggests on‐axis mode of formation for the Ninetyeast Ridge by 100 Ma. The present study favors the two‐phase breakup model for the evolution of these plates.

Petrogenesis and Metallogenesis of Late Cretaceous Adakites in the Nuri Large Cu-W-Mo Deposit, Tibet, China: Constraints from Geochronology, Geochemistry, and Hf Isotopes

The Gangdese metallogenic belt in Tibet is an important polymetallic metallogenic belt formed during the subduction of the Neo-Tethys Ocean and subsequent India–Asia collision. Adakitic rocks are widely distributed in this belt and are considered to be closely related to porphyry–skarn Cu-Mo polymetallic mineralization. However, the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt remain controversial. In this study, we focus on the quartz diorite in the Nuri Cu-W-Mo deposit along the southern margin of the eastern Gangdese belt. LA-ICP-MS zircon U-Pb dating yields a Late Cretaceous age of 93.6 ± 0.4 Ma for the quartz diorite. Whole-rock geochemistry shows that the quartz diorite possesses typical adakitic signatures, with high SiO2, Al2O3, and Sr contents, but low Y and Yb contents. The relatively low K2O content and high MgO, Cr, and Ni contents, as well as the positive zircon εHf(t) values (+6.58 to +14.52), suggest that the adakites were derived from the partial melting of the subducted Neo-Tethys oceanic slab, with subsequent interaction with the overlying mantle wedge. The Late Cretaceous magmatic flare-up and coeval high-temperature granulite-facies metamorphism in the Gangdese belt were likely triggered by Neo-Tethys mid-ocean ridge subduction. The widespread occurrence of Late Cretaceous adakitic intrusions and associated Cu mineralization in the Nuri ore district indicate a strong tectono-magmatic-metallogenic event related to the Neo-Tethys subduction during this period. This study provides new insights into the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt, and has important implications for Cu polymetallic deposit exploration in this region.

Morphological characteristics and relationship with spreading rates in Mid-Ocean ridges

Mid-ocean Ridges is where plate spreads, serving as a window to explore the interior earth and its dynamics. This research investigates the morphological characteristics of mid-ocean ridges and their relationship with spreading rates, magma activity, and magma supply. The study reveals that the morphology of mid-ocean ridges is closely related to the stability and adequacy of magma supply. Mid-ocean ridges with rapid spreading exhibit relatively stable and uniform crustal thickness, thus developing axial high and smoother morphology. While those with slow and ultra-slow spreading have a cold thermal structure and thick brittle/plastic layers where fluctuations between segments are obvious and deeper axial valley shapes within segment. Additionally, the slope variation of mid-ocean ridges is associated with fluctuations in magma activity, with regions of higher magma activity often having steeper slopes. The study also finds a positive correlation between slope and axial depth, with faster spreading rates corresponding to greater axial depth.

Subduction of active spreading ridges and the disappearance of Andean-type cordilleras

Subduction of active spreading ridges and the disappearance of Andean-type cordilleras

Identification of variable contributions to arc rocks by Li[sbnd]Mo and Sr–Nd–Hf–O isotope compositions: A study of the Mafan mafic rocks in the Hong'an orogen, Central China

The geochemistry of arc rocks provides a window to probe compositional variability in mantle wedge and slab-derived inputs. However, quantitative estimation of their contribution is a great challenge. Here we report an integrated study for determination of variable contributions to early Paleozoic arc plutons (the Mafan diorites and gabbros), from the northern Hong'an orogen, central China. Zircon grains from the diorites yielded crystallization age of 445 ± 3 Ma, whereas the gabbros have zircon UPb age of 432 ± 3 Ma. Although both the two mafic rocks suites exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li–Mo–O and radiogenic Sr–Nd–Hf isotope compositions, as well as major and trace element features. Geochemically, the diorites are medium-K calc-alkaline series with high and positive whole-rock εNd(t) (+3.8 to +4.5) and zircon εHf(t) values (+12.4 to +14.3), and normal mantle-like zircon δ18O values (5.1 ± 0.2 ‰). Contrarily, the gabbros are tholeiitic and have relatively low whole-rock εNd(t) (+ 2.3 to +3.1) and zircon εHf(t) (+5.0 to +8.5) values with normal mantle-like zircon δ18O values (5.4 ± 0.3 ‰). The diorites have δ98Mo values of −0.15 to −0.03 ‰, while the gabbros have δ98Mo values of −0.28 to −0.12 ‰. These values are slightly higher than those of the mid-ocean ridge basalts (MORB) (−0.20 ± 0.01 ‰). Whereas, δ7Li values of the studied diorites (2.15–4.49 ‰) and gabbros (2.89–5.33 ‰) are mostly consistent within the range of MORB. In combination with the above results, the primary magma of diorites was inferred to be derived from 1 to 5% partial melting of depleted lithospheric mantle metasomatized by slab-derived fluid with minor sediment-derived melts. While the gabbros were suggested to be generated by 5–10% partial melting of more depleted lithospheric mantle that had undergone perhaps extra 0.3% melt extraction, which had further been metasomatized by more subducted materials. In summary, both the slab-derived components and nature of the mantle wedge sources are quantitatively constrained for the mafic rocks in the Mafan plutons. Our new data suggest that the Mafan plutons exhibited the Mariana arc-affinity and represented products of an intra-oceanic subduction system in the northern Hong'an orogen. Combined with previously published data for the early Paleozoic mafic magmatic rocks along the Qinling-Tongbai-Hong'an orogenic belt, there exists an oceanic arc in the east (Hong'an orogen) and an Andean-type continental arc in the west (Qinling-Tongbai orogen).

Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab

Abstract Estimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between −6 (Kasai, Congo and Udachnaya, Siberia) and −0.1 (Udachnaya, Siberia) log units (relative to the fayalite–magnetite–quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/∑Fe values range between 0.03 and 0.19 for garnet and 0.18–0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/∑Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of ± 0.02 and ± 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0–5.2 (± 0.4) GPa and 720–1050 (± 60) °C for both high-MgO and low-MgO eclogites, slightly lower than previous P–T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, ∆logfo2 (FMQ) calculated using the available oxythermobarometric model varies from −1.7 to −0.6 log units for high-MgO eclogites and from −2.9 to 0.9 log units for low-MgO eclogites. Samples recording ∆logfo2 (FMQ) ≤ −1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of −1.2 (± 0.4) log units for high-MgO and −0.6 (± 1.1) log units for low-MgO eclogites suggest different redox conditions of basaltic protoliths during subduction worldwide. Previous geochemical studies on the same rock samples reported evidence of cryptic metasomatism in both garnet and clinopyroxene that we demonstrate being not recorded by their major elements, while modal metasomatism evidenced by the presence of phlogopite as a product of interaction with a kimberlitic melt only affects the MgO of the bulk rock. Therefore, we suggest that high Fe3+/∑Fe ratios for garnet (> 0.10) and for reconstructed bulk rocks in the case of both low-MgO and high-MgO samples cannot be due to metasomatic interaction with an oxidized fluid, but rather are the consequence of Fe3+ redistribution in an unusually oxidized mafic protolith upon metamorphism. Our results highlight the redox variability of eclogites of Archaean age at conditions more oxidized than present-day mid-ocean ridge basalts (MORBs) and imply an oxidizing nature of the convective mantle source where magma was formed with consequent speciation of C in the form of carbonate fluid explaining, therefore, the lack of eclogitic diamonds in V. Grib kimberlite pipe.

Comment on , Lithos 456-457, 107325, Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

An article (Gennaro et al., 2023, 456–457, 107325) recently published in Lithos proposes a model for the transcrustal magmatic plumbing system at the Marsili Seamount, a massive and still active volcanic centre at the Southern Tyrrhenian back-arc spreading ridge. Their model is based on a combination of geophysical and petrochemical data, the former confined to the axial zone of the Marsili volcano while the latter mostly refer to literature data dealing with samples collected from various sectors of the volcano. This comment questions the rationality of the proposed model, documenting that it is poorly constrained and, at places, incorrect.

Reply to the comment by on , Lithos 456–457, 107325, Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

Reply to the comment by on , Lithos 456–457, 107325, Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

Tellurium Transport and Enrichment in Volcanogenic Massive Sulfide Deposits: Numerical Simulations of Vent Fluids and Comparison to Modern Sea-Floor Sulfides

Abstract Volcanogenic massive sulfide deposits may represent a significant future source of Te, which is a critical element important for the green energy transition. Tellurium is enriched in these settings by up to 10,000 times over its crustal abundance, indicating that fluids in sea-floor hydrothermal systems may transport and precipitate Te. The major element composition of these hydrothermal fluids is controlled by fluid-rock interaction and is well documented based on experimental, modeling, and natural studies; however, controls on Te mobility are still unknown. To better understand Te enrichment in this deposit type, numerical simulations of the mafic-hosted Vienna Woods and the felsic-hosted Fenway sea-floor vents in the Manus basin were performed to predict Te mobility in modern sea-floor hydrothermal vent fluids and Te deposition during sulfide formation. These simulations demonstrate that the mobility of Te in sea-floor hydrothermal systems is primarily controlled by fluid redox and temperature. Tellurium mobility is low in reduced hydrothermal fluids, whereas mobility of this metal is high at oxidized conditions at temperatures above 250°C. Numerical simulations of the reduced vent fluids of the mafic-hosted Vienna Woods site at the back-arc spreading center in the Manus basin yielded Te concentrations as low as 0.2 ppt. In contrast, the more oxidized model fluids of the felsic-hosted Fenway site located on Pual Ridge in the eastern Manus basin contain 50 ppt Te. The models suggest that Te enrichment in these systems reflects rock-buffer control on oxygen fugacity, rather than an enriched source of Te. In fact, the mafic volcanic rocks probably contain more Te than felsic volcanic rocks. The association of elevated Te contents in the felsic-hosted Fenway system likely reflects magmatic volatile input resulting in lower pH and higher Eh of the fluids. More generally, analysis of sulfide samples collected from modern sea-floor vent sites confirms that redox buffering by the host rocks is a first-order control on Te mobility in hydrothermal fluids. The Te content of sulfides from sea-floor hydrothermal vents hosted by basalt-dominated host rocks is generally lower than those of sulfides from vents located in felsic volcanic successions. Literature review suggests that this relationship also holds true for volcanogenic massive sulfides hosted in ancient volcanic successions. Results from reactive transport simulations further suggest that Te deposition during sulfide formation is primarily temperature controlled. Modeling shows that tellurium minerals are coprecipitated with other sulfides at high temperatures (275°–350°C), whereas Te deposition is distinctly lower at intermediate (150°–275°C) and low temperatures (100°–150°C). These predictions agree with geochemical analyses of sea-floor sulfides as Te broadly correlates positively with Cu and Au enrichment in felsic-hosted systems. The findings of this study provide an important baseline for future studies on the behavior of Te in hydrothermal systems and the processes controlling enrichment of this critical mineral in polymetallic sulfide ores.

Pb isotopic constraints on the evolution of the South Adamastor paleo-oceanic remnants, SW gondwana paleocontinent

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

U-Pb calcite ages date oblique rifting of the Arctic–North Atlantic gateway

Abstract Miocene breakup of Svalbard from Greenland formed a deep oceanic gateway that enabled circulation between the Arctic and Atlantic Oceans, significantly changing the global climate. However, the timing of events remains unclear. An excellent opportunity to constrain this timing is found onshore western Svalbard, where the Sarsbukta fault forms the eastern margin of the Eocene–Oligocene Forlandsundet basin. Here, we present new results from U-Pb dating of calcite precipitated in fault-related veins to constrain the timing of Sarsbukta fault deformation and the evolution of the basin. Our oldest calcite age is Permo-Triassic, suggesting long-lived deformation along the fault. A cluster of ages between 41 and 33 Ma overlaps with fossil-based depositional ages from parts of the Forlandsundet basin. These data indicate that onshore transtension partly pre-dated the well-established Chron 13 (magnetic polarity time scale; 35.5–33.7 Ma) reorganization of spreading ridges in the North Atlantic. Our youngest age of 13 Ma indicates that faulting persisted long after the preserved basin fill was deposited. If seafloor spreading marked the end of extension of continental crust, Molloy Ridge spreading during Chron 5 (19.6–9.8 Ma) may have initiated after 13 Ma.

Strain localization instabilities and the genesis of multiple axes of seafloor spreading in the Carmen basin, southern Gulf of California

We present new insight into the tectonic evolution of the Carmen basin (CB) in the southern Gulf of California (GC) from high-resolution bathymetry and two-dimensional seismic reflection data. Our goal is to document the seafloor morphology and sub-surface acoustic characteristics to understand the structure and crustal lithology across the CB. We identify three sub-basins with distinct geometries and evolutionary histories, with basement structures displaying a strong affinity with highly reflective, sigmoidal-shaped layers and the emplacement of high-amplitude tabular material underlying sediments with varying stratigraphic thicknesses. From the extent of new oceanic crust accreted along the CB, we estimate the age of the basin using a seafloor spreading rate of 52 mm/year, which is the average velocity of the relative plate motion between the Baja California microplate and the North American plate, as documented by previous authors. The southern and central sub-basins of the CB are mainly abandoned, while the northern one is currently the locus of seafloor spreading. This is evidenced by the juxtaposition of oceanic crust younger than ~ 1.9 Ma against older oceanic crust correlating in age with the adjacent Guaymas and Farallon basins to the northeast and southwest, respectively. We propose that mantle upwelling beneath the CB is a northward continuation of the East Pacific Rise, resulting in a fast-evolving system with sharp variations in strain localization within the seafloor spreading centers of the CB.Graphical

Massive crustal carbon mobilization and emission driven by India underthrusting Asia

The active Himalayan-Tibetan orogen, where India underthrusts into Asia, is an important geological source of carbon dioxide (CO2) emission into Earth’s atmosphere. However, the extent to which Indian underthrusting could stimulate the mobilization of deeply-sourced carbon and its subsequent emission remains unknown. Here, we use a combination of field observations coupled with in-situ CO2 flux measurements and helium and carbon isotopic data, to study the controls on CO2 origins and fluxes in a 400-kilometre-long rift transecting northern Himalaya and southern Tibet. High diffuse CO2 fluxes sustained by pure crustal fluids are confined to rift segments in the northern Himalaya, while toward southern Tibet, CO2 fluxes become lower but mantle fluid inputs are identified. Such rift-related CO2 degassing profile suggests metamorphic decarbonation and release of carbon-bearing fluids enhanced by the underthrusting Indian lower crust, agreeing well with Himalayan metamorphism and orogen-parallel lithospheric extension. Deep CO2 fluxes from extensional tectonics in northern Himalaya and southern Tibet, primarily of crustal origins, are comparable to mantle CO2 fluxes from global mid-ocean ridges. Our findings demonstrate that geophysical and geo-tectonic responses to continental underthrusting could facilitate massive crustal carbon mobilization and emission, making active collisional orogens globally important carbon sources.