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TS1.2

EDI
Exploring frontiers in ocean-plate tectonics and marine geosciences

This session invites multi-disciplinary, multi-scale contributions integrating seafloor mapping and sampling and regional geophysical studies that provide important constraints on the life cycle of oceanic lithosphere from formation at mid ocean ridges to destruction in subduction trenches, and all the processes modifying between its birth and death. By studying the seafloor and the underlying oceanic crust and lithosphere across diverse plate tectonic environments, we can gain a deeper appreciation of the role of ocean-plate tectonics for the large-scale structure, evolution and functioning of Earth.

It was the pioneering work of Marie Tharp, who’s physiographic maps of the seafloor revealed some of the largest, yet previously unknown bathymetric features on Earth: mid-ocean ridges, fracture zones, transform faults, seamounts and hotspot tracks, and trenches. Compiling bathymetric profiles along shiptracks, she carried out detailed, systematic mapping and produced a striking visualization of these features, the ‘physiographic diagrams’, that had a profound and lasting contribution to plate tectonics and marine geosciences. Today, initiatives such as the Nippon Foundation - GEBCO Seabed 2030 project and IODP 2050 open new avenues to address unresolved questions in oceanic plate tectonics, in addition to mapping and filling the gaps in the world ocean’s bathymetric maps.

Contributions to this session may address (but are not limited to) the following questions:

how does oceanic crust form at both fast and slow spreading mid-ocean ridges?
what controls seafloor morphology and how the different processes interact?
how does the asthenospheric mantle get structurally exhumed at ultraslow spreading ridges
how do subduction zones initiate?
how do marginal and full-scale ocean basins evolve?
how does oceanic crust accrete and vary along ~60,000 km of the mid-ocean ridges?
how is oceanic crust chemically, physically, and biologically altered as it matures; and what fraction may or may not be recycled into the mantle?
why is the oceanic crust so variable if the process forming it is the same in principle?

Public information:

Marie Tharp Medal Recipient 

Including Marie Tharp Medal Lecture
Convener: Derya GürerECSECS | Co-conveners: Mathilde Cannat, Javier Escartin, Lucia Perez-DiazECSECS, Paola Vannucchi
Presentations
| Tue, 24 May, 08:30–11:50 (CEST)
 
Room D1
Public information:

Marie Tharp Medal Recipient 

Tue, 24 May, 08:30–10:00

Chairpersons: Paola Vannucchi, Derya Gürer, Lucia Perez-Diaz

08:30–08:35
Session introduction

08:35–08:45
Laudation EGU TS Medalist

08:45–08:55
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EGU22-13304
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solicited
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Marie Tharp Medal Lecture
Francesca Funiciello

The use of experimental tectonics (also known as analogue-, laboratory, or physical modelling) to study tectonic processes is not a novelty in Earth Science. Following Sir James Hall’s pioneer work (1815), many modellers squeezed, stretched, pushed and pulled a wide range of materials – e.g., sand, clay, oil, painters’ putties, gelatins, wax, paraffin, syrups, polymers – to unravel a wide range of tectonic processes to determine parameters controlling their geometry, kinematics and dynamics. However, only recently experimental analogue modelling has definitively transformed from a qualitative to a quantitative technique, thanks to appropriate scaling relationships, the improvement in the knowledge of the rheology of both natural and analogue materials and the use of high-resolution monitoring techniques to quantify morphology, kinematics, stress, strain and temperature.

Here, I specifically review the experimental work performed to study one of the most intriguing aspects of plate tectonics: the subduction process. Subduction provides the dominant engine for plate tectonics and mantle dynamics. Moreover, it has also societal importance playing a key role on hazard at short (i.e., earthquakes and mega-earthquakes, tsunami, effusive and explosive volcanic activities with impact on aviation safety) and long time scales (i.e., local and global climate change). Over the last decades, a noteworthy advance in the quality and density of global geological, geophysical and experimental data has allowed us to provide systematic quantitative analyses of global subduction zones and to speculate on their behaviour. These constraints have been integrated into a mechanical framework through modelling.

I will bring you to a journey through the past, the present and the future of analogue modelling and related efforts, results and perspectives for the study of the subduction process. It will be shown how analogue models, with their inherent 3D character and behaviour driven by simple and natural physical laws, contribute to successfully unravelling the subduction process, inspiring new ideas. Challenging ongoing perspectives of analogue models imply the possibility to compare time and space scales, allowing to merge, within the same model, both short- and long-term and shallow and deep processes.

How to cite: Funiciello, F.: Analogue modelling of subduction: yesterday, today and tomorrow., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13304, https://doi.org/10.5194/egusphere-egu22-13304, 2022.

08:55–09:05
Q & A EGU TS Medalist

09:12–09:14
Questions

09:14–09:21
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EGU22-13027
Marta Pérez-Gussinyé et al.

The processes that lead to the transition from continental extension and break-up to steady-state mid-ocean ridge formation are not well understood. Particular unknowns are the paleo-water depths at which the continental lithosphere breaks up, the nature of the crust at the so-called continent-ocean transition and when and how a steady-state mid-ocean ridge is established. To understand these questions we use numerical models that couple tectonic deformation, sedimentation, hydrothermal cooling, serpentinisation and melting, as a virtual laboratory. We present results of models run with different velocities that simulate natural examples observed in nature such as the South China sea and the West Iberia-Newfoundland margins. We focus on the evolution of subsidence, heat-flow and nature of the basement as the rift transforms into a steady-state mid-ocean ridge and show how the interplay between tectonics and hydrothermal cooling lead to the different configurations observed in nature.

 

How to cite: Pérez-Gussinyé, M., García-Pintado, J., Liu, Z., and Mezri, L.: How Continents Break-up and New Ocean Ridges are Established , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13027, https://doi.org/10.5194/egusphere-egu22-13027, 2022.

09:21–09:23
Questions

09:23–09:30
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EGU22-7065
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ECS
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Marie Eide Lien et al.

Activity along the Arctic Mid-Ocean Ridge (AMOR) has been progressively explored over the past 20 years. Along the ridge, processes such as dike intrusion and faulting cause earthquakes. We focus on an area around the Loki’s castle hydrothermal vent field (LCVF), located on the Mohn ultra-slow spreading ridge. Ultra-slow spreading systems are strongly controlled by tectonic processes, which provide an opportunity to study almost exclusively the effect of tectonism on a hydrothermal vent field.

In June 2019, we deployed a network of eight broadband ocean bottom seismometers (OBS) in an area of about 20 by 20 km around the LCVF. The OBSs were deployed for a one-year monitoring period until July 2020. We processed the OBS data using an automatic detection routine and machine learning approach to pick phases, and then located the local earthquakes based on a 1D velocity model. This provided an earthquake catalogue that was interpreted to understand the seismicity in terms of spatial and temporal distribution, and to identify fault structures. Within the broader tectonic system we aim to enhance our understanding of the LCVF.

How to cite: Lien, M. E., Pilot, M., Schlindwein, V., Ottemöller, L., and Barreyre, T.: Arctic Mid-Ocean Ridge seismicity: Results from an OBS deployment at Loki’s Castle , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7065, https://doi.org/10.5194/egusphere-egu22-7065, 2022.

09:30–09:32
Questions

09:32–09:39
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EGU22-7720
Mathilde Cannat et al.

The thermal regime of mid-ocean ridges determines their spreading modes (i.e., the combination of mid-ocean ridge tectonic, magmatic and hydrothermal processes that control the composition and structure of the oceanic lithosphere). It is determined by the balance of heat supply and heat loss in the axial region. Most heat is supplied through magma, while hydrothermal energy fluxes depend on the permeability and depth extent of the hydrothermal cooling domain, and on the thickness of the conductive boundary layer at its base. At fast spreading ridges, the flux of melt is high, the thermal regime is hot, and melt resides at depths of only a few kms in a steady state fashion, well within the reach of vigourous axial hydrothermal convection. At slow ridges, the melt flux is lower, the thermal regime is colder, so that melt can only reside durably at depths that are commonly > 10 km, out of the reach of vigourous (high permeability) hydrothermal systems. Melt there, however, is commonly injected higher up in the axial lithosphere, forming transient melt bodies in colder host rocks and triggering high temperature, black smoker, hydrothermal systems.

Here we report on numerical models that explore the thermal effects of varying both the melt flux and the depth of magma emplacement, a parameter that previously published mid-ocean ridge thermal models did not take into account. Our models do predict the large variability in thermal regime that is documented at slow ridges, from cold detachment-dominated settings, to hotter melt-rich segment centers. We discuss these results and the strengths and limitations of the modelling approach. We also explore the potential effects of varying the melt flux and melt emplacement depth with time at a given slow spreading ridge location, on crustal construction processes and on the respective roles of faults and melt intrusions to accommodate plate divergence.  

How to cite: Cannat, M., Chen, J., and Olive, J. A.: The thermal regime of mid-ocean ridges: geological perspectives and numerical modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7720, https://doi.org/10.5194/egusphere-egu22-7720, 2022.

09:39–09:41
Questions

09:41–09:48
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EGU22-854
Anke Dannowski et al.

The Ascension fracture zone (AFZ) is a double transform fault system and offsets the Mid-Atlantic Ridge axis by 230 km at 7 °S. The transform fault system is consisting of two parallel transform faults, the North and South Ascension Fracture Zone, sandwiching a ~20 km long ridge segment, which we name Ascension Fracture Zone segment. The segment shows strong topographical variations and corrugated surfaces typical for detachment faults that form oceanic core complexes. An elongated massif approximately 50 km east of the ridge axis with transform-parallel striations of over 100 km on top, indicate a detachment fault active for several million years. This would be one of the longest transform-parallel corrugated surface observed anywhere in the oceans. The question arises whether the corrugations belong to one OCC, representing a rather stable crustal accretion, or if several OCCs have been developed, representing a rather variable crustal accretion. Changes in melt supply influence the crustal structure, which in turn can be recognised by seismic methods.

RV Meteor (cruise M62-4) set out to acquire seismic refraction and wide-angle reflection data along a 265 km long spreading parallel transect to image the crustal velocity distribution and the crustal thickness of the intervening short AFZ segment. Densely spaced, every ~9.25 km, ocean bottom seismometers recorded P-wave and converted S-wave energy emitted from a 64 l G-gun cluster at a shot interval of 60 s, equal to ~125 m shot distance.

The results reveal P-wave velocities that vary along the profile from 3.5 km/s to 5 km/s at the seafloor and reach 7.2 km/s in ~6 km depth at the ridge axis and at 3 km to 4 km depth under the ridge shoulders. At larger offsets to the ridge axis. S-wave velocities vary from 2 km/s to 2.5 km/s at the seafloor and increase to 3.5 km/s in ~2 km depth east of the ridge axis, while the S-wave velocities west of the ridge axis show a lower velocity gradient and reach 3.5 km/s in 3 km to 4 km depth. A Vp/Vs ratio >1.9 is observed in areas where seafloor corrugations have been observed. These areas are interpreted as serpentinised mantle material. However, the high Vp/Vs ratio seems to be limited to the upper 1.5 km to 2 km of the subsurface, indicating that the hydration of the seafloor is limited to that depth. The eastern ridge flank is dominated by a high Vp/Vs ratio for offsets larger than 40 km from the ridge axis, however, it is interrupted by small stripes of Vp/Vs <1.9. Thus, in the short AFZ segment, detachment faulting seem to occur continuously over a long period with short interruptions when the magmatic budged exceeds a certain upper limit.

How to cite: Dannowski, A., Grevemeyer, I., Baehre, V., Bialas, J., and Reston, T.: Crustal evolution and oceanic core complexes at the Ascension fracture zone – MAR 7° S, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-854, https://doi.org/10.5194/egusphere-egu22-854, 2022.

09:48–09:50
Questions

09:50–09:57
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EGU22-4929
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ECS
Flora Hochscheid et al.

Fluid-rock interactions in mantle rocks that turns peridotite into serpentinite has been widely documented during the past two decades, for geological settings such as mid-ocean ridges (MOR) and subduction zones. In contrast, serpentinization at rifted margins has received much less attention, while serpentinites at these settings are largely involved in geochemical and tectonic processes that occur from continental break-up to the establishment of a steady-state MOR. This study presents new petrological and mineralogical investigations on peridotites that were part of the subcontinental mantle exhumed along a former Ocean-Continent Transition (OCT) of the Jurassic Alpine Tethys, nowadays exposed as ophiolitic nappes (Platta, Tasna and Totalp) in the southeastern part of the Swiss Alps. These peridotites experienced various degrees of serpentinization, from moderately to completely serpentinized. At Totalp, initially located close to the continent, serpentinization forms a typical lizardite-bearing mesh texture that surrounds relics of primary minerals. Locally, the association of andradite and polyhedral serpentine occurs as alteration products of clinopyroxene, which may be interpreted in terms of low temperature serpentinization and near-isochemical conditions. At lower Platta, which represents the oceanwards (distal) domain of the OCT, serpentinization is extensive and, similarly to Totalp, predominantly formed by mesh lizardite. For the two previously mentioned sites, the typical mesh texture suggests a fluid-rock interaction with a low water-to-rock ratio. At Tasna and upper Platta, which both correspond to more proximal domains of the OCT (i.e., continentwards), serpentinites are characterized by several superimposed serpentinization events marked by successive generations of serpentine-filling veins with distinct morphologies and textures, forming the following sequence: Mesh texture —> Banded veins (V1) —> Crack seals (V2) —> Lamellar veins (V3). The V1 banded veins are made of several serpentine species including chrysotile, polygonal serpentine, polyhedral serpentine and lizardite. They formed as a result of gradual opening during exhumation of the mantle from a supersaturated solution. The progressive evolution from chrysotile to polygonal serpentine and then lizardite is attributed to more intense fluid-rock interactions and a lower fluid saturation with decreasing depth. V2 crack seals consist of chrysotile veins formed at shallow depth after strain release and under high water/rock ratios. Surprisingly, antigorite was identified as the latest vein generation (V3). Trace element compositions for V3 are comparable to those of earlier vein generations, but strongly differ from those attributed to the Alpine convergence, excluding their formation during prograde subduction metamorphism. Rather, we propose that antigorite veins formed as a result of compressive stresses generated by apparent unbending of the footwall during final exhumation. This result shows that antigorite is not only restricted to convergent domains, and that it may be more common in rifted margins and (ultra-)slow spreading centers than previously thought.

How to cite: Hochscheid, F., Ulrich, M., Muñoz, M., Lemarchand, D., and Manatschal, G.: Petrology of Alpine Tethys serpentinites: New insights on serpentinization at passive margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4929, https://doi.org/10.5194/egusphere-egu22-4929, 2022.

09:57–09:59
Questions

Tue, 24 May, 10:20–11:50

Chairpersons: Lucia Perez-Diaz, Mathilde Cannat, Derya Gürer

10:20–10:22
Introduction block II

10:22–10:29
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EGU22-5911
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ECS
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Souradeep Mahato and Mathilde Cannat

The 64°E region of the eastern SWIR is a melt poor end member region of the MOR system. Magma focusing to axial volcanoes leaves >50 km wide along axis corridor, where seafloor spreading occurs almost fully via successive alternate polarity detachment faults (Sauter et al., 2013). The present-day south-dipping young (~300 kyrs; Cannat et al., 2019), active detachment fault cuts through an older north dipping detachment fault. The active detachment emergence can be traced over 32 km from the shipboard bathymetry data (for comparison, the scoped-shaped finely corrugated 13°20’N exhumed detachment surface at the Mid-Atlantic Ridge extends only ~5-6 km along-axis; Escartin et al., 2017). HR micro-bathymetry maps acquired on the west and east sides of this emergence line indicate an along-strike variation of the fault structure and geometry. In the east, the fault emerges at an overall angle of ~26°-30° and the emerging fault surface is irregular, with undulations at hectometer to km scales, close to parallel to spreading direction, and rare occurrences of decameter-scale corrugations, up to 20° oblique to spreading. In the west, the detachment emerges in the form of two distinct fault splays, ~400 m apart, both at an angle ~40°-50°.  This western region receives some magma input, resulting in localized patches of basalt and hummocky ridges.

Near fault deformation structures, documented by Remotely Operated Vehicle (ROV) dives and sampling, also differ between east and west. In the west, sigmoidal blocks (~5-10 m) of moderately fractured serpentinized peridotite, some with gabbro dikes, are observed below the emerging faults, which consist of <1.5 m thick zone of serpentinite breccia, and micro-breccia, with cm-thick intervals of gouge. In the east, the highly strained intervals are thicker (up to 8 m), with a greater proportion of serpentinite gouge and microbreccia. The more coherent rocks below the fault are also more pervasively fractured, with planar decimeter to meter-spaced south-dipping joints. ROV dives near the detachment breakaway offer an opportunity to study the deformation below a more mature region of the previously active detachment. Steep landslide head scarps there expose vertical sections, up to 70 m thick, with several intervals of serpentine gouge and micro-breccia, intercalated with coarser brecciated serpentinized peridotite, and with sigmoidal, meter to decameter sized blocks of serpentinized peridotite. Together, these observations point to a heterogeneous structure and to a variable thickness of the strain localization/damage zone associated with the emerging portions of the 64°40'E SWIR detachment. Based also on the seismic reflection structure of the fault zone at depth (Momoh et al., 2017), we propose that the material that emerges samples distinct regions of a kilometer-thick heterogeneously deformed damage zone, leading to different geometries and structure of the emerged fault surface(s).

How to cite: Mahato, S. and Cannat, M.: Early stages of evolution of an axial detachment fault at the ultraslow spreading mid-ocean ridge (64°40'E SWIR), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5911, https://doi.org/10.5194/egusphere-egu22-5911, 2022.

10:29–10:31
Questions

10:31–10:38
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EGU22-6115
Alex Hughes et al.

10:38–10:40
Questions

10:40–10:47
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EGU22-7217
Alessio Sanfilippo et al.

The prominent Charlie Gibbs right-lateral multi-transform system (52°-53°N) offsets the Mid Atlantic Ridge (MAR) by ~340 km. The transform system is formed by two distinct transform faults linked by a short ~40 km-long intra-transform spreading centre (ITR). The two adjacent MAR segments are influenced by both the Azores and the Iceland mantle plume. Recently, high resolution multibeam surveys and a dense sampling program of the entire transform system, including the adjacent southern and northern MAR segments, were carried out during expeditions of R/V Celtic Explorer (2015, 2016 and 2018) [1], R/V A.N. Strakhov (2020) and A.S. Vavilov (2021) [2]. The new surveys show widespread occurrence of large structures with corrugated surfaces and exhumed lower crust and mantle rocks on both sides of the intra-transform spreading axis. Morphological analyses of the intra-transform domain and magnetic data indicate that crustal accretion was driven by flip-flop detachment faulting [3], with minimal ridge melt supply and little axial volcanism. The tectonic spreading persisted for tens of millions of years. Along axis MORB chemistry shows that changes in seafloor accretion styles are mirrored by variations in melt supply, in turn dependent on mantle temperature and by a large-scale mantle heterogeneity. Charlie Gibbs is a key case study of how seafloor accretion modes at a spreading segment is critically dependent on mantle thermal state but also on its intrinsic compositional heterogeneity.

[1] Georgiopoulou A. and CE18008 Scientific Party, 2018. Tectonic Ocean Spreading at the Charlie-Gibbs Fracture Zone (TOSCA): CE18008 Research Survey Report. Marine Institute of Ireland, Dublin, pp 1-24. [2] Skolotnev, S. et al., 2021. Seafloor Spreading and Tectonics at the Charlie Gibbs Transform System (52-53ºN, Mid Atlantic Ridge): Preliminary Results from R/V AN Strakhov Expedition S50. Ofioliti, 46(1). [3] Cannat, Met al., 2019. On spreading modes and magma supply at slow and ultraslow mid-ocean ridges. Earth and Planetary Science Letters, 519, 223-233.

How to cite: Sanfilippo, A., Skolotnev, S., Ligi, M., and Peyve, A. and the A.N. Strakhov Expedition S50 and A.N. Vavilov Expedition V53 Science Parties: Seafloor spreading modes across the Charlie Gibbs transform system (52°N, Mid Atlantic Ridge), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7217, https://doi.org/10.5194/egusphere-egu22-7217, 2022.

10:47–10:49
Questions

10:56–10:58
Questions

11:05–11:07
Questions

11:07–11:14
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EGU22-8780
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ECS
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Soumya Bohidar et al.

Lucky Strike volcano is the central edifice of the Lucky Strike segment, Mid-Atlantic Ridge. Its summit overlies an axial magma chamber (AMC), 3-3.8 km beneath the seafloor, and hosts one of the largest known deep-sea hydrothermal fields. Local seismicity beneath the hydrothermal field has been monitored since 2007 as a part of the EMSO (European Multidisciplinary Seafloor and water column Observatory)-Azores observatory by 5 OBSs with yearly redeployments. From the 2007-2019 earthquake catalog, the primary process for the seismicity observed beneath the volcano region is proposed to be thermal contraction at the base of the hydrothermal circulation. In this interpretation the most seismically active zones represent the domains of maximum heat extraction at the base of the hydrothermal system. Here we present the evolution of the hydrothermal system controlled by magmato-tectonic interactions in the frame of this interpretation.

First, we observe two shifts of the most seismically active zones from ~1.4km North-Northwest of the hydrothermal field as documented in 2007-2009 to ~0.7km to North of the field in between 2010-2013 and then Eastward for about ~0.6km from 2010-2013 to 2015-present. These shifts, of the order ~600-700 meters, occurring at time scales of a few years, might be driven by one or several of the following mechanisms: the relocation of the maximum heat extraction zone to a shallower region of the AMC after significant heat extraction, the relocation to a recent magmatic injection,  and/or a tectonically-driven change in the hydrothermal fluid pathways.

Second, we observe three main Higher Seismic Activity (HSA seismic rate > 18 events/week) periods: April-June 2009, August-September 2015 and April-May 2016. The 2009 HSA period lasted ~13 weeks and the events clustered just above the AMC, while the 2015 and 2016 HSA periods lasted ~4-5 weeks, with events forming a narrow, dike-shaped cluster between the AMC  and just few meters below seafloor. HSA periods are characterized by deeper events and the occurrence of a few higher magnitude events (ML > 1.0). In between HSA periods, the seismicity tends to align along the trace of an inward dipping fault that bounds the narrow axial graben to the west, at the top of the volcano. The HSA periods can thus be interpreted as periods of maximum heat extraction by the hydrothermal circulation, possibly obscuring the background fault-related seismicity that is detected in periods of lesser seismic activity.

How to cite: Bohidar, S., Crawford, W., and Cannat, M.: Seismic constraints on the hydrothermal circulation and magmato-tectonic interactions beneath Lucky Strike volcano, Mid-Atlantic Ridge, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8780, https://doi.org/10.5194/egusphere-egu22-8780, 2022.

11:14–11:16
Questions

11:16–11:23
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EGU22-10702
Fernando Martinez and Richard Hey

Mantle melting at mid-ocean ridges is thought to strengthen residual mantle by extracting water (hydroxyl defects) thereby increasing its viscosity by over two orders of magnitude to create a “compositional” lithosphere. Although water is strongly partitioned into basaltic melt relative to olivine, mantle dehydration also requires that melt extraction be efficient. Otherwise, retained low-degree hydrous melts will rapidly reinfuse surrounding mantle with hydrogen on solidifying owing to its high diffusivity in mantle materials. The pattern of mantle melting at ridges varies strongly both within segments and with spreading rate. We examine these patterns along the northern Mid-Atlantic Ridge and the adjoining Reykjanes Ridge using mantle Bouguer anomalies (MBAs). The Reykjanes Ridge has a linear axis, no transform faults, and a continuous MBA low, indicating continuous axial mantle melting. In contrast the adjoining Mid-Atlantic Ridge is segmented with transform and non-transform discontinuities and has pronounced “bulls-eye” MBA lows indicating focused mantle melting beneath each segment. We hypothesize that the pattern of mantle melting explains the absence or occurrence of transform faults on these systems. Segmented mantle melting results in dry, depleted, and strong mantle beneath ridge segment interiors but at segment ends, low extents of melting and inefficient melt extraction preserve damp and weak mantle.  Since the rheological changes created by segmented melting develop rapidly near the ridge axis and extend from the Moho to the dry solidus depth, a pronounced rheological banding is formed in the mantle. The weak segment ends localize shear zones oriented in the spreading direction where transform faults may form whereas the ridges, flanked by strong compositional lithosphere, will be oriented orthogonally.  Our hypothesis also explains the variation of transform fault spacing with spreading rate or their absence. At ultra-slow ridges, overall melting is limited and irregular and melt extraction is inefficient so that no systematic rheological bands form and transform faults are not favored. At slow spreading rates, mantle melting forms three-dimensional diapiric instabilities at typical spacings of ~40-80 km so that transform faults also have this spacing. As spreading increases to fast rates mantle melting becomes two-dimensional and typical magmatic segment length and corresponding transform spacing increases to >100 km.  At ultra-fast ridges (>145 km/my) mantle melting is ubiquitous and melt extraction is everywhere efficient so that a systematic rheological banding does not form and transform faults are again not favored. Our model implies that beyond cooling and strengthening with age, the pattern of mantle melting shapes the rheological structure of oceanic lithosphere and the geometry of plate tectonics. Reference:  Martinez and Hey, 2022, https://doi.org/10.1016/j.epsl.2021.117351

How to cite: Martinez, F. and Hey, R.: Segmented mantle melting, lithospheric strength, and the origin of transform faults: Insights from the North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10702, https://doi.org/10.5194/egusphere-egu22-10702, 2022.

11:23–11:25
Questions

11:25–11:32
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EGU22-11888
Michele Paulatto et al.

3D full waveform inversion (FWI) has been applied to the seismic refraction data of the MARINER (Mid-Atlantic Ridge INtegrated Experiment at Rainbow) experiment to create a robust high-resolution model of the seismic velocity structure of the Rainbow massif. The Rainbow massif is an oceanic core complex located on a non-transform discontinuity (NTD) in a magma-starved region of the mid-Atlantic Ridge. Despite the low magmatic input, the core complex hosts a high-temperature hydrothermal vent field  (>340°C) that requires a long-lived magmatic heat source. The FWI results show that deep within the massif, ∼3-8 km below the seafloor, is a low-velocity body that represents a partially molten sill complex with >20% gabbro intrusions. The complex extends out north to the AMAR Minor N segment suggesting an increased magmatic input into this segment, forcing the NTD to migrate southwards. Extensive magmatic intrusion into the core complex was likely responsible for the termination of slip on the detachment fault. Above the sill complex, we image a channel of lower velocity material that cuts through the main hydration front to the deep sill region. Velocity values and micro-seismicity correlation suggests that this channel consists of 10-30% serpentinized peridotite and fracturing from serpentinization reactions create fluid pathways for fluids to exchange between the deeper partially molten heat source and the fluid network of the hydrothermal vents. A high-velocity chimney below the extinct vent sites of the massif may represent the abandoned stockwork of these extinct hydrothermal systems.

How to cite: Paulatto, M., Oxford, T., and Bardner, R.: An intrusive complex imaged within the roots of an oceanic core complex using 3D full-waveform inversion , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11888, https://doi.org/10.5194/egusphere-egu22-11888, 2022.

11:32–11:34
Questions

11:34–11:41
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EGU22-12802
Leila Mezri et al.

Why do some mid-ocean ridges have morphology that expresses oceanic core complexes with large gabbro bodies, while others have smooth seafloors with large exposures of serpentinized mantle or rough seafloors, while lavas are observed almost everywhere?

Over the past few decades, numerical models have inferred that the fundamental mechanism controlling the wide diversity of lithology, crustal thickness, and ridge morphology is the balance between magmatism and tectonics. Key controls on this modeled equilibrium are the melt supply rate, which varies to account for the discontinuous volcanism observed on slow ridges, and the thermal structure, which depends on the balance between heat injected during magmatic accretion and heat removed by hydrothermal cooling, modulated by the spreading rate.

Based on this paradigm, it has been established that the fraction of melt that is accreted into the crust controls the formation of large oceanic core-complexes and flip-flop detachments, with the former being formed at fractions corresponding of half of spreading rate and the latter being formed when the melt supply is much smaller.

However, several fundamental questions remain poorly understood or unanswered. Why can slip on oceanic detachment faults continue and why does it stop? How do serpentinization and magmatic intrusions play a role in crustal growth and how do they interact? How and why do mechanisms related to magma supply switch from magmatic to detachment dominated mode during oceanic accretion?

Here we present self-consistent numerical simulations of the development of mid-ocean ridges, starting  from continental rifting and breakup. In our models melt supply varies dynamically with extension velocity and is affected by faulting. We focus on understanding how tectonism, melting, serpentinisation and hydrothermal cooling interact to form smooth-seafloor, core complexes and normal igneous seafloor, and their diverse crustal lithology.



How to cite: Mezri, L., Gracià-Pintado, J., Pérez-Gussinyé, M., Liu, Z., and Wolfgang, B.: Dependencies of morphology and lithological variations on tectonics, thermal structure and ocean loading at ultra-slow and slow ridges., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12802, https://doi.org/10.5194/egusphere-egu22-12802, 2022.

11:41–11:43
Questions

11:43–11:50
Open Discussion & Wrap Up