Mid-ocean ridges (MORs) form as a result of upwelling and partial melting of the underlying mantle, leading to seafloor spreading and new lithosphere formation. They result from an interplay between different geological forces shaping ocean seafloors and offer insights into Earth's mantle convection and lithospheric evolution. Recent advances in numerical models contributed to describe oceanic rift processes, although complex geodynamic settings remain relatively unexplored.
Knipovich and Mohns ultraslow spreading ridges are located in the Arctic Ocean, separated from Kolbensey and Gakkel ridges by the Jan Mayen transform and Lena Trough. They do not present any evidence of transform fault along their entire length and are characterized by a high obliquity (~35°-50°) with respect to their spreading direction, constituting some of the most intriguing MORs worldwide. At their intersection, geophysical data revealed a focused mantle upwelling along a narrow, oblique, and strongly asymmetric zone, coinciding with uneven surface uplift. These asymmetrical features have been associated to i) the control on passive upwelling of slow and asymmetric motion of the North America and Eurasia plates, or ii) the results of a major spreading reorganization in the area. However, asymmetries are tipically observed in other geodynamic settings, such as for example subduction zones, where they have been related to the relative motion of lithospheric plates with respect to the asthenosphere. In this work we carried out 3D numerical models reproducing the geodynamic evolution of a ~800-km long segment of the Knipovich and Mohns ridges (extending from ~76°N to ~71°N), including their migration with respect to the asthenosphere. The model uses a visco-plastic rheology which approximate both the asthenospheric and the lithospheric mantle, providing information on the temperature and deformation patterns within the mantle. We also computed the degrees of melting beneath each area of the MOR segment. In agreement with previous geophysical and petrological data, our results suggest that mantle upwelling is focused in a narrow zone, where the MOR makes a sharp bend, providing the inferred asymmetric patterns. On this basis, we propose a mechanism which could have led to the asymmetrical features (e.g., topography, spreading rate, mantle temperature and composition, etc.) characterizing the Knipovich-Mohns segments area. 

How to cite: Ficini, E., Marco, C., Marco, L., Edie, M., and Alessio, S.: Evolution history of the Knipovich-Mohns ridge intersection (Artic Ocean) during the last 20 Ma, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13004, https://doi.org/10.5194/egusphere-egu24-13004, 2024.

Understanding the depths of the marine substrate is of vital importance for an increasing variety of fundamental purposes that contribute to the comprehension of our planet's functioning. The primary objective of this research was to map the seafloor through multibeam bathymetry, at Latitude 1º N of the mid-Atlantic ridge, NW of the Archipelago of São Pedro and São Paulo. Data were collected aboard the Hydrographic and Oceanographic Research Ship (NpqHOc) Vital de Oliveira, within the scope of the QWHALES, SeabedMap and PQ MapMar projects. An EM-122 multibeam echosounder was used, in the 12 kHz frequency range, with an opening of 60º and at a speed of 7 knots. Raw data processing was performed with Caris HIPS & SIPS version 11.4.24 software. Until this study, the selected area had not been mapped, meaning that mapping a previously unexplored region of the ocean floor represents a crucial advancement, highlighting the importance of understanding the complexity of the marine environment. The obtained results allowed the generation of a bathymetric surface map at a pixel resolution of 50 meters. With the bathymetric map, it was possible to identify the morphology of slow-spreading mid-ocean ridges. Through elevation profile, an axial valley delimited by edge faults to the axial valley was interpreted. Along the valley, there is a discontinuity with an offset of approximately 17 km from the axis in the mapped area. This metric, associated with the morphology of the expansion axis that develops a narrow and deep axial valley, allowed the classification of non-transforming displacement or second-order displacement. Finally, it was also possible to identify that this discontinuity is located between the South American and African tectonic plates. 

How to cite: Francicleide Sousa da Silva, R., Vital, H., and Farias do Nascimento, A.: Interpretation of a second-order discontinuity at 1°N latitude of the Mid-Atlantic Ridge, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22147, https://doi.org/10.5194/egusphere-egu24-22147, 2024.

The equatorial Atlantic transform faults are among the largest and most complex in the world’s oceans. Among them, the St. Paul Transform System (SPTS) is a large multi-faulted system formed by four slow-slipping transform faults (Transforms A, B, C, and D), accumulating ∼630 km of axial offset (Maia et al., 2016). Transform A is the northernmost fault which contains the Atoba Ridge Zone (ARZ), a large transpressive ridge formed at a large stepover (Maia et al., 2016). St. Peter and St. Paul (SPSP) islets sit at the ARZ summit, where is installed a single broadband seismograph (ASPSP) recording local seismicity since 2011 (de Melo and do Nascimento., 2018). Here, we produce a new catalog of the local seismicity recorded around the ARZ between 2011 and 2016. For the epicenter location, we process an initial picking of the P and S waves referent to 359 earthquakes identified manually using SEISAN package applying a 2-12 Hz band-pass filter. Next, we follow a single station approach to locate the epicenters. We estimate the source-receiver distance based on the differential S-P time and the back azimuth from the polarization of P wave recordings, whenever this shows a high rectilinearity coefficient (Montalbetti and Kanasewich., 1970; Cesca et al., 2022). A total of 245 earthquakes were cataloged again using the new improved location process. 54 earthquakes were also identified also by EquatorialAtlanic hydroacoustic catalog (Parnell-Turner et al., 2022) and 12 by the International Seismological Centre. Our results reveal that a large part (174 earthquakes) of the local seismicity is clustered on the west flank of the ARZ, located from 2.18 to 22.58 km southwest of the SPSP islets. Rocks sampled along the ARZ are peridotite mylonites exhumed during the transpressional push-up tectonism of the ARZ. Other minor events are located on the east flank of the ARZ where sample deformation shows strong control of seawater fluid percolation (Bickert et al., 2023), enabling weakening of the rheology and possibly contributing to maintain an aseismic behavior on the faults.

Bickert, M., Kaczmarek, M. A., Brunelli, D., Maia, M., Campos, T. F., & Sichel, S. E. (2023). Fluid-assisted grain size reduction leads to strain localization in oceanic transform faults. Nature Communications, 14(1), 4087.

Cesca, S., Sugan, M., Rudzinski, Ł., Vajedian, S., Niemz, P., Plank, S., ... & Dahm, T. (2022). Massive earthquake swarm driven by magmatic intrusion at the Bransfield Strait, Antarctica. Communications Earth & Environment, 3(1), 89.

de Melo, G. W., & Do Nascimento, A. F. (2018). Earthquake magnitude relationships for the Saint Peter and Saint Paul archipelago, equatorial atlantic. Pure and Applied Geophysics, 175, 741-756.

Maia, M., Sichel, S., Briais, A., Brunelli, D., Ligi, M., Ferreira, N., ... & Oliveira, P. (2016). Extreme mantle uplift and exhumation along a transpressive transform fault. Nature Geoscience, 9(8), 619-623.

Montalbetti, J. F., & Kanasewich, E. R. (1970). Enhancement of teleseismic body phases with a polarization filter. Geophysical Journal International, 21(2), 119-129.

Parnell‐Turner, R., Smith, D. K., & Dziak, R. P. (2022). Hydroacoustic monitoring of seafloor spreading and transform faulting in the equatorial Atlantic Ocean. Journal of Geophysical Research: Solid Earth, 127(7), e2022JB024008.

How to cite: Sampaio de Melo, G. W., Maia, M., Cesca, S., Grevemeyer, I., and do Nascimento, A.: Local seismicity surrounding the Atobá Ridge in the slow-slipping St. Paul transform system, equatorial Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21861, https://doi.org/10.5194/egusphere-egu24-21861, 2024.

Rheological weakening mechanisms play a crucial role in plate tectonics by locally dropping lithospheric strength and leading to the formation of new plate boundaries, including new subduction zones or transform boundaries. Despite the abundance and persistence of large-offset oceanic transform faults, understanding their formation, the role of their preceding continental rifting history and their preservation is less understood. Furthermore, the role of different rheological weakening mechanisms is still debated.

In this study, we aim to better understand the contribution of different rheological weakening mechanisms, including both brittle and ductile processes, to the development of rifts and transform fault zones. To achieve this, we are running a comprehensive series of high-resolution 3D petrological-thermomechanical models (i3ELVIS). These models include elasto-visco-plastic rheology with strain-rate induced weakening, partial mantle melting, oceanic crustal growth, thermal contraction and mantle grain size evolution. To investigate the influence of weakening processes, we compare model evolutions that include strain-induced and strain-rate induced plastic weakening parameters. A particular focus is made on the evolution of locally high plastic strain rate values during the evolution and stabilization of transform faults following rifting and during oceanic spreading. New insight from this study can then be applied on a natural example such as the Romanche Transform Fault Zone located in the equatorial Atlantic.

How to cite: Ritter, S., Balázs, A., and Gerya, T.: The impact of brittle and ductile weakening mechanisms on strain localization and the stabilization of transform fault zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6198, https://doi.org/10.5194/egusphere-egu24-6198, 2024.

Little is known about the geometry of oceanic transform faults.  Although their surface trace and curvature along kinematic small circles has been known since the advent of plate tectonics, their structure at depth remains poorly constrained. The classical assumption is that oceanic transform faults are vertical and delimited at depth by the 600°C isotherm. It is only recently that the deployment of local OBS arrays on major oceanic transform faults have allowed us to investigate their geometry at depth and the link with their seismicity.  Seismic moment tensors of teleseismic events also contain first order information about the geometry of oceanic transform faults, giving us access to the dip of the fault plane that has ruptured.  Abercrombie and Ekström (2001) investigated focal mechanisms along the Chain transform fault (TF) in the equatorial Atlantic Ocean, which indicated a consistent northward dip of the fault along its entire 300-km length.
In this study, we take advantage of the increasing data from global seismic catalogs and conduct a statistical exploration of the dip variations of strike-slip focal mechanisms along more than 80 oceanic transform faults.  Most of them are either vertical to subvertical, depending on the local variability of the data, or show no preferential dip towards a given side of the fault. Although the optimal dip for a strike-slip fault in a classical Andersonian stress state is vertical, we show here that the case of the Chain TF is not isolated and several other oceanic transform faults show a similar deviation to the vertical, including Owen TF in the Indian Ocean, Vema TF in the Atlantic Ocean, and Tharp TF along the Pacific-Antarctic ridge. The measured deviations to the vertical show a maximum at ~20° (dip 70°).   We discuss our observations within the tectonic setting and history of each of these plate boundaries, and speculate on the implications of this maximum dip for the mechanical properties and/or stress conditions in oceanic lithosphere. 

How to cite: Janin, A., Behn, M. D., and Tian, X.: Global variations in the dip geometry of oceanic transform faults, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4461, https://doi.org/10.5194/egusphere-egu24-4461, 2024.

Fracture zones were recognized to be an integral part of the seabed long before plate tectonics was established. Later, plate tectonics linked fracture zones to oceanic transform faults, suggesting that they are the inactive and hence fossil trace of transforms. Yet, scientist have spent little time surveying them in much detail over the last three decades. Recent evidence (Grevemeyer, I., Rüpke, L.H., Morgan, J.P., Iyer, K, and Devey, C.W., 2021, Extensional tectonics and two-stage crustal accretion at oceanic transform faults, Nature, 591, 402–407, doi:10.1038/s41586-021-03278-9) suggests that the traditional concept of transform faults as being conservative (non-accretionary) plate boundary faults might be wrong. Instead, transform faults are always deeper than the associated fracture zones and numerical modelling results suggest that transform faults seem to suffer from extensional tectonics below their strike-slip surface fault zone. In 2021, we tested this hypothesis by collecting, in a pilot study, micro-seismicity data from the Oceanographer transform fault which offsets the Mid-Atlantic Ridge by 120-km south of the Azores near 35°N. Analysis of 10-days of seismicity data recorded at 26 ocean-bottom-seismometers and hydrophones showed 10-15 local earthquakes per day. Furthermore, a sparse network recorded micro-earthquakes for three months. Joint interpretation of the data shows that earthquakes away from the ridge-transform intersections cluster along the fault trace imaged in bathymetric data and focal mechanisms support strike-slip motion. However, at the ridge-transform intersections seismicity does not mimic a right-angular plate boundary; instead, seismicity occurs below the inside corner and focal mechanism indicate extensional tectonics. In addition, we put published micro-earthquake data from surveys conducted in the 1970 to 1980s from the Oceanographer, Kane and Vema transform fault in a new context by plotting them onto modern swath-bathymetric data. In concert, micro-seismicity supports features found in numerical simulations, revealing that transform faults have an extensional as well as a strike-slip component.

How to cite: Grevemeyer, I., Lange, D., Ruepke, L., Klauke, I., Bienest, A., Gómez de la Peña, L., Ren, Y., Hilbert, H.-S., Li, Y., Murray-Bergquist, L., Unger Moreno, K., Hansteen, T., and Devey, C. W.: Extensional tectonics at ridge-transform intersections – constraints from micro-seismicity and bathymetric data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4970, https://doi.org/10.5194/egusphere-egu24-4970, 2024.

The role of transform faults, significant plate boundaries located on the seafloor, in influencing and modifying the spreading processes of adjacent mid-ocean ridges has long been a subject of investigation. However, the reciprocal impact of spreading rate and magma supply on the development and demarcation of transform faults has not been fully addressed. Observations from the Atlantic further suggest that long offset transform faults tend to remain stable upon variations in magma supply at the adjacent ridge segments, while shorter transforms are frequently abandoned during axis reorganizations by e.g. propagating ridges. In this study, we developed a three-dimensional model to examine the response of transform faults to variations in magma supply at the adjacent ridges.  In a suite of model runs, we change offset, transform fault rheology, and magma supply to evaluate if shorter transforms are more likely to be abandoned or replaced by non-transform offsets than longer transforms. We confront our modeling insights with observations from the North Atlantic, particularly between latitudes 30°N and 35°N, where the Atlantis, Hayes, and Oceanographer transforms appear to have been stable on long time scales, while the shorter segmentations in between have experienced multiple reorganizations.

How to cite: Zhang, Y., Ruepke, L., Zhang, F., Liu, S., and Grevemeyer, I.: Modeling the Evolution of Transform Faults: Influence of Mid-Ocean Ridge Spreading Dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20491, https://doi.org/10.5194/egusphere-egu24-20491, 2024.

The Mid-Atlantic Ridge (MAR) is a complex system comprising spreading centers, transform faults and associated volcanic features, and it is divided into distinctive accretionary segments by axial discontinuities (Sempere et al., 1990). Our focus is on two specific sections of the MAR recently surveyed during several cruises, the central MAR and the equatorial MAR. Our aim is to investigate the complex interplay between tectonics, volcanism and hydrothermal processes in the building of the axial lithosphere.

At present, our investigation focuses on the central Mid-Atlantic Ridge (24° N to 24°40’ N), surveyed by the HERMINE 1 & 2 cruises. This area is characterized by extensive faulting demonstrating a broad spectrum of lengths and thrust magnitudes. The fault lengths vary from tens of meters to several kilometers, while thrust ranges from a few meters to 1.2 kilometers. The volcanic characteristics in this area encompass narrow, axis-parallel ridges emplaced on the rift valley floor as well as a range of volcanic features, from minor cones to substantial volcanoes.

The rift valley comprises a sequence of intermittent deep basins featuring minor linear topographic elevations on the valley floor associated with recent volcanism, suggestive of recent dike emplacement. The terrain is notably uneven, particularly in the northern section, where talus accumulates adjacent to a large normal fault of 9.2 kilometers long with a vertical offset of 930 meters, located at the segment end, possibly a detachment fault. Volcanic summits are present near this fault, suggesting a potential failed detachment. The spreading axis exhibits a slight clockwise rotation at 24°30’ N. Samples collected during the HERMINE 1 and 2 cruises through dredging and dive methods include various rock types such as basalts, gabbros and serpentinized/mineralized peridotites. The presence of serpentinized/mineralized peridotites provides clues about hydrothermal circulation and exhuming mantle rocks to the seafloor.

By addressing a joint study of the bathymetry and the sample petrology, we will study the evolution of this portion of the ridge axis in connection with tectonic, magmatic, and hydrothermal processes.

How to cite: Rajeevan, R., Maia, M., Rospabé, M., Olive, J. A., and Pelleter, E.: Role of lithosphere and mantle composition on tectonic and magmatic variability of oceanic accretion at slow spreading ridges, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5134, https://doi.org/10.5194/egusphere-egu24-5134, 2024.

Fault scarps at Mid-Ocean Ridges (MOR) are well recognized on the seafloor and often measured to estimate the tectonic component of plate spreading. However, tectonic strain estimates based on the dimensions of fault scarps that can be traced on seafloor topographic maps (which we refer to as apparent tectonic strain) differ from the actual whole tectonic strain. This is clearly the case at relatively melt-poor slow-ultraslow ridge segment ends, where strain is accommodated by detachment faults that do not produce linear fault scarps at the seafloor. This contribution explores the relation between actual and apparent tectonic strain in magma robust MOR regions (at fast, intermediate spreading ridges, and in the magmatically robust segment centers of slow-ultraslow ridges). We use high-resolution (1-2 m) bathymetry data at 8 MOR sites, which span a broad range of spreading rates (14-110 km/Ma) and melt fluxes. To the first degree, apparent tectonic strain is highest at slow spreading ridges, which have the lowest melt fluxes, and decreases as melt flux increases (fast spreading ridges). We examine how faults nucleate and evolve on the young axial seafloor, while establishing the relationships to volcanism. Apparent tectonic strain derived from the dimensions of fault scarps on the young seafloor is reduced due to lava flows that cover pre-existing faults. Apparent tectonic strain also includes a component of strain that is not related to far-field tectonic stresses but to stalled dike intrusions that induce extensional faults in the shallow crust. This mechanism is probably responsible for the high apparent tectonic strain estimated at domal volcanos found at the center of intermediate-slow-ultraslow ridge segments. Apparent tectonic strain at and near MOR thus poorly reflects the real tectonic component of plate divergence, instead relating to the interplay between tectonic and magmatic processes over different time scales.

How to cite: Cannat, M., Chen, J., and Escartin, J.: Fault scarps and tectonic strain in young seafloor, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17326, https://doi.org/10.5194/egusphere-egu24-17326, 2024.

Large-offset detachment faults are commonly observed at slow-spreading mid-ocean ridges (MORs), typically in areas with a moderate to low magma supply (e.g., 13º20'N on the Mid-Atlantic Ridge). Detachments are also found at nearly amagmatic sections of ultraslow MORs (e.g., 64ºE on the Southwest Indian Ridge), where the seismogenic lithosphere is unusually thick (> 15 km). There, detachments of opposing polarity form in sequence and cross-cut each other in a "flip-flop" regime. Prior studies have shown that marked strength contrasts, resulting from reduced cohesion and/or friction in fault zones, promote stable detachments. Here we present 2-D thermo-mechanical models based on geological observations to examine how strength contrasts between fault zones and the adjacent lithosphere impact the modes of faulting at an ultraslow and nearly amagmatic ridge axis.

We model the brittle lithosphere as a Mohr-Coulomb elasto-plastic material, where cohesion and friction diminish with increasing plastic strain. We explore a broad range of cohesion and friction contrasts between deformed and intact material. We also consider the influence of a strong, viscous lower lithosphere on the brittle deformation of the upper lithosphere by comparing simulations that use a dry olivine flow law with models where the brittle lithosphere sharply transitions into a low-viscosity asthenosphere. Fluid circulation in the shallow axial lithosphere is also considered, parameterizing both the cooling and the mechanical effect of hydrothermal circulation.

Our simulations produce three distinct regimes: (1) sequential development of horsts bound by two active antithetic faults, (2) formation of intersecting “flip-flopping” detachments, (3) runaway detachments. The latter case describes models in which a single detachment remains active. In nature, this endmember case is not observed, probably because it results in an excessive migration of the detachment toward its hanging wall. We show that these 3 regimes transition over a narrow range of cohesion and friction contrasts between deformed and intact material (the contrast in friction coefficient over which our simulations transition from regimes 1 to 3 is only 0.1- 0.2). Distributed footwall damage produces antithetic proto-faults, but their ability to mature as major seafloor-breaching faults depends on the degree of rheological weakening. A stronger lower lithosphere promotes such distributed faulting and modifies the onset of the persistent detachment regime to greater strength contrasts. The impact of hydrostatic fluid pressure on tectonic styles is relatively minor compared to fault weakening.

The results of these simulations are consistent with an analytical force balance model that compares the (localizing) loss of fault strength in the detachments to the (delocalizing) flexural force that develops in the surrounding lithosphere. Detachments persist when the magnitude of fault strength loss exceeds the maximum bending force. We find that runaway detachments require a total loss of integrated strength in excess of 1.5e12 N.m, equivalent in our models to a drop in friction coefficient by ~0.25–0.3 in fault zones. Thus, even a moderate frictional weakening, such as that allowed by the presence of lizardite in the fault zone (frictional strength of 0.45) enables large-offset (>15 km) faulting.

How to cite: Demont, A., Cannat, M., and Olive, J.-A.: Modes of detachment faulting at slow and ultraslow mid-oceanic ridges, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15593, https://doi.org/10.5194/egusphere-egu24-15593, 2024.

Oceanic detachment faults (ODFs) are critical drivers of plate separation in slow-to-ultraslow spreading mid-ocean ridges (MORs). Numerous previous studies have shown that the anatomy of ODFs varies significantly between more magmatic and nearly amagmatic ridges sections. More magmatic ODFs are typically dome-shaped, corrugated, and face volcanic seafloor on the hanging wall side, while the footwall exhumes upper crustal and mantle-derived rocks, including gabbro, serpentinized peridotite, basaltic breccia, and diabase. In contrast, nearly amagmatic ODFs (e.g., at 64°E Southwest Indian Ridge SWIR) are characterized by long (up to ~95 km) broad and smooth ridges with no visible corrugations in the shipboard bathymetry. These ODFs form in alternate polarity and exhume serpentinized peridotite to the seafloor on both plates. Here, we present the anatomy of the exposed fault zone in the footwall of D1, a young active ODF in the nearly amagmatic 64°35'E region of the SWIR, utilizing shipboard bathymetry, micro-bathymetry, and ROV dive observations to document their footwall geology, deformation patterns, and along-strike variations.

The axial valley wall in the study area corresponds to the footwall of D1, and high-resolution bathymetry shows that it exposes two distinct domains. The western domain displays corrugations similar to those documented in more magmatic, domal ODFs, while the eastern domain is smooth. The western, corrugated domain also displays small offset ESE-trending antithetic normal faults and several hecto-to-kilometers-wide NNE-trending ridges, interpreted as mega-corrugations that formed due to hecto-to-kilometer-scale phacoids between linked fault splays in the detachment damage zone. These ridges and the antithetic minor faults are absent in the smooth eastern domain. Outcrop scale ROV dive observations show one significant difference in the geology of the exposed fault zone between the two domains: in the east, the fault zone comprises up to 10 m-thick intervals of serpentine microbreccia and chrysotile gouge, while in the west, these highly deformed horizons are a few decimeter-thick at the most, surrounding phacoids of less deformed serpentinites and therefore less pervasive at the outcrop scale. These observations suggest a stronger fault and footwall in the corrugated region. Microstructural observations also suggest that hydrous fluids facilitated the formation of the gouges and that non-brittle mechanisms (serpentine dissolution and precipitation) were involved.

Compared with more magmatic, domal corrugated ODFs, the smooth eastern part of our study area exposes thicker and more pervasive intervals of cataclastic microbreccia and gouge. In contrast, previous work on domal ODFs shows that strongly deformed intervals there consist mostly of talc±tremolite±chlorite±serpentine schist. Experimental studies suggest that the frictional strength of talc and chrysotile gouge at the sample scale are comparable. However, the gouge outcrops in the eastern part of the D1 footwall are thick and promote the formation of a planar (smooth) exposed fault surface. By contrast, highly deformed talc-bearing schists at domal ODFs are found around meter to decameter-scale phacoids of less deformed rocks, which are probably the cause of the observed corrugations.

How to cite: Mahato, S. and Cannat, M.: More magmatic versus less magmatic oceanic detachment fault zone anatomy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17664, https://doi.org/10.5194/egusphere-egu24-17664, 2024.

At ultra-slow spreading ridges, with full spreading rates less than ~15-20 mm/yr, spreading is accommodated both by limited, highly spatially and temporally segmented magmatism, and by tectonic extension along large-scale oceanic detachment faults, which cut from the seafloor through into the upper mantle and exhume ultramafic material to the seafloor. Detachment faulting is highly asymmetric and alternates in polarity over time, producing a “flip-flopping” effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed “smooth seafloor” comprising elongate, broad ridges, which have peridotite/serpentinite lithologies distinct from the typical basalt-gabbro layered oceanic crustal structure. We refer to the outer layer, above the mantle, in this case as the “crustal section”.

We conducted tomographic travel-time inversion of a 3-D wide-angle seismic dataset acquired over a region of smooth seafloor around 64˚30’E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. The resulting velocities support a non-magmatic origin for the crustal section, up to 100% alteration of originally ultramafic compositions to serpentinite, and a near-constant thickness of ~3.4 km into a transitional Moho zone which overlies the unaltered mantle. Patterns of velocity anomalies are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid-rock interaction, controlled by faulting and tectonic damage processes and progressive porosity infill. The detachment faults show limited along-axis extent and are not simple planar structures at depth, instead mirroring the shapes of the bathymetric ridges they exhume. The boundaries between smooth seafloor and adjacent more magmatic segments are not vertical at depth, suggesting that detachment processes extend laterally at depth beyond their mapped extent seen at the seafloor. Magmatic input is overall highly limited and dominantly takes the form of individual flows forming superficial veneers, but there is one region on the lower part of an exhumed detachment footwall where the magmatic section is up to ~1.5 km thick, which may reflect changes in larger-scale magma segmentation which could contribute to detachment abandonment.

How to cite: Robinson, A., Watremez, L., Leroy, S., Minshull, T., Cannat, M., and Corbalán, A.: A 3-D Seismic Tomographic Study of Spreading Structures and Smooth Seafloor Generated by Detachment Faulting – the Ultra-Slow Spreading Southwest Indian Ridge at 64˚30’E, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11449, https://doi.org/10.5194/egusphere-egu24-11449, 2024.

The 70-km long Lucky Strike segment at 37^oN on the Mid-Atlantic Ridge is characterized by a well-defined median valley bounded by ridge-ward dipping faults and a volcano located at the segment centre. The central volcano is fed by an axial magma chamber at ~3 km depth below seafloor. Away from the axial valley, the seafloor morphology is dominated by fault-controlled abyssal hills. Several active hydrothermal vents have been observed on the summit of the Lucky Strike volcano, and the axial magma chamber has been suggested to be the source supplying heat. Seismic velocities of the subsurface provide constraints on porosity and thus permeability, as well as the distribution of hot or molten rock. Therefore, determining fine-scale velocity structure of crust formed on the Lucky Strike segment is critical for understanding the interactions between magmatic, tectonic and hydrothermal processes during crustal accretion.

We performed two-dimensional elastic full waveform inversion (FWI) to wide-angle ocean bottom seismometer (OBS) data to constrain the velocity of the crust beneath a profile along the entire Lucky Strike segment and another across the axis extending to ~50 km distance on both flanks. The seismic data were acquired during the SEISMOMAR survey in 2005. The OBS intervals vary between 4.5 and 14.0 km, with denser OBS deployed around the central volcano. Both profiles were shot multiple times, with shot spacings of 425 m and 150 m. Starting models for FWI were obtained from travel time tomography of the OBS dataset. The FWI results show a low velocity anomaly (LVA) beneath the Lucky Strike volcano at ~3 km depth below seafloor, just below the axial magma chamber reflector imaged in seismic reflection data. The LVA extends ~10 km along axis and ~4 km across axis and is ~0.7-1 km thick in depth. Taking the depth of 6.5 km/s velocity contour as the base of upper crust, the upper crust thickens along the axis from 2.2 km at segment centre to 3.5 km at the segment ends. In contrast, the crustal thickness reduces from ~8.3 km at the segment centre to 4.0-4.5 km at distal ends, assuming the 7.1 km/s velocity contour corresponds to the crustal base. This contrast is due to the significant reduction in lower crustal thickness towards the segment ends, where the upper-to-lower crustal thickness ratio increases from ~0.4 to >3.5 from segment centre to ends. These observations suggest the presence of focused magma supply to the segment centre along the Lucky Strike segment and that the igneous crust at the segment ends is formed primarily by magma eruption and/or diking. The upper crustal thickness has smaller variations in the across-axis direction to ~40 km distance, suggesting the current magmatic accretion mode could have been going for ~3.5 Myr. Beneath the central volcano, the crustal velocity is higher in the along axis direction above the LVA, suggesting seismic heterogeneity in the upper crust.

How to cite: Wang, Z., Minshull, T. A., and Singh, S. C.: Fine-scale crustal velocity structures along and across the Lucky Strike segment of Mid-Atlantic Ridge from full waveform inversion of wide-angle seismic data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12161, https://doi.org/10.5194/egusphere-egu24-12161, 2024.

Intense discharge of high temperature fluids through focused vents at black smoker hydrothermal fields creates local entrainment of cold seawater into the shallow sub-seafloor. This secondary hydrothermal circulation generates lower temperature diffuse vents that surround the black smokers, carry a large part of the total hydrothermal heat flux, and facilitate mineral precipitation in the substratum. Pontbriand and Sohn (2014) constrained this secondary circulation beneath the Trans-Atlantic Geotraverse (TAG), by characterizing shallow microearthquakes which were located by a small ~200 m aperture short-period Ocean Bottom Seismometer (OBS) network. These microearthquakes were proposed to have been triggered by reaction-driven cracking in response to anhydrite precipitation from heated seawater in the secondary circulation system.

To detect possible shallow microearthquakes associated with the secondary circulation at the Tour Eiffel (TE) vent site, a small-scale ~150 m aperture 4 hydrophones network was deployed in 2016 as part of the EMSO-Azores observatory. TE is the largest vent site of the Lucky Strike hydrothermal field. It has a massive ~15 m high sulfide edifice, bearing several black smokers and surrounded by diffuse flow areas. The total heat flux, including both discrete and diffuse venting, from the TE vent site is estimated to be at least 20 MW, with more than 95% of the heat coming from diffuse venting.

The first one year of data (September 2016 - September 2017) recorded by the hydrophone network includes shallow near vents events, whale songs and earthquakes originated outside the network. We therefore developed criteria based on waveform characteristics, number of phases, frequency spectra and synthetic waveform modelling to select only the shallow microearthquakes. We detected only 740 shallow microearthquakes, yielding a seismicity rate of only ~3 events/day and an average local magnitude of -2.48. The number of shallow events, and their magnitudes, are much smaller than those documented beneath the TAG hydrothermal mound (~ 243 events/day with average local magnitude = -0.95). The small number of shallow microearthquakes detected near TE over the one-year survey suggests that heating of entrained seawater and anhydrite precipitation are less prevalent than at TAG. This hypothesis is supported by time-series analysis of diffuse fluid samples, which mostly show no chemical evidence for anhydrite precipitation. It is also consistent with the TE vent site being smaller and having a lower estimated heat flux compared to the TAG mound (~1 GW).

How to cite: Bohidar, S., Crawford, W., and Cannat, M.: Near vent seismicity at the Tour Eiffel vent site, Lucky Strike hydrothermal field, Mid-Atlantic Ridge, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7636, https://doi.org/10.5194/egusphere-egu24-7636, 2024.

The structure of the lithosphere and the associated magmatic systems found in different locations along slow-spreading ridges can vary dramatically, from melt-starved to magmatically robust segments. A growing number of studies suggest that the evolution of the magmatic crust being governed solely by fractional crystallization is too simplistic. Reactions between migrating melts and their surroundings play a key role during accretion, yet the full extent of their impact is still to be resolved. We present here the results of a petrological, microstructural, and in situ geochemical study of two drilled magmatic sequences from the Kane Megamullion and Atlantis Massif oceanic core complexes. Our results show that mineral-melt interactions generate locally strong textural and/or geochemical heterogeneity at the cm-scale, but their impact can also be reconstructed at the 100m-scale. We found evidence for assimilation at various degrees of primitive lithologies of potential mantle origin within gabbros (sensu lato) at both locations, in addition to typical melt-mush reactions previously described in other slow-spread magmatic systems. Numerical modeling shows the sameness of the reaction equations to be considered for both sequences. Yet, the regime of the reactions (ranges of assimilation over crystallization ratios) varies between Kane Megamullion and Atlantis Massif, variations which likely result from differences in melt fractions present during mineral-melt interactions. We infer, relying on our observations, available thermodynamic modeling, and previous studies, that the regime of the reactions is most likely controlled by the melt flux during the formation of the two sections.

How to cite: Boulanger, M., Godard, M., Ildefonse, B., and Bakouche, M.: A comparative study of mineral-melt interactions at Kane Megamullion and Atlantis Massif: Identifying universal processes building the slow-spreading lithosphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7463, https://doi.org/10.5194/egusphere-egu24-7463, 2024.

Slow and ultraslow spread oceanic lithospheres consist of a mixture of magmatic rocks and mantle rocks with variable alteration degrees. However, the nature, extent and distribution of alteration mineral assemblages are not well constrained. Understanding this alteration pattern at ridges is key to determining the nature of hydrothermal fluids and the seismic structure of the oceanic lithosphere, with implications for seismogenesis at ridges, transform fault zones, and subduction zones. Here, we present 2D numerical models that aim to explore the nature of alteration mineral assemblages and the seismic structure of the oceanic lithosphere during ultraslow magma-poor spreading. For this, we couple thermodynamic calculations with a visco-elasto-plastic model. We simulate the formation of the oceanic lithosphere, ongoing faulting, magmatism, hydrothermal cooling and hydration reactions, starting from continental extension to oceanic spreading. We compare our results with the Gakkel Ridge and the magma-poor section of the Southwest Indian Ridge at 64°30’ East; as both present similarities in the magma production rate and mineral assemblages suggesting similar conditions of hydrothermal alteration [1-3]. Our model reproduces the observed seismic structure of this part of the oceanic lithosphere and its alteration mineral assemblages. Importantly, we show that the interaction between faulting, hydrothermal cooling and hydration reactions results in a complex compositional nature of the oceanic lithosphere. In particular, we find a correlation between the spatial distribution of seismicity peaks and changes in mineral stability fields at mid-ocean ridges. We discuss the impact of such a compositional complexity on hydrothermal vent chemistry and the seismogenic behavior of the oceanic lithosphere.

1- Patterson, S.N., et al., High temperature hydrothermal alteration and amphibole formation in Gakkel Ridge abyssal peridotites. Lithos, 2021. 392: p. 106107. 

2- Bickert, M., M. Cannat, and D. Brunelli, Hydrous fluids down to the semi-brittle root zone of detachment faults in nearly amagmatic ultra-slow spreading ridges. Lithos, 2023. 442: p. 107084.

3- Dessimoulie, L., et al., Major and trace elements exchanges during fluid-rock interaction at ultraslow-spreading oceanic lithosphere: Example of the South West Indian Ridge (SWIR). Lithos, 2020. 352: p. 105233.

How to cite: Mezri, L., Ferrand, T. P., Diehl, A., García-Pintado, J., Bickert, M., and Pérez-Gussinyé, M.: Hydration of the Oceanic Lithosphere: Impact on Hydrothermal Fluid Chemistry and Seismicity., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7861, https://doi.org/10.5194/egusphere-egu24-7861, 2024.

After nearly 50 years of research on hydrothermal circulation, the global hydrothermal on-axis element turnover is still not well constrained. Existing estimates of hydrothermal element fluxes typically invoke a basalt-hosted black smoker archetype hydrothermal vent fluid that is imposed to be responsible for the global hydrothermal cooling of oceanic lithosphere. The diversity of hydrothermal vent fluid compositions, especially to be found at slow to ultra-slow spreading mid-ocean ridges (due to varying degrees of fluid rock interaction with peridotites), has not been properly addressed yet.

Here we present a study that for the first time considers the diversity of hydrothermal vent fluids by analyzing a global database of hydrothermal vent fluid compositions (MARHYS Database Version 3.0). We derive a proper weighting of these fluid types by analyzing strike lengths and substrate types of the mid-ocean ridge system and estimate the partitioning of these hydrothermal fluid types to improve quantification of hydrothermal element fluxes at mid-ocean ridges. We show that the element‑to‑energy flux ratio in peridotite-hosted (or peridotite-influenced) hydrothermal vent fluids is significantly different to the one in purely basalt-hosted fast spreading ridges. Consequently, for many compounds significantly higher (e.g. H₂, CH₄, Fe) or lower (e.g. H₂S, CO₂) element fluxes are found to be associated with hydrothermal cooling at slow- and ultra-slow spreading ridges. Our results show that, despite their lower power output (compared to fast spreading ridges), slow and ultra-slow spreading centers, with their serpentinization‑derived hydrothermal fluids, play a major role for the element transfer between the ocean crust and the ocean.

How to cite: Diehl, A. and Bach, W.: Revising hydrothermal element fluxes at mid-ocean ridges: The role of slow and ultra-slow spreading centers regarding the global hydrothermal element budget., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8149, https://doi.org/10.5194/egusphere-egu24-8149, 2024.

The Pompeii hydrothermal field was discovered in July 2022 during the HERMINE2 cruise [1]. It is located on an inside corner high (21°20 N) at the northern end of a “doomed” ridge segment [2] located just south of the TAMMAR propagating rift. The inside corner high is a domal bathymetric high with gentle slope ridgewards and spreading-parallel lineations (corrugations) characteristic of an oceanic core complex (OCC). The OCC is dissected by several faults including a ridge-perpedicular fault and a series of smaller ridge-parallel faults.

The main Pompeii hydrothermal site is located on the corrugated surface atop a spreading-perpendicular rubble ridge. The mound is about 150 m in diameter and 30-40 m high and mainly composed of sulfide-bearing rocks partly covered by Fe-Mn hydrothermal crusts. Sulfide-bearing mineralization mainly consist of quartz and pyrite and are characterized by low copper and zinc concentrations (i.e. <0.1 wt.%). At least three smaller satellite mounds (< 40m in diameter) located north, south and west of the main site are composed of silica-rich slabs and/or talc-rich mineralizations. Talc-dominated mineralizations are composed of talc with variable amount of microcistalline silica and rare fully-oxidized sulfides. Mineralogy and chemistry of the talc-rich mineralization is similar to that described for the deep active Van Damm hydrothermal field [3]. The main hydrothermal still exhibit a very weak hydrothermal activity (up to 4.25 °C) with H₂ concentrations ranging from 45 to 90 nmol/L indicating interaction with a gabbro-peridotite basement. While sulfide-rich mineralization suggest high-temperature interaction in the reaction zone, talc-rich hydrohtermal deposits point out moderate-temperature interaction with mafic/ultramafic rocks [3]. The tight spatial association between these two different types of deposits (i.e. talc-rich and sulfide-bearing deposits) within the Pompeii hydrothermal field raises the question of the evolution and dynamics of hydrothermal circulation over time in an OCC setting.

[1] Pelleter and Cathalot (2022),

https://doi.org/10.17600/18001851

[2] Dannowski et al., (2018) J. Geophys. Res. 123, 941-956

[3] Hodgkinson et al. (2015) Nat. Commun 6:10150

doi: 10.1038/ncomms10150 .

How to cite: Pelleter, E.-L., Cathalot, C., Rospabe, M., Giunta, T., Dupre, S., Maia, M., Boissier, A., Cheron, S., Rovere, M., Germain, Y., Guyader, V., and Donval, J.-P.: Hydrothermal processes at the Pompeii hydrothermal field: insights from the association of a large sulfide deposit and talc-rich hydrothermal mounds (Mid-Atlantic Ridge) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17466, https://doi.org/10.5194/egusphere-egu24-17466, 2024.