TS1.2

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.

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.

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.

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.

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.

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.

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.

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.