Déformation des unités métamorphiques de haute pression de la subduction à l'exhumation. (Exemple des Cyclades, Grèce)

Highpressure-­‐lowtemperaturemetamorphicrocksarewitnessofthebehaviourofsubductionzonesatlithospherescale.Theserocksundergoprogradeandretrogrademetamorphismduringburialandexhumation,respectively.Duringexhumation,deformationevolvesfromductiletobrittle.ThepresentthesisconsistsinananalysisofthesuccessivedeformationsrecordedbytheCycladicBlueschistUnit(CBU)inGreeceduringasubduction-­‐exhumationcycle,intheframeoftheAegeanback-­‐arcextension.ThestudyofductiledeformationsrecordedbytheCBUwasusedtoseparatethoseassociatedtoburial,duringsubduction,fromthoserelatedtoexhumationandtocharacterisetheirkinematicsintheframeoftheAegeanextensiondynamics.Arestorationofpost-­‐12MadeformationsshowsthatinearlytomiddleMiocenedeformationisduetoonlyonedetachment(theNorthCycladicDetachment)controlledonlyonecorecomplex(theCentralCycladesCoreComplex).Laboratoryexperimentsonsmall-­‐scalemodelshavebeenusedtoarguethatAegeanextension,since12Ma,resultedfromaninteractionbetweentheHellenicsubductionretreattowardtheSWandthewestwarddisplacementofAnatoliaatleast7yearsearlierthanwhatwasthoughtuptonow.ThefoldeddomainofthecentralCycladesisadirectwitnessofthisinteraction.TheexperimentsalsoshowthattheVardarsuturehasplayedanimportantlocalisingroleinthisprocessasapre-­‐existingweakzoneLesrochesmétamorphiquesdehautepression–bassetempératuresontdestémoinsdufonctionnementdeszonesdesubductionàl’échelledelalithosphère.Lorsdeleurenfouissementcesrochessubissentunmétamorphismeprograde,puisretrogradependantleurexhumation,etuneévolutiondesdéformationsdeductilesàfragiles.Cettethèseprésenteuneanalysedesdéformationssuccessivesenregistréesparl’unitédesSchistesBleusCycladiques(SBC)enGrècependantuncyclesubduction-­‐exhumation,danslecadredel’extensionarrière-­‐arcduDomaineEgéen.L’étudedesdéformationsductilesenregistréesparlesSBCapermisdedistinguercellesassociéesàl’enfouissement,pendantlasubduction,decellessynchronesdel’exhumationetdecaractériserleurcinématiquedanslecontextedynamiquedel’extensionégéenne.Unerestaurationdesdéformationspostérieuresà12Mamontrequel’extensionMiocèneinférieuràmoyenn’estduequ’àunseuldétachement,leDétachementNordCycladique,contrôlantledéveloppementd’unseuldômeextensif,leCoreComplexdesCycladesCentrales.Desexpériencesdelaboratoiresurmodèlesréduitsontpermisd’argumenterqueladéformationégéenne,depuis12Ma,résultaitd’uneinteractionentreleretraitversleSWdelasubductionHelleniqueetledéplacementversl’ouestdel’Anatolie,aumoins7Maplustôtqu’onlepensaitjusqu’àprésent.LazoneplisséeducentredesCycladesenestuntémoingéologiquedirect.LesexpériencesontaussimontréquelasutureduVardarajouéunrôleimportantdansceprocessus,entantquezonedefaiblessemécanique

The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene

International audienceThe Aegean domain is a key area for understanding the processes of back-arc extension. Observed deformation pattern and present day kinematics result from the interaction between the southward retreat of the Hellenic trench and the westward escape of Anatolia. Lithosphere-scale analogue models were employed to display that the overall pattern of Aegean extension requires not only the combination of trench retreat and Anatolia escape since middle Miocene but also the presence of an inherited lithosphere-scale mechanical discontinuity: the Vardar Suture Zone (VSZ). The reactivation in dextral shear of the eastern branch of the VSZ accommodates both the trench retreat (NS stretching) and the westward escape of Anatolia (EW shortening) in the Cyclades area since middle Miocene. Additionally, our model shows that the North Anatolian Fault (NAF) is a late structure in the evolution of the Aegean, initiated around 10 Ma after the onset of Anatolia escape. The displacement field at the surface of the model allows the identification of sub-domains, which result from strain partitioning instead of being "rigid microplates", directly comparable to the present-day displacement field (GPS) of the Aegean and western Anatolia. Our model provides a simple but powerful way to look at the dynamics of Aegean extension in two main stages. From middle Eocene to middle Miocene, extension was only driven by the southward retreat of the Hellenic trench at a rate lower than 1 cm*y− 1. Since middle Miocene, the combination of slab rollback with Anatolia westward escape resulted in a southwest direction of trench retreat, with an accelerating rate of up to 3 cm*y− 1

Obliquity along plate boundaries

International audienceMost of the plate boundaries are activated obliquely with respect to the direction of far field stresses, as roughly only 8% of the plate boundaries total length shows a very low obliquity (ranging from 0 to 10°, sub-orthogonal to the plate displacement). The obliquity along plate boundaries is controlled by (i) lateral rheological variations within the lithosphere and (ii) consistency with the global plate circuit. Indeed, plate tectonics and magmatism drive rheological changes within the lithosphere and consequently influence strain localization. Geodynamical evolution controls large-scale mantle convection and plate formation, consumption, and re-organization, thus triggering plate kinematics variations, and the adjustment and re-orientation of far field stresses. These geological processes may thus result in plate boundaries that are not perpendicular but oblique to the direction of far field stresses. This paper reviews the global patterns of obliquity along plate boundaries. Using GPlate, we provide a statistical analysis of present-day obliquity along plate boundaries. Within this framework, by comparing natural examples and geological models, we discuss deformation patterns and kinematics recorded along oblique plate boundaries

Kinematic records of subduction and exhumation in the Ile de Groix blueschists (Hercynian belt; Western France)

International audienceDeciphering between deformations related to subduction and exhumation in HP metamorphic rocks represents a challenge for the understanding of the dynamic processes involved and more particularly, the exhumation mechanisms. An analysis of shear criteria in the Ile de Groix blueschists in the Hercynian Belt of southern Brittany (Western France) is carried out in terms of relative chronology and in relation to lithology (mainly micaschists and glaucophane-bearing metabasites). Structural mapping shows that foliations are flat-lying, bearing stretching lineations oriented in a N150°E ± 30° direction. Associated shear criteria are dominantly top-to-northwest and locally, in the southeast part of the island, both top-to-northwest and -southeast. Both shearing deformations lead to intense finite strains as demonstrated by the occurrence of sheath folds and the general orientation of fold axes parallel to the stretching lineation. Detailed mapping in this area showing opposite senses of shear, demonstrates that only top-to-southeast shear is observed in rocks containing well-shaped lawsonite pseudomorphs. In rocks affected by top-to-northwest shear, the lawsonite pseudomorph shape is destroyed by intense stretching. The top-to-southeast shear is coeval with prograde HP metamorphism and precedes a top-to-northwest shear coeval with a retrograde metamorphic path. In the frame of the hercynian belt of southern Brittany, the southeast directed shear is attributed to thrusting related to a northward dipping subduction and the northwest directed shear to exhumation controlled by a northward dipping extensional detachment

Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)

International audienceThe Cycladic Blueschist Unit (CBU) represents the northern passive margin of the Adria continental block and ophiolites that are the remnants of the Pindos Ocean, which were affected by high pressure-low temperature metamorphism in the blueschist and eclogite facies during the Eocene. Prior to the Lutetian, the ophiolitic and margin units were thrust towards the SW inside a NE-dipping subduction zone. Two subsequent exhumation stages are characterized by top-to-the-NE senses of shear: i) from mantle depths to lower crustal levels, before 37 Ma, which was accommodated by an extensional inversion of the Vardar suture zone, and ii) from deep crustal levels to the near-surface, between 30 and 20 Ma, which was accommodated by the North Cycladic Detachment. It is during this late stage of ductile exhumation that the central Cyclades Core Complex was exhumed. Segmentation of the CBU by normal faults started at 12 Ma, as recorded by the low temperature thermochronometers, and was coeval with block rotation and folding with NS-trending axes, which were predominantly controlled by the Myrthes-Ikaria strike-slip fault in the centre of the Cyclades. A map restoration of the CBU geometry, prior to segmentation, has been carried out using available structural and paleomagnetic data. The restoration shows that, following the CBU exhumation from mantle depths to crustal levels, a single core complex was exhumed along a single NE dipping detachment

Layer-parallel shear during subduction and exhumation of HP-UHP metamorphic rocks units

International audienc

Structure and kinematics of the Sumatran Fault System in North Sumatra (Indonesia)

International audienceLithospheric-scale faults related to oblique subduction are responsible for some of the most hazardous earthquakes reported worldwide. The mega-thrust in the Sunda sector of the Sumatran oblique subduction has been intensively studied, especially after the infamous 2004 Mw 9.1 earthquake, but its onshore kinematic complement within the Sumatran subduction, the transform Sumatran Fault System, has received considerably less attention. In this paper, we apply a combination of analysis of Digital Elevation Models (ASTER GDEM) and field evidence to resolve the kinematics of the leading edge of deformation of the northern sector of the Sumatran Fault System. To this end, we mapped the northernmost tip of Sumatra, including the islands to the northwest, between 4.5°N and 6°N. Here, major topographic highs are related to different faults. Using field evidence and our GDEM structural mapping, we can show that in the area where the fault bifurcates into two fault strands, two independent kinematic regimes evolve, both consistent with the large-scale framework of the Sumatran Fault System. Whereas the eastern branch is a classic Riedel system, the western branch features a fold-and-thrust belt. The latter contractional feature accommodated significant amounts (c. 20%) of shortening of the system in the study area. Our field observations of the tip of the NSFS match a strain pattern with a western contractional domain (Pulau Weh thrust splay) and an eastern extensional domain (Pulau Aceh Riedel system), which are together characteristic of the tip of a propagating strike-slip fault, from a mechanical viewpoint. For the first time, we describe the strain partitioning resulting from the propagation of the NSFS in Sumatra mainland. Our study helps understanding complex kinematics of an evolving strike-slip system, and stresses the importance of field studies in addition to remote sensing and geophysical studies

The interaction between Aegean back-arc extension and Anatolia escape since Late Miocene

International audienceThe Aegean domain is a key area for understanding the processes of back-arc extension. Observed deformation pattern and present day kinematics result from the interaction between the southward retreat of the Hellenic trench and the westward escape of Anatolia. Using laboratory experiments designed to study lithosphere-scale deformation, we show that the overall pattern of Aegean extension requires not only the combination of trench retreat and Anatolia escape since 15 Ma but also the presence of an inherited lithosphere-scale mechanical discontinuity: the Vardar Suture Zone (VSZ). The reactivation in dextral shear of the eastern branch of the VSZ accommodates both the trench retreat (NS stretching) and the westward escape of Anatolia (EW shortening) in the Cyclades area since 15 Ma. Additionally, our model shows that the North Anatolian Fault (NAF) is a late structure in the evolution of the Aegean, initiated 10 Ma after the onset of Anatolia escape. The model displays displacement sub-domains, which result from strain partitioning instead of being “rigid microplates”, directly comparable to the present-day displacement field (GPS) of the Aegean and western Anatolia. Our modelling provides a simple way to look at the dynamics of Aegean extension in two main stages. From middle Eocene to middle Miocene, extension was only driven by the southward retreat of the Hellenic trench at a rate lower than 1 cm.y-1. Since middle Miocene, the combination of slab rollback with Anatolia westward escape, resulted in a southwest direction of trench retreat, with an accelerating rate of up to 3 cm.y-1

Re-orientation of the extension direction and pure extensional faulting at oblique rift margins: comparison between the Main Ethiopian Rift and laboratory experiments

In this study, we draw on a unique combination of well-resolved fault-slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending Main Ethiopian Rift, East Africa. These data show that both boundary and internal faults – oblique and orthogonal to the plate divergence (PD) respectively – exhibit almost pure dip-slip motion, and indicate significant local deflection in orientation of the extension direction at rift margins. Scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep-seated, pre-existing weakness – oblique to the direction of PD – that is able to cause a local rotation in the orientation of the extension direction at rift margins. Minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems, as supported by existing and new palaeomagnetic data from the rift

Origin and time evolution of subduction polarity reversal from plate kinematics of Southeast Asia

International audienceWe present a regional model of plate geometry and kinematics of Southeast Asia since the Late Cretaceous, embedded in a global plate model. The model involves subduction polarity reversals and sheds new light on the origin of the subduction polarity reversal currently observed in Taiwan. We show that this subduction zone reversal is inherited from subduction of the proto–South China Sea plate and owes its current location to triple junction migration and slab rollback. This analysis sheds new light on the plate tectonic context of the Taiwan orogeny and questions the hypothesis that northern Taiwan can be considered an older, more mature equivalent of southern Taiwan