Ridge Axis Research Articles

Variations in Volcanism and Tectonics Along the Hotspot‐Influenced Reykjanes Ridge

AbstractMapping and sampling four sections of the slow‐spreading Reykjanes Ridge provide insight into how tectonic and volcanic activity varies with distance from the Iceland plume. The studied areas are characterized by significant variations in water depth, lava chemistry, crustal thickness, thermal structure, and ridge morphology. For each study area, fault pattern and dimension, tectonic strain, seamount morphology, and density are inferred from 15 m‐resolution bathymetry. These observations are combined with geochemical analysis from glass samples and sediment thickness estimations along Remotely Operated Vehicle‐dive videos. They reveal that (a) tectonic and volcanic activity along the Reykjanes Ridge, do not systematically vary with distance from the plume center. (b) The tectonic geometry appears directly related to the deepening of the brittle/ductile transition and the maximum change in tectonic strain related to the rapid change in crustal thickness and the transition between axial‐high and axial valley (∼59.5°N). (c) Across‐axis variations in the fault density and sediment thickness provide similar widths for the neo‐volcanic zone except in regions of increased seamount emplacement. (d) The variations in seamount density (especially strong for flat‐topped seamounts) are not related to the distance from the plume but appear to be correlated with the interaction between the V‐shape ridges (VSR) flanking the ridge and the ridge axis. These observations are more compatible with the buoyant upwelling melting instability hypothesis for VSR formation and suggest that buoyant melting instabilities create many small magma batches which by‐pass the normal subaxial magmatic plumbing system, erupting over a wider‐than‐normal area.

The Role of On‐ and Off‐Axis Faults and Fissures During Eruption Cycles and Crustal Accretion at 9°50′N, East Pacific Rise

AbstractFissures and faults provide insight into how plate separation is accommodated by magmatism and brittle deformation during crustal accretion. Although fissure and fault geometry can be used to quantify the spreading process at mid‐ocean ridges, accurate measurements are rare due to insufficiently detailed mapping data. Here, fissures and faults at the fast‐spreading 9°50′N segment of the East Pacific Rise were mapped using bathymetric data collected at 1‐m horizontal resolution by autonomous underwater vehicle Sentry. Fault dip estimates from the bathymetric data were calibrated using co‐registered near‐bottom imagery and depth transects acquired by remotely operated vehicle Jason. Fissures are classified as either eruptive or non‐eruptive (i.e., cracks). Tectonic strain estimated from corrected fault heaves suggests that faulting plays a negligible role in the plate separation on crust younger than 72 kyr (<4 km from the ridge axis). Pre‐ and post‐eruption surveys show that most fissures were reactivated during the eruptions in 2005–2006. Variable eruptive fissure geometry could be explained by the frequency with which each fissure is reactivated and partially infilled. Fissure swarms and lava plateaus in low‐relief areas >2 km from the ridge are spatially associated with off‐axis lower‐crustal magma lenses identified in multichannel seismic data. Deep, closely spaced fissures overlie a relatively shallow portion of the axial magma lens. The width of on‐axis fissures and inferred subsurface dike geometry imply a ∼9‐year long diking recurrence interval to fully accommodate plate spreading, which is broadly consistent with cycle intervals obtained from estimates of melt extraction rates, eruption volumes, and spreading rate.

Shallow Low‐Velocity Layer in the Hyuga‐Nada Accretionary Prism and Its Hydrological Implications: Insights From a Passive Seismic Array

AbstractShear wave velocity (Vs) estimations of accretionary prisms can pose unique constraints to the physical properties of rocks, which are hard to obtain from compressional velocities (Vp) alone. Thus, it would help better understand the fluid processes of the accretion system. This study investigates the Vs structure of the Hyuga‐nada accretionary prism using an array of ocean‐bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green's functions and a surface wave dispersion curve are inverted to one‐dimensional Vs structures using transdimensional inversion. The results indicate the presence of a low‐velocity zone 3–4 km below the seafloor. The reduced Vs corresponds to a reduced Vp feature obtained in a previous seismic refraction survey, and the high Vp/Vs ratio suggests the presence of high pore fluid pressure. In addition, the resolved lithological boundary exhibits a sharp offset that extends laterally across the OBS array. We attribute this offset to a blind fault below while acknowledging other possibilities, such as due to mud diapirism or intense fracturing. The predicted fault is located at the Kyushu–Palau Ridge flank, oriented roughly parallel to the ridge axis, and thus likely caused by ridge subduction. This fault may act as a fluid conduit, contributing to the formation of a fluid reservoir beneath the compacted sediment layers.

Quantifying the influence of magmatism and tectonism on ultraslow-spreading-ridge hydrothermal activity: Evidence from the Southwest Indian Ridge

Hydrothermal activity in mid-ocean ridges (MORs) is an important intermediary for the mass and heat exchange between the ocean and lithosphere. The development of hydrothermal activity on MORs is primarily controlled by coupled magmatic and tectonic activities. In ultraslow-spreading ridges, deep-dipping low-angle normal faults with large offsets, typically detachment faults in the inside corners of ridge offsets, favor the formation of tectonic-related hydrothermal activities, whereas volcanic-related hydrothermal fields are typically developed in neovolcanic zones in this category of the ridge system. However, whether tectonic or magmatic activity is dominant and to what extent they control the formation of hydrothermal activities on ultraslow-spreading ridges remain unclear. Segments in the west and east of the Gallieni transform fault (TF) located in the ultraslow-spreading Southwest Indian Ridge (SWIR), namely, western area (WA) and eastern area (EA), exhibit distinct magma-supply conditions that provide favorable conditions for examining the influence of magmatic and tectonic activities. We generated prediction models for these areas using the spatial analysis of the water depth, minor faults, large faults, ridge axis, nontransform discontinuity (NTD) inside corners, TF inside corners, Bouguer gravity anomaly, magnetic anomalies, and seismic activities. By employing the weights of evidence method, we reported that the formation of seafloor hydrothermal systems in SWIR was primarily correlated to the NTD inside corner, ridge axis, and minor fault (i.e., contrast values (C) of 4.186, 3.727, and 3.482 in WA and 4.278, 3.769, and 3.135 in EA). Furthermore, EA was significantly affected by the TF inside corner (C = 3.501), whereas WA was influenced by large faults (C = 4.062). Our results demonstrated that tectonism was the primary controlling factor in the development of hydrothermal activities in the study area, and the contribution of magmatism was secondary, even in WA, which has a relatively robust magma supply. We delimited prominent prospecting areas at each side based on posterior probability. Our results provided insights into the formation mechanisms of hydrothermal activities and support prospecting in MORs.

Hydrostatic and Non‐Hydrostatic Baroclinic Instability in the Dynamical Core of the DOE Global Climate Model

AbstractThe dynamical core of the Department of Energy global climate model is used to understand the role of non‐hydrostatic dynamics in the simulation of dry and moist baroclinic waves. To reduce computational cost, the Diabatic Acceleration and REscaling approach is adopted. Scale analysis and numerical simulations suggest that the model solution is not distorted by the change of spherical metric terms due to the change of Earth radius, neither are the inter‐scale interactions strongly altered as indicated by the analysis of spectral flux. Compared with hydrostatic simulations, the onset of baroclinic instability is delayed under non‐hydrostatic dynamics as the associated weaker vertical motions tend to increase the critical wavelength and narrow the range of waves that can be baroclinically unstable. During the development of baroclinic waves, non‐hydrostatic dynamics tends to induce vertical motions in the upper troposphere, accelerating the eastward propagation of upper‐level ridges through their impact on local vorticity tendency. These processes reduce the westward tilt of the vertical ridge axes and suppress the conversion of mean flow available potential energy to eddy kinetic energy, leading to weaker baroclinic eddies than in hydrostatic simulations. The contrasts between hydrostatic and non‐hydrostatic settings hold in supplemental experiments that vary the background flow Rossby number, the amount of water vapor content and vertical resolution. We also find that mesoscale and smaller‐scale activities can be considerably under‐represented when the vertical resolution is limited to that typically used in global climate models.

Oceanic Core Complex or not? When bathymetric structures challenge seafloor spreading models

We use regional magnetic and local multibeam bathymetric data to investigate the geological nature of a prominent high located off-axis near the junction between the Mohns and the Knipovich ridges, in the Norwegian-Greenland Sea. Given the location of this hill, near the bend marking the junction between these two ultraslow spreading centers and the bathymetric asymmetry on both sides of the ridge, an Oceanic Core Complex, corresponding to the outcropping part of an underlying detachment, was proposed as a possible interpretation. Nevertheless, the magnetization distribution over this hill is compatible with a basaltic environment and magnetic gradients are also identical to those existing in basaltic contexts, pointing towards a similar geology. Finally, a combined plate and magnetic reconstruction reveals that this hill has a conjugate structure and that regional magnetic anomalies are symmetric. These observations rule out the Oceanic Core Complex hypothesis, as detachment surfaces are often associated with a marked spreading asymmetry. We therefore propose that this protruding bathymetric feature should rather be seen as a basaltic hill and the depth asymmetry likely results from seafloor subsidence triggered by the massive accumulation of sediments from the Bear Island Fan, on the eastern flank of the ridge axis.

The spreading rate dependence of the distribution of axial magma lenses along mid-ocean ridges

Seismically imaged axial melt lenses (AMLs) are seen almost everywhere along the axis of fast-spreading ridges but at only a few localized segment centers on slow-spreading ridges. Standard models assuming that AMLs form when melt percolating upward pools where freezing produces an impermeable cap do not explain this fundamental observation. To tackle this long-standing problem, we combine a crustal density model and a thermal model with a recent mechanical model for sill formation. The mechanical model predicts that AMLs form below the axial lithosphere but only if the average density of the axial brittle lithosphere is not greater than the magma density. For standard thermal models, crustal density structures inferred from seismic velocity data and normal crustal thicknesses, AMLs are found to be stable along all of a ridge segment for spreading rates greater than about 50 mm/y. To explain slow-spreading observations, we assume that a share of the melt produced by the mantle upwelling all along a segment is focused to the segment center. Some of this melt partially crystallizes, releasing latent heat, before the evolved magma flows along the axis to build the crust away from the segment center. This "extra" heat, beyond what is supplied by the magma that builds the crust near the segment center, results in the lithosphere thin enough for stable melt lenses at the segment center. Our results are consistent with observations and offer a quantitative explanation of the marked difference in the distribution of AMLs along fast- versus slow-spreading centers.

Temporal and spatial variation of seafloor spreading at ultraslow spreading ridges: Contribution of marine magnetics

The heterogenous magma supply at ultraslow spreading ridges creates diverse seafloor morphologies and lithospheric structures, which in turn generate a large variability in marine magnetic anomalies. The variability brings difficulties to interpret the evolution of oceanic lithosphere. On the other hand, different magnetic signatures of different seafloors provide an opportunity to identify the modes of seafloor spreading on the ultraslow spreading ridges. Here, we modeled several across-axis magnetic profiles selected from the Gakkel Ridge, Southwest Indian Ridge and Mid-Cayman Spreading Center to explore the lithospheric structure and seafloor spreading processes. Considering conjugate flanks, we observed three modes of seafloor spreading, Magmatic vs Magmatic, Magmatic vs Tectonic, and Tectonic vs Tectonic, on the three ultraslow spreading ridges. These three spreading modes reflect a strong, intermediate, and starved magma supply, respectively. Furthermore, four alternances of the different spreading modes were identified including the Magmatic vs Magmatic to Magmatic vs Tectonic, Magmatic vs Tectonic to Tectonic vs Magmatic, Tectonic vs Tectonic to Magmatic vs Magmatic, and Mixed. These alternances of spreading modes in across-axis direction suggest oscillations of the magma supply at different levels. Variation in the modes of seafloor spreading on four nearby profiles over the Mid-Cayman Spreading Center reveals the evolution of magma supply along the axis of this short ridge.

The off-axis plume–ridge interaction model: Confirmation from the mineral chemistry of Cretaceous basalts of the Ontong Java Plateau

The Ontong Java Plateau (OJP) is the largest known igneous province on Earth. The proposal that its origins involved off-axis plume–ridge interactions remains highly controversial. Here we present new in-situ major (EPMA), trace element (LA-ICP-MS) and Sr isotopic (LA-MC-ICP-MS) characterization of clinopyroxene and plagioclase phases present in OJP tholeiitic basalts from Sites 1183, 1186, 1185, 1187. Site 1187 and Upper Site 1185 have yielded olivine tholeiites containing olivine, Ca-rich augite, Ca-deficient pigeonite, and plagioclase, inferred to be derived from higher degrees of partial melting of dominantly peridotitic mantle and reflecting relatively low crystallization temperatures. Sites 1183 and 1186 and Lower Site 1185 yielded tholeiitic basalts consisting mainly of plagioclase and Ca-rich augite without olivine, interpreted to originate from lower degrees of partial melting of pyroxenite- and peridotite-derived melts with relatively high clinopyroxene and plagioclase crystallization temperatures. Site 1187 and Upper Site 1185 plagioclase, clinopyroxene, and equilibrium melts have much lower REE contents but slightly higher in-situ Sr isotopic ratios compared to those phases at Sites 1183 and 1186 as well as Lower Site 1185. Sites 1186 and 1187 plagioclases and clinopyroxenes display normal zoned textures which are attributed to the process of fractional crystallization and extensive magma mixing beneath the heterogeneous mantle source. In-situ Sr isotopic disequilibrium between plagioclase phenocrysts and plagioclase clusters further indicates an enriched compositional heterogeneity. Due to the strong tensile forces expected at the spreading ridge, the OJP off-axis mantle plume is inferred to be drawn towards the ridge axis, leading to shifts in mineral compositions, degrees of partial melting, and crystallization temperatures inferred for the volcanics of Sites 1183, 1186, 1185, and 1187 as a function of distance from the spreading ridge. The enriched invariability of in-situ Sr isotopic compositions is likely due to the relatively long half-life of the radiogenic elements. These mineral chemistries are consistent with off-axis ridge–plume interaction.

Accretion Processes of Oceanic Crust in Slow‐spreading Ridges: Plagiogranite Perspective of the Xigaze Ophiolite, South Tibet

AbstractStructural and petrological data suggest that the Xigaze ophiolite from the Yarlung Zangbo Suture Zone (YZSZ) in south Tibet was a typical slow‐spreading ridge. A new field, geochemical, mineral, and U‐Pb zircon dataset of plagiogranite intrusions were used to constrain the dynamic processes of oceanic accretion in this slow‐spreading ridge. Plagiogranites mainly occur as dykes or intrusions intruded into the whole sequence of the ophiolite and have a similar orientation to the dolerite dykes developed in the late stage of detachment faulting. U‐Pb zircon ages of 122–123 Ma were obtained for two types of plagiogranites and associated dolerite dykes. Detailed geochemical and mineralogical examinations suggest that the plagiogranites are the product of low‐pressure (2–3 kbar) fractional crystallization of mid‐ocean ridge basalt‐like magma and unlikely to have been derived from the partial melting of hydrous gabbroic rocks. The complex cross‐cut relationship between the plagiogranites and ophiolite sequence reflects that they are controlled by small discontinued melt lenses rather than a big magma chamber under the ridge axis and reveals multiple injections during the oceanic crust accretion. The formation of plagiogranites possibly reflects the complex characteristic of oceanic accretion at slow‐spreading ridges, time‐dependent on structural (external) and magmatic (internal) processes.

Temperatures and Timescales of Crystallization in the “Mush Zone” Beneath Fast‐Spreading Ocean Ridges

AbstractQuantifying the timescales of magma solidification and eruption beneath mid‐ocean ridges is critical to understanding how melts are emplaced and crystallized, and how long these magmas and their crystals remain in a semi‐molten “mush‐like” state. Here, we present a new method to quantify the crystallization and solidification temperatures of individual gabbros in the lower crust of the Wadi Tayin massif (Samail ophiolite). Cotectic crystallization temperatures of olivine + plagioclase + clinopyroxene are constrained by our clinopyroxene Mg# thermometer, and supported by clinopyroxene‐olivine Fe‐Mg equilibration temperatures. We find that lower crustal gabbros reach the cotectic at approximately 1,160 ± 16°C, and are fully solidified between 781 and 1011°C, depending on cooling rate. Our results show that lower crustal gabbros spend, on average, 7,500–11,500 years in a partially molten state with no systematic change in duration with depth in the crust. Samples with very fast cooling rates result in solidification only a few tens of meters off‐axis from their emplacement location and likely indicate solidification near the distal edge of the on‐axis melt lens or in off‐axis melt lenses. Incorporating a uniform 1 km wide emplacement region directly on‐axis, as inferred geophysically, and a half‐spreading rate of 55 mm/yr, our results suggest that the Mush Zone at fast‐spreading ridges remains partially molten for 0–3 km away from the ridge axis, but up to 4–7 km considering all possible sources of uncertainty. Both geophysical and geochemical estimates of the Mush Zone are inconsistent with conductive cooling of the lower oceanic crust.

Ocean expansion: The role of detachment faults

AbstractDetachment faulting at slow spreading ocean ridge axes is recognized as a major surface creation mechanism, yet the structural relationships of these faults with feeder‐dykes of on‐axis volcanoes remained unresolved. This summary study shows that surface creation leading to ocean widening is exclusively controlled by detachment faults, shallow tracers of tectonic stresses induced by the westward drift of plates. Volcanoes are fed by feeder‐dykes following on‐axis rotational detachment faults. Once formed volcanoes are dragged along the detachment whose footwall is made of mantle material sometimes hosting gabbro sills. Due to the faster drift of the uppermost lithospheric layer, the feeder‐dykes are then intersected by active deeper low‐angle detachments, become inactive and are replaced by new ones on‐axis. Rooted vertically on either side of the plate boundary, the detachment flexing is all the earlier and more progressive as the faults are far from the axis, positioning gabbro as sills at shallow level within a deformed mantle interspersed with cataclasite horizons. Correlations between shallow and deep lithospheric processes are then clarified.

Association of Optical Coherence Tomography–Measured Fibrovascular Ridge Thickness and Clinical Disease Stage in Retinopathy of Prematurity

Accurate diagnosis of retinopathy of prematurity (ROP) is essential to provide timely treatment and reduce the risk of blindness. However, the components of an ROP examination are subjective and qualitative. To evaluate whether optical coherence tomography (OCT)-derived retinal thickness measurements at the vascular-avascular junction are associated with clinical diagnosis of ROP stage. This cross-sectional longitudinal study compared OCT-based ridge thickness calculated from OCT B-scans by a masked examiner to the clinical diagnosis of 2 masked examiners using both traditional stage classifications and a more granular continuous scale at the neonatal intensive care unit (NICU) of Oregon Health & Science University (OHSU) Hospital. Infants who met ROP screening criteria in the OHSU NICU between June 2021 and April 2022 and had guardian consent were included. One OCT volume and en face image per patient per eye showing at least 1 to 2 clock hours of ridge were included in the final analysis. Comparison of OCT-derived ridge thickness to the clinical diagnosis of ROP stage using an ordinal and continuous scale. Repeatability was assessed using 20 repeated examinations from the same visit and compared using intraclass correlation coefficient (ICC) and coefficient of variation (CV). Comparison of ridge thickness with ordinal categories was performed using generalized estimating equations and with continuous stage using Spearman correlation. A total of 128 separate OCT eye examinations from 50 eyes of 25 patients were analyzed. The ICC was 0.87 with a CV of 7.0%. Higher ordinal disease classification was associated with higher axial ridge thickness on OCT, with mean (SD) thickness measurements of 264.2 (11.2) μm (P < .001), 334.2 (11.4) μm (P < .001), and 495.0 (32.2) μm (P < .001) for stages 1, 2, and 3, respectively and with continuous stage labels (ρ = 0.739, P < .001). These results suggest that OCT-based quantification of peripheral stage in ROP may be an objective and quantitative biomarker that may be useful for clinical diagnosis and longitudinal monitoring and may have implications for disease classification in the future.

Fractal analysis of lineaments using CryoSat-2 and Jason-1 satellite-derived gravity data: Evidence of a uniform tectonic activity over the middle part of the Central Indian Ridge

The availability of high-accuracy altimetry and satellite gravity data leads to the development of high-resolution global gravity model data. It helps in understanding the structure and evolution of remote oceanic regions. The present study utilizes fractal analysis of structural patterns identified from the Sandwell (2014) gravity data to understand the geological implication over the Central Indian Ridge (CNIR). The normalized standard deviation (NSTD) edge detection technique has been used to identify different possible lineaments/structural patterns. Generally, the Northern, Middle, and Southern blocks of CNIR are characterized by ENE-WSW, NE-SW, NNE-SSW, and NW-SE lineaments. However, the Southern block is additionally characterized by few E-W-oriented lineaments. The lineament density map indicates Southern block exhibit a maximum lineament concentration than other two blocks. The fractal dimensions of lineaments in the Northern, Middle, and Southern blocks are in the order of 1.64, 1.627, and 1.666, respectively. The high fractal dimension and lineament density values of the Southern block indicate a concentrated distribution of lineaments. The low fractal dimension and lineament density values over the Middle block indicate a greater degree of uniformity in the lineament distribution than the other two blocks, representing a relatively stable tectonic setup over the Middle block. The high multifractal width of the Southern block is characterized by variable spreading rates and frequent ridge jumps. The multifractal studies demonstrate the presence of medium-small scale lineaments over the CNIR. The large numbers of small and discontinuous lineaments over the CNIR indicate strong but less frequent tectonic activity along the ridge axis.

Deep hydration and lithospheric thinning at oceanic transform plate boundaries.

Transform faults accommodate the lateral motions between lithospheric plates, producing large earthquakes. Away from active transform boundaries, former oceanic transform faults also form the fracture zones that cover the ocean floor. However, the deep structure of these faults remains enigmatic. Here we present ultra-long offset seismic data from the Romanche transform fault in the equatorial Atlantic Ocean that indicates the presence of a low-velocity anomaly extending down to ~60 km depth below sea level. We performed 3D thermal modelling that suggests the anomaly is likely to be due to extensive serpentinization down to ~16 km depth, overlying a hydrated, shear mylonite zone down to 32 km depth. The water is considered to be sourced from seawater-derived fluids that infiltrate deep into the fault. Below 32 km is interpreted to be a low-temperature, water-induced melting zone that elevates the lithosphere-asthenosphere boundary, causing significant thinning of the lithosphere at the transform fault. The presence of a thinned lithosphere at transform faults could explain observations of volcanism, thickened crust and intra-transform spreading centres at transform faults. It also suggests that migration and mixing of water-induced melt with the high temperature melt may occur beneath the ridge axis.

A new freshwater snail genus and species (Gastropoda: Caenogastropoda, Cochliopidae) with extremely spinous shells from sub-recent spring deposits in northeastern Mexico.

A new monotypic genus of freshwater snail from late Holocene spring deposits in Viesca, Coahuila (Mexico), is described based on shell morphology. Spinopyrgus luismaedai n. gen. et n. sp. has two to three carinate shells with long and wide shovel-shaped spines, strong axial ridges and a pointed protoconch. All sculptural ornamentations on the teleoconch are part of the calcareous shell material and not projections of the periostracum. This combination of shell features and their almost marine-like appearance is unknown among North American recent and fossil freshwater snails. Because of its shell characteristics, we placed the new genus tentatively in the Cochliopidae. The springs of Viesca dried up in the second half of the 20th century so that any living occurrence of this species in neighboring areas is unlikely, rendering the new genus and species possibly extinct.

The Role of Crustal Accretion Variations in Determining Slab Hydration at an Atlantic Subduction Zone

AbstractWe present a 2D P‐wave velocity model from the outer rise region of the Lesser Antilles island arc, the first wide‐angle seismic study of outer rise processes at an Atlantic subduction zone. The survey consists of 46 OBS receivers over a 174 km profile with velocities resolved to 15 km below top basement. The final velocity model, produced through tomographic inversion, shows a clear decrease in the velocity of the lower crust and upper mantle of the incoming plate as it approaches the trench. We attribute this drop to outer rise bend‐related hydration, similar to Pacific cases, but superimposed on spatial variations in hydration generated at the slow‐spreading ridge axis. In thin, tectonically controlled crust formed under magma‐poor spreading conditions the superposition of these sources of hydration results in compressional velocities as low as 6.5 km s−1 beneath the PmP reflector. In contrast, segments of crust interpreted as having formed under magma‐rich conditions show velocity reductions and inferred hydrous alteration more like that observed in the Pacific. Hence, variations in the style of crustal accretion, which is observed on 50–100 km length scales both along and across isochrons, is a primary control over the distribution of water within the slab at Atlantic subduction systems. This heterogeneous pattern of water storage within the slab is likely further complicated by along strike variations in outer rise bending, subducting fracture zones and deformation at segment ends and may have important implications for our understanding of long‐term patterns of hazard at Atlantic subduction systems.

Hydroacoustic Monitoring of Seafloor Spreading and Transform Faulting in the Equatorial Atlantic Ocean

AbstractSeismicity along mid‐ocean ridges and oceanic transform faults provides insights into the processes of crustal accretion and strike‐slip deformation. In the equatorial Atlantic ocean, the slow‐spreading Mid‐Atlantic Ridge is offset by some of the longest‐offset transform faults on Earth, which remain relatively poorly understood due to its remote location far from land‐based teleseismic receivers. A catalog of T‐phase events detected by an array of 10 autonomous hydrophones deployed between 2011 and 2015, extending from 20°N to 10°S is presented. The final catalog of 6,843 events has a magnitude of completeness of 3.3, compared to 4.4 for the International Seismic Center teleseismic catalog covering the same region, and allows investigation of the dual processes of crustal accretion and transform fault slip. The seismicity rate observed at asymmetric spreading segments (those hosting detachment faults) is significantly higher than that of symmetric spreading centers, and 74% of known hydrothermal vents along the equatorial Mid‐Atlantic Ridge occur on asymmetric spreading segments. Aseismic patches are present on nearly all equatorial Atlantic transform faults, including on the Romanche transform where regional rotation and transpression could explain both bathymetric uplift and reduction in seismic activity. The observed patterns in seismicity provide insight into the thermal and mechanical structure of the ridge axis and associated transform faults, and potentially provide a method for investigating the distribution of hydrothermal vent systems.

Off-rift Axis Channelized Melt and Lithospheric Metasomatism along Mid-ocean Ridges—A Case Study from Iceland on the Limits of Melt Channelling

AbstractMagmatism in Iceland is classically explained by the interaction of the mid-Atlantic ridge with the Iceland plume. The growth of Iceland through time is the result of volcanic activity at the rift axis. However, Holocene volcanism (0–11.5 ka) is not restricted to the rift zone (RZ) but also occurs off-axis, specifically in the western Snæfellsnes Volcanic Belt (SVB) and in the Southern Flank Zone (SFZ). The rift and off-axis postglacial volcanic zones are separated by a gap of ~60–80 km. While the volcanic activity of the SFZ seems correlated with the actual location of the Iceland plume, the plume relation to the SVB is uncertain. To address the origin and relationships between rift and off-rift magmas, we present new data from two transects perpendicular to the Reykjanes rift. The lavas in the SVB are characterized by transitional to alkaline compositions, with elevated incompatible trace element content. In contrast, the RZ volcanic rocks have tholeiitic compositions with trace element signatures slightly more enriched than Mid-Ocean Ridge Basalt (MORB). Rift and off-rift Iceland lavas are all characterized by positive Ba and Nb anomalies, particularly in alkaline rocks. Tholeiitic and alkaline lavas show distinct differentiation sequences, with the main difference being the delayed crystallization of plagioclase in the fractionating assemblage of alkaline magmas. We apply these sequences to calculate primary magma compositions, which are then used to constrain melting conditions. Geochemical modelling indicates that Iceland rift and off-rift magmas can be produced from a peridotitic mantle source if lithospheric processes are involved. We demonstrate that recycled crust in the form of pyroxenite is not required to generate Snæfellsnes alkaline lavas. The low solidus temperature and high productivity of pyroxenite favour early and more extensive melting producing primary magmas that are not sufficiently enriched in incompatible trace elements to explain the compositional variation of Snæfellsnes magmas. An alternative mechanism to involve pyroxenite in the source of Snæfellsnes lavas relates to the hybridization of recycled oceanic crust with peridotite, but such reacted pyroxenite requires specific compositions to reproduce the Ba and Nb anomalies. As an alternative, we suggest that Snæfellsnes alkaline lavas are the result of channelized low-degree melts produced on the periphery of the melting column at distances exceeding 65 km from the ridge axis. These melts accumulate and percolate into the lithosphere producing amphibole ± phlogopite-bearing hydrous cumulates. Incongruent melting of these cumulates via renewed magmatic activity and melt-peridotite reaction can reproduce the alkaline compositions observed in the SVB, including the Ba and Nb anomalies. Numerical simulations of melt extraction below mid-ocean ridges suggest that low-degree melts produced as far as ~65 km from the central ridge axis rise vertically to the base of the lithosphere and are then focussed towards the ridge axis in decompaction channels. We propose that these melts interact with hydrous cumulates previously formed during the development of decompaction channels at the lithosphere–asthenosphere boundary and acquire specific Ba and Nb anomalies. The mixing of these distal enriched melts with more depleted melts extracted from the central part of the melting regime explains the composition of RZ lavas. Alkaline lavas observed in the SFZ show numerous analogies with the Snæfellsnes magmas, suggesting that similar lithospheric processes control their chemistry. The high thermal regime in Iceland and thick lithosphere explains the difference between Icelandic tholeiite and typical MORB. Our results highlight the importance of mantle dynamics below mid-ocean ridges and lithospheric interaction to produce off-axis magmatism with enriched alkaline signatures.

Generation and evolution of the oceanic lithosphere in the North Atlantic

Half a century ago, our view of the Earth shifted from that of a Planet with fixed continents and ancient stable ocean basins to one with wandering continents and young, active ocean basins, reviving Wegener’s Continental Drift that had rested dormant for years. The lithosphere is the external, mostly solid and relatively rigid layer of the Earth, with thickness and composition different below the oceans and within the continents. We will review the processes leading to the generation and evolution of the Earth’s lithosphere that lies beneath the oceans. We will discuss how the oceanic lithosphere is generated along mid-ocean ridges due to upwelling of convecting hot mantle. We will consider in particular lithosphere generation occurring along the northern Mid Atlantic Ridge (MAR) from Iceland to the equator, including the formation of transform offsets. We will then focus on the Vema fracture zone at 10°–11° N, where a ~ 300 km long uplifted and exposed sliver of lithosphere allows to reconstruct the evolution of lithosphere generation at a segment of the MAR from 25 million years ago to the Present. This axial ridge segment formed 50 million years ago, and reaches today 80 km in length. The degree of melting of the subridge mantle increased from 16 million years ago to today, although with some oscillations. The mantle presently upwelling beneath the MAR becomes colder and/or less fertile going from Iceland to the Equator, with “waves” of hot/fertile mantle migrating southwards from the Azores plume. Scientific revolutions seem to occur periodically in the history of Science; we wonder when the next revolution will take place in the Earth Science, and to what extent our present views will have to be modified.