NH2.8 | Subduction Zone Observation through Time, Space and Scales: a multidisciplinary initiative
Subduction Zone Observation through Time, Space and Scales: a multidisciplinary initiative
Co-organized by GD4/GMPV4
Convener: Marianne Conin | Co-conveners: Paola Vannucchi, Mathilde Radiguet, Thomas P. FerrandECSECS, Marco Scambelluri
Orals
| Tue, 29 Apr, 16:15–18:00 (CEST)
 
Room 1.14
Posters on site
| Attendance Wed, 30 Apr, 10:45–12:30 (CEST) | Display Wed, 30 Apr, 08:30–12:30
 
Hall X3
Orals |
Tue, 16:15
Posters on site
Wed, 10:45
Subduction zones generate numerous natural hazards, including volcanism, earthquakes and tsunamis, and shape the landscape through a series of processes lasting from seconds to millions of years. Their dynamics are driven by complex feedbacks between stress, strain, rock transformation and fluid migration along and across the plate interface, from shallow to deep environments. Despite their utmost importance, the intricate time-sensitive thermo–hydro–mechanical–chemical (THMC-t) processes remain largely puzzling. This is essentially due to the complexity of integrating observations across multiple spatial, magnification and temporal scales (from the nanoscale and the grain boundary size to the plate interface, and from seconds to millions of years).

Our session aims, therefore, at gathering recent advancements in observatory techniques, monitoring and high-resolution imaging of i) the plate interface kinematics, ii) the accretionary wedge, iii) the subducting slab, and iv) the mantle wedge in active and fossil subduction interfaces. This includes studies from a wide range of disciplines, such as seismology and geodesy, geodynamics, marine geosciences, field-based petrology and geochemistry and microstructure, rock mechanics and numerical modelling. We particularly encourage initiatives that foster collaboration between communities to achieve a comprehensive understanding of subduction systems through space and time.

Orals: Tue, 29 Apr, 16:15–18:00 | Room 1.14

Chairpersons: Marianne Conin, Paola Vannucchi, Thomas P. Ferrand
16:15–16:35
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EGU25-13950
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ECS
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solicited
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On-site presentation
Laboratory Friction Experiments and Modeling Reveal the Mechanism of Shallow Slow Slip Events Observed in the Nankai Trough, Southwest Japan 
Junli Zhang and Matt Ikari

Slow slip events (SSEs) are the slowest type of discrete slip within the full spectrum of fault-slip behaviors and have been confirmed by both geodetic (e.g., Dragert et al., 2001) and laboratory data (e.g., Ikari, 2019). They have attracted considerable attention due to their mutual interaction with earthquake processes, and multiple approaches have been employed to investigate different aspects of SSEs. Here, we present a study that combines laboratory friction experiments and numerical modeling to explore the mechanisms of SSEs observed through geodetic and borehole data.

We conducted velocity-stepping friction experiments on intact core samples retrieved from the major reverse fault zones of the Nankai Trough, southwest Japan. These experiments were performed under both in-situ effective stress conditions and at 10 MPa, with slip velocities ranging from 1.6 nm/s (plate tectonic driving rates) to 30 μm/s. Our results reveal that fault zone samples transition from velocity-weakening to velocity-strengthening behavior as slip velocities increase, and some rate-and-state friction (RSF) parameters exhibit a dependence on sliding velocity. Numerical models (Zhang and Ikari, 2024) using velocity-dependent RSF parameters, constrained by our experimental data, successfully replicate SSEs comparable to those observed in the Nankai Trough (Araki et al., 2017; Yokota and Ishikawa, 2020) by assuming fault patches at depth ranges and sizes consistent with observational data. In contrast, models based on non-transitional frictional behavior (constant RSF parameters) or near-neutral stability (constant RSF parameters with extremely small velocity weakening) generate slip events that are several orders of magnitude faster than observed SSEs. We therefore propose that the transitional frictional behavior with increasing slip velocity is a key mechanism of shallow SSEs in the Nankai Trough.

Our study demonstrates that laboratory data obtained from centimeter-scale samples can be used to predict the frictional behavior of real faults on the scale of tens of kilometers. By integrating methodologies from multiple disciplines, we can achieve a more comprehensive understanding of the dynamics governing fault slip behavior.

How to cite: Zhang, J. and Ikari, M.: Laboratory Friction Experiments and Modeling Reveal the Mechanism of Shallow Slow Slip Events Observed in the Nankai Trough, Southwest Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13950, https://doi.org/10.5194/egusphere-egu25-13950, 2025.

16:35–16:45
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EGU25-17524
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On-site presentation
Aftershock activity following the 2011 Tohoku earthquake suggests near-stationnary afterslip rate at depth 
David Marsan and Blandine Gardonio

We analyze the evolution of the interplate earthquake rate along the Japan Trench following the 2011 Tohoku earthquake. Nearly 14 years of aftershock activity allows to constrain with good accuracy how the rate relaxes after an initial jump, and how this relaxation depends on location, and most notably on depth. We find that specific intermediate depth areas display very little relaxation, i.e., that the rate of earthquake post-2011 stays constant at an elevated rate throughout the >10 years. This behaviour is specific to small, isolated areas, that tend to host repeating earthquakes, and that are located within the (large) GPS-inverted afterslip zone. The relaxation is found to be faster, tending to a classical Omori-like type, when averaging over larger areas. Our observations suggest that (1) afterslip kinematics following the 2011 megathrust is highly spatially dependent (showing significant variability at the kilometric scale), (2) that the usually accepted 1/t afterslip relaxation is only valid when averaged over large areas, (3) that the relaxation can be very slow in areas characterized by small, isolated asperities, in the transition zone between the locked updip fault and the deeper fault where no interplate activity is observed. This overall trend can be seen as caused by the stress-‘screening’ of rapidly healing asperities at shallow depth that cause the post-seismic deformation to quickly relax, while the slip rate / deformation remains nearly stationnary when moving away from these asperities.

How to cite: Marsan, D. and Gardonio, B.: Aftershock activity following the 2011 Tohoku earthquake suggests near-stationnary afterslip rate at depth, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17524, https://doi.org/10.5194/egusphere-egu25-17524, 2025.

16:45–16:55
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EGU25-19185
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On-site presentation
  Loading rate changes following megathrust earthquakes explored with viscoelastic models  
Anne Socquet, Juliette Cresseaux, bertrand lovery, and mathilde radiguet

Viscoelastic relaxation following large subduction earthquakes is known to last from years to decades , and affect the interseismic loading rate up to hundreds of kilometers in the trench perpendicular direction. Post seismic relaxation also generates a rotation pattern close to the edges of the ruptured asperity. Recently, several observations reported an accelerated loading rate coeval with megathrust ruptures, at along-trench distances from the epicenter of hundreds of kilometers.

 

Proposed models involved so far viscoelastic relaxation in the mantle wedge and the oceanic mantle, as well as a weak oceanic LAB layer. However those models often fail to explain simultaneously the amplitude and the spatio-temporal patterns of the observations.

Here we perform 3D viscoelastic models of post seismic relaxation and explore various structural and rheological settings in order to test the mechanisms responsible for the complex loading variations observed. These involve a Burgers rheology, a contrast of viscosity between the continental and the oceanic mantles, a weak LAB, and a low viscosity layer atop the slab.

The pertinence of these different models is discussed against the fit to observations done after several earthquakes along the Chile-Peru subduction, in order to assess the importance of the different mechanisms.

How to cite: Socquet, A., Cresseaux, J., lovery, B., and radiguet, M.:   Loading rate changes following megathrust earthquakes explored with viscoelastic models , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19185, https://doi.org/10.5194/egusphere-egu25-19185, 2025.

16:55–17:05
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EGU25-9520
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ECS
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On-site presentation
Arc migration driven by subduction dynamics: a possible origin for the Chon Aike magmatic province 
Jorge Sanhueza-Soto, Joaquín Bastias-Silva, and Jesús Muñoz-Montecinos

The spatial-temporal evolution of volcanic arcs provides valuable insights into the deep melting processes occurring in the mantle wedge. The dehydration of the subducting slab is key because these fluids directly affect the melting temperatures of the mantle wedge. Fluids in this region (partial melts and released fluids from the slab) migrate to the corner of the wedge, where pressure/temperature conditions are optimal for magma production. Changes in the locus of the volcanic arc can be thus related to the position or changes in the physicochemical properties of the mantle wedge at depths, which is drastically dependent on subduction dynamics in time. The dip of the subducting slab is one of the key factors affecting the relative location of the mantle wedge, which can migrate the volcanic arc several hundreds of kilometers into the continent during flat slabs periods. However, the transition to a normal subduction angle or even processes such as slab break-off will migrate the mantle wedge, and the volcanic arc, to the trench and potentially generating large magmatic provinces in the lifespan of an active margin.

The scope of this preliminary study is to track the location of the magmatic arc in time driven by different subduction styles (e.g., low/high angle subduction, slab break-off) and the generation of magmatic provinces in the continent. We conducted a series of 2D geodynamics models using the code I2ELVIS feeded with ad hoc thermodynamic pseudosection modelling with the Perple_X software, to reproduce different subduction angles and the transition between them. The timings and mechanisms of the arc migration is applied to the well-documented exposure of Jurassic igneous rocks along the Antarctic Peninsula and Patagonia in the Chon Aike magmatic province. Recent debate postulates an active margin origin of these rocks, which is supported by geochemical signatures of typical slab-dehydration reactions and a mixed magmatic source that resided in the continental crust. Even though, the subduction dynamics are not constrained, the location and age of these rocks suggest several episodes of arc migration during the Jurassic, making this an exceptional study case to understanding the mechanisms of arc migration and the role of subduction dynamics.

Preliminary results of our modelling tracked the position of the mantle wedge by the presence of partial melts and the maximum depth of dehydration of the subducting slab. Explored scenarios consisted on periods of flat slab subduction triggered by the subduction of aseismic ridges and the return to a normal subduction. During the flat slab period, we also tested the generation of slab break-off, which induced local mantle upwelling and melting. Finally, we expect to reproduce the magmatic history of Antarctic Peninsula and Patagonia in the Jurassic to support the active margin hypothesis for the generation of the Chon Aike magmatic province.

How to cite: Sanhueza-Soto, J., Bastias-Silva, J., and Muñoz-Montecinos, J.: Arc migration driven by subduction dynamics: a possible origin for the Chon Aike magmatic province, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9520, https://doi.org/10.5194/egusphere-egu25-9520, 2025.

17:05–17:15
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EGU25-18778
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On-site presentation
Transient creep in subduction zones explained by reaction-induced rheological switches 
Mathieu Soret, Jorge Jara, Julien Gasc, Giuseppe Costantino, Nadaya Cubas, Alexandre Schubnel, Harsha Bhat, and Romain Jolivet

Despite extensive research over the years, the weakening mechanisms that govern strain localization along deep subduction interfaces are still debated. These mechanisms span from the downdip boundary of the seismogenic zone (∼350°C) to the mechanical coupling transition with the upper plate mantle near sub-arc depths (>600°C). Current thermo–mechanical models posit that rock rheology is primarily stress- and rate-temperature-sensitive in the absence of mineral reactions. Strain is accommodated by stable creep, within several km-thick shear zones and at very low strain rates (< 10-11 s-1). However, geophysical observations of active subduction zones have outlined, over the last two decades, that deep plate interfaces are likely to be dominated by unstable creep characterized by episodic events of aseismic slips (“slow slip events”) occurring at relatively high strain rates (> 10-7 s-1). Meanwhile, geological (i.e. petro-structural) observations of deep subduction interfaces have shown that strain is generally localized within < 10–100’s m-thick shear zones. These shear zones are also known to concentrate metamorphic reactions and episodic fluid flow that have both significant influence on the rock strength. Yet, quantifying the effects of these chemo–mechanical transformations on the transient aseismic slips of deep plate interfaces remains hindered by the complexity of integrating geophysical and geological observations and the general lack of high-pressure deformation experiments.

 

Drawing on novel deformation experiments conducted at 2 GPa (eclogite-facies conditions) using a new generation Griggs-type apparatus, we reveal that unstable creep can be steered by local transient changes of rheology from dislocation creep to dissolution–precipitation creep (DPC) during mineral reactions. These changes of rheology can cause rock weakening by several orders of magnitude if intergranular fluid transfer is efficient. Such a weakening is a transient process since reaction rates tend to be intermittent / episodic at great depths. Moreover, we show that fluid concentration during viscous strain localization promotes extensive fracturing that may correspond to tremors (i.e., low frequency earthquakes) observed during slow slip events. Indeed, thermodynamic modeling of mafic and sedimentary rocks along pressure/temperature (P/T) gradients of active subduction zones worldwide reveals that slow slip events and tremors preferentially occur in horizons undergoing major dehydration reactions, and thus potential transient changes in rock rheology.

How to cite: Soret, M., Jara, J., Gasc, J., Costantino, G., Cubas, N., Schubnel, A., Bhat, H., and Jolivet, R.: Transient creep in subduction zones explained by reaction-induced rheological switches, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18778, https://doi.org/10.5194/egusphere-egu25-18778, 2025.

17:15–17:25
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EGU25-8793
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ECS
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On-site presentation
Importance of understanding the evolution of crustal permeability for the apparent friction coefficients in Japanese subduction zones 
Dongwoo Han, Changyeol Lee, and Claire Nichols

The Nankai subduction zone in Southwest Japan is vulnerable to megathrust earthquakes posing a significant risk to the infrastructure and population it accommodates. This region has gained recent interest after the Hyuga-nada earthquake on 8th August 2024, because a megathrust earthquake, which has not occurred for the last 80 years despite its cycle known as 100 – 150 years, can be triggered by the event. Understanding earthquake mechanisms can mitigate the potential damage. The frictional condition at the plate interface is one of the key factors in estimating the location and magnitude of the potential megathrust earthquake. A previous study used numerical modelling that includes frictional heat to find the best apparent friction coefficient (μ') to explain the observed seafloor heat flow. However, hydrothermal circulation (HC) was not considered in this previous model although it significantly affects the thermal structure and the seafloor heat flow by redistributing heat energy. Therefore, we conducted numerical modelling that includes HC to find μ' values for the two subduction zones known for high risks of potential megathrust earthquakes – the Nankai and Tohoku (Northeast Japan) subduction zones. The results show that a wide range of μ' (0.00 – 0.30 and 0.00 – 0.12 for the Nankai and Tohoku subduction zones, respectively) can explain the observed seafloor heat flow depending on the vigour and extent of HC. This indicates that μ' cannot be constrained using heat flow observations before the evolution of the aquifer permeability is understood. Here, we suggest that the age of the oceanic crust and bending-induced faulting play a crucial role in the evolution of the aquifer permeability, resulting in a slowly decreasing permeability. Therefore, to better understand the frictional condition within a subduction zone, various fields of research – magnetic and seismological surveys, field and laboratory measurements, etc. – should work together as well as computational modelling.

How to cite: Han, D., Lee, C., and Nichols, C.: Importance of understanding the evolution of crustal permeability for the apparent friction coefficients in Japanese subduction zones, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8793, https://doi.org/10.5194/egusphere-egu25-8793, 2025.

17:25–17:35
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EGU25-12831
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On-site presentation
Fault rheology near the downdip limit of the seismogenic zone: new insights from microstructural and geochemical studies in fault cores from the Kodiak Central Belt, Alaska 
Hugues Raimbourg, Kristijan Rajič, Vincent Famin, Donald M. Fisher, Kristin Morell, and Ida Di Carlo

Geophysical evidence of high fluid pressures and the presence of fluidized microstructures provide two independent arguments supporting the existence of fluid-like materials within the core of slipping fault zones of the crust. The nature of these materials varies depending on the specific case, including H2O-rich fluid, ultra-comminuted rock, and melt formed after frictional slip. The persistence of such fluid-like materials over several episodes of slip is questionable, because high fluid pressures may decrease after slip and associated host-rock damage, while frictional melts solidify almost instantaneously.

To shed light on this issue, we investigated several fault zones from the Kodiak Central Belt, Alaska, which were active under peak metamorphic conditions (3.0 ± 0.4 kbar, 320 ± 20 °C). At outcrop scale, these faults cut across metamorphosed turbidites and extend for tens of meters, with fault cores up to 5 cm of thickness. Their kinematics indicate a top-to-the-SE motion, consistent with the main deformation stage in the Kodiak Central Belt. Injections of the core material into dm-long cracks in the host rock, perpendicular to the main slip plane, are locally present.

At thin section-scale, the fault cores show a multilayered structure, indicative of multiple slip events. The microstructures of these layers are variable, including cataclasites with clasts of various size surrounded by a quartz-rich cement, as well as quartz or calcite veins. The fault slip surfaces, within layers dominated by quartz, are underlined by aligned micrometric chlorite and titanium-rich inclusions.

The cement is to a large extent composed of idiomorphic quartz crystals that exhibit successive growth increments, highlighted by rims of micrometric chlorite inclusions. These chlorite inclusions share the same composition as the larger grains forming the metamorphic foliation of the host rock. The growth history of idiomorphic quartz crystals is further revealed by sharp variations in the concentration in Al, accompanied by corresponding changes in cathodoluminescence intensity. Most crystals display isotropic growth microstructures, indicating that the crystal growth occurred without steric constraints or application of a significant deviatoric stresses. Additionally, crack-seal microstructures formed in a dilatation jog along a microfault slip plane show similarly cyclical variations in Al content of the quartz cement.

These microstructures indicate that quartz crystal growth spanned multiple slip events and occurred under variable physico-chemical conditions, which influenced the differential incorporation of Al and solid inclusions into the quartz. The geometry of the growth microstructures suggests that the density and viscosity of the fluid were sufficiently high to prevent the crystals from settling down by gravity during their growth. Based on these observations, we propose that the fault core remained predominantly in a fluid state over multiple slip cycles, with viscosity variations resulting primarily from the progressive growth of crystals within the fluid. This mechanical behavior, characterized by persistently low viscosity, may correspond to the sequence of repeated slow-slip events observed in subduction zones.

How to cite: Raimbourg, H., Rajič, K., Famin, V., Fisher, D. M., Morell, K., and Di Carlo, I.: Fault rheology near the downdip limit of the seismogenic zone: new insights from microstructural and geochemical studies in fault cores from the Kodiak Central Belt, Alaska, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12831, https://doi.org/10.5194/egusphere-egu25-12831, 2025.

17:35–17:45
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EGU25-17113
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ECS
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On-site presentation
Syn-collisional exhumation of eclogites at the E margin of the Adula nappe (San Bernardino pass area, S Switzerland)
(withdrawn)
Chiara Montemagni, Riccardo Monti, Nadia Malaspina, Paola Vannucchi, and Stefano Zanchetta
17:45–17:55
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EGU25-18629
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ECS
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On-site presentation
Identifying tsunamigenic megathrust events in the Japan Trench through sedimentary biomarkers 
Piero Bellanova, Sara Trotta, Morgane Brunet, Natascha Riedinger, Christian März, Troy Rasbury, Martin Koelling, Rui Bao, Min Luo, Michael Strasser, Ken Ikehara, and Klaus Reicherter

The 2011 Tohoku-Oki earthquake highlighted substantial deficiencies in our understanding and an underestimation of the hazard potential of megathrust earthquakes and their cascading effects, including tsunamis. Offshore deep-sea paleoseismology evolved from the need to better understand mechanisms and depositional processes within megathrust subduction zones. The examination of sedimentary records has demonstrated effectiveness in reconstructing complex historical seismic events resulting in multi-pulse depositional sequences. However, reliably identifying individual turbidite sequences and delineating precise boundaries of distinct events remains challenging. This is especially true for the upper limit of turbidite-homogenite sequences where the contact between the homogenite and the background sedimentation is gradual and visually not detectable. Advances in organic geochemistry (e.g., high-resolution GC-MS and lower detection limits) can overcome and push such limitations. Organic sedimentary biomarkers, such as n-alkanes, polycyclic aromatic hydrocarbons, and fatty acids, serve as robust proxies for identifying allochthonous, earthquake-related strata and differentiating them from (hemi-)pelagic deposits. The high source-specificity of sedimentary biomarkers allows for obtaining sediment provenance information and the reconstruction of transport processes and depositional history.

In the Japan Trench, hadal seismic sediments result from turbidity currents transferring substantial amounts of material from shallow marine and coastal regions (e.g., tsunami backwash) into deep hadal basins. Initial sedimentary biomarker results from n-alkanes, steranes, and hopanes present a distinct marine signature from planktonic sources for the background sediments. However, turbidites and homogenite deposits linked to seismic events present increases in terrigenous signals, suggesting input of remobilized material from shallower marine environments or through a tsunami backwash.

This study highlights the application of organic sedimentary biomarkers as proxies to identify, characterize, and reconstruct past megathrust earthquakes (MW≥9) in the Japan Trench. By bridging current knowledge gaps, this approach advances seismic hazard assessment and supports the future development of improved mitigation strategies through an enhanced understanding of paleoseismological records.

How to cite: Bellanova, P., Trotta, S., Brunet, M., Riedinger, N., März, C., Rasbury, T., Koelling, M., Bao, R., Luo, M., Strasser, M., Ikehara, K., and Reicherter, K.: Identifying tsunamigenic megathrust events in the Japan Trench through sedimentary biomarkers, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18629, https://doi.org/10.5194/egusphere-egu25-18629, 2025.

17:55–18:00
Discussions

Posters on site: Wed, 30 Apr, 10:45–12:30 | Hall X3

Display time: Wed, 30 Apr, 08:30–12:30
Chairpersons: Mathilde Radiguet, Marco Scambelluri, Marianne Conin
X3.12
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EGU25-4246
Brittle behaviour and petrologic change of the subducting oceanic lithosphere 
Marco Scambelluri, Giovanni Toffol, Enrico Cannaò, Donato Belmonte, Nicola Campomenosi, Serena Cacciari, and Giorgio Pennacchioni

Metamorphism causes major changes in the mineralogy and rheology of the lithosphere. However, without coupled deformation and fluid flow, the unaltered lithosphere remains long time stiff and metastable, thus sustaining large differential stresses. This is relevant to subduction of oceanic lithosphere, where fluid presence vs absence affects seismicity and eclogitization. The subduction-zone behavior of hydrated oceanic slabs has been deeply studied in the recent years; differently, the unaltered lithosphere from the inner slab is much less known, though italso hosts earthquakes and its eclogitization can drive the slab pull.

Aim of this contribution is providing field-based evidence of the main structural and metamorphic changes affecting the dry portions of subducting oceanic slabs. The ophiolitic gabbro-peridotite of the Lanzo Massif (W. Alps) largely escaped Alpine subduction metamorphism due to poor oceanic hydration. This made these rocks dry, stiff asperities in the subduction complex, which locally developed pseudotachylytebearing faults and widespread meso- to micro-faulting at intermediate-depth depths. In the field, thin, flat-lying metric faults cause centimetre-scale offsets of gabbro dykes: such faults contain sub micrometric “annealed” ultracataclasite of fresh olivine and pyroxene locally overgrown by secondary chlorite. Cataclastic plagioclase is progressively altered into high-pressure zoisite + paragonite up to become the most intensively eclogitized mineral domain in the studied samples. The fault planes thus developed at dry conditions in the olivine stability field; localized fluid access promoted fault hydration and massive plagioclase replacement by high-pressure assemblages. By means of LA-ICP-MS element trace analyses, we also identified the internal redistribution of fluid-mobile elements. This implies that the subduction zone eclogitization of the slab mantle is triggered by fluid access along pervasive fault discontinuities and reactive minerals. The faulted Lanzo lithospheric mantle can represent slab domains affected by minor slip events and close to areas of faulting and pseudotachylyte formation during major earthquakes.

How to cite: Scambelluri, M., Toffol, G., Cannaò, E., Belmonte, D., Campomenosi, N., Cacciari, S., and Pennacchioni, G.: Brittle behaviour and petrologic change of the subducting oceanic lithosphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4246, https://doi.org/10.5194/egusphere-egu25-4246, 2025.

X3.13
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EGU25-8755
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ECS
New Geochronological Results from Glaucophane bearing Metabasalts and Metadacites from the Nan - Uttaradit mafic-ultramafic complex, NE-Thailand 
Pornchanit Sawasdee, Christoph A. Hauzenberger, John E. Booth, Etienne Skrzypek, Daniela Gallhofer, and Zsolt Benko

The Nan – Uttaradit mafic-ultramafic complex, associated with which are two outcrops of epidote blueschists, forms the linear core of the Nan back-arc basin. We have also found, as float, higher grade blue amphibole – garnet gneisses and white mica – garnet schists, from which we report newly obtained U-Pb zircon and allanite protolith ages, with K/Ar metamorphic ages from phengitic white micas.

The two outcrops of epidote blueschists are some 130 km apart; in the stream Huai Sak, 20 km east of Nan Noi town, and along a mountain ridge just north of highway 102, some 15 km west of Uttaradit city. The gneiss float samples were found in the stream Huai Phi Rong, 1 km east of Huai Sak, and on point bars of the Wa river east of Mae Charim town.

The blueschists, commonly retrogressed to greenschists, have the mineral assemblage Gln/Rbk/Act – Ep – Chl – Ph – Ab – Qz ± Ttn ± Rt ± Ilm ± Hem. Whole-rock geochemistry points towards basic igneous protoliths of tholeiitic affinity. The gneisses are coarse grained, with garnets up to 1 cm diameter. They have mineral assemblages Grt + Gln/Rbk/Win + Ep + Ph + Chl + Qz ± Stp ± Ap ± Ttn ± Rt ± Zrc. Geochemistry indicates dacite to andesite protoliths of calc-alkaline affinity.

Zircons large enough to analyse have been found only in the gneisses and garnet – white mica schists. They are euhedral to subhedral grains, 30 to 100 μm in length, with magmatic oscillatory zoning. U–Pb isotopic compositions of zircons from 11 samples were obtained using LA-(MC)ICP-MS. There are no indications of metamorphic rims, with all ages in the range 330 to 310 Ma. One sample also contained an older cluster around 360 Ma. Allanite, of magmatic origin, occurs in metabasites and gneisses as euhedral to subhedral grains, 100 to 400 μm in length, some with metamict cores and patchy zoning. U-Pb analysis by LA-MC- ICP-MS constrains their ages to 340 – 320 Ma, in good agreement with the zircon dates.

To determine the age of the HPLT event that affected these rocks, white micas and amphiboles were separated from five samples for K/Ar dating. Mineral inhomogeneity means that no reliable ages were obtained from the amphiboles, which will now be dated using the Ar/Ar method. However, two phengitic white mica samples gave consistent ages of ~327 and ~317 Ma.

It is concluded that subduction of the Nan basin was ongoing by the mid Carboniferous, with some igneous rocks being subducted very soon after emplacement. Further, if the Nan basin is indeed a back arc basin formed by rifting off the Sukhothai terrane from Indochina, then the precursor volcanic arc must have been formed at least in the Early Carboniferous and more likely in the Late Devonian. It is notable that the subduction of the Nan basin began at least some 100 my before the first recognized events of the Indosinian orogeny, which occurred around the end of the Middle Triassic.

How to cite: Sawasdee, P., Hauzenberger, C. A., Booth, J. E., Skrzypek, E., Gallhofer, D., and Benko, Z.: New Geochronological Results from Glaucophane bearing Metabasalts and Metadacites from the Nan - Uttaradit mafic-ultramafic complex, NE-Thailand, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8755, https://doi.org/10.5194/egusphere-egu25-8755, 2025.

X3.14
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EGU25-5330
Lithospheric structure of the Fiordland plutonic block controls the transition from transpression to subduction along the southwestern New Zealand plate boundary 
Donna Eberhart-Phillips, Sandra Bourguignon, Cedric De Meyer, Calum Chamberlain, and Jack Williams

In southwestern Zealandia, the plate boundary transitions from the Puysegur oblique subduction zone to the 600-km long transpressive Alpine Fault and Southern Alps uplift zone.  Utilizing abundant earthquake observations, we construct a 3D seismic velocity model to 130-km depth that demonstrates that the strong lithosphere of the Fiordland block defines the character of deformation along the plate boundary zone.  Highly oblique convergence combined with the relatively-weak young Puysegur slab enables sharp slab bending as it is translated northward around the Fiordland block. 

The Fiordland block contains plutonic rock from the 500-100 Ma Gondwana Cordillera, and its Grebe shear zone is a long-lived boundary, with a geochemically indicated Precambrian lithospheric keel underlying the Eastern Domain.  The Grebe shear zone is imaged as a boundary to 80-km depth, with Eastern Domain lithosphere abutting the deeper Australian slab, where it bends to vertical below 75-km depth.  Western Fiordland Orthogneiss lower crust, uplifted in the Miocene along reactivated shear zones, is imaged as a rigid/strong high-velocity feature pushed up above the 30-70-km depth Australian slab. In the crust, seismicity is distributed from the offshore Alpine Fault to eastern Fiordland, with partitioning along various structures including reactivated shear zones.

In southernmost Fiordland, south of Dusky Sound, the Puysegur slab maintains its moderately dipping subduction continuous with its offshore extent, and the overlying Pacific plate shows moderate seismic velocity material with the deep keel located further east than the slab.  In northern Fiordland, the impacting Pacific lithospheric base has an additional strong component, with Cretaceous underplated Hikurangi igneous plateau. This collision further steepens the young Australian slab which exhibits abundant deep seismicity 70-150-km depth. Overlying the deep vertical slab, our model suggests crustal thickening between the George Sound and Indecision Creek shear zones with exhumed high-velocity orthogneiss (Vp~6.5 km/s) overlying mid-crustal Vp of ~6.0 km/s.

How to cite: Eberhart-Phillips, D., Bourguignon, S., De Meyer, C., Chamberlain, C., and Williams, J.: Lithospheric structure of the Fiordland plutonic block controls the transition from transpression to subduction along the southwestern New Zealand plate boundary, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5330, https://doi.org/10.5194/egusphere-egu25-5330, 2025.

X3.15
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EGU25-12332
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ECS
Spatial extent of deep slab slicing events: Insights from the Phyllite-Quartzite paleo-accretionary complex (SE-Peloponnese and Kythira, Greece) 
Maïlys Bouhot, Armel Menant, Clément Ganino, Samuel Angiboust, Onno Oncken, Damien Deldicque, Laurent Jolivet, and Nikolaos Skarpelis

The massive transfer of material at depth significantly influences the long-term morphology of active subduction zones. However, the process of basal accretion (or tectonic underplating), when active, remains challenging to observe directly, due to the low resolution of geophysical imaging at high depth and the lack of spatial and temporal constraints on its tectonic and topographic signature in fore-arc domains. To tackle this issue, we aim at constraining the size of accreted tectonic slices that were stacked at high pressure/low temperature (HP/LT) conditions to build an accretionary complex.

To provide such constraints, we carried out a multidisciplinary study on the now-exhumed Phyllite Quartzite paleo-accretionary complex dated to the Oligo-Miocene, which crops out discontinuously along the active Hellenic subduction zone (Greece). This natural laboratory represents a key site for studying deep accretion processes as it remains in a fore-arc position and has not undergone a strong overprinting by later tectonic events.

A petro-structural study was therefore undertaken to identify the different sub-units of the Phyllite Quartzite complex. Detailed mapping of Kythira and southeastern Peloponnese, combined with structural measurements, petrological observations, Raman spectroscopy of carbonaceous material, and thermobarometric modeling, revealed several tectono-metamorphic sub-units forming this nappe stack. These units are distinguished by their petrological characteristics, the orientation of finite deformation markers, and their pressure-temperature history.

The results highlight two HP/LT sub-units in southeastern Peloponnese, which are also likely present on the island of Kythira, where one or two additional sub-units have been identified. These sub-units exhibit a distinct metamorphic evolution characterized by an increasing peak temperature from the base to the top of the HP/LT nappe stack. These observations suggest that the Phyllite-Quartzite paleo-accretionary complex was formed through a minimum of three successive episodes of basal accretion in this area. To better constrain the geometry of these units, spatial correlations with the neighboring regions where the nappe stack crops out are proposed, providing a minimum estimate of the size of the HP/LT units. The slices are estimated to measure tens of kilometers by hundreds of kilometers in the trench-perpendicular and trench-parallel directions, respectively. This study thus represents a first key step for constraining the characteristic size and the dynamics of tectonic underplating, which may still be active along the Hellenic margin and is observed in many active subduction zones worldwide.

How to cite: Bouhot, M., Menant, A., Ganino, C., Angiboust, S., Oncken, O., Deldicque, D., Jolivet, L., and Skarpelis, N.: Spatial extent of deep slab slicing events: Insights from the Phyllite-Quartzite paleo-accretionary complex (SE-Peloponnese and Kythira, Greece), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12332, https://doi.org/10.5194/egusphere-egu25-12332, 2025.

X3.16
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EGU25-163
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ECS
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An analytical approach for modeling the initiation and early development of the Rayleigh-Taylor gravitational instability in subduction settings  
Dimitrios Papadomarkakis and Maria-Sotiria Frousiou

In the past decades, the comprehension of major geodynamic processes has mostly been dominated by computational and numerical models, with researchers generally avoiding the usage of analytical methods. The main reason for the latter lies in the fact that geodynamic systems and processes can be very challenging, and sometimes even impossible, to model analytically due to their high complexity and unknown factor. However, with the proper assumptions, the processes can be simplified in a way that analytical approaches can be utilized to model the occurring phenomenon, without compromising accuracy and realism. Overall, a subject that has been studied by various researchers, and as a result a great number of computational models have been proposed in the last two decades, is the development of the Rayleigh-Taylor gravitational instability in the interface between the subducting plate and the flowing mantle. This instability is induced by the density contrast between the two aforementioned layers, and particularly the fact that a denser fluid, in this case the flowing mantle, overlies a lighter fluid, the subducting plate. It has been illustrated that overtime with the development of the instability, characteristic plume-like shapes are formed that enter the hot flowing mantle and at some point even detach completely from the subducting plate. These plumes are then subjected to high, or even ultra high, pressure and temperature conditions making them newly formed metamorphic rocks that at some point in time are likely to get exhumed. The initiation and early development of the above discussed phenomenon was modeled in the present work by using linearised Navier-Stokes equations for two viscous fluids, with different density and viscosity values. From this analytical approach a basic methodology is proposed, capable of estimating the required growth rate of the instability in its early stages and also the critical wavelength, after which the plume is considered to have been fully formed and probably even detached from the plate. Additionally, the introduced function for the amplitude of the instability was correlated with the detachment potential of the plume from the downgoing plate. Furthermore, the proposed model was applied to the subduction setting of the Mediterranean ridge, located south of the island of Crete. Lastly, macroscopic observations from the broader Hellenides region were employed, by mostly examining the existing literature, to ascertain whether any such metamorphic rocks had indeed surfaced, thus confirming their exhumation.  

How to cite: Papadomarkakis, D. and Frousiou, M.-S.: An analytical approach for modeling the initiation and early development of the Rayleigh-Taylor gravitational instability in subduction settings , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-163, https://doi.org/10.5194/egusphere-egu25-163, 2025.

X3.17
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EGU25-3791
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ECS
Numerical modeling of deformation associated with seamounts subduction.Implications for the seismic cycle. 
Alexis Gauthier, Nadaya Cubas, and Laetitia Le Pourhiet

Subduction zones are frequently affected by the subduction of seamounts (Wessel et al., 2010). Numerous studies have proposed that seamount subduction could significantly influence the seismic cycle of subduction zones (Wang & Bilek, 2014). In recent years, seamounts have been increasingly linked to the induction of fluid overpressures that trigger shallow slow slip events (SSEs) (Saffer & Wallace, 2015), contributing to the aseismic behavior of subduction zones.

However, rigorously establishing a connection between the seismic cycle and seamount subduction remains challenging due to the limited availability of observations. Identifying subducted seamounts is particularly difficult: seismic reflection methods are limited to depths of a few kilometers, while gravimetric techniques rely on inverse modeling, which introduces substantial uncertainties.

In this study, we performed numerical simulations to investigate the deformations associated with multiple seamount subductions in accretionary wedges. Our objective is to improve our understanding of the relationship between seamounts and the seismic cycle by:

  • Determining new structural criteria to better locate seamounts along mega-thrusts, thereby increasing the number of observations of wedges deformed by seamounts.
  • Providing mechanical constraints on the link between the seismic cycle and the subduction of seamounts.

We used the pTatin2d thermo-mechanical code (May et al., 2014, 2015), considering lithospheric flexure and surface processes (Jourdon et al., 2018). Our simulations explored variations in basal friction, seamount size, and lithospheric elastic thickness.

We showed that, contrary to previous thought (Wang & Bilek, 2014; Ruh et al., 2016), seamounts can be cut off during their subduction. This primarily depends on their size, as only smaller seamounts can be cut off. More surprisingly, it also depends on the timing of the seamount's arrival at the deformation front relative to the backthrust-forethrust succession.

The tectonic structures of the wedge are strongly influenced by the deformation mode of the seamount. If it is cut off, the structural inheritances of the wedge are preserved, with slices and basins that reflect past seamount subductions. If it is not cut off, gravitational collapse occurs. Additionally, the structural inheritances are not preserved but deformed during seamount burial. Only the structures associated with the subduction of the most recent seamount remain visible, consisting of a basin, a slice, and mass transport deposits at the surface.

We also investigated the stress state within the wedge. Once cut off, seamounts have no influence on the stress state. On the other hand, non-cut off seamounts induce significant tectonic overpressure landward and underpressure seaward (Ruh et al., 2016). Landward of the seamount, an undeformed sediment zone is identified (Wang et al., 2021). This zone is favorable for fluid burial since it is not drained by faults. Additionally, the horizontal orientation of the principal stresses is also favorable for the buildup of fluid overpressure (Sibson, 1990), which may induce SSE nucleation (Leeman et al., 2018). This study provides mechanical explanations for the observations of shallow SSEs landward of seamounts, as observed at Hikurangi (Bell et al., 2014; Barker et al., 2018;  Todd et al., 2018) and Nankai (Takemura et al., 2023).

How to cite: Gauthier, A., Cubas, N., and Le Pourhiet, L.: Numerical modeling of deformation associated with seamounts subduction.Implications for the seismic cycle., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3791, https://doi.org/10.5194/egusphere-egu25-3791, 2025.

X3.18
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EGU25-11648
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ECS
Numerical simulations of gravitational perturbations due to pre-seismic deep slab deformations before the 2011 Mw 9.1 Tohoku earthquake 
Rajesh Parla, Isabelle Panet, Hom Nath Gharti, Roland Martin, Dominique Remy, and Bastien Plazolles

The numerical simulation of gravity perturbations associated with deep slab deformations during the seismic cycle of great subduction earthquakes remains a significant challenge. This study presents a novel approach for simulating gravity anomalies induced by short-term slab deformations using the Spectral-Infinite-Element (SIE) method, implemented in the SPECFEM-X tool. Geodynamic models involving different fault settings are developed within a realistic 3D earth structure. The simulation includes a layer of infinite boundary elements surrounding the models in order to mimic a semi-infinite extent of the domain. Sensitivity analyses are carried out to assess the influence of the fault slip parameters (magnitude, mechanism, and location) as well as the density and velocity structure. The approach is first validated through synthetic benchmarks and then applied to a real-world scenario of the 2011 Mw 9.1 Tohoku earthquake. For this case, we design a 3D Earth model, incorporating a realistic Pacific slab in the region of the earthquake, and calculate the gravity anomalies induced by a sudden episode of slab extension, which is hypothesized to have occurred months before the rupture. The modelled gravity changes due to these pre-seismic deformations are compared with GRACE satellite gravity observations. This work highlights the importance of numerical simulations in satellite gravimetry and geodesy, offering new insights into the deformation processes that may result in gravity anomalies during the seismic cycle.

How to cite: Parla, R., Panet, I., Gharti, H. N., Martin, R., Remy, D., and Plazolles, B.: Numerical simulations of gravitational perturbations due to pre-seismic deep slab deformations before the 2011 Mw 9.1 Tohoku earthquake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11648, https://doi.org/10.5194/egusphere-egu25-11648, 2025.

X3.19
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EGU25-13515
XRF Core Scanning Based Chemostratigraphic Correlation for Paleoseismology in the Central Japan Trench 
Jyh-Jaan Steven Huang, Jun-Ting Lin, Ken Ikehara, and Michael Strasser

Megathrust earthquakes in subduction zones, such as the 2011 Mw 9.1 Tohoku-oki earthquake, are rare but pose significant threats to society. Their long recurrence intervals and limited historical records make reconstructing recurrence models challenging. The International Ocean Discovery Program (IODP) Expedition 386 addressed this by recovering over 800 meters of sediment cores from 11 trench-fill basins along the Japan Trench, providing a unique opportunity to extend paleo-earthquake records. Despite this, achieving reliable spatiotemporal correlations of event deposits remains a complex task. Here we show that high-resolution chemostratigraphic correlations using X-ray Fluorescence Core Scanning (XRF-CS), Principal Component Analysis (PCA), and Cluster Analysis (CA) effectively link event deposits across cores M0083D and M0089D in the northern basin and M0090D in the southern basin of the central Japan Trench. We identify eight event deposits in the northern basin, characterized by higher Ca and Sr with upward-decreasing trends, or elevated Si, Rb, and K without such trends, indicating distinct compositional differences and depositional processes of the turbidity currents. Across basins, M0090D deposits exhibit consistent clustering with M0089D but differ in internal structures and elemental trends, suggesting spatially similar sediment sources but varying erosion and transport mechanisms. Temporal chemical variations further suggest surficial sediment remobilization, rather than landslides, as the dominant trigger for turbidity currents, as it transports slope material that evolves compositionally over time. This insight reinforces the reliability of chemostratigraphy for event-stratigraphic correlation. Moreover, the spatial distribution of event deposits further highlights potential rupture areas and turbidity current pathways. Southward thinning of high Si, Rb, and K deposits suggests a northern source, while thicker Ca and Sr deposits in the southern core may imply a southern rupture zone. These findings establish a robust chemostratigraphic framework, enhancing our understanding of paleo-earthquake dynamics along the Japan Trench. The approach provides a valuable tool for reconstructing earthquake histories in other subduction zones, contributing to global paleoseismology research.

How to cite: Huang, J.-J. S., Lin, J.-T., Ikehara, K., and Strasser, M.: XRF Core Scanning Based Chemostratigraphic Correlation for Paleoseismology in the Central Japan Trench, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13515, https://doi.org/10.5194/egusphere-egu25-13515, 2025.

X3.20
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EGU25-17027
Diatom and radiolarian biostratigraphy in the vicinity of the 2011 Tohoku earthquake source fault: IODP Hole 343-C0019E of JFAST 
Masao Iwai, Isao Motoyama, Weiren Lin, Reishi Takashima, Yasuhiro Yamada, Minori Hagino, and Nobuhisa Eguchi

The frontal prism in the Japan Trench on the 2011 Tohoku-Oki earthquake (Mw 9.0, March 11, 2011) rupture zone had been drilled during the Integrated Ocean Drilling Program (IODP) Expeditions 343 and 343T. We investigated fossil diatoms and to determine age constraints on the cored sediments and reveal the behavior of sediment deformation history. Although diatoms and radiolarians abundances are varied in samples from common to rare with poor to moderate preservation in studied sediments, general biostratigraphic schemes in the North Pacific are applicable and well constrain the age of those sediments, except samples from fault clay in which fossils were barren. These results suggest that there are three large stratigraphic gaps at ~830 mbsf between the Cretaceous chert and the upper Miocene pelagic clay, at ~820 mbsf between the upper Miocene and the Pliocene-Quaternary, and at ~670 mbsf between the upper Miocene and the Pliocene-Quaternary. The former likely represents a hiatus or unconformity derived by tectonic erosion just above the incoming Pacific Plate, and the latter two correspond to an injection of material above the plate boundary fault due to increasing of volcanic activity in the NE Japan arc after 8 Ma. The Upper Miocene pelagic sequence below the plate boundary décollement comprises reversed stratigraphy, suggesting deformation by thrusting, slumping, folding etc.

How to cite: Iwai, M., Motoyama, I., Lin, W., Takashima, R., Yamada, Y., Hagino, M., and Eguchi, N.: Diatom and radiolarian biostratigraphy in the vicinity of the 2011 Tohoku earthquake source fault: IODP Hole 343-C0019E of JFAST, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17027, https://doi.org/10.5194/egusphere-egu25-17027, 2025.