Subduction drives plate tectonics, generating the major proportion of subaerial volcanism, releasing >90% seismic moment magnitude, forming continents, and recycling lithosphere. Therefore, it is the most important geodynamical phenomenon on Earth and the major driver of global geochemical cycles. Seismological data show a fascinating range in shapes of subducting slabs. Arc volcanism illustrates the complexity of geochemical and petrological phenomena associated with subduction.

Numerical and laboratory modelling studies have successfully built our understanding of many aspects of the geodynamics of subduction zones. Detailed geochemical studies, investigating compositional variation within and between volcanic arcs, provide further insights into systematic chemical processes at the slab surface and within the mantle wedge, providing constraints on thermal structures and material transport within subduction zones. However, with different technical and methodological approaches, model set-ups, inputs and material properties, and in some cases conflicting conclusions between chemical and physical models, a consistent picture of the controlling parameters of subduction-zone processes has so far not emerged.

This session aims to follow subducting lithosphere on its journey from the surface down into the Earth's mantle, and to understand the driving processes for deformation and magmatism in the over-riding plate. We aim to address topics such as: subduction initiation and dynamics; changes in mineral breakdown processes at the slab surface; the formation and migration of fluids and melts at the slab surface; primary melt generation in the wedge; subduction-related magmatism; controls on the position and width of the volcanic arc; subduction-induced seismicity; mantle wedge processes; the fate of subducted crust, sediments and volatiles; the importance of subducting seamounts, LIPs, and ridges; links between near-surface processes and slab dynamics and with regional tectonic evolution; slab delamination and break-off; the effect of subduction on mantle flow; and imaging subduction zone processes.

With this session, we aim to form an integrated picture of the subduction process, and invite contributions from a wide range of disciplines, such as geodynamics, modelling, geochemistry, petrology, volcanology and seismology, to discuss subduction zone dynamics at all scales from the surface to the lower mantle, or in applications to natural laboratories.

Co-organized by GMPV2/SM2/TS7
Convener: Oğuz H Göğüş | Co-conveners: Taras Gerya, Ágnes Király, Wim Spakman
| Attendance Wed, 06 May, 08:30–10:15 (CEST)

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Chat time: Wednesday, 6 May 2020, 08:30–10:15

D1356 |
Armel Menant, Samuel Angiboust, Taras Gerya, Robin Lacassin, Martine Simoes, and Raphael Grandin

Subduction zones are the loci of huge mass transfers, including accretion and erosion processes responsible for the long-term formation (and destruction) of fore-arc margins. Study of now-exhumed deep portions of the fore-arc crust revealed km-scale tectonic units of marine sediments and oceanic crust, which have been underplated (i.e. basally accreted) to the overriding plate. However, geophysical observations of this deep process in active subduction zones are unclear and the dynamics of tectonic underplating, as well as its existence, along most of active margins remain controversial. We attempt to shed light on this critical process from the plate interface where tectonic slicing is triggered, to the surface where topographic variations are expected in response to such a mass transfer.

Using high-resolution visco-elasto-plastic thermo-mechanical models, we present with unprecedented details the dynamics of formation, preservation and destruction of underplated crustal nappes at 10-40-km depth in subductions zones. Our results show that subduction segments exhibiting an increasing frictional behaviour control deep accretionary dynamics and that the long-term frictional zonation of the plate interface is stable due to a positive feedback between fluid distribution and effective stress. As a result, discrete underplating events follow one after another for tens of Myr, leading to the formation of a thick duplex structure supporting a coastal topographic high. The rise of this high topography is cadenced by Myr-scale uplift-then-subsidence cycles, characterising each underplating event and the subsequent period of wedge re-equilibration. This periodical evolution is significantly modified by changing the rheological properties of the material entering the subduction zone, suggesting that tectonic underplating is likely a transient process active along most of active margins, depending on severe variations of the hydro-mechanical properties of the plate interface at Myr timescale.

How to cite: Menant, A., Angiboust, S., Gerya, T., Lacassin, R., Simoes, M., and Grandin, R.: Fore-arc building and destruction: a critical interplay between fluid flow, megathrust strength and tectonic underplating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1783, https://doi.org/10.5194/egusphere-egu2020-1783, 2020.

D1357 |
Gordon Lister

The slab-sheet-slump hypothesis postulates the existence of relatively weak sheets of partially-hydrated and dehydrating mantle that slide down the face of lithospheric slabs as they subduct, at a rate slightly faster than the overall rate of subduction. The slab-sheet-slump hypothesis takes note of arrays of otherwise inexplicable landward-dipping tilt-blocks. These typically form and/or accentuate in the uppermost 20-25 km of slabs as they enter the subduction zone, in the time preceding, or in the immediate aftermath, of large megathrust earthquakes. The slab-sheet-slump hypothesis suggests that displacement on these headwall faults connects to detachment faults or ductile shear zones at depth, and that this detachment partially uncouples the slumping sheet from the rest of the subducting lithosphere. The dimensions vary. The width of the slump channel may range from 30—100 km. The depth extent is determined by the geometry of the paired seismic zone that forms 20-30 km beneath the slab-asthenosphere boundary.

The slab-sheet-slump hypothesis further suggests that seismogenic failure within the interior of a slumping slab-sheet leads to paired seismic zones. The surface of the slab-sheet (dominated by the oceanic crust) may fail in a brittle fashion, with fault orientation predicted by the Coulomb-Mohr failure criterion. The base of the slab-sheet may fail as the result of boudinage, with the shallowly-dipping orientation of semi-brittle or ductile faults predicted by a maximum moment condition. Occasionally, but rarely, the magnitude of stored elastic potential energy may allow major earthquakes, and these more accurately decorate the structure of the slab sheet. The 2006-2007 Kuril Islands rupture showed the first example of a Mw>8 earthquake on the sidewall of a slab-sheet slump. The 2011 Great Earthquake was accompanied by accelerated motion in the inferred slab sheet beneath. Earthquakes within the slab sheet occasionally exceed Mw 7, allowing delineation of the rupture. In the upper plane, some orientations may reflect the structuring caused by the original landward-dipping normal faults. Fault orientations in the lower levels of the slab sheet may reflect structuring caused by boudinage.  

Paired seismic zones otherwise present an enigma. Estimates of the elastic thickness of unstructured lithosphere range from 60-120 km. Yet paired seismic zones are rarely more than 20-30 km apart. Flexure of an uncoupled slab sheet allows explanation of this paradox, while the bending or unbending of unstructured lithosphere does not. Moment tensor data are consistent with the existence of two aseismic shear zones, one adjacent to the slab surface, with the same sense of shear as required by subduction, while the basal shear zone has the opposite sense, consistent with that required by the slab-sheet-slump hypothesis. These structures appear to be persistent over long time periods, so they match the geomorphology of individual segments of the adjacent subduction megathrust.

How to cite: Lister, G.: Further data in support of the slab-sheet slumping hypothesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1712, https://doi.org/10.5194/egusphere-egu2020-1712, 2020.

D1358 |
Bernd Schurr, Lukas Lehmann, Christian Sippl, and Wasja Bloch

Subduction zone forearcs deform transiently and permanently due to the frictional coupling with the converging lower plate. Transient stresses are mostly the elastic response to the spatio-temporally variable plate coupling through the seismic cycle. Long-term deformation depends e.g., on the plate convergence geometry, where obliqueness or change in obliqueness play important roles. Here we use the Integrated Plate Boundary Observatory Chile (IPOC) and additional temporal networks to determine source mechanisms for upper plate earthquakes in the northern Chile subduction zone. We find that earthquakes in the South American crust under the sea and under the Coastal Cordillera show a remarkably homogenous north-south, i.e. trench-parallel, compressional stress field. Earthquake fault mechanisms are dominated by east-west striking thrusts. Further inland, where the lower plate becomes uncoupled, the stress field is more varied with direction east-west to southeast-northwest (approx. convergence parallel) dominating. The peculiar stress-regime above the plate-coupling-zone almost perpendicular to plate convergence direction may be explained by a change in subduction obliqueness due to the concave shape of the plate margin.

How to cite: Schurr, B., Lehmann, L., Sippl, C., and Bloch, W.: Trench-parallel compressive upper plate stress field in the northern Chile forearc from earthquake source mechanisms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2706, https://doi.org/10.5194/egusphere-egu2020-2706, 2020.

D1359 |
Ebru Şengül Uluocak, Russell N. Pysklywec, Oğuz H. Göğüş, and Emin Ulugergerli

Southeast Carpathians with deep basins (e.g., Transylvania and Focsani) and the mountain chain (SE Carpathians Mountain with ~1.5 km elevation) are characterized by unique morphological features.  The highly-variable subsurface structures (e.g., Vrancea slab) related to post-collisional tectonics are imaged by geophysical studies. Numerical modeling studies are performed to understand the deformation linked with active geodynamic processes developing in the east part of the region. Here, we present our multi-dimensional (2D-3D) thermo-mechanical modeling results with varying temperatures and crustal configurations. We analyze modeling results together with the observations in terms of possible mantle flow components of the surface topography in Southeast Carpathians. In addition to residual topography calculations, non-isostatic compensation of the elevation is interpreted based on admittance functions between free-air gravity and topography. Our results indicate that mantle flow induced dynamic forces beneath the region modify the elevation with positive amplitudes over the Transylvania Basin (0.8-1 km) and the high SE Carpathian Mountains (~ 1 km) and subsidence of the Focsani Basin (0.5-1 km).

How to cite: Şengül Uluocak, E., Pysklywec, R. N., Göğüş, O. H., and Ulugergerli, E.: Surface Topography Anomalies Induced by Geodynamic Processes in the Southeast Carpathians, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-212, https://doi.org/10.5194/egusphere-egu2020-212, 2020.

D1360 |
| solicited
Bernhard Steinberger and Douwe van Hinsbergen

Identifying the geodynamic processes that trigger the formation of new subduction zones is key to understand what keeps the plate tectonic cycle going, and how plate tectonics once started. Here we discuss the possibility of plume-induced subduction initiation. Previously, our numerical modeling revealed that mantle upwelling and radial push induced by plume rise may trigger plate motion change, and plate divergence as much as 15-20 My prior to LIP eruption. Here we show that, depending on the geometry of plates, the distribution of cratonic keels and where the plume rises, it may also cause a plate rotation around a pole that is located close to the same plate boundary where the plume head impinges: If that occurs near one end of the plate boundary, an Euler pole of the rotation may form along that plate boundary, with extension on one side, and convergence on the other.  This concept is applied to the India-Africa plate boundary and the Morondova plume, which erupted around 90 Ma, but may have influenced plate motions as early as 105-110 Ma. If there is negligible friction, i.e. there is a pre-existing weak plate boundary, we estimate that the total amount of convergence generated in the northern part of the India-Africa plate boundary can exceed 100 km, which is widely thought to be sufficient to initiate forced, self-sustaining subduction. This may especially occur if the India continental craton acts like an “anchor” causing a comparatively southern location of the rotation pole of the India plate. Geology and paleomagnetism-based reconstructions of subduction initiation below ophiolites from Pakistan, through Oman, to the eastern Mediterranean reveal that E-W convergence around 105 Ma caused forced subduction initiation, and we tentatively postulate that this is triggered by Morondova plume head rise. Whether the timing of this convergence is appropriate to match observations on subduction initiation as early as 105 Ma depends on the timing of plume head arrival, which may predate eruption of the earliest volcanics. It also depends on whether a plume head already can exert substantial torque on the plate while it is still rising – for example, if the plate is coupled to the induced mantle flow by a thick craton.

How to cite: Steinberger, B. and van Hinsbergen, D.: An analytical model concerning the possible initiation of subduction between the India and Africa plates, caused by the Morondova plume, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6050, https://doi.org/10.5194/egusphere-egu2020-6050, 2020.

D1361 |
Yajian Gao, Frederik Tilmann, Dirk-Philip van Herwaarden, Sölvi Thrastarson, Andreas Fichtner, and Bernd Schurr

We present a new seismic tomography model from multi-scale full seismic waveform Inversion for the crustal and upper-mantle structure beneath the Central Andes (16°-30° S), where the oceanic Nazca plate is subducting beneath the South American continent. The Central Andes is characterized by significant along-strike changes in crustal shortening and thickening, arc migration, subduction erosion and catastrophic earthquakes (e.g. the 2014 Iquique M8.1 earthquake). A high resolution seismic velocity model would bring new insights into the geodynamics of this region, especially for the effects on the seismicity and volcanic arc from the serpentinization in the mantle wedge and dehydration effects from the subducting oceanic crust. 

Our model is derived from multi-scale full waveform inversion, including multiple time period stages (40-80 s, 30-80 s, 20-80 s, 15-80 s and 12-60 s). In order to avoid the risk of falling into the local minima of optimization, we started our inversion from the lowest frequency signals costing lower computational resources. Specifically, the forward and adjoint simulation based on a 3D model are accomplished with Salvus (Afanasiev et al., 2018), which is a suite of spectral-element method solver of the seismic wave equation. We invert waveforms from 117 events, which are carefully selected for good data coverage of the study region and depth range. We take advantage of the adjoint methodology coupled with conjugate gradients and L-BFGS optimization scheme to update the velocity model. We adopt a time-frequency phase shift as misfit functional with adjoint sources in the first four period-stages, and cross-correlation coefficient in the final stage after most of the phase shifts has been eliminated. The cross-correlation coefficient can capture distorted body wave seismograms, not only the phase shift. We also provide a resolution analysis through the computation of the point-spreading functions and validation dataset with a misfits evolution chart, demonstrating the robustness of our final model.

Through full-waveform inversion, we provide a new higher resolution P and S wave velocity model from the middle crust to the upper mantle around 300 km depth. The subducted Nazca slab in the upper mantle beneath the Central Andes is fully imaged, with dip angle variations from the north to the south. We could also observe a strong low velocity band in the middle crust and uppermost mantle from 80 to 100 km beneath the volcanic arc, correlating with the volcano distributions and recent intermediate depth seismicity relocation results. An offset of this low velocity band between 20°-21°S is conspicuous, both in the middle crust and uppermost mantle, indicating a weak extent of the dehydration from 20°-21°S, resulting in the weak intermediate depth seismicity and absent volcanic activity in the same latitude range. We also imaged strong low velocity anomalies in the middle crust beneath the Altiplano-Puna Volcanic Complex and South Puna, giving strong evidence supporting the magmatic underpinnings and reservoirs. Meanwhile, low velocity beneath Puna tectonic units down to 100 km may represent the lithospheric detachment, resulting in the melting and upwelling fluids from the Nazca plate.

How to cite: Gao, Y., Tilmann, F., van Herwaarden, D.-P., Thrastarson, S., Fichtner, A., and Schurr, B.: SeisAndes: A High Resolution Crust and Upper Mantle Seismic Velocity Model beneath the Central Andes from 16° S to 30° S from Full Waveform Inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6936, https://doi.org/10.5194/egusphere-egu2020-6936, 2020.

D1362 |
Menno Fraters, Wim Spakman, Cedric Thieulot, and Douwe Van Hinsbergen

The eastern Caribbean Lesser-Antilles subduction system is a strongly arcuate subduction system. We have investigated the dynamics of this system through numerical modelling, demonstrating the developed capabilities and computational feasibility for assessing the 3D complexity and geodynamics of natural subductionsystems and applied this to the eastern Caribbean region. We show the geodynamic feasibility of westward directed trench-parallel slab transport through the mantle, i.e. slab dragging, on the northern segment of the slab, while the eastern segment of the slab is subducting by a mantle-stationary trench. The resistance of the mantle against slab dragging by the North American plate motion, as well as the deformation associated with the arcuate geometry of the slab, creates a complex 3D stress field in the slab that deviates strongly from the classical view of slab-dip aligned orientation of slab stress. More generally this means that the process of slab dragging may reveal itself in the focal mechanisms of intermediate and deep earthquakes. The characteristics of the arcuate subduction such as slab dragging and a complex 3D stress field as studied in the Caribbean region can be more generically applied to other arcuate subduction systems as well, such as the Izu-Bonin-Marianas or the Aleutians-Alaskasystems, where anomalous focal mechanisms of slabs are observed.

How to cite: Fraters, M., Spakman, W., Thieulot, C., and Van Hinsbergen, D.: Assessing the geodynamics of strongly arcuate subduction zones in the eastern Caribbean subduction setting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6173, https://doi.org/10.5194/egusphere-egu2020-6173, 2020.

D1363 |
Zoltán Erdős, Ritske S. Huismans, and Claudio Faccenna

Both divergent and convergent plate boundaries had been studied extensively throughout the last five decades. Among a host of other aspects came the realization, that given the right circumstances, a broad extensional basin can form behind a convergent plate boundary. The exact mechanisms triggering back-arc extension and why they are episodic, lasting only for tens of millions of years is still debated. The absolute and relative velocities of the plates, the age of the subducting oceanic plate and the inherited rheological properties of the back-arc lithosphere are all thought to be key players, shaping the dynamics of the fore-arc - back-arc systems.

Here we use 2D mantle scale plane-strain thermo-mechanical model experiments to investigate how the accretion of small continental crustal terrains onto the overriding plate affect the dynamics of the subducting slab and the deformation of the overriding plate.

Our results suggest that slab-retreat and back-arc extension can be achieved through the combination of slow convergence and micro-continent accretion. Back-arc extension during fast convergence is also possible through the subsequent accretion of more than one micro-continental terrain. Moreover, even the accretion of one such terrain can produce short (1-5 My) episodes of extension-contraction-quiescence in the overriding plate. These episodes are connected to slab break-off events, slab-interaction with upper mantle phase-change boundaries and variations in slab-pull due varying slab thickness.

Our model experiments also result in complex structures in the overriding plate where discrete outcrops from a single oceanic basin are preserved on the surface hundreds of kilometres apart. This indicates that in nature a simple accretion scenario could produce a surface geological record that is difficult to decipher. Our results compare favourably to observations from the Aegean back-arc basin.

How to cite: Erdős, Z., Huismans, R. S., and Faccenna, C.: Overriding-plate deformation during micro-continent accretion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7167, https://doi.org/10.5194/egusphere-egu2020-7167, 2020.

D1364 |
Marzieh Baes, Stephan Sobolev, Taras Gerya, and Sascha Brune

The formation of new subduction zones is a key component of global plate tectonics. Initiation of subduction following the impingement of a hot buoyant mantle plume is one of the few scenarios that allow breaking the lithosphere and recycling a stagnant lid without requiring any pre-existing weak zones. According to this scenario, upon arrival of a hot and buoyant mantle plume beneath the lithosphere, the lithosphere breaks apart and the hot mantle plume materials flow atop of the broken parts of the lithosphere. This leads to bending of the lithosphere and eventually initiation of subduction. Plume-lithosphere interaction can lead to subduction initiation provided that the plume causes a critical local weakening of the lithospheric material above it, which depends on the plume volume, its buoyancy, and the thickness of the lithosphere. Previous modeling studies showed that plume-lithosphere interaction can result in initiation of multi- or single-slab subduction zones around the newly formed plateau. However, they did not explore the parameters playing key roles in discriminating between the single- and multi-slab subduction scenarios. Here, we investigate factors controlling the number and shape of retreating subducting slabs formed by plume-lithosphere interaction. Using 3d thermo-mechanical models we show that the response of the lithosphere to arrival of a mantle plume beneath it depends on several parameters such as age of oceanic lithosphere, thickness of the crust, large-scale lithospheric extension rate, relative location of plume head and plateau edge and mantle temperature. The numerical experiments reveal that plume-lithosphere interaction in present day Earth can result in three different deformation regimes: (a) multi-slab subduction initiation, (b) single-slab subduction initiation and (c) plateau formation without subduction initiation. On early Earth (in Archean times) plume-lithosphere interaction could result in formation of either multi-slab subduction zones, very efficient in production of new crust, or episodic short-lived circular subduction. Extension eases subduction initiation caused by plume-lithosphere interaction. Plume-induced subduction initiation of old oceanic lithosphere with a plateau with thick crust is only possible if the lithosphere is subjected to extension.

How to cite: Baes, M., Sobolev, S., Gerya, T., and Brune, S.: Plume-induced subduction initiation: single- or multi-slab subduction?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7783, https://doi.org/10.5194/egusphere-egu2020-7783, 2020.

D1365 |
Kiran Chotalia, George Cooper, Fabio Crameri, Mathew Domeier, Caroline Eakin, Antoniette Greta Grima, Derya Gürer, Ágnes Király, Valentina Magni, Elvira Mulyukova, Kalijn Peters, Boris Robert, Grace Shephard, and Marcel Thielmann

Numerous studies have provided insights into one of the key problems of the Earth Sciences: subduction zone initiation (SZI). The insights into SZI are both numerous and diverse with evidence from multiple disciplines in Earth Sciences. SZI studies exploit the geological record, reconstruct regional or global plate motion back in time, interpret seismic tomography to identify the tip depth of sunken plate portions, and diagnose theoretical and numerical models of rock and plate deformation based on known physics.

Getting and keeping an overview over the many discipline-specific advances is challenging for many reasons: one being the dispersed sources of information, another being the missing communication across the individual disciplines. The latter shortcoming also arises from the multiple incompatible scientific jargons currently in use.

The SZI database now unifies the scientific jargon, and brings together old and new insights relating to SZI into a common, community-wide platform online (www.SZIdatabase.org). The SZI database builds bridges between individual communities, opening a community-wide discussion by making SZI data readily available and understandable. This keeps data and knowledge up-to-date, and can therefore provide the most complete picture of our current understanding of SZI.

In this presentation, we outline where to find, how to use, and why to contribute to the SZI database. This community-wide project has already yielded interesting results regarding the fascinating question about how and where SZI occurs on present-day Earth and back to around 100 Ma. Work thus far suggests ‘subduction breeds subduction’, highlighting the beginning of crucial insights from these ongoing cross-disciplinary efforts.

How to cite: Chotalia, K., Cooper, G., Crameri, F., Domeier, M., Eakin, C., Grima, A. G., Gürer, D., Király, Á., Magni, V., Mulyukova, E., Peters, K., Robert, B., Shephard, G., and Thielmann, M.: The trans-disciplinary and community-driven subduction zone initiation (SZI) database, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15910, https://doi.org/10.5194/egusphere-egu2020-15910, 2020.

D1366 |
| solicited
Ben Maunder, Saskia Goes, Julie Prytulak, and Mark Reagan

Plate tectonics requires the formation of plate boundaries. Particularly important is the enigmatic initiation of subduction: the sliding of one plate below the other, and the primary driver of plate tectonics. A continuous, in situ record of subduction initiation was recovered by the International Ocean Discovery Program Expedition 352, which drilled a segment of the fore-arc of the Izu-Bonin-Mariana subduction system, revealing a distinct magmatic progression with a rapid timescale (approximately 1 million years). Here, using numerical models, we demonstrate that these observations cannot be produced by previously proposed horizontal external forcing. Instead a geodynamic evolution that is dominated by internal, vertical forces produces both the temporal and spatial distribution of magmatic products, and progresses to self-sustained subduction. Such a primarily internally driven initiation event is necessarily whole-plate scale and the rock sequence generated (also found along the Tethyan margin) may be considered as a smoking gun for this type of event.

How to cite: Maunder, B., Goes, S., Prytulak, J., and Reagan, M.: Vertically Driven Dynamics and Magmatism of Rapid Subduction Initiation in the Western Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19611, https://doi.org/10.5194/egusphere-egu2020-19611, 2020.

D1367 |
Nicholas Schliffke, Jeroen van Hunen, Frederic Gueydan, Valentina Magni, and Mark B. Allen

Jumps in the location of back-arc spreading centres are a common feature of back-arc basins, but the controlling factors are not understood. In several narrow subduction zones with a long subduction history, such as the Scotia arc or Tyrhennian Sea, several spreading centres have been active in the course of history with regular, quasi-instantaneous jumps towards the retreating trench. A prominent feature of these regions are large bounding transform (‘STEP’) faults. However, whether STEP faults influence the (unknown) dynamics spreading centre jumps remains to be explored.


We therefore run 3D-models to simulate a long narrow subducting slab, bound by continents, which retreats and creates necessary STEP-faults self-consistently. The results offer a new mechanism for back-arc spreading jumps: After the creation of a back-arc spreading centre in the retreating subduction system, transform faults between trench and back-arc basin form. Spreading jumps are thus a consequence of the fact that these constantly elongating transform faults, which decouple the overriding plate from neighbouring plates, fail to remain active once a threshold length (~1.3x plate width) is reached. Subsequently, the back-arc basin and neighbouring plates are strongly coupled, and ongoing trench retreat localizes stresses and rapidly ruptures the overriding plate closer to the trench while the old spreading centre is abandoned.  In a parameter study, the results further explain why the narrowest subduction zones, such as the Calabrian Arc, experience more frequent and closer spreading jumps than the long-period jumps of a wider subduction zone such as the Scotia Arc. The widest subduction zones should not undergo any back-arc spreading jumps with this mechanism, consistent with other natural examples.

How to cite: Schliffke, N., van Hunen, J., Gueydan, F., Magni, V., and Allen, M. B.: The role of transform faults during back-arc spreading centre jumps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16952, https://doi.org/10.5194/egusphere-egu2020-16952, 2020.

D1368 |
Martin Vallée, Raphaël Grandin, Jean-Mathieu Nocquet, Juan-Carlos Villegas, Sandro Vaca, Yuqing Xie, Lingseng Meng, Jean-Paul Ampuero, Patricia Mothes, and Paul Jarrin

According to GlobalCMT, the 2019/05/26 North Peru earthquake is the largest event since 1976 in the wide depth range between 70km and 550km. Its hypocentral location (at about 130km depth) inside the Nazca slab geometry, together with its normal focal mechanism, favor an origin related to slab bending. Owing to its magnitude and depth, this earthquake generated large coseismic displacements over a broad area, that were geodetically measured by InSAR and GNSS. By combining these observations with regional and teleseismic data, we invert for the rupture process of the event, and first focus on the actual focal plane. Inversion reveals that the steeper plane (dipping 55-60° to the East) is preferred. A clear northward propagation is also imaged, with rupture traveling ~200km in 60s, and with little extent in the dip direction. This narrow rupture aspect implies that the stress drop is significant, even if a simple duration-based measurement would not indicate so. These inversion results obtained at relatively low frequency (below 0.2Hz) are then thoroughly compared with back-propagation images obtained at higher frequency (at 0.5-4Hz), which also highlight the dominantly northward rupture propagation with an average rupture speed of about 3 km/s. Implication in terms of earthquake rupture dynamics and occurrence of such large intermediate depth earthquakes in slabs will finally be discussed.

How to cite: Vallée, M., Grandin, R., Nocquet, J.-M., Villegas, J.-C., Vaca, S., Xie, Y., Meng, L., Ampuero, J.-P., Mothes, P., and Jarrin, P.: Rupture characteristics of the 2019 North Peru intraslab earthquake (Mw8.0), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10429, https://doi.org/10.5194/egusphere-egu2020-10429, 2020.

D1369 |
Uğurcan Çetiner, Oğuz Göğüş, and Antoine Rozel

The evolution from stagnant/episodic lid to modern-day plate tectonics on earth is not well understood. Geochemical and geomorphological findings indicate that Archaean Eon is the most likely candidate for the onset of plate tectonics. In order to have plate tectonics, the oceanic lithosphere has to be denser than the asthenosphere and subducting slabs must be rheologically strong so that it would stay intact/undeformed during subduction. Our study focuses on investigating the initiation of subduction on the margins of an Archaean craton/continent based on the subcretion tectonic model of Bédard (2018). Here, we use 2-D mantle convection models (StagYY) to understand the controlling parameters for possible subduction or lithospheric downwellings. A 230 km thick craton accompanied by a 60 km thick oceanic lithosphere on both sides is introduced into the model setup. The model domain is divided by 64 vertical cells and 512 lateral cells corresponding to 660 km depth and 2000 km length. Both for the upper and lower boundary, free-slip surface conditions are used. Left and right boundaries are periodic. Velocities are forced to be zero until a critical depth of 60 km, after that a sub-lithospheric mantle flow of 4 cm/yr imposed into the model which is a proxy for a disturbance generated within the mantle by the “overturn upwelling zones”. Our results indicate that cratonic keels can be mobilized by the sub-lithospheric mantle winds and what happens afterward is highly dependent on the surface yield stress, eclogite phase transition depth, deformation mechanism and, most importantly, reference mantle viscosity. Lower viscosity (1019 Pa s) models resulted in a stagnant-lid regime while the others with the increased viscosity (1020 Pa s – 1021 Pa s) yielded in a transition from stagnant to plate-like behaviors.

How to cite: Çetiner, U., Göğüş, O., and Rozel, A.: From Drip To Plate: Did Subduction Start in Archaean?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19761, https://doi.org/10.5194/egusphere-egu2020-19761, 2020.

D1370 |
Giridas Maiti, Joyjeet Sen, and Nibir Mandal

Subduction zones witness exhumation of deep crustal rocks metamorphosed under high pressure (HP) and ultra-high pressure (UHP) conditions, following burial to depths of 100 km or more. The exhumation dynamics of HP and UHP rocks still remains a lively issue of research in the Earth science community. We develop a new tectonic model based on the lubrication dynamics to show the exhumation mechanism of such deep crustal rocks in convergent tectonic settings. Our model suggests subducting plate motion produces a dynamic pressure in the subduction wedge, which supports the excess gravitational potential energy of the thicker and relatively denser overriding plate partly lying over the buoyant subduction wedge. A drop in the dynamic pressure causes the overriding plate to undergo gravitational collapse and forces the wedge materials to return to the surface. Using lubrication theory we estimate the magnitude of dynamic pressure (P) in the wedge as a function of subduction velocity (us), convergence angle (α) and wedge viscosity (µ). We also conduct thermo-mechanical numerical experiments to implement the lubrication model in subduction zones on a real scale. Our analysis suggests that drop in wedge dynamic pressure below a threshold value due to decease in us  and µ, or by other processes, such as slab rollback and trench retreat facilitate exhumation of deep crustal rocks. Finally we discuss their implications for the exhumation of deep crustal rocks in different subduction setups such as the Himalayan continental subduction, the Mediterranean realm (Calabria–Apennine and Aegean belts) and Western Alps.

How to cite: Maiti, G., Sen, J., and Mandal, N.: Lubrication Dynamics for Exhumation of high-pressure Rocks in Subduction Zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-314, https://doi.org/10.5194/egusphere-egu2020-314, 2020.

D1371 |
Agustina Pesce and Victor Sacek

One challenge to numerically simulate the subduction of cold oceanic lithosphere under continental lithosphere is the preservation of the decoupling between the subducting and upper plates for tens of millions of years. One strategy to simulate the persistence of the decoupling is the continuous entrainment of a weak layer (i.e. with low effective viscosity) at the top of the oceanic plate, representing a lubrication between both plates. However, variations on the thickness and rheological structure of this weak layer affect the geodynamic evolution of the subducting plate, modifying the geometry and degree of interactions between the lithospheric plates. 

In the present work we evaluated how the variation of the geometry, viscosity and density of the weak layer, relative to the surrounding lithosphere, can affect the lubrication between the two lithospheric plates. We performed a series of 2D numerical simulations using a finite element code for thermochemical convection. The code solves the Stokes flow for a fluid using the Boussinesq approximation in a Cartesian coordinate system, considering that the viscosity varies exponentially as a function of the temperature. In the present visco-plastic approach, the effective viscosity is determined by the combined effect of a viscous component, assuming the Frank-Kamenetskii rheology, and plastic deformation, following the Byerlee's friction law. 

In our numerical scenarios, the subduction is produced by the negative buoyancy of the cold oceanic lithosphere, without the imposition of an external velocity as boundary conditions. The time range of the simulation is of the order of 50 million years. In the initial simulation, a weak zone is imposed in the region between the two plates. This zone presents low viscosity and density relative to the surrounding lithosphere. As the oceanic slab is subducted, the weak zone is deformed and dragged. This removes the lubrication until utterly coupling the lithospheric plates, generating the thickening of the continental lithosphere below the trench region. To preserve the decoupling along all the simulation time, an extra continuous weak layer on top of the oceanic plate is added with low density and viscosity. In this scenario, the first weak zone is still dragged by the subducting plate, but the additional weak layer keeps a lubrication zone between the plates, preventing the coupling of the two lithospheric plates. Therefore, adding a continuous weak layer on top of the oceanic crust together with a weak zone prevents the coupling of the subducting and overriding plates when the effective viscosity of the weak layer is smaller than ~1019 Pa s. These numerical scenarios are used to analyse the subduction pattern of the Nazca plate observed in the southeastern portion of South America, using as constraints the slab geometry of the subducting oceanic plate derived from the Slab2 model.

How to cite: Pesce, A. and Sacek, V.: Evaluation of the presence of a weak layer in the numerical simulation of lithospheric subduction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-734, https://doi.org/10.5194/egusphere-egu2020-734, 2020.

D1372 |
Yossi Mart

Oceanic core complexes are lithological assemblages of peridotites and serpentinites, embedded in the basaltic oceanic crust at active or dormant intersections of several slow-spreading oceanic accreting rifts with fracture zones. These occurrences are presumed to derive from the upper mantle, emplaced by low-angle and large-throw normal detachment faults. The abundant serpentinites are attributed to alteration of the ultramafic peridotites during its long ascent from the upper mantle. However the absence of both high-pressure lithologies in the oceanic core complexes and the rareness of earthquakes generated by low-angle normal faulting cast doubt on the validity of this conventional model. Alternately, analog tectonic experiments showed that subduction is a probable process for the generation of oceanic core complexes, because it could develop between two juxtaposed tectonic slabs if their density contrast will exceed 200 kg/m3 with no lateral converging pressure, if the friction between the slabs were low. Indeed oceanic core complexes occur in unique oceanic domains where two basaltic slabs of contrasting densities are juxtaposed across a weakness zone of low friction. Density of fresh basalt at the accreting ridge is approximately 2700 kg/m3 and that of the older basalts, juxtaposed across the fracture zone, is ca. 2900 kg/m3. Slow spreading rates of some ridges would set slabs of significant density contrast across the fracture zone even if the transform offsets are not large. Furthermore, the thermal gradient under the ridge is some 1300/km, enabling the metamorphism of the oceanic basalts either to serpentinites or to peridotites at similar P-T constraints, depending on the availability of water. Therefore, it seems that the serpentinites are not secondary products of source-rock alteration, but genetic equivalents to the peridotites. It is presumed therefore that the pliable serpentinite would ascend diapirically through cracks in the over-riding basaltic slab and reach the seafloor, carrying along large blocks of peridotite to produce the serpentinite-peridotite petrology, that lithological association of oceanic core complexes.

How to cite: Mart, Y.: The geodynamics of oceanic core complexes: would subduction occur at ridge-transform intersections? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1565, https://doi.org/10.5194/egusphere-egu2020-1565, 2020.

D1373 |
Yang Liu, Ziyin Wu, Jihong Shang, Dineng Zhao, and Jieqiong Zhou

Different tectonic backgrounds often produce different subduction mechanisms. The Mariana subduction zone is a typical erosive margin, and the mode of material transportation is mainly controlled by subduction erosion, while the subduction process in the northern Manila subduction zone is dominated by subduction accretion. However, there are little comparative investigation about the subduction mechanisms between the Mariana subduction zone and northern Manila subduction zone. In this study, the high-resolution bathymetric data obtained by using the multi-source data fusion method and collected multichannel seismic profiles are used to research the subduction mechanisms and to develop the subduction modes for the Mariana subduction zone and northern Manila subduction zone. We propose that the Mariana subduction zone formed at the intra-oceanic convergent margins with rare continental sediments tends to occur subduction erosion. A rough seafloor morphology (e.g. seamounts, horst and graben topography) of the subducting Pacific Plate, with a convergence rate of 8.4 cm/yr, and the steep slope of the inner trench, promote subduction erosion at the Mariana margin. The northern Manila subduction zone is the result of the convergence of ocean-continent plates. The continental sediments of the overlying plate usually undergo subduction accretion during the subducting process, forming an accretionary wedge along the northern Manila margin. With the continuously subducting of the continental crust, a series of folds and thrust faults are formed inside the accretionary wedge. Both the Mariana subduction zone and northern Manila subduction zone are distinctive types of the convergent margins in the world. The comparison of subduction mechanisms has important reference significance for the study of the subduction process, evolution and inter-plate interaction of global intra-oceanic and ocean-continent convergent margins.

How to cite: Liu, Y., Wu, Z., Shang, J., Zhao, D., and Zhou, J.: Subduction processes on the Mariana trench and northern Manila trench: implications for the intra-oceanic and ocean-continent convergent margins, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1787, https://doi.org/10.5194/egusphere-egu2020-1787, 2020.

D1374 |
Christian Sippl, Timm John, and Stefan Schmalholz

The origin of double seismic zones (DSZs), parallel planes of intraslab seismicity observed in many subduction zones around the globe, is still highly debated. While most researchers assume that fluid release from prograde metamorphic reactions in the slab is an important control on DSZ occurrence, the role of slab unbending is currently unclear.
Slab bending at the outer rise is instrumental in hydrating the downgoing oceanic plate through bend faulting, and is evident from earthquake focal mechanisms (prevalence of shallow normal faulting events). Observations from NE Japan show that focal mechanisms of DSZ earthquakes are downdip compressive in the upper and downdip extensive in the lower plane of the DSZ, which strongly hints at slab unbending. This coincidence of slab unbending and DSZ seismicity in NE Japan has given rise to several models in which unbending forces are a prerequisite for DSZ occurrence.

To globally test a potential correlation of slab unbending with DSZ seismicity, we derived downdip slab surface curvatures on trench-perpendicular profiles every 50 km along all major oceanic slabs using the slab2 grids of slab surface depth. We here make a steady-state assumption, i.e. we assume that the slab geometry is relatively constant with time, so that the downdip gradient of slab curvature corresponds to slab (un)bending. We compiled the loci and depth extent of all DSZ observations avalable in literature, and compare these to the slab bending or unbending estimates.

Preliminary results indicate that while there is a clear correspondence between the depth of slab unbending to DSZ seismicity in the Japan-Kurile slab, most other slabs do not show this correlation. Moreover, some DSZs deviate from the above-mentioned focal mechanism pattern and exhibit downdip extension in both planes (e.g. Northern Chile, New Zealand). It appears that the global variability of slab geometries in the depth range 50-200 km is larger than anticipated, and DSZ seismicity is not limited to slabs where unbending is prevalent at these depths. The Northern Chile case is especially interesting because focal mechanisms there not only do not fit the pattern observed in NE Japan, but also can not be explained with the current slab geometry alone. This could indicate a direct influence of ongoing metamorphic reactions on focal mechanisms (e.g. via volume reduction and densification), or it may be a hint that our steady-state assumption is invalid for the Nazca slab here (i.e. that it is in the process of changing its geometry).

How to cite: Sippl, C., John, T., and Schmalholz, S.: A closer look at the relationship between slab (un)bending and double seismic zone seismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2000, https://doi.org/10.5194/egusphere-egu2020-2000, 2020.

D1375 |
Nestor G. Cerpa and Diane Arcay

Topography at subduction zones is the result of multiple processes operating at various temporal and spatial scales. At intermediate wavelengths (~100 km), the models predict the formation of dynamically-induced flexural topography that affects the overriding plate (OP) from the trench to the back-arc [Davies 1981, Crameri et al., 2017]. In our study, we assess how the velocity of the OP affects such a non-isostatic topography by using numerical mechanical models of subduction. We particularly investigate the effects of changes in OP velocity on the evolution of topography.

Our models consist of two converging visco-elastic plates with free surfaces. Friction is imposed along the planar subduction interface. We consider an isoviscous upper mantle with an impermeable barrier at a 660-km depth. We consider cases where the subducting plate (SP) has reached the bottom of the upper mantle and has a stationary motion. The models are performed with the code ADELIM [Cerpa et al., 2014].

We first characterize the main topographic features at a constant OP velocity, using spatial definitions that are based on estimations of the volcanic arc position. The models exhibit the formation of a bulge in the forearc area followed landwards by a depression and a smaller second bulge, the latter two of which are predicted to bracket the arc region. The steady-state distance to the trench of these three flexural features increase with OP velocity. Their amplitude is more sensitive to kinematics when the interplate friction is high and less when the SP viscosity is low.

We next test the effect of sudden changes in OP velocity. An OP acceleration yields a transient topographic tilt, during which the outer forearc quickly subsides whereas the arc region uplifts. The tilt is followed by reverse slower motions. An OP slowdown induces opposite motions. The rates of elevation during the tilt are approximately proportional to velocity variations and mainly sensitive to the SP strength. They are higher than 0.1 mm/yr for velocity changes higher than 1 cm/yr. We suggest that topographic accommodations of OP velocity changes should be considered when quantifying non-isostatic topography.

How to cite: Cerpa, N. G. and Arcay, D.: The influence of overriding-plate velocity on surface topography in subduction zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4770, https://doi.org/10.5194/egusphere-egu2020-4770, 2020.

D1376 |
Hongwei Zheng

The Tongbai-Dabie Orogenic belt formed in the Middle-to-Late Triassic through a collision between the Yangtze Block (YB) and North China Block (NCB) and is a key component of the Central Orogen of China, which is famous on the most extensive high and ultrahigh pressure (HP/UHP) metamorphic zone in the world and marks the irregular suture between the YB and NCB. It is an ideal place to study the ancient orogenic processes between collided continents. In this study, we used a large number of P-wave arrival times recorded by portable and permanent seismic stations to reveal the structure of the crust and upper mantle beneath the Tongbai-Dabie orogenic belt and its adjacent region. Our images show the south-dipping high-velocity anomalies beneath the Tongbai-Dabie orogenic belt and the east-dipping high-velocity anomalies beneath the Tanlu Fault, which represent the southeastward subducted NCB in Mesozoic. While a huge high-velocity anomaly beneath the Wudang Moutin region extending down to 250 km is possible the ancient lithosphere of the Yangtze Craton remnant since the Paleoproterozoic. The northward subducted YB is only limited in the Eastern Dabie terrane and Yangtze foreland. Break-off retained Paleo-Tethyan oceanic slab are revealed at depths from the upper mantle 250 to 400 km. The structure of the crust and upper mantle suggests that the southeastward subduction of NCB resulted in the collision of NCB with YB.

How to cite: Zheng, H.: Seismic evidence for the subduction of North China Block to Yangtze Block beneath Tongbai-Dabie Orogenic belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6341, https://doi.org/10.5194/egusphere-egu2020-6341, 2020.

D1377 |
| Highlight
Valentina Magni and Manel Prada

The morphology of back-arc basins shows how complex their formation is and how pre-existing lithospheric structures, rifting and spreading processes, and subduction dynamics all have a role in shaping them. Often, back-arc basins present multiple spreading centres that form one after the other (e.g. Mariana subduction zone), propagate and rotate (e.g., Lau Basin) following trench retreat. Episodes of fast and slow trench retreat can cause rift jumps, migration of magmatism, and pulses of higher crustal production (e.g., Tyrrhenian Basin). The evolution of a back-arc basin is not only tightly linked to subduction dynamics, but it is likely that the composition and the pre-existing structure of the lithosphere play a role in shaping the basin too. In this work, we investigate the interplay between these features with numerical models of lithospheric extension with a visco-plastic rheology. We use the finite element code ASPECT to model the rifting of continental and oceanic lithosphere with boundary conditions that simulate the asymmetric type of extension caused by the trench retreat. We perform a parametric study in which we systematically change key parameters such as crustal composition and thickness, initial thermal structure and rheology of the lithosphere, and rate of extension. These models aim at understanding how pre-existing lithospheric structures affect back-arc rifting and spreading and what processes control spreading centres jumps in back-arc settings. Preliminary results show that time-dependent boundary conditions that simulate episodes of fast trench retreat, thus fast extension, play an important role into the style of lithospheric back-arc deformation. Finally, we will compare our model results with the location and timing of back-arc rifting and spreading in different active and inactive back-arc basins.

How to cite: Magni, V. and Prada, M.: Linking subduction dynamics to back-arc deformation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7092, https://doi.org/10.5194/egusphere-egu2020-7092, 2020.

D1378 |
Michaël Pons and Stephan Sobolev

The Andean orogeny is a subduction-type orogeny, the oceanic Nazca Plate sinks under the continental South American Plate. While the subduction has been active since ~180 Ma, the shortening of the Andes initiated at ~50 Ma or less.

In a oceanic-continental subduction system, the absolute velocity of the overriding-plate (OP) largely controls the style of subduction (stable, advancing, retreating), the geometry of the slab (dipping angle, curvature) and the style of deformation (shortening or spreading) within the OP. In the case of the Central Peru-Chile subduction, the South American plate is advancing westwards whereas the Nazca plate is anchored into the transition zone (~660 km). As a consequence, the trench is forced to retreat and the Nazca plate to roll-back. The dip of the slab decreases meanwhile the Andes experienced a maximum shortening of ~300 km at ~19-21°S latitudes.

Previous study have shown that the strain localizes within areas of low strength and low gravitational potential of energy. In central Andes, weakening mechanisms of the OP such as lithospheric delamination have intensified the magnitude of tectonic shortening and contributed to formation of the Altiplano-Puna plateau. The deformation between the plateau and the foreland occurs in the form of pure shear or simple shear and is expressed in terms of different tectonic styles in the foreland basin, thick-skinned (e.g the Puna) and thin-skinned (e.g the Altiplano), respectively. Nevertheless, the influence of the strength variations of the OP on the subduction dynamics in the case of the central Andes has been poorly explored so far. Our hypothesis is that lateral variations of OP strength result in variable rates of trench roll-back. To test it, we have built 2D high-resolution E-W cross sections along the Altiplano and Puna latitudes (12-27°S) including the subduction of the Nazca plate. For that purpose, we used the FEM geodynamic code ASPECT. Our model includes visco-plastic rheology in addition to gabbro-eclogite phase transition. These preliminary results contribute to the discussion on the nature of the magnitude of shortening in a subduction system. They are also a first step to derive a 3D model of the entire region and to consider additional surface processes such as erosion, transportation and sedimentation.

How to cite: Pons, M. and Sobolev, S.: The nature of the North-South change of the magnitude of tectonic shortening in Central Andes at Altiplano-Puna latitudes: a thermomechanical modeling approach., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8177, https://doi.org/10.5194/egusphere-egu2020-8177, 2020.

D1379 |
Taco Broerse, Ernst Willingshofer, Dimitrios Sokoutis, and Rob Govers

Tearing of the lithosphere at the lateral end of a subduction zone is a consequence of ongoing subduction. The location of active lithospheric tearing is known as a Subduction-Transform-Edge-Propagator (STEP), and the tearing decouples the down going plate and the part of the plate that stays at the surface. STEPs can be found alongside many subduction zones, such as at the south Caribbean or the northern end of the Tonga trench. Here we investigate what controls the evolution and geometry of the lithospheric STEP. Furthermore we study the type of lithosphere deformation in the vicinity of STEPs.


We study the ductile tearing in the process of STEP evolution by physical analogue models, which are dynamically driven by the buoyancy of the subducting slab. In our experiments, the lithosphere as well as asthenosphere are viscoelastic media in a free subduction setup. A stress-dependent rheology plays a major role in localization of strain in tearing processes of lithosphere such as slab break-off. Therefore we developed and tested analogue materials that can serve as mechanical analogues for the stress-dependent lithosphere rheology, such as has been inferred by rock laboratory test for dislocation creep of olivine.


We show the influence of age and integrated strength of the lithosphere and its contrasts across the passive margin, on the timing, depth, and the degree of localization of the tearing process. When tearing of the lithosphere is dominated by ductile deformation, we find that gradual necking of the passive margin precedes tearing. In many of our models we find that tearing at the lateral ends of the subduction zones is resisted by the lithospheric strength, such that tearing is delayed with respect to rollback of the slab. This has consequences for the shape of the subduction zone, and for the separation between the subducted slab and the surface lithosphere. We study the type of deformation in the vicinity of the STEP of the lithosphere that stays at the surface, and relate this to deformation observed beside STEP fault zones along the Hellenic slab, the Lesser Antilles slab, and the New Hebrides slab. 

How to cite: Broerse, T., Willingshofer, E., Sokoutis, D., and Govers, R.: Lithosphere deformation due to tearing at STEPs: an analogue model approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8721, https://doi.org/10.5194/egusphere-egu2020-8721, 2020.

D1380 |
Lisa Eberhard, Philipp Eichheimer, Wakana Fujita, Marcel Thielmann, Michihiko Nakamura, Gregor J. Golabek, and Daniel J. Frost

The process of dehydration in subduction zones is important for (i) element recycling, (ii) hydration of the mantle wedge, as well as (iii) melting processes related to arc volcanism. Additionally, the release of fluids is also proposed to be related to the origin of deep earthquakes. The transport of fluids from the slab into the mantle requires a sufficiently high permeability. Due to shear deformation in subduction zones a strong foliation will be developed by preferred orientation of serpentinite minerals, which might influence the permeability. Measurements of permeability up to 100 MPa indeed showed that the foliation yields a strong anisotropy in serpentinites. The permeability parallel to the foliation is one order of magnitude higher than in the perpendicular direction.

Here we present a method to estimate the fluid flux in serpentinites at mantle conditions combining laboratory experiments, X-ray CT scans and numerical modelling. For this purpose, we performed HP-multi-anvil experiments at temperatures of 500 °C to 700 °C and pressure up to 2.5 GPa. As starting material we used a natural antigorite sample showing a strong foliation. A cylindrical drill core is placed into an MgO sleeve. The MgO is hydrated to brucite at the PT conditions at which serpentine is still stable, i.e. serpentine partially dehydrates and brucite is formed as the released fluid moves into the MgO sleeve. After the experiment the location and proportion of brucite formed allows the preferred fluid flux to be determined. The formation of 5 times less volume per unit area of brucite in the direction perpendicular to the foliation indeed indicates a preferred fluid flow parallel to the preferred orientation.

In a second step we employ CT scans to obtain data on the pore space of the samples. Finally, using numerical methods, we determine both the porosity as well as the permeability of the recovered samples. Combined, these methods can be used to obtain a model of fluid flow in subduction zones.

How to cite: Eberhard, L., Eichheimer, P., Fujita, W., Thielmann, M., Nakamura, M., Golabek, G. J., and Frost, D. J.: Permeability of serpentinites at high PT: towards fluid flow determination in subduction zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9624, https://doi.org/10.5194/egusphere-egu2020-9624, 2020.

D1381 |
István Bozsó, Ylona van Dinther, Liviu Matenco, and István Kovács

Numerous subduction systems in the Meditteranean realm are derived from the subduction of narrow oceanic domains, which are too narrow to generate the means of a fully coupled two-dimensional thermo-mechanical numerical model that takes into account the visco-elasto-plastic properties of different lithospheric domains. The results show that the narrow extent of the Ceahlau-Severin Ocean commonly assumed by paleogeographic reconstruction cannot generate roll-back upon subduction, in particular for models that must assume that slabs do not penetrate the 660 km discontinuity. Therefore, we propose that the subduction of the Carpathians system must have an inherited component from a previous orogenic evolution, which will ensure sufficient slab-pull to generate roll-back in the Carpathians realm. The model is constrained by recent results in terms of mantle structure and geodynamic reconstructions, while multiple compositional, thermal distribution and geometrical scenarios are tested in successive models. In all of our models, roll-back is achieved, which indicates that the proposed inherited component can sufficiently explain the roll-back subduction of the aforementioned narrow oceans. The subducting oceanic slab does not penetrate the 660 km discontinuity, this is in agreement with seismic tomographic results from various Mediterranean subduction zones. The exact onset and dynamics of the roll-back are mostly controlled by the thermic age of the ocean and the convergence kinematics of the continental slabs. An outlook on possible future improvements to the model, such as taking into account pre-existing rheological weakness zones in the lithosphere, is discussed and the opportunity of a seismo-thermo-mechanical modelling to investigate the seismic cycle in the Vrancea-zone is highlighted.

How to cite: Bozsó, I., van Dinther, Y., Matenco, L., and Kovács, I.: Towards understanding the roll-back subduction of narrow oceanic domains: inferences from the modelling of Carpathians subduction zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16395, https://doi.org/10.5194/egusphere-egu2020-16395, 2020.

D1382 |
Ben Maunder, Saskia Goes, and Jeroen van Hunen

The subduction zone coupling transition depth, CTD, marks the transition from frictional/ductile decoupling between the two plates to viscous coupling between the subducting plate and convecting mantle. This depth plays an important role in the state of stress, earthquake potential, and the location of the volcanic arc. Based on previous studies of heat flow and seismic structure of circum-Pacific subduction zones, the CTD has been inferred to at a constant 70-80 km. The mechanism for this constancy remains elusive, although models have reproduced the sharpness of the CTD as a consequence of the evolving strength contrast between a frictional (damage) type rheology along the interface and temperature and stress dependent viscosity in the plates and mantle . Using kinematically driven subduction models with such rheology, we find a relationship between the CTD, slab age and velocity that predicts that 91 % of Pacific subduction zones should have an CTD between 65 and 80 km depth, consistent with observations. However, some other zones are predicted to have significantly deeper or shallower CTD.  For example, a 120 km CTD recently found in the Lesser Antilles can be explained by our models . Sub-arc slab depth is bound by a similar age-velocity relation to that derived for the CTD, but offset to ~50 km larger depths. Hence rheology exerts the primary control on the CTD, and the coupling transition depth is in fact not constant but varies with plate age and convergence.

How to cite: Maunder, B., Goes, S., and van Hunen, J.: The coupling transition depth in subduction zones: rheologically controlled and not constant, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21869, https://doi.org/10.5194/egusphere-egu2020-21869, 2020.

D1383 |
Hao Kuo-Chen, Zhuo-Kang Guan, Wei-Fang Sun, and Chun-Rong Chen

The Taiwan orogeny is forming along a complex plate boundary in which the Eurasian Plate (EUP) is subducting eastward beneath the Philippine Sea Plate (PSP). This complex plate boundary is situated in eastern Taiwan and results in large earthquakes occurred frequently in this region. For instance, in 1951, 1972, 1986, 2003, 2006, 2013, 2018, and 2019, earthquakes with magnitude greater than 6 occurred near or along the plate boundary and most of them caused serious damages. However, due to the complexity of the plate boundary from south to north of eastern Taiwan, the seismogenic structures for those events are very different. In order to understand the tectonic structures thoroughly in eastern Taiwan, we planned a integrated geophysical experiment, including seismic reflection, dense seismic array deployments, and magnetic survey from 2016 to 2020. There are 8 seismic reflection profiles along the Longitudinal valley from north to south. As a result, the seismic images show that the sedimentary deposits can reach ~1 km thickness in the northern part and is shallower toward to the southern part. The rocks below the sedimentary deposits are from the east flank of the Longitudinal valley, which belongs to the Eurasian plate. The dense array deployments from 2016-2019 around eastern Taiwan with 1-5 km spacing and totally more than 300 short-period stations deployed. During the deployments, we have captured two aftershock sequences in the north of eastern Taiwan in 2018 and 2019. The seismogenic zones with high-resolution tomography from dense seismic array data sets reveal that the plate interaction between the EUP and PSP. The physical behaviors of the seismogenic zones are related to the collision to subduction along the plate boundary from south to north. Also, the results of the magnetic survey in eastern Taiwan show that the high magnetic anomalies only sparsely distribute, which indicates the volcanic arc may not widely occupy than previous geological investigation. The results of this experiment provide a new thought of the tectonic processes along the plate boundary in eastern Taiwan.

How to cite: Kuo-Chen, H., Guan, Z.-K., Sun, W.-F., and Chen, C.-R.: Integrated geophysical data for investigating the tectonic structures in eastern Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21008, https://doi.org/10.5194/egusphere-egu2020-21008, 2020.

D1384 |
Leoncio Cabrera, Sergio Ruiz, Piero Poli, Eduardo Contreras-Reyes, Renzo Mancini, and Axel Osses

We investigate the differences of the seismic source and aftershock activity using kinematic inversions and template matching respectively, for the six largest intraslab intermediate-depth earthquakes occurred in northern Chile (Mw ~6.3) since 2010 at depths between 90 and 130 km and recorded by dense strong-motion and broad-band seismic networks. In addition, we developed a thermal model using the finite element method in the study region with the aim of analyze the impact of temperature on seismic behavior as the oceanic plate subducts. Our results show that geometries of rupture zones are similar, with semi-axis for an elliptical patch approach about 5 km, and stress drop values between 7 and 30 MPa. On the other hand, the number of aftershocks exhibits clear differences, and their amount decreases with increasing the depth within the slab bounded by the 450 ºC isotherm, which represents a limit between a high-hydrated and a dry or low-hydrated region. Furthermore, mainshocks occur at distances from the top of the slab from 7 to 40 km, and all of them exhibit normal focal mechanisms suggesting that the extensional regimen deepens within the slab to the 700-750 ºC isotherm-depth. We suggest that in northern Chile the abrupt decrease of aftershocks in the lower part of the extensional regimen is caused by the absence of a hydrated slab at those depths.

How to cite: Cabrera, L., Ruiz, S., Poli, P., Contreras-Reyes, E., Mancini, R., and Osses, A.: Aftershock Activity at Intermediate-Depth Earthquakes in Northern Chile Controlled by Plate Hydration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11909, https://doi.org/10.5194/egusphere-egu2020-11909, 2020.

D1385 |
Joaquin Bastias, Richard Spikings, Alexey Ulianov, Teal Riley, Anne Grunow, Massimo Chiaradia, Urs Schaltegger, and Alex Burton-Johnson

We present new geochemical, isotopic and geochronological analyses of Late Triassic-Jurassic volcanic and intrusive rocks of the Antarctic Peninsula and Patagonia. Whole-rock geochemical data suggest that all of these igneous units formed in an active margin setting. This conclusion challenges the current paradigm that Jurassic magmatism of the Chon Aike province formed by the migration of the Karoo mantle plume from Africa towards the Pacific margin (Pankhurst et al., 2000). KDE analysis of 98 crystallisation ages reveals four main pulses of magmatism (V0: ~223-200 Ma; V1: ~188-178 Ma; V2: ~173-160 Ma; V3: ~157-145 Ma), which are approximately coincident with the episodic nature of the Chon Aike Magmatic Province reported by Pankhurst et al. (2000). Some magmatic units in eastern Patagonia are distal to the hypothetical paleo-trench relative to most active margin magmatism. These rocks have geochemical and geochronological characteristics that are indistinguishable from active margin-related rocks located ~200km from the palaeo-trench. Thus, we propose that a segment of the slab formed a flat-slab along southwestern Gondwana during the Late Triassic-Jurassic. This flat-slab is probably a temporal extension of the flat-slab episode suggested by Navarrete et al. (2019) for the Late Triassic (V0 episode) in eastern Patagonia. The progressive migration of the flat-slab magmatism to the southwestern margin of Patagonia suggest an evolution of its architecture during the Jurassic. Further, we propose that the flat-slab magmatism present in eastern Patagonia was triggered by slab failure, where foundering of the slab drove upwelling of hot mantle, forming a broad arc in an inland position in eastern Patagonia. Flat-slab subduction finished during the V3 episode (~157-145 Ma), with a continuation of an active margin along the western margin of the Antarctic Peninsula and Patagonia. Coeval extension in the South Atlantic and in western Patagonia lead to sea floor spreading, the formation of the Weddell Sea (~155-147 Ma; e.g. Konig & Jokat. 2006) and the Rocas Verdes Basin (~150 Ma; e.g. Calderon et al., 2007), respectively. The paleogeographic reconstructions juxtapose the northern Antarctic Peninsula and southern Patagonia during the Late Jurassic (e.g. Jokat et al., 2003), which suggest that the Rocas Verdes Basin and the Weddell Sea are oriented by a ~120° angle and potentially meet in southern Patagonia. This junction of sea-floor spreadings corresponds to the limits of the southern Rocas Verdes Basin with the eastern Weddell Sea oceanic lithosphere. We suggest that these rifts formed part of a triple junction, while the third rift arm should be located with a sub north-south orientation in the Antarctic Peninsula. Vast regions of the Antarctic Peninsula remain unexplored beneath the ice-cap, although we speculate that the third arm may correspond to the Eastern Palmer Land Shear Zone, which currently has a lateral extension of ~1500km (Vaughan & Storey, 2000). This new triple junction would be a Ridge-Ridge-Transform Fault intersection.

Calderon et al. 2007. JGS,164: 1011-1022.

Jokat et al. 2003. JGR, 108: 2428.

Konig & Jokat. 2006, 111: B12102.

Pankhurst et al. 2000. JP, 41(5): 605-625.

Navarrete et al. 2019. ESR, 194: 125-159.

Vaughan & Storey. 2000. JGS, 157: 1243-1256.

How to cite: Bastias, J., Spikings, R., Ulianov, A., Riley, T., Grunow, A., Chiaradia, M., Schaltegger, U., and Burton-Johnson, A.: Late Triassic-Jurassic active margin magmatism in southwestern Gondwana: implications for the tectonic evolution of the Antarctic Peninsula, Patagonia and the Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19892, https://doi.org/10.5194/egusphere-egu2020-19892, 2020.

D1386 |
Sascha Zertani, Johannes C. Vrijmoed, Frederik Tilmann, Timm John, Torgeir B. Andersen, and Loic Labrousse

Eclogitization occurs deep in subduction and collision zones inaccessible to direct observation. Field-based studies dealing with crustal material previously transformed at eclogite-facies conditions and exhumed to the surface provide information from the micro scale up to a few kilometers. On the other hand, geophysical methods aim at imaging the ongoing processes in-situ. However, these methods are limited by the achievable resolution and typically only sensitive to structures a few kilometers in size, leaving a large gap between the scales at which observations are interpreted. In this study we try to discern the implications of structures mapped in field-based studies to interpretations of geophysical imaging. We therefore calculated effective anisotropic P wave velocities for a suite of representative structural associations using the finite element method. The structural associations are directly extracted from observations of partially eclogitized assemblages on the island of Holsnøy in the Bergen Arcs of western Norway. Physical properties of the constituting lithologies are taken from laboratory measurements of the same rocks and the calculations are performed on a variety of scales, from the 20-m scale up to the kilometer scale to be able to predict how the effective seismic properties change with varying scale. Our results show that the P wave velocity of the effective medium is solely controlled by the volumetric fraction of the constituting lithologies and their elastic properties. We find that the structural relationship of the different lithologies has no significant influence on the resulting seismic velocities. P wave anisotropy, however, is controlled by the constituting lithology with the highest initial anisotropy and to a lesser extent by the modal abundance of the different lithologies. Further, our results show that seismic anisotropy is largely transferable across scales validating the assumptions often made when measuring seismic velocities on centimeter-sized sample volumes. On the kilometer scale, a scale that is potentially resolvable by geophysical methods, our results show that an eclogite-facies shear zone network such as the one exposed on Holsnøy would indeed produce a significant P wave anisotropy on a crustal scale. This anisotropy is produced by the eclogite-facies shear zones themselves even though eclogites are typically considered to be low-anisotropy rocks. Comparison of our results with active settings of continental collision and subduction zones reveals that eclogite-facies shear zones have the potential to produce a significant backazimuthal bias of the retrieved signal in geophysical imaging and underline the significance of seismic anisotropy as a tool to further increase the sensitivity of seismological methods to lithological variations.

How to cite: Zertani, S., Vrijmoed, J. C., Tilmann, F., John, T., Andersen, T. B., and Labrousse, L.: P wave anisotropy caused by partial eclogitization of descending crust demonstrated by modeling effective petrophysical properties , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7456, https://doi.org/10.5194/egusphere-egu2020-7456, 2020.