Éditeur(s) : HAL CCSD
Résumé : International audience
EGU General Assembly 2014
Vienne, Austria

insu-01113701
https://hal-insu.archives-ouvertes.fr/insu-01113701

Éditeur(s) : HAL CCSD
Résumé : National audience
Les semelles métamorphiques sont situées à la base desgrandes ophiolites obductées. Elles sont principalement composéesd’amphibolites (métabasaltes à amphiboles ± grenat,clinopyroxène, plagioclase) fortement cisaillées et dont les conditionsmaximum de métamorphisme sont estimées à 800±100Cet 1.0±0.2 GPa. Leur formation a lieu au cours des 2 Ma suivantl’initiation des zones de subduction. Ces amphibolites sont interprétéescomme des portions de lithosphère océanique subduite,portions qui sont métamorphisées, dis-je, à haute températurepar transfert de chaleur depuis le coin de manteau sus-jacent quiformera la base de l’ophiolite obductée. Leur importante déformationau pic de métamorphisme est interprétée comme synchroneà leur accrétion au manteau de la plaque supérieure. Les semellesmétamorphiques sont donc des témoins directs de la dynamiqueprécoce des zones de subduction océanique. Leur pétrologie etleur déformation fournissent des contraintes majeures, et rares,sur l’évolution de la structure thermique et sur le comportementmécanique de l‘interface de subduction naissante. Cette étudeprésente des données structurales à plusieurs échelles, couplant desanalyses de terrain aux analyses EBSD sur des échantillons à amphibole± plagioclase ± clinopyroxene ± grenat de la semelle métamorphique.Les grains d’amphibole et de plagioclase sont caractériséspar une forte orientation préférentielle parallèle à la foliationet à la linéation. Cette fabrique est accompagnée par une forteorientation cristallographique préférentielle des amphiboles marquéepar un axe [001] marquant la linéation et la normale au plan(001) perpendiculaire à la foliation. Les plagioclases montrentune orientation cristallographique préférentielle faible voire nulle.Cette fabrique est similaire à celle observée dans les amphibolitescisaillées de base de croûte continentale. Les clinopyroxèneset les grenats présentent des degrés d’orientation intermédiaires.Ces deux phases forment des boudins dont l’axe long est orientéparallèlement à la foliation. Les porphyroclastes de clinopyroxèneprésentent une extinction ondulante et le développement de sousgrainsà leur périphérie. Les clinopyroxènes et les grenats agissentdonc comme phases ” dures ”, augmentant la résistance de laroche à la déformation. À l’initiation de la subduction, la partiesupérieure de la plaque plongeante est affectée par l’augmentationprogressive du degré de métamorphisme. Ceci entraîne une fortedéshydratation et la cristallisation de phases ” faibles ” (quartz,amphibole, plagioclase) et ” fortes ” (clinopyroxene, grenat). Larhéologie de l’interface de la subduction en est fortement affectée.Les unités de la semelle métamorphique (avec des minéralogies etdes conditions P-T-t différentes) accrétées aux péridotites de laplaque supérieure témoignent de changements rhéologiques majeuresà l’interface de la subduction au cours de l’histoire précocedes zones de subduction.
RST 2016, livre des résumés
Caen, France

insu-01388730
https://hal-insu.archives-ouvertes.fr/insu-01388730

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Subduction processes play a major role in plate tectonics and the subsequent geological evolution of Earth. This special issue focuses on ongoing research in subduction dynamics to a large extent (oceanic subduction, continental subduction, obduction…) for both past and active subduction zones and into mountain building processes and the early evolution of orogens. It puts together various approaches combining geophysics (imaging of subduction zones), petrology/geochemistry (metamorphic analysis of HP-UHP rocks, fluid geochemistry and magmatic signal, geochronology), seismology and geodesy (present-day evolution of subduction zones, active tectonics), structural geology (structure and evolution of mountain belts), and numerical modelling to provide a full spectrum of tools that can be used to constrain the nature and evolution of subduction processes and orogeny. Studies presented in this special issue range from the long-term (orogenic cycle) to short-term (seismic cycle).
ISSN: 0264-3707

insu-01287985
https://hal-insu.archives-ouvertes.fr/insu-01287985
https://hal-insu.archives-ouvertes.fr/insu-01287985/document
https://hal-insu.archives-ouvertes.fr/insu-01287985/file/rolland-2016.pdf
DOI : 10.1016/j.jog.2016.03.006

Éditeur(s) : HAL CCSD
Résumé : International audience
Metamorphic soles correspond to m to 500m thick tectonic slices welded beneath most of the large-scale ophiolites.They typically show a steep inverted metamorphic structure where the pressure and temperature conditions ofcrystallization increase upward (from 500100ºC at 0.50.2 GPa to 800100ºC at 1.00.2 GPa), with isogradssubparallel to the contact with the overlying ophiolitic peridotite. The proportion of mafic rocks in metamorphicsoles also increases from the bottom (meta-sediments rich) to the top (approaching the ophiolite peridotites). Thesesoles are interpreted as the result of heat transfer from the incipient mantle wedge toward the nascent slab (associatedwith large-scale fluid transfer and possible shear heating) during the first My of intra-oceanic subduction(as indicated by radiometric ages). Metamorphic soles provide therefore major constraints on early subductiondynamics (i.e. thermal structure, fluid migration and rheology along the nascent slab interface).We present a detailed structural and petrological study of the metamorphic sole from 4 major cross-sections alongthe Oman ophiolite. We show precise pressure–temperature estimates obtained by pseudosection modelling andEBSD measurements performed on both the garnet-bearing and garnet-free high-grade sole. Results allow quantificationof the micro-scale deformation and highlight differences in pressure–temperature–deformation conditionsbetween the 4 different locations, showing that the inverted metamorphic gradient through the sole is not continuousin all locations.Based on these new constraints, we suggest a new tectonic–petrological model for the formation of metamorphicsoles below ophiolites. This model involves the stacking of several homogeneous slivers of oceanic crust leadingto the present-day structure of the sole. In this view, these thrusts are the result of rheological contrasts between thesole and the peridotite as the plate interface progressively cools down. These slivers later underwent several stagesof retrogression (partly mediated by ascending fluids from the slab) from amphibolite- to prehnite/pumpellite-faciesconditions.
Geophysical Research Abstracts
Vienne, Austria

insu-01309369
https://hal-insu.archives-ouvertes.fr/insu-01309369

Éditeur(s) : HAL CCSD
Résumé : International audience
Understanding subduction rheology in both space and time has been a challenge since the advent of plate tectonics.We herein focus on "subduction infancy", which corresponds to the first 0-2 My immediately following subductionnucleation, when a newly born slab penetrates into the overriding plate mantle and heats up.The only remnants of this critical, yet elusive, geodynamic step are thin metamorphic soles, commonly found beneathpristine, 100-1000 km long portions of oceanic lithosphere emplaced on top of continents (i.e. ophiolites).In this study, we show how, during subduction infancy, transient mechanical properties of both the mantle and crustacross the subduction plate interface (during 100s ky) control and hinder the penetration of tectonic plates intothe mantle, and how this results in strong peaks of resistance and even slicing of their surface — leaving behindthin, chopped-off metamorphic slivers (i.e. metamorphic soles).These findings constrain the mechanical behaviour of the subduction plate interface (with implications for couplingprocesses and earthquake generation) as well as the properties of the crust and mantle. They also highlight the roleof fluids in enabling subduction to overcome this early resistance.
Geophysical Research Abstracts
Vienne, Austria

insu-01309386
https://hal-insu.archives-ouvertes.fr/insu-01309386

Éditeur(s) : HAL CCSD
Résumé : International audience
Metamorphic soles correspond to m to 500 m thick highly strained metamorphic rock units found beneath myloniticbanded peridotites at the base of large-scale ophiolites, as exemplified in Oman. Metamorphic soles aremainly composed of metabasalts deriving from the downgoing oceanic lithosphere and metamorphosed up togranulite-facies conditions by heat transfer from the mantle wedge. Pressure–temperature peak conditions areusually estimated at 1.00.2 GPa and 800100ºC. The absence of HP–LT metamorphism overprint implies thatmetamorphic soles have been formed and exhumed during subduction infancy. In this view, metamorphic soleswere strongly deformed during their accretion to the mantle wedge (corresponding, now, to the base of the ophiolite).Therefore, metamorphic soles and banded peridotites are direct witnesses of the dynamics of early subductionzones, in terms of thermal structure, fluid migration and rheology evolution across the nascent slab interface.Based on fieldwork and EBSD analyses, we present a detailed (micro–) structural study performed on samplescoming from the Sumeini window, the better-preserved cross-section of the metamorphic sole of Oman. Largedifferences are found in the deformation (CPO, grain size, aspect ratio) of clinopyroxene, amphibole and plagioclase,related to mineralogical changes linked with the distance to the peridotite contact (e.g., hardening due to theappearance of garnet and clinopyroxene).To model the incipient slab interface in laboratory, we carried out 5 hydrostatic annealing and simple-shear experimentson Griggs solid-medium apparatus. Deformation experiments were conducted at axial strain rates of 10-6s-1. Fine-grained amphibolite was synthetized by adding 1 wt.% water to a (Mid-Ocean Ridge) basalt powder asa proxy for the metamorphic sole (amphibole + plagioclase + clinopyroxene garnet assemblage). To synthetizegarnet, 2 experiments were carried out in hydrostatic conditions and with deformation at 800C with confiningpressure of 2 GPa. Another simple-shear experiment has been carried out at 800C and 1 GPa with fined-grainednatural garnet. With the aim of mimicking the early slab interface (between the metamorphic sole and bandedperidotites at the base of the ophiolite), 2 simple-shear deformation experiments with 2 layers have been carriedout at 800C and confining pressure of 1 GPa. The bottom layer was made of hydrated basalt powder and the toplayer was made of olivine.Fined-grained garnet-free amphibolite is significantly weaker than dunite but the appearance of harder mineralsin the amphibolite (garnet and clinopyroxene) has major implications on its rheological evolution. These resultsallow liking field observations of strain localization at the interface to the metamorphic sole formation
European Geosciences Union General Assembly 2016
Vienne, Austria

insu-01309394
https://hal-insu.archives-ouvertes.fr/insu-01309394

Éditeur(s) : HAL CCSD
Résumé : International audience
Understanding subduction rheology in both space and time has been a challenge since the advent of plate tectonics. We herein focus on "subduction infancy", that is the first ~1-5 My immediately following subduction nucleation, when a newly born slab penetrates into the upper plate mantle and heats up. The only remnants of this critical yet elusive geodynamic step are thin metamorphic soles, commonly found beneath pristine, 100-1000 km long portions of oceanic lithosphere emplaced on continents (i.e., ophiolites).Through the (i) worldwide compilation of pressure-temperature conditions of metamorphic sole formation augmented by pseudosection thermodynamic modeling, (ii) calculations of the viscosity of materials along the plate interface and (iii) generic numerical thermal models, we provide a conceptual model of dynamic plate interface processes during subduction infancy (and initiation s.l.).We show in particular how major rheological switches across the subduction interface control slab penetration, and the formation of metamorphic soles. Due to the downward progression of hydration and weakening of the mantle wedge with cooling, the lower plate (basalt, sediment) and the upper plate (mantle wedge) rheologies equalize and switch over a restricted temperature-time-depth interval (e.g., at ~800°C and ~1 GPa, during 0.1-2 My, for high-temperature metamorphic sole formation). These switches result in episodes of maximum interplate mechanical coupling, thereby slicing the top of the slab and welding pieces (basalt, sediment) to the base of the mantle wedge. Similar mechanical processes likely apply for the later, deeper accretion and exhumation of high-temperature oceanic eclogites in serpentinite mélanges, or for the accretion of larger tectonic slices.This model provides constraints on the effective rheologies of the crust and mantle and general understanding, at both rock and plate scale, for accretion processes and early slab dynamics.
AGU Fall Meeting 2015
San Francisco, United States

insu-01242103
https://hal-insu.archives-ouvertes.fr/insu-01242103

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Subduction infancy corresponds to the first few million years following subduction initiation, when slabs start their descent into the mantle. It coincides with the transient (yet systematic) transfer of material from the top of the slab to the upper plate, as witnessed by metamorphic soles welded beneath obducted ophiolites. Combining structure–lithology–pressure–temperature–time data from metamorphic soles with flow laws derived from experimental rock mechanics, this study highlights two main successive rheological switches across the subduction interface (mantle wedge vs. basalts, then mantle wedge vs. sediments; at ∼800 °C and ∼600 °C, respectively), during which interplate mechanical coupling is maximized by the existence of transiently similar rheologies across the plate contact. We propose that these rheological switches hinder slab penetration and are responsible for slicing the top of the slab and welding crustal pieces (high- then low-temperature metamorphic soles) to the base of the mantle wedge during subduction infancy. This mechanism has implications for the rheological properties of the crust and mantle (and for transient episodes of accretion/exhumation of HP-LT rocks in mature subduction systems) and highlights the role of fluids in enabling subduction to overcome the early resistance to slab penetration.
ISSN: 0012-821X

hal-01383368
http://hal.upmc.fr/hal-01383368
http://hal.upmc.fr/hal-01383368/document
http://hal.upmc.fr/hal-01383368/file/Agard_2016_Plate_interface.pdf
DOI : 10.1016/j.epsl.2016.06.054