The Gakkel Ridge is an ultraslow-spreading mid-ocean ridge located beneath perennial Arctic sea ice at water depths exceeding 5,000 m. Its extremely low spreading rate, sparse magmatism, and permanent ice cover have long limited geophysical detection, direct observation, and sampling. Exploration is further constrained by drifting sea ice that prevents emergency surfacing, under-ice navigation and communication challenges, extreme cold, short operational windows, and the absence of nearby rescue infrastructure. As a result, large portions of the ridge have remained poorly explored for decades.
Here we report the scientific and technical achievements of a recent Chinese-led expedition to the eastern Gakkel Ridge, representing the first intensive manned geological and biological investigation of this remote polar environment. Using the deep-diving human-occupied vehicle Fendouzhe, more than 43 successful dives were conducted beneath Arctic sea ice, reaching maximum depths greater than 5,200 m. These dives enabled unprecedented in situ observations and direct sampling of seafloor geology, hydrothermal features, and associated ecosystems. We present initial geological and biological results and discuss their implications for understanding crustal accretion, hydrothermal activity, and ecosystem development at ultraslow-spreading ridges in polar settings.
Rerences:
Alexandra Witze, Nature 647, 564-565 (2025) doi: https://doi.org/10.1038/d41586-025-03679-0
How to cite: Huang, X.: Pioneering Human Dives to the Gakkel Ridge in the Arctic Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4430, https://doi.org/10.5194/egusphere-egu26-4430, 2026.
While it is well established that ultraslow-spreading ridges exhibit both regions of unusually thick crust and exhumed mantle domains along their axes, the temporal scales governing crustal thickness variations remain poorly constrained, and the processes controlling these long-term variations remain unclear. The Gakkel Ridge, characterized by the slowest spreading rate globally, represents an ideal natural laboratory for investigating such crustal thickness variations. However, the presence of sea ice cover over the Gakkel Ridge poses a significant challenge to conducting seafloor surveys targeting crustal thickness variations perpendicular to the ridge axis, thereby limiting the ability to draw robust conclusions regarding these lateral variations. Here we use high-resolution active-source ocean-bottom refraction/reflection seismic profiling perpendicular to the ridge axis over a 50 km long section at 100°E on the Gakkel Ridge to show the crustal evolution over the past 10 Myr. This study employs 2.5-dimensional first-arrival P-wave tomography to image the evolution of the crustal structure. The results reveal an initial phase of thick oceanic crust (8.5 km) during 0–2 Myr, followed by a transition to thin oceanic crust (4 km) between 2–4 Myr. Subsequently, the period of 4–8 Myr is characterized by the exhumation of serpentinized mantle, before crustal thickening resumes from 8 to 10 Myr. These marked temporal variations in crustal thickness are interpreted as indicative of periodic fluctuations in melt supply. We propose that these variations were driven by mantle temperature perturbations of approximately 30–40°C over an 8-million-year period.
How to cite: Niu, X., Li, J., Sauter, D., Ding, W., Zhang, T., Yu, Z., Tan, P., and Sun, Q.: Temporal Crustal Structure at 100°E on the Ultraslow-Spreading Gakkel Ridge, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15683, https://doi.org/10.5194/egusphere-egu26-15683, 2026.
Melt distribution along ultraslow spreading ridges is characterized by strong focusing at widely spaced volcanic centers, rather than uniform axial accretion. This localized magmatism supports unusually frequent hydrothermal activity and high-temperature venting, posing a fundamental question: how is sufficient heat supplied and melt zones maintained within otherwise cold lithosphere? We present new local earthquake tomography results from two networks of eight ocean-bottom seismometer deployed around two confirmed hydrothermal vent fields on the Arctic Mid-Ocean Ridge—Aurora and Loki’s Castle. We inverted P- and S-phase arrival times of several thousand microearthquakes recorded over almost one year for P- and S-wave velocity structure and vp/vs ratio. Our tomography reveals heterogeneous lithospheric structures at both sites, with no clear evidence of large, sustained melt reservoirs. This contrasts with prominent low-velocity (vp/vs) anomalies at the Logachev and Segment 8 volcanic centers, which are indicative of extensive melt zones and are accompanied by seismic gaps, swarm activity, and circular magnetic anomalies.
The geophysical characteristics of Aurora and Loki’s Castle vent fields, located at ridge bends near regions of robust magmatism, differ significantly from those of Logachev and Segment 8. Despite the apparent absence of significant melt volumes, these sites exhibit long-lived hydrothermal activity. We propose that these differences may reflect distinct temporal stages in the life cycle of ultraslow spreading ridges or be related to the specific tectonic setting at ridge bends. Our findings highlight the complex interplay between magmatism, tectonics, and hydrothermal processes in ultraslow spreading environments.
How to cite: Vera, S., Jakovlev, A., and Pilot, M.: Structure of volcanic centres at ultraslow spreading ridges revealed from local earthquake tomography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1731, https://doi.org/10.5194/egusphere-egu26-1731, 2026.
Flip-flop detachment fault systems characterize magma-starved regions of ultraslow mid-ocean ridges (MOR). They involve the succession of large-offset normal faults that face alternatively to one then to the other diverging plate,accommodate most of the plate divergence and consistently expose mantle-derived serpentinized peridotites on the seafloor. Currently the best documented MOR flip-flop detachment fault system is located in the 64°E region of the Southwest Indian Ridge (SWIR). Here, we report on two recent research projects focused on this region of the SWIR.
One project uses thermo-mechanical models to investigate which overall, plate boundary-scale, strength contrasts between the fault zones and the surrounding lithosphere favor the flip-flop faulting mode. It highlights how relatively modest rheological contrasts (equivalent to a 0.1-0.2 reduction in frictional strength for a cohesion loss of 20-25 MPa) between intact and deformed lithosphere enables large-offset flip-flop faulting in the thick lithosphere of magma-starved and ultraslow MOR regions. To better understand the flip-flop mode, this modelling project also develops an energy minimization analysis of a configuration with two antithetic faults, one older, and fully weakened, and the other new and not yet fully weakened, but cutting through the thinned footwall of the first fault. It shows that the rate of fault weakening in this new fault is a key parameter to determine whether or not it takes over as the new detachment.
The other project is based on studying actual rock samples and submersible dive videos from the exposed fault zone of the presently active SWIR 64°E axial detachment. It shows that deformation in the upper regions of the fault (at temperatures consistent with serpentine stability) is primarily brittle but that the most highly strained horizons are serpentinite gouges that exhibit syn-tectonic chrysotile fiber growth and dissolution-precipitation textures, indicating fluid-assisted semi-brittle deformation. While these gouges probably have extremely low frictional strength, it is their distribution at outcrop to fault zone scales, their thickness, and interconnectedness, along with the availability of hydrous fluid, that likely control the overall strength of these upper, serpentinized, regions of the fault zone. Further, several of these characteristics are likely influenced by prior distributed brittle and semi-brittle deformation in the deeper, hotter and non-serpentinized regions of the fault.
The study of natural samples therefore indicates that the strength of the axial lithosphere in the nearly amagmatic 64°E SWIR region is controlled by complex interactions between brittle failure, ductile deformation, fluid percolation and hydrous mineralogical transformations in and around fault zones and across a range of depths and temperatures. Numerical models suggest that, overall, these processes result in a moderate integrated rheological contrast between intact rocks and strain weakened fault zones. Yet it is likely that they also cause spatial and temporal variations of fault weakening rates, with consequences on whether and when new antithetic faults successfully take over.
How to cite: Cannat, M., Demont, A., Mahato, S., and Olive, J. A.: Rheology of mid-ocean ridge flip-flop detachment fault systems : numerical models and field observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11951, https://doi.org/10.5194/egusphere-egu26-11951, 2026.
The Mid-Atlantic Ridge (MAR) north of the Kane Transform Fault (MARNOK) provides an
ideal setting to investigate the interplay between magma supply, faulting, and lithospheric
structure at a slow-spreading mid-ocean ridge (MOR). Along this section, two orthogonal
segments and four oblique segments bounded by non-transform discontinuities show
contrasting accretion styles. Orthogonal segments 1 and 6, located at the southern and
northern ends of the study area, show symmetrical spreading, and progressive thinning of the
crust with decreasing distance to the axis (from 8-9 km in ~1.12-Myr old lithosphere to 6-7 km
on-axis). These segments also display closely spaced, elongated normal faults, and their
Mantle Bouguer anomaly (MBA) and Residual Mantle Bouguer anomaly (RMBA) are lower
than that of the adjacent oblique segments. The lack of axial volcanic ridges in segments 1
and 6 along with the decreasing crustal thickness towards the axis indicate a reduction in melt
supply in recent geological time, and possible fluctuations of the magma supply on
characteristic time scales of ~1.12 Myr in this part of the MAR.
The oblique segments (Segments 2 to 5) show a mixed tectono-magmatic regime that reflects
the structural complexity of the MARNOK region. Detachment faults at the inside corners of
segments 2 and 5 along with thin crust indicate earlier asymmetrical, low-magma accretion
typical of oblique MAR segments. Present-day magmatism forms discontinuous, sigmoidal,
and locally focused axial volcanic ridges that resemble those observed on other oblique MOR
segments such as Mohns ridge, and certain oblique areas of the Southwest Indian Ridge.
Short, widely spaced faults and irregular volcanic constructions indicate that magma is
currently contributing to plate separation. Even though the axial volcanic ridges are aligned
with the strike of orthogonal segments 1 and 6, the melt budget of segments 2–5 does not
appear sufficient to reorganize these segments into orthogonal spreading.
Petrological observations reveal that melt–rock interaction is pronounced in tectonically
dominated MARNOK domains. This result along with structural and gravity, observations
indicating transient, localized melt focusing occurs within the MARNOK mantle. These
findings support observations from other slow and ultraslow ridges showing that magmatic
accretion is highly variable and controlled by mantle fertility, detachment-related cooling, and
intermittent melt supply. Overall, the results indicate that crustal formation in the MARNOK
region is shaped not simply by spreading rate, but by the combined influence of obliquity, melt
availability, faulting, and thermal structure. This integrated tectono-magmatic framework
provides new insight into how slow-spreading lithosphere evolves north of the Kane Transform
Fault and highlights the rapid temporal and spatial variability that characterizes magmatic and
tectonic processes at the Mid-Atlantic Ridge.
How to cite: Rajeevan, R., Maia, M., Rospabé, M., Pelleter, E., Besson, F., Olive, J.-A., Principaud, M., and Alix, A.-S.: Accretion Dynamics of the Oblique section of the Mid-Atlantic Ridge North of the Kane Transform Fault (23°50’N-25°15’N) , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-393, https://doi.org/10.5194/egusphere-egu26-393, 2026.
Mid-oceanic ridges are the sites of oceanic crust formation, accommodating plate divergence through a combination of tectonic extension, magmatic accretion, and hydrothermal circulation. The thickness of the oceanic crust produced at these ridges is a first-order indicator of mantle melting processes and melt supply, and is traditionally linked to spreading rate, mantle temperature, mantle composition, and the efficiency of melt extraction. Fast-spreading ridges are typically associated with relatively uniform crustal thicknesses due to a 2D sheet-like mantle upwelling, whereas slow- and ultraslow-spreading ridges exhibit greater spatial variability due to enhanced tectonic strain and heterogeneous melt focusing. Despite the well documented observations and geodynamic modeling of mantle upwelling, the role of short-lived or transient changes in ridge geometry on melt production and crustal thickness remains poorly constrained. Using high resolution seismic reflection data from the Wharton Basin in the Indian Ocean, we show that the crustal thickness decreases smoothly from a normal crustal thickness of ~ 6 km to ~ 4 km and then back to ~ 6 km over a distance of ~120 km. This distance corresponds to a time span of 1-2 Myrs for a crust formed at the super-fast Wharton spreading centre. The dramatic change in crustal thickness is associated with an anticlockwise rotation of the magnetic anomaly Chron 29 (64.4 - 65.1 Ma), which is temporally coincident with the separation of Seychelles from the Indian sub-continent and the Deccan flood basalt volcanism caused by the La Réunion mantle plume. It is likely that this major plate tectonic event in the Indian Ocean caused a temporary change in the spreading rate and spreading direction. We suggest that a rapid rotation in the spreading direction could divert the melt focusing away from the ridge axis, decreasing the melt delivery and thus decreasing the crustal thickness. Within a span of 1 - 2 Myr, the spreading ridge returned to its original geometry and the regime stabilised to a uniform upwelling directly beneath the ridge axis, giving rise to a normally thick crust of 5.5 - 6 km. Our findings show that changes in ridge orientation can significantly influence melt fluxes on relatively short geological timescales, without requiring large-scale changes in mantle temperature or composition. This underscores the sensitivity of magmatic systems at spreading ridges to evolving plate kinematics.
How to cite: Singh, S., Rohilla, S., and Carton, H.: Change in crustal thickness due to localised rotation caused by a long-distance tectonic event in the Indian Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3974, https://doi.org/10.5194/egusphere-egu26-3974, 2026.
Over geological times, the growth of the ocean floor involves magmatic and tectonic extension at mid-ocean ridges. Because seismo-geodetic monitoring of these submarine plate boundaries remains challenging, little is known on how these systems operate on yearly timescales. Here we report the first in-situ observation of a rifting event at a mid-ocean ridge segment, that combines hydroacoustic, direct-path ranging and bottom pressure measurements, with repeated seafloor mapping.
The event started on April 26, 2024 at the axis of the Southeast Indian Ridge near 37˚S, two months after instruments had been deployed across the ridge axis and nearby Amsterdam transform fault. The event began as a rapidly migrating swarm of extensional seismicity along the axial valley. It caused 4 m of subsidence of the valley floor, and over a meter of horizontal extension across the valley. We interpret this as the deflation of a magma reservoir feeding propagating dykes and inducing aseismic slip on a valley-bounding fault. The dyke eventually led to the outpouring of ~150 million m3 of lava at the seafloor, while triggering seismic activity on the abutting transform faults.
Using 2-D elastic dislocation models, we randomly sampled 10 million combinations of sill, dyke and fault geometries to assess how well they could account for the observed displacements. Out of these, about 2200 yielded a satisfactory root mean squared (RMS) misfit (< 20 cm), which have all in common: (i) a sill at least 3500 m deep compacting by 10-20 m; (ii) a dyke rooted at the sill and extending to sub-seafloor depths of tens of meters with a metric opening; (iii) a metric slip on an axial-valley bounding fault down to a few km; and (iv) a total horizontal extension of 2 to 4 m, distributed between the dyke and the fault. Most models favour the dyke taking up more extension than the fault. The measured and modelled horizontal displacements are equivalent to 31 to 63 years of spreading at the average rate of 6.3 cm/yr inferred from space geodesy. They are considerably larger than the centimetric offsets caused by the swarm of Mw≈5 earthquakes and must therefore have accrued aseismically during the early stages of the spreading event.
This unique set of observations provides a detailed chronology of a seafloor spreading event, and, along with modelling, suggests that aseismic slip plays a major role during such events, thereby explaining the well-documented earthquake deficit on normal faults at mid-ocean ridges.
How to cite: Royer, J.-Y., Olive, J.-A., Bazin, S., Ballu, V., Briais, A., Raumer, P.-Y., Retailleau, L., and Lenhof, E. and the OHA-GEODAMS Scientific party: A seafloor spreading event captured by in-situ seismo-geodesy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4456, https://doi.org/10.5194/egusphere-egu26-4456, 2026.
Mid-ocean ridges (MORs) are extremely active volcanic systems where dike intrusions and eruptions recur on decadal time scales. Their submarine setting has long made in-situ observations of active deformation extremely challenging, hindering insight into sub-seafloor deformation sources. Recent progress in seafloor geodesy is rapidly changing this state of affairs, by providing measurements of rapid seafloor displacements throughout the MOR eruption cycle. These novel datasets therefore call for the development of new models to fully realize their potential. Importantly, MOR plumbing systems have been particularly well imaged and typically comprise shallow reservoirs termed axial melt lenses (AMLs) lying above, and embedded within a lower crustal mush zone. Leveraging this knowledge, we design 2-D (forward) finite-element models of the active seafloor deformation that should characterize a cycle of steady AML inflation followed by an instantaneous dike intrusion and AML drainage. The AML lies at the base of an elastic lithosphere, and atop a Maxwell viscoelastic mush zone, with viscosity ηM, that reaches Moho depths and is laterally confined to the axial domain. The underlying asthenosphere is viscoelastic with a viscosity of 10¹⁸ Pa.s.
Our models treat AMLs as a tensile dislocation that opens at a specified rate, corresponding to a constant replenishment flux. AML replenishment manifests as distributed seafloor uplift. When ηM ≥ 10¹⁸ Pa.s, our models resemble elastic half-space end-members. Lower values of ηM however exert a damping effect on seafloor uplift rates, which slow down significantly from beginning to end of a replenishment phase. When the AML suddenly drains and/or when a dike suddenly opens, low mush zone viscosities result in a transient phase of post-drainage and post-diking relaxation, manifesting as steadily vanishing seafloor uplift and seafloor subsidence, respectively.
We use our numerical simulations to revisit estimates of AML inflation at the East Pacific Rise (9°50′N) using seafloor uplift rates (up to ~7 cm/yr) measured by Nooner et al. (2014) between 2009 and 2011, i.e., 4 to 6 years following the 2005-2005 eruption. If we assume a strong mush (ηM >10¹⁸ Pa.s), the observed uplift requires an AML replenishment rate of ~150 m³/yr per meter along the ridge axis, whereas a very weak mush (ηM <10¹⁶ Pa.s) requires rates as large as ~350 m³/yr/m. Interestingly, the observed cross-axis profile of seafloor displacements appears incompatible with our post-eruption relaxation models, implying either that such relaxation did not take place, or that it was effectively over within 4 years. If the latter is true, then the effective viscosity of the axial mush zone should be close to, or slightly less than 10¹⁶ Pa.s, consistent with micro-mechanical models of gabbroic mush flow, and large-scale thermo-mechanical models of MOR thermal structure.
How to cite: Boulze, H. and Olive, J.-A.: Seafloor displacements across the mid-ocean ridge eruption cycle modulated by mush zone viscosity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9023, https://doi.org/10.5194/egusphere-egu26-9023, 2026.
Fast-spreading oceanic ridges are characterized by magmatic systems with a lower crustal magma reservoir containing predominantly mush (i.e. a crystal-rich magma), punctuated by melt-rich lenses and overlain by a shallow Axial Magmatic Lens (AML). This mush-melt system plays a central role in oceanic crustal accretion, melt migration, and magmatic differentiation. After solidification away from the ridge axis, the lower crust shows a vertical layered structure from bottom to top consisting of layered gabbro (several km thick), foliated gabbro (1-2 km) and varitextured gabbro (tens to several hundreds of meters). Two end-member models have been suggested for the formation of the lower crust: the gabbro-glacier model, involving the subsidence of crystals from the AML, and the sheeted-sill model, requiring in situ crystallization of injected melt sills and ascending melts. The foliated gabbro unit, which remains relatively understudied, plays a key role in magma transfer and percolation between the different lower crustal units, as it is located at an intermediate stratigraphic position between the layer gabbro and varitextured gabbro. To better constrain accretionary processes, we selected key samples from the foliated gabbro unit of the Oman ophiolite (Wadi Tayin massif, and ICDP OmanDP Hole GT2) that represents one of the best natural analogues of fast-spreading oceanic ridges. In this study, we take advantage of the outcropping of entire crustal section and follow an integrated approach combining petrographical characterization of rocks textures and crystal morphologies with major and trace element chemical maps and spot measurements.
The results reveal the heterogeneity of the unit in terms of both textures and chemistry. Distinct differentiation paths can be identified in the thin sections. We identify a background mush composed of relatively evolved clinopyroxene generally displaying normal or inverse zoning. The associated plagioclases are overall homogeneous. This background mush is overprinted by less evolved melts. The zones that most clearly record these less evolved melts signatures are characterized by plagioclases recording cyclic zoning, whereas clinopyroxenes commonly display resorbed cores similar to the background one and inverse or more complex zoning patterns. These features are frequently associated with strongly poikilitic textures. In addition, we observe in some places inherited plagioclase cores with very low An (Anorthite) contents closely associated with accessory mineral phases that are typical of the greenschist facies. In the uppermost foliated gabbro, skeletal cores are commonly observed in plagioclase, and clinopyroxenes display cyclic zoning or sector zoning.
Our results highlight that foliated gabbros record repeated episodes of recharge of magma reservoir by less evolved melts. Recharge melts then either interacted locally with previously hydrothermally altered crustal material or evolved within a mush through processes combining magma mixing and reactive porous flow. Plagioclase zoning indicates that fast-growth crystal morphologies are restricted to shallow levels and does not support the transfer of shallow crystals to deeper crustal levels. These observations provide new constraints on accretion models and support a significant role for melt percolation through the lower crust rather than crystal subsidence.
How to cite: Cadoux, L., France, L., Boulanger, M., Laubier, M., Koepke, J., and Singh, S.: Magmatic processes recorded in the shallow plutonics of the Oman ophiolite (fast spreading oceanic centre): Implications for crustal accretion models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17848, https://doi.org/10.5194/egusphere-egu26-17848, 2026.
Fast slipping oceanic transform faults show a quasi-periodic occurrence of large (M>5.5) earthquakes. For example, at the Gofar transform fault in the East Pacific Ocean, slipping at a high rate of ~14 cm/yr, some segments rupture every 5-6 years in a Mw~6 earthquake while other segments remain quite in the global record. Based on the regularity of the seismic cycles, US American researchers deployed an ocean bottom seismograph (OBS) network to capture the predicted 2008 Mw 6.0 event. Indeed, the event was record on 12th September 2008, providing a unique seismological dataset of stations spaced at 10-20 km and OBS operating for 2 months to 13 months with 12 OBS out of 16 OBS covering the full observation period and recording three component data. Previous studies analysed the seismic behaviour (McGuire et al., Nature Geoscience, 2012; Gong and Fun, G-cubed, 2022), focusing solely on micro-seismicity. Here, we re-analysed the archived dataset down-loaded from the EarthScope Consortium (www.iris.edu) and (i) used a machine-learning technique developed to study large datasets of OBS data (PICK-Blue, Bornstein et al., EPS, 2024) to reveal seismicity pattern covering the interseismic phase before the earthquake, the co-seismic and post-seismic phase. In addition, (ii) we searched for similar earthquakes rupturing periodically the same patch of the rupture zone and found two classes of events: repeating at very short time intervals and hence within one day, which we call “bursts”, and events repeating within more than seven days, which we call “repeaters”. Such earthquakes are generally used to reveal seismic creep. Last, (iii) we search for time-dependent features in the continuous recordings and found evidence for spontaneous velocity-changes and gradual healing which we interpret in terms of slow slip events. We found that most bursts and repeaters occurred throughout the year at the segment eastward of the 10 km long mainshock area, while most other segments show little evidence for repeaters, except the segment to the west of the mainshock showing repeaters in April and May 2008. In addition, we observed slow slip in the mainshock area and at the two segments towards the east, while the other segments showed no evidence for prominent velocity changes within the fault zone. We conclude that the occurrences of creep on adjacent segments and slow slip loaded the later mainshock area over several months, subsequently issuing the mainshock. Most striking, the Gofar transforms shows contrasting seismogenic behaviour at adjacent segments: one accommodating plate motion by creep while the other issues large earthquakes.
How to cite: Grevemeyer, I., Ren, Y., and Lange, D.: Setting the stage for the 2008 Mw 6 earthquake at the Gofar transform fault, Pacific Ocean: slow slip, repeating earthquakes, interseismic and co-seismic activity from OBS data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7854, https://doi.org/10.5194/egusphere-egu26-7854, 2026.
The oceanic lithosphere along oceanic transform faults (OTFs) forms at ridge–transform intersections (RTIs) through the interplay of magmatic, tectonic, and hydrothermal processes, and can subsequently evolve via deformation within the transform fault zone itself. We investigate these processes along the southern side of the Vema OTF, which segments the slow-spreading Mid Atlantic Ridge (MAR), thus focusing on a magmatically robust RTI that contrasts with most MAR transform faults.
Significant magmatic supply to the MAR segment south of Vema is indicated by a well-developed basaltic upper crustal section exposed in the transform wall and by abyssal ridge morphology of the adjacent seafloor. The south wall of the Vema transform, and to a lesser extent its valley, have been extensively sampled. Gabbros crop out primarily at the base of the wall. Submersible observations document a steep, transform-parallel fault contact between gabbros and foliated serpentinised peridotites further down the wall.
We studied gabbroic rocks from 25 dredges and 2 dives from the base of transform wall, with more deformed ones mainly collected from depths greater than 4000 m below sea level and towards the western part of the OTF. Twenty-one representative samples were selected for petrological, geochemical and thermobarometric analyses, allowing us to identify four successive deformation regimes.
(1) A high-temperature viscous regime characterized by mylonitic shear bands with brown amphibole (Amp), ilmenite-magnetite, plagioclase (Pl), clinopyroxene (Cpx), orthopyroxene (Opx) ± apatite. Amp-Pl thermobarometry indicates deformation at ~850-950℃. Mineral textures and Ti-rich amphibole suggest melt-assisted deformation.
(2) A high-temperature semi-brittle regime marked by shear zones, cataclastic zones and fractures containing green-Amp, secondary Pl, sphene-ilmenite and Cl-rich apatite. Amp-Pl thermobarometry leads to temperatures of ~650-750℃ and pressure of 1.5-3 kbar. Significant amount of chlorine (700-2400 ppm) together with low Ti (0.065- 0.23 a.p.f.u) in the green hornblende suggest a hydrothermal fluid origin.
(3) A medium temperature semi-brittle regime with formation of green-Amp, chlorite, and sphene within fractures Cl-rich green Amp (up to 6000 ppm) again involves hydrothermal fluids. Amp-Pl thermometry gives temperatures of around 500 ℃ consistent with greenschist facies assemblage.
(4) A low-temperature brittle regime characterized by fracturing and brecciation, with syn/post deformational globular zeolite crystallization, reflecting interaction with seawater at ~200 ℃.
These gabbros formed at the magmatically robust east-RTI, although actual contacts have not been observed, they crop out adjacent to and structurally below a well-documented upper crustal sequence of basalt lava and dikes. This suggests crystallization, and subsequent deformation, at relatively shallow depths for these gabbros, consistent with the low pressure estimated from mineral thermobarometry. Our interpretation is that the succession of deformation regimes documents the deformation style and hydrothermal alteration at relatively shallow depths in the transform. And that the large range of temperature covered by the 4 deformation regimes corresponds mostly to progressive cooling and hydrothermal alteration of lower crustal rocks during aging and lateral transport away from the RTI, with limited contribution from tectonic exhumation within the transform fault.
How to cite: Mukherjee, S., Prigent, C., and Cannat, M.: Deformation and hydrothermal alteration of gabbroic rocks in the Vema oceanic transform fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12446, https://doi.org/10.5194/egusphere-egu26-12446, 2026.
Oceanic transform faults (OTFs) have long been viewed exclusively as vertical, strike-slip structures offsetting mid-ocean ridges, yet their deep geometry and structural complexity remain poorly constrained. Thus, key questions persist, including whether OTFs are single-stranded and continuous, whether they maintain vertical dip angles, if they accommodate mixed-mode slip, and what factors control their geometry. Our study addresses these questions through a global statistical analysis of teleseismic earthquake focal mechanisms from 150 OTFs across diverse tectonic settings. We introduce 'stack maps', a novel method that quantifies fault dip and rake, providing a graphical representation of average focal mechanisms. Our findings reveal that while OTFs tend to conform to the classical vertical, strike-slip model, nearly half exhibit deviations, either in dip or motion, challenging the simplified view of these plate boundaries. We identify four distinct OTF categories: (1) those adhering to the standard model, (2) non-vertical faults with transtensive/transpressive components, (3) non-vertical faults accommodating strike-slip motion, and (4) vertical faults with a vertical component of motion. Tectonic regime shifts emerge as a primary driver of structural changes, with non-vertical geometries persisting even after the regime reverts to pure strike-slip motion. This structural memory suggests that fault geometry, once established, remains stable over geological timescales of several tens of Myr. By reconciling previously 'unusual' focal mechanisms with fault structure and dynamics, this work demonstrates that global seismic catalogues, when analysed statistically, offer robust insights into OTF geometry and tectonic regimes.
How to cite: Janin, A., Behn, M., and Tian, X.: Geometry, structure, and tectonic regime of oceanic transform faults revealed by teleseismic earthquake focal mechanisms, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1651, https://doi.org/10.5194/egusphere-egu26-1651, 2026.
Most of Earth’s eruptive volcanism occurs along mid-ocean ridges (MORs), yet direct observations of eruptions and their immediate hydrothermal consequences remain rare. On 29 April 2025, the science party of R/V Atlantis Expedition AT50-36 directly observed a long-anticipated eruption at the East Pacific Rise (EPR) 9°N only hours after its onset, representing the most rapidly detected and documented deep-sea MOR eruption to date. The expedition occupied the area from 10 April to 3 May 2025, allowing characterization of hydrothermal and water-column conditions from ~14 days before the eruption to ~96 hours afterward. In this communication we present on-board fluid geochemistry measurements documenting both pre-eruption vent fluid chemistry and post-eruption water-column responses. High-temperature and diffuse-flow fluids were sampled at the Bio9, P Vent, Tica, BioVent, and YBW vent fields during DSV Alvin dives conducted in the days preceding the eruption. Post-eruption bottom waters were investigated using a CTD–rosette system equipped with an in situ electrochemical analyzer. Pre-eruption measurements of dissolved Fe (dFe), H₂S, dissolved Mn (dMn), and pH show elevated H₂S:T and H₂S:dFe ratios relative to previous years at EPR 9°N, consistent with subsurface phase separation and volatile-enriched hydrothermal fluids prior to eruption. Immediately following the eruption, high-temperature vent sources could not be accessed due to aborted Alvin dives; however, CTD profiles revealed pronounced bottom-water anomalies in at least one of pH (up to 0.8 units), H₂S (up to 70 µM), or dFe (up to 841 nM) at CTD stations conducted over Bio9, P Vent, Tica, BioVent vent fields. These geochemical anomalies were spatially widespread along the ridge axis and extended to at least 10 m above the seafloor, with pH and temperature perturbations closely coupled to elevated H₂S concentrations. Although temperature anomalies in bottom waters decayed within four days of the eruption, pH, H₂S, and dFe anomalies persisted. A CTD cast conducted four days post-eruption revealed, via the rosette-mounted electrochemical analyzer, H₂S concentrations of up to 40 µM coincident with turbidity and redox potential anomalies extending to at least 600 m above the seafloor, indicating the development of a vertically extensive hydrothermal megaplume. Such concentration ranges are typically confined to the immediate vicinity of black smoker orifices, highlighting the exceptional spatial scale of hydrothermal discharge following this eruptive event. The near-real-time observation of this MOR eruption provides new constraints on eruption-triggered hydrothermal fluxes, plume formation, and the role of episodic volcanic events in modulating ridge-axis hydrothermal systems.
How to cite: Yücel, M., Wozniak, A., Shah Walter, S., Wagner, S., Katz, S., Tüzün, S., Alımlı, N., Demir, N. Y., Cura, H., McNichol, S., and Luther, G.: Near-Real-Time Geochemical Constraints on the April 2025 Mid-Ocean Ridge Eruption at the East Pacific Rise 9°N, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6546, https://doi.org/10.5194/egusphere-egu26-6546, 2026.
Hydrothermal circulation at mid-ocean ridges is permitted by the highly permeable young oceanic lithosphere and the presence of a shallow heat source, both of which can fluctuate on different time scales in response to tectonic and magmatic activity. Seafloor observatories increasingly allow us to quantify how hydrothermal discharge responds to these changes, by continuously measuring key properties of vent fluids such as temperature, chemical composition, or flow rate. Barreyre et al. (2025, PNAS) for example showed that hydrothermal vent temperatures at the East Pacific Rise (EPR) 9º50’N steadily increase between eruptions, as the axial melt lens inflates. The models used to interpret these measurements, however, have thus far assumed a uniform permeability along the fluid upflow path, when magmatic inflation likely imparts depth-dependent changes to the permeability field.
To remedy this, we developed SAPHYR, a semi-analytical workflow to study the behavior of an axisymmetric (1-D) hydrothermal upflow zone with a depth-dependent permeability profile, subjected to lateral heat loss. SAPHYR specifically predicts the steady-state temperature and velocity of upwelling fluids, from heat source to seafloor, given a basal heat input and background permeability profile. It is benchmarked against standard models that assume both uniform and exponentially-decaying permeability profiles.
We use SAPHYR to assess how exit fluid temperatures may evolve in response to depth-dependent perturbations of the upflow zone permeability profile. At the EPR, such perturbations could stem from changes in the mean stress of the upper oceanic crust caused by an inflating axial melt lens. To test this idea, we run a large parametric study where we compare the state of the hydrothermal discharge zone before and after imposing a perturbation, and do so for a wide range of basal heat inputs, background permeability profiles, and degree of lateral heat loss. We find that an inflating melt lens can either drive an increase or a decrease in hydrothermal vent temperatures depending on the basal heat-flow and the vent location with respect to the inflating body. Our findings explain why neighboring hydrothermal vents may respond differently to the same sub-seafloor deformation process, as was documented at the EPR. They further open a path to inverting changes in sub-seafloor permeability and stress from time series of black smoker temperatures.
How to cite: Moutard, K., Olive, J.-A., Barreyre, T., and Marjanović, M.: Sensitivity of hydrothermal vent temperatures to changes in crustal permeability profiles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8232, https://doi.org/10.5194/egusphere-egu26-8232, 2026.
Hydrothermal circulation at mid-ocean ridges represents one of the largest points of exchange of energy and chemistry between Earth’s surface and interior. In basalt-hosted systems, black smoker chimneys vent metal-rich fluids at up to ~400°C that fuel unique ecosystems and produce massive sulfide deposits. Recharging seawater reacts with the surrounding basalt at increasing pressure and temperature, drastically changing fluid chemistry. Yet, the extent and shape of hydrothermal recharge pathways remain poorly constrained.
Here, we present a series of geochemical models, investigating these processes through equilibrium thermodynamics: In a system of 16 elements (Si, Ti, Al, Fe, Mg, Cu, Pb, Zn, Ca, Na, K, S, C, Cl, H, O), we test a broad range of hydrothermal recharge pathways with various pressure and temperature profiles and fluid/rock ratios. Multi-pass sequential reactor chain models are set up using xgems (https://github.com/gemshub/xgems), the Python package derived from GEMS [1], and the MINES thermodynamic database [2]. Simplified recharge pathways are varied in circulation depth (1–5 km below seafloor), peak temperature (370–430°C) and integrated fluid/rock ratio. Using fluids derived from these models, a second set of models is run, reproducing the basalt alteration patterns observed in rocks below the TAG hydrothermal field. Based on comparison to measured TAG vent fluids, these models offer three main conclusions:
- For significant metal leaching matching black smoker fluids, peak temperatures of hydrothermal circulation need to exceed 400°C.
- Relatively shallow circulation (< 3 km bsf), and thus shallow heat sources, favorably result in fluid compositions matching black smokers.
- Black smoker fluids only result from rock-buffered reactions. This implies that recharge pathways must contain a significant fraction of fresh basalt throughout the lifetime of a hydrothermal system.
References
[1] Kulik, D. A., Wagner, T., Dmytrieva, S. V., Kosakowski, G., Hingerl, F. F., Chudnenko, K. V., & Berner, U. R. (2013). GEM-Selektor geochemical modeling package: revised algorithm and GEMS3K numerical kernel for coupled simulation codes. Computational Geosciences. https://doi.org/10.1007/s10596-012-9310-6
[2] Gysi, A. P., Hurtig, N. C., Pan, R., Miron, D. G., & Kulik, D. A. (2023). MINES thermodynamic database. New Mexico Bureau of Geology and Mineral Resources, Version 23. https://doi.org/10.58799/mines-tdb
How to cite: Engelmann, J., Gysi, A., and Rüpke, L.: Geochemical Modeling Insights into the Formation of Black Smoker Fluids, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21577, https://doi.org/10.5194/egusphere-egu26-21577, 2026.
Hatiba Mons is the largest axial dome-shaped volcano in the ultra-slow spreading Red Sea rift. It hosts recently discovered (2022) widespread hydrothermal activity consisting of extensive iron deposits in the form of iron mounds. Two of these vent fields were investigated in detail during an expedition in 2023, with ROV observations as well as gravity coring of metalliferous sediments, massive sulfides, and background carbonates. A multidisciplinary approach was applied to first establish a geochemical and mineralogical framework of the new system, which is then linked to microbiological and pore fluid analyses of the sediments. This was achieved through the implementation of X-ray fluorescence, instrumental neutron activation analysis, inductively coupled plasma mass spectrometry, X-ray diffraction, petrological microscopy, electron-microprobe analysis, sulfur isotope analysis, and microthermometry. Whole-genome metagenomic sequences and morphological studies (scanning electron microscopy) are currently analyzed to elucidate the role of microbial communities in mound formation and/or degradation and mineral precipitation. The pore fluid chemistry will further enhance our understanding of the formation of the hydrothermal system at Hatiba Mons and the processes responsible for the chemical variability within the mounds.
Our study provides the first detailed description of an active Red Sea hydrothermal vent system outside the metalliferous brine-pool muds such as those of the Atlantis-II Deep. Hydrothermal precipitates at Hatiba Mons resemble MOR basalt-hosted deposits elsewhere. However, given the close proximity (<10km) of Miocene evaporites, the presence of small brine-filled depressions at the volcano summit and near-saturation salinities in fluid inclusions indicate a substantial contribution of dissolved evaporites to the hydrothermal system, influencing metal solubility, transport, and precipitation. This is reflected in some unusual high metal concentrations (e.g., Zn, Au, Ag, Cd, Sb). The mineral composition and paragenetic sequence, as well as microthermometric results suggest a waning hydrothermal system that experienced high-temperature hydrothermalism (250-300°C) in the past and current temperatures within the mounds (130-150°C) that are well above the currently measured in situ temperatures of 31°C and 51°C venting and core temperatures, respectively. Furthermore, we provide a detailed assessment of the first polymetallic massive sulfide occurrence associated with active hydrothermal venting in the Red Sea.
The deposit at Hatiba Mons formed at high temperatures, clearly showing that the fundamentals of hydrothermal activity in the Red Sea are not entirely different from other mid-ocean ridges; however, the elevated salinities may provide evidence that the geological setting allows for greater variability in the mineral deposits currently not observed in other modern seafloor hydrothermal systems, but common in the fossil rock record. The Red Sea spreading center remains an exploration target for the discovery of further sulfide occurrences and/or high-temperature vent sites. The presence of current low-temperature fluid venting and microbial mats, along with high-temperature precipitates within the mound, suggests a complex and dynamic hydrothermal activity at Hatiba Mons volcano and in the Red Sea. These findings contribute to a deeper understanding of the formation of marine mineral deposits, the evolution of hydrothermal systems, and their broader implications for deep-sea geochemistry and microbial ecology.
How to cite: Diercks, I., Petersen, S., Follmann, J., Augustin, N., van der Zwan, F. M., and Sander, S. G.: Discovery of high-temperature hydrothermal mineralisation at Hatiba Mons volcano in the Red Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3813, https://doi.org/10.5194/egusphere-egu26-3813, 2026.
Seafloor massive sulfide (SMS) deposits formed at ultramafic-hosted hydrothermal systems along slow- and ultraslow-spreading ridges are among the most metal-rich known on the seafloor, yet the processes governing metal transport and deposition in these environments remain poorly constrained. The Semenov hydrothermal field at 13°30′N on the Mid-Atlantic Ridge is one of the largest known ultramafic-hosted SMS systems, comprising multiple sulfide mounds developed on an oceanic core complex with a long-lived hydrothermal history (~124 kyr). This setting provides a valuable opportunity to link present-day hydrothermal fluid chemistry with the formation and preservation of extensive sulfide deposits.
Here we present the first detailed geochemical characterisation of hydrothermal fluids from the active Semenov-2 vent field, based on samples collected from three high-temperature vent sites (Ash Lighthouse, Phantom Urchin, and Yellow Submarine). Fluids were analysed for major elements, trace metals, volatiles, and isotopes, alongside mineralogical characterisation of associated chimney material. The chemical composition of end member vent fluids, calculated by extrapolation to zero magnesium, are similar across all three vents, consistent with a shared hydrothermal source. Relative to other ultramafic-hosted systems, Semenov fluids are characterised by elevated CO₂ concentrations but comparatively low metal and H₂S contents.
Chimney material recovered from the vent orifices were dominated by sulfate minerals (anhydrite-gypsum), with sulfide phases present only in minor amounts in the recovered chimney material. Together, the fluid and mineralogical data suggest that metal precipitation may occur predominantly beneath the seafloor, potentially driven by evolving pH-temperature conditions, redox state, and fluid-rock interaction associated with serpentinization. Alternatively, these signatures may reflect a waning or evolving hydrothermal system in which reduced or migrating heat input limits the transport of metals and reduced sulfur to the seafloor. These observations highlight the importance of subsurface processes in controlling metal fluxes and the development of SMS deposits in ultramafic-hosted hydrothermal systems.
How to cite: Portlock, G., Shannon, J., Steigenberger, S., Hillegonds, D., Murton, B., Yeo, I., and James, R. H.: Hydrothermal fluid chemistry and implications for sulfide deposit formation at the ultramafic-hosted Semenov vent field, Mid-Atlantic Ridge (13°30′N), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19621, https://doi.org/10.5194/egusphere-egu26-19621, 2026.
Serpentinization is a common geochemical process in ultramafic-hosted hydrothermal systems, where the hydration of mantle rocks releases heat and hydrogen that can support hydrothermal circulation and chemosynthetic ecosystems. Most current understanding of serpentinization kinetics and thermodynamic limits is primarily derived from closed-system laboratory experiments. Here, we investigate how this reaction operates within dynamically circulating fluid systems in nature. A simplified model for serpentinization as a function of temperature and fluid velocity was developed and implemented via three-dimensional numerical simulations using the IC-FERST flow simulator. Flow simulations explore how serpentinization interacts with fluid circulation and responds to variations in rock porosity and permeability. We apply this framework to a geologically realistic model of the Rainbow hydrothermal field (north Mid-Atlantic Ridge) to evaluate the combined effects of a deep magmatic heat source and reaction-driven heat generation. Results indicate that while the high vent temperatures and heat fluxes observed at Rainbow require a magmatic driver, serpentinization works synergistically with magmatic heat to temporarily elevate vent temperatures (by up to 50°C) and substantially increase seabed heat and fluid fluxes. Rather than being a uniformly progressing front, the serpentinization reaction is most effective in permeable regions surrounding the upwelling plume, where temperatures remain within an optimal thermodynamic window. Heat released by serpentinization has the unexpected effect of making upwelling plumes more stable in space and time, potentially contributing to sustaining black smoker vent fields over long periods of time (>10k years). By capturing key characteristics of the observed discharge at Rainbow, this study highlights how chemical reactions and fluid circulation jointly regulate hydrothermal activity and hydrogen production in ultramafic systems.
How to cite: Lyu, W., Paulatto, M., Jacquemyn, C., and Jackson, M.: Effects of Serpentinization on Hydrothermal Systems: Modelling the Ultramafic-Hosted Rainbow Hydrothermal Field, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15639, https://doi.org/10.5194/egusphere-egu26-15639, 2026.
There are relatively few measurements of oceanic crust formed at the Reykjanes Ridge south of Iceland. During the IMPULSE experiment of 2024, we acquired two wide-angle seismic profiles using dense arrays of ocean bottom seismometers (OBSs). One profile, presented here, deployed 89 OBSs along an approximately 400 km flow line centered on the ridge axis at 60°17’ N, extending to plate ages of over 18 million years on either side. The second profile consists of 51 OBSs deployed along a 550 km axial chron line. Travel times of crustal (Pg) and mantle (Pn) refractions, and the wide-angle reflections from Moho (PmP) were picked and inverted using the TOMO2D software package to map crustal and upper mantle structure along the flow line. The results reveal an igneous crustal thickness varying between 6 and 9 km at intervals of 25-50 km from the ridge axis. Seismic velocities near the base of the thickest crust reach ~7.5 km/s away from the ridge axis, but can be as low as ~7.1 km/s for the thinnest crust on the profile. Variations of both crustal thickness and seismic velocity with distance are similar on either side of the ridge axis, suggesting that they are controlled by axial processes. At the ridge axis, the crust is approximately 9 km thick. However, lower crustal velocities within 10 km of the ridge axis are ~0.5 km/s slower than those observed at locations with similar crustal thicknesses only 50 km away. This observation suggests that the thick axial crust is anomalously hot, consistent with the diminished earthquake seismicity observed along this segment of the Reykjanes Ridge. Our results support the hypothesis that a hot transient pulse of asthenosphere lies beneath the Reykjanes Ridge at 60° N.
How to cite: Dhabaria, N., Henstock, T., M Jones, S., and White, N.: The IMPULSE experiment: Oceanic crust formed beneath the Reykjanes Ridge at 60° N, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14998, https://doi.org/10.5194/egusphere-egu26-14998, 2026.
International Ocean Discovery Program Expeditions 384, 395C and 395 investigated ocean crust formation at the Reykjanes Ridge, the variable influence of the nearby Iceland plume, the origin of V-shape ridges and troughs marking the flanks of the ocean ridge, and the alteration of basaltic crust with time. These Expeditions collected cores from a transect of five drill sites along a plate-spreading flowline spanning seafloor ages from 2.8 to 32 Ma. Combined, over 400 m of oceanic basalt core was recovered, and downhole logging collected physical property measurements in the crust, and resistivity and ultrasound images of the boreholes. These datasets provide a unique record of volcanic and tectonic characteristics of the uppermost basaltic crust, and of the progressive basalt alteration. Here we use downhole logging images along with observations from the recovered cores to characterize the lava morphology and quantify flow types in basement holes, and to investigate the fracturing and alteration of the basalts. This analysis complements the observations from the cores especially where basalt recovery was low. Data from the different sites along the flowline allows us to analyze how these physical characteristics vary with age, and to compare the flows emplaced at V-shaped ridges with those emplaced in the troughs. We estimate the fluid circulation paths from downhole images and compare with the basalt alteration observed from the cores.
How to cite: Briais, A., McNamara, D., Hochmuth, K., Eason, D., Pasquet, G., Dodd, J., Murton, B., Parnell-Turner, R., Levay, L., and Expedition_395, S.: Variability of volcanic constructions along the Reykjanes Ridge: Observations from downhole imaging at IODP395C/395 basement sites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16039, https://doi.org/10.5194/egusphere-egu26-16039, 2026.
The physical properties of oceanic crust evolve significantly with age as the lithosphere cools, densifies, and subsides. At the crustal scale, the oceanic crust undergoes a progressive reduction in porosity and permeability (due to pore space and fracture infill), leading to an overall increase in seismic velocity. In particular, alteration of basaltic crust by low-temperature hydrothermal fluids produces the largest modification to the upper oceanic crust. This means that understanding the impact of porosity changes is critical for quantifying crustal physical property evolution through time.
Here, we present a new dataset of physical property measurements from the upper oceanic crust recovered during the South Atlantic Transect (IODP Expeditions X390–393), spanning basalt ages of approximately 6 to 61 Ma. The dataset includes P-wave velocity (Vp), pycnometry measurements, and X-ray micro-CT image analyses. The new dataset, integrated with existing shipboard data, provides a comprehensive view of low-temperature alteration processes.
Micro-CT analyses reveal that basalt samples exhibit a highly heterogeneous porosity structure. Primary porosity is dominated by vesicles that are variably filled with secondary minerals; many vesicles remain partially unfilled or display clay coatings, indicating incomplete calcite precipitation. Secondary porosity occurs as micro-porosity (< 10 micron) associated with volcanic glass, olivine and plagioclase alteration, as well as fracture networks. Two generations of cross-cutting fractures are identified, filled by clay and calcite, respectively, reflecting multiple stages of fluid circulation and mineral precipitation.
Variations in porosity are closely linked to volcanic emplacement style and microstructural characteristics, including groundmass grain size, phenocryst abundance, vesicle distribution, and are positively proportional to the degree of alteration.
Our findings provide new constraints on the mechanisms governing physical property evolution in ageing oceanic crust and have important implications for upscaling models of CO₂ sequestration in basaltic formations, where porosity, permeability, and fracture connectivity are critical parameters.
How to cite: Amadori, C., Robustelli Test, C., Harris, M., Alvarez-Borges, F., Coggon, R., and Teagle, D.: Porosity-dependent physical property changes of the oceanic crust at the South Atlantic Transect (IODP X390-393), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21080, https://doi.org/10.5194/egusphere-egu26-21080, 2026.
The South Atlantic Transect (SAT) ocean drilling expeditions (IODP Expeditions 390 & 393) recovered basaltic lavas formed between ~7 and 61 Ma along the western flank of the Mid-Atlantic Ridge at 31°S. Mid-ocean-ridge basalt (MORB) recovered during the SAT preserves primary magmatic characteristics and evidence of varying extents of reaction with seawater-derived hydrothermal fluids. This transect offers a unique opportunity for studying accretion of upper oceanic crust, off-axis hydrothermal processes over time, and the influence of rock alteration on long-term variations in seafloor magnetization.
Magnetic minerals in basaltic lava flows are known to reflect primary volcanic features, such as magma composition and emplacement style, and on- and off-axis hydrothermal processes.
In this study we performed detailed rock magnetic investigations to characterize the magnetic mineral assemblages and grain-size variations among fresh and altered basalts. Across the ridge flank (i.e., with increasing age), the magnetic properties highlight a strong dependence of magnetic mineral grain-sizes and composition on the nature of the volcanic units and their evolution during hydrothermal alteration. For example, fresh MORB displays Ti-rich titano-magnetite with finer and coarser grains in pillows and massive lava flows, respectively. Fluctuations in remanent magnetization and magnetic susceptibility intensities are also strictly dependent on primary textures and emplacement style.
Magnetic mineral compositions (e.g., changes in Ti-content) and grain-sizes vary across distinct types of alteration halos, with a general decrease in magnetization. As alteration evolves, the magnetic properties demonstrate a progressive oxidation of the primary titano-magnetite into titano-maghemite coupled with magnetic mineral grain-size reduction associated with various extents of groundmass and phenocryst replacement. Strongly altered basalts reveal a paramagnetic contribution related to the formation of secondary clays associated with Fe-oxyhydroxides (i.e., goethite).
Overall, the variation of magnetic properties across the South Atlantic ridge flanks provides constraints on the complex interplay of volcanic stratigraphy and the evolution of hydrothermal alteration as the upper oceanic crust ages, linking petrology with the long-term variation of marine magnetic anomalies.
How to cite: Robustelli Test, C., Amadori, C., Harris, M., Belgrano, T., Jonnalagadda, M., Evans, A., Grant, L., Albers, E., Coggon, R., Teagle, D., and Zanella, E.: Rock magnetic constraints on primary igneous features and hydrothermal alteration of MORBs along the South Atlantic ridge flanks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13036, https://doi.org/10.5194/egusphere-egu26-13036, 2026.
The Aegir Ridge was active in the northeastern Atlantic between Norway and Greenland from early Eocene (~55 Ma) until its cessation in late Oligocene (~26–24 Ma). The ridge remains understudied in the literature despite its importance in reconstructing tectonic history of the Northern Atlantic. Although portions of the ridge axis are visible in modern bathymetric grids, much of its morphology is subdued by sedimentary cover. Numerous seamounts are evident near the former spreading axis, serving as key indicators of magmatic and tectonic processes. However, away from the ridge axis, the seamounts are buried beneath sediments and are therefore undetectable in bathymetry alone, necessitating an integrated geophysical approach to locate them.
In this study, we perform systematic mapping of seamounts across the extinct Aegir Ridge by integrating publicly available bathymetric, gravity and vintage seismic reflection datasets. While bathymetry reveals seamounts primarily near the spreading center, we utilize gravity data to identify buried or sediment-covered edifices away from the ridge. To do that, we enhance gravity data and determine the signal from known bathymetric seamounts. We then identify and map similar filtered anomaly responses as “gravity seamounts”. To validate these features, we analyze seismic reflection profiles obtained from the GeoMap App. This allows us to confirm “seismic seamounts” where the structures rise above the basement but are covered by sediments. Due to limited seismic coverage, not all “gravity seamounts” can be validated. Therefore, we categorize seamounts into “bathymetric”, “gravity” and “seismic” ones and compare them with previously published bathymetric seamounts and igneous complexes.
We further analyze patterns in gravity and magnetic anomalies to delineate individual spreading segments of the extinct Aegir Ridge. Our analysis shows that most mapped seamounts align with the spreading center, while some display oblique orientations. These oblique seamounts correspond to offsets between ridge segments. In addition, magnetic anomalies exhibit characteristic distortions in the polarity reversals that are aligned with those oblique seamounts. These are characteristic of pseudofaults and propagator wakes, which form when two ridge segments compete with each other for magma supply. Our integrated geophysical mapping enables identification of previously unrecognized volcanic features and tectonic elements and suggests that ridge propagation occurred during the active lifespan of the Aegir Ridge.
How to cite: Salman, M. A. and Filina, I.: Geophysical Mapping of Seamounts and Tectonic Elements over the Extinct Aegir Ridge, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-312, https://doi.org/10.5194/egusphere-egu26-312, 2026.
The Galápagos Spreading Center (GSC) is characterized by an intermediate spreading rate, and influenced by the nearby Galápagos hotspot, resulting in a pronounced along-axis gradient in magma supply that decreases by ~40% from east to west. Between 92°W and 97°W, the axial morphology shifts from a high to a valley, as the seismic crustal thickness decreases from 7.5 to 5.6 km, and the seismically-imaged axial melt lens (AML) deepens from 1.4 km at 92°W to 3 km at 94°W, beyond which it becomes undetectable, e.g., at 97°W (Blacic et al., 2004, doi: 10.1029/2004jb003066). However, a P-wave low-velocity anomaly persists along the GSC between 92°W and 97°W, suggesting the widespread presence of an axial crystal-rich mush zone (Canales et al., 2014, doi: 10.1002/9781118852538.ch17). These along-GSC variations provide an ideal laboratory to explore the impact of melt flux on the dynamics (e.g., depth, transience, and eruptibility) of magma (crystal-poor) – mush (crystal-rich) systems at the axis of mid-ocean ridges.
We use a 2-D numerical thermal model, multiporo-magma, which couples repeated, instantaneous melt emplacement events in the lower crust, parameterized magma convection within individual magma bodies, and hydrothermal circulation (porous flow) in the uppermost crust. Our reference model predicts that, from 92°W to 97°W, decreasing melt flux leads to a deepening of the crystal mush zone (from 1.5 to 3.5 km), and to the formation of increasingly smaller and more transient melt-rich magma bodies within the mush zone. These results highlight that higher melt fluxes (e.g., 92°W) support nearly steady-state magma bodies capable of sustaining frequent eruptions, whereas lower melt fluxes (e.g., 97°W) result in deeper, short-lived magma bodyies with reduced eruptive potential. Importantly, we show that the absence of a seismically-imaged AML at any given time can reflect increased transience in the thermal state of the axis, and does not require that the 1000ºC isotherm lies below the Moho, as previous thermal models had postulated. Our simulations further reveal how the behaviour of the crystal mush zone is modulated by the efficiency of hydrothermal cooling and the size of individual melt sills.
How to cite: Chen, J., Olive, J.-A., and France, L.: Depth, transience and eruptibility of magma-mush reservoirs modulated by varying magma supply along the Galápagos Spreading Center, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3929, https://doi.org/10.5194/egusphere-egu26-3929, 2026.
Ocean floor formed at intermediate spreading ridges typically consists of volcanic effusion products (80-90%) and regularly-spaced normal fault scarps (10-20%) that shape elongated abyssal hills. This fabric forms over millions of years as the divergence of two tectonic plates induces discrete events of magmatic intrusion and fault slip at the ridge axis, which can last from several seconds to several months. Little is known, however, on how the repetition of such events ultimately shapes the partitioning of tectonic and magmatic strain that is encoded in the morphology of the seafloor. To address this, we quantify the amount of fault slip and magmatically-accommodated extension during the early days of the April 2024 rifting event that took place on the Southeast Indian Ridge at 37°S, and was documented by the OHA-GEODAMS seismo-geodetic observatory (Royer et al. EGU26-GD5.1).
Using elastic dislocation modelling in a Bayesian framework, we find that the rifting event accounted for 2–4 m of horizontal extension, of which ∼85% involved the emplacement of a magmatic fracture that propagated along the axis within less than 2 hours. We attribute the remainder of the extension to dominantly aseismic slip on axial valley bounding faults. This "instantaneous" fraction of magmatic extension is strikingly similar to that revealed by bathymetric analyses (M∼90%), which quantify deformation averaged over hundreds of thousands of years. We therefore propose that the long-term "M-fraction" that characterizes intermediate-spread seafloor could be determined at the scale of individual rifting events, possibly by static stress transfers between a propagating dike and adjacent faults. At the Southeast Indian Ridge, such events likely recur every ∼50 years and are separated by periods of seismic quiescence, as mid-ocean ridge normal faults may primarily grow when triggered by magmatic activity.
How to cite: Olive, J.-A., Royer, J.-Y., Bazin, S., Ballu, V., Briais, A., Raumer, P.-Y., Retailleau, L., Lenhof, E., Beesau, J., Daniel, R., Dausse, D., Furst, S., Gros-Martial, A., Guerin, C., Klein, E., Pacaud, D., Poitou, C., Tanrin, J., and Testut, L.: Partitioning of magmatic and tectonic extension from hours to millions of years at the Southeast Indian Ridge, 37°S , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6354, https://doi.org/10.5194/egusphere-egu26-6354, 2026.
Seismic imaging of magmatically robust mid-ocean ridges (MORs) reveals the presence of sill-shaped axial melt lenses (AMLs) located a few kilometres below the seafloor, overlying and embedded within mush zones. AMLs are active features: they must undergo rapid replenishment to provide the heat that fuels high-temperature hydrothermal convection. Ocean bottom pressure sensors have shown that this replenishment causes steady uplift of the seafloor over decadal time scales, which is partially or completely reversed during MOR eruptions. Previous studies of this phenomenon have typically modelled seafloor displacements by imposing overpressurisation rates in a tensile deformation source embedded in a (visco-)elastic half-space. Very few, however, have focused on the physical mechanisms that enable overpressurisation of a magma pocket in a mush zone.
To address this gap, we test the hypothesis that AMLs represent boundary layers formed by the decompaction of partially molten rocks beneath a permeability barrier (e.g., the brittle-ductile transition). Using numerical two-phase poro-viscous flow models, we calculate the buoyant load exerted by a decompacting boundary layer on its overlying permeability barrier. By systematically varying the solid and liquid shear viscosities, bulk viscosity exponent, background porosity, and grain size, we obtain a range of overpressure buildup rates that show strong agreement with a simple scaling analysis. The bulk viscosity exponent, background porosity, and grain size exert the strongest control on the rate of overpressure buildup. We then convert our computed loading rates to seafloor uplift rates using elastic dislocation models and compare them with data from the East Pacific Rise at 9°50’N. By doing so, we demonstrate that the decompaction of magmatic mush is a viable mechanism for AML overpressurisation and seafloor inflation. Future work will aim to incorporate more realistic rheologies for the magma-mush system and assess their impact on the rates of AML inflation.
How to cite: Cserép, A., Olive, J.-A., Aharonov, E., Duretz, T., and Boulze, H.: Decompaction-Driven Overpressurisation of Mid-Ocean Ridge Magma Lenses, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10877, https://doi.org/10.5194/egusphere-egu26-10877, 2026.
The Gakkel Ridge is the slowest-spreading axial ridge on Earth extending across the Arctic Ocean for ~1800 km. It was subdivided into a western and an eastern magmatically robust zone separated by a central sparsely magmatic zone, based on rock recovery during the AMORE expedition in 2001. During the same expedition, at least nine discrete hydrothermal sources were inferred from water-column plume detections. Due to the perennial ice cover limiting the deployment of underwater vehicles, only two of these plumes have ever been traced to their seafloor sources: the Aurora vent field, at the westernmost end of the Gakkel Ridge, and the Polaris vent field, in the Eastern Volcanic Zone.
In this study, we present an integrated high-resolution multibeam and optical dataset acquired onboard RV Polarstern at the axial volcanic high hosting the Polaris vent site (56°E) in 2016 using the towed camera system OFOBS. The combination of bathymetric data and photogrammetric reconstruction from optical imagery reveals pronounced morphological and geological heterogeneity across the study area. Based on these observations, we classify the seafloor into three main terrain types: (1) an axial volcanic summit dominated by pillow basalt, indicative of volcanic emplacement; (2) a faulted zone where tectonic structures focus hydrothermal activity, corresponding to the location of the Polaris vent field; and (3) a distal domain characterized by larger-scale tectonic structures with no clear evidence for recent volcanism or active hydrothermal venting.
By providing one of the few high-resolution bathymetric datasets of a hydrothermally hosted axial volcanic high, this dataset allows us to examine the relationship between hydrothermal venting and fine-scale seafloor morphology on the ultraslow-spreading Gakkel Ridge.
How to cite: Isler, T., Schlindwein, V., Albers, E., and German, C. R.: Association of hydrothermal venting with seafloor morphology from high-resolution bathymetry at the Polaris vent site, Gakkel Ridge, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6443, https://doi.org/10.5194/egusphere-egu26-6443, 2026.
Estimates of on‑axis hydrothermal element fluxes commonly assume that basalt‑hosted, black smoker‑type vent fluids dominate global hydrothermal cooling of the oceanic lithosphere. However, hydrothermal vent fluids exhibit substantial compositional diversity related to different substrate types (basaltic, ultramafic, sedimented, intermediate‑felsic) and geological settings (mid-ocean ridges, back‑arc spreading centers, volcanic arcs), which has not yet been adequately incorporated into global flux estimates.
Here, we account for this diversity by analyzing the current plate boundary configuration and a global database of hydrothermal vent fluid compositions (MARHYS Database, Version 4.0). We calculate weighting factors for the relative contributions of different hydrothermal fluid types to lithospheric cooling by integrating ridge and arc strike lengths, spreading rates, and substrate distributions across plate boundary types. Using these weighting factors, we estimate the partitioning of vent fluid types and quantify global submarine on-axis hydrothermal element fluxes.
We show that element-to-energy flux ratios vary significantly among geological settings and differ markedly from characteristics of purely basalt-hosted, fast-spreading ridges. As a result, substantially different fluxes are obtained for several key elements (e.g., H₂, CH₄, Fe) associated with hydrothermal cooling across diverse plate boundaries and substrate types. Our results demonstrate that oceanic element fluxes are regionally variable and that the partitioning of plate boundary types (e.g., ultraslow versus fast‑spreading ridges; volcanic arcs and back‑arc spreading centers versus mid‑ocean ridges) plays a major role in regulating element transfer between the oceanic crust and the ocean over geological timescales.
How to cite: Diehl, A. and Bach, W.: Global on-axis hydrothermal element fluxes at submarine plate boundaries, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21979, https://doi.org/10.5194/egusphere-egu26-21979, 2026.
Mantle plumes interacting with mid-ocean ridges (MORs) produce prominent geophysical and geochemical anomalies in oceanic lithosphere. However, the role of oceanic transform faults (OTFs), major discontinuities within MOR systems, in modulating along-axis plume dispersion remains poorly understood. Here, we combine a global dataset of 24 plume–ridge–transform systems with 3D geodynamic modeling to investigate the geometric and kinematic controls on plume behavior along segmented ridges.
Based on spatial relationships among plumes, ridge segments, and transforms, we define three end-member interaction modes: (1) on-ridge, (2) off-ridge, and (3) on-transform– fracture zone plumes. Systematic geodynamic models reveals that OTFs may exert one of three primary roles depending on plume location and system geometry: (i) barriers, which impede along-ridge plume dispersion when long transform offsets create lithospheric discontinuities; (ii) bridges, which permit relatively unimpeded dispersion when plumes lie near transform–ridge junctions or beneath fracture zones; and (iii) boosters, where transform-centered or inside-corner plumes enhance plume transport via strike-slip-induced mantle flow acceleration near the transform fault.
We demonstrate that transform offset length, plume–ridge and plume–transform distances, and ridge spreading rate collectively determine the efficacy of plume dispersion along ridge axes. The proposed framework offers a geometric basis for interpreting observed asymmetries in natural plume–ridge systems and highlight the complex, context-dependent nature of transform fault influence. These insights challenge the classical “transform damming” hypothesis and emphasize the necessity of considering 3D mantle flow dynamics in plume–ridge–transform interactions.
How to cite: Liu, S., Zhang, F., Rüpke, L., Luo, Y., Chen, M., Zhang, X., Zhao, L., Zhang, Y., Chen, Z., and Lin, J.: Oceanic Transform Faults as Barriers, Bridges, and Boosters: Geometric Controls on Plume Dispersion Along Segmented Mid-Ocean Ridges, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7543, https://doi.org/10.5194/egusphere-egu26-7543, 2026.
Ridge-transform intersections (RTIs) display distinct topographic morphologies, yet the origin remains debated. Standard thermal models treat the RTI as a juxtaposition of an old and cold plate against a young and warm spreading ridge such that this contact deepens the RTI with age offset, while another classic view attributed RTI bathymetry to spreading rate dependence of magma supply. These models explain the systematically deepening of RTI bathymetry with age offset and decreasing spreading rate, but fail to account for the highly variable RTI bathymetry with comparable age offset and spreading rate.
We analyzed multibeam bathymetric data of 101 RTIs at 65 OTFs at ultraslow- to fast-spreading ridges and conducted 3D numerical simulations of plate separation and dike injection at a ridge-transform-ridge system by using the geodynamic code LaMEM (Lithosphere and Mantle Evolution Model). We treat a dike injection to occur when differential stress that defined as the difference between magmatic overpressure and tectonic stress overcomes lithosphere pressure, which yields an effective M value that represents time-averaged fraction of plate separation accommodated by magmatic emplacement in a time scale of 10-100 yr. We show the variability in RTI depth can be related to brittle lithosphere thickness, where a thinner brittle lithosphere can generate the M value in a wider range and eventually leads to distinct topographic morphologies. This results in the systematically deepening of RTI bathymetry with age offset and its increasing variability with decreasing age offset. Furthermore, our result suggests that the systematical variations in RTI depth mainly reflects the age offset dependence of plate cooling, instead of spreading rate. More generally, it implies that the interactions between spreading ridge and the juxtaposed old plate determine time-averaged magma supply that reshapes seafloor morphology when oceanic transform faults pass mid-ocean ridges and evolve into fracture zones.
How to cite: Chen, M., Rüpke, L., Grevemeyer, I., Ren, Y., and Liu, S.: Magmatism controls bathymetry at global mid-ocean ridge-transform intersections, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10683, https://doi.org/10.5194/egusphere-egu26-10683, 2026.
Oceanic transform faults offset spreading axes by tens to hundreds of kilometers and are among the most prominent tectonic features in deep ocean basins. The Gofar transform fault system (GTFS) is a major left-lateral ridge-crest discontinuity connecting segments of the fast-spreading East Pacific Rise. This highly segmented transform fault system is characterized by high-relief flanks, J-shaped structures at ridge-transform intersections, and deep troughs connecting three fault segments (G1, G2, and G3, from east to west). Over the past two decades, the western G3 segment has been extensively studied through multidisciplinary approaches including near-field observations and numerical modeling, revealing along-strike variations in seismicity patterns, slip behavior, and potential governing factors. However, the segmentation of the entire GTFS and its relationship with intra-transform spreading centers and/or pull-apart basins remain poorly understood, as seismic behavior of the eastern G1 and G2 segments has not been sufficiently well constrained by near-field observations.
Between November 2019 and February 2022, 30 ocean bottom seismometers (OBS) were deployed to monitor seismic activity along the eastern GTFS (G1 and G2 segments). We first evaluated the performance of multiple deep-learning phase pickers on this OBS dataset, including EQTransformer, PhaseNet, and PickBlue. PickBlue, specifically trained for OBS data, demonstrated superior event detection performance compared to pickers trained on onshore datasets. We then applied the non-linear oct-tree grid-search algorithm (NonLinLoc) with source-specific station terms (SSST) to obtain precise absolute event locations. Our results reveal high seismicity density along the G1 and G2 transform segments, as well as distributed deformation within the deep trough connecting these segments, showing features resembling continental pull-apart basins. Notably, the OBS network captured a magnitude 6 earthquake in the study area, providing unique insights into fault slip behavior before and after the mainshock at oceanic transform faults.
How to cite: Ren, Y., Lange, D., and Grevemeyer, I.: Segmentation and Seismicity of the Eastern Gofar Transform Fault System Revealed by 30-Month Ocean Bottom Seismometer Deployment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8441, https://doi.org/10.5194/egusphere-egu26-8441, 2026.
The Azores-Gibraltar Plate Boundary (AGPB) materializes the present-day westernmost segment of the Africa (Nubia)-Eurasia plate boundary, and connects the Azores triple junction, at the west, to the Gibraltar orogenic arc, at the east. The Gloria Fault corresponds to its central and transform segment, trending E-W to WNW-ESE between 24° W and 14°30’W and showing dextral strike-slip motion. This feature corresponds to one of the rare examples of a ridge-transform fault-orogenic arc plate configuration worldwide. The Gloria Faults has also been the site of great-magnitude earthquakes, such as the 25 November 1941 (Mw8.4), the second-largest oceanic strike-slip event recorded worldwide. In spite of the studies carried out in this AGPB segment, the seismotectonics of the Gloria Fault is still poorly known. In this work we present a detailed morphostructural characterization of the Gloria Fault and discuss the relationship between the main morphostructures and seismicity, based on i) the geomorphological analysis of unpublished multibeam bathymetry collected in the scope of the Project of Extension of Continental Shelf; ii) correlation of the main morphostructures identified with instrumental seismicity and microseismicity records available from public catalogues (SHARE, USGS, IPMA) and published by several authors, and iii) profile gravity analysis based on SGG-UGM-2 satellite gravity data compilation.
Based on the morphostructural, seismotectonic and gravimetric analysis we propose the existence of a Gloria Fault Transform System, which includes the several morphological features relate to its transcurrent motion (e.g., central valley, transverse ridges, restraining bend, Western Gap, Eastern Ridge), and the two main seismically active structures in the area, located at north and south of the Gloria Fault. This suggests that, at present, the stress due to the motion of the Africa-Eurasia plates is accommodated by seafloor deformation along a wide E-W stripe.
This work is supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020 - https://doi.org/10.54499/LA/P/0068/2020 , UID/50019/2025, https://doi.org/10.54499/UID/PRR/50019/2025, UID/PRR2/50019/2025.
How to cite: Roque, C., Manzoni, S., Duarte, J., Gonçalves, S., Batista, L., and Souto, M.: The Africa-Eurasia transform plate boundary – Insights from the morphostructure of the Gloria Fault, NE Atlantic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12584, https://doi.org/10.5194/egusphere-egu26-12584, 2026.
The Oceanographer Transform Fault is a 120 km long and E-W oriented transform fault located southwest of the Azores. We have detailed geological and morphological information of the area through high-resolution bathymetry and an extensive collection of rock samples. There we see different seafloor types (magmatic dominated volcanic seafloor, tectonic dominated smooth seafloor and core complexes) that indicate variations in the magmatic productivity. Our results show that seafloor morphology is linked to magma supply rates.
Now, we work at showing a complete geotectonic evolution of the Oceanographer Transform Fault area. The new data presented here, include radiometric age dates, which put constraints on the timing of processes, and magnetic signatures.
The magnetic anomalies were analyzed by 2D profile forward models. Weak magnetic patterns are observed above areas where mainly mantle-derived rocks occur. On the other hand, magmatic robust segments which are predominantly basaltic, are characterized by well-defined magnetic anomalies. Based on these magnetic anomaly analyses, we estimate seafloor spreading rates. Crustal accretion is asymmetric at both axes and varies in space and time.
To verify our magnetic anomaly results, we conducted U-Pb dating on zircons in five gabbroic samples collected by dredging. Obtained crystallization ages range between 3 Ma to 8 Ma. Not all results align with the seafloor ages, some geochemical ages are younger than the dates derived from the magnetic anomaly, which might be due to secondary magmatism.
How to cite: Unger Moreno, K. A., Beniest, A., Rüpke, L. H., Hansteen, T. H., Devey, C. W., Nikogosian, I. K., and Grevemeyer, I.: Geotectonic evolution of the Oceanographer Transform Fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18924, https://doi.org/10.5194/egusphere-egu26-18924, 2026.
