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- Welcome, i build and test numerically stories of deformation processes in geodynamic and volcanology settings, involv... moreWelcome, i build and test numerically stories of deformation processes in geodynamic and volcanology settings, involving elastic, plastic, viscous and fluid rock mechanics that might explain some mysterious observations!edit
Une modélisation numérique préliminaire permet de contraindre la façon dont la déformation s'accumule sur les marges convergentes structurées. Nous décrivons quelques modèles mécaniques simples portant sur les relations entre... more
Une modélisation numérique préliminaire permet de contraindre la façon dont la déformation s'accumule sur les marges convergentes structurées. Nous décrivons quelques modèles mécaniques simples portant sur les relations entre tectonique et minéralisations pour les cas des Andes et de la Papouasie, et nous évaluons l'intérêt de la modélisation pour résoudre ce genre de problèmes. (Résumé d'auteur
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Determining the mechanisms that promote large silicic eruptions is one of the biggest challenges in volcanic hazard assessment. The 2011-2012 Cordón-Caulle eruption in Chile was one of the largest silicic eruptions of the 21st century and... more
Determining the mechanisms that promote large silicic eruptions is one of the biggest challenges in volcanic hazard assessment. The 2011-2012 Cordón-Caulle eruption in Chile was one of the largest silicic eruptions of the 21st century and was characterized by a rapid change from explosive to effusive behavior. This eruption was preceded by inflation from 2007 to 2009, followed by two years of barely any ground deformation. Despite intensive monitoring by geodetic and seismological data, its trigger remains undetermined. Here, we benefit from SAR imagery over the Puyehue Cordón-Caulle Volcanic Complex acquired by ALOS-1, ENVISAT and SENTINEL-1 data, to analyze the temporal and spatial behavior of ground displacements before, during and after the eruption. First, we find that a similar prolate spheroidal source explains the ground deformation for the pre-eruptive and post-eruptive periods. Then, we use 3D numerical elasto-plastic models to assess the failure conditions resulting from the pre-eruptive magma injection. Our results show that such a magma injection was too small to trigger the eruption. Therefore we explore other eruption triggers. Analytical elastic inversion models show that the ground displacements observed during the explosive phase may have been produced by slip motion along a NNW-striking dextral-strike slip, double-branch fault of the north-trending Liquiñe-Ofqui Fault System (LOFS), or along a single southern branch fault of the LOFS and collapse of the caldera. When investigating the elasto-plastic deformation pattern resulting from dextral slip along this branch-fault system, we obtain a sub-vertical dilatational plastic zone that connects the reservoir wall to the surface in a location that coincides with that of the 2011 eruption. Hence, we propose that this LOFS branch-fault eventually destabilized (perhaps weakened by the 2007-2009 episode of magma injection), and then slipped in a way that opened channels for fluid migration from the magma reservoir up to the surface.
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Silicic systems generate the most explosive eruptions on Earth. In contrast to basaltic systems, they can accumulate large volumes of magma without systematically erupting, confronting the classical interpretation that a volcano inflates... more
Silicic systems generate the most explosive eruptions on Earth. In contrast to basaltic systems, they can accumulate large volumes of magma without systematically erupting, confronting the classical interpretation that a volcano inflates when a magmatic intrusion occurs. Understanding the mechanisms of volcanic inflation and unrest is thus one of the most important challenges in volcanic risk assessment. Laguna del Maule (LdM) in the Southern Volcanic Zone (SVZ) of Chile, is one of the most active Holocene silicic complexes in the world and it has been inflating since 2007, accumulating 2 meters of uplift without erupting. Several geophysical and geochemical studies conclude that a large crystal rich reservoir would be residing beneath LdM, in consistency with other multi-disciplinary studies showing that such crystal-rich reservoirs (“mush zones”) can be maintained beneath silicic volcanoes, fed by mafic magma recharge from below. Nevertheless, the mechanical state of such reservoirs remains unclear. Here, we characterize for the first time the mechanical properties of such a mush reservoir, able to promote large surface displacements such as those measured at LdM. Using a 3D finite element method we simulate a recharge of magma at the base of a crystal rich reservoir, by assuming an overpressurized source surrounded by a large viscoelastic shell. Inversion results show that this model fits the observed temporal and spatial evolution of ground displacements measured with InSAR data and GNSS data between 2007 and 2017. We interpret the temporal behavior of ground displacement at LdM as resulting from two contributions. A magma recharge occurred within the first 4 years of the active inflation, followed by the viscous response of the large viscoelastic shell, set to a viscosity of 1017 Pa.s. Compared to a purely elastic solution, our model suggests that up to 50 % of the accumulated surface displacement during the ten-year period can be explained by this viscous response, and predicts ongoing displacements 50 years after the onset of inflation. This model agrees with geophysical and geochemical observations and offers a simple explanation of the temporal evolution of surface displacements. It further allows to reconsider the mechanical behavior of large partially crystallized domains in the upper crust; such asignificant transient stress transfer over large viscoelastic areas should thus be accounted for in other studies of silicic volcanic complexes.
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The Chilean Andes extend north–south for about 3000 km over the subducting Nazca plate, and show evidence of local rheological controls on first-order tectonic features. Here, rheological parameters are tested with numerical models of a... more
The Chilean Andes extend north–south for about 3000 km over the subducting Nazca plate, and show evidence of local rheological controls on first-order tectonic features. Here, rheological parameters are tested with numerical models of a subduction driven by slab-pull and upper plate velocities, and which calculate the development of stress and strain over a typical period of 4 Myr. The models test the effects of subduction interface strength, arc and fore-arc crust rheology, and arc temperature, on the development of superficial near-surface faulting as well as viscous shear zones in the mantle. Deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities. Upper plate shortening and trench advance are both correlated, and favored, to a first-order by upper plate weakness, and to a second-order by interface strength. In cases of a strong interface, a weak fore-arc crust is dragged downward by “tectonic erosion”, a scenario for which indications are found along the northern Chilean margin. In contrast for a resistant fore-arc, the slab-pull force transmits to the surface and produces topographic subsidence. This process may explain present-day subsidence of the Salar de Atacama basin and/or the persistence of a Central Depression. Specific conditions for northern Chile produce a shear zone that propagates from the subduction zone in the mantle, through the Altiplano lower crust into the Sub-Andean crust, as proposed by previous studies. Models with a weak interface in turn, allow buoyant subducted material to rise into the continental arc. In case of cessation of the slab-pull, this buoyant material may rise enough to change the stress state in the continental crust, and lead to back-arc opening. In a case of young and hydrated oceanic plate forced by the slab-pull to subduct under a resistant continent, this plate is deviated and indented by the continental mantle, and stretches horizontally at ∼100 km depth. This situation might explain the flat Wadati–Benioff zone of Central Chile.
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Abstract: Compression of the entire continental lithosphere is considered using twodimensional numerical models, in order to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages.... more
Abstract: Compression of the entire continental lithosphere is considered using twodimensional numerical models, in order to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages. The models are based on the central section of ...
Research Interests: Geology, Heat Transfer, Multidisciplinary, New Zealand, Crustal Deformation and tectonic activities, and 15 moreCrustal Deformation studies using GPS, Earth Surface Processes, Continental collision, Geophysical, Crust, Fault Localization, Numerical Model, Lithosphere, Elasto Visco Plasticity, Electric Conductivity, Collisional Tectonics, Crustal Deformation, Crustal stresses, Localised Deformation, and Lithospheric strength control
Research Interests: Geology, Geophysics, Southern Alps, New Zealand, Gravity Anomaly, and 13 moreThree dimensional modeling and simulation, Geomatic Engineering, COLLISION, Continental collision, Geophysical, Crust, Continental Crust, Three Dimensional, Lithosphere, Elasto Visco Plasticity, Oblique Case, Thickening, and three-dimensions
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Research Interests: Earth Sciences, Geology, Seismology, Plate Tectonics, Shear bands, and 13 moreKinematics, Physical sciences, Mantle plumes, Volcanism, Numerical Model, Elasto Visco Plasticity, Boundary Condition, Velocity Field, Oceanic lithosphere, Plate Boundary, Oceanic hotspots and intraplate volcanism, Shear deformation, and present day
This paper presents an interdisciplinary study of the northern Chile double seismic zone. First, a high‐resolution velocity structure of the subducting Nazca plate has been obtained by the tomoDD double‐difference tomography method. The... more
This paper presents an interdisciplinary study of the northern Chile double seismic zone. First, a high‐resolution velocity structure of the subducting Nazca plate has been obtained by the tomoDD double‐difference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth, and the two seismic planes is 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated with this DSZ are deduced by using current thermal‐petrological‐seismological models and are compared to pressure‐temperature conditions provided by a numerical thermomechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions. In the seismic upper plane located within the upper crust, these reactions would affect material of basaltic (MORB) composition and document different metamorphic reactions occurring within high‐P (>2.4...
Research Interests: Earth Sciences, Geology, Geochemistry, Seismology, Tomography, and 12 moreSubduction Zone Processes, Physical sciences, High Resolution, Thermodynamical and thermomechanical modeling, Northern Chile, Subduction Zone, Seismic Tomography, Seismic Velocity, Double Difference, Subduction Zone Fluids, Radius of Curvature, and double seismic zone
Geothermal systems are commonly genetically and spatially associated with volcanic complexes, which in turn, are located nearby crustal fault systems. Faults can alter fluid flow in their surroundings, potentially acting as barriers or... more
Geothermal systems are commonly genetically and spatially associated with volcanic complexes, which in turn, are located nearby crustal fault systems. Faults can alter fluid flow in their surroundings, potentially acting as barriers or conduits for fluids, depending on their architecture and slip-rate. However, this fundamental control on fluid migration is still poorly constrained. Most previous modeling efforts on volcanic and hydrothermal processes consider either only fluid flow in their formulations, or only a mechanical approach, and seldom a full, monolithic coupling between both. In this work, we present a poro-elasto-plastic Finite Element Method (FEM) to address the first-order, time-dependent control that a strike-slip crustal fault exerts on a nearby geothermal reservoir. For the model setting, we selected the Planchón-Peteroa geothermal system in the Southern Andes Volcanic Zone (SAVZ), for which the geometry and kinematics
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Crustal-scale convection and diapiric upwelling of a partially molten orogenic root (Naxos dome, Greece). The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tecto(2017),
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Migmatites and granitic-gneisses exhumed in Archean to Phanerozoic segments are former partially molten crustal roots, display typical domes structures ranging in size from kilometers to decakilometers, and are often interpreted as... more
Migmatites and granitic-gneisses exhumed in Archean to Phanerozoic segments are former partially molten crustal roots, display typical domes structures ranging in size from kilometers to decakilometers, and are often interpreted as resulting from the development of diapiric or convective gravitational instabilities. In previous work (part I), we determined various regimes of gravity-driven segregation, by considering a thick continental crust heated from below and containing melt related heterogeneities. These heterogeneities, represented by inclusions of distinct densities and viscosities with respect to the ambient partially molten material, can be entrained into convection cells (in the "suspension" and "layering" regimes) and/or accumulate as clusters (in the "layering" and "diapirism" regimes). Here we further investigate the specific conditions that allow for the formation and preservation of domes resulting from diapirism at the top of convective cells. We show that both the cessation of basal heating and the freezing of the buoyant inclusions density favor their stacking and preservation at ca. 15 km depth, within about 10 Myr. The buoyant inclusions form domes, 5-20 km in size, that also record several convective cycles at velocities ranging from 0.5-4 cm/yr. Three-dimensional models demonstrate their radial geometrical nature. The influence of the size and concentration of the inclusions is also assessed, complementing the characteristics of crustal heterogeneity in driving its differentiation and the formation of migmatite domes.
Research Interests: Convection and Migmatites
Bedrock failure around an inflating magma chamber is an important factor that controls the occurrence of volcanic eruptions. Here, we employ 3-D numerical models of elasto-plastic shear failure around an inflating crustal reservoir, to... more
Bedrock failure around an inflating magma chamber is an important factor that controls the occurrence of volcanic eruptions. Here, we employ 3-D numerical models of elasto-plastic shear failure around an inflating crustal reservoir, to study how the induced failure patterns depend on the geometry of the chamber, on the host rock strength and on the gravitational field. Our simulations show that either localized or diffuse plastic failure domains develop in 3 stages. Failure initiates (stage 1) after a critical overpressure is reached, the value of which depends on effective host rock strength. Next, and with increasing applied overpressure, either distributed (for zero friction angle) or localized plastic failure zones (for 30 friction angle) form (stage 2), until they finally connect to the surface (stage 3). Cylindrical chambers develop prismatic shear zones that merge from the surface and chamber walls. For spherical and prolate chambers, diffuse conical zones of failure develop from the chamber’s crest, whereas for oblate symmetrical chambers, shear bands initiate at the horizontal tips but bend back above the center of the chamber to reach the surface. In contrast for asymmetrical oblate chambers, shear bands initiate in their cylindrical section and vanish along the elongated direction. Here, magmatic fluids may migrate both through diffuse elastic dilation zones at the tips, and through localized shear zones from the crest. Our results thus suggest how natural observations may be used to constrain the mode of failure occurring underneath a volcano. We discuss several natural examples in this context.
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Slab dip controls the state of stress in an overriding plate and affects mountain building. Analog and numerical models have shown variations in tectonic regime induced by slab folding over the 660 km depth discontinuity zone in... more
Slab dip controls the state of stress in an overriding plate and affects mountain building. Analog and numerical models have shown variations in tectonic regime induced by slab folding over the 660 km depth discontinuity zone in orthogonal convergence. Here using a three-dimensional model of oblique subduction (30°) and accounting for free top surfaces, we show how slab folding generates an along-strike slab dip segmentation, inducing variations in topography of the overriding plate. When the subducting plate begins to curve forward, the elevation height rises inland and varies along the trench from 5 km to 2 km. The Andes are a suitable natural zone to compare our results with because of its linear margin and well-constrained plates kinematics. Thus, we provide a new explanation to the general decrease in elevation from the central to southern Andes, which still remains to be combined with other 3-D mechanisms to explain the actual Andean topography ...
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The aim of this study is to quantify the relationship between the kinematics of subduction, deformation in the overriding plate and the evolution of slab geometry. A 2-D finite element numerical code is used, and a first objective... more
The aim of this study is to quantify the relationship between the kinematics of subduction, deformation in the overriding plate and the evolution of slab geometry. A 2-D finite element numerical code is used, and a first objective consists in benchmarking previously published analogue models. Far-field plate velocities are applied, and once the subducting plate reaches the 660 km discontinuity, modelled as a rigid base, we obtain two different forms or styles of subduction that depend on the overriding plate velocity v op : if v op > 0, the slab lies forwards on the 660 km discontinuity (style 1), and if v op ≤ 0, the slab lies backwards on the discontinuity
(style 2). We also obtain a cyclic pattern with the slab folding on itself repeatedly when v_sp > 0 and 2v_op + v_sp > 0 (where v sp is the subducting plate velocity). These conditions result from the analysis of several simulations in which the subduction velocities and plate viscosities are varied. When the slab periodically folds on the 660 km discontinuity, periods of shallow slab dip and compression in the overriding plate are followed by periods of slab steepening and relative extension in the overriding plate. Folding periodicity is controlled by the slab viscosity and subduction velocity. When a low-viscosity zone is included in the overriding plate, the trench motion is effectively decoupled from the overriding plate velocity, therefore allowing it to be directly controlled by the deep dynamics of the slab. For the cyclic style corresponding to forward folding of the slab, the low-viscosity region in the overriding plate increases the stress amplitudes oscillations, the trench motion and the folding periodicity with time. Therefore the strength of the entire overriding plate is shown to directly control the dynamics of subduction. Using the Nazca and South American plate velocities we produce models of cyclic folding with a period of ca. 22 Ma and a minimal dip angle of ca. 10 ◦ . Episodic folding of the slab on the 660 km discontinuity would produce the necessary changes in slab dip and overriding plate deformation that explain episodes of volcanic quiescence alternating with greater rates of shortening along the Andes.
(style 2). We also obtain a cyclic pattern with the slab folding on itself repeatedly when v_sp > 0 and 2v_op + v_sp > 0 (where v sp is the subducting plate velocity). These conditions result from the analysis of several simulations in which the subduction velocities and plate viscosities are varied. When the slab periodically folds on the 660 km discontinuity, periods of shallow slab dip and compression in the overriding plate are followed by periods of slab steepening and relative extension in the overriding plate. Folding periodicity is controlled by the slab viscosity and subduction velocity. When a low-viscosity zone is included in the overriding plate, the trench motion is effectively decoupled from the overriding plate velocity, therefore allowing it to be directly controlled by the deep dynamics of the slab. For the cyclic style corresponding to forward folding of the slab, the low-viscosity region in the overriding plate increases the stress amplitudes oscillations, the trench motion and the folding periodicity with time. Therefore the strength of the entire overriding plate is shown to directly control the dynamics of subduction. Using the Nazca and South American plate velocities we produce models of cyclic folding with a period of ca. 22 Ma and a minimal dip angle of ca. 10 ◦ . Episodic folding of the slab on the 660 km discontinuity would produce the necessary changes in slab dip and overriding plate deformation that explain episodes of volcanic quiescence alternating with greater rates of shortening along the Andes.
Our study compares the seismic properties between the flat and normal subduction regions in central Chile, to better understand the links between the slab geometry, surface deformation and the deeper structures. In comparison with... more
Our study compares the seismic properties between the flat and normal subduction regions in central Chile, to better understand the links between the slab geometry, surface deformation and the deeper structures. In comparison with previous studies, we show the most complete 3-D regional seismic tomography images for this region, in which we use (1) a larger seismic data set compiled from several short-term seismic catalogues, (2) a denser seismic array allowing a better resolution of the subduction zone from the trench to the backarc and into the upper ∼30 km of the slab and (3) a starting 1-D background velocity model specifically calculated for this region and refined over the years. We assess and discuss our tomography results using regional seismic attenuation models and estimating rock types on the basis of pressure and temperature conditions computed from thermomechanical models. Our results show significant seismic differences between the flat and normal subduction zones. As expected, the faster seismic velocities and increased seismicity within the flat slab and overriding lithosphere are generally consistent with a cooler thermal state. Our results are also consistent with dehydration of the mantle above the subducted Juan Fernandez Ridge at the eastern tip of the flat slab segment, indicating that the latter retains some fluids during subduction. However, fluids in the upper portion of the flat slab segment are not seismically detected, since we report instead fast slab seismic velocities which contradict the argument of its buoyancy being the cause of horizontal subduction. The forearc region, above the flat slab, exhibits high Vs and very low Vp/Vs ratios, uncorrelated with typical rock compositions, increased density or reduced temperature; this feature is possibly linked with the aftershock effects of the Mw7.11997 Punitaqui earthquake, the flat slab geometry and/or seismic anisotropy. At the surface, the seismic variations correlate with the geological terranes. The Andean crust is strongly reduced in seismic velocities along the La Ramada–Aconcagua deformation belt, suggesting structural damage. Slow seismic velocities along the Andean Moho match non-eclogitized hydrated rocks, consistent with a previous delamination event or a felsic composition, which in turn supports the extent of the Chilenia terrane at these depths. We confirm previous studies that suggest that the Cuyania terrane in the backarc region is mafic and contains an eclogitized lower crust below 50-km depth. We also hypothesize major Andean basement detachment faults (or shear zones) to extend towards the plate interface and canalize slab-derived fluids into the continental crust.
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Internal contrasts in strength are responsible for lithospheric buckling. These are quantified by comparing the Indian Ocean data to two-dimensional visco-elasto-plastic numerical models where the material properties depend on temperature... more
Internal contrasts in strength are responsible for lithospheric buckling. These are quantified by comparing the Indian Ocean data to two-dimensional visco-elasto-plastic numerical models where the material properties depend on temperature and pressure. The central Indian Basin is known for its intraplate seismicity and long wavelength undulations of the sea floor and associated gravity signatures. To simulate the amplitudes of undulations that reach 1 km within about 11 Ma of compression and 60 km of shortening, the required mean yield strength of the lithosphere is 400 MPa. If either a hydrostatic fluid pressure is considered in the crust or a mechanical decoupling at the depth of the Moho, small crustal wavelengths are superimposed on the long wavelength deformation, in agreement with observations. It is then possible to match the alternative indication that buckling commenced only 4 Ma ago, with a total amount of shortening of 30 km, and with a required yield strength of 200 MPa....
Research Interests: Earth Sciences, Geology, Faulting (Structural Geology), Rheology, Stress, and 12 moreGeodynamics, Indian Ocean, Physical sciences, Buckling, Thermal Conductivity, Plastic deformation, Elasto Visco Plasticity, Yield Strength, Material Properties, Serpentinite, Intraplate Deformation, and Oceanic lithosphere
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We are pleased to present in this Special Issue “Earthquakes in subduction zones, a multidisciplinary approach” of the journal Physics of the Earth and Planetary Interiors, a selection of papers presented during the International... more
We are pleased to present in this Special Issue “Earthquakes in subduction zones, a multidisciplinary approach” of the journal Physics of the Earth and Planetary Interiors, a selection of papers presented during the International Conference Montessus de Ballore held in Santiago, Chile, in November 2006. This conference was organized by the Department of Geophysics, University of Chile, and the Institute of Physics of the Globe of Paris, in memory of the great 1906 earthquake of Valparaiso. It was sponsored by INSU/CNRS (Institut National des Sciences de l'Univers/Centre National de la Recherche Scientifique), the Department of Geophysics of the University of Chile, by IPGP (Institute of Physics of the Globe of Paris), by IRD (Institut pour la Recherche et le Developpement), MESR (French Ministry of Research and Higher Education), and MAE (French Ministry of Foreign Affairs). This conference has been an excellent opportunity to gather the seismological community working on subduction zone topics, more particularly in the South American context.
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Compression of the entire continental lithosphere is considered using two-dimensional numerical models to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages. The numerical scheme... more
Compression of the entire continental lithosphere is considered using two-dimensional numerical models to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages. The numerical scheme incorporates brittle-elastic-ductile rheology, heat transfer, surface processes, and fault localization. Models are based on the central section of the New Zealand Southern Alps, where continental collision has occurred along the Alpine Fault since about 7 Ma. The results are compared to the surface relief, the GPS convergence velocity, the measured electrical conductivity, and the geometry of the crustal root imaged from seismic velocity measurements. The crustal deformation is characterized by localized uplift at the plate boundary (Alpine Fault) and by two secondary zones of faulting. One is located $60-80 km east of the Alpine Fault, at the start of upper crust bending (or tilting), and the other is located $100-130 km east of the Alpine Fault as a result of shear strain propagating to the surface through the ductile lower crust. The observed asymmetric shape of the crustal root is best reproduced for mantle lithosphere strength of the order of 200 MPa and an intermediate rate of strain softening. A lower strength of the mantle lithosphere can produce symmetric thickening, but the amplitude of the crustal root is too small when compared to observations. The observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced. This offset is most likely due to the three dimensionality of oblique collision in the Southern Alps. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 8120 Tectonophysics: Dynamics of lithosphere and mantle-general; 8159 Tectonophysics: Rheology-crust and lithosphere; 9355 Information Related to Geographic Region: Pacific Ocean; KEYWORDS: mechanical modeling, rheology of the lithosphere, strain localization, decoupled crust and mantle, continental collision, Southern Alps Citation: Gerbault, M., S. Henrys, and F. Davey, Numerical models of lithospheric deformation forming the Southern Alps of New Zealand,
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S U M M A R Y Crustal thickening in an oblique continental collision, such as in the South Island of New Zealand, necessarily involves deformation processes in three dimensions (3-D). We have investigated the role played by the strength... more
S U M M A R Y Crustal thickening in an oblique continental collision, such as in the South Island of New Zealand, necessarily involves deformation processes in three dimensions (3-D). We have investigated the role played by the strength of the lower crust using simplified 3-D mechanical models. These models show that crustal thickening occurs away from the area of maximum compression, along an axis inclined to the plate boundary (about 10 •-20 • to the plate boundary in the case of the South Island), and perpendicular to the convergence direction. Furthermore, the specific geometry of the relatively old and strong Australian lithosphere versus the Pacific lithosphere also controls the location of crustal thickening. These conditions could explain the observed mismatch between the locations of maximum elevation and minimum gravity in South Island, New Zealand, as a consequence of decoupled deformation owing to low-viscosity lower crust.
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This paper presents an interdisciplinary study of the northern Chile double seismic zone. First, a high-resolution velocity structure of the subducting Nazca plate has been obtained by the tomoDD double-difference tomography method. The... more
This paper presents an interdisciplinary study of the northern Chile double seismic zone. First, a high-resolution velocity structure of the subducting Nazca plate has been obtained by the tomoDD double-difference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth, and the two seismic planes is 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated with this DSZ are deduced by using current thermal-petrological-seismological models and are compared to pressure-temperature conditions provided by a numerical thermomechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions. In the seismic upper plane located within the upper crust, these reactions would affect material of basaltic (MORB) composition and document different metamorphic reactions occurring within high-P (>2.4 GPa) and low-T (<570°C) jadeite-lawsonite blueschists and, at greater depth (>130 km), lawsonite-amphibole eclogite conditions. The lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions. The Vp and Vs characteristics of the region in between both planes are consistent with a partially ($25-30 vol % antigorite, $0-10% vol % brucite, and $4-10 vol % chlorite) hydrated harzburgitic material. Discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions. While various geophysical indicators evidence particularly cold conditions in both the descending Nazca plate and the continental fore arc, thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400°C (±50°C) isotherm. Lower plane earthquakes are predicted to occur in the slab's flexural neutral plane, where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity. Fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane, thus producing a sheeted layer of free fluids, or a serpentinized layer. Therefore earthquakes may present either downdip compression and downdip tensile characteristics. Numerical tests indicate that the slab's thermal structure is not the only factor that controls the occurrence of inner slab compression. (1) A weak ductile subduction channel and (2) a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab. (3) Decreasing the radius of curvature of the slab broadens the depth of inner slab compression, whereas (4) decreasing upper plate convergence diminishes its intensity. All these factors indicate that if DSZs indeed contour inner slab compression, they cannot be G 3
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The South Central Pacific is the location of an abnormal concentration of intraplate volcanism. Noting that this volcanism is present from the Kermadec Tonga trench to the Easter microplate and forms a wide east-west channel, we propose... more
The South Central Pacific is the location of an abnormal concentration of intraplate volcanism. Noting that this volcanism is present from the Kermadec Tonga trench to the Easter microplate and forms a wide east-west channel, we propose to explain its occurrence in relation to the Pacific plate geometry and kinematics. We construct 2D numerical models of stress and strain within the Pacific plate using its velocity field and boundary conditions. The models indicate a shear band, associated to a change from compressional stresses to the south to tensional stresses to the north, which develop after 10 Myr between the Australian plate corner and the Easter microplate. We propose that the Central Pacific intraplate volcanism is related to this process, and may represent the first step of a future plate reorganization which will eventually break the Pacific plate in a southern and a northern plate due to intraplate stresses. Present-day intraplate volcanism would define break up spots of the future border.
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Reverse faults in northern Chile have formed 20300 m high scarps that contain open fractures which occur in a zone of 201600 m wide. Two-dimensional numerical models were used to explore the geometrical and mechanical parameters needed... more
Reverse faults in northern Chile have formed 20300 m high scarps that contain open fractures which occur in a zone of 201600 m wide. Two-dimensional numerical models were used to explore the geometrical and mechanical parameters needed to produce extension within ...
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A number of observations reveal large periodic undulations within the oceanic and continental lithospheres. The question if these observations are the result of large-scale compressive instabilities, i.e. buckling, remains open. In this... more
A number of observations reveal large periodic undulations within the oceanic and continental lithospheres. The question if these observations are the result of large-scale compressive instabilities, i.e. buckling, remains open. In this study, we support the buckling hypothesis by direct numerical modeling. We compare our results with the data on three most proeminent cases of the oceanic and continental folding-like deformation (Indian Ocean, Western Gobi (Central Asia) and Central Australia). We demonstrate that under reasonable tectonic stresses, folds can develop from brittle faults cutting through the brittle parts of a lithosphere. The predicted wavelengths and finite growth rates are in agreement with observations. We also show that within a continental lithosphere with thermal age greater than 400 My, either a bi-harmonic mode (two superimposed wavelengths, crustal and mantle one) or a coupled mode (mono-layer deformation) of inelastic folding can develop, depending on the strength and thickness of the lower crust.
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La plasticite non-associee est un facteur important si ce n'est primordial dans le processus de localisation de la deformation cassante. Un code numerique particulierement adapte a reproduire les comportements mecaniques tres varies... more
La plasticite non-associee est un facteur important si ce n'est primordial dans le processus de localisation de la deformation cassante. Un code numerique particulierement adapte a reproduire les comportements mecaniques tres varies des roches nous permet de modeliser efficacement l'initiation et la propagation des deformations localisees a travers la lithosphere. La region de l'Ocean Indien est un cas spectaculaire de flambage lithospherique , accompagne d'une activite sismique intense. Plusieurs modeles analytiques, analogiques et numeriques rendent compte du processus de ambage. On developpe un modele numerique qui pour la premiere fois reproduit le developpement de ambage simultan'avec celui de failles. En utilisant une rheologie elasto-visco-plastique (critere de Mohr-Coulomb et loi de puissance dependant du geotherme), la fracturation di use a ecte d'abord l'ensemble de la lithosphere competente, est suivie de ambage et simultanement de failles qui ...
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We present here a model applied to the Pacific plate for a mechanism governing plate motion related to the plate geometry and kinematics. We start from the observation that from the Kermadec Tonga trench to the Easter microplate, a group... more
We present here a model applied to the Pacific plate for a mechanism governing plate motion related to the plate geometry and kinematics. We start from the observation that from the Kermadec Tonga trench to the Easter microplate, a group of recent and presumed non-deep Pacific hotspots forms a wide east-west channel, and hypothesize that this is not a coincidence. We develop plane strain numerical models of an area corresponding to the Pacific plate from the mid-oceanic ridge to the subduction zone under the Australian plate, with differential velocities applied on the northern and southern part of the plate because of absolute trench motions. Our 2D models indicate a shear band, associated to a change from compressional stresses to the south to tensional stresses to the north, which develop after 10 Myr between the Australian plate corner and the Easter microplate. We propose that the South Central Pacific (SCP) intraplate volcanism is related to this process, and may represent the...
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Two-dimensional numerical models are developped that integrate the entire context of oceanic subduction in contact with continental lithosphere and mantle down to about 300 km depth. 'Ongoing' Andean subduction is chosen with... more
Two-dimensional numerical models are developped that integrate the entire context of oceanic subduction in contact with continental lithosphere and mantle down to about 300 km depth. 'Ongoing' Andean subduction is chosen with initial conditions of surface relief in isostatic equilibrium with the (pre-existing) subducting plate. Because these models incorporate elastic-brittle-viscous, pressure and temperature dependent behaviours, the range of possible rheological parameters is large and requires numerous preliminary tests. We study deformation of the continental lithosphere during slab advance on the time-scale of several millions years. Computed deformation, shear zones, and strength distribution are compared with present day knowledge from data, and previous modeling studies that include only some of the parameters included in our models. Elastic properties imply that there is a component of homogeneous compression, but they also control the slab rigidity versus frictio...
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(1) IRD UR 104, 38 rue des 36 Ponts, 31400 Toulouse, FRANCE (gerbault@lmtg.ups-tlse.fr) (2) LMTG Université Paul Sabatier, 38 rue des 36 Ponts, 31400 Toulouse, FRANCE ... KEY WORDS: faulting, lithospheric deformation, rheology, plate... more
(1) IRD UR 104, 38 rue des 36 Ponts, 31400 Toulouse, FRANCE (gerbault@lmtg.ups-tlse.fr) (2) LMTG Université Paul Sabatier, 38 rue des 36 Ponts, 31400 Toulouse, FRANCE ... KEY WORDS: faulting, lithospheric deformation, rheology, plate boundary, numerical modelling
Two-dimensional numerical models are developped that integrate the entire context of oceanic subduction in contact with continental lithosphere and mantle down to about 300 km depth. 'Ongoing' Andean subduction is chosen with... more
Two-dimensional numerical models are developped that integrate the entire context of oceanic subduction in contact with continental lithosphere and mantle down to about 300 km depth. 'Ongoing' Andean subduction is chosen with initial conditions of surface relief in isostatic equilibrium with the (pre-existing) subducting plate. Because these models incorporate elastic-brittle-viscous, pressure and temperature dependent behaviours, the range of possible rheological parameters is large and requires numerous preliminary tests. We study deformation of the continental lithosphere during slab advance on the time-scale of several millions years. Computed deformation, shear zones, and strength distribution are compared with present day knowledge from data, and previous modeling studies that include only some of the parameters included in our models. Elastic properties imply that there is a component of homogeneous compression, but they also control the slab rigidity versus friction along the subduction plane; migration of the subduction trench is also tested with speed of applied plate convergence rate and rheology.In the continent, crustal thickening, fault localisation, and propagation of deformation from the lithospheric wedge into the intraplate domain in general are characterised. A preliminary attempt is made on identifying general constants in the parametric domain and specific observed variations, such as the development and evolution of a single or two Cordilleras in the forearc domain (as observed along the Chilean and South Peruvian margin). Crustal or lithospheric scale shear zones are observed, together with possible horizontal ductile flow in the lower crust.
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ABSTRACT Indentation during collision invokes strength contrasts across plate boundaries as de- duced from the analysis of structures at the surface. In this study we investigate the mechanical properties of the lower crust to cause... more
ABSTRACT Indentation during collision invokes strength contrasts across plate boundaries as de- duced from the analysis of structures at the surface. In this study we investigate the mechanical properties of the lower crust to cause indentation and examine the influ- ence of different lower crustal rheologies on the evolution of collision zones. The strength of the 2D three layer model (upper and lower crust, and lithospheric man- tle) varies laterally such that a weak central crustal part is surrounded by relatively stronger crust to the left and the right side representing a weak orogenic zone, a strong foreland plate, and a strong indenter in nature, respectively. Upon horizontal compres- sion the model with a strong lower crust yields narrow (width 160 km) but high ( 6 km) orogens. Although being ductile the lower crust decouples from the upper crust and indents the weak orogenic wedge. Strain in the upper crust is concentrated within two mega-shear zones defining a crustal-scale pop-up structure. Strain also concen- trates in the thickening central part of the crust, and in the bends of the flexed mantle lithosphere. In contrast, the model with a weak lower crust predicts wide ( 400 km) but less high ( 5 km), plateau-type, mountains. The weakness of the lower crust prohibits a lower crustal indenter to develop. Instead, horizontal channel flow of the lower crust directed towards the foreland causes widening of the orogen. Strain in the upper crust is distributed over wider areas in both the weak orogenic zone and in the foreland re- gions. Shear is diffusely distributed in the lower crust and is highest at the border of flow channel. Models with a strong lower crust on one side and a weak lower crust on the other side produce asymmetric orogens, the development of one prominent up- per crustal shear zone verging towards the strong side, and outward flow of the lower crust towards the weak side. These preliminary results emphasize that the strength of the lower crust exerts a first-order control on the geometry of an orogen, the concentra- tion and localisation of strain and the direction of flow within the crust. Furthermore, lower crustal indentation appears to depend on relative strength contrasts and does not imply brittleness of the lower crust.
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The Chilean subduction margin presents a large-scale constancy as it spreads along 3000 km from north to south, linked to large scale relatively constant conditions between the subducting Nazca plate and the south american continent. In... more
The Chilean subduction margin presents a large-scale constancy as it spreads along 3000 km from north to south, linked to large scale relatively constant conditions between the subducting Nazca plate and the south american continent. In greater detail, the variable height and width of the Andean mountains or the existance of a Central Depression in the fore-arc indicate the role of rheological differences at depth. A 2D numerical approach calculates the distribution of the stress field and deformation in a model 2000 km wide per 200km deep, in which a basal traction is applied to a flexed oceanic lithosphere, simulating slab-pull over a time-span of 2 Myrs. The influence of the strength of the subduction channel and that of the overriding continent are tested, with the use of elastic-viscous-brittle temperature dependent rheology. Loading of the model is accompanied by the development of localised shear strain from the subduction zone through the continent up to the free surface. A strong subduction interface and a strong continental crust favor the development of a subsiding depression in the forearc, as the crust is dragged downwards coupled to the descending plate. Such a scenario could be valid for the Atacama basin (24°S). The role of the continental mantle is discussed, since high friction along a subduction channel suggests resistant normal forces acting from the continental side at depths corresponding to recorded earthquakes. When such a strong continental mantle wedge is accounted for, it can act as an indenter to the subducting plate, making it deviate and flatten. Finally the models illustrate how potential fluids may use channels of localised shear in the continental mantle to migrate upwards.
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This work aims at investigating the thermo-mechanical conditions required for the development of convective instabilities and polydiapirism in the partially molten root of orogenic belts. First, we tested the volume-of-fluid method (VOF)... more
This work aims at investigating the thermo-mechanical conditions required for the development of convective instabilities and polydiapirism in the partially molten root of orogenic belts. First, we tested the volume-of-fluid method (VOF) implemented in codes OpenFOAM (open source) and Jadim (in-house IMFT code). Comparison of theoretical and numerical solutions of Rayleigh-Taylor and Rayleigh-Benard instabilities show that Openfoam is most satisfactory in terms of speed and mass conservation (Louis-Napoleon et al., 2020).
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Diverse mechanical processes involved in build-up and segmentation of the Andes have been addressed (i.e. fore-arc composition, shortening rates, back-arc strength, etc.). In this work we use the 2-D thermo-mechanical simulation code... more
Diverse mechanical processes involved in build-up and segmentation of the Andes have been addressed (i.e. fore-arc composition, shortening rates, back-arc strength, etc.). In this work we use the 2-D thermo-mechanical simulation code PARAVOZ to study the effects of rheological differences between the upper and lower crust in the fore-arc, on localisation of deformation. Results show that a relatively weak lower crust leads to wide and homogeneous thickening of the crust, associated to an increase of the western component of descendent crustal flow. A weak lower crust consequently achieves lower topography than the opposite case, and a better transmission of stress and deformation to the west of the growing topography. Furthermore, testing the thermal perturbation corresponding to an active-arc, shows an important control over the down-west flow of the lower crust, at isotherms 400-500'C around 40 km depth. High temperatures rise the brittle-ductile transitions, favor decoupling...
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Sous le Chili central et l’ouest de l'Argentine (29°-35°S), la plaque oceanique Nazca, en subduction sous la plaque continentale Amerique du Sud, change radicalement de geometrie : inclinee a 30°, puis horizontale, engendree par la... more
Sous le Chili central et l’ouest de l'Argentine (29°-35°S), la plaque oceanique Nazca, en subduction sous la plaque continentale Amerique du Sud, change radicalement de geometrie : inclinee a 30°, puis horizontale, engendree par la subduction de la chaine de volcans de Juan Fernandez. Le but de mon etude est d'evaluer, la variation de nature et de proprietes physiques de la lithosphere chevauchante entre ces deux regions afin de mieux comprendre (1) sa structure profonde et (2) les liens entre les deformations observees en surface et en profondeur. Pour repondre a cette thematique, j’utilise une approche originale couplant la sismologie, la thermometrie, et la petrologie. Je montre ainsi des images 3-D de tomographie sismique les plus completes de cette region par rapport aux etudes precedentes, qui integrent (1) de nombreuses donnees sismiques provenant de plusieurs catalogues, (2) un reseau de stations sismiques plus dense permettant de mieux imager la zone de subduction. J’apporte la preuve que la plaque en subduction se deshydrate dans deux regions distinctes : (1) le coin mantellique, et (2) le long de la ride subduite avant que celle-ci ne replonge plus profondement dans le manteau. La croute continentale au-dessus du flat slab possede des proprietes sismiques tres heterogenes en relation avec des structures de deformation profondes et des domaines geologiques specifiques. La croute chevauchante d’avant-arc, au-dessus du flat slab, est decrite par des proprietes sismiques inhabituelles, liees a la geometrie particuliere du slab en profondeur, et/ou liees aux effets du seisme de 1997 de Punitaqui (Mw 7.1). Mes resultats, confirmant les etudes anterieures, montrent que : - le bloc Cuyania situe plus a l’est, dans la zone d’arriere-arc est plus mafique et contient une croute inferieure eclogitisee ; quant a, la croute continentale inferieure sous l’arc Andin, est epaisse et non-eclogitisee, decrivant surement le bloc felsique de Chilenia.
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The Chilean subduction margin presents a large-scale constancy as it spreads along 3000 km from north to south, linked to large scale relatively constant conditions between the subducting Nazca plate and the south american continent. In... more
The Chilean subduction margin presents a large-scale constancy as it spreads along 3000 km from north to south, linked to large scale relatively constant conditions between the subducting Nazca plate and the south american continent. In greater detail, the variable height and width of the Andean mountains or the existance of a Central Depression in the fore-arc indicate the role of rheological differences at depth. A 2D numerical approach calculates the distribution of the stress field and deformation in a model 2000 km wide per 200km deep, in which a basal traction is applied to a flexed oceanic lithosphere, simulating slab-pull over a time-span of 2 Myrs. The influence of the strength of the subduction channel and that of the overriding continent are tested, with the use of elastic-viscous-brittle temperature dependent rheology. Loading of the model is accompanied by the development of localised shear strain from the subduction zone through the continent up to the free surface. A ...
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We present here a model applied to the Pacific plate for a mechanism governing plate motion related to the plate geometry and kinematics. We start from the observation that from the Kermadec Tonga trench to the Easter mi-croplate, a group... more
We present here a model applied to the Pacific plate for a mechanism governing plate motion related to the plate geometry and kinematics. We start from the observation that from the Kermadec Tonga trench to the Easter mi-croplate, a group of recent and presumed non-deep Pacific ...
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Diverse mechanical processes involved in build-up and segmentation of the Andes have been addressed (i.e. fore-arc composition, shortening rates, back-arc strength, etc.). In this work we use the 2-D thermo-mechanical simulation code... more
Diverse mechanical processes involved in build-up and segmentation of the Andes have been addressed (i.e. fore-arc composition, shortening rates, back-arc strength, etc.). In this work we use the 2-D thermo-mechanical simulation code PARAVOZ to study the effects of rheological differences between the upper and lower crust in the fore-arc, on localisation of deformation. Results show that a relatively weak lower crust leads to wide and homogeneous thickening of the crust, associated to an increase of the western component of descendent crustal flow. A weak lower crust consequently achieves lower topography than the opposite case, and a better transmission of stress and deformation to the west of the growing topography. Furthermore, testing the thermal perturbation corresponding to an active-arc, shows an important control over the down-west flow of the lower crust, at isotherms 400-500'C around 40 km depth. High temperatures rise the brittle-ductile transitions, favor decoupling...
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The Variscan orogeny is the result of an oceanic subduction followed by a continental collision that ended with a late-collisional high temperature – low pressure event at the scale of the whole orogen. The singularity of the Pyrenean... more
The Variscan orogeny is the result of an oceanic subduction followed by a continental collision that ended with a late-collisional high temperature – low pressure event at the scale of the whole orogen. The singularity of the Pyrenean segment is the predominance of this late high temperature event which is characterized by an intense deformation synchronous with the high temperature metamorphism and an abundant and varied magmatism. The aim of this study is to determine pressure-temperature-time trajectories in the Saint Barthelemy Massif, and to discuss the origin of the geothermal gradients. The Saint Barthelemy Massif is one of the granulitic north-Pyrenean massifs constituting the deepest relics of the Variscan crust. It is composed of two main units separated by a low angle detachment. The upper unit is composed of Paleozoic metasediments, micaschists, migmatites and small plutonic bodies and represents the Variscan upper crust. The basal unit, made of granulitic to amphiboliti...