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Discrete (mm- to m-scale) mylonitic shear zones in the northeastern Harquahala metamorphic core complex, Arizona, show evidence of fluid-mineral interactions catalyzing deformation and metamorphism. Many contain a deformed central epidote vein with adjacent bleached haloes and flanking paired shear zones that indicate significant fluid-rock interaction during deformation. An integration of structural and geochemical methods was employed to understand timing, metamorphic conditions, and physiochemical processes responsible for producing the discrete shear zones. Field and microstructural evidence suggest the zones initiated on antecedent fractures. Electron backscatter diffraction (EBSD) analyses show a significant coaxial contribution to the shear, and quartz deformation predominately by prism <a> slip, along with some rhomb <a> slip, suggesting amphibolite-facies conditions during shearing. Fourier Transform Infrared spectroscopy analyses of quartz reveal higher water contents within shear zones than within country rocks, indicating fluid infiltration synchronous with shearing. Stable isotope analyses of quartz and feldspar from mylonites are consistent with an igneous or metamorphic fluid origin. Microstructural observations suggest that the zone morphology with epidote veins, bleached haloes, and flanking discrete paired shear zones was developed predominantly from reaction softening mechanisms. The increase in deformation from bleached rock to flanking shear zones is marked by progressive modal increases in biotite and myrmekite, and modal decreases in K-feldspar, and locally epidote and titanite. Myrmekitic textures recrystallized readily and resulted in progressively greater grain size reduction of feldspar, which aided in the progressive alignment and linkage of the biotite grains, which together concentrated the deformation in bands. Volume reduction resulting from some of the metamorphic reactions may have led to a positive feedback cycle among fluid infiltration, metamorphism and deformation. U-Pb isotope analyses of syn-metamorphic titanite yield an age of ~70 Ma, suggesting the shear zones formed during cooling of the Late Cretaceous (75.5±1.3 Ma) Brown’s Canyon pluton, consistent with their top-to-the-southwest sense of shear, rather than during top-to-the-northeast directed Miocene metamorphic core complex exhumation. Petrography, EBSD analyses, and U-Pb dating of titanite from other (non-discrete) mylonites in the area imply most formed synchronously with the discrete shear zone mylonites. Only rare, scattered mylonites show features consistent with metamorphic core complex exhumation. / text

Les mécanismes d’exhumation des roches de haute-pression basse-température (HP-BT) au sein des zones de subduction sont nombreux, complexes et toujours activement débattus. L’étude des zones de subduction fossiles permet alors de mieux comprendre ces mécanismes, montrant notamment que l’exhumation des roches métamorphiques HP-BT est en grande partie accommodée le long de zones de cisaillement ductiles d’échelle crustale. Ce travail de thèse vise à contraindre la géométrie, la cinématique et la durée de l’histoire tectono-métamorphique des roches de HP-BT du complexe de subduction des Schistes Bleus Cycladiques située en Grèce. Un des objectifs est de contraindre, à différentes échelles, le calendrier de la localisation de la déformation au cours de l’exhumation dans le but de mieux comprendre le comportement mécanique des zones de subduction. Trois méthodes principales d’investigation ont été utilisées : i) une étude de terrain permettant de caractériser la géométrie, la cinématique et la distribution de la déformation, montrant notamment une localisation progressive de la déformation vers la base de l’unité des Schistes Bleus Cycladiques au cours de l’exhumation et le long de zones de cisaillement, ii) des outils de pétrologie métamorphique permettant de contraindre l’évolution P-T des roches métamorphiques au sein de la zone de subduction, et iii) des datations ⁴⁰AR/³⁹AR afin de déterminer le calendrier de l’histoire tectono-métamorphique des Schistes Bleus Cycladiques et de la localisation de la déformation au sein de zones de cisaillement d’échelle kilométrique à millimétrique associées à des degrés de rétromorphose variés. Une corrélation nette est clairement observée entre l’intensité de la déformation, le degré de rétromorphose et les âges les plus jeunes. Un des résultats de ce travail est que la préservation à l’affleurement d’éclogites et schistes bleus n’implique pas forcément une exhumation rapide. En effet, nos résultats suggèrent que l’unité des Schistes Bleus Cycladiques a enregistré une histoire d’exhumation longue d’environ 30 Ma. Le taux d’exhumation n’est donc pas le seul paramètre contrôlant le degré de rétromorphose des unités HP-BT, la remontée le long d’un gradient métamorphique froid dans le canal de subduction et la localisation progressive de la déformation ductile au cours de l’exhumation étant également des facteurs majeurs. / Exhumation mechanisms of high-pressure low-temperature (HP-LT) metamorphic rocks in subduction zones are complex and actively discussed. The study of fossilized subduction zones allows a better comprehension of these mechanisms, showing that exhumation of HP-LT rocks is mainly accommodated along crustal-scale ductile shear zones. This study aims at constraining the geometry, the kinematic and the timing of the tectonometamorphic history of the HP-LT Cycladic Blueschist Unit (CBU) cropping out in Greece. A main objective is to constrain the timing of strain localization at different scales during exhumation to better understand the mechanical behaviour of subduction zones. Three principal methods of investigation have been used, including i) a structural fieldwork that allows to characterize the geometry, the kinematic and the distribution of deformations, highlighting progressive strain localization during exhumation toward the base of the CBU and along shear zones, ii) a metamorphic petrology study aiming at determining the P-T evolution of the CBU, and iii) ⁴⁰AR/³⁹AR dating used to constrain the timing of the tectonometamorphic evolution of the CBU and the timing of strain localization within kilometre- to millimetre-scale shear zones showing different degrees of retrogression. We observe an obvious correlation between the intensity of finite deformation, the degree of retrogression and youngest mica ages. A major result of this thesis work is that the preservation of eclogite and blueschist-facies rocks does not necessarily imply fast exhumation rates. Our results instead suggest that the exhumation history of the CBU is relatively long, spanning over ca. 30 Ma. Consequently, it appears that the exhumation rate is not the main parameter controlling the degree of retrogression of HP-LT metamorphic rocks in the CBU compared to progressive strain localization during exhumation along a cold retrograde P-T evolution within the subduction channel.

Made available in DSpace on 2015-02-24T19:48:34Z (GMT). No. of bitstreams: 1 FabioSSC_capa_ate_secao4.pdf: 2789704 bytes, checksum: 672a97f59de08b590b598f925828b920 (MD5) Previous issue date: 2007-06-06 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The studied region, named Forquilha and localized in northwestern Central Cear? domain (northern portion of Borborema Province), presents a lithostratigraphic framework constituted by paleoproterozoic metaplutonics, metasedimentary sequences and neoproterozoic granitoids. The metasedimentary rocks of Cear? group occupy most part of the area. This group is subdivided in two distinct units: Canind? and Independ?ncia. Canind? unit is represented basically by biotite paragneisses and muscovite paragneisses, with minor metabasic rocks (amphibolite lens). Independ?ncia sequence is composed by garnetiferous paragneisses, sillimanite-garnet-quartz-muscovite schists and quartz-muscovite schists, pure or muscovite quartzites and rare marbles. At least three ductile deformation events were recognized in both units of Cear? group, named D1, D2 and D3. The former one is interpreted as related to a low angle tangential tectonics which mass transport is southward. D2 event is marked by the development of close/isoclinal folds with a N-S oriented axis. Refolding patterns generated by F1 and F2 superposition are found in several places. The latest event (D3) corresponds to a transcurrent tectonics, which led to development of mega-folds and several shear zones, under a transpressional regime. The mapped shear zones are Humberto Monte (ZCHM), Po?o Cercado (ZCPC) and Forquilha (ZCF). Digital image processing of enhanced Landsat 7-ETM+ satellite images, combined with field data, demonstrate that these penetrative structures are associated with positive and negative geomorphologic patterns, distributed in linear and curvilinear arrangements with tonal banding, corresponding to the ductile fabric and to crests. Diverse color composites were tested and RGB-531 and RGB-752 provided the best results for lineament analysis of the most prominent shear zones. Spatial filtering techniques (3x3 and 5x5 filters) were also used and the application of Prewitt filters generated the best products. The integrated analysis of morphological and textural aspects from filtered images, variation of tonalities related to the distribution of geologic units in color composites and the superposition over a digital elevation model, contributed to a characterization of the structural framework of the study area. Kinematic compatibility of ZCHM, ZCPC, ZCF shear zones, as well as Sobral-Pedro II (ZCSPII) shear zone, situated to the west of the study area, was one of the goal of this work. Two of these shear zones (ZCHM, ZCPC) display sinistral movements, while the others (ZCSPII, ZCF) exhibit dextral kinematics. 40Ar/39Ar ages obtained in this thesis for ZCSPII and ZCPC, associated with other 40Ar/39Ar data of adjacent areas, indicate that all these shear zones are related to Brasiliano orogeny. The trend of the structures, the opposite shear senses and the similar metamorphic conditions are fitted in a model based on the development of conjugate shear zones in an unconfined transpression area. A WNW-ESE bulk shortening direction is infered. The geometry and kinematic of the studied structures suggest that shortening was largely accommodated by lateral extrusion, with only minor amounts of vertical stretch / A regi?o de Forquilha localiza-se no extremo noroeste do Dom?nio Cear? Central (por??o setentrional da Prov?ncia Borborema) e apresenta um arcabou?o litoestratigr?fico formado por rochas metaplut?nicas paleoproteroz?icas, seq??ncias metasupracrustais e granit?ides neoproteroz?icos. As rochas metassedimentares do Grupo Cear? ocupam a maior parte da ?rea e s?o subdivididas em duas unidades distintas: Canind? e Independ?ncia. A unidade Canind? ? formada basicamente por biotita paragnaisses e muscovita paragnaisses, envolvendo rochas metab?sicas (lentes de anfibolitos). A seq??ncia Independ?ncia abrange paragnaisses granad?feros, intercalados com silimanita-granada-quartzo-muscovita xistos e quartzo-muscovita xistos, quartzitos puros ou com muscovita e, mais raramente, m?rmores. Foram identificados e caracterizados pelo menos tr?s eventos de deforma??o d?ctil, denominados de D1, D2 e D3. O evento D1 pode ser interpretado como correspondente ? uma tect?nica tangencial de baixo ?ngulo com transporte tect?nico para sul. O evento D2 ? caracterizado pelo desenvolvimento de dobras fechadas a isoclinais com eixos aproximadamente N-S. Padr?es de superposi??o de dobras F1 e F2 foram verificadas em diversos locais. O terceiro evento (D3), desenvolvido em um regime transpressional, gerou dobras megasc?picas com eixos NNE a NE e zonas de cisalhamento transcorrentes. Essas zonas transcorrentes D3 s?o denominadas de Zona de Cisalhamento Humberto Monte (ZCHM), Zona de Cisalhamento Po?o Cercado (ZCPC) e Zona de Cisalhamento Forquilha (ZCF). O processamento digital de imagens de sat?lite Landsat 7-ETM+, combinado com dados de campo, demonstrou que essas estruturas penetrativas desenvolveram padr?es geomorfol?gicos positivos e negativos, distribu?dos em arranjos lineares e curvilineares com bandamento tonal, associados ao fabric d?ctil das rochas e a cristas. Diversas composi??es coloridas foram testadas, sendo que RGB-531 e RGB-752 proporcionaram os melhores resultados para an?lise de lineamentos correspondentes ?s principais zonas de cisalhamento. T?cnicas de filtragem espacial (filtros 3x3 e 5x5) tamb?m foram utilizadas, sobressaindo-se os produtos resultantes da aplica??o de filtros Prewitt na banda PC1. A an?lise integrada dos aspectos morfol?gicos e texturais presentes nas imagens filtradas, somados ?s varia??es de cores assumidas pelas unidades geol?gicas nas composi??es coloridas e ? sobreposi??o sobre um modelo digital de eleva??o, permitiram uma caracteriza??o em mega-escala da conforma??o estrutural da ?rea de estudo. Um dos objetivos desse trabalho foi compatibilizar a cinem?tica das zonas de cisalhamento ZCHM, ZCPC, ZCF, juntamente com a zona de cisalhamento (lineamento) Sobral-Pedro II (ZCSPII), situada na adjac?ncia, a oeste da ?rea estudada. As idades 40Ar/39Ar obtidas nesta tese para a ZCSPII e a ZCPC, associadas com outros dados 40Ar/39Ar de zonas de cisalhamento do Estado do Cear?, s?o indicativas de que todas essas zonas estejam relacionadas ? orog?nese Brasiliana. Al?m disso, a orienta??o geral das estruturas, os sentidos de cisalhamento opostos (sinistral na ZCHM e na ZCPC; dextral na ZCSPII e na ZCF) e as condi??es metam?rficas semelhantes conduziram ? proposta de um modelo evolutivo das zonas de cisalhamento da regi?o de Forquilha a partir do desenvolvimento de zonas de cisalhamento d?cteis conjugadas em uma ?rea de transpress?o n?o-confinada, com uma dire??o geral de encurtamento WNW-ESE. A geometria e a cinem?tica do conjunto de estruturas analisadas sugerem que o encurtamento tenha sido amplamente acomodado por extrus?o lateral, com menores quantidades de estiramento vertical

Les zones de cisaillement constituent des objets structuraux communs de la lithosphère. À grande échelle, elles sont le siège principal des déplacements entre plaques tectoniques, accommodant de grandes quantités de déformation. La compréhension de leur comportement mécanique dans le temps et l'espace est donc essentielle pour la connaissance générale de la dynamique de la lithosphère. La température joue un rôle majeur sur la loi de comportement rhéologique qui caractérise le domaine ductile (en profondeur), réduisant alors efficacement la résistance mécanique. Chaque roche possède en outre des propriétés mécaniques intrinsèques qui varient en fonction de sa composition minéralogique, de sa texture et de sa structure interne. Or, en l'absence de grandeurs directement mesurables en profondeur, la rhéologie de la lithosphère demeure sujette à diverses interprétations. Le comportement mécanique des zones de cisaillement est d'autant plus méconnu qu'elles sont le siège d'intenses changements de la nature des roches et de perturbations thermiques majeures. En particulier, l'énergie mécanique qui y est convertie en chaleur (shear heating) peut engendrer une étroite interrelation entre thermique et mécanique. Ce travail de thèse vise à contribuer à la connaissance générale de la rhéologie des zones de cisaillement lithosphérique. Une approche originale a été mise en place, se basant sur l'évolution thermique aux abords et au sein des zones de cisaillement. Sur la base de modèles numériques thermo-cinématiques 2-D et de développements analytiques, la variabilité de premier ordre de l'évolution et de la perturbation thermique est analysée et quantifiée au regard de l'influence des trois processus thermiques majeurs que sont la diffusion, l'advection et le shear heating. Les résultats sont confrontés aux signatures thermiques métamorphiques associées aux chevauchements intra-continentaux pour lesquels les influences des processus d'accrétion et d'érosion sont également examinées. Le cas du Main Central Thrust (Himalaya), associé à une inversion thermique métamorphique bien développée, est pris comme exemple de référence. Nos résultats quantitatifs mettent en avant le rôle crucial du shear heating, notamment de la variabilité de la résistance mécanique des zones de cisaillement. L'accent est mis sur l'importance des paramètres de fluage des roches. L'étude de zones de cisaillement centimétriques développées au sein de la granodiorite du Zillertal (fenêtre des Tauern, Alpes) à la faveur de faibles variations de la composition minéralogique révèle l'extrême sensibilité de la rhéologie des roches ignées représentatives de la croûte continentale. Les conséquences de cette variabilité intense à petite échelle sont finalement discutées au regard des rhéologies classiquement considérées dans les modèles qui s'intéressent aux processus qui régissent la dynamique de la lithosphère. / Shear zones are common structural features in the lithosphere and occur at various scales (from microscopic to lithospheric). At the lithospheric scale, they concentrate most of the relative movements between tectonic plates, and therefore, accommodate a high amount of strain. Consequently, the understanding of both their spatial and temporal mechanical behaviour is crucial for the general knowledge of the lithosphe dynamics. Rheology of rocks, which define their mechanical behaviour, is controlled by physical laws that predict how they deform under some stresses. Temperature plays a major role in the creep-dislocation behaviour, which characterizes the ductile domain (in depth), decreasing efficiently the rock strength. Furthermore, each rock has intrinsic mechanical properties, which depend on its mineralogical composition, texture and internal structures. However, due to the lack of data directly measurable deeper than a few kilometres, the lithosphere rheology, and in particular the continental lithosphere remains subject to drastically different interpretations. The mechanical behaviour of major shear zones is not fully understood, as they are the location of intense changes of both the rock internal nature and major thermal perturbations. Especially, the mechanical energy, converted into heat (shear heating) causes a close interaction between thermal ad mechanical evolutions. This thesis aims to better understand the rheological state of lithospheric scale shear zones. For this purpose, we used an original approach, based on the temperature field evolution around and within such shear zones. From 2D numerical thermo-kinematic models and analytical developments, the first order variability of thermal evolution and perturbation is anal- ysed and quantified with respect to the impact of three major thermal processes, defined as diffusion, advection and shear heating. Results are compared to metamorphic thermal signatures associated to intra-continental thrust zones for which the influence of both accretion and erosion was also investigated. The case of the Main Central Thrust (MCT) in the Himalayas, whose the inverse metamorphic thermal zonation has been extensively studied, was chosen as the main natural analogue. Our quantitative results highlight the crucial role of shear heating, and more particularly of mechanical strength variability within shear zones. We thus emphasise on the importance of rock creep parameters. The study of centimetre-scale shear zones, which developed within the granodiorite of the Zillertal nappe (Tauern window, Tyrol, Alps) thanks to little local variations of the mineralogical composition, reveals the extreme sensitivity of igneous rocks rheology, representative of the continental crust. The consequences of such an intense variability, revealed at small scale are finally discussed with regard to rheologies usually considered in models that focus on processes controlling lithosphere dynamics.

Στην παρούσα Μεταπτυχιακή Διατριβή αναλύονται τα παραμορφωτικά και συστασιακά (πετρογραφικά - ορυκτοχημικά) χαρακτηριστικά μυλωνιτιωμένων λιθολογιών από την πλαστική ζώνη διάτμησης του Φελλού (Βορειοδυτική Άνδρος). Η ζώνη διάτμησης του Φελλού είναι μια μεσοσκοπικής κλίμακας δομή, ΒΒΑ διεύθυνσης, με δομικό πάχος της τάξεως των 200 μέτρων. Το κεντρικό τμήμα της ζώνης διάτμησης φιλοξενείται σε μετα-πηλιτικές λιθολογίες και οριοθετείται από έντονα παραμορφωμένα μετα-υπερβασικά και μετα-βασικά λιθολογικά σώματα. Οι μυλωνιτιωμένες λιθολογίες της ζώνης διάτμησης χαρακτηρίζονται σαν S έως S – L τεκτονίτες. Η τεκτονική χαρτογράφηση στην περιοχή μελέτης αποκάλυψε ότι η μυλωνιτική ζώνη αναπτύχθηκε υπο-παράλληλα με το αξονικό επίπεδο ενός ήπια κεκλιμένου συγκλινοειδούς το οποίο έχει υποστεί καθολικό μετασχηματισμό. Οι ρεολογικές αντιθέσεις μεταξύ του πυρήνα της ζώνης διάτμησης και των περιθωρίων της σε συνδυασμό το τεκτονικό περιβάλλον ενός μετασχηματισμένου συγκλινοειδούς έχουν συμβάλλει στον εντοπισμό της παραμόρφωσης στην θέση ανάπτυξης της ζώνης διάτμησης. Σε χαρτογραφική κλίμακα ο προσανατολισμός των γραμμώσεων έκτασης μεταβάλλεται από ΝΝΔ , παράλληλα στη διεύθυνση της ζώνης διάτμησης, σε ΔΒΔ διεύθυνση βύθισης. Η μεταβολή στον προσανατολισμό των γραμμώσεων έκτασης δείχνει ότι η διεύθυνση κίνησης εντός της ζώνης διάτμησης διαμερίζεται χωρικά σε συνιστώσες κίνησης παράλληλα και κάθετα στην παράταξη της μυλωνιτικής φολίωσης. Ο κινηματικός διαμερισμός στην πλαστική ζώνη διάτμησης του Φελλού είναι χαρακτηριστικό γνώρισμα ζωνών διάτμησης πλάγιας συστολής. Ο κινηματικός διαμερισμός που αναγνωρίστηκε, ερευνήθηκε περαιτέρω με την εξέταση της χωρικής κατανομής των κρυσταλλογραφικών αξόνων [c] του χαλαζία, από δείγματα χαλαζιακών φλεβών και χαλαζιακών μυλωνιτών, προσανατολισμένα παράλληλα στη μυλωνιτική φολίωση και κάθετα στη γράμμωση έκτασης (ΧΖ επίπεδο ελλειψοειδούς παραμόρφωσης). Τα ιστολογικά διαγράμματα [c] αξόνων χαλαζία παρουσιάζουν μεταβολές στην τοπολογία τους σε διαφορετικά δομικά επίπεδα της ζώνης διάτμησης και διακριτές διαφορές με τα πρότυπα κρυσταλλογραφικά διαγράμματα [c] αξόνων μονοκλινικής συμμετρίας. Διακριτό χαρακτηριστικό , ειδικά για τα ανώτερα δομικά επίπεδα της ζώνης διάτμησης, είναι η εστίαση των [c] αξόνων σε σημειακά μέγιστα που προβάλλονται σε περιφερειακές θέσεις των κρυσταλλογραφικών διαγραμμάτων. Η βάση , δομικά, της ζώνης διάτμησης βρίσκεται στην ρεολογική διεπιφάνεια μετα-πηλιτών – μετα-βασιτών. Στην επαφή αυτή, ιστολογικά διαγράμματα από γειτονικά δείγματα χαλαζιακών φλεβών παρουσιάζουν συστηματικά διαγράμματα τύπου μικρών κύκλων. Η συστηματικότητα αυτή υποδηλώνει εντοπισμό της γεωμετρίας της παραμόρφωσης στην λιθολογική επαφή των μετα-πηλιτών - μετα-βασίτων. Η ποσοτική ανάλυση της παραμόρφωσης έδειξε ότι η παράμετρος Rxz μεταβάλλεται από 2.1 έως 6.45 ενώ η παράμετρος Flinn κυμαίνεται από 0.01 έως 0.93, υποστηρίζοντας και ποσοτικά ότι η γεωμετρία της παραμόρφωσης ανήκει στο πεδίο της πλάτυνσης. Η γεωμετρία της παραμόρφωσης στο πεδίο της πλάτυνσης είναι ένα επιπλέον διαγνωστικό χαρακτηριστικό των ζωνών διάτμησης πλάγιας συστολής. Η πετρογραφικη εξέταση μυλωνιτιωμένων λιθολογιών εστίασε στην αναγνώριση ζωνωμένων ορυκτών φάσεων με σκοπό την στοιχειοθέτηση των ορυκτοχημικών χαρακτηριστικών τους. Η πετρογραφική εξέταση δειγμάτων από μια επιφανειακή εμφάνιση μεταβασιτών από την βάση της μυλωνιτικής ζώνης συνέβαλλε στην αναγνώριση, σε κλίμακα λεπτής τομής, δύο λιθολογικών τύπων οι οποίοι είναι (α) Γρανατούχος – γλαυκοφανιτικός σχιστόλιθος και (β) Επιδοτιτικός - γλαυκοφανιτικός σχιστόλιθος. Η ορυκτοχημεία ζωνωμένων γρανατών και αμφιβόλων από τους δύο λιθολογικούς τύπους εξετάστηκε μέσω SEM/EDS.Η ορυκτοχημική ανάλυση οδήγησε στην αναγνώριση τριών τύπων ζώνωσης στις αμφιβόλους με τα εξής χαρακτηριστικά (α) Σκούροι πράσινοι πυρήνες σιδηρο-Κεροστίλβης με κροσσιτικά περιθώρια (β) Παλίνδρομη ζώνωση μεταξύ μπλε (σιδηρο-Γλαυκοφανής) και πράσινων αμφιβόλων (σιδηρο-Βαρροϊσίτης) και (γ) Απομονωμένοι κρύσταλλοι με κροσσιτικούς πυρήνες και ριβεκιτικά περιθώρια. Τα περιθώρια των ζωνωμένων γρανατών που εξετάστηκαν βρίσκονται σε ιστολογική ισορροπία με μπλέ αμφιβόλους, το γεγονός αυτό σε συνδυασμό με τα ορυκτοχημικά πειστήρια της κλασμάτωσης του μαγγανιου στους πυρήνες αποδεικνύει ότι η βλάστηση των γρανατών συνέβη σε αυξανόμενες συνθήκες πίεσης και θερμοκρασίας κατά την διάρκεια του υψηλής πίεσης μεταμορφικού γεγονότος. / The subject of the present contribution is the analysis of deformation in the Fellos ductile shear zone, which crops out in the island of Andros. Fellos shear zone (FSZ) is an outcrop scale NNE –striking structure with a total structural thickness of, approximately, 200 m. The central main domain of the shear zone is defined by mylonitized metapelitic rocks, bounded by strongly deformed meta-basic and meta–ultrabasic rocks both on top and bottom. Mylonitic rocks in the shear zone can be classified as S to S – L tectonites. Structural mapping revealed that the Fellos shear zone resulted from the complete transposition of a gently inclined synform the axial plane of which is orientated sub – parallel to the mylonitic foliation. The rheological contrasts between the core of the shear zone and its margins in conjunction with the structural framework of a transposed synform are first order influences to the localization of deformation. The map scale pattern of the stretching lineations in the shear zone shows that the lineation swings from a NNE orientation, trending parallel to the strike of the shear zone, to an ESE orientation. This variation shows that transport orientation in the shear zone is spatially partitioned into strike parallel and strike normal movements. This kinematic partitioning in the Fellos shear zone is a characteristic feature of transpressional high strain shear zones. The recognized partitioning was further investigated by examining the pattern of quartz [c]-axes fabrics in quartz veins oriented parallel to the mylonitic foliation as well as in quartz –rich mylonites. Quartz [c] axes fabric diagrams shows variations in their topology at different structural levels of the shear zone and distinct differences with the [c] axes patterns of monoclinic strain symmetry. Distinct feature, especially at the uppermost structural levels of the shear zone, is the point maxima clustering of the [c] axes in peripheral position of the crystallographic diagrams. At the base of the shear zone, in the rheological interface of metapelites with metabasites, the fabric diagrams from neighboring quartz vein specimens shows consistently small circle girdle pattern. This observation denotes localization in the geometry of deformation (flattening) at this lithological contact. Finite strain analysis shows that Rxz varies from 2.1 to 6.45, while the Flinn parameter range between 0.01 and 0.93, supporting quantitatively the flattening geometry of strain. Flattening strain is another diagnostic criterion of transpressional shear zones. The means to constrain the compositional features of the mylonitic rocks was mineral chemistry and petrologic analysis. Petrographic examination, specifically, of a metabasite outcrop from the base of the shear zone revealed, in thin section scale, two lithologic types : (a) Garnet blueschist and (b) Epidote blueschist. The mineral chemistry of zoned garnets and amphiboles from these petrographic types has been examined using SEM/EDS. The microchemical analysis revealed three different zoning patterns in the amphiboles which are characterized by (a) dark green Ferro – hornblende cores with crossite rims (b) Amphiboles with compositional zoning and alternations of blue (Ferro – glaucophane) and green amphiboles (Ferro – barroisite) and (c) Isolated grains with crossite cores and riebeckite rims. Compositional mapping of zoned garnets witnessed fractionation of manganese to the cores, which is the Prima facie evidence of growth zoning. In addition, the almandine rims of the zoned garnets are in textural equilibrium with blue amphiboles, observation which implies that these garnets nucleated during the HP event.

Cette étude de terrain du massif des Ecrins (Alpes Occidentales, zone externe) montre que le raccourcissement Alpin est essentiellement accommodé dans le socle par des zones de cisaillement inverse top-vers-l'ouest ductile-fragile (probablement localisé par la présence de demi-graben) ainsi que par les nappes de charriage de la Meije et du Combeynot à l'Est du massif. Les failles normales N-S ne semblent pas avoir été réactivées. Ces zones de cisaillement sont caractérisées par une phengitisation des feldspaths le long de bandes anastomosées dont la géométrie traduit un gradient de déformation qui permet d'expliquer la formation des "plis de socle" soulignée par la géométrie des téguments de Trias. Les températures maximum d'enfouissement liées au charriage des unités internes sont proches de l'isotherme 335°C pour un gradient géothermique compris entre 20 et 25°C.km-1 depuis Bourg d'Oisans au Front Pennique. Les zones de cisaillement ont été datées entre 33 et 25 Ma (40Ar/39Ar sur phengites syn-cinématiques), ce qui suggère qu'elles ont été initiées très tôt après l'enfouissement de la zone externe qui débute vers 34Ma. Les phyllonites présentent des âges plus jeunes que les mylonites, ce qui traduit une localisation de la déformation le long de ces bandes anastomosées de faible résistance entre 30 et 25Ma. Le raccourcissement NO-SE observé le long du Front Pennique semble synchrone des déformations E-O à NESO du massif des Ecrins. La présence d'une zone de cisaillement transpressive senestre diffuse entre le massif des Ecrins et les Alpes Ligures, issu de la réactivation d'une zone de transfert Liasique, permettrait d'expliquer en partie la cinématique d'édification Oligocène des Alpes Occidentales. Au Miocène, la déformation se localise le long d'un plan de chevauchement sous le massif de Belledonne, donnant naissance au Vercors, ainsi que le long du Front Pennique réactivé en faille normale. / This field-based study of the Ecrins massif (Western Alps, external zone) show that the Alpine shortening is accomodated within the basement by brittle-ductile top-to-the-west reverse shear zones (probably localized by half-graben) as well as by the thrust sheets of La Meije and Combeynot to the east of the massif. The N-S normal fault do not seems to be reactivated. These shear zones are characterized by a phengitisation of feldspars along anastomosing planes whose geometry reflect a strain gradient which explain the "basement folds" formation underlined by the geometry of Triassic teguments. The maximum temperatures reach by the cover, due to the burial under the internal nappes are close to the isotherms 335°C for a geothermal gradient close to 20-25°C.km-1 from Bourg d'Oisans to the Penninic Frontal Thrust (PFT). The shear zones were dated between 33 to 25 Ma (40Ar/39Ar on syn-kinematics phengites), suggesting that they were initiated slightly after the burial which start close to 34 Ma. The phyllonites show younger ages than mylonite; that argue a localization of the deformation along these anastomosing planes between 30 to 25Ma and traduce a weak crust. The NW-Se shortening observed alonf the PFT seems to be coeval with the E-W to NE-SW shortening of the Ecrins massif. A sinistrial transpressive diffuse shear zones between the Ecrins massif and the Ligurian Alps, which is probably a reactivated Liassic transfer zone, can explaina part of the Oligocene building kinematics of the Western Alps. During Miocence, the deformation is localized along a crustal thrust under the Belledonne massif, which has given the Vercors massif, and along the PFT, reactivated as a normal fault.

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