GD5.1 | Magmatic, tectonic and hydrothermal processes at mid-oceanic ridges and transform faults: new insights from observations, experiments and numerical modelling
EDI
Magmatic, tectonic and hydrothermal processes at mid-oceanic ridges and transform faults: new insights from observations, experiments and numerical modelling
Co-organized by TS2
Convener: Philipp Brandl | Co-conveners: Marcia Maia, Eleonora Ficini, Antoine Demont, Florent Szitkar
Orals
| Mon, 15 Apr, 08:30–12:30 (CEST)
 
Room -2.47/48
Posters on site
| Attendance Tue, 16 Apr, 10:45–12:30 (CEST) | Display Tue, 16 Apr, 08:30–12:30
 
Hall X2
Orals |
Mon, 08:30
Tue, 10:45
Relative motions at plate boundaries allow the Earth’s crust to compensate forces driven by global plate tectonics. Mid-oceanic ridges (MORs) provide the unique opportunity to study two of the three plate boundaries: divergent at the ridge axis and strike-slip at transform faults (off-setting the ridge axis). Knowledge of the active and past processes building and altering the oceanic lithosphere has increased over the past 20 years due to advances in deep sea research technologies and numerical modelling techniques. Yet, several questions remain open, such as the relative role of magmatic, tectonic and hydrothermal processes in the formation of the oceanic lithosphere at the ridge axis, especially at slow and ultra-slow spreading ridges and at their intersection with transform faults. Transform faults and their extension into fracture zones, for example, remain largely under-studied features. For a long time, they were considered as cold and inactive; however, evidence for magmatism emerged recently inside both features. Given the complex network of faults associated with these structures, they represent ideal pathways for hydrothermal percolation into the Earth’s lithosphere and may therefore play a significant role in the chemical and the thermal budget of the planet, as well as in the chemical exchange with the ocean (e.g., nutrients). This session aims at fostering the scientific exchange across all disciplines studying mid-oceanic ridge axes, transform faults and fracture zones. Studies building on the use of cutting-edge deep-sea research technology are particularly welcome. The session also welcomes recent developments in thermo-mechanical models, which integrate geophysical and geological data with numerical modelling tools, bridging the gap between observations and numerical models.

Orals: Mon, 15 Apr | Room -2.47/48

Chairpersons: Philipp Brandl, Marcia Maia, Florent Szitkar
08:30–08:35
08:35–08:55
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EGU24-12596
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solicited
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Virtual presentation
Thibaut Barreyre, Lars Rüpke, Jean-Arthur Olive, Eoghan Reeves, Lars Ottemöller, Jill McDermott, Ross Parnell-Turner, and Daniel Fornari

Hydrothermal systems along mid-ocean ridges (MORs) are a crucial interface between Earth’s deep interior, the seafloor, and the overlying ocean. Although hydrothermal systems are typically thought of as steady-state flow environments, field-based observations indicate that flow rates and temperatures are highly variable over a wide range of spatial and temporal scales. These observations show that flow systems respond to sub-surface processes such as earthquakes, magmatic activity, dissolution/precipitation of fluid minerals, and tidal loading of the oceanic crust and sediments. This variability in subsurface phenomena associated with seafloor flow systems directly impacts both the transfer of heat and matter and therefore, productivity of associated hydrothermal ecosystems.

Moving beyond an empirical assessment, however, remains challenging because a complete theoretical framework relating tectonic and magmatic fluctuations to hydrothermal output is currently lacking. Understanding the relationship between tectonic- and magma-induced stress and strain transients, crustal permeability, and the thermo-chemical state of hydrothermal fluids before, during and after an earthquake and magmatic emplacement event is particularly crucial. To address this issue, we curated time series of vent temperatures at the Loki’s Castle and EPR 9°50'N hydrothermal systems, and analyzed them alongside microseismicity catalogs, intermittent sampling of vent fluid chemistry, and a large body of geological, geophysical and biological observations, including proxies for crustal permeability.

Results suggest that both short-term (sec to hours) and long-term (decadal) variability in hydrothermal venting is controlled by fluctuations in the permeability field of the underlying crust, which can itself be related to changes in the crustal stress regime. We capture co-seismic, dike-induced and inter-eruptions changes in the fluid flow records indicating tectonic and magmatic control on hydrothermal vent temperature and flow discharge. Using simple analytical models for hydrothermal discharge temperature, elastic stress changes, and permeability-stress relations, we argue that temperature fluctuations can result from changes in permeability caused by either passing seismic waves, magmatic reservoir inflation (/deflation), or intrusions. Our observations and models further imply that short- and long-term fluctuations in tectonic and magmatic activity can modulate hydrothermal output, with potential consequences for deep sea ecosystems. This methodology has the potential to track tectonic and magmatic induced deformation transients in the sub-seafloor from hydrothermal flow records. 

How to cite: Barreyre, T., Rüpke, L., Olive, J.-A., Reeves, E., Ottemöller, L., McDermott, J., Parnell-Turner, R., and Fornari, D.: Smokers under stress: new insights into the tectonic, magmatic and oceanic modulation of hydrothermal discharge at mid-ocean ridges, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12596, https://doi.org/10.5194/egusphere-egu24-12596, 2024.

08:55–09:05
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EGU24-2899
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ECS
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On-site presentation
Fei Zhou, Chunhui Tao, Tao Wu, and Jerome Dyment

Rock types of basement determine the magnetic signature of hydrothermal fields. Low magnetization zone (LMZ) is commonly observed at the basalt-hosted hydrothermal fields due to the fluid-rock interaction destroying the magnetic minerals inside basalt. We here report a near-seafloor magnetic survey conducted by the Autonomous Underwater Vehicle (AUV) over a basalt-hosted hydrothermal field on the East Pacific Rise (EPR). Inversed magnetization and Reduced-To-the-Pole (RTP) magnetic anomaly both show negative reduced magnetic signature centered on the hydrothermal field, reflecting enhanced demagnetization alteration process. Meanwhile, we delineate the range of the LMZ and compare it with the previous high-resolution near-seafloor magnetic studies on the fast-spread EPR, slow-spread Mid-Atlantic Ridge, and ultra-slow-spread Southwest Indian Ridge and Mohns Ridge. The statistical result shows that the diameter of LMZ increases with the decreasing spreading rate, suggesting the stable tectonic environment and focused melt supply at slower spreading ridge favor the birth of larger hydrothermal field.

How to cite: Zhou, F., Tao, C., Wu, T., and Dyment, J.: A near-seafloor study reveal a smaller low magnetization zone of a basalt-hosted hydrothermal field at East Pacific Rise, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2899, https://doi.org/10.5194/egusphere-egu24-2899, 2024.

09:05–09:15
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EGU24-20662
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ECS
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On-site presentation
Szu-Ying Lai, Gaye Bayrakci, Bramley Murton, Tim Minshull, Emma Gregory, and Isobel Yeo

We presented results from a new seismic refraction experiment at the Semenov hydrothermal area at 13°30’ N on the western flank of Mid-Atlantic Ridge. The survey was carried out on Cruise JC254 on RRS James Cook in November 2023. Semenov is a typical ultramafic-hosted field consisting of five active and extinct hydrothermal sites (Semenov-1 to 5) associated with massive sulphide mounds (SMS), hosted on a 20-km-long oceanic core complex (OCC). The OCC offers an exceptional opportunity to observe deep-seated ultramafic rocks exposed on the seafloor by detachment faulting. Semenov field is an ideal location to investigate the link between OCC-related detachment and SMS deposit formation. Here, we aim to determine the Semenov OCC crustal structure, geometry, and extent of subseafloor SMS mineralisation.

Our seismic refraction survey revealed a detailed 2-D P-wave velocity structure beneath Semenov. We target the Semenov-3 and Semenov-4 hydrothermal sites sitting at either side of the seabed termination of the detachment fault. We focused on profiles crossing the OCC in E-W and N-S directions, shot with two GI guns (250G and 105I cubic inch) every 30 m. The data were recorded by a network of 18 ocean bottom nodes (OBX) at 0.4 to 1 km spacing, showing clear first-arrival refractions from beneath the OCC and the hanging wall. We expect to define the seismic structure down to 2 km beneath the seabed. The derived velocity model could give information to the lithology beneath the Semenov OCC-related detachment and possibly driving source for the hydrothermal circulation. Lastly, we compare our result with another detachment-related hydrothermal system at TAG, where the OCC is thought to be at the initial stage and the hydrothermal system is basalt-hosted.

How to cite: Lai, S.-Y., Bayrakci, G., Murton, B., Minshull, T., Gregory, E., and Yeo, I.: 2-D Oceanic Core Complex Structure at the Semenov hydrothermal field on the Mid-Atlantic Ridge from New Wide-Angle Seismic Data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20662, https://doi.org/10.5194/egusphere-egu24-20662, 2024.

09:15–09:25
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EGU24-5337
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On-site presentation
Anke Dannowski, Martin Engels, Bettina Schramm, Michael Schnabel, Oscar Lucke, Udo Barckhausen, Ingo Heyde, Stefan Ladage, Rüdiger Lutz, Christian Filbrandt, Anna Jegen, and Ingo Grevemeyer

Three tectonic plates meet at the Rodriguez Triple Junction in the Central Indian Ocean. The plates are separated by the Central Indian Ridge (CIR), the South-East Indian Ridge (SEIR) and the South-West Indian ridge (SWIR), which all show highly different spreading behaviours. While the northernmost segment of the SEIR is magmatically robust, the eastern tip of the SWIR is highly amagmatic. The CIR appears to oscillate between opening mechanisms, associated either with magmatic or magma-starved spreading processes, which can be observed over a very confined stretch of crust. Even though the area has been studied thoroughly, using a variation of geophysical and geological methods in the past decades, seismic images of the region were missing. From November 2023 to January 2024, RV Sonne (SO301 - SCIROCCO) set out for a seismic reflection and refraction survey to fill this gap and to provide a database for a better understanding of the tectonic setting and evolution of the area. A special focus was put on studying the structure and extent of the Oceanic Core Complex (OCC) at 25 °S.

Here we present preliminary results of an east-west trending 150 km long profile crossing the OCC and the CIR. Along the profile, 33 ocean bottom seismometers were deployed with a spacing of 4-5 km that grew denser over the OCC. The shot spacing was between 50-110 m. Clear crustal refracted P- and S-phases were observed to offsets of up to 40 km in the shot sections and mantle reflections, as well as Pn-phases could be identified sporadically. First results of travel time tomographies, which were executed separately for P- and S-waves, and used for the calculation of a Vp/Vs-ratio section indicate a strongly variable crustal construction. Highly fractured areas seem to interchange with highly hydrated areas within short distances. Correlations of the new bathymetric data to the seismic images and the integration of the new gravimetric and magnetic data will sharpen the geophysical image and its tectonic interpretation along the profile.

How to cite: Dannowski, A., Engels, M., Schramm, B., Schnabel, M., Lucke, O., Barckhausen, U., Heyde, I., Ladage, S., Lutz, R., Filbrandt, C., Jegen, A., and Grevemeyer, I.: Seismic velocity structure of the 25 °S OCC north of the Rodriguez Triple Junction at the Central Indian Ridge extracted from ocean bottom seismometer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5337, https://doi.org/10.5194/egusphere-egu24-5337, 2024.

09:25–09:35
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EGU24-7543
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Highlight
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On-site presentation
Milena Marjanovic, Jie Chen, Javier Escartín, Ross Parnell-Turner, and Jyun-nai Wu

Mid-ocean ridges host the most extensive magmatic system on Earth, where ~60% of the lithosphere is formed. Fast spreading segments such as the East Pacific Rise (EPR) 9º50’N (full spreading rate >80 mm/yr) represent only ~20% of the global ridge network, but contribute ~50% of the total oceanic crustal accretion.  At these ridge segments, magma accumulates in on-axis, quasi-steady-state axial magmatic lenses (AML), typically found 1-2 km below the seafloor, 2 km wide on average, and <0.1 km thick. AMLs are highly three-dimensional in geometry, marked by alternating lineated ridges and troughs where dikes originate and connect with the seafloor through the systems of the faults and fissures (Marjanović et al., 2023). A couple of kilometers away from the ridge axis, the seafloor is dominated by abyssal hills, which are bounded by faults resulting from unbending, cooling, and extension of the lithosphere. Within the critical region between the axial summit trough (AST) and the first abyssal hill bounding faults, sparse mapping has shown that prominent faults can exist, but the mechanism for their origin and contribution to tectonic strain has remained elusive.

At the EPR 9º50’N, we combine meter-scale bathymetric mapping with the highest-resolution seismic imagery of an AML to date (horizontal resolution 25 x 25 m2) to reveal a remarkable vertical alignment between the AML and seafloor fault scarps. This genetic link we observe for four distinct cases. Each AML-fault pair is aligned asymmetrically with respect to the ridge axis and is associated with confirmed and possible records of hydrothermal venting observed in the results of recent seafloor and water column mapping (Wu et al., 2023). Along most of such faults’ scarps, the emplacement of magma through various eruption episodes is evident, helping build the crust outside the AST. Our observations at 9º48’N support a mechanism by which these asymmetric, tectonic-magmatic features originate from shallow magma injection sites. After initial magma injection, the surrounding crustal stresses are perturbed thus promoting further crack propagation aligned with the orientation of the underlying magma body. Finally, by joint analyses of faults exposed on the seafloor and seismically-imaged in the subsurface, we also show that their collective contribution to the overall tectonic component of seafloor spreading is less than 0.5%, with a close to negligible role of the lava-covered faults, much smaller than previously proposed.

Marjanović, M. et al., 2023, Insights into dike nucleation and eruption dynamics from high-resolution seismic imaging of magmatic system at the East Pacific Rise: Science advances, v. 9, p. eadi2698, doi:10.1126/sciadv.adi2698.

Wu, J.-N., Parnell-Turner, R., Fornari, D.J., Barreyre, T., and McDermott, J., 2023a, Oceanic heat transfer by diffuse and focused flow through off-axis vents at 9°50’N, East Pacific Rise, in AGU Fall Meeting Abstracts, v. 2023, p. V43B-0174.

How to cite: Marjanovic, M., Chen, J., Escartín, J., Parnell-Turner, R., and Wu, J.: Magma-induced tectonics at the East Pacific Rise 9º50’N: Evidence from high-resolution characterization of seafloor and subseafloor , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7543, https://doi.org/10.5194/egusphere-egu24-7543, 2024.

09:35–09:45
09:45–09:55
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EGU24-14978
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On-site presentation
Roi Granot and Carmen Gaina

The Central Atlantic oceanic crust documents the evolution of seafloor spreading along the Central Mid-Atlantic Ridge. Previous works have identified marine magnetic anomalies (i.e., isochrons) every ~7 Myr for the pre-Neogene time, thus, the existing Nubia-North America plate kinematic models suffer from rather crude temporal resolution. Here, we present preliminary results from a detailed plate kinematic investigation aiming to reconstruct the kinematics of seafloor spreading at ~1.5 Myr time intervals. Our model is constrained by ~11000 identifications of 40 magnetic reversals younger than C34y (83.6 Ma) and older than C6no (19.7 Ma). We also investigated the fracture zones using multibeam bathymetry and satellite-derived gravity data. These identifications are confined by the Fifteen-Twenty fracture zone in the south and the Azores triple junction in the north. We invert these identifications and fracture zone crossings to estimate a set of finite rotation poles and stage rotations. We confirm the validity of our plate kinematic solutions by comparing a set of synthetic flowline tracks to the location of fracture zone traces. Based on these new poles, we present a detailed kinematic analysis that sheds new light on the evolution of the Central Atlantic since the Late Mesozoic.

 

How to cite: Granot, R. and Gaina, C.: High-resolution kinematic study of the Late Mesozoic to Late Cenozoic seafloor spreading at the Central Mid-Atlantic Ridge., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14978, https://doi.org/10.5194/egusphere-egu24-14978, 2024.

09:55–10:05
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EGU24-11890
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ECS
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On-site presentation
Attila Balázs, Taras Gerya, and Gabor Tari

The ocean floor shows variable morphological features, transtensional and transpressional structures, magmatic and amagmatic domains. Surprisingly, continental blocks separated from the continental margins from 100s or 1000s km distance have been occasionally reported, however, their origin remains debated.

We conducted 3D magmatic-thermo-mechanical numerical experiments with the code I3ELVIS to simulate the dynamics of continental rifting, continental proto-transform fault zones, and eventually the formation of persistent oceanic transform faults and their connection to mantle melting. Numerical modelling results allow to analyze the first order features of passive and transform margins and oceanic basins. Our models explain the evolution of continental blocks entrapped between oceanic spreading ridges bounded by strike-slip fault zones inherited from the preceding continental rifting stage. The formation of such continental slivers is controlled by the relative timing between the onset of oceanic spreading and strain localization along strike-slip fault zones. This is connected to the rheology of the plates and also linked to different thermal gradients, divergence velocities, melting conditions and surface processes. Furthermore, we discuss the formation of zero-offset V-shaped oceanic fracture zones and the along-ridge variation of oceanic crustal thicknesses. Our model results are compared with observational data from the Romanche transform of the Equatorial Atlantic, the East Greenland Ridge and Newfoundland Ridge in the northern Atlantic, the Zabargad Islands in the Red Sea and the Davie Fracture Zone.

How to cite: Balázs, A., Gerya, T., and Tari, G.: Continental slivers in oceanic transform faults controlled by tectonic inheritance, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11890, https://doi.org/10.5194/egusphere-egu24-11890, 2024.

10:05–10:15
Coffee break
Chairpersons: Eleonora Ficini, Antoine Demont, Philipp Brandl
10:45–11:05
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EGU24-13609
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solicited
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Highlight
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Virtual presentation
Jill M. McDermott, Connor C. Downing, Jada M. Siverand, Esmira Bibaj, Thibaut Barreyre, Daniel J. Fornari, Ross Parnell-Turner, Jeffrey S. Seewald, Eoghan P. Reeves, Drew D. Syverson, Dalton S. Hardisty, and Alysia D. Cox

Multidisciplinary studies at the 9°50’N East Pacific Rise (EPR) hydrothermal field span three decades and encompass two periods of volcanic activity in 1991-1992 and 2005-2006. Shifts in the pressure and temperature of hydrothermal circulation induced by the magmatic cycle drive changes in the composition of venting fluids. Previous geothermobarometric model approaches used quartz solubility and fluid Cl concentrations to estimate pressure and temperature conditions in the zone where hydrothermal fluids originate [1, 2]. A geothermometer based on dissolved Fe/Mn [3] now provides additional insight on fluid origin temperatures. Consequently, application of an updated geothermobarometric model is possible.

We estimate the pressure and temperature conditions of fluid formation at historic high temperature vents in the 9°50’N EPR area between 2018 and 2023 and compare them with time series data since 1991. These calculations focus on six vents that span 7 km north to south along the axial summit trough, including M, Bio9, P, V, L, and L-Hot8 vents.

Immediately following eruptions, fluid origin pressures are considerably shallower at Bio9, M, and P vents (25-27 MPa) than during periods of lower magmatic activity (30-35 MPa). Additionally, fluid origin temperatures at the same vents rise from 390-400 °C for 1-2 years after eruptions to 410-430 °C during the periods between eruptions. Despite repeated observation of a continuous warming trend in vent fluid exit temperatures in the years preceding eruptions, fluid origin temperatures are relatively stable at a given vent location over the same time periods. 

These results support previous assertions that the upflow zone may experience enhanced permeability immediately following an eruption, leading to seawater entrainment, and cooling prior to venting. These inferences are also supported by a significantly greater proportion of radiogenic, seawater-derived Sr isotope input to circulating fluids in the 1-2 years after eruptive events, followed by a shift toward less radiogenic, more basalt-derived Sr isotope signatures.  The vent fluids are presently circulating into the sheeted dikes and attaining maximum depths and temperatures similar to those previously observed leading up to the 2005/2006 eruption. The chemical behavior and formation conditions of these hydrothermal fluids will be tracked through 2025, with the goal to understand the hydrothermal response preceding the next magmatic event. 

[1] Von Damm (2004) AGU Mono; [2] Fornari et al. (2012) Oceanography; [3] Pester et al. (2011) Geochim. Cosmochim. Acta.

How to cite: McDermott, J. M., Downing, C. C., Siverand, J. M., Bibaj, E., Barreyre, T., Fornari, D. J., Parnell-Turner, R., Seewald, J. S., Reeves, E. P., Syverson, D. D., Hardisty, D. S., and Cox, A. D.: Geochemical insights into conditions of vent fluid origin and water-rock interaction over two eruptive cycles at 9° 50´N East Pacific Rise, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13609, https://doi.org/10.5194/egusphere-egu24-13609, 2024.

11:05–11:15
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EGU24-10653
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On-site presentation
Lars-Eric Heimbürger-Boavida, Natalia Torres-Rodriguez, Jingjing Yuan, Sven Petersen, Aurélie Dufour, David Gonzalez-Santana, Valerie Chavagnac, Hélène Planquette, Milena Horvat, David Amouroux, Cecile Cathalot, Ewan Pelleter, Ruoyu Sun, Jeroen Sonke, and George Luther

The UNEP Minamata Convention on Mercury aims to reduce human exposure to toxic mercury through the reduction of anthropogenic emissions. We are primarily exposed via the consumption of fish that bioaccumulate mercury from the ocean. The current paradigm is that anthropogenic mercury emissions (present-day 3,100 tons per year) have increased the global oceanic mercury reservoir by 21%. This estimate is flawed because we do not know how much natural mercury resided in the ocean before anthropogenic emissions started. We are similarly unable to quantify how anthropogenic emissions have affected fish mercury levels. Hydrothermal venting is the only direct source of natural mercury to the ocean. Previous studies, based on vent fluid measurements alone, suggested that hydrothermal mercury inputs could range from 20 and 2,000 tons per year. We use observations of vent fluids, plume, sea water and rock cores from the Trans-Atlantic Geotraverse (TAG) hydrothermal vent at the Mid-Atlantic ridge aquired during three dedicated oceanographic cruises. The combined observations suggest that the majority (67–95%) of the mercury enriched in the vent fluids (4,966 ± 497 picomoles per litre) is diluted into sea water  to reach background seawater levels (0.80 picomoles per litre) and a small fraction is scavenged locally (2.6–10%). An extrapolation of our results suggests that the global hydrothermal mercury flux from mid ocean ridges is small (1.5 - 65 tons per year) compared to anthropogenic mercury missions. While this suggests that most of the mercury present in the ocean is of anthropogenic origin, it also gives hope that the strict implementation emission reductions in the framework of the Minamata Convention could effectively reduce fish mercury levels and human exposure.

Torres-Rodriguez, N., Yuan, J., Petersen, S., Dufour, A., González-Santana, D., Chavagnac, V., Planquette, H., Horvat, M., Amouroux, D., Cathalot, C., Pelleter, E., Sun, R., Sonke, J. E., Luther, G. W., and Heimbürger-Boavida, L.E.: Mercury fluxes from hydrothermal venting at mid-ocean ridges constrained by measurements, Nat. Geosci., 1–7, https://doi.org/10.1038/s41561-023-01341-w, 2023.

 

How to cite: Heimbürger-Boavida, L.-E., Torres-Rodriguez, N., Yuan, J., Petersen, S., Dufour, A., Gonzalez-Santana, D., Chavagnac, V., Planquette, H., Horvat, M., Amouroux, D., Cathalot, C., Pelleter, E., Sun, R., Sonke, J., and Luther, G.: Mercury fluxes from hydrothermal venting at mid-ocean ridges constrained by measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10653, https://doi.org/10.5194/egusphere-egu24-10653, 2024.

11:15–11:25
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EGU24-22120
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On-site presentation
Muriel Andreani, Clément Herviou, Gilles Montagnac, Clémentine Fellah, Bénédicte Ménez, Céline Pisapia, Marvin Lilley, and Gretchen Früh-Green

In nature, very few organic compounds are recognized as abiotic. Abiotic methane (CH4) is the most abundant, and can be accompanied by short-chain hydrocarbons (ethane, propane) or organic acids (formate, acetate) in fluidsoccurring in molecular hydrogen (H2)-enriched hydrothermal systems where olivine-bearing rocks are altered via serpentinization reactions, such as along slow and ultra-slow spreading ridges. In addition to those volatiles and dissolved organic species, studies of oceanic serpentinites have highlighted low temperature (T), abiotic formation of organic compounds such as amino acids or various carbonaceous compounds within the rock substrate. This suggests the availability of more diverse abiotic organic reactants than previously expected on Earth, notably in the subseafloor, and questions the reaction paths at their origin.

Here we present an rocky road to abiotic organic synthesis and diversification in hydrothermal environments, which involves magmatic degassing and water-consuming mineral reactions occurring in olivine fluid inclusions. This combination gathers key gases (N2, H2, CH4, CH3SH) and various polyaromatic materials associated with nanodiamonds and mineral products of olivine hydration (serpentinization). This endogenous assemblage results from re-speciation and drying of cooling C-O-S-H-N fluids entrapped below 600°C-2kbars in rocks forming the present-day oceanic lithosphere. Samples have been drilled at the Atlantis Massif (30°N Mid-Atlantic Ridge) during IODP Expeditions 304-305, five km to the north of Lost City hydrothermal field where the discharge of abiotic H2, CH4 and formate have been observed in fluids. Fluid inclusions served as a closed microreactor in which serpentinization dries out the system toward macromolecular carbon condensation, while olivine pods keep ingredients trapped until they are remobilized for further reactions at shallower levels. Results greatly extend our understanding of the forms of abiotic organic carbon available in hydrothermal environments and open new pathways for organic synthesis encompassing the role of minerals and drying. Such processes are expected in other planetary bodies wherever olivine-rich magmatic systems get cooled down and hydrated.

How to cite: Andreani, M., Herviou, C., Montagnac, G., Fellah, C., Ménez, B., Pisapia, C., Lilley, M., and Früh-Green, G.: Abiotic synthesis of volatile and condensed organic compounds in the deep oceanic lithosphere , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22120, https://doi.org/10.5194/egusphere-egu24-22120, 2024.

11:25–11:35
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EGU24-14965
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ECS
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On-site presentation
Camilla Sani, Alessio Sanfilippo, Sergey Skolotnev, Marco Ligi, Felix Genske, and Andreas Stracke

The Doldrums transform system (TS), located in the Equatorial Mid Atlantic Ridge (MAR) at 7-8°N, is a 110 km-wide multi-fault shear zone, with five active transform faults separated by four short intra-transform ridge segments (ITRs). The medial ITRs are substantially deeper than the peripheral rift segments, which indicate differences in the thermal conditions of the sub-ridge mantle. New chemical and radiogenic isotope data from on-axis lavas erupted across the entire transform domain reveal that the basalts from the shortest and deepest ITRs are enriched comparatively in alkalis (Na2O+K2O= 4.3 wt%; Na8 up to 3.7) and light rare earth elements (La/Sm)N = 0.86 -0.97), likely suggesting the presence of an extremely cold mantle region characterised by low degrees of partial melting. The enriched incompatible element compositions, however, are coupled with the lowest Sr and Pb isotopes in the Equatorial Atlantic magmatism (i.e., 87Sr/86Sr ~ 0.70237 and 206Pb/204Pb ~ 18) and relatively high Nd and Hf isotope ratios (143Nd/144Nd = 0.51315-0.51325; 177Hf/176Hf = 0.2832-0.28325), which indicates that incompatible element enriched components are less abundant in the mantle source of the central ITRs. Hence we infer that the mantle under the central ITRs has been melted at the MAR axis before being transported laterally into the central ITR domain during the formation of the Doldrums transform system. This mantle portion melted a second time, and to a low extent, during the opening of the cold ITR, revealing its depleted geochemical character. Therefore, MORB from intra-transform ridge segments provide a rare opportunity to constrain the isotopic composition of the depleted peridotitic mantle, a ubiquitous, but otherwise often concealed component of Earth’s mantle.

How to cite: Sani, C., Sanfilippo, A., Skolotnev, S., Ligi, M., Genske, F., and Stracke, A.: Revealing mantle heterogeneity in a cold intra-transform spreading segment (7-8°N at Mid Atlantic Ridge), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14965, https://doi.org/10.5194/egusphere-egu24-14965, 2024.

11:35–11:45
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EGU24-16093
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On-site presentation
Alexandra Yang Yang, Xiaohan Huang, Siyu Zhao, and Taiping Zhao

Ultra-slow spreading ridges are unique in global ridges in its highly heterogeneous crustal thickness, numerous economically significant hydrothermal vents despite of extremely limited melt supply, and exposure of lower crust gabbro and mantle peridotite at seafloor particularly at amagmatic spreading segments. Although amagmatic accretionary segments are considered to be the key component of ultraslow-spreading ridges (Dick et al., 2003), how limited melt supply accommodates such a tectonically-dominated spreading system and heats the hydrothermal system remain unclear. And the key resides in the evolution of gabbro sills which preserves the history of frozen melt in the cold lithosphere and also provides the heat necessary for hydrothermal systems at such a condition. Therefore, for a better understanding of the lithosphere accretion history of amagmatic segments, we conduct systematic petrographic and geochemical analyses on a variety of samples collected from so far the best sampled and mapped amagmatic segment -- 53°E Southwest Indian Ridge, including abyssal peridotite, primitive to evolved cumulates (olivine-rich troctolite, gabbro, and oxide gabbro) and MORB.

We identify several unique chemical compositions of the minerals which was never reported in ocean ridges before, especially in the olivine-rich troctolite located to the south of rift valley within a massive exposure of peridotite up to ~3200 km2 (Zhou and Dick, 2013). 1. Unique NiO vs. Fo for olivine in the olivine-rich troctolite record the reaction between highly evolved magma (with Mg#~20) and dunitic mush. 2. Highly evolved trace element signatures with high Mg# for clinopyroxene and orthopyroxene in the gabbro vein cutting the troctolite confirm that the highly evolved magma intruding into the dunitic mush is felsic in composition. 3. The occurrence of oxide gabbro in the gabbro core complex (~380 km2) to the north of the rift valley indicates the presence of highly evolved gabbro sill in the north, which is most likely the parental magma of the evolved felsic melt invading the primitive troctolite in the south. 4. The occurrence of small volume of primitive troctolite with a crystallization temperature of ~1192°C in the massive peridotite in the south, and large volume of variably differentiated gabbro in the gabbro core complex with crystallization temperature as low as 998°C in the north reveal the unique cooling history of gabbro sills with different size in the newly-formed lithosphere in the amagmatic spreading segment.

The unique chemical compositions and 200°C variation in temperature suggests two juxtaposed gabbro sills in the lithosphere in amagmatic segments can vary greatly with different cooling and crystallization history. We propose that bigger gabbro sills in amagmatic spreading systems would more likely have a prolonged cooling history to crystallize more evolved lithologies, which would provide the necessary heat supply for potential hydrothermal systems. Future exploration on the occurrence of gabbro sills beneath hydrothermal systems is recommended to better understand how varied cooling history of individual gabbro sill control the formation and evolution of hydrothermal systems at ultraslow-spreading ridges.

References:

Dick, H. J. B., et al. (2003). Nature 426(6965): 405-412.

Zhou, H. Y. and H. J. B. Dick (2013). Nature 494(7436): 195-200.    

How to cite: Yang, A. Y., Huang, X., Zhao, S., and Zhao, T.: Unique chemical compositions of cumulates from 53°E Southwest Indian Ridge reveal distinct evolution and cooling history of gabbro sills in amagmatic accretionary segments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16093, https://doi.org/10.5194/egusphere-egu24-16093, 2024.

11:45–11:55
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EGU24-2122
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ECS
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On-site presentation
Thomas Gyomlai and Cecile Prigent

Along slow and ultra-slow spreading detachment and transform faults, a large abundance of oxide gabbro is documented and thought to represent the injection of a differentiated Fe-Ti-(V)-(P) saturated nelsonitic melt in gabbroic mushes. The oxide-rich gabbro carapace observed on detachment faults is interpreted as a deformation-assisted melt migration playing a critical role in rock weakening, strain localization, exhumation and evolution of core complexes. Fe-Ti-rich metagabbros are also often found in exhumed high-pressure low-temperature oceanic metamorphic units which allow to understand how they transform during subduction. The aim of this study is to constrain the composition and deformation of oxide gabbros from transform faults and to compare it to (1) oxide gabbros and processes at detachment faults and (2) subducted and exhumed oxide gabbros to characterize their variability and infer their chemical and rheological impacts on the subduction system.

This study focuses on samples collected in the Atlantic Ocean with the submersible Nautile along the northern and southern Vema transform valley walls during the Vemanaute campaign. The nelsonitic melt led to the crystallization of large amount (~10-60%) of V-rich ilmenite and titanomagnetite, F-rich amphibole, olivine and variable amount of apatite, which pervasively intrude the primary gabbro. Thermometric estimates on amphiboles suggest a crystallization at around 800-900°C. Some samples are mylonitic and textures suggest that deformation was coeval with melt infiltration and crystallization. Fe-Ti oxides do not show any internal deformation suggesting rock hardening following melt-rock reaction. Vema oxide gabbros are nonetheless impacted by subsequent hydrothermal alteration with the pseudomorphic replacement of pyroxene into Cl-rich amphibole (~500-700°C) and late alteration phases (e.g., clay).  

Oxide gabbros therefore play a similar role in weakening the oceanic lithosphere on both transform and detachment faults. However, apatite-rich oxide gabbros (with apatite content up to 50% of the melt products) found at the Vema transform fault are not described in detachment faults. Furthermore, melt-rock interactions produce amphibole and olivine in transform faults whereas pyroxene is formed at detachment faults. This indicates important differences on the composition of the nelsonitic melt, with potential differences on the melt source or degree of differentiation. In the case of transform faults, the presence of hydrated phases indicates a hydrated source.

In both cases, the nelsonitic melt intrusion induces a localized drastic change in the rheology and bulk composition of gabbros which will likely hold significant chemical and rheological implications during subduction, particularly along the subduction interface. This phenomenon can be further explored in exhumed ophiolite, such as in Syros, Greece, interpreted as a preserved coherent fragment of a discontinuous, slow-spreading oceanic domain. There, Fe-Ti-rich gabbros consist of blocks distributed in a serpentinite matrix. They play a major role as a calcium source and as a lithological discontinuity localizing fluid pathways which allow to pervasively metasomatize the serpentinite matrix (half of it) into a tremolite-chlorite-talc schist. Such diffuse transformation of a serpentinite unit is bond to impact the chemical and rheological behavior of the subduction interface.

How to cite: Gyomlai, T. and Prigent, C.: Oxide gabbro: from detachment/transform faults to subduction., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2122, https://doi.org/10.5194/egusphere-egu24-2122, 2024.

11:55–12:05
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EGU24-7999
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On-site presentation
Gang Lu, Ritske Huismans, and Dave May

Emplacement of magmatic crust at mid-ocean ridges (MORs) is confined in a narrow neovolcanic zone on the seafloor, whereas geophysical observations suggest that mantle melting occurs over a broad region. How melt is transported horizontally towards the ridge axis, i.e. melt focusing, remains incompletely understood. Here we present numerical models, theoretical decomposition, and scaling analysis, to isolate melt focusing mechanisms, and focus in particular on ridge suction and on the permeability barrier. We show that shear deformation induced dynamic pressure leads to large decompaction pressure, which increases porosity, instead of generating ridge suction as previously expected. We further demonstrate that a permeability barrier resulting from cold lithosphere systematically leads to a horizontal compaction pressure gradient that focuses melt toward the ridge axis, which may explain widespread melt focusing at global MORs as well as the three-dimensional melt distribution at ultra-slow spreading centers.

How to cite: Lu, G., Huismans, R., and May, D.: On the causes of melt focusing at mid-ocean ridges: Ridge suction versus permeability barrier, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7999, https://doi.org/10.5194/egusphere-egu24-7999, 2024.

12:05–12:15
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EGU24-16143
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Virtual presentation
Nibir Mandal, Joyjeet Sen, and Shamik Sarkar

Mid-ocean ridge (MOR) systems form multi-layered mechanical structures, constituted by a solid elastic crustal layer and an underlying melt-rich mush complex (MC) in the mantle. This article presents a new integrated solid-fluid modelling approach to show the development of complexly heterogeneous stress field in MORs. The modelling is implemented in two steps: 1) simulation of multi-ordered 3D convective circulations, produced by decompression melting in the mushy region, subjected to random thermal perturbations, and 2) mechanical coupling of the sub-ridge mushy regions with the overlying elastic crustal layer within a mathematical framework of fluid-structure interaction (FSI) mechanics. Using an enthalpy-porosity-based fluid-formulation of uppermost mantle the model accounts for a one-way FSI interaction for transmission of viscous forces of the MC region to the overlying upper crust. It is demonstrated from the model runs that a MOR spontaneously develops strongly heterogeneous stress fields on a time scale of million years, characterized by their segmented patterns. The stress mapping reveals a distinct 30 km wide axial zone of ridge-normal tensile stresses ( < 250 MPa), flanked by ridge-parallel linear belts of ridge-normal compression (median < 100 MPa). The FSI model results suggest that ridge-parallel compression belts can develop in MORs without involving flexural bending of lithospheric plates. In addition, a MOR system produces narrow along-axis compressional zones transverse to the ridge axis, resulting in segmentation of the stress field on a wavelength of 40-150 km. These segmented stress fields conforms to the second-order magmatic segmentation patterns of MORs, as reported in the literature.

How to cite: Mandal, N., Sen, J., and Sarkar, S.: Calculations of the 3D stress fields in mid-ocean ridge systems: a fluid-structure interaction (FSI) modelling approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16143, https://doi.org/10.5194/egusphere-egu24-16143, 2024.

12:15–12:25
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EGU24-10389
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ECS
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On-site presentation
Daniel Hafermaas and Daniel Koehn

In the quest to deepen our understanding of mid-ocean ridge dynamics, this study presents coupled numerical simulations focused on the intricate processes of ridge formation and propagation leading to micro-plate creation, their rotation and the formation of transform faults. Our numerical approach to the problem is based on a 2.5D approximation with a fracturing brittle and a ductile viscous layer coupled to a temperature field. The growth of new oceanic material is modelled by the introduction of new hot particles in opening fractures and the plates cool by temperature diffusion. Thermo-mechanical coupling is induced by a reduction of breaking strength, elastic constants and viscosity of the solid as a function of temperature leading to weakening of material whereas a healing function that is reconnecting broken bonds leads to hardening. Initially we are inserting seeds for offset ridges so that overlaps and potential transform faults are predefined, however, we also observe the first self-developing transform faults in the system. The model is not as complex as some existing full 3D models, however it offers to study the complexity of the brittle processes and the growth of ridges in detail.

Central to our investigation is the comprehensive simulation of mid-ocean ridge systems under varying spreading rates and the creation of micro-plates versus stable transform faults as well as the comparison to natural settings. Fast spreading rates lead to hot ridges in nature and in the model, because hot material is added faster than the heat can diffuse, whereas slow ridges remain relatively cool. Higher temperature is thought to lead to faster healing in our model, which counteracts the weakening induced by higher temperature. We modelled the formation and evolution of microplates and transform faults, uncovering the critical role of healing and weakening rates in shaping these features. Faster healing leads to micro-plate formation whereas more weakening, especially the reduction of the breaking strength, induces stable transform faults. The interplay between ridges is very dynamic with a continuous process of microplate rotation versus micro-plate splitting, their integration in the mid-ocean ridge and their destruction when transform faults form. The important parameters in the simulations that prefer micro-plate formation are higher breaking strength, fast healing, low viscosity and larger lateral distance between opening ridges.

The integrated analysis from our numerical simulation enriches the existing understanding of mid-ocean ridge dynamics. It highlights the nuanced interplay between spreading rates, lithospheric stress, and thermo-mechanical coupling in shaping the oceanic crust. The findings from our study, particularly the spinning of microplates and the formation of transform faults, provide a new dimension to our comprehension of these geological features.

This research contributes significantly to marine geology, offering a framework for future explorations and a benchmark for comparison with natural ridge systems. The detailed insights gained from our simulation pave the way for more informed interpretations of mid-ocean ridge processes and underscore the potential of numerical modelling in advancing our knowledge of Earth's dynamic systems.

How to cite: Hafermaas, D. and Koehn, D.: Bridging Theory and Nature: Numerical Simulations to Understand Mid-Ocean Ridge Formation, Transform Faults and Microplates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10389, https://doi.org/10.5194/egusphere-egu24-10389, 2024.

12:25–12:30

Posters on site: Tue, 16 Apr, 10:45–12:30 | Hall X2

Display time: Tue, 16 Apr 08:30–Tue, 16 Apr 12:30
Chairpersons: Philipp Brandl, Eleonora Ficini, Marcia Maia
X2.1
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EGU24-13004
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ECS
Eleonora Ficini, Cuffaro Marco, Ligi Marco, Miglio Edie, and Sanfilippo Alessio

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.

X2.2
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EGU24-22147
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ECS
Raissa Francicleide Sousa da Silva, Helenice Vital, and Aderson Farias do Nascimento

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.

X2.3
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EGU24-21861
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ECS
Guilherme Weber Sampaio de Melo, Marcia Maia, Simone Cesca, Ingo Grevemeyer, and Aderson do Nascimento

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 Communications14(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 & Environment3(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 Geophysics175, 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 Geoscience9(8), 619-623.

Montalbetti, J. F., & Kanasewich, E. R. (1970). Enhancement of teleseismic body phases with a polarization filter. Geophysical Journal International21(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 Earth127(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.

X2.4
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EGU24-6198
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ECS
Sandrine Ritter, Attila Balázs, and Taras Gerya

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.

X2.5
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EGU24-4461
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ECS
Alexandre Janin, Mark D. Behn, and Xiaochuan Tian

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.

X2.6
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EGU24-4970
Ingo Grevemeyer, Dietrich Lange, Lars Ruepke, Ingo Klauke, Anouk Bienest, Laura Gómez de la Peña, Yu Ren, Helene-Sophie Hilbert, Yuhan Li, Louisa Murray-Bergquist, Katharina Unger Moreno, Thor Hansteen, and Colin W. Devey

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.

X2.7
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EGU24-20491
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ECS
Yinuo Zhang, Lars Ruepke, Fan Zhang, Sibiao Liu, and Ingo Grevemeyer

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.

X2.8
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EGU24-5134
Remisha Rajeevan, Marcia Maia, Mathieu Rospabé, Jean Arthur Olive, and Ewan Pelleter

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.

X2.9
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EGU24-17326
Mathilde Cannat, Jie Chen, and Javier Escartin

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.

X2.10
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EGU24-15593
Antoine Demont, Mathilde Cannat, and Jean-Arthur Olive

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.

X2.11
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EGU24-17664
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ECS
Souradeep Mahato and Mathilde Cannat

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.

X2.12
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EGU24-11449
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ECS
Adam Robinson, Louise Watremez, Sylvie Leroy, Timothy Minshull, Mathilde Cannat, and Ana Corbalán

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.

X2.13
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EGU24-12161
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ECS
Zhikai Wang, Tim A. Minshull, and Satish C. Singh

The 70-km long Lucky Strike segment at 37oN 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.

X2.14
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EGU24-7636
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ECS
Soumya Bohidar, Wayne Crawford, and Mathilde Cannat

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.

X2.15
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EGU24-7463
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ECS
Marine Boulanger, Marguerite Godard, Benoit Ildefonse, and Malissa Bakouche

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.

X2.16
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EGU24-7861
Leila Mezri, Thomas P. Ferrand, Alexander Diehl, Javier García-Pintado, Manon Bickert, and Marta Pérez-Gussinyé

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.

X2.17
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EGU24-8149
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ECS
Alexander Diehl and Wolfgang Bach

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.

X2.18
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EGU24-17466
Ewan-Loiz Pelleter, Cecile Cathalot, Mathieu Rospabe, Thomas Giunta, Stephanie Dupre, Marcia Maia, Audrey Boissier, Sandrine Cheron, Mickaël Rovere, Yoan Germain, Vivier Guyader, and Jean-Pierre Donval

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.