This session seeks to provide an integrated understanding of subduction zone processes, combining insights from global subduction zones with a detailed focus on the western margin of South America, one of the Earth's most significant subduction systems. While advancing a broad framework for the initiation, evolution, and dynamics of subduction zones globally, this session will also highlight distinctive features of the South American margin, such as its accretionary orogen, occurrences of flat-slab subduction, and its history of major seismic events. This presents an opportunity to examine a variety of processes, such as the generation and migration of volatiles and melts, seismic activity, magmatism, and crustal deformation.
We invite contributions from disciplines including geodynamics, geochemistry, petrology, volcanology, seismology, and geophysics to discuss subduction zone dynamics at all scales from the surface to the lower mantle, and in applications to natural laboratories, especially in relation to the Andes. With its broad focus, this session aims to foster interactions across traditional disciplinary boundaries.
The process of subduction initiation is still debated and caused a great deal of controversy such as: Can a subduction zone initiate without any external forcing? Is the thicker and more buoyant lithosphere really the more likely to subduct? To try to answer these questions, a database of 70 cases of Cenozoic subduction zone initiation was built and analyzed in 2021. We find that initiation of subduction zone succeeded in reaching the mature stage for 72% of the cases, usually in less than ∼15 Myr, and that compositional heterogeneities are essential to localize convergence. Interestingly, we observe that the plate age offset when convergence starts is very low (close to zero) in half the cases; otherwise the incipient downgoing plate is as often the younger lithosphere as the older one, and that it could have any age. This indicates that the buoyancy contrast does not determine the subduction zone polarity.
We then build a numerical experimental setup to try to explain this observation. We consider the simple set-up of an oceanic transform fault (TF) or a fracture zone and perform 2D thermomechanical simulation, by combining a non-Newtonian ductile and a pseudo-brittle rheologies. We carry out three different and complementary studies.
We first study the feasibility of ’spontaneous’ subduction initiation, i.e., gravitational collapse of the older lithosphere, at a TF. Simulations show that the main mechanical parameters have to be tuned to quite extreme values to trigger the old lithosphere collapse. The comparison to the geological records of the 3 most likely candidates of ’spontaneous’ subduction initiation (Izu-Bonin-Mariana, Yap, and Matthew & Hunter) leads us to conclude that this mode of initiation at a TF is unlikely in modern Earth conditions.
Second, we simulate normal convergence symmetrically imposed on the two oceanic plates forming the TF to study the ’forced’ mode of subduction initiation. Surprisingly, the range of conditions leading to the older plate subduction is quite limited, whereas the subduction of the younger plate is much more frequently simulated. We find that the success of initiation, as well as the subduction zone polarity strongly depends on the plate age pair and on the initial structure of the TF. The rheological properties and the plate ability to be deformed and sheared may be the first order parameters controlling the subduction initiation mode for rather stiff lithospheres. The model predictions are in good agreement with different Cenozoic records (Gagua, Mussau, and Hjort).
However, this modeling forecasts that the older plate subduction cannot occur at a typical TF for large plate age offsets, in disagreement with what is observed at Izu-Bonin-Mariana, Matthew & Hunter or Palau. We note that, in these cases, a thicker crust made of continental or oceanic terranes (fossil arc or plateau) was always present near the inter-plate domain when convergence started. The third numerical study investigates how such a lithologic ‘raft’ might affect subduction initiation for high plate age contrasts. We find that the raft dimensions and location basically control the under-thrusting of the older and thicker plate.
How to cite: Arcay, D. and Lallemand, S.: Conditions for subduction zone initiation in present-day Earth in the light of Cenozoic examples and numerical simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3662, https://doi.org/10.5194/egusphere-egu25-3662, 2025.
Within the last two-million years, subduction has initiated at the southern end of the New Hebrides trench along the ~E-W trending Matthew-Hunter section of the trench (Patriat et al., 2015; 2019). This part of the subduction system originated as a subduction-transform edge propagator (STEP) fault, a transcurrent plate boundary that terminates Australian plate subduction at the southern end of the New Hebrides trench at a slab tear and allows its rapid southwestward rollback (Govers and Wortel, 2005). The down warped torn lithospheric edge of the STEP fault dips northward in the same direction as the absolute plate motion of the Australian plate in a hotspot reference frame. This creates a strong southward mantle flow (~55 km/Myr) against the already failed and weak northward dipping STEP fault, promoting further down bending and subduction. Through this mechanism, subduction and southward rollback of the STEP fault edge has begun, initiating a subduction zone in an extensional stress regime without requiring initial convergence between the Australian plate and the North Fiji Basin. In fact, the North Fiji Basin is in extension, forming rifts and spreading centers and volcanically accreting crust unusually close to the Matthew-Hunter trench. Subduction initiation at the Matthew-Hunter trench has effectively terminated the STEP fault and slab tear, so that subduction now takes place continuously around the corner from the New Hebrides to the Matthew-Hunter section of the trench. This model proposes that STEP faults are favorable tectonic boundaries for subduction initiation, provided that mantle flow induced by absolute plate motion is oriented correctly, as shown by the opposing example of the Tonga step fault, which displays no evidence of initiating subduction despite a much larger lithospheric age contrast (Martinez, 2024).
Govers, R., and M. J. R. Wortel (2005), Lithosphere tearing at STEP faults: Response to edges of subduction zones, Earth and Planetary Science Letters, 236, 505-523.
Martinez, F. (2024), Subduction initiation (or not) due to absolute plate motion at STEP faults: The New Hebrides vs. the Tonga examples, in EGU General Assembly 2024, Vienna, Austria, https://doi.org/10.5194/egusphere-egu24-4189
Patriat, M., et al. (2015), Propagation of back-arc extension into the arc lithosphere in the southern New Hebrides volcanic arc, G-Cubed, 16(9), 3142-3159.
Patriat, M., et al. (2019), Subduction initiation terranes exposed at the front of a 2 Ma volcanically-active subduction zone, Earth and Planetary Science Letters, 508, 30-40.
How to cite: Martinez, F.: Subduction initiation at the New Hebrides STEP fault induced by absolute plate motion and mantle flow, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13469, https://doi.org/10.5194/egusphere-egu25-13469, 2025.
By far the largest plate-circuit misfit on the planet for geologically current plate motion is that of the Cocos, Nazca, and Pacific plates. This plate motion circuit fails closure by a linear velocity of 12 mm a–1 ±4 mm a–1 (DeMets et al., 2010, Zhang et al., 2017). Here we investigate this nonclosure. In an initial test, we omit the spreading rates along the Cocos-Pacific plate boundary north of the Orozco transform fault where it appears that the Pacific and Rivera plates are separated by a diffuse boundary. With this omission, the non-closure linear velocity shrinks to 9 mm a–1 ±4 mm a–1 (95% confidence limits) with a non-closure angular velocity of 0.22° Ma–1 (± 0.12° Ma-1; 95% confidence limits) about a pole at 22°N, 92°W. The size of the misfit remains too large to be explained by any known processes of intraplate deformation and suggests that there is an unrecognized plate boundary somewhere in the circuit.
We argue that undiscovered plate boundaries (or intraplate deformation large enough to explain the observed non-closure) within the Pacific plate and most of the Nazca plate are implausible, which leaves either a boundary within the traditionally defined Cocos plate or possibly a boundary within the northeast corner of the currently defined Nazca plate. If the spreading rates and transform faults along the traditionally defined Cocos-Nazca plate boundary east of ≈87°W are eliminated from the Cocos-Nazca data set, the non-closure velocity is reduced to 3 mm a–1 ±4 mm a–1 (95% confidence limits), small enough to be within uncertainty or to be explained by expected horizontal thermal contraction.
This result indicates that the traditionally defined Cocos-Nazca plate boundary east of ≈87⁰W may not record motion between the Cocos and Nazca plate after all, but instead records motion between a small previously unrecognized plate and either the Cocos or Nazca plate. The distribution of earthquakes suggests that the better candidate is a small plate within the traditionally defined Cocos plate. We propose to call this hypothesized plate the Kahlo plate. A possible location for a hypothesized narrow plate boundary and an alternative hypothesized diffuse plate boundary will be presented and discussed.
How to cite: Gordon, R., Zhang, T., and Wang, C.: What Causes the Non-closure of the Cocos-Nazca-Pacific Plate Motion Circuit?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14301, https://doi.org/10.5194/egusphere-egu25-14301, 2025.
The subduction of the Nazca plate beneath the South American plate in Northern Chile offers a unique opportunity to investigate processes associated with intermediate-depth intraslab seismicity. Microseismic catalogs (e.g., Sippl et al., 2023) have revealed a downdip transition from a well-defined double seismic zone to a ~30 km thick seismogenic volume where the distinction between the upper and lower seismic planes vanishes near 80 km depth. Understanding the underlying mechanisms of these phenomena can shed light on the factors and processes driving intermediate-depth seismicity. Seismic wavespeeds can provide insights into the state of the downgoing lithosphere, in terms of petrology, fluid distribution, and phase transitions. In order to investigate these factors, we conducted a high-resolution local earthquake tomography study to obtain a detailed seismic velocity distribution of the downgoing slab, using 14 years of travel-time data.
We selected a study area between 20.4°S–22.5°S and 68.0°W–70.0°W, particularly focusing on the seismogenic volume. Events from this region were considered down to depths of 200 km. The dataset includes 18,426 events recorded by 190 seismic stations, with 293,846 P-wave and 83,900 S-wave arrivals from 2007 to 2021. Data were sourced from the IPOC network (Sippl et al., 2023), augmented by additional picks from temporary networks generated using EQTransformer on 60-second time windows starting at each event’s origin time. Event selection prioritized spatial homogeneity and data quality, employing declustering techniques to ensure a balanced distribution. Tomographic inversion is performed using the SIMUL23 algorithm, and checkerboard tests with different grid sizes are used to check the reliable sizes of anomaly as seen in the tomography results, in different parts of the study area. Ray coverage maps and synthetic resolution tests validate the robustness and interpretability of our results.
Our 3D velocity models reveal a number of P- and S-wave as well as Vp/Vs anomalies across and above the Nazca slab. Most prominently, we retrieve low P-wavespeeds and significantly elevated Vp/Vs in the uppermost slab as well as in the overlying mantle wedge, which indicate the presence of fluids or melt in these areas. In contrast, the deeper portions of the downgoing slab feature high Vp and low Vp/Vs. We will present a detailed description of the retrieved anomalies, as well as their tentative interpretation in terms of petrology and fluid processes.
How to cite: Hassan, N. and Sippl, C.: Visualizing Dehydration Processes with High-Resolution Local Earthquake Tomography of the Nazca Slab in Northern Chile, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8499, https://doi.org/10.5194/egusphere-egu25-8499, 2025.
Oceanic subduction zone is the dominant (if not the only) pathway for transporting carbon into the interior of the Earth, and thus plays a critical role in deep carbon cycling. Several mechanisms have been proposed for slab decarbonation process, with two primary ones being metamorphic decarbonation and carbonate dissolution. The metamorphic decarbonation has been widely analyzed by numerical models in the closed system (i.e., with constant water content). However, the water and carbon evolutions in subduction zone are strongly coupled together, leading to an open system in which the water cycling not only affects the metamorphic decarbonation, but also controls the dissolution of carbonates. However, the decarbonation efficiency and the contributions of different decarbonation mechanisms to slab carbon removal remain controversial. Here, we develop a coupled thermo-metamorphic-dissolution model to investigate physicochemical decarbonation processes. Systematic numerical models with variable thermal parameters (Φ = slab age × subduction velocity / 100) have been conducted in both closed and open systems. The results indicate that the metamorphic carbon outflux in open system is lower than that in closed system, whereas the dissolved carbon outflux in open system is approximately three times higher due to fluid infiltration. Moreover, the metamorphic carbon outflux decreases exponentially with Φ in both closed and open systems. In contrast, the dissolved carbon outflux exhibits a nearly linear increase with Φ < 13 km, followed by an exponential decrease with Φ ≥ 13 km. The new models provide systematic and quantitative constraints for the deep carbon cycling in subduction zones.
How to cite: Wang, Y., Li, Z., and Zhang, H.: Quantification of thermally-controlled metamorphic decarbonation and carbonate dissolution in subduction zones, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14427, https://doi.org/10.5194/egusphere-egu25-14427, 2025.
Geodynamic models suggest that plate boundary shear zones require mechanically weak materials to form. However, peridotites in proto-plate boundary hanging walls are inherently strong and experience cooling from >1000°C to <500°C over ~10 Myr during subduction initiation. Without a micro-physical or metasomatic mechanism to weaken the olivine-rich mantle, it will resist strain localization with cooling. Serpentinites are often credited with facilitating lithosphere-scale strain localization, but proto-interface temperatures exceed ~550°C at 20–30 km depth, and therefore are too hot for serpentine to be stable. What, therefore, are the roles of both olivine and serpentine in plate boundary formation?
To address this, we present structural and geochemical data from a fossilized subduction interface at Mont Albert (Québec, Canada). This Ordovician ophiolite records subduction initiation and subsequent obduction during the Taconian Orogeny (~450–500 Ma). Field and microstructural observations show that spinel peridotites in distributed shear zones evolved from mylonitic to ultramylonitic fabrics under increasingly hydrous conditions toward the paleo-plate contact. Olivine Crystallographic Preferred Orientation (CPO) transitions from A- and D-type fabrics in mylonites to weaker AG- and B-type fabrics in ultramylonites, accompanied by grain size reduction from ~60–80 μm to ≤20 μm, and phase mixing of olivine-orthopyroxene metasomatic layers. These transitions are consistent with a mechanical switch from dislocation creep to diffusion-accommodated creep, with sustained grain size reduction through phase mixing and growth of hydrous phases such as chlorite and amphibole.
At the paleo-plate contact, a ~10–20 m thick zone of ultramylonites is heavily serpentinized (75–90%). This zone contains finely layered, well-aligned lizardite (confirmed with Raman spectroscopy), Fe-oxides (hematite and magnetite), and relict olivine ± orthopyroxene, amphibole layers. No antigorite was identified. We interpret serpentinization as largely static and post-kinematic with respect to the incredibly strong fabrics in contact ultramylonites, supported by observations of undeformed lizardite mesh textures and hematite-decorated grain boundaries in coarser lizardite aggregates.
Bulk rock geochemical analyses along a 40 m transect in the hanging wall of the paleo-plate boundary reveal mantle Al2O3 (wt%), chondrite-normalized [Yb], and HREE concentrations all decrease systematically with distance from the contact, highlighting pimary compositional layering. Ce, Sr, and Pb show subtle enrichment at the contact where rocks are most heavily serpentinized. However, LREE and other fluid-mobile element distributions are highly variable, suggesting limited chemical overprinting associated with the serpentinizing fluid.
Our findings suggest that high-temperature ductile deformation initially localized due to hydrous phase introduction, facilitating deformation near the paleo-plate contact despite cooling conditions through shifts in deformation mechanisms. Based on the chemical data and the micro-textural observations of static lizardite, we infer that plate boundary serpentinization was late-stage and occurred under very low-temperatures (<300°C) from a highly oxidizing fluid. Serpentinites therefore did not aid strain localization or obduction but instead formed post-kinematically, locking the shear zone and forcing obduction-related strain to migrate elsewhere.
How to cite: Kotowski, A., Keats, A., Smit, H., van Broekhoven, J., Tarling, M., Godard, M., Plümper, O., Drury, M., and Hellebrand, E.: Mechanical and metasomatic evolution of a developing mantle wedge from subduction initiation to obduction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13559, https://doi.org/10.5194/egusphere-egu25-13559, 2025.
The Ecuadorian Cenozoic arc developed upon autochthonous continental and allochtonous accreted oceanic terranes. It provides a unique opportunity to explore the processes governing arc magmatism and crustal evolution. Using a multi-proxy approach, combining zircon petrochronology (U-Pb geochronology, trace element geochemistry, and isotopic analysis) with whole-rock geochemistry, we trace the tectono-magmatic evolution of the northernmost segment of this arc.
Our results define two distinct magmatic episodes: ~41–16 Ma and ~14–7 Ma. The older episode comprises tonalitic rocks exhibiting zircon δ18O (6.4 – 3‰) and εHf values (+17 – +12), as well as trace element ratios, indicating derivation from juvenile sources. Magma genesis during this period is believed to occurred within the amphibole stability field, in a moderately thick crust (~35 km). In contrast, the younger episode is dominated by granodioritic rocks derived from more enriched reservoirs (δ18O: 8.2 – 5.8‰ and εHf: +13 – +7). Zircon and whole-rock trace element and isotopic data suggest magma genesis in the garnet stability field, within a thickened crust (~60 km).
The transition to a thicker crust and enriched sources occurred around 14 Ma, coinciding with the tectonic reorganization associated with the arrival of the young (and buoyant) Nazca plate at the South American margin. This event likely induced a shallower subduction angle, increased compressional stresses, and facilitated melting of an evolved oceanic crust.
These findings highlight the dynamic interplay between tectonics, crustal processes, and magmatic evolution in shaping Cordilleran arcs. They also demonstrate the efficacy of zircon petrochronology as a tool for resolving crustal-scale processes, providing insights into the mechanisms driving continental growth and orogenesis.
How to cite: Iglesias Flores, J., Witt, C., Poma, O., Bruguier, O., Bosch, D., Bosse, V., Zattin, M., Seyler, M., Hernandez, M. J., Chanier, F., and Averbuch, O.: Geochronology and geochemistry of Cenozoic magmatic intrusions in the north-western Ecuadorian Andes: the role of crustal thickness, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13936, https://doi.org/10.5194/egusphere-egu25-13936, 2025.
Continental subduction beneath an overriding oceanic plate is known to occur in nature, following the arrival of a continental margin at an intra-oceanic subduction zone, and often implying synthetic (i.e., Tethyan type) obduction. However, the main geodynamic constraints and geological/geophysical parameters governing this process, its viability and likelihood, are still not fully understood.
In the present work, we use 2D geodynamic numerical modelling to specifically investigate the geodynamic causes that might determine the amplitude of the subduction-exhumation (time-depth) cycle, as well as the viability of ophiolite emplacement and associated inward continental reach of ophiolitic nappes.
Using the finite-element code Underworld (Moresi et al., 2007) we constructed a 2D model with top free surface boundary conditions (to account for obduction-related topography build-up), lateral periodic boundary conditions (to compensate for the absence of asthenospheric mantle toroidal flow), and no-slip basal boundary conditions (to simulate subducting slab anchoring at the upper-lower mantle discontinuity when the slab reaches this interface at 660 km depth).
All simulations considered an initial intra-oceanic subduction zone, in which the subducting plate is linked to a trailing continental segment that eventually arrives at the subduction trench. To evaluate the geodynamic viability and efficiency of subsequent continental subduction and ophiolite emplacement, we used buoyancy driven models (i.e., without any externally imposed velocity boundary conditions), and investigated the following variable parameters: existence vs. absence of a weak (serpentinized) crustal layer in the overriding plate; variable age of the oceanic overriding plate (10, 20 and 60 Myrs) vs. a constant 70 Myrs subducting plate; different length of the oceanic segment of the subducting plate; and fixed vs. free subducting plate trailing edge boundary conditions.
Our preliminary results reveal a clear facilitation of ophiolite emplacement by the considered weak (serpentinite) crustal layer (in the overriding plate). Also, younger, less dense, and relatively weaker, overriding plates are shown to likewise favour more efficient obduction, including ophiolitic nappe allochthonous transport and formation of ophiolitic thrust windows and klippen. Finally, a higher length of the oceanic segment of the subducting plate and fixed trailing edge boundary conditions are shown to better comply with the geodynamic requirements assisting efficient, more realistic, amplitude subduction-exhumation cycles during continental subduction, as well as associated ophiolite obduction processes.
Acknowledgements:
This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UID/50019/2025 and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020).
References:
Moresi, L., Quenette, S., Lemiale, V., Mériaux, C., Appelbe, B., & Hans-Bernd Mühlhaus (2007). Computational approaches to studying non-linear dynamics of the crust and mantle. Physics of the Earth and Planetary Interiors, 163 (1), 69-82. (Computational Challenges in the Earth Sciences) doi: 488 10.1016/j.pepi.2007.06.009
How to cite: Rosas, F., Gomes, A., Schellart, W., Nicolas, R., Duarte, J., and Almeida, J.: 2D Numerical modelling of continental subduction and synthetic obduction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10088, https://doi.org/10.5194/egusphere-egu25-10088, 2025.
The present study investigates the tectonic evolution of the western extremity of Chhotanagpur Gneissic Complex (CGC), within the eastern part of E-W trending Central Indian Tectonic Zone (CITZ). The study has been done along a N-S stretch (mostly within Chhattisgarh, India) extending from Sanawal in the north to Pali in the South. The reportedly Meso- to Neo-Proterozoic litho-package of the area, comprise metasedimentaries, metavolcanics and younger granite gneiss-granitoids, is mapped in details and records five episodes of deformation (named D1 to D5). The D1 is manifested by rarely preserved recumbent Class-2 folds (F1), while D2 is represented by E-W trending, low-plunging, upright to inclined and Class-1A, 1B, 1C geometry F2 folds. The D3 exhibits ESE-WNW trending, reclined to near vertical, Class-2 geometry F3 folds and D4 is brittle-ductile shear zone. D5 related F5 is a N-S cross warping. We prove that the first (D1), third (D3) and fourth (D4) episodes of deformation in CGC are due to thrust movement, which is manifested by development of shear zones and related folds. In one such northerly dipping thrust zone, named Balangi-Sanawal thrust zone (BSTZ, considered as a splay of the Son-Narmada South Fault, SNSF), enderbite and khondalite (of the Makrohar Granulite belt, MGB) are seen to be present as discrete bodies within the CGC granitoids. These granulite occurrences and adjacent CGC has been geologically mapped, which show that the enderbites of MGB have been thrusted over the D2 related F2 folds developed within the amphibolites of CGC. In the proximal zones of all these shear zones sheared porphyroclastic augen syenogranite is emplaced along numerous narrow channels as lensoidal bodies. These sheared syenogranite, along with the associated alternate amphibolite layers, exhibit D3 related near-vertical F3 folds.
Existing literatures from the southern part of CITZ, reveal granulites (Balaghat-Bhandara granulites, BBG and Chhatuabhavna granulites, CBG) occur with the greenschist-amphibolite facies CGC rocks against southerly dipping Central Indian Shear (CIS) zone along with emplacement of aforementioned syn-tectonic porphyroclastic augen syenogranite during ~1.62-1.42 Ga (Bhowmik et al., 2011). The field evidences suggest that this CIS thrusting event also marks regional D3 episode of shearing in CGC. Glancing through these geological evidences, we correlate the northerly dipping BSTZ in the northern part of CITZ to be at ~1.62-1.42 Ga. In the central part of CITZ, CGC rocks got juxtaposed with Ramakona-Katangi granulites (RKG) along northerly dipping Gavilgarh-Tan shear zone (GTSZ) placed at ~1.04-0.93 Ga (Chattopadhyay et al., 2020). We propose that this GTSZ, represents D4 episode of deformation within the CGC. This has produced a peculiar ‘Ramp and Flat’ geometry with imprints of brittle-ductile shearing in the study area. It is interesting to note that at ~1.62-1.42 Ga, during the D3 deformation phase, both northerly and southerly subduction of the Central Indian block (CIB) (now preserved as CGC) occurred along the northern and southern boundary of CIB respectively. This prompts us to think of a unique either way subduction accompanied by thrusting of deep-crustal granulites through hanging wall block over the Central Indian continental landmass.
How to cite: Ghosh, A., Talukdar, D., Mondal, S., Sen, K., Maity, A., Ghosh, B., and Dasgupta, N.: Meso–Proterozoic tectonic evolution of Chhotanagpur Gneissic Complex (CGC): Existence of an either way subduction within Central Indian Tectonic Zone (CITZ), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12060, https://doi.org/10.5194/egusphere-egu25-12060, 2025.
The southern Central Andes (29°S-39°S) is a key area for understanding the interplay between the oceanic plate and the continental plate and its resulting surface expressions in a subduction zone. In this area, the dip of the oceanic plate changes from normal subduction (~30° between 33°S and 35°S) in the south to flat subduction (< 5° between 29° and 33°S) in the north. This region displays remarkable along- and across- strike variations in both tectonic and seismic deformation patterns. In this context, the relative contributions of each plate on the localization of the long- and short-term deformation along the mountain belt and its neighbouring regions have been a matter of long-standing debate. To address this issue, we investigated the relative contribution of various key factors to strain localization in the Southern Central Andes, including compositional and thickness variations in the upper plate, sedimentary basins, surface topography, frictional strength of the subduction interface and changes in the dip geometry of the lower plate. Using multiple geophysical approaches and data sources, we have built a series of structural, density, thermal, rheological and integrated them in a thermomechanical geodynamic model to quantify the relative importance of these key factors to strain localization at tectonic and seismic timescales. This forward data-driven modelling approach allows us to reconcile long- and short-term deformation as close as possible with geophysical and geological measurements.
We found that the compositional and thickness configuration of the upper plate, weak inherited faults associated with weak sediments, topography and thickness of the radiogenic crust plays a prominent role in modulating strain location between the flat and steep subduction segments. The flat slab in the northern part of the region, cools and further strengthens the upper plate, preventing the plate from pronounced deformation and propagating the deformation far inland to the eastern edge of the broken foreland. A complex broad shear zone developed at the transition between flat to steep subduction which is associated to the development of a thick to thin skinned foreland deformation style transition at the surface. In addition, the strength of the upper plate ultimately controls the spatial distribution of the short-term deformation occurs above the modelled transition from brittle to ductile conditions and seismicity is localised in regions at the transition between rigid and weak lithospheric blocks, such as the front of the forearc, which acts as a rigid indenter. These results highlight the importance of considering the interactions between the upper and lower plate to better understand multiscale scale deformation processes in subduction zones and their resulting surface expression.
How to cite: Rodriguez Piceda, C., Pons, M., Scheck-Wenderoth, M., Cacace, M., Bott, J., and Strecker, M.: Contributions of plate strength and dip geometry on the localization of deformation in Central Andes: a data-driven modelling approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5759, https://doi.org/10.5194/egusphere-egu25-5759, 2025.
Flat or near-horizontal subduction of oceanic lithosphere is suggested to occur for ~10% of Earth’s subduction zones. While it is therefore not the dominating geometry, it has been suggested to have significant impact on tectonic processes both currently and in the geologic past. As an example, the ongoing subduction of the aseismic Nazca Ridge beneath South America has been associated with the onset of flat subduction and the termination of arc volcanism in Peru.
In this study, we investigate the impact of flat-slab subduction on the mantle flow and deformation in the larger asthenosphere-lithosphere system beneath the northern portion of the South American subduction zone. Strain in the asthenospheric and lithospheric mantle causes an alignment of intrinsically anisotropic mantle minerals, particularly olivine. The resulting bulk anisotropy can be measured as splitting of core-mantle converted phases, parameterized by the delay time and the fast splitting direction. While shear phases are commonly investigated for average splitting parameters, the tomographic inversion of shear wave splitting data for upper mantle anisotropy has been a longstanding challenge for classical analysis techniques. Recent developments involve the calculation of finite-frequency sensitivity kernels for SKS splitting intensity observations, which allow us to take advantage of overlapping sensitivity kernels at adjacent stations to localize anisotropic structure at depth.
Here we apply probabilistic, finite-frequency SKS splitting intensity tomography to all available datasets across the Andes in Peru and Bolivia to improve our understanding of mantle flow and deformation in the lithosphere in the complex flat slab subduction scenario. While the data sets are mostly comprised of dense lines of seismic stations, the broad lateral distribution of the different networks allows us to combine the data set in a 3D tomographic inversion for upper mantle anisotropy.
How to cite: Link, F. and Long, M. D.: Lithospheric deformation and Mantle flow in the asthenosphere-lithosphere system of the flat slab subduction beneath the Peruvian Andes with probabilistic finite-frequency SKS splitting intensity tomography, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20645, https://doi.org/10.5194/egusphere-egu25-20645, 2025.
The subducted Nazca plate flattens at ~100 km depth beneath the south-central Andes (29˚S-32˚S), known as the Pampean flat slab. This flat slab results in a volcanic gap, active basement-cored uplifts of the overriding South American Plate, and a high rate of crustal seismicity at depths below 50 km distinct from the subducting slab seismicity. Previous studies suggest extensive slab dehydration and partial eclogitization of the overriding plate’s lower crust. In this study, we analyze seismic data recorded by the SIMEBRA and ESP projects (deployed between 2007 and 2010) with a modified spectral decomposition method to constrain the source spectra of hundreds of earthquakes in the study area. We further estimate the earthquakes’ corner frequencies and stress drops, revealing similar stress drops between the slab and overriding-plate events, with little depth dependence in the region. In addition, we observe low stress drops in the slab at ~150 km depth, coinciding with a low-velocity and high Vp/Vs zone, potentially indicating slab dehydration.
How to cite: Zhang, Y., Wei, S. S., Tian, D., Linkimer, L., and Beck, S.: Characterizing Source Spectra and Stress Drops of Earthquakes in the Pampean Flat Slab and Overriding Plate , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20505, https://doi.org/10.5194/egusphere-egu25-20505, 2025.
Slab geometry and structures are critical to understanding subduction processes, regional tectonics, and arc volcanism. Located at the convergent plate boundary between the Cocos, Nazca, and Caribbean plates (locally the Panama microplate), the Costa Rica subduction zone is featured by the aseismic subduction of the Cocos Ridge initiated at ~2-3 Ma, and the spatial coincidence with the arc volcanoes that ceased activities since ~5-8 Ma and the uplift of the Talamanca Mountain since ~3 Ma. These phenomena were interpreted by the flat subduction of the Cocos Ridge that has a thick ocean crust. However, this interpretation has been challenged recently by geophysical imaging, which suggests alternative models involving the steep Cocos slab, the doubly convergent Caribbean plate, and the stagnant Nazca slab fragment, leaving the dominant factor driving the regional tectonics enigmatic.
Here, we propose a new teleseismic receiver function (RF) method, Dip Direction Searching Plus (DDS+), to detect weak RF signals associated with dipping interfaces. DDS+ estimates dip directions by fitting the back-azimuthal variations in both radial and transverse RFs. Applying DDS+ to teleseismic data recorded by 17 broadband seismic stations across Costa Rica, we identify positive RF phases with clear back azimuthal variations, indicating dipping interfaces with dip directions of ~N8˚-57˚E (±12˚ on average) beneath 11 stations. The dip direction estimates are consistent to the Cocos Slab2 model. The estimated depths of these interfaces (~13-113 km; ±2.8 km on average) align with the Cocos Slab2 model and the intra-slab seismicity, suggesting the phase are probably Ps conversions from the Moho of the Cocos plate. While the Cocos Moho extends to the depth of ~110 km beneath the northern Talamanca Mountain, it is absent at stations to the south where the slab is expected to subduct beyond 50 km depths. Additionally, we observe a mysterious positive RF phase indicating an interface at ~40-60 km depths in the mantle. This phase was interpreted as either the subducting Caribbean plate Moho (southwestern dipping) or the stagnant Nazca plate Moho (flat) beneath the Talamanca Mountain. Our result reveals no prominent dipping features for this phase, therefore favoring the stagnant Nazca plate Moho interpretation.
Different from previous studies debating continuously flat or steep Cocos subduction, our analysis indicates a steeply dipping Cocos slab to the north and a flat (or truncated) geometry to the south. Therefore, the flat Cocos subduction model cannot explain the volcanic cessation and Talamanca uplift across the entire region. Instead, we propose that the stagnant Nazca slab fragment plays a key role in barricading mantle magma upwelling and thus ceases the arc volcanism. Our study provides new insights into the slab geometry and structures within the Costa Rica subduction zone and the dominant factor shaping the orogenesis and volcanism.
How to cite: Feng, M. and Wei, S.: Distinct lateral slab geometry and structures in the Costa Rica subduction zone revealed by teleseismic receiver functions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3978, https://doi.org/10.5194/egusphere-egu25-3978, 2025.
In this study, we assemble body wave arrival times from earthquakes occurring in the central Chile between 2014 and 2019, and Rayleigh wave phase velocity maps at periods of 5-80 s from ambient noise Empirical Green's functions in Chile. By jointly using body wave arrival times and surface wave dispersion data, we refine the Vs model and improve earthquake locations in central Chile. Compared to other velocity models in the region that are determined by individual data type, our joint inversion Vs model shows better consistency with the intraslab seismicity distribution as well as the Moho and slab interfaces. Our Vs model clearly images an eastward dipping high velocity band of 40-50 km thick, corresponding well to the thickness of the Nazca plate estimated by receiver function imaging and thermal modelling.
Overall, the intraslab seismicity distribution spatially correlates well with the slab high velocity anomalies except for along the subduction paths of the Copiapó Ridge and Juan Fernández Ridge. Additionally, parallel low-velocity stripes are imaged beneath the subducting plate, which are likely associated with the accumulated melts. The joint inversion velocity model also resolves widespread low-velocity anomalies in the crust beneath the Central Volcanic Zone of the central Andes, likely representing crustal magma chambers for various volcanoes.
How to cite: Chen, Z., Zhang, H., Gao, L., Yang, S., Liu, Y., and Comte, D.: Joint inversion of subduction zone velocity structure of central Chile by body wave arrival times and surface wave dispersion data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10496, https://doi.org/10.5194/egusphere-egu25-10496, 2025.
Understanding the inner structure of the crust and upper mantle is essential to evaluate those mechanisms driving Earth’s dynamics. Usually, surface deformation provides valuable constraints on viscoelastic parameters. Postseismic deformation following large megathrust earthquakes, offers a unique opportunity to explore the viscoelastic properties of the shallower earth structure since it is strongly influenced by viscoelastic relaxation processes. This postseismic deformation is often recorded by GNSS stations, which offer high temporal resolution and therefore are useful to constrain the relaxation time along convergent margins. However, the spatial coverage of GNSS networks is often sparse, inhibiting our ability to study the large scale variations in viscoelastic properties of the medium.
To solve these issues, we rely on InSAR time series which provide continuous spatial resolution of surface deformation. In this work, we exploit the FLATSIM project (Thollard et al., 2021) initiative considering Sentinel-1 data over Central Chile that has been processed using the NSBAS processing chain (Doin et al., 2013). Particularly, we focus on Central Chile, with special emphasis on the 2015 8.3 Mw Illapel earthquake. The InSAR data spans 8 years and has been corrected using the global atmospheric models ERA-5. Complementary, we use GNSS time series from 25 stations deployed over the Illapel rupture area, combining stations from Centro Sismologico Nacional and the DeepTrigger project.
Since both data sets contain the contribution from multiple tectonic and non-tectonic processes, we employ different techniques to isolate the postseismic deformation of the 2015 Illapel earthquake. Actually, for GNSS, we apply Independent Component Analysis while for InSAR time series, we perform a parametric decomposition pixel by pixel. Our findings reveal a very strong postseismic signal with a typical logarithmic decay, lasting at least 8 years.
In this work, in order to investigate the underlying rheological properties of the medium, we exploit the PyLith software, a finite-element model that can take into account the complex rheological structure of the system. To do so, we impose the co-seismic slip model coming from averaged slip solutions, thereby initiating the model to distinguish between viscoelastic and afterslip contributions. By reproducing the surface deformation patterns given jointly by GNSS and InSAR data, we aim to determine the geometrical and rheological variations beneath the Illapel rupture area, particularly those viscoelastic parameters characterizing the crust and upper mantle regions. Our analysis provide insights to better understand how these properties affect both the seismic cycle and long-term deformation patterns at local and regional scales.
How to cite: Molina, D., Lovery, B., Radiguet, M., Doin, M.-P., and Socquet, A.: Rheological insights from Illapel postseismic deformation through GNSS and InSAR time series analysis , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18961, https://doi.org/10.5194/egusphere-egu25-18961, 2025.
The South American margin, where the Nazca Plate subducts below the South American plate is a highly seismogenic region. In recent decades, it has been the focus of abundant studies with the deployment of temporal and permanent seismic networks. These efforts have generated large datasets that are challenging to process with traditional methods. To take advantage of the large volume of data available, we pair modern machine learning picking and association methods with traditional location and relocation techniques to create a dense, high resolution seismic catalogue. We process data from the CSN, OVDAS and other smaller permanent networks between 2017 and 2021 to obtain over 650.000 double-difference relocated events, at least 10 times more than any other regional catalogue in our study area. This implies at least a 1 order of magnitude reduction of the magnitude of completeness (Mc).
Our catalogue is designed to ensure temporal consistency (i.e. the selected stations are active for most or all of the study period) and the processing workflow is the same for the whole region (in contrast to joining catalogues resulting from independent local or regional networks). This consistency paired with the catalogue’s high resolution, allows us to observe spatial and temporal variations in seismicity and to improve our understanding of processes that may be studied through micro-seismicity. One application of such a catalogue is the observation that, although the Chilean subduction zone is known for its megathrust earthquakes, intraslab events make up the bulk of seismicity (>80% of the events), with two particularly active clusters. One is located in northern Chile, inland of the subduction of the Iquique Ridge, at an “unusual subduction segment” documented by Sippl et al. (2018) and another inland of the subduction of the Juan Fernandez Ridge. Furthermore, the slab and plate interface are most active in northern Chile an seismicity diminishes towards the south, especial from 36°S. This decrease in seismicity is likely related to changes in Nazca Plate age and temperature and/or to the influence of 1960 Valdivia and 2010 Maule mega-earthquakes.
How to cite: Riedel-Hornig, M., Sippl, C., Tassara, A., Ruiz, S., Potin, B., Puente, J., Morales, C., Carcamo, F., and Castro, C.: Seismicity of the south-western South American margin through a machine learning automated approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14177, https://doi.org/10.5194/egusphere-egu25-14177, 2025.
The Atacama segment in Northern Chile (24⁰S to 31⁰S) is a mature seismic gap with no major event (Mw≥8) since 1922. Nonetheless, the region regularly releases stress through shallow and deep slow slip events, and hosts recurring seismic swarm activity. To investigate this seismic gap and its complex seismic-aseismic behaviour, we instrumented the region with almost 200 seismic and geodetic stations between November 2020 and February 2024. Using machine learning techniques, we derived a dense, high-resolution seismicity catalog, encompassing over 165,000 events with double-difference relocated hypocenters. Within the network, we achieve relative location uncertainties below 50 m, enabling the resolution of fine-scale structures. Our catalog details the outer rise, interface, intraplate and upper plate seismicity. Furthermore, we capture anthropogenic sources from mine blasting and offshore active seismic experiments. Here, we focus on three aspects:
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The new slab geometry and it’s influence on the large scale seismic segmentation
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The fine scale space-time segmentation of the subduction interface
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The complex seismic swarms around the 2023 shallow slow slip event in Copiapó, highlighting in detail the underlying mechanisms of slow-to-fast earthquake interaction
Our results provide a holistic view of this complex subduction zone, while at the same time giving insights into fine-scale structures and processes.
How to cite: Münchmeyer, J., Molina, D., Marsan, D., Langlais, M., Baez, J.-C., Heit, B., González-Vidal, D., Moreno, M., Tilmann, F., Lange, D., and Socquet, A.: Characterising the Northern Chile subduction zone (24⁰S - 31⁰S) with > 165,000 earthquakes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11906, https://doi.org/10.5194/egusphere-egu25-11906, 2025.
The serpentinized mantle wedge corner above subducting slabs has been long considered as mostly aseismic. However, mantle wedge seismicity (MWS) has been observed in different subduction zones such as Japan, New Zealand, Lesser Antilles, South America, Colombiaand the Hellenic subduction zone. Several hypothesis have been made to explain such seismicity: (i) a graveyard of piled seamounts that are detached from the subducting plate and underplates the forearc lithosphere; (ii) plume underplating leading to higher viscosity parts in the mantle corner; (iii) serpentine dehydration embrittlement and (iv) pulses of fluids released from the plate interface. Several subduction zones exhibit seismicity gaps on the interface that might indicate a diversion of the seismicity through vents. The rheological and mechanical behaviors of the mantle wedge and its possible interactions between its seismicity and the interface still remain largely unclear. In this study, we take advantage of the recent catalogue obtained with machine learning on Chile from November 2020 to February 2024 and analyse the spatio-temporal distribution and the statistics of the mantle wedge seismicity in this area.
We find that the MWS is mostly active between -27°S and 31°S latitude and shows a Gutenberg-Richter b-value of 1.4 which is higher than the interface seismicity (around 1). It presents a magnitude of completeness of 1.6 and is gathered as clusters of events that behave as swarms rather than mainshock-aftershocks sequences. Some clusters are triggered after a large event (magnitude > 5) occurring on the interface. The detailed analysis of the distribution of the MWS compared to the interface shows that the MWS is mainly located in a band between 130 and 160km away from the trench while the interface seismicity is mainly located in a band of 60km to 100km away from the trench. While the interface seismicity gap present just above the MWS might confirm the presence of vents that would deviate the seismicity, the difficulty to track potential fluid paths from the intraplate seismicity at depth to the MWS might rule out fluids as the origin of the MWS in Chile. Rather, this gap might indicate the importance of the mineralogical contact between the interface and the mantle wedge.
How to cite: Gardonio, B., Socquet, A., and Münchmeyer, J.: The spatio-temporal behavior of the Mantle Wedge Seismicity and its relationship with the interface in Chile., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12184, https://doi.org/10.5194/egusphere-egu25-12184, 2025.
Posters on site: Fri, 2 May, 14:00–15:45 | Hall X1
The Philippine Sea Plate (PSP) is located in the convergence zone of the Eurasian Plate, the Pacific Plate, and the Indo-Australian Plate, and is almost entirely surrounded by deep-sea trenches. Due to its special tectonic location, influenced by multiple tectonic factors such as plate subduction, seafloor spreading, and mantle plume activity, the Philippine Sea has always been a hot - spot area in the international geoscience community. It is a natural laboratory for reconstructing the plate tectonic pattern and studying the initiation mechanism of plate subduction and other cutting - edge scientific issues of the Earth. It is also the best place to develop and improve the plate tectonic theory.
This study utilized the acquired shallow - layer profile survey data, deep - reflection multi - channel seismic data, and combined with some borehole data from DSDP Leg 59 to comprehensively reveal the sedimentary and tectonic characteristics of several major tectonic units in the study area (the West Philippine Basin, the Kyushu - Palau Ridge, and the Parece Vela Basin), and established an initial geological model of the study area. The study found that in the West Philippine Sea Basin near the Kyushu - Palau Ridge (KPR), there are two sets of sedimentary covers of different origins, upper and lower. The thickness of the lower stratum varies greatly, mostly consisting of volcanic materials, and it continuously thickens in the direction of the KPR. At the foot of the mountain near the KPR, a large set of volcaniclastite aprons has developed. The thickness of the upper sequence is relatively stable, being a set of deep - water fine - grained sediments of the ocean. The crustal thickness of the West Philippine Sea Basin and the Parece Vela Basin is approximately 6-7 km, which is close to the global average oceanic crust thickness. The Moho discontinuity in the West Philippine Sea Basin is in the shape of a gentle fold, undulating basically in sync with the oceanic crust basement beneath the sediments. The depth of the Moho discontinuity shows a trend of gradually rising towards the spreading ridge of the central sea basin. Both the seismic profiles and drillings in the West Philippine Sea Basin have revealed a tectonic compression event during the Eocene period. The initiation of subduction along the ancient IBM might be an induced subduction caused by the far - field effect of the India - Asia collision. The subduction process was accompanied by lateral propagation and a continuous compressional stress field, until the island - arc rifting that started around 30 Ma.
Keywords: tectonic–sedimentary features; subduction initiation; seismic reflection; Philippine Sea Plate
How to cite: Qin, K.: Tectonic and Sedimentary Characteristics of the Philippine Sea Plate and the Initiation of Subduction: A Comprehensive Deep Reflection Seismic Study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7902, https://doi.org/10.5194/egusphere-egu25-7902, 2025.
After large earthquakes, aftershocks are observed globally as a time-dependent phenomenon. In subduction zones, aftershocks occurring in the upper plate are particularly hazardous, as they often take place near densely populated areas, increasing the risk to structures already weakened by the mainshock. The number of aftershocks typically decreases over time, following a pattern described by the empirical Omori-Utsu law. Despite this well-documented behavior, the physical mechanisms driving this decay remain uncertain. While coseismic static stress transfer cannot explain the non-linear time dependence of aftershocks, transient postseismic processes such as afterslip and viscoelastic relaxation have been proposed as possible mechanisms. Alternatively, considering the temporal decay of aftershock sequences and the similar behavior observed in induced seismicity caused by wastewater injection, we explore the hypothesis that pore-pressure diffusion plays a key role in controlling the spatial and temporal distribution of natural earthquake aftershocks.
In this study, we investigate the 2014 Mw 8.2 Iquique event to test our hypothesis, using an approach that integrates geodetic and seismological data, as well as geological, frictional, rheological, and hydraulic constraints. Using a 4D (space and time) modeling approach considering realistic rock material properties, we first reproduce the 3D postseismic deformation time series observed by continuous GNSS stations. We then disaggregate the individual contributions of the three dominant postseismic processes, i.e., afterslip, viscoelastic, and poroelastic relaxation, to the deformation signal. In particular, poroelastic deformation substantially affects the observed vertical geodetic signal in the near field. We then compute and analyze the spatiotemporal stress changes produced by the individual postseismic processes using the Coulomb Failure Stress (CFS) parameter. By comparing these CFS changes to the distribution of upper-plate aftershocks, we find that stress changes produced by pore-pressure changes best correlate in space with increased upper-plate aftershock activity. Furthermore, increased pore pressure reduces the effective fault normal stresses independently of the fault orientation and consequently triggers all faulting styles. This explains the higher diversity of faulting styles observed in upper-plate aftershocks. Finally, we find a very strong temporal correlation (>0.98) between the exponential increase of the cumulative number of upper-plate aftershocks and pore-pressure changes. This finding suggests that the unclear physical basis for Omori-type aftershock decay may relate to the hydraulic properties (e.g., rock permeability and porosity) of the upper plate. Thus, our work offers a deeper understanding of the hydro-mechanical behavior of the upper plate during large earthquakes and may open new avenues for physics-based aftershock forecasting.
How to cite: Peña, C., Heidbach, O., Schurr, B., Metzger, S., Moreno, M., Bedford, J., Oncken, O., and Faccenna, C.: Decoding Upper-Plate Aftershocks: The Critical Role of Pore-Pressure Diffusion following the 2014 Iquique Earthquake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6504, https://doi.org/10.5194/egusphere-egu25-6504, 2025.
We present preliminary results of the analysis of the interseismic coupling at the southern Peru subduction zone, with special focus on the Nazca Ridge and the Nazca fracture zone. This study is based on the analysis of GPS data from over 120 permanent and field GPS stations collected during the last decade. The obtained GPS velocity field shows the current state of interseismic deformation of the Peruvian subduction margin. The inversion of the geodetic displacements allowed us to estimate the interseismic coupling at the plate interface. Our results show that the interseismic coupling is heterogeneous, with two areas of significant low to weak coupling coefficient, one located over the Nazca ridge and the other in front of the Nazca fracture zone. These results are compared with the spatial distribution of the seismicity recorded by the IGP national seismic network and a temporary seismic network installed as part of the project for the period 2022-2024, which accounts for more than 100,000 events. The analysis reveals a remarkable correlation of the areas where high interseismic coupling is observed with lack of seismicity, whereas in the areas with low interseismic coupling intense seismic activity is observed. These results confirm the hypothesis that the Nazca Ridge acts as a persistent barrier against the propagation of earthquake rupture, and suggest that the interseismic coupling patterns could be associate with the seismic activity. This ongoing work provides valuable information for understanding the tectonic processes in the region and their implications for the earthquake potential.
How to cite: Villegas-Lanza, J.-C., Socquet, A., Sanchez-Reyes, H., Chalumeau, C., Lovery, B., and Chlieh, M.: Relation Between Interplate Locking and Microseismicity in the southern Peru subduction , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19402, https://doi.org/10.5194/egusphere-egu25-19402, 2025.
In the last decade, a large network of permanent seismic stations in Northern Chile (Integrated Plate boundary Observatory Chile network – IPOC) has enabled a range of studies that provided constraints on the geometry of the subduction zone in this region. Larger seismicity compilations and tomography studies have led to a better definition of the downgoing slab, and receiver function studies have illuminated the shape of the continental Moho. This calls for an effort to summarize these diverse constraints in a gravity-based 3D model of the region.
We compiled a 3D integrated geophysical model for Northern Chile in the IGMAS+ software based on the Complete Bouguer Anomaly computed at the Calculation Service of the International Centre for Global Earth Models (ICGEM) from the EIGEN-6C4 Global Gravity Field Model. The 3D gravity-based model represents an update of a similar model by Tassara and Echaurren (2012), which we used as an initial constraint of the geometry and physical properties of our model. The plate interface and slab surface geometry is updated based on the most recent IPOC seismic catalog, and offshore active seismic results. Other significant geophysical interfaces, namely the Moho and the lithosphere-asthenosphere boundary (LAB) in both oceanic and continental domains, were constrained by recently published results from receiver functions, active seismics, seismic tomography, as well as joint inversion and isostatic studies. These studies show considerable uncertainty in the geometry of the mantle wedge near the plate interface. To fit the gravity observations, we had to address the tradeoff between assumed geometry and density distribution, which we did by trying out a range of different shapes and petrophysical properties.
This contribution aims to offer a better understanding of the impact of geometry adjustments, namely in the mantle wedge area, on the gravity response of our 3D model of the North Chilean subduction zone. The final obtained model offers a data-driven 3D geometry that can be used for a wide range of future regional or larger-scale studies.
How to cite: Godová, D., Sippl, C., and Tassara, A.: Building a 3D gravity-based model of the North Chilean subduction zone constrained by recent seismic results, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8366, https://doi.org/10.5194/egusphere-egu25-8366, 2025.
The Río Loa earthquake (Mw 6.2), which occurred at a depth of ~56 km in northern Chile on September 11, 2020, was inferred to have happened within the South American upper plate although it was located in the direct vicinity of the plate interface. This was mostly due to its strike-slip focal mechanism, which is distinct from the typical megathrust seismicity observed along the subduction interface. According to Tassara et al. (2022), this earthquake may reflect the fluid-driven rupture of a fault zone and the release of megathrust fluids into the upper plate, a process similar to that observed in the aftershock sequence of the 1995 Antofagasta earthquake (Mw 8.0).
In this study, we aim to perform a comprehensive analysis of temporal variations in seismic attenuation by conducting a comparative 3D tomography of the region before and after the Río Loa earthquake. This approach aims to detect potential changes in the attenuation structure, which could provide insights into stress redistribution, fluid migration, and fault zone evolution triggered by the event.
Seismic attenuation is highly sensitive to temperature variations, fluid presence, and the degree of fracturing within the crust and mantle. Changes in attenuation following a significant seismic event can indicate perturbations in these properties, reflecting enhanced permeability or increased pore fluid pressure in the surrounding rock. This study leverages data from a recent extension of the seismicity catalog of Sippl et al. (2023), which comprises over >200,000 events recorded between 2007 and 2023, with dense station coverage from the Integrated Plate boundary Observatory Chile (IPOC) and temporary deployments.
By applying the coda normalization method and the Multi-Resolution Attenuation Tomography (MuRAT) algorithm (Sketsiou et al., 2021), we obtain high-resolution attenuation models of the forearc region surrounding the Río Loa earthquake. Our inversion process uses ray paths traced through the 3D velocity model of Hassan et al. (2024) to estimate total-Q values. A key focus is the analysis of anomalies in attenuation that may coincide with the mainshock rupture plane or regions exhibiting aftershock clustering.
How to cite: Castro-Melgar, I. and Sippl, C.: Temporal Variations in Seismic Attenuation: A 3D Pre- and Post-Event Tomography of the region around the 2020 Río Loa Mw 6.2 Earthquake (Chile), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8387, https://doi.org/10.5194/egusphere-egu25-8387, 2025.
Northern Chile, located at the boundary of the South American and Nazca plates, is one of the most seismically active regions in the world. To better understand the seismicity and tectonic processes of this complex subduction zone, we analyzed and processed data from the Integrated Plate Boundary Observatory Chile (IPOC) as well as all available temporary seismic stations, spanning the period from 2007 to 2023 (total: 243 seismic stations).
Using EQTransformer, a deep learning-based phase picker, we identified P and S wave arrivals with high precision across a vast dataset of seismic waveforms. We originally utilized the version of EQTransformer pre-trained on the INSTANCE dataset (available from SeisBench), but achieved better results by applying transfer learning based on hand-picked IPOC data. In total, 93,721,745 P- and 71,296,129 S- phases were obtained in this step. The selected phases were then processed with PyOcto, an advanced association and location tool, to group the phases into seismic events. This workflow resulted in a catalog of ~2.5 million events, about 10 times as many as the most complete regional catalog to date. Finally, we relocated the catalog using first Simul2000 and a 2D velocity model, then hypoDD to obtain relative locations, which provide a detailed view of the seismicity in the region.
We present a summary of the retrieved catalog, as well as zooms into potentially interesting subregions. Our catalog offers the potential for numerous follow-up studies, e.g. in statistical seismology or seismic tomography.
How to cite: Najafipour, N., Sippl, C., Kasravi, J., Folesky, J., and Schurr, B.: A deep-learning based seismicity catalog for Northern Chile(2007–2023) containing >2 million events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8522, https://doi.org/10.5194/egusphere-egu25-8522, 2025.
Along the Andean margin, oblique subduction of the Nazca Plate is accommodated by slip on the subduction interface and deformation of the overriding South American Plate. Active plate boundary deformation, in particular due to strain partitioning, is analyzed using elastic block modelling constrained by compiled GPS velocities to estimate plate motions, fault slip rates, and spatially variable interplate coupling on the Nazca-South American subduction interface. In the block modelling approach, interseismic GPS velocities are assumed to be the sum of rigid block rotation and elastic strain accumulation on block-bounding faults. Therefore, the western South American margin is divided into smaller blocks, primarily based on active faults traces. The block model geometry is adjusted to minimize the misfit between observed and modeled velocities.
The preferred model shows strain partitioning of varying degrees along the Andean margin. In the North, the margin-parallel component of convergence is partially accommodated by right-lateral slip on a strike-slip system that extends from the Gulf of Guayaquil off southern Ecuador to western Venezuela. This results in the northeastward motion of the North Andean Block with respect to stable South America. In Peru, the model confirms the existence of the southeastward moving Inca Sliver, that is bounded by the trench and the Subandean fold-and-thrust belt. Along the central Chilean margin, oblique convergence is partially accommodated by minor right-lateral slip along the Subandean thrust fault. In southern Chile, right-lateral transpression along the intra-arc Liquiñe-Ofqui Fault Zone results in the northward translation of the Chiloé Sliver.
The separation of the North Andean Block, the Peruvian Inca Sliver, and the Chilean Andean orogen is related to the curvature of the Andean margin and the associated changes in the sense of convergence obliquity. The differing directions of movement of these blocks result in extension in the Gulf of Guayaquil, where the Andean margin is seaward convex. In contrast, on the Altiplano, at the concave bend of western South America, the rotational velocities of the Peruvian and Chilean blocks are converging into a similar direction.
The spatial distribution of interplate coupling as estimated by our block modelling shows that the Andean margin is segmented into strongly and weakly coupled zones. Epicenters of major thrust earthquakes correlate fairly well with areas of strong interplate coupling.
How to cite: Kusche, F. and Kukowski, N.: Partitioning of deformation along the Andean margin: insights from elastic block modelling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12857, https://doi.org/10.5194/egusphere-egu25-12857, 2025.
In a subduction system where an oceanic lithosphere dips beneath a continental lithosphere, the convergence speed (CS) is predominantly governed by ridge push and slab pull forces. However, numerical models have shown significant sensitivity to the geometry under the same physical parameters. This study aims to shed light on how subduction dynamics is affected by changes in both geometry and rheology, and explore an approach for simulating subduction that makes convergence speeds more consistent and stable by incorporating an effective partial melt region. A series of 2D simulations was conducted to investigate how the kinematics of subduction zones evolve in a self-sustained manner, where no external forces were applied to drive subduction. To achieve this, we used the geodynamic numerical code LaMEM to solve the set of constitutive equations of momentum, mass, and energy suited for geological processes. We also used the mineral assemblage code MAGEMin to compute density changes in relevant lithospheric and asthenospheric rocks. Furthermore, a pyrolytic composition was employed to parameterize the phase change from the asthenospheric mantle to the lower mantle, adopting a Clapeyron slope. In this study, an oceanic plate subducts over a low-viscosity region (LVR) representing a partial melt region. The goal was to demonstrate how the convergence speed varies as a function of both the LVR and the asthenosphere viscosities. To minimize friction between the lithospheric plates, the oceanic plate slides beneath a weak zone. The role of the oceanic plate geometry was studied by varying its horizontal length at the surface. We observed that the CS is inversely correlated with the length of the oceanic plate at the surface. Our study indicates that the LVR makes the convergence speeds more stable over time, simplifies adjustments, and reduces the drag force influence on the overall kinematics of the descending plate. In summary, such an approach minimizes the role of the plate length on the overall evolution of the system in numerical studies and facilitates more stable convergence speeds.
How to cite: Assunção, J., Kaus, B., Riel, N., Picollo, A., and Sacek, V.: Influence of Geometry and Rheology on Convergence Speed in Self-Sustained Andean-Type Subduction Systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13268, https://doi.org/10.5194/egusphere-egu25-13268, 2025.
Subduction zone plate boundary interfaces are some of the largest shear zones on our planet and are host to the largest earthquakes, plus other diverse seismic and aseismic slip phenomena. These zones are often highly heterogenous mélanges. Accreted and exhumed subduction interface mélanges therefore provide a ‘window’ into the conditions and processes within these otherwise inaccessible environments. The geometries of blocks, the proportion of blocks to matrix, and the relative mechanical properties between different block populations and between blocks and matrix have been demonstrated to control the physical behaviour of these mélange zones, including their propensity towards seismicity. Here we report a detailed multi-scale 3D characterisation of the material properties, block geometries and fracture networks within the Chrystalls Beach mélange, New Zealand.
3D structural analysis utilised a tiled photogrammetric model constructed from ca. 12,500 images and consists of detailed and systematic analysis of the mélange fabric, block geometries, and distribution and orientation of faults, fractures and veins. In-situ rock mechanics tests were performed using a Schmidt rebound hammer with measurement sites located to cm-accuracy in the field and on the 3D model. Samples were collected from these same sites for point-load strength tests and laboratory-based triaxial shear experiments. Through this approach, we aim to identify systematic relationships between measurable physical properties of the exhumed rock and the inferred original rheological behaviour of this mélange.
The Chrystalls Beach mélange consists of centimetre – decametre-scale blocks of sandstone, chert, and siltstone with minor altered basalt within a pelitic matrix and has been deformed within the shallow portion of the subduction zone. In-situ strength measurements show that the strength of blocks vary from up to twice as strong as the matrix to similar to — or in places below — the strength of the surrounding matrix. The matrix is also heterogenous in its material properties with two distinct matrix types defined on the basis of matrix lithology, included block populations, and material properties.
Patterns of fractures and brecciation of the blocks provide a structural indication of the comparative rheology of each of the block populations during deformation, with each lithology exhibiting distinct behaviour. Blocks in the mélange are either high-aspect-ratio, boudinaged, dismembered beds or variably rounded brecciated fragments, with stronger lithologies forming more angular, higher-sphericity, and less aligned fragments. This mélange is pervasively cut by several centimetre-thick veins which form an anastomosing network, often at the boundaries of the chert and sandstone blocks which they are deflected around.
This preliminary analysis has revealed varied deformation styles operate between blocks of different mechanical properties and that this deformation style depends both on the rheologies of the individual components and also on the difference in rheology between the blocks and the matrix. The patterns of the thick veins reveal the locations of the greatest slip localisation throughout the mélange and show that veins localise at the margins of blocks with the greatest rheological contrast. This analysis therefore provides the material and geometrical input parameters and end results which provide real-world constraints for future simulations of deforming mélange zones.
How to cite: Clarke, A., Fenske, S., and Toy, V.: A Glimpse into the Subduction Zone Plate Interface: 3D structural and mechanical mapping of the Chrystalls Beach mélange, New Zealand, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16257, https://doi.org/10.5194/egusphere-egu25-16257, 2025.
Ophiolites are exposed remnants of oceanic lithosphere that are critical to our understanding of the structure, composition, and evolution of the oceanic lithosphere.
Some ophiolites (e.g., some Tethyan-type ophiolites) originate in the oceanic forearc of an intra-oceanic subduction system (i.e., in the overriding plate). The forearc is then placed on top of the subducting continental passive margin. Subsequently, the buoyant crustal domains of the continental passive margin undergo a burial-exhumation cycle, during which the exhuming continental crust can drag and detach the tip of the overlaying oceanic forearc, creating an allochthonous ophiolitic nappe. Ophiolites of this type, and associated host assemblages, are invaluable to comprehending the evolution of subduction systems, as they record many key aspects of subduction initiation and forearc development, through to the closure of the oceanic basin and slab break-off.
However, the processes leading to obduction are still poorly understood. For instance, the possible control exerted on ophiolite emplacement by pre-existing weak zones within the oceanic forearc is still largely unexplored. Yet, it is expected that the paleo-forearcs (from which the Tethyan-type ophiolites in the geological record originated) were subject to faulting and other mechanical and chemical weakening prior to the emplacement process, since such structures are ubiquitous in present-day oceanic forearcs.
Physical and chemical weakening of the forearc is, however, not uniformly distributed through space. For example, significant variations in chemical weakening intensity and fault distribution are expected in both trench-parallel and trench-normal directions. If pre-existing weak domains in the forearc do in fact determine the mechanisms of ophiolite emplacement, then the three-dimensional distribution of such structures will exert a considerable control on obduction dynamics, as well as on the final tectonic architecture of the ophiolite and continental-basement assemblage.
Here, we present a set of novel 3D buoyancy-driven numerical models using LaMEM, to study the role of pre-imposed forearc weak structures on the ophiolite emplacement process. Specifically, we systematically test different initial spatial distributions for the weakened domains within the forearc (varying in both trench-parallel and trench-normal directions).
Preliminary results show that spatial variation of pre-existing weakened domains in the oceanic forearc have a first order effect on the Tethyan-type ophiolite emplacement process.
This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), and through scholarship SFRH/BD/146726/2019.
How to cite: Gomes, A., Riel, N., Rosas, F., Schellart, W., and Duarte, J.: The effect of oceanic forearc serpentinization on ophiolite emplacement: Insights from 3D geodynamic models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10715, https://doi.org/10.5194/egusphere-egu25-10715, 2025.
Cycling incompatible elements and fluids into the mantle plays a crucial role in shaping its compositional heterogeneity through time and crustal evolution. Ocean island basalts (OIBs) and silicate inclusions in diamonds are enriched in incompatible fluid-mobile elements (FME) relative to normal mid-ocean ridge basalt (N-MORB) and primitive mantle, which is commonly interpreted to reflect the presence of recycled oceanic crust (the HIMU endmember) and/or sediment (EM endmembers) in their lower mantle sources. However, the specific mineral phases that transport these FME into the lower mantle are poorly understood. Carbonatized serpentinites have attracted relatively little attention. These rocks represent a major source of FME that may be recycled into the deep mantle. In addition, magnesite is the main carbonate phase in subducted carbonatized serpentinites. It has been found to be an inclusion in deep diamonds and, with microdiamonds, in carbonatized peridotite and can be stable at depths of at least 700 km. Here, we present a comprehensive mineralogical and geochemical investigation of magnesite (MgCO3) within subducted Neoproterozoic carbonatized serpentinites from the Arabian–Nubian Shield, which is enriched in FME (e.g., B, Sb, As, Pb, and Mo) relative to primitive mantle. Atom probe tomography shows that these elements are more-or-less homogeneously distributed within magnesite and, thereby, structurally bound. Given that the experimentally determined stability of magnesite extends to lower mantle pressures, our findings indicate that magnesite is a major carrier of fluid-mobile elements (including carbon) into Earth’s deep interior, where it contributes to the lower mantle source of some ocean island basalts (OIBs) and superdeep diamonds.
How to cite: Gamaleldien, H., Qu, Y.-R., Johnson, T., Liu, S., Abu-Alam, T., Fougerouse, D., Reddy, S., Evans, N., and Gong, T.-N.: Subducted magnesite in serpentinite carries fluid-mobile elements and carbon into the lower mantle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3005, https://doi.org/10.5194/egusphere-egu25-3005, 2025.
Discriminating between fore-arc crust [1] and wedge serpentinite [2] contributions in arc magmas is critical for understanding mass recycling mechanisms in subduction zones but remains challenging because fore-arc crust may include serpentinite signatures from prior subduction events. Here we present molybdenum (Mo) isotope and concentration data, along with reanalyzed and published geochemical data, for common representatives of circum-Pacific high-Mg andesites and adakites. Elevated δ98/95Mo values (-0.13‰ to 0.00‰) in Kamchatka and Aleutian high-Mg andesites, accompanied by high Mo/Ce (0.026 to 0.075), Ba/Th (138 to 808), and Sb/Ce (0.0026 to 0.0192) ratios, as well as depleted mantle-like Sr-Nd-Hf-Pb isotopes and moderate δ18O values (+6.6‰ to +7.8‰), indicate slab-derived aqueous fluids via fore-arc serpentinites. In contrast, Cascadia and Setouchi high-Mg andesites, along with adakites from Fiji and the Austral Volcanic Zone, show decreasing δ98/95Mo (-0.07‰ to -0.48‰), Mo/Ce, and Sb/Ce ratios, coupled with higher Sr/Y (15 to 207) and altered oceanic crust-like Sr-Nd-Hf-Pb-O isotopic compositions, reflecting melts from subducted oceanic crust. Nine adakites from the Aleutians, Fiji, Panama, and the Austral Volcanic Zone exhibit intermediate δ98/95Mo (-0.19‰ to -0.04‰) with low Mo/Ce and Sb/Ce ratios, but high Sr/Y (57 to 295), radiogenic Nd-Hf isotopes, and low δ18O (+6.3‰ to +6.5‰), suggesting origins from fore-arc crust dragged by subducting slabs. These results link δ98/95Mo variations to partial melting of oceanic and fore-arc crust, highlighting dehydration and melting [3, 4] as key processes in subduction zones.
[1] Liu et al. (2023) Geology; [2] Li et al. (2021), Nature Communications; [3] Elliott (2003) Inside the Subduction Factory; [4] Liu et al. (2024) Chemical Geology.
How to cite: Liu, H.-Q., Tian, F., Hoernle, K., Li, J., Huang, X.-L., Zhang, L., Bindeman, I., and Xu, Y.-G.: Molybdenum isotope insights into mass recycling in subduction zones, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2959, https://doi.org/10.5194/egusphere-egu25-2959, 2025.
The collision of the Indian Plate with the Eurasian Plate has led to the building of the Himalayas, the highest mountain range and one of the most seismically active regions in the world. The supercontinent Pangea began to break at around 200Ma, and the Indian plate moved northwards toward the Eurasian plate at 9-16cm/year. At around 50Ma, the velocity of the Indian plate slowed down to about 4-6cm/year. This slowdown is the beginning of the collision between the plates, the Tethys Ocean's closing, and the uplifting of the Himalayas. The Indian plate is still moving with a velocity of nearly 5cm/year, causing a rise in the height of the Himalayas at approximately 4-10mm/year, which is the cause of the extensive seismicity in the nearby region. There has been extensive research on tectonics and seismicity in the Himalayas; however, it is one of the most geologically complex regions, and much of it is still unfathomable and thus requires insight through further studies. This study attempts to find the variation in the physical properties at the subduction zone due to the variation in the collision velocities of the plates. In this study, we have used a numerical simulation of the collision and subduction using finite difference modelling in MATLAB. We have compared physical parameters such as pressure, stress, strain, and temperature for the profiles at different velocities of the colliding blocks at the subduction zone. This geodynamic study focuses on enhancing the understanding of the tectonics and collision of the Indian and Eurasian plates and the formation of the Himalayas.
How to cite: Banerjee, A. and Roy, P. N. S.: A study on the docking of the Indian plate with the Eurasian plate through Numerical Modelling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12064, https://doi.org/10.5194/egusphere-egu25-12064, 2025.
The mantle near Earth's subduction zones experiences significant deformation, forming anisotropic rock textures. These textures can be detected using seismic methods and simulated in geodynamic models. This study employs time-series clustering to examine tracers in subduction models, identifying regions with similar deformation histories, olivine crystallographic-preferred orientation (CPO) development, and CPO-induced anisotropic viscosity. We compare the evolution of olivine textures predicted by various numerical methods (e.g. D-Rex, MDM, and MDM+AV) for both retreating and stationary trench subduction settings.
Our modeling shows notable variations in olivine texture around the slab and as a function of subduction dynamics. These variations, which are illuminated by the clustering analysis, show that texture, seismic, and viscous anisotropy can vary greatly within the mantle wedge, sub-slab, and subducting plate regions of the upper mantle. In the retreating-trench model, the strongest textures are observed in the mid-depth mantle wedge region and beneath the slab at the 660 km transition zone. Trench-normal olivine a-axis orientations are predominant in the center of subduction zones, while toroidal flow around slab edges produces a mix of trench-normal, trench-parallel, and oblique fast seismic directions. On the other hand, in the stationary-trench model, the trench-normal signal in front of the slab is weaker while there are stronger trench-normal signals behind the slab at shallow depths between 100 and 300 km. At the edge of the slab, weak toroidal flow produces trench-oblique orientations while trench-parallel and trench-normal orientations are missing. In general, the retreating trench model exhibits stronger textures and anisotropy due to increased deformation from trench motion.
These results provide valuable insights into seismic anisotropy in subduction zones and underscore the importance of considering texture heterogeneity when interpreting geodynamic models and seismic data. The use of time-series clustering algorithms highlights the intricate pattern of evolution and the relationship between deformation history, CPO, and CPO-induced viscous anisotropy occurring within subduction zones.
How to cite: Wang, Y., Király, Á., Conrad, C., and Maupin, V.: Subduction dynamics and mantle anisotropy: modeling and clustering of olivine textures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19151, https://doi.org/10.5194/egusphere-egu25-19151, 2025.
The subduction of the Juan de Fuca Plate at the Cascadia subduction zone significantly influences the mantle dynamics and the structure of the overlying North American Plate. In southwest Canada, the Cordillera lithosphere is thin (60-70 km) with high surface heat flow, low mantle seismic velocity, and low mantle electrical resistivity for ~500 km inboard of the subduction zone. Magmatism and geological observations suggest that the Cordillera lithosphere has been thin for at least 30 Myr. The eastern limit of thin lithosphere approximately underlies the Rocky Mountain Trench. East of this, the Laurentian Craton is thick (>200 km), and recent seismic data show that the Cordillera Craton boundary is marked by subvertical to west-dipping lithospheric step.
In this study, we investigate the effects of subduction and the lithosphere step on mantle dynamics and the evolution of the Cordillera lithosphere over the last 40 Myr. We use 2D thermal-mechanical models of ocean-continent subduction, where the domain is 3000 km wide and 660 km deep. We first test models where subduction of the Juan de Fuca plate occurs below a 60 km thick continent with no lateral variations, representing the Canadian Cordillera. These models show that if the mantle rheology is based on dry olivine, it has a relatively high viscosity, and the mantle flow field is dominated by subduction-driven corner flow. This results in a slow thickening of the backarc continental mantle lithosphere to nearly 90 km within 40 Myr. If a weaker (more hydrated) olivine rheology is used for the mantle, backarc thickening is inhibited by the development of small-scale convection (SSC). To maintain a ~65 km lithosphere, our models predict that the backarc mantle must be hydrated and weak (viscosity of 1018 – 1019 Pa s). In the second set of models, 200 km thick Craton lithosphere is added to the models. The presence of the lithosphere step at the Cordillera-Craton boundary induces edge-driven convection (EDC), which is enhanced for a hydrated mantle or weak craton mantle lithosphere. We find that EDC had only a secondary influence on the Cordillera lithosphere in the arc and central back arc regions, but EDC may be important for maintaining a sharp thermal contrast between the Cordillera and Craton.
In the final set of models, we investigate the effects of subduction termination on mantle dynamics, using the model structure that includes the Craton lithosphere step. After plate convergence ceases, SSC and EDC continue for tens of millions of years, and these slow the cooling and thickening of the continent. However, even with a hydrated mantle, the Cordillera thickens to ~80 km after 40 Myr. This suggests that the central Canadian Cordillera lithosphere (north of the current subduction zone), where subduction terminated in the Eocene, may be somewhat cooler than the modern backarc to the south. Future work will focus on how the slab edge geometry of the Juan de Fuca plate influences mantle flow patterns and lithospheric structure in the Canadian Cordillera.
How to cite: Baruah, A. J. and Currie, C. A.: Investigating mantle dynamics and lithospheric evolution in the Southern Canadian Cordillera: Insights from numerical modeling of the Cascadia subduction zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14257, https://doi.org/10.5194/egusphere-egu25-14257, 2025.
The Siletzia oceanic plateau was accreted to the western margin of North America in the Eocene, marking the most recent accretion event in this area and the formation of the modern Cascadia subduction zone. Siletzia formed on or near the spreading ridge of two oceanic plates, and its chemical composition shows that its origin is a mixture of mid-ocean ridge basalts and hotspot volcanism likely associated with the Yellowstone plume. The plateau formed between 56 and 49 Ma, and accretion to the continent occurred at around 50 Ma. Plate reconstructions date the age of the oceanic plate during accretion at about 10 Ma. Therefore, Siletzia accretion occurred in a unique environment where the oceanic plate was young and likely hot and weak. Observation of modern equivalent plateaus show a conduit of hot, weak, partially melted mantle below the islands, which creates an especially weaken zone below the plateau.
Using 2D thermo-mechanical numerical models, we explore the dynamics as an oceanic plateau is carried into a subduction zone to determine the conditions under which the plateau is accreted to the overlying continent. Our models examine the effects of variations in age of the oceanic plate, weakening of the plate due to the plateau creation and the structure of the continent. We also test the effect two different boundary conditions: (1) forced plate convergence at 4 cm/yr and (2) free subduction, where plate convergence is driven dynamically by the negative buoyancy of the oceanic plate.
Results show that in models with an old oceanic plate (>50 Ma), the plateau is readily subducted into the deeper mantle with little disruption to the subduction system for both boundary conditions. In contrast, for a young oceanic plate (~10 Ma), subduction stalls as the plateau enters the subduction zone, leading to accretion of the plateau and parts of the oceanic lithosphere to the continental margin. With no imposed convergence, all plate motions cease, whereas forced convergence is accommodated by formation of a new subduction zone outboard of the terrane when the plate is weakened by the formation of the plateau. Otherwise, deformation occurs within the interior of the oceanic plate, causing the oceanic plate to break 900 km seaward of the subduction zone. These models demonstrate that Siletzia accretion to North America may have occurred due to the young plate age, but in some models, accretion only lasts for around 10 Myr as continued plate convergence causes entrainment and subducting of the plateau. If the modern Cascadia subduction zone formed as a new plate boundary west of Siletzia, continued plate convergence may have been driven by the older subducting plate to the south. Ongoing work is using 3D models to assess this in more detail.
How to cite: Urban, M. and Currie, C.: Oceanic plateau accretion for young oceanic plates: Geodynamics models of Siletzia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7317, https://doi.org/10.5194/egusphere-egu25-7317, 2025.
