Orals: Wed, 6 May, 16:15–18:00 | Room D1
How to cite: Penniston-Dorland, S., Wada, I., Harvey, K., Bullock, E., Dragovic, B., and van Keken, P.: Thermal evolution of the subduction interface: Coupled petrologic and geodynamic study of high-pressure rocks of the Rio San Juan Complex, Dominican Republic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22560, https://doi.org/10.5194/egusphere-egu26-22560, 2026.
The maximum depth of decoupling or MDD is the depth at which subducting oceanic plates—or slabs—become fully viscously coupled with the overlying mantle wedge and has a strong influence on the thermal structure of subduction zones. In many models, this depth is assumed to be around 80 km, based on comparisons between model results and measured surface heat flow data. However, very few convergent margins have a dense enough network of heat flow measurements to provide reliable constraints on this depth. As a result, the range of possible MDD for different regions and different times remains poorly constrained and the suitability of using a fixed value for Dc in thermal models of subduction in unclear.
We propose an alternative method for estimating MDD based on the rock record of subduction-type metamorphic belts. As rocks move along the subduction interface and pass through this depth, they transition from a cold domain—where the thermal structure is dominated by the advection of cool lithosphere—to a much hotter domain, where induced inflow of hot mantle towards the subduction interface leads to significant warming. This transition should result in a sharp increase in temperature over a relatively small increase in depth. If this thermal bend can be recognized in subduction-type metamorphic belts, its depth can serve as a valuable MDD indicator in ancient subduction zones. An important caveat to our proposed approach is that high thermal gradients can also result from shear heating at shallower depths, and these must be distinguished to make a reliable estimate.
We have identified several examples of thermal bends from ancient subduction zone settings. These all suggest that MDD occurs at depths 70–90 km. Our results support the idea that MDD varies little between different subduction zones or over geological time.
How to cite: Wallis, S., Hoshi, H., and Ito, T.: Maximum Decoupling Depths in Subduction Zones From the Rock Record, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18355, https://doi.org/10.5194/egusphere-egu26-18355, 2026.
The architecture of deep subduction zones governs the mechanical behavior and rheology of the subduction interface, influencing processes from long-term mountain building to earthquake rupture dynamics. Traditionally, mélanges have been considered chaotic assemblages resulting from high-strain tectonic mixing, yet recent studies challenge this view. The Kampos Belt on Syros Island has been regarded as a prime example of a subduction mélange, where high-viscosity eclgoties, blueschsits and metagabbros are embedded in a lower-viscosity metasomatic matrix composed of chlorite tremolite schist. However, the formation mechanisms of this structure remain debated. This has significant implications for understanding how, where and when strain is accommodated along megathrust shear zones.
In this study, we present new high-resolution field mapping, structural and petrological analyses, along with thermodynamic modeling to refine the spatial distribution of lithologies, deformation styles, and metasomatic processes that contributed to the structure of the Kampos Belt. Our findings suggest that rather than a chaotic mélange, the Kampos Belt represents a coherent stack of variably deformed metamafic slivers juxtaposed due to localized deformation along shear zones, where lithological variations largely reflect pre-subduction heterogeneities. These localized shear zones originate from different sources, including: metasedimentary slivers (mica schist), relict peridotitic lenses (antigorite schist), and metasomatic horizons associated with relict mafic lenses (chlorite-tremolite schist). Moderate- to high-strain domains are preferentially localized along metasomatic chlorite tremolite schist shear zones, which formed through fluid-assisted reactions at prograde to early-exhumation conditions. These metasomatic zones played a key role in strain localization, weakening the subduction interface and shaping the observed shear zone architecture. Our results challenge the classical interpretation of Kampos as a mélange. We suggest that the architecture of the belt is unlikely to have formed through large-scale tectonic mixing, instead we support a model where pre-existing lithological heterogeneity and fluid-assisted deformation (e.g., continued metasomatism along fractures) controlled the shear zone fabric. These findings have broad implications for subduction zone rheology, as they highlight the role of lithology dependent strain partitioning and fluid-induced weakening in deep megathrust shear zones.
How to cite: Munoz-Montecinos, J., Behr, W., hildebrandt, D., and Tokle, L.: Understanding Chaos: Fabric-Forming Processes from the Kampos Belt "Mélange" (Syros) and Implications for Megathrust Rheology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3595, https://doi.org/10.5194/egusphere-egu26-3595, 2026.
Seamounts are ubiquitous features of oceanic plates and are commonly subducted at convergent margins, where they can significantly deform the overriding plate. Numerous studies have proposed that subducting seamounts can influence megathrust slip behavior, either by promoting aseismic creep or acting as persistent barriers to earthquake rupture propagation. However, the interplay between long-term structural evolution and short-term seismicity remains poorly understood.
To investigate this relationship, we couple the long-term geodynamic code pTatin2d with the seismic cycle code Tandem. We first perform long-term geodynamic simulations with pTatin2d, focusing on the effects of subducting multiple seamounts. These simulations allow us to track the evolution of fault geometries, stress fields, and structural complexities in the upper plate over millions of years. At selected stages of seamount subduction, we extract the geometry of faults and the associated stress distribution to initialize seismic cycle simulations with Tandem.
To elucidate the role of each extracted parameter, and thereby develop a methodology linking geodynamic simulations to seismic cycle models, we systematically and independently investigate the effects of normal stress heterogeneity, topography, basal fault geometry, and upper-plate faulting on the seismic cycle. Specifically, we observe that variations in normal stress can act both as barriers to earthquake propagation and as asperities where earthquakes can nucleate. The upper plate faults also play an important role. Our simulations show that multiple splay faults can be activated during a single megathrust event. Rupture can also nucleate on a splay fault and subsequently propagate onto the main fault.
We then consider the combined influence of all extracted parameters, allowing us to assess how inherited structural and stress conditions control earthquake recurrence, magnitude, and the spatial distribution of seismic events. Our results provide new insights into how bathymetric highs modulate seismic behavior in subduction zones, bridging long-term geodynamics and short-term seismic processes.
How to cite: Gauthier, A., May, D., Cubas, N., Gabriel, A., and Le Pourhiet, L.: Linking geodynamic simulations of seamount subduction to seismic cycle modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7758, https://doi.org/10.5194/egusphere-egu26-7758, 2026.
To assess seismic hazard along subduction zones, which host Earth’s largest earthquakes, geodesists routinely measure interseismic surface deformation rates and invert them to estimate distributions of slip deficit along the plate interface. The resulting geodetic coupling models highlight portions of the megathrust that are “locked” and accumulating strain, thus identifying likely candidates for future rupture. However, inherent limitations in these models arise due to poor resolution of offshore observations leading to substantial uncertainty in shallow coupling estimates. Furthermore, recent geodetic records indicate that coupling can change significantly over just a few years, challenging the assumption that it remains stationary over interseismic periods, a critical caveat given that geodetic measurements typically span only a few decades. Beyond these observational challenges, studies have shown that high coupling is not necessarily a prerequisite for dynamic rupture: slip can penetrate creeping regions, and strongly coupled fault segments may act as rupture barriers.
To evaluate whether, and to what extent, geodetically inferred coupling correlates with coseismic slip, we perform a global comparison of slip deficit models and finite-fault slip distributions. We compile the first unified dataset of coupling models including twelve subduction zones and 61 finite-fault models of megathrust earthquakes that ruptured these margins, with a cumulative moment magnitude of 470. We discretize each slip model into a point cloud reflecting its slip distribution, allowing us to quantitatively link slip with coupling values to evaluate their correlation.
Our slip-coupling analysis reveals consistent global patterns: large megathrust earthquakes (Mw ≥ 7.5) preferentially rupture highly coupled regions, whereas smaller events show weaker coupling-slip correlations. Comparison with the null hypothesis in which slip-coupling correlation is completely random highlight that observed slip-coupling correlations are statistically significant. These findings highlight the complex interplay between coupling and rupture behavior, demonstrating that strong coupling alone does not unequivocally predict future earthquake slip patterns.
How to cite: Oryan, B. and Gabriel, A.: Do coupled megathrusts rupture? A Global Comparison of Megathrust Coupling and Earthquake Slip, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8678, https://doi.org/10.5194/egusphere-egu26-8678, 2026.
Jadeitites are commonly found in serpentinite mélanges and form by fluid flow across the subduction interface. Petrological analysis of jadeitites from various localities (Myanmar, Guatemala, Cuba, Russia, and Iran) coupled with structural characterization enabled the identification of successive jadeite/omphacite generations with subordinate amphibole and mica. These parageneses reflect metasomatism coeval with brittle and/or ductile deformation and complex crosscutting relationships. The composition of fluid inclusions (salinity, gas, δ18O, δD) reveals a wide range of fluid species pointing to a diversity of jadeitite-forming metasomatism. In situ trace element analysis and isotopic measurements (δ11B, 87Sr/86Sr, δ18O) indicate a fluid source dominated by altered oceanic crust (AOC) with a minor sedimentary component. Despite marked changes in major element content during protracted metasomatism, trace elements evolve only moderately while isotopes are virtually homogeneous, recording only small variations of fluid composition over time. Jadeitite evolution is strongly related to the ongoing serpentinization of the mantle wedge, promoting a longer fluid time-residence at the interface associated with chemical exchange and pore-pressure build-up. This suggests that (i) First jadeite generations formed by percolation of highly channelized AOC-derived-fluids in a dry mantle wedge, while later generations record fluid interaction with sediments and the serpentinized mantle. (ii) Fluid pulses across the subduction interface and rheological behavior of the near interface mantle wedge are not controlled by drastic changes in the nature of the slab input, but rather by the cooling of the serpentinizing subduction environment. (iii) The re-use of the same fluid pathways above the slab promotes the re-equilibration of isotopic signatures. (iv) Overpressures may build up upon jadeitite formation and promote brittle deformation events. This may lead to switches in deformation style and variations in permeability, thus changing fluid flow mode along the base of the mantle wedge.
How to cite: Minnaert, C., Angiboust, S., Herviou, C., Melis, R., Glodny, J., Cambeses, A., Raimondo, T., and Garcia-Casco, A.: Tracking fluid sources in mantle wedge jadeitites: petro-geochemical constraints and implications for fluid venting above the subduction interface, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11699, https://doi.org/10.5194/egusphere-egu26-11699, 2026.
Long-term submarine observations are critical for understanding subduction zones because the slip of great earthquakes occurs offshore. Geophysical observations suggest that the Cascadia megathrust is locked from the coastline to the deformation front in many places, but off central Oregon they are consistent with a narrowly locked megathrust near the deformation front and creeping behavior beneath the shelf where there are two clusters of earthquakes near the plate boundary, including repeating and very low frequency earthquakes. In this region, scientific objectives include understanding how megathrust locking transitions between the deformation front and the coastline, determining whether there is transient slip behavior, improving constraints on how shallow offshore earthquake clusters are linked to the megathrust, and characterizing the baseline deformation rate and fault slip behavior of the accretionary prism. This summer, the Cascadia Offshore Subduction Zone Observatory (COSZO), an infrastructure project funded by the US National Science Foundation, will add seismic and geodetic instruments to the Ocean Observatories Initiative (OOI) Regional Cabled Array (RCA) off Newport, Oregon. New seafloor science junction boxes, with updates to the RCA design, will be connected to three primary nodes on the continental slope and shelf that currently do not support seafloor geophysical observations. At each new junction box and a fourth site on the shelf where there is an existing science junction box but no geophysical instruments, COSZO will install a Nanometrics Atlantis Cabled Observatory ocean bottom seismic package comprising a buried broadband seismometer, a strong-motion accelerometer, a low-frequency hydrophone, and a differential pressure gauge. The project incorporates two types of calibrated absolute pressure gauges that utilize Paroscientific resonant quartz crystal sensors. The Geodetic and Seismic Sensor Module combines a triaxial accelerometer with two pressure gauges that are periodically calibrated against the internal pressure of the housing measured by a barometer. The Self-Calibrating Pressure Recorder also includes two pressure gauges but performs calibrations with a reference pressure close to ambient generated by a piston gauge. COSZO will also install uncalibrated absolute pressure gauges and Nortek Vector 3-component ocean current meters. Together with sensors already on the OOI RCA at the Slope Base and Hydrate Ridge sites and autonomous long-term geodetic observations, the COSZO infrastructure will form a critical mass observatory on the Cascadia Subduction Zone to support scientific studies and efforts to prototype offshore earthquake and tsunami early warning. COSZO will stream data into EarthScope Data Services and a workshop is planned for spring 2027 to engage early career scientists. Looking forward, each science junction box includes open ports and any unspent COSZO funds and independent PI-driven proposals can add to the suite of cabled instruments. The OOI RCA has also hosted three short fiber sensing experiments, demonstrating the potential for single- and multi-span distributed acoustic sensing concurrent with observatory operations. Implementing permanent fiber sensing on the OOI RCA would complement COSZO by adding additional observations over an expanded footprint.
How to cite: Wilcock, W., Harrington, M., Schmidt, D., Kelley, D., Tobin, H., Denolle, M., Thompson, M., Manalang, D., Cram, G., McGuire, C., Tilley, J., Zumberge, M., Sasagawa, G., Cook, M., Lipovsky, B., Krauss, Z., Hartog, R., and Bodin, P.: Sustained Cabled Seafloor Observations of the Cascadia Subduction Zone off Central Oregon, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15285, https://doi.org/10.5194/egusphere-egu26-15285, 2026.
Subduction zones generate the largest and most devastating earthquakes and tsunamis on Earth as a result of seismic slip on the megathrust fault. In addition to being capable of generating magnitude 8+ earthquakes, megathrusts also accommodate plate convergence via aseismic creep processes including episodic slow slip events. Above the megathrust, a portion of the overall plate convergence is accommodated as finite permanent strain (shortening) via slip along upper plate faults, tectonic folding, and reduction of sediment porosity (compaction). The most seaward expression of tectonic shortening in a subduction zone is focused within the outer accretionary wedge, but can also extend seaward of the main frontal thrust into sediments of the trench. Quantifying the strain budget among these different processes is essential for a better understanding of the partitioning between permanent inelastic strain and elastic strain accumulation as part of the seismic cycle – and thus ultimately toward an improved picture of subduction zone behavior and tsunami hazard. In this study, we use exceptionally detailed seismic reflection depth imaging and P-wave velocities to characterize sediment compaction within the outer wedge and trench along a profile of the southern Hikurangi subduction margin. Complementing these data with new constraints on stratigraphy, lithology and sediment physical properties, we provide the first quantifications of tectonic shortening attributable to sediment compaction on the Hikurangi margin. Our results demonstrate a broad region of compaction that extends more than 15 km seaward of the outermost faults. Future work beyond this study will explore relationships between pore scale compaction, proto-thrust development and active creep near the trench, in an attempt to provide a holistic understanding of strain accumulation in the outer wedge and trench.
How to cite: Crutchley, G., Klaeschen, D., Tozer, B., Wallace, L., and Saffer, D.: Tectonic shortening in the subduction trench and outer wedge, southern Hikurangi margin, New Zealand, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18257, https://doi.org/10.5194/egusphere-egu26-18257, 2026.
Vertical deformation is the most discriminating observable of the seismic cycle in subduction zones, but its complex measurement and contamination by multiple controlling processes have limited its exploitation. While horizontal displacements primarily reflect interplate coupling, vertical signals encode the competition among elastic megathrust loading, plate tectonic forcing, mantle flow and relaxation, and long-wavelength mass redistribution within the upper plate and forearc. Advances in satellite geodesy now allow vertical deformation rates to be resolved with sufficient precision and spatial coherence to provide new constraints on long-standing conceptual models of interseismic deformation. Recent observations reveal the existence of a secondary zone of interseismic subsidence (SZIS) in Cascadia, Nankai, Japan Trench, and Southern Chile. We use new data and models to assess the persistence of a SZIS and quantify and unravel its physical controls.
Using multi-track InSAR rate maps in northern Chile, we identify an inter-seismic secondary zone of subsidence landward of the primary coastal uplift belt. The presence of this SZIS supports the existence of a persistent secondary zone of interseismic subsidence. However, within the classical backslip framework, the elastic half-space predicts a slow monotonic transition from coastal uplift to inland subsidence. We show that it cannot reproduce the observed secondary trough without invoking unphysical coupling distributions or implausible fault geometries. The discrepancy is therefore not parametric, but conceptual. Interestingly, the existence of a SZIS was first predicted by our numerical seismotectonic models (van Dinther et al., PAGEO, 2019). We use these cross-scale visco-elasto-plastic models to demonstrate the critical role of a visco-elastic lower crust, which allows for an elastic upper crust that is thin enough to bulge under compression transferred across a coupled megathrust. We find that this mechanism is important, but it is not the only relevant mechanism. To quantify and detangle the physical mechanisms in more detail, we build a data-driven model of Northern Chile and aim to explain lateral variations along our observed segment. We integrate high-resolution earthquake catalogue, seismic tomography, and gravity anomaly observations to constrain slab geometry, forearc rheology, density structure, and seismogenic zone dimensions. Our fully dynamic visco-elasto-plastic earthquake cycle model with invariant rate-and-state friction resolves sequences of quasi-periodic earthquakes and can build topography over them. Through that, we aim to explain the presence of a SZIS also in our early geomorphic and geological interpretations of upper-plate deformation. Those multi-scale observations support that vertical surface displacements are not only governed by elastic rebound of megathrust faulting but also include a long-term long-wavelength deformation signal possibly related to position-dependent buckling of the upper plate.
We argue that, together with the existence of a persistent secondary zone of coseismic uplift of the largest earthquakes, such a secondary zone of deformation is a persistent and characteristic feature of seismic cycle deformation in subduction zones. This primary diagnostic will allow for a reinterpretation of the mechanics of subduction through an extension of the canonical backslip surface-deformation model.
How to cite: van Dinther, Y., Kosari, E., Koelzer, A., Sippl, C., and Godová, D.: On the Secondary Zone of Interseismic Subsidence and the Mechanics of Subduction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21059, https://doi.org/10.5194/egusphere-egu26-21059, 2026.
Posters on site: Thu, 7 May, 14:00–15:45 | Hall X2
The transfer of material from the downgoing plate to the overriding plate at depth exerts a first-order control on the mechanical and thermal evolution of subduction zones. However, the geometry, size, and temporal organization of deep tectonic slices formed during basal accretion remain poorly constrained, due to the limited resolution of geophysical imaging and the rarity of geological analogues preserving deep accretionary architectures formed during continental subduction.
Here, we investigate the spatial extent and stacking dynamics of tectonic slices by reconstructing the architecture of the deep paleo-accretionary wedge through the study of the exhumed Phyllite-Quartzite (PQ) nappe, derived from microcontinental protoliths, which was formed by basal accretion during the Oligo-Miocene along the Hellenic subduction zone. We carried out a multidisciplinary study of this now-exhumed PQ nappe, which crops out discontinuously from Crete to the Peloponnese (Greece). Preserved in a fore-arc position and weakly overprinted by later tectonic events, this natural laboratory provides direct access to deep accretionary processes.
An integrated petro-structural study conducted across southeastern Peloponnese and Kythira combines detailed mapping, structural analysis, petrological observations, Raman spectroscopy of carbonaceous material, and thermobarometric modeling. This approach allows us to distinguish several tectono-metamorphic sub-units within the PQ nappe stack, each recording a distinct P-T evolution that constrains the depth of basal accretion for successive episodes. Hypotheses of lateral continuity between these sub-units provide first-order constraints on their present-day spatial extent and on the minimum size of individual accretionary slices.
In southeastern Peloponnese, two HP-LT sub-units are identified within the PQ nappe stack, while at least two equivalent sub-units are recognized on Kythira. These sub-units record a systematic increase in peak temperature from the base to the top of the HP-LT nappe stack, consistent with successive episodes of basal accretion. Reconstructed P-T conditions indicate that basal accretion occurred at depths of ~50-60 km along the subduction interface. Based on spatial correlations between structurally equivalent HP–LT sub-units exposed in neighboring regions along strike, we infer a minimum present-day lateral continuity of individual accretionary slices. On this basis, deep tectonic slices formed during basal accretion are inferred to currently extend over several tens of kilometers in the trench-perpendicular direction and up to a hundred kilometers along strike.
This study provides new quantitative constraints on the depth, lateral extent, and dynamics of tectonic underplating, with direct relevance for the Hellenic margin, where such processes may still be active, and for 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 wedge (Hellenic subduction zone), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6595, https://doi.org/10.5194/egusphere-egu26-6595, 2026.
Subducting bathymetric reliefs, such as seamounts, modify the slip behavior of megathrusts, thereby potentially dictating seismic segmentation, rupture dynamics, and the structural evolution of the subduction channel and upper plate. While geodetic data often suggest that the megathrust near subducting seamounts is weakly coupled and dominated by aseismic creep or microseismicity, several "seamount earthquakes" have been documented. The role of subducting topography in governing fault coupling, rupture dynamics, and the spatial distribution of rupture remains poorly understood.
Laboratory seismotectonic experiments provide an effective means of simulating earthquake cycles and observing fault slip behaviour with high spatiotemporal precision, thereby overcoming the limitations of sparse onshore and missing offshore geodetic networks, as well as short historical records. In our experiments, a topographic high with seamount geometry was subducted along a 15° dipping, velocity-weakening seismogenic zone accompanied by hundreds of analogue earthquake cycles. The model upper plate is a wedge composed of an elastoplastic granular material that can respond to seismic cycles and seamount-induced stresses. We constrained the interface slip distribution by combining analogue geodetic slip inversion of surface displacement with direct monitoring of the interface via side-view imaging.
The results reveal that during the early stages of seamount subduction, when the seamount has partially subducted beneath the upper plate, along-strike rupture propagation is arrested at the seamount, which acts as a barrier, producing partial ruptures. Progressively, as the main portion or the entire seamount becomes subducted, another consistent spatial pattern emerges: coseismic slip concentrates at the leading downdip edge of the seamount, while the center and updip regions remain largely aseismic, with minor shallow slip reflecting slope instabilities triggered by upper-plate extensional structures. This pattern aligns well with interseismic high-coupling patches, which can also extend to the deep flank of the seamount.
Our findings indicate that, while subducting seamounts inhibit earthquake nucleation and broadly arrest rupture propagation, they still allow slip to extend onto the seamount-bearing interface. This explains why the deeper flank of a subducting seamount or ridge remains seismically active. A series of earthquakes (1996 Mw 6.7 and Mw 6.8; 2024 Mw 7.1; 2025 Mw 6.8) systematically occurred around the downdip edge of a Kyushu-Palau Ridge. Similar rupture behavior has been documented for a series of Mw ~ 7 events in the southern Japan Trench and for the two Mw > 8 events in central Nankai. This spatial pattern is further supported by geological evidence of pseudotachylytes, which are only localized on the downdip side of the exhumed fossil seamount.
Beyond slip kinematics, our experiments demonstrate that subducting seamounts perturb the megathrust stress field, leading to heterogeneous stress accumulation along dip, consistent with previous numerical mechanical-hydrological modeling studies. This suggests that seamount-induced coupling enhances upper-plate deformation and long-term structural features, including forearc uplift, fault reactivation, and localized fracturing. The short- and long-term upper-plate deformation patterns provide a key means of identifying subducted topographic features and assessing their impact on earthquake and tsunami hazards.
How to cite: Tan, H., Kosari, E., Rosenau, M., Gao, X., and Oncken, O.: Spatial Patterns of Megathrust Seismogenic Behavior Modulated by a Subducting Seamount, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9476, https://doi.org/10.5194/egusphere-egu26-9476, 2026.
Current studies on seamount subduction propose contrasting effects on megathrust behavior. Some suggest that subducting seamounts increase normal stress and promote large earthquakes, while others argue that seamounts fracture the upper plate, enhancing microseismicity and aseismic creep that may inhibit major ruptures. Field- and microscale observations from the Azuero Accretionary Complex, an exhumed intra-oceanic accretionary complex in Panama, provide new constraints on the deformation processes associated with seamount subduction.
Coastal exposures on the Azuero Peninsula expose the contact between the autochthonous Azuero Plateau and the allochthonous Azuero Accretionary Complex. The Azuero Plateau forms part of the Caribbean Large Igneous Province and consists mainly of massive to pillowed oceanic plateau basalts with minor Upper Cretaceous chert. In contrast, the accretionary complex comprises massive to pillowed basalts and volcanic breccias with ocean island affinity, locally interbedded with Paleogene carbonates. A ~3 km wide deformation zone, the Azuero Mélange, separates these units, and is inferred to be a deformed portion of the plateau based on new field observations and geochemical data.
The accreted seamount lithologies show pervasive faulting, cataclasites, abundant zeolite veins, and a chlorite-rich shear zone located ~60 m below the mélange. These rocks lack evidence for large displacement through-going faults. In contrast, plateau-derived rocks record both brittle faulting and ductile deformation. Ductile strain localized within the Azuero Mélange, where clay-rich cataclasites accommodated deformation through cataclastic flow and dissolution–precipitation creep. At the structural base of the mélange, a ~10 m thick shear zone composed of foliated cataclasites with basalt and limestone clasts within a clay-rich matrix is observed, interpreted to be sheared seamount lithologies.
Fluids and alteration played a crucial role in localizing strain within the upper plate and the décollement, enhancing mechanical weakening and diffusive mass transfer. Cataclasis increased permeability, enabling fluid infiltration and the formation of mechanically weak phases such as clays and chlorite. These processes promoted strain localization and facilitated deformation by cataclastic flow and dissolution-precipitation creep. The interplay between these alteration and deformation processes likely favored aseismic creep during seamount subduction.
How to cite: Maglalang, E. J., Fagereng, A., Buchs, D., and Toffol, G.: Deformation and alteration during seamount subduction: Insights from an exhumed intra-oceanic accretionary complex, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14561, https://doi.org/10.5194/egusphere-egu26-14561, 2026.
Understanding the structure of the incoming Pacific Plate is essential for reconstructing the tectonic evolution of the Japan Trench. International Ocean Discovery Program Expedition 405, “JTRACK,” drilled through the frontal prism and plate boundary fault zone of the Japan Trench and into the underlying Pacific Plate at Site-C0019, located close to the hypocentre of the 2011 Mw 9.0 Tohoku‑Oki earthquake. Here, we present new petrographic and geochemical results from five basaltic samples recovered from below the plate boundary fault zone (930–946 mbsf), which show the first direct geological evidence for a subducted volcanic body beneath this segment of the margin.
Core and image logs from Expedition 405 indicate that the drilled interval comprises a repeated sequence of sheeted dykes, massive basalts, and pillow lavas, with at least one interbedded sedimentary horizon separating the successions. This architecture is incompatible with simple ocean-plate stratigraphy and instead indicates a later phase of volcanic activity interacting with pre-existing crust. Of the five samples analysed, one was taken from below the intercalated sedimentary horizon and four from above it. The four samples from above the sediments are systematically more evolved than the sample below. In the samples above, plagioclase shows anorthitic cores overprinted by albitised rims, pyroxenes are Mg-rich, and ilmenite is abundant. Several samples contain K-rich clays, disseminated sulphides, and Zr-rich domains, including possible baddeleyite. These features suggest interaction with sedimentary cover and progressive evolution of magma chemistry during the later stages of emplacement, alongside post-emplacement hydrothermal alteration. Whole-rock major and trace element data show enrichment in incompatible elements (K, Pb, Cs, Rb, Ba, Th, U) relative to typical MORB. K-metasomatism discrimination diagrams indicate that these signatures are not produced by alteration, supporting a primary magmatic origin. Together, the mineralogical and chemical characteristics point to a volcanic body formed off-axis on the Pacific Plate rather than at a spreading ridge.
Integration with seismic interpretations suggests that this volcanic body, likely a seamount chain, was faulted prior to subduction and now lies partially perched on a horst, forming a structural high directly beneath the plate boundary fault zone. These results provide the first physical confirmation of a subducted, faulted seamount chain beneath the Japan Trench, offering new constraints on the structure and evolution of the incoming Pacific Plate.
How to cite: Gough, A., Jurado, M.-J., Ishikawa, T., Fukuchi, R., Webb, M., Nakamura, Y., Yamaguchi, A., Conin, M., Nicholson, U., Gürer, D., Rasbury, T., Fulton, P., Kirkpatrick, J., Kodaira, S., Regalla, C., Ujiie, K., Eguchi, N., Maeda, L., Okutsu, N., and Toczko, S. and the Expedition 405 Scientists: Petrological and Geochemical Evidence for a Subducted Off-Axis Seamount Chain Beneath the Japan Trench Plate Boundary Fault Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9894, https://doi.org/10.5194/egusphere-egu26-9894, 2026.
Accurately assessing strain accumulation and release in subduction zones is contingent upon robust detection and characterization of locking and slip along the megathrust. However, the distribution of slip on shallow, offshore plate boundaries is not well-resolved with onshore GNSS networks. At the Hikurangi Subduction Zone offshore Aotearoa-New Zealand, extensive investment has been made into seafloor geodetic techniques such as seafloor pressure and GNSS-acoustic, which have significantly improved observation and characterization of offshore SSEs. Despite their utility, oceanographic noise limits the ability of these seafloor techniques to detect SSEs. Formation pore pressure changes (as a proxy for volumetric strain) detected in borehole observatories have an enhanced signal-to-noise ratio and can reliably resolve deformation at the 10s of nanostrain-level, providing an improved view of shallow crustal deformation offshore.
Here, we report on a suite of SSEs observed in two IODP borehole observatories in the northern Hikurangi Subduction Zone between 2018 and 2023 and model their slip distribution and magnitude. During this time, five SSEs were clearly recorded in the borehole pore pressure data. Four of these occurred spontaneously, and the borehole pressure changes correlate with surface displacement observed at onshore GNSS stations. In contrast, in early 2021, the Mw 7.2 East Cape earthquake triggered a near-trench SSE that was only captured by the observatories. We jointly invert changes in pore pressure with onshore GNSS displacements and seafloor pressure (when available) for slip distribution along a 2D transect for each of the events. Our inversions incorporate realistic elastic properties constrained by high-resolution seismic velocity models and logging-while-drilling data, which is crucial for accurately resolving slip distribution and magnitude. We find large differences in slip initiation and evolution characteristics during the 2021 triggered SSE compared to the spontaneous events. We also find that, in total, SSEs accommodate most (>80%) of the plate convergence budget along the shallow (<10 km) megathrust. The 2021 triggered event was particularly important for filling in a slip deficit near the trench. Our results have implications for the role of SSEs in accommodating the megathrust strain budget near the trench at subduction zones.
How to cite: Carlson, M., Wallace, L., Saffer, D., and Williams, C.: Slow Slip Accommodates the Full Plate Convergence Budget at the Northern Hikurangi Subduction Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8339, https://doi.org/10.5194/egusphere-egu26-8339, 2026.
Sulawesi and Borneo are tectonically complex islands in South East Asia that were assembled from numerous crustal fragments throughout the Cenozoic. Past P wave tomography studies of this region have used land stations and hence image seismic velocity structure primarily beneath the two islands. By adding data from Ocean Bottom Seismometers deployed between 2019 and 2020 in the Makassar Strait, which lies between Sulawesi and Borneo, and incorporating core-going converted P phases with steeper ray paths (PKP and PKIKP), we illuminate the mantle beneath this offshore region to depths of around 800 km. The new tomographic velocity model robustly images a high-velocity north-west dipping tabular anomaly at depths between ~300 and ~660 km beneath the strait, which is interpreted as an aseismic ancient slab. By combining a slab age-depth relationship with a plate tectonic reconstruction, we uncover the palaeosubduction boundary responsible for this slab, thereby providing conclusive evidence for the previously hypothesised north-westward subduction of the Celebes Sea beneath Northern Borneo around ~15 Ma (plus/minus a few million years). Following slab break-off and northward plate migration of Sulawesi, sinking of this northwest Celebes Sea slab may have contributed to the initiation of subduction of the Celebes Sea southwards beneath Northern Sulawesi, which today is confidently imaged by a Benioff zone in addition to seismic tomography.
How to cite: Rawlinson, N., Li, Y., Pilia, S., Kesumastuti, L., Lü, C., Widiyantoro, S., and Hao, T.: Cenozoic Subduction Polarity Reversal Within the Celebes Sea Inferred from Teleseismic Tomography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13387, https://doi.org/10.5194/egusphere-egu26-13387, 2026.
Slip behavior on the shallowest part of the subduction zone is heavily influenced by the lithostratigraphy of the incoming plate. Coseismic slip that propagates toward the seafloor is a major tsunami hazard as seen in the 2011 Tohoku-Oki earthquake. In Nankai Trough, scientific drilling combined with seismic facies mapping has allowed scientists to characterize the lithostratigraphy of the Shikoku Basin which in turn have been linked with the geotechnical properties, structural architecture, and seismicity. Seismic reflection surveys have been conducted here for more than 25 years covering over 730 x 150 km and mapping the sediment input using the traditional visual interpretation at this scale is inefficient. Taking advantage of this large seismic dataset combined with scientific drilling to map the fault properties can help improve hazard assessments. This study presents a framework for a semi-automatic classification using 2D seismic reflection profiles acquired by different surveys. Three trench-parallel lines within the accretionary wedge acquired by three different surveys were used for the case study. The amplitudes of the western and eastern profiles were first scaled based on the seafloor reflection of the central profile. Features were extracted using a rectangular window measuring 500 m above the top of the oceanic basement with varying widths measuring 1 km, 3 km, and 5 km. A total of 121 features divided into three groups were extracted. The statistical group (13) describes the strength of reflections, the spectral group (30) describes the presence or absence of internal reflections, and textural group (78) describes the continuity of reflections. The three principal components of each group were extracted and altogether subjected to K-Means clustering with 6 clusters. The 5 km window showed the most comparable classification with visual interpretation and the consistent classification in the overlap between profiles indicate a satisfactory performance of our method. Comparing the classification with previous drilling, Cluster 0 located in the overlap between the central and eastern profile is associated with turbidites occurring in basement lows. Cluster 1 is classified as noise. Cluster 2 in the western and central profile are likely siliciclastic turbidites from the Kyushu Fan. Cluster 3 and 4 in the eastern profile appears to have no drilling analog. Cluster 5 in the central profile is associated with hemipelagic mudstones. These initial results appear promising and will be tested in larger datasets and other subduction zones in the future.
How to cite: Flores, P. C., Fujie, G., Shiraishi, K., Nakamura, Y., Kimura, G., Su, J., Agata, R., and Kodaira, S.: An unsupervised classification scheme for seismic facies mapping of sediment input in Nankai Trough using reflection amplitudes from 2D profiles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11054, https://doi.org/10.5194/egusphere-egu26-11054, 2026.
Numerous seismic surveys have been conducted to uncover the geophysical structure and fluid distribution in subduction zones, since seismic velocities are primarily influenced by pore fluids within rocks. To link seismic velocity and the pore fluid state, laboratory measurements of elastic wave velocity using rocks samples from subduction zone environments have been conducted, revealing the role of microscopic pores and cracks in controlling seismic velocity. However, natural geological systems are heterogeneous and contain defects of different scales at each scale of observation. Therefore, the velocities observed at in situ scales should be affected not only by microscopic pore structures but also by larger-scale defects, such as fractures and faults. Such large-scale defects should also play a role in fluid drainage system, since permeability of rocks depends strongly on the dimensions of conduits. In this study, we compare laboratory-measured ultrasonic velocity measured on core samples from the Susaki area in the Shimanto accretionary complex, SW Japan, with sonic velocity measured by borehole logging experiments. P-wave velocities were measured at a frequency of 1 MHz under dry conditions at 5 cm intervals along core sections spanning a total length of 128 m. The measured values were then converted to velocities under wet conditions using an effective medium model, enabling comparison with sonic velocities acquired under groundwater-saturated conditions. Results show that P-wave velocity decreases from the laboratory (~6 km/s) to the borehole scales (~5 km/s). This scale-variant effect can be explained by an effective medium model whereby mesoscale porosity that is undetectable at the ultrasonic wavelength is introduced into the matrix phase with microscale porosity. Assuming typical apertures for micro- and mesoscale fractures, we estimate that the effective permeability can increase to 10–12–10–11 m2 with increasing in the mesoscale porosity and decreasing P-wave velocity down to 4–5 km/s. These results indicate that seismic velocity anomalies and related seismic activity are associated with the presence of mesoscale fractures in subduction zones.
How to cite: Akamatsu, Y., Okuda, H., Kitamura, M., and Sawai, M.: Scale-dependent seismic velocity and permeability in subduction zones caused by mesoscale fractures, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4431, https://doi.org/10.5194/egusphere-egu26-4431, 2026.
Understanding how deformation is distributed within accretionary prisms is essential to constrain their structural evolution, internal organisation and seismo-tsunamigenic potential. Researchers would typically use seismic images to characterise large-scale subsurface structure, but accretionary prisms often appear internally chaotic-to-transparent in such data, lacking coherent reflectors. This is likely due to strong lateral heterogeneity and stratal disruption around the scale of the seismic resolution, resulting from intense deformation of accreted sediments and remobilised mass transport deposits.
IODP Expedition 405 “JTRACK” (October-December 2024) drilled the frontal prism of the Japan Trench subduction zone at Site C0019, targeting the decollement that hosted extremely large slip (>50 m) to the trench during the 2011 Mw 9.1 Tohoku-Oki earthquake. The expedition involved continuous coring and logging-while-drilling (LWD) through the prism, resulting in an improved time-depth tie with regional 2-D seismic profiles. At this site the seismic images show a highly chaotic internal prism, which introduces large uncertainties in structural interpretation and inhibits the upscaling and lateral extrapolation of borehole-scale observations from Site C0019.
Here we present an approach to characterise this heterogeneous internal structure by inverting for scale-independent geostatistical parameters (vertical and lateral correlation lengths, dip) from the combined seismic image and LWD sonic velocity data. We use a Bayesian Markov Chain Monte Carlo approach that gives spatially varying, probabilistic estimates of the geostatistical parameters. The lateral correlation length, specifically, can be used as a proxy for the degree of stratal disruption/deformation, as shorter correlation lengths reflect more intense deformation. While the aspect ratio of the correlation lengths is generally well-constrained for seismic data, the estimate of vertical correlation length from the borehole logs is necessary to resolve the other individual geostatistical parameters away from the borehole.
Initial results reveal that the degree of deformation varies significantly within the frontal prism, with the lateral correlation length varying between around 10-50 m. The degree of deformation appears to be compartmentalised by steeply dipping structures that could correspond to fault zones also observed in cores from C0019. These distinct changes in lateral correlation length correspond to lithological units interpreted from core observations. They also coincide with significant changes in vertical correlation length from the sonic log, anisotropy of seismic velocity from core samples and bedding orientation from borehole images. The results demonstrate that seismically-derived geostatistical parameters can delineate internal compartmentalisation of the frontal prism, providing a framework for tectonic and mass transport deposit interpretation and for the extrapolation of core-scale observations. Future work will extend this analysis using parallel profiles along the Japan Trench axis, allowing for mapping of frontal prism internal deformation in three dimensions away from existing drill sites.
How to cite: Ford, J., Nakamura, Y., Jurado, M.-J., Schaible, K., Nicholson, U., Doan, M.-L., Pei, P., Hamada, Y., Miyakawa, A., Conin, M., Fulton, P., Kirkpatrick, J., Kodaira, S., Regalla, C., Ujiie, K., Eguchi, N., Maeda, L., Okutsu, N., and Toczko, S. and the Expedition 405 Scientists: Geostatistical characterisation of internal deformation of the Japan Trench frontal prism using seismic and logging-while-drilling data (Site C0019), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12829, https://doi.org/10.5194/egusphere-egu26-12829, 2026.
Making a link between hydraulic properties and lithology is important for understanding fluid flow and pore pressure evolution in subduction zone sediments. However, it's a difficult challenge to get the in-situ downhole hydraulic responses due to the complex lithological variations and strong vertical heterogeneity. IODP Expedition 405 - JTRACK, drilled Site C0026 in the Japan Trench, penetrating a thick sedimentary sequence by hemipelagic mud, pelagic clay, chert and basalt intervals. Continuous logging-while-drilling (LWD) and coring operations provide a unique opportunity to examine lithology-related hydraulic properties in the sedimentary structures.
In this study, we integrate the interpretated downhole annular pressure (DHAP) from LWD time series dataset and other physical properties, including gamma ray, resistivity, sonic velocity, and caliper logs, together with interpretated lithogical logging units. Comparing forward-modelled DHAP with measured DHAP data, the results indicates that the in-situ fluid pressure evolution is highly correlated with lithological variability. In the hemipelagic mud at shallow depth, apparent inflow is largely influenced by borehole enlargement, where caliper increases strongly affect flow modelling and make it difficult to identify the real formation inflow. In the pelagic clay interval, little to no fluid inflow or loss is observed, indicating the absence of significant overpressure or under pressure. Localized fluid loss is identified within chert layers, consistent with their brittle behaviour and increased caliper values, suggesting fracture-controlled fluid escape. In contrast, a clear and active inflow is observed in the basalt interval, where fractures identified from borehole image logs provide efficient pathways for fluid flow to the borehole.
These observations indicate that hydraulic properties at Site C0026 are strongly controlled by lithological contrasts, leading to vertically variations fluid flow behavior. This study highlights the importance of integrating with borehole logs lithological information to constrain fluid transport processes in subduction zone sedimentary sequences.
How to cite: Pei, P. and Doan, M.-L. and the Expedition 405 Scientists: Lithology-related hydraulic properties of subduction zone sediments at Japan Trench, IODP Exp.405 Site C0026, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13415, https://doi.org/10.5194/egusphere-egu26-13415, 2026.
The rheological and compositional evolution of a subducting plate critically depends on its thermal structure. The temperature evolution of the subducting plate depends on its history prior to subduction, on its interaction with the overriding plate, and on how it interacts with the ambient mantle. Many processes that are associated with subduction such as deep seismicity and fluid release, which are responsible for arc-volcanism, can be understood through the temperature evolution of the slab. Studying the temperature evolution of the subducting slab, however, is not straightforward because of the lack of direct observations and a complete subduction record. Thus, to assist the interpretation of the available data it is necessary to use forward geodynamic modelling (FWG).
FWG can either study a full dynamic system or study the evolution of the slab using a kinematic model, in which the slab geometry and velocity are fully prescribed as boundary conditions. Kinematic models are more suitable to study specific subduction zones, as the dynamic models would require high computational cost to fit the available data. However, kinematic models suffer from several assumptions that oversimplify the complexity of the subduction process. For example, kinematic models are frequently performed with constant convergence velocity, age of subducting plate, and thermal properties.
In this contribution, we aim to test whether the pressure- and temperature-dependent thermal properties improve our ability to interpret the natural data, and if the improvements are worth the additional complexity.
How to cite: Piccolo, A., Craig, T. J., Van Zelst, I., and Thieulot, C.: The effect of pressure-temperature dependent material properties on thermal evolution of the slab, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14028, https://doi.org/10.5194/egusphere-egu26-14028, 2026.
The Sierra del Convento mélange in southeastern Cuba is one of a limited number of jadeite jade occurrences worldwide. This exhumed, pervasively serpentinized fossil subduction interface hosts tectonic blocks up to several tens of meters in scale that were reworked during intense metasomatism. The flat-lying mélange spans a 300 m thick vertical exposure, progressing from a block-rich lower portion toward a block free, serpentinite-dominated upper region. We conducted structurally-controlled sampling of blocks and matrix to produce a spatially-resolved geochemical and microstructural profile through the thickness of the Convento mélange. Pale green, nearly pure jadeitite constitutes the dominant block population in the southern part of this mélange, and may contain up to 30% epidote and/or white mica by volume. Minor metasedimentary and metamafic block populations, recording variable degrees of HP metamorphism, coexist alongside the jade blocks. We report a newly identified metre-scale zoning within the jadeitite bodies, consistent with that documented in other jade localities. Green jadeitite locally occurs surrounding an older core of zoisitite containing relict jadeite crystals rimmed by omphacite. Green jade is cross-cut by fractures infilled by dark-colored brecciated jade, which is in turn rimmed by a late, pyroxene-free rind composed mainly of weakly foliated phengite + albite ± epidote. The latter facies occupies a similar position to chloritite blackwalls previously described from the Convento jade occurrence. All jade varieties except for these latest phengite-albite rinds and chloritite blackwalls are cross-cut by fractures infilled by jadeite and omphacite. Parts of the main jade bodies exhibit prismatic radial and comb jadeite microstructures, consistent with descriptions of P-type jade, which precipitate directly into open fractures from hydrothermal fluids. However, Convento jade contains paragonite with up to 2 wt.% K2O and jadeite-omphacite exsolution domains brecciated into jigsaw-like fragments recemented by jadeite and/or omphacite. These observations are consistent with at least part of the jade in the Sierra del Convento mélange representing near-total high-temperature metasomatic replacement of high-pressure anatectic trondhjemite protoliths, which originated as partial melts of garnet amphibolite at ~15 kbar, resembling an R-type (replacement) jade paragenesis. To overcome the considerable ambiguity inherent to geochronology datasets from HP igneous and metasomatic rocks from this locality (105 to 115 Ma, U-Pb zircon), we are conducting detailed multi-mineral and multi-system geochronology, including further U-Pb on zircon, titanite, and apatite, Ar/Ar on white mica, and Rb-Sr on white mica. This multi-chronometric approach will establish relative and numerical chronology for the diverse jade facies of the Convento occurrence, resolving timescales of the multiple associated fluidization events within the subduction channel.
How to cite: Ducharme, T., Angiboust, S., Cambeses, A., Peverelli, V., Raimondo, T., Núñez-Cambra, K., Blanco-Quintero, I. F., Cárdenas-Párraga, J., and Garcia-Casco, A.: Timescales of metasomatism in a hot subduction channel: a microstructural and (radio-)isotopic study of Sierra del Convento jade, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16928, https://doi.org/10.5194/egusphere-egu26-16928, 2026.
