Showing papers on "Mid-ocean ridge published in 2023"

Disparate crustal thicknesses beneath oceanic transform faults and adjacent fracture zones revealed by gravity anomalies

Abstract: Plate tectonics describes oceanic transform faults as conservative strike-slip boundaries, where lithosphere is neither created nor destroyed. Therefore, seafloor accreted at ridge-transform intersections should follow a similar subsidence trend with age as lithosphere that forms away from ridge-transform intersections. Yet, recent compilations of high-resolution bathymetry show that the seafloor is significantly deeper along transform faults than at the adjacent fracture zones. We present residual mantle Bouguer anomalies, a proxy for crustal thickness, for 11 transform fault systems across the full range of spreading rates. Our results indicate that the crust is thinner in the transform deformation zone than in either the adjacent fracture zones or the inside corner regions. Consequently, oceanic transform faulting appears not only to thin the transform valley crust but also leads to a secondary phase of magmatic addition at the transition to the passive fracture zones. These observations challenge the concept of transform faults being conservative plate boundaries.

Microseismicity and lithosphere thickness at a nearly-amagmatic oceanic detachment fault system

Abstract: Oceanic detachment faults play a central role in accommodating the plate divergence at slow-ultraslow spreading mid-ocean ridges. Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow spreading Southwest Indian Ridge (SWIR) at 64°30'E accommodate ~100% of plate divergence, with mostly ultramafic smooth seafloor. Here we present microseismicity data, recorded by ocean bottom seismometers, showing that the axial brittle lithosphere is on the order of 15 km thick under the nearly-amagmatic smooth seafloor, which is no thicker than under nearby volcanic seafloor or at more magmatic SWIR detachment systems. Our data reveal that microearthquakes with normal focal mechanisms are colocated with seismically-imaged damage zones of the active detachment fault and of antithetic hanging-wall faults. The level of the hanging-wall seismicity is significantly higher than that documented at more magmatic detachments of slow-ultraslow ridges, which may be a unique feature of nearly-amagmatic flip-flop detachment systems.

Sea‐Level Fall Driving Enhanced Hydrothermal and Tectonic Activities: Evidence From a Sediment Core Near the Tectonic‐Controlled Tianxiu Vent Field, Carlsberg Ridge

Abstract: Hydrothermal activity on mid-ocean ridges plays an important role in shaping marine chemistry, yet the variability of hydrothermal venting and its forcing mechanism remain elusive. Here, we analyzed a sediment core obtained near the tectonic-controlled Tianxiu vent field, Carlsberg Ridge, to reconstruct the hydrothermal venting history. The core documented two significant hydrothermal events (H1 and H2) in the past 30 ka. H1 occurred at 24.1–24.5 ka during the Last Glacial Maximum (LGM), while H2 occurred at 10.5–11.6 ka during the deglacial period. Compared to H2, H1 was relatively weak, and it occurred concurrently with a tectonic event. We suggest that H2 was caused by the increased melt production associated with the decompression melting of the upper mantle during sea-level fall, which is consistent with previously published records, whereas H1 was likely triggered by an intense tectonic event associated with depressurization during the LGM, which was previously unrecognized.

Ignition of the southern Atlantic seafloor spreading machine without hot-mantle booster

Abstract: The source of massive magma production at volcanic rifted margins remains strongly disputed since the first observations of thick lava piles in the 1980s. However, volumes of extruded and intruded melt products within rifted continental crust are still not accurately resolved using geophysical methods. Here we investigate the magma budget alongside the South Atlantic margins, at the onset of seafloor spreading, using high-quality seismic reflection profiles to accurately estimate the oceanic crustal thickness. We show that, along ~ 75% of the length of the Early-Cretaceous initial spreading centre, the crustal thickness is similar to regular oceanic thickness with an age > 100 Ma away from hot spots. Thus, most of the southernmost Atlantic Ocean opened without anomalously hot mantle, high magma supply being restricted to the Walvis Ridge area. We suggest that alternative explanations other than a hotter mantle should be favoured to explain the thick magmatic layer of seaward dipping reflectors landward of the initial mid-oceanic ridge.

Plume–ridge interactions: ridgeward versus plate-drag plume flow

Abstract: Abstract. The analysis of mid-ocean ridges and hotspots that are sourced by deep-rooted mantle plumes allows us to get a glimpse of mantle structure and dynamics. Dynamical interaction between ridge and plume processes have been widely proposed and studied, particularly in terms of ridgeward plume flow. However, the effects of plate drag on plume–lithosphere and plume–ridge interaction remain poorly understood. In particular, the mechanisms that control plume flow towards vs. away from the ridge have not yet been systematically studied. Here, we use 2D thermomechanical numerical models of plume–ridge interaction to systematically explore the effects of (i) ridge-spreading rate, (ii) initial plume head radius and (iii) plume–ridge distance. Our numerical experiments suggest two different geodynamic regimes: (1) plume flow towards the ridge is favored by strong buoyant mantle plumes, slow spreading rates and small plume–ridge distances; (2) plume drag away from the ridge is in turn promoted by fast ridge spreading for small-to-intermediate plumes and large plume–ridge distances. We find that the pressure gradient between the buoyant plume and spreading ridge at first drives ridgeward flow, but eventually the competition between plate drag and the gravitational force of plume flow along the base of the sloping lithosphere controls the fate of plume (spreading towards vs. away from the ridge). Our results highlight that fast-spreading ridges exert strong plate-dragging force, which sheds new light on natural observations of largely absent plume–lithosphere interaction along fast-spreading ridges, such as the East Pacific Rise.

Seismic observation of an active detachment faulting system beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge

Abstract: Hydrothermal processes in detachment settings at slow and ultraslow spreading ridges differ greatly from those at melt-rich faster spreading ridges. Active detachment faulting provides the possibility for off-axis high-temperature hydrothermal vents located far away from the heat source beneath the axial volcanic ridge. Seismic data from the slow spreading Mid-Atlantic ridge revealed that hydrothermal fluids may exploit detachment faults to extract heat from a melt zone near the crust-mantle interface. However, knowledge of the subsurface structure and the kinematic processes of detachment faults, and their interaction with hydrothermal fields at the slowest spreading ridges is still insufficient. Here, we report new insights on the seismicity beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge from three ocean bottom seismometer monitoring experiments. The seismicity outlines the subsurface geometry of a detachment faulting system (DF1 and DF2). The strongly flexed DF1 (45°) is a mature detachment fault with a domed-shaped OCC where ultramafic rocks are exposed on the seafloor. An active young DF2 with intense earthquake activity along the steep subsurface (67°) suggests the initiation phase of rotation. The diversity of hydrothermal activities in the Longqi detachment-hydrothermal systems is closely related to the evolution of detachment faults. Additionally, both the non-transform discontinuity on the western margin of the Longqi-1 field and local faults facilitate hydrothermal circulation. Our study provides baseline observations for a Longqi-type of hydrothermal circulation at the inside corner associated with detachment faulting and non-transform offsets.

Evolution of a Cold Intra‐Transform Ridge Segment Through Oceanic Core Complex Splitting and Mantle Exhumation, St. Paul Transform System, Equatorial Atlantic

Abstract: Accretionary processes at mid-ocean ridge segments with low magma input have seldom been investigated over the long term. The evolution of such magma-starved segments over time is still largely unknown. We present a study on the structure and evolution of the southernmost intra-transform ridge segment of the St. Paul Transform Fault System in the Equatorial Mid-Atlantic Ridge, based on new bathymetry, gravity, and rock sampling data. We show that this area evolves differently from previously described tectonics along ridge segments of similar spreading rate. On the flanks of the axial ridge segment, we observe a succession of structures exhumed by detachment faulting, evolving from east-facing, long-lived, corrugated oceanic core complexes (∼6 Ma ago), to short-lived detachment faults exposing lower crust and mantle rocks and facing alternatively east and west in the more recent part of the segment. The oldest detachment faults have been repeatedly split and partially transferred to the opposite flank through the formation of new detachments into the footwall. The terminations of three old, east-facing detachments are observed on the east flank of the segment. The westward relocations of the plate boundary appear to compensate for the asymmetry of accretion through detachment faulting, overall creating the same amount of lithosphere on both flanks of the ridge. We interpret the observed changes in the time of the accretionary processes to reflect a decrease of the melt supply over the last ∼6 Myr.

Preliminary rock magnetic and anisotropy of magnetic susceptibility results from the Reykjanes Ridge basalts, Atlantic Ocean (IODP Expeditions 384 and 395C)

Abstract: The Reykjanes Ridge is located in the North Atlantic Ocean, southwest of Iceland. Here, the oceanic crust is characterized by a series of V-shaped ridges (VSRs) and V-shaped troughs (VSTs), the formation of which has been linked to three alternative hypotheses: i) thermal pulsing, ii) propagating rifts, and iii) buoyant mantle upwelling. During International Ocean Discovery Program Expeditions 384 and 395C, a transect of five sites were drilled eastwards of the modern Mid-Atlantic Ridge (between 20-30°W) at a latitude of ~60°N, to investigate VSTs/VSRs formation. In this preliminary study, we analyze basalt samples from four sites, two from VSRs and two from VSTs, using rock magnetic and anisotropy of magnetic susceptibility (AMS) techniques, to investigate the differences between VSRs and VSTs. We analyzed the samples at the CIMaN-ALP (Peveragno) and INGV (Rome) Laboratories of paleomagnetism through bulk susceptibility, AMS, stepwise demagnetization of natural remanent magnetization through alternating field and temperature, hysteresis loops and FORC diagrams, and susceptibility vs temperature curves. Rock magnetism was used to determine the rock magnetic properties of each sample and investigate its correlation with the degree of alteration observed in the basalts. The AMS was measured to determine the magnetic fabric as a proxy of the magmatic fabric, where, for instance, lava flow-like fabric would be typical of an unaltered basalt. Preliminary results suggest that basalts from VSTs are generally characterized by higher susceptibility values, while the AMS shows a mixed behavior (well defined or dispersed) independently from the structural position. Further rock magnetic data, integrated with petrological, structural and geochemical data will be correlated to the pervasiveness of alteration in each site, the age of basalts and their distance from the Mid-Atlantic Ridge to test the three hypotheses.

Volcanic evolution of an ultraslow-spreading ridge

Abstract: Abstract Nearly 30% of the ocean crust forms at mid-ocean ridges where the spreading rate is less than 20 mm per year. According to the seafloor spreading paradigm, oceanic crust forms along a narrow axial zone, becomes inactive, and is transported away from the rift valley. However, because quantitative age data of volcanic eruptions are lacking, constructing geological models for the evolution of ultraslow-spreading crust remains a major challenge. Here, we use sediment thicknesses acquired from more than 4000 km of sub-bottom profiler data combined with C14 ages from sediment cores to determine the age of the ocean floor of the oblique ultraslow-spreading Mohns Ridge at the segment scale and reveal a systematic pattern of young volcanism occurring outside axial volcanic ridges. We present the first age map of a mid-ocean ridge and find that nearly half of the 6-17 km wide inner rift valley floor has been rejuvenated by volcanic activity during the last 25 Kyr. High-resolution bathymetric observations of young volcanic structures at the rift flanks indicate that crustal accretion occurs across the width of the axial valley and implies that formation of ocean crust may take more than one million years at ultraslow-spreading ridges.

Ocean Bottom Seismic Survey in the Knipovich Ridge area

Abstract: The structure of the oceanic crust generated by the ultraslow-spreading mid-ocean Knipovich Ridge still remains relatively uninvestigated compared to the other North Atlantic spreading ridges further south. The complexity of the Knipovich Ridge, with its oblique ultraslow-spreading and segmentation, makes this end-member of Spreading Ridge Systems an important and challenging ridge to investigate. The aim of this work is to better understand the lithospheric structure beneath the rare ultraslow-spreading ridges, using as example the Knipovich Ridge along its spreading direction. Ultraslow spreading ridges are characterized by a low melt supply. At spreading rates below 20 mm/y, conductive cooling effectively reduces the mantle temperature and results in less melt produced at larger depths. The Ocean Bottom Seismometer (OBS) data along a refraction/reflection profile (~280 km) crossing the Knipovich Ridge off the western Barents Sea was acquired by use of RV G.O. Sars on July 24 - August 6, 2019. The project partners are University of Bergen, Institute of Geophysics, Polish Academy of Sciences, and Hokkaido University. The seismic energy was emitted every 200 m by an array of air-guns with total volume of 80 l. To receive and record the seismic waves at the seafloor, ocean bottom seismometers were deployed at 12 positions with about 15-km spacing in 2 deployments. All the stations were recovered and correctly recorded data. Seismic energy from airgun shots were obtained up to 50 km from the OBSs. The profile provides information on the seismic crustal structure of the Knipovich Ridge and oceanic and continental crust in the transition zone. This profile is a prolongation of the previously acquired profile AWI-20090200 (Hermann & Jokat 2013) and together allow for the modeling of ~535 km long transect crossing the Knipovich Ridge from the American to the European plate. Seismic record sections were analyzed with 2D trial-and-error forward seismic modeling. This work is supported by the National Science Centre, Poland according to the agreement UMO-2017/25/B/ST10/00488. The cruise was funded by University of Bergen. Hermann, T. and Jokat, W., 2013. Crustal structures of the Boreas Basin and the Knipovich Ridge, North Atlantic. Geophys. J. Int., 193, 1399–1414, doi: 10.1093/gji/ggt048 

The role of plume-ridge decoupling on rapid plate motion and intraplate volcanism

Abstract: The migration of mid-ocean ridges is driven by asymmetric plate motions on either ridge flank transmitted from far-field subduction forces. Within this model, the geometry and location of mid-ocean ridges are independent of lower-mantle dynamics. However, this fails to recognise the attraction between mid-ocean ridges and mantle plumes. Using numerical models of mantle convection, we show that plumes with high buoyancy flux (> 6000 kg/s) can capture mid-ocean ridges within a 1000 km radius and anchor them in place. If the plume buoyancy flux wanes below 1000 kg/s the ridge may be released, potentially resulting in rapid migration rates that trigger a major plate reorganisation. Plume-ridge interactions are commonly preserved as conjugate large igneous provinces (LIPs), which form on each flank of a mid-ocean ridge as new crust is created. The decoupling of ridges from plumes are demarcated by a switch from conjugate LIPs, formed by a plume beneath a spreading ridge, to trails of intraplate hotspot volcanoes signifying the plume and ridge have separated. We demonstrate that the waning buoyancy flux of the Kerguelen plume, inferred from the geochemistry of eruption products, resulted in its decoupling with the SE Indian Ridge spurring rapid northward migration of the Australian plate. Our modelling predicts that following plume-ridge decoupling, the waning plume can tilt 15° within the upper mantle towards the migrating ridge, providing an explanation for diffuse volcanism and low eruption volumes along the Kerguelen Archipelago. Our results have significant implications for other plume-ridge interactions globally such as the Iceland, Tristan, and Easter plumes, and the generation of intraplate hotspot volcanoes proximal to mid-ocean ridges.

Evidence of ghost plagioclase signature induced by kinetic fractionation of europium in the Earth’s mantle

Abstract: Abstract Crustal recycling in the Earth’s mantle is fingerprinted by trace-element and isotopic proxies in oceanic basalts. Positive Eu and Sr anomalies in primitive lavas and melt inclusions that are not otherwise enriched in Al 2 O 3 are often interpreted as reflecting the presence of recycled, plagioclase-rich oceanic crust in their mantle source – referred to as “ghost plagioclase” signatures. Here, we report natural evidence of Eu anomalies and extreme crystal-scale heterogeneity developed kinetically in mantle peridotite clinopyroxene. Numerical modelling shows that diffusional fractionation between clinopyroxene and melts can account for this intra-crystal heterogeneity and generate Eu anomalies without requiring plagioclase. We demonstrate that kinetically induced Eu anomalies are likely to develop at temperatures, redox conditions and transport timescales compatible with the genesis of mid-ocean ridge and ocean island basalts. Our results show that, in the absence of converging lines of evidence such as radiogenic isotope data, ghost plagioclase signatures are not an unequivocal proxy for the presence of recycled crust in oceanic basalt sources.

Trace Element Evidence of Subduction-Modified Mantle Material in South Mid-Atlantic Ridge 18–21°S Upper Mantle

Abstract: Mid-ocean ridge basalts (MORBs), produced at mid-ocean ridge where the continents and subduction zones are distant, are the product of partial melting of the upper mantle and their chemical composition can provide information about the mantle itself. The geochemical characteristics of MORBs enable us to be more informed about the geological processes of the upper mantle below the mid-ocean ridge, and assist us in understanding mantle heterogeneity and geodynamic processes. In this paper, new data of major elements, trace elements, and Nd-Hf isotopes of south mid-Atlantic ridge (SMAR) 18–21°S MORBs are presented. TAS diagram shows that the samples belong to subalkaline basalt compositional field. Trace elements (e.g., (La/Sm)N = 0.49–0.79) show that the samples are N-MORBs. However, the primitive mantle-normalized trace element patterns showed that the studied samples were clearly enriched in Rb, U, Pb, and other fluid-mobile elements. Meanwhile, the trace element ratios, such as Nb/U and Ce/Pb, are also significantly different from the typical N-MORB. Combined with the Nd-Hf isotopic composition, we propose that these anomalies are not related to continental crust material, delaminated subcontinental lithospheric mantle (SCLM), recycled sediments, direct supply of mantle plume, nor are they the result of subduction directly affecting the mantle source, but are caused by the incorporation of mantle material modified by subduction.

Upper Crustal Structure of Superfast‐Spread Oceanic Crust Exposed at the Pito Deep Rift: Implications for Seafloor Spreading

Abstract: A tectonic window into the upper 2,000 m of oceanic crust generated at the superfast spreading (∼142 mm/yr) southern East Pacific Rise exposes a continuous layered structure of basaltic lavas and sheeted dikes over gabbroic rocks. This relatively simple structure is in accord with expectations for crustal accretion at a very fast spreading rate and high magma budget where magmatic construction keeps pace with plate separation. Detailed observations show that basaltic lava flows dip progressively more steeply inward (toward the spreading axis where they were erupted). Underlying sheeted dikes are faulted and tectonically rotated to dip steeply outward. These structures are interpreted in terms of subsidence beneath the axis of the southern East Pacific Rise during crustal construction that allowed the lava unit to thicken to >400 m without creating comparable relief at the spreading center. Transitional units above and below the sheeted dike complex show that the thickness of upper crustal rock units is modified by tectonic and intrusive processes during accretion. The crustal structure shows that even approaching the superfast spreading end-member of seafloor spreading, crustal accretion involves dramatic tectonic processes that are not obvious from the surface geology of spreading centers.

Spreading Magnetic Anomalies Separation of the South China Sea Based on the Nested Elliptical Directional Filters

Abstract: Spreading magnetic anomalies recorded the paleo-geomagnetic field variation that has great significance in the investigation of the extension process of ocean basins. Interpreting spreading magnetic anomalies under complex geological environments is challenging, especially for marginal sea basins. We proposed nested elliptical directional filters to separate the spreading magnetic anomalies of the South China Sea (SCS). The results show that the spreading magnetic anomalies separated by the nested elliptical directional filters depict the expansion process of the oceanic crust, and the interference magnetic anomalies are effectively suppressed. The separated spreading magnetic anomalies indicate that the expansion process of the SCS is affected by the interactions between the surrounding plates. The spreading magnetic anomalies of the SCS are warped, interrupted, and not strictly parallel. The pattern of the spreading magnetic anomalies reflects multiple ridge jumps during the expansion process and the post-spreading magmatic disturbances. The long-wavelength magnetic anomalies indicate lithospheric fractures and Curie surface variations in the SCS, which are affected by the post-spreading magmatic rejuvenation. The magnetic anomalies of the SCS resulted from the superposition of magnetic anomalies in the ocean crust and the uppermost mantle.

The role of magma supply in fragmentation of oceanic lithosphere

Abstract: Observations suggest that the oceanic lithosphere is shaped by dike intrusions and faulting in proportions that depend on the spreading rate (Carbotte et al., 2016). Yet it remains unclear how the interplay between magmatism and faulting during seafloor spreading affects mid-ocean ridge (MOR) axial morphology, fault spacing, and the pattern of abyssal hills (Buck et al., 2005, Huybers et al., 2022). Here we present two-phase flow numerical models of oceanic lithosphere extension that reconcile the nonlinear brittle behaviour of the lithosphere with mantle melting and magma transport through the lithosphere.&#160;Fast-spreading ridges show symmetric normal faulting and axial highs, while slow-spreading ridges show an asymmetric fault pattern and axial valleys. Previous work has focused on explaining the MOR fault pattern by tectonic or magmatic-induced deformation. In the first scenario, faults result from tectonic stretching of the thin axial lithosphere during amagmatic periods (Forsyth 1992), while in the second scenario, dike-injection may create stresses that activate extensional faults (Carbotte et al., 2016). Current state-of-the-art models (i.e., Buck et al., 2005) use a single-phase formulation for the deformation of oceanic lithosphere in which a prescribed axial dike may accommodate both magmatic and tectonic extension. In these models, the fault pattern depends on M &#8211; the fraction of plate separation rate that is accommodated by magmatic dike opening. While M-models are able to explain a number of observations, M represents a simple parameterization of complex fracture dynamics of sills, dikes, and faults. In particular, M-value models neglect fault&#8211;dike interaction and other modes of melt transport and emplacement in the lithosphere (Keller et al., 2013).&#160;Here we build a 2-D oceanic lithosphere extension model that incorporates a new poro- viscoelastic&#8211;viscoplastic theory with a free surface (Li et al., in review) to robustly simulate plastic representations of dikes and faults in a two-phase magma/rock system. We hypothesise that magma supply controls the pattern of dike&#8211;fault interaction in oceanic extension settings. We present simplified model problems to compare results with those from M-value models. These enable us to address the significance of M in terms of fundamental magma and lithospheric processes. We then focus on development of fault patterns, magma pathways and crustal production at fast-/slow-spreading ridges.&#160;ReferencesBuck et al., 2005, Nature, doi:10.1038/nature03358.Carbotte et al., 2016, Geol. Soc. London, doi:10.1144/SP420.Forsyth, 1992, Geology, doi:10.1130/0091-7613(1992)020<0027:FEALAN>2.3.CO;2.Huybers et al., 2022, PNAS, doi:10.1073/pnas.2204761119.Keller et al., 2013, GJI, doi:10.1093/gji/ggt306.Li, Y., Pusok, A., Davis, T., May, D., and Katz, R., (in review). Continuum approximation of dyking with a theory for poro-viscoelastic&#8211;viscoplastic deformation, GJI.

Forced Subduction Initiation Near Spreading Centers: Effects of Brittle‐Ductile Damage

Abstract: Although positive buoyancy of young lithosphere near spreading centers does not favor spontaneous subduction, subduction initiation occurs easily near ridges due to their intrinsic rheological weakness when plate motion reverses from extension to compression. It has also been repeatedly proposed that inherited detachment faults may directly control the nucleation of new subduction zones near ridges subjected to forced compression. However, recent 3D numerical experiments suggested that direct inversion of a single detachment fault does not occur. Here we further investigate this controversy numerically by focusing on the influence of brittle-ductile damage on the dynamics of near-ridge subduction initiation. We self-consistently model the inversion of tectonic patterns formed during oceanic spreading using 3D high-resolution thermomechanical numerical models with strain weakening of faults and grain size evolution. Numerical results show that forced compression predominantly reactivates and rotates inherited extensional faults, shortening and thickening the weakest near-ridge region of the oceanic lithosphere, thereby producing ridge swellings. As a result, a new megathrust zone is developed, which accommodates further shortening and subduction initiation. Furthermore, brittle/plastic strain weakening has a key impact on the collapse of the thickened ridge and the onset of near-ridge subduction initiation. In contrast, grain size evolution of the mantle only slightly enhances the localization of shear zones at the brittle-ductile transition and thus plays a subordinate role. Compared to the geological record, our numerical results provide new helpful insights into possible physical controls and dynamics of natural near-ridge subduction initiation processes recorded by the Mirdita ophiolite of Albania.

Pre-Deccan Campanian acidic volcanism on Laccadive ridge: Implications on the nature of its underlying crust

Abstract: The Laccadive-Chagos Ridge (LCR) is a prominent aseismic ridge in the Indian Ocean. The origin and nature of the crust beneath the LCR have been debated. Based on Ar-Ar geochronology of the volcanic basement rocks from ODP wells 713 and 715 from Chagos and Maldives ridges, a hotspot trail model was proposed for the genesis of the LCR. On the other hand, based on geophysical studies, the LCR has been inferred as a continental sliver/hyperextended continental crust which has undergone heavy underplating and Reunion hotspot volcanism. Even though these two different hypotheses differ on the genesis and nature of the LCR, both agree that the ridge experienced extensive Reunion hotspot trail volcanism, making the crust's nature challenging to decipher. We report Ar-Ar ages of two rhyolite rock samples from a well drilled on the Padua bank located on the northern extent of the Laccadive ridge. These acidic rocks are at a depth of ~1700m and ~2300m and underlie the Tertiary sediments forming the basement in this well. The previously determined K-Ar geochronology ages are 60.2 Ma and 102 Ma. The precise Ar-Ar dates of these rocks determined in the present study are 76.2 ± 0.4 Ma and 77.5 ± 0.5 Ma (2σ) respectively. These acidic rocks (78-76 Ma) coupled with onshore acidic rocks of the St-Mary's island (87-84 Ma) and Ezhimala (95-93 Ma) suggest continuous extension between the India and Laccadive ridge after the Indo-Madagascar continental breakup (88 ma). Contrary to the earlier hypothesis, the acidic rocks of Laccadive Ridge are older than the Reunion Plume trail volcanism (62-60 Ma). The geophysical and geochronological evidence suggests that the Laccadive ridge is continental in nature, and the volcanism predates the Reunion plume volcanism. These evidences suggest that the Laccadive Ridge is a continental thinned crust that has undergone rift-related volcanism rather than a hotspot trail.

The Past and the Future of Plate Tectonics and Other Tectonic Regimes

Abstract: Plate tectonics is the theory that considers the Earth's rigid outermost shell, the lithosphere, to be broken into a number of relatively large tectonic plates that slowly move on top of a mechanically weak asthenosphere (part of the upper mantle). Along convergent boundaries, at subduction zones, cold and dense oceanic plates sink into the mantle. The volume of subducted material is balanced by the formation of new (oceanic) crust along divergent margins and the growth of the lithosphere itself due to heat diffusion. This dynamics is intrinsically related to mantle convection, that is, the slow creeping motion of the Earth's solid mantle. This relatively new theory was only generally accepted in the 1960s and transformed the way we think about the evolution and dynamics of the Earth and other rocky planets and moons. Advances in numerical modeling allowed not only a better understanding of plate tectonics, but also the recognition of other tectonic regimes that can explain observations from other Solar System bodies. These are stagnant (a one-plate planet), episodic (where the lithosphere is usually stagnant and sometimes overturns into the mantle), ridge-only (where a large ridge can form and slightly compress the surrounding lithosphere without forming subduction zones), and plutonic-squishy lids (characterized by a thin lithosphere, and a set of small strong plates or blocks separated by warm and weak regions generated by plutonism). In this work, we further describe the different tectonic regimes and their applications to the Earth and other bodies, and discuss how we can keep moving forward in our understanding on the evolution of planets.

Air-Sea CO$_2$ Fluxes Localized By Topography in a Southern Ocean Channel

Abstract: Air-sea exchange of carbon dioxide (CO$_2$) in the Southern Ocean plays an important role in the global carbon budget. Previous studies have suggested that flow around topographic features of the Southern Ocean enhances the upward supply of carbon from the deep to the surface, influencing air-sea CO$_2$ exchange. Here, we investigate the role of seafloor topography on the transport of carbon and associated air-sea CO$_2$ flux in an idealized channel model. We find elevated CO$_2$ outgassing downstream of a seafloor ridge, driven by anomalous advection of dissolved inorganic carbon. Argo-like Lagrangian particles in our channel model sample heterogeneously in the vicinity of the seafloor ridge, which could impact float-based estimates of CO$_2$ flux.

Unbending connects sea level to faulting at fast-spreading mid-ocean ridges

Abstract: Topographic spectra of abyssal hills from fast-spreading mid-ocean ridges have concentrations of power at Milankovitch frequencies and, in particular, around 1/(41 ka) [1].&#160; This frequency corresponds to variations in Earth&#8217;s obliquity and is prominent in many climate records, including Pleistocene sea-level variations. Sea-level variations are understood to induce variations in magma supply to the ridge axis [2]. How might these magma-supply variations pace the faulting that creates abyssal hills?&#160; We hypothesise that magma-supply variations introduce a perturbation to elastic plate thickness that is correlated with crustal thickness [3]. Building on Roger Buck&#8217;s theory for plate unbending and faulting at fast-spreading ridges [4], we show how thickness perturbations lead to concentrations in bending stresses in thinner parts of the plate.&#160; These concentrations can be significant relative to background unbending stresses and may therefore pace faulting, depending on their amplitude and wavelength.&#160; Using perturbation analysis and numerical solutions of Euler-Bernoulli beam theory, we develop predictions for fault spacing as a function of spreading rate, amplitude of magma supply variations, and other physical parameters.[1] Huybers, Peter, et al. "Influence of late Pleistocene sea-level variations on mid-ocean ridge spacing in faulting simulations and a global analysis of bathymetry." PNAS https://doi.org/10.1073/pnas.2204761119&#160;[2] Cerpa, Nestor G., David W. Rees Jones, and Richard F. Katz. "Consequences of glacial cycles for magmatism and carbon transport at mid-ocean ridges." EPSL https://doi.org/10.1016/j.epsl.2019.115845&#160;[3] Boulahanis, Bridgit, et al. "Do sea level variations influence mid-ocean ridge magma supply? A test using crustal thickness and bathymetry data from the East Pacific Rise." EPSL https://doi.org/10.1016/j.epsl.2020.116121&#160;[4] Buck, W. Roger. "Accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs." JGR https://doi.org/10.1029/2000JB900360&#160;

Sensitivity of mid-ocean ridge hydrothermal system controlled by the detachment fault to the glacial cycle

Abstract: Hydrothermal activity in the mid-ocean ridge facilitates the chemical exchange of seawater with new oceanic crusts. This activity mostly occurs on the detachment fault of the asymmetric accretion segment in the slow-ultraslow spreading ridge, which is characterised by limited magma supply. Deep faults can readily extract heat from deeper heat sources. Moreover, the repeated movement of faults activates the permeable fluid channels of the overlying oceanic crust, thus driving long-life hydrothermal circulation. Recent studies have found that the response time of the hydrothermal activity of the intermediate-fast spreading ridges differs from that of the slow-spreading ridge to the glacial cycle, and a unified model is expected to explain it. Also, the response of hydrothermal activity to the glacial cycle must consider the differences between oceanic ridges with different spreading rates and types of hydrothermal systems.Here, based on two sediment cores collected near the Yuhuang hydrothermal field (HF)on ultraslow-spreading Southwest Indian ridge, we obtained high-resolution sediment history records spanning three glacial periods, understood the 160 ka history of hydrothermal, volcanic and tectonic activities in the region and attempted to reveal the response mechanism of hydrothermal activities controlled by detachment faults to the glacial cycle. We discovered that in the Yuhuang HF controlled by detachment faults, hydrothermal activity increased significantly during the glacial period, and more active detachment fault activity appeared at the same time. At the end of the glacial period, both activities are reduced at the same time. We believe that in the slow-ultraslow spreading ridge, the magmatism regulated by sea level changes may regulate the evolution of detachment faults and the hydrothermal circulation, which are recorded in the sediments near the hydrothermal field.We established a response model of Sea level change&#8211;Magmatism&#8211;Detachment fault activity&#8211;Hydrothermal activity&#160;and concluded that the magmatism of slow-ultraslow spreading ridges is more sensitive to sea level changes; with the synchronous effect of detachment faults, the hydrothermal activity responds faster to the glacial cycle.

Gravity signature in the mid-ocean ridge-transform system: Insights from deep mantle rheology and shallow crustal structure

Abstract: Gravity signals over the mid-ocean ridge-transform system reflect the distribution of underlying crustal and upper mantle mass anomalies. The gravity measurement, especially &#8216;residual&#8217; gravity anomalies, relies on the gravitational corrections of both seafloor relief and lithospheric thermal structure. Lithospheric thermal correction typically uses a 1D plate cooling approximation or a 3D passive flow model that assumes isoviscous mantle rheology. As this rheological approximation is oversimplified and physically complex, how sensitive gravity anomalies are to an increasingly complex/accurate approximation for mantle rheology is still unresolved. Here we systematically examine the residual gravity anomaly discrepancies caused by assumptions of different mantle rheologies on 16 natural ridge-transform systems ranging from ultraslow- to fast-spreading. Our calculations show that estimated residual gravity anomalies are significantly lower (e.g., ~21 mGal lower at mid-ocean ridges) in the isoviscous flow models than in the static plate cooling models, primarily due to the effects of lateral heat advection and conduction. When the assumed mantle rheology is changed from uniform viscosity to a non-Newtonian viscosity with brittle weakening in cooler (faulting) regions, the mantle upwelling intensifies and local near-surface temperature generally increases, resulting in an increase in the residual anomaly. This increase is distributed uniformly along the ultraslow-and slow-spreading ridge axes, but is concentrated along transform faults at intermediate- and fast-spreading ridges. The amount of the rheology-induced gravity difference is most closely linked to transform age offset instead of spreading rate or transform offset length alone. Our analysis reveals that oceanic transform faults exhibit higher gravity anomalies than adjacent fracture zones, which may reflect thinner crust in the transform deformation zone.

Highly Asymmetric Seismicity in a System of Tectonic Extension and Hydrothermal Venting at the Mohn-Knipovich Ridge Bend

Abstract: In recent years hydrothermal vent systems were found in unexpectedly high abundance along ultraslow spreading ridges, despite their overall decreased magma supply. Thin oceanic crust and resulting shallow heat sources can drive hydrothermal fluid circulation and detachment faults can act as fluid pathways, resulting in e.g., serpentinization of the oceanic crust. So far, no long-term recording of seismicity around hydrothermal vent systems along ultraslow spreading ridges have been reported. Here, we present results from a ~1-year local Ocean Bottom Seismometer deployment between 2019 - 2020 at Loki&#8217;s Castle hydrothermal vent field (LCVF) along the Arctic Mid Ocean Ridge. LCVF is located at a water depth of ~2500m on top of the axial volcanic ridge (AVR) at the Mohn-Knipovich Ridge bend where spreading is highly asymmetric from west to east.For the processing we use a combination of an automatic event detection algorithm (Lassie), a deep-learning phase picking model (PhaseNet) and partial manual re-evaluation of phase picks. Additionally, selected clusters of events are cross-correlated and relocated using hypoDD. The resulting earthquake catalogue consists of a total of 12368 events with 6719 manually re-evaluated and 5649 automatically picked events.From the results we see that most of the plate divergence at the Mohn-Knipovich Ridge bend is accommodated by a young detachment fault west of the AVR. Most of the seismicity occurs between depths of ~2-8km in a bended band that steepens up to 70&#176; with depth and follows the local topography. However, the described detachment fault differs from reported mature detachment faults at the Mid-Atlantic Ridge or Southwest Indian Ridge. Within the footwall we observe episodical, clustered seismicity with extensional faulting mechanisms, indicating that the detachment could be cross-cut by normal faults. Along strike, the seismicity of the fault plane appears highly heterogeneous, with the central part showing only sparse seismicity at depths below 3km while other segments show episodical shallow seismicity. Towards LCVF seismicity below the AVR increases and the maximum depth of earthquakes shallows by about ~2km. This could indicate the presence of a shallow heat source below LCVF as a driving factor for the hydrothermal circulation.

Evolution of Icelandic rift zones geometry as result of MOR-plume interaction

Abstract: Rift zones of Iceland large igneous province (LIP) have complicated interior geometric pattern expressing in several parallel extension centers. It significantly differs from adjacent Reykjanes (RR) and Kolbeinsey (KR) mid-oceanic ridges (MOR) that only have small overlappings between separate neovolcanic centers. At small scale, rift zones connect with each other by broad transform zones with distributed strain pattern instead of typical narrow transform faults. Those transform zones have very different structure varying from simple book-shelf fault zones of South Iceland seismic zone to sophisticated system of magmatic and amagmatic structures of Tj&#246;rnes transform zone. The whole system drastically differs from typical structure and geometry of ultra-slow MOR. Iceland rift zone evolution commenced at 25 Ma and strongly influenced by thermal pulses of Iceland plume each 6-7 My and slightly asymmetric spreading. Another challenge of this region lies in asymmetric thermal influence of Icelandic plume. RR is affected by plume at distance of at least 800 km, whereas Kolbeinsey ridge at distance of ca. 600 km. To reveal the ridge-plume interaction through Iceland evolution and possible causes of Icelandic plume influence asymmetry we used a method of physical modelling. The extending setting comprises mineral oils mixture that have numerical resemblance with oceanic crust in density, shear modulus and thickness. Two-layered model have elastic bottom layer, brittle top one and local heating source (LHS) corresponding to Icelandic plume pulses. The first experiment type configuration includes two sections corresponding to RR and KR. At their joint, the LHS melts the modelling lithosphere creating analogue of LIP. The LHS periodically switched on and transported to another position, which is similar to plume pulses in asymmetric spreading conditions. The general pattern of each cycle is as following. Initially within LIP two rift branches propagate to each other forming an overlapping. A block between two rift branches rotates as horizontally as vertically. These blocks express in Iceland topography as uplifted peninsulas of its northwestern part. In some time, overlapping transforms to oblique transfer zone and rift zones change their structure of several extension centers to one-axis structure and have direct connection. Then new plume pulse rejuvenates the cycle. If incipient offset between rift branches is quite small, then overlapping structure passes to oblique transform zone with several extension centers and small overlappings. Thermal pulses of less volumes have considerable influence as well, but current data cannot permit to correctly them. As a result, we created a conceptual model of Iceland rift zones evolution also using data of other researchers. The second model had the same initial configuration, but thermal pulses extend downward to modelling Reykjanes ridge. This migration caused by density heterogeneities of upper layers due to deep thermal differences. The resulting geometry is very similar to natural one. There are different segmentation pattern at both spreading ridges and some rift zones. Developed transform zones confine rotating blocks and have structure varying from book-shelf fault zone to overlapping as in nature. We infer that modeled asymmetry and origination can reflect the natural ones.

Spatiotemporal and thermal variability in upper mantle flow as a result of mid-ocean ridge migration

Abstract: Complex tectonic plate motions and thermal heterogeneity in the Earth’s mantle give rise to three-dimensional, time-varying patterns in material transport towards the surface near mid-ocean ridges. The commonly accepted view of mid-ocean ridges is that they are passive features, where plate divergence results in vertical upwelling from deep inside the mantle. In this model, formation of the crust is uniform and symmetric about the ridge axis. Recent plate motion models have revealed that mid-ocean ridges are not stationary in time (Whittaker et al., 2015). Their migration may influence the pathlines of mantle rock motion towards the ridge axis. Both geophysical and geochemical observations further reveal that the mantle does not have a uniform temperature distribution. Spatial variations in mantle temperature can be either local (i.e., mantle plumes) or regional (i.e., basin-scale). Together, the driving force of the plate motions and the heterogeneous heat distribution in the mantle can impact melting, volcanism, and crustal formation at mid-ocean ridges.The first chapter investigates the influence of mid-ocean ridge migration on isothermal upper mantle flow. We build a unique new laboratory apparatus and fluid reservoir that are ideally suited to studying this problem. Using this setup, experimental simulations of ridge migration are precisely controlled and repeatable. Through image processing and particle tracking, we find that the source region for new crust is constrained to much shallower depths than previously thought. Calculations of mantle melting indicate that melting may be suppressed at high migration rates and that melt production is not symmetric about the ridge axis. However, comparisons of natural ridge migration rates with published geochemical and geophysical data yield few meaningful correlations. The second chapter assesses how buoyant thermal upwellings interact with the upper mantle flow field generated by a migrating ridge. We use a circular heater placed at the base of the fluid reservoir to simulate mantle plumes in a controlled way. Stationary ridges bisect the plume as it surfaces and plume material spreads laterally beneath the plate. Beneath a migrating ridge, the flow field can deform mantle plumes and thereby reduce their buoyancy. This results in a loss of heat through diffusion. The deformation further inhibits the full surfacing of a mantle plume, such that portions of the plume may rise towards the surface but not melt. This suppresses the total volcanic output that we expect from plumes. These hot sections of plume material contribute to the global distribution of thermal heterogeneity in the upper mantle. The variability in volcanic output from deformation of rising mantle plumes has a natural counterpart. Large igneous provinces (LIPs) are thought to be the solidified lava outpourings resulting from melting of plume heads, which can vary in volume by several orders of magnitude. While controlled plumes yield valuable insight into the behavior of the largest buoyant upwellings in the Earth’s mantle, it is important to understand how thermal boundary layers naturally generate buoyant upwellings (active flow) that interact with (passive) plate-driven flow. We use a large rectangular heater at a scaled depth of 670 km to form a thermal boundary layer at the base of the mantle transition zone and allow the system to evolve naturally. We find two novel behaviors related to the formation and destruction of sheet-like upwellings. First, sheet-like upwellings collapse into plume-like conduits in the wake of a migrating ridge. Buoyant flow within the conduit stalls at depths of ~150 km, such that melt is generated but may be retained within the solid upper mantle. Depending on the melting model used, maximum melt fractions may range from 0.1 – 0.24 with a melt column of 30-75 km height. Ambient upper mantle material is entrained by the flow of upwellings and may contribute in part to melting near the surface, especially if thermal diffusion has played a role during upwelling. This is dependent on the strength of the thermal boundary layer and the plate rates. Second, once vertical pathways for thermal boundary layer material have been established, a thermal upwelling will not form again directly beneath the ridge axis. Over the course of 36 Myr, we observe that material within the thermal boundary layer flows laterally towards the nearest upwelling, regardless of passive vertical flow beneath the ridge. This has implications for our understanding of the origins of thermal heterogeneity and the stability of thermal features over long timescales.