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International audience; Ce livre foisonnant est un hommage collectif à la mesure de Robert Ilbert, historien d'Alexandrie, enseignant passionnant, fondateur de la Maison méditerranéenne des sciences de l'homme (MMSH) et de l'Institut méditerranéen de recherches avancées (IMéRA), intellectuel insatiable guidé depuis l’enfance par une extraordinaire soif de liberté. Les 60 auteurs réunis pour l’occasion forment une assemblée joyeuse et indocile, ouverte à toutes les audaces théoriques. Des historiens, des philosophes, des sociologues, des linguistes, des anthropologues, des physiciens, des poètes : toutes les humanités dépassent ici leurs cloisonnements disciplinaires pour questionner l’étrange métier de chercheur. Au détour de ces textes décalés on croisera les thématiques essentielles de l’œuvre d’Ilbert – Alexandrie, le cosmopolitisme, la citadinité, les migrations, les Empires, la Méditerranée – mais aussi des objets plus inattendus, des ponts, des détroits, des canaux, des synagogues désaffectées ou des piétions désorientés… Un état du monde et de la recherche, sans prétention à l’exhaustivité, mais avec le souci du plaisir partagé.

ResumeLe but de la presente etude est de caracteriser la deformation recente subie par la Provence occidentale, situee entre la nappe de Digne a l’est et la faille de Nimes a l’ouest. Dans un premier temps, la deformation subie par un marqueur sedimentaire qui est a la fois le plus recent possible et le plus uniformement represente dans la zone etudiee est consideree. Ce marqueur sedimentaire est le Miocene. La caracterisation de la deformation passe par l’etude de la localisation de celle-ci et donc par l’identification des structures actives responsables de la deformation du Miocene. Ces caracteristiques permettent d’estimer la quantite de raccourcissement total accumule lors des derniers 20 millions d’annees. Dans un deuxieme temps, on distingue la part de la deformation postMiocene de cette deformation totale. Les marqueurs utilises sont des surfaces et des objets geomorphologiques reperes. Dans ce travail, nous montrons que la deformation en Provence a ete de faible importance entre le Miocene et fac...

Archaean tectonic regimes can be constrained by structural analysis of greenstone belts and adjacent granite-gneiss masses. The late Archaean Bababudan greenstone belt (Dharwar craton, South India) contains a low-grade volcanosedimentary basin sequence (the Dharwar Supergroup) which unconformably overlies a previously eroded basement made up mainly of granite and gneiss. Fabric trajectories, kinematic indicators and superimposed deformation patterns show that greenstones have undergone a radial converging downward flow with respect to underlying basement rocks, as well as strike-slip shearing. This is attributed to the progressive sagduction (gravity-sinking) of the greenstone belt into its basement along a decollement horizon and to late transcurrent shearing. We propose a deformation model for the Bababudan belt that may explain common features of many greenstone belts (such as decollements and superimposed upright fold patterns) which are attributed to the progressive sinking of the supracrustals into their basement. The results of our study and other field investigations in the crust of the Dharwar craton, as well as structural evidence from other cratons (Zimbabwe and Pilbara), raise the possibility that a unique continental tectonic regime existed during the Archaean, controlled by the emplacement of dense volcanic traps, mantle underplating and thermal blanketing of the continental crust. This tectonic regime differs from that observed within the modern lithosphere as it is characterized by the pervasive development of gravity (i.e. Rayleigh-Taylor) instabilities between the volcanic traps and the rest of the continental crust. It allowed heat and material transfer through the crust and has to be seen as an efficient recycling mechanism for the young continental crust during periods of mantle underplating. The diapiric phenomenon is independent of the boundary forces acting at the limits of the protocontinents and is associated in time and space with large-scale accretion events of the continental crust, perhaps in hot-spot environments.

The In Ouzzal granulitic massif is composed mainly of various meta-igneous rocks which, in spite of Rb, U, Th, Cs and some K and Sr mobility, can be dated and generally classified according to their chemical composition as follows. Basic and ultrabasic granulites interlayered with the metasediments correspond to (1) ultrabasic cumulates from dislocated tholeiitic bodies, (2) ancient komatiitic to high-Mg tholeiitic basalts similar to the suites found in Archaean greenstone belts and (3) calcalkaline protoliths of high-K andesitic composition. No geochronological constraints are available apart from the depositional age of some associated sediments which is younger than 2.70 Ga detrital zircons, and the Nd model age of the andesitic granulites of c. 3.4 Ga. In spite of the high-grade metamorphism, the acidic magmatic precursors of the charnockites can be divided in three groups. (1) The most juvenile acid orthogneisses are trondhjemitic or tonalitic in composition, being similar to the TTG suites which are classically considered to be formed by partial melting of mantle-derived protoliths. The 3.3–3.2 Ga TDM indicates a possible age of separation from the mantle reservoir while the plutons may have been emplaced between 3.3 and 2.7 Ga (U–Pb zircon & Nd ages). (2) A group of alkaline granitic gneisses, similar in composition to rift-related-granites, were emplaced at 2650±10 Ma (U–Pb & Rb–Sr ages) in a thick continental crust. (3) Calcalkaline granodioritic and monzogranitic suites derived from the partial melting of continental precursors (3.5–3.3 Ga), in lower to middle levels of the continental crust. They were emplaced close to 2.5 Ga during crustal thickening. The very high-temperature metamorphism occurred at 2002±7 Ma from the age of synfoliation intrusions and was probably related to major overthrusting. Retrogressive metamorphism is dated at 1.95 Ga from garnet-Nd ages. In spite of the very high-temperature conditions, partial melting during granulite facies metamorphism may be restricted to scarce cordierite-bearing monzogranitic gneisses. The 2.0 Ga VHT metamorphism could be related to overthrusting, extensional or underplating processes.

Models simulating the role of gravity in the Earth's crust are the main source of criteria used for the field identification of diapirs. After a short review of the distribution of finite strain in experimental diapir models, we describe an example where differential erosion of a tilted craton has provided 3D geometrical information on patterns in a diapiric system at various depths (from greenschist to granulitic facies conditions) within the Archaean continental crust of the Dharwar craton, South India. Diapirs and anti-diapirs (‘sagducted’ greenstones) are observed at different levels: structures and strain are consistent with all the field identification criteria inferred from experimental simulation, but some peculiar structural features are also observed which are not predicted by the models. These features, such as strain trajectory triple junctions, result from (i) the progressive deformation of diapiric domes or sagducted basins, (ii) interferences between diapiric structures (body forces) and (iii) the superimposition of diapiric strain fields on regional horizontal shortening (boundary forces).

New structural and geochronological data are used to constrain the evolution of the Precambrian In-Ouzzal block situated on the eastern side of the West African Craton. The results of this study can be summarized as follows: 1. (i) The block can be subdivided into two series; one is mainly composed of Archaean granitic gneisses (3.2–2.5 Ga old), while the other contains metasedimentary and basic to ultrabasic rocks which can be compared with those described in Archaean supracrustal terrains. 2. (ii) The entire block is affected by very high temperature granulite-facies metamorphism. 3. (iii) The metamorphism is associated with two main deformational phases, the first of which is responsible for a regional foliation foliation refolded during the second phase. 4. (iv) The major tectono-metamorphic event in the area is related to a 2.0 Ga-old (Eburnian) orogeny which ended with carbonatite emplacement. Nearly all evidence of the earliest crustal evolution events (i.e. the initial emplacement and deformation of the granitic and supracrustal materials) has been erased by the Eburnian event.

Qualitative strain data are spatially integrated in order to define Archaean foliation trajectories, and variations in strain intensity and ellipsoid type, in the Holenarsipur area, Dharwar craton, southern India. The study provides constraints on the bulk geometry, the progressive deformation and the kinematic history of this region of Archaean continental crust. Two major structural events have shaped the study area: (i) a regional diapiric event contemporaneous with a magmatic event, and (ii) a later sinistral transcurrent ductile shear event which has locally reoriented both foliations and stretching lineations. The entire Holenarsipur supracrustal belt underwent a common tectono-metamorphic history that included two metamorphic events. Each event corresponds temporally to a distinct strain field. The first, a regional amphibolite facies event, was contemporaneous with the main tectonic event and probably with the intrusion of Halekote trondhjemites. The second occurred under greenschist-facies conditions and was restricted to a late transcurrent shear zone at the eastern boundary of the greenstone belt. The strain pattern prior to transcurrent shearing cannot be explained by a thrusting event unless all trace of this event had been erased by the diapirism. We interpret all structures older than the transcurrent shearing to be the direct result of vertical movements linked to body forces, augmented by minor horizontal crustal shortening.

The Pyrenees are a part of the Alpine chain of Western Europe which runs from the northern Iberian margin in the West to the Alps in the East (Figure 1). This E-W belt is considered to be a shortened crustal domain that occupies the site of the boundary between separate Iberian and Euro­ pean plates during Cretaceous times. In their central part, the Pyrenees display a fanlike cross-section: most of the faults are northward directed thrusts in the North , while large southward translations of Mesozoic and Cenozoic series can be observed in the South. The range is divided into several zones , bounded by major faults. From North to South one can distinguish

The 250 km long, deep Seismic reflection survey across the Pyrenees from one foreland (Ebro Basin) to the other (Aquitaine Basin) was designed to study the structure of the deformed Mesozoic boundary between the Iberian and European Plates. The results are: 1. (1) the entire crust shows well-defined reflectors with a general fan-shape geometry; 2. (2) a deep layered domain is evident above the Moho discontinuity; 3. (3) near the surface in the external domains, reflectors accurately define the geometry of major thrusts and structures affecting the Mesozoic and Cenozoic cover of the Pyrenees; 4. (4) the middle crust shows two major sets of Seismic events interpreted as Hercynian structures. Using these results, a balanced structural section of the Pyrenean belt is proposed. The mode of thickening of the crust during the Alpine orogeny is briefly discussed; only the Iberian crust was significantly thickened by a southward stacking of slabs.

The contribution of deep seismic profiling to our understanding of the pre-Mesozoic crust in France and adjacent areas is of the utmost importance. Despite limitations which will be discussed in Section 1.2, the method can obtain images of the crust down to and deeper than 50 km. It is the only way to observe the deep structure of the crust in some detail, to identify features such as thrusts, decollements or detachments, decoupling levels, deep faults, etc., to trace the Mohorovicic Discontinuity and image upper mantle discontinuities in favorable cases.

[1] The structural study of the Kolar greenstone belt and surrounding granite-gneiss terrains combined with U-Pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern Dharwar craton developed during a major magmatic accretion event between 2550 and 2530 Ma. The granite-greenstone pattern resulted from the interference of gravity-driven sagging of the greenstones (i.e., diapirism), E-W bulk inhomogeneous shortening combined with horizontal N-S stretching, and syntectonic juvenile pluton emplacement. Bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive, N-S trending vertical foliation, shallow stretching lineation, and conjugate strike-slip shear zone pattern within and outside the greenstone belt, resulting in regional horizontal pure shear deformation. The plutons around the greenstone belt record regional contraction by developing one set of strike-slip C-S fabrics of the shear zone pattern. The development of the granite-greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion, melting, and granulite metamorphism in the lower crust. The Kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain, due to mechanical homogeneity and low viscosity provided by large-scale melting during the accretion event. This example further suggests specific boundary conditions to the craton that allowed E-W inhomogeneous shortening to be accommodated by N-S stretching and spreading of the crust without significant tectonic thickening. Such tectonoplutonic pattern is specific to the Archean and may develop as a consequence of mantle plume activity in intracontinental settings.

The central part of the Aar Granite (Aar Valley) shows lens-shaped domains of low strain separated by anastomosing domains of high strain. A comparable pattern is found on all scales of observation. Deformation gradients are outlined by a gradual change from isotropic granite to orthogneiss and locally ultramylonites. Ultramylonites are concentrated within conjugate ductile shear zones. Where moderate, the strain is concentrated in very narrow small-scale faults which affect relatively undeformed granite. Within foliated domains, finite strain measurements on deformed xenoliths indicate that (i) strain intensity increases gradually with grain size reduction and (ii) finite-strain ellipsoids are of flattening type irrespective of the degree of mylonitization. The pattern of small-scale faulting, the trends of the regional foliation and the orientation of ductile shear zones are consistent with a bulk strain field of flattening type throughout progressive deformation.

During the last decade, it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures (e.g. pressure shadows, shear bands, C-S structures, fabrics, tension gashes, folds, veins). The correspondence between coaxial or non-coaxial deformation, and symmetric or asymmetric particle velocity fields, respectively accounts for the use of structural symmetry as an indicator of strain history. The application of this symmetry concept to various field examples emphasizes that: (i) the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system; and (ii) the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern, even where the progressive deformation results in a complex pattern which cannot be directly interpreted.

International audience; Extensional tectonics cover a wide range of crustal structures from narrow continental rifts to large continental areas. If we except continental and oceanic rifts, stretched portions of the crust exhibit complex faulting patterns at upper levels with high‐angle and low‐angle normal faults. Very little is known concerning the geometry of structures and physical processes at depth. It is therefore difficult to extrapolate surface observations to the crustal scale. Five general models are proposed that combine continuous and discontinuous deformation and geometrical and geological tests are proposed. Two natural examples, the West Armorican Atlantic margin and Eldorado Mountains (Basin and Range Province), are presented. They are characterized by an association of high‐angle normal faults, tilted blocks, and low‐angle normal faults. Field and geometrical arguments are given to demonstrate that low‐angle faults are controlled by décollement surfaces along preexisting interfaces. Geometrical implications of the tilted block pattern are presented and the gravity gliding hypothesis is proposed as a possible explanation of block tilting when associated with low angle normal faults. In such cases, the amount of stretching cannot be related to bulk thinning of the crust.

Geological deformations which are statistically homogeneous at bulk scale (e.g. the macroscale) are often localized into arrays of narrow shear zones at a smaller scale (e.g. the mesoscale). This paper shows that shear zone patterns can be used to estimate both a bulk finite strain ellipsoid and aspects of the bulk deformation history. We describe examples of heterogeneously deformed granitic rocks which reveal the following features. (1) Shear zones show preferred orientations. (2) There are correlations between shear zone orientations and directions and senses on shear. (3) For areas that have undergone coaxial deformation histories, shear zone patterns have orthorhombic symmetries directly related to strain ellipsoid shape. (4) For areas that have undergone non-coaxial deformation histories, shear zone patterns have a lower symmetry. (5) For areas that have undergone bulk simple shear, shear senses on shear zones are consistent with the bulk shear sense. Results are compared with predictions of kinematic models involving slip along inextensible fibres and sheets. According to these models, preferred orientations of slip surfaces track surfaces of no finite extension of the bulk strain ellipsoid, whereas slip directions track directions of large shear in the bulk strain ellipsoid. There are good correlations between preferred orientations of shear zones and fibre models.

During the Fall of 1979, a manned submersible program, utilizing DSRV ALVIN, was carried out at the intersection of the East Pacific Rise (EPR) with the Tamayo Transform boundary. A total of seven dives were completed in the vicinity of the EPR/Tamayo intersection depression and documented the geologic relationships that characterize the juxtaposition of these types of plate boundaries. The young volcanic terrain of the EPR axis can be traced into and across the Tamayo Transform valley but becomes buried by sedimentary talus that is being shed from sediment scarps along the unstable sediment slope that defines the north side of the intersection depression. Within 4 km of the transform boundary, the dominant trend (000°) of the fissures and faults that disrupt the rise-generated volcanics is markedly oblique to the regional direction of sea floor spreading (120°). Since no evidence was found to suggest that these structures accommodate significant amounts of strike-slip displacement, they are taken to reflect a distortion of the EPR extensional tectonic regime by a transform generated shear couple. The floor of the Tamayo Transform valley in this area is inundated by mass-wasted sediment, and the principal transform displacement zone is characterized at the surface by a narrow (

A model of evolution for the western segment of the Alpine chain is presented. Available data concerning the structural, metamorphic and geochronological evolution in the majorunits involved in the thickening processes are takeninto account IO constrain the model. The successive stages of the collisional history are correlated with the evolution of thermal structure: the African paleomargin was deformed at the onset of collision just after the abduction of the oceanic crust. After these initial stages, progressive stacking affected more and more external domains of the European plate. Lastly, the deformation of the entire pile is linked IO the thermal reequilibration of the considered thickened crust. The Alpine chain is a good example of a collision zone where the interaction of thermal and mechanical effects can be treated quantitatively.

The deformation of two granitic massifs along a dextral wrench fault zone (the South Armorican Shear Zone) is examined at the sample and grain scales. We note that an association of continuous-discontinuous deformation mechanisms is responsible for the mylonitisation at the grain scale. The general characteristics of the deformation (strain, ellipsoid type, strain regime, non-coaxial model) and quartz fabrics measured in these mylonites at each of the mylonitisation stages described, are also discussed.

A submersible study of a fast spreading (12 cm yr-1) Mid-Ocean ridge segment (East Pacific Rise near 12°50′ N) led to the discovery of intense hydrothermal activity. Twenty four sites with active vents and sixty inactive hydrothermal deposits were found within a narrow graben averaging about 300 m in width along a 20 km long segment of the ridge crest. The graben is floored with fresh basaltic sheet flows including collapsed pits or lava lake structures. From both deep towed camera stations and manned submersible observations, it is estimated that the average spacing between the ridge axis hydrothermal deposits averaging in size 10–50 m in diameter lies between 100 to 200 m.

The Ardoukoˆba Rift, subaerially exposed for ∼12 km between the Ghoubbat-al-Kharab and Lake Asal in the French Territory of the Afars and Issas (northeast Africa), has intrinsic features and a regional setting consistent with arguments that it is the site of crustal accretion at approximately the same rate as that found along the rifted Mid-Oceanic Ridge (∼2 cm/yr). The ∼11 km wide Central Zone of the Ardoukoˆba Rift has an internal relief of less than ∼300 m and is set between step-like ridges standing up to 800 m above the deepest part of the rift. The lower inward-facing scarps of the Central Zone border a narrow Inner Floor. The Central Zone of the typically ∼25–35 km wide oceanic Rift Valley can have a greater and rougher relief and has a width of ∼8–16 km, but deep areas with an internal relief of

We attempt to reconstruct the kinematics (deformation and motion) of the Western Alpine Arc between the Mercantour and Aar Massifs. First, we compile a map showing principal extension (X) direction within three major structural units. In the external zone, X directions are broadly radial to the Alpine Arc; in the internal zone, they are uniform or smoothly varying in some domains; whereas in the intermediate zone they often show much scatter. Next, we use geochronological data and P-T conditions, estimated from metamorphic mineral reactions, to time the deformation. On each of a series of interval maps, we show X-directions for strain accumulated during a chosen time interval. We argue that these incremental X-directions are approximately equivalent to segments of particle paths, showing displacements relative to stable Europe. For 120–100 Ma, the displacement direction was about N150 and coeval with eclogite facies metamorphism; it is interpreted as a direction of overthrusting by the African Plate. Around 40 Ma, the displacement in the south part of the arc became westerly. Finally, between 25 and 15 Ma, displacement directions were generally radial to the arc.

Data from rock-dredging have often been used to infer that oceanic fracture zones provide a 'window' into layers of the oceanic crust lying at a depth below the surface that is approximately equivalent to the vertical offset of the fracture zone, and thus permit the reconstruction of a crustal stratigraphy for the whole of acoustic layer 2 (commonly considered to have an average thickness of ~2 km) and, in some interpretations, for the upper part of layer 3. Alternatively, it has been suggested that fracture zones are preferential sites of serpentinite mega-dykes differing in composition from layer 3 but containing inclusions of the third layer. The published data indicate that basalts and basaltic rubble are abundant in fracture zones and, on analysis, do not justify the assumptions that have been made. The structure of fracture zones limits the possible extent of crustal sections exposed on their walls. Moreover, it is suggested that rocks of different layers of the lithosphere can be emplaced in the transform domain due to the dynamic of the transform fault system, itself.

Using the diving saucer “Cyana”, a portion of the East Pacific Rise between 12°38′ and 12°54′N has been studied and mapped. Four main zones are defined: (1) an active volcanic zone located in the central graben; the graben displays a variable relief along strike from less than 10 m up to 55 m and has an en-echelon pattern in plan view; (2) an active tectonic zone comprising the central graben and extending up to about 2 km from either side of the rise axis; (3) an inactive tectonic zone starting 2 km away from the axis and formed by discontinuous horst and graben type of structures; and (4) off-axis volcanoes (seamounts) located less than 20 km on either side of the rise axis. At 12°50′N, the active tectonic zone is extremely narrow and the mode of deformation is through fissuring in strong contrast with that observed in slow or intermediate rate ridges. The presence or absence of a magma reservoir at shallow depth explains the difference in the width of the active tectonic zone. The state of isostatic equilibrium or lack of equilibrium explains the different modes of deformation.

The results of 26 dives with the diving saucer Cyana in the Gulf of Tadjoura and Ghoubbet al Kharab (Republic of Djibouti) are presented. One can demonstrate that the sites of recent volcanism, tectonics and hydrothermal activity within the axial part of the Gulf of Tadjoura coincide with topographic highs trending at a high angle (azimuth 135°) with respect to the average trend of the axial trough of the Gulf (azimuth 080°). The highs owe their relief to both volcanism and normal faulting along a trend of 130–140°. Recent faulting on the bounding walls of the axial trough is also found along a trend of 130–140° where the faults interfere with another set of apparently older faults trending 070° parallel to the axial trough. Spacing between the active zones of the Gulf is regular and about 30 km. No evidence for transform faulting has been found, in contradiction to all previous kinematic models of the Gulf of Tadjoura. There is evidence that the presently active phase of opening associated with 130–140° faults is less than 0.7 my old and that there was an older phase associated with 070°–080° faults creating the main trough of the Gulf. The basaltic lavas created during the two phases have evolved from transitional to tholeiitic.

In the south-east border of French Central Massif, near Anduze, limestone series of Upper Jurassic age are affected by an association of deformations which occurred during a single tectonic phase. Thus chronologically appeared: 1. (1) Conjugated shear planes with vertical intersection (dextral and sinistral microdisplacement). 2. (2) Concentric folds with approximately east-west axis trend associated with stylolitic joints. 3. (3) Conjugated shear planes with horizontal intersections (microthrusts to north or south). The study of these different deformations gives way to the reconstitution of both successive states of stress during time and local variations in direction of the stress ellipsoid axis.

Secondary crystallization at the extremities of non-deformable object within a schistose rock, form of this zone

Massive ore-grade zinc, copper and iron sulphide deposits have been found at the axis of the East Pacific Rise.Although their presence on the deep ocean-floor had been predicted there was no supporting observational evidence. The East Pacific Rise deposits represent a modern analogue of Cyprus-type sulphide ores associated with ophiolitic rocks on land.They contain at least 29% zinc metal and 6% metallic copper.Their discovery will provide a new focus for deep-sea exploration, leading to new assessments of the concentration of metals in the upper layers of the oceanic crust. [NOT CONTROLLED OCR]

The south-arm orican leucogranites are progressively deformed in a major dextral hercy-nian shear. The quartz fabric is shown at different stages of the strain history defined by the spatial evolution of the planar structures at microtectonic scale. Non coaxial deformation occurs and progressive rotation of c axis orientations is demonstrated. The influence of heterogeneities on this evolution is briefly discussed., Les leucogranites sud-armoricains sont progressivement orthogneissifiésle long d'un cisaillement dextre hercynien. Les orientations préférentielles du quartz sont mesurées à divers stades de l'évolution de la déformation, définie à l'échelle microtectonique par l'évolution spatiale des structures planaires. Dans le régime de déformation non coaxiale envisagé, on met en évidence une rotation progressive de la fabrique et on discute brièvement l'influence des hétérogénéités sur l'évolution des orientations préférentielles des axes c du quartz., Berthé Didier, Choukroune Pierre, Gapais Denis. Orientations préférentielles du quartz et orthogneissification progressive en régime cisaillant : l'exemple du cisaillement sud-armoricain. In: Bulletin de Minéralogie, volume 102, 2-3, 1979. Mécanismes de déformation des minéraux et des roches.

The different hypotheses proposed for the creation of the Bay of Biscay are reviewed. New geological and geophysical data collected in the last two years in the Bay and in the Pyrenean domain give new insight into the tectogenesis of the Pyrenees. Geological data of the Pyrenean area provide tight constraints on the hypothesis of formation of the Bay. The most probable hypothesis is an opening by rotation of the Iberian Peninsula around a pole of rotation situated near Paris, which resulted in strike-slip motion dong the North Pyrenean fault during the Upper Mesozoic. A progressive westward migration of the pole initiated in the late Cretaceous blocked the motion dong the fault and led to the main Eocene tectogenetic Pyrenean phase. [NOT CONTROLLED OCR]

On Saddle Island a detachment fault of Tertiary age separates two structurally different plates. In the lower plate, Precambrian amphibolite and gneiss are strongly overprinted by a subhorizontal mylonitic foliation that may have formed during east-directed overthrusting of Sevier age. The mylonitic foliation is in turn overprinted by a later brittle deformation expressed by the detachment fault and by two gently north-dipping zones of anastomosing faults that outline crude lenslike masses of unfractured rock. These fault zones are probably an expression of the brittle deformation that dominates during a detachment event and are themselves cut by north-south–trending, high-angle normal faults related to a second brittle deformation. Our field measurements of the orientation of fault planes and striae on these fault planes suggest that the orientation of the least principal stress direction (σ 3 ) changed during the brittle faulting event from north-northwest to northwest-southeast. Neither of these directions of extension is compatible with the strain field associated with the earlier ductile shearing responsible for the development of the mylonitic foliation.

In this thesis, we present the results of structural analysis in different segments of Archaean continental crust (>2.5 Ga) in which strain patterns are compatible with the development of crustal-scale gravity instabilities. We also present analog experiments simulating the development of these gravity instabilities in order to constrain their thermomechanical significance within the Archaean continental lithosphere. Structural results A structural study of several greenstone belts from the Dharwar Supergroup (Dharwar craton, India) leads to a progressive deformation model for the initiation of gravit y instabilities between the basin and its basement located in the upper structural lev el of the crust. The basal unconformity of the Dharwar Supergroup acted as a décollement zone. The kinematic criteria within this décollement are aIl compatible with a displacement of the supracrustal rocks toward the internaI parts of the greenstone belts. The décollement-related structures are progressively refolded ab ove the zone of maximum subsidence. In sorne of the greenstone belts, the gravity strain pattern interfers with the regional strain field, which is characterized by predominant E-W shortening and N-S trending strike-slip shear zones. In the Dharwar intracratonic basin, the sagging episode was contemporaneous with the last tectonometamorphic event recognized within the craton, c.a. 2.5 Ga ago. This episode is characterized by the formation of granulites at depth as weIl as by large-scale migmatization associated with the emplacement of a large amount of juvenile material. We consider that the high mantle heat flux and partial melting led to the softening of the continental crust and this allowed the development of the gravity instabilities. Within the western Reguibat rise (Mauritania), structural analysis reveals the occurrence of dome-and-basin structures. The strain patterns associated with these structures are interpreted as the result of interference between the diapiric rise of granitoid domes within the greenstone sequences and regional E-W shortening. At the scale of the southern part of the Archaean Reguibat rise, the strain trajectories have the same overall trends in both the high-grade and medium-grade terrains. No major tectonic break has been identified between these metamorphic domains. This implies that the formation of diapiric domes was penecontemporaneous with granulite metamorphism and regional deformation within the lower crust, which occurred c.a.2.7 Ga ago. Experimental results We used temperature-dependent viscosity material (Gum Rosin) and an new experimental apparatus to simulate unstable granite/ greenstone layers. A vertical temperature gradient was maintained, and the deformation was driven by gravit y alone. For a fixed temperature at the bottom of the model, the relative thicknesses of the different layers were changed from one experiment to another. Insome experiments, a brittle material (sand) was introduced as a superficial layer. The experiments show that : (1) the triggering of gravity instabilities is possible only if the materials at the unstable boundary (cover/basement contact) are ductile and (2) the growth rate of gravity instabilities is an exponential fonction of the temperature at the unstable interface and the tempe rature gradient at the top of the uns table layer. These results suggest that this type of gravity instability could not develop within a modern continental lithosphere., Dans cette thèse, on présente les résultats de l'analyse structurale de différentes portions de croûte continentale archéenne (> 2.5 Ga) dans lesquelles les champs de déformation sont compatibles avec le développement d'instabilités gravitaires d'échelle crustale. Sont présentés également des modèles expérimentaux visant à simuler le développement de telles instabilités gravitaires, afin de comprendre leur signification thermomécanique à l'intérieur de la lithosphère continentale archéenne. Résultats de l'étude structurale L'étude structurale de différentes ceintures du Super.groupe de Dharwar (craton de Dharwar, Inde), permet de décrire, dans un modèle de déformation progressive, les premiers stades du développement d'instabilités gravitaires entre le bassin et son socle dans les niveaux structuraux supérieurs de la croûte archéenne du craton. La discordance basale du Supergroupe de Dharwar est tectonisée en un niveau de décollement dont la cinématique traduit un déplacement des séries supracrustales vers le coeur des ceintures de roches vertes. Les structures développées aux marges des ceintures sont replissées à l'aplomb des zones de subsidence maximale. Selon la ceinture de roches vertes considérée, le phénomène gravitaire a pu interagir avec le champ de déformation régional caractérisé par un raccourcissement E-W et des cisaillements transcurrents de direct'ion moyenne N-S. Cette déformation gravitaire du bassin intracratonique de Dharwar est associée au dernier épisode tectonométamorphique ayant affecté le craton, il y a environ 2.5 Ga. Cet épisode est caractérisé par la formation généralisée de granulites dans la croûte profonde ainsi que par une migmatisation régionale associée à la mise en place d'une quantité importante de matériel juvénile. On considère que l'important flux de chaleur mantellique et la fusion partielle permettent l'amollissement de la croûte continentale, favorisant ainsi le développement des instabilités gravitaires décrites. Dans la Dorsale Reguibat occidentale (Mauritanie), l'analyse structurale révèle la présence de structures en dôme-at-bassin. Les champs de déformation sont interprétés comme le résultat de l'interférence entre la mise en place diapirique de dômes de granitoïdes dans les séries de roches vertes et un raccourcissement régional E-W. A l'échelle de la partie méridionale de la Dorsale Reguibat archéenne, les trajectoires de déformation sont compatibles dans les terrains granulitiques et dans les terrains de degré métamorphique intermédiaire. Entre ces deux domaines métamorphiques, aucune discontinuité structurale majeure n'est identifiée. Ceci impliquerait que la formation des dômes diapiriques soit sub-synchrone de la formation des granulites et de la déformation régionale dans la croûte inférieure, c'est-à-dire vers -2.7 Ga. Résultats expérimentaux Grâce à l'emploi d'une resme thermosensible et d'un dispositif permettant le chauffage des modèles expérimentaux, il a été possible de réaliser des expériences simulant les bicouches granite/roches vertes gravitairement instables, soumis à un gradient vertical de température et aux seules forces gravitaires. Pour une température fixée à la base des modèles, les paramètres que l'on a fait varier d'une expérience à l'autre sont les épaisseurs relatives des différentes couches. On a pu également introduire dans les modèles une couche superficielle à rhéologie fragile (sable). Les expérie"ces révèlent (1) que le déclenchement d'instabilités gravitaires n'est possible que si les matériaux situés au niveau de l'interface instable (contact socle/couverture) ont des rhéologies ductiles et (2) que la vitesse du développement des instabilités est une fonction exponentielle de la température au niveau de l'interface instable et du gradient de température au sommet de la couche instable. Ces résultats suggèrent que ce type d'instabilités gravitaires ne puisse pas se développer à l'intérieur d'une lithosphère continentale moderne.

The aim of this thesis is to characterize the tectonic and metamorphic evolution of an Archaean continental segment (>2.5 Ga). The studied area is the Dharwar craton (South India) where deeper and deeper structurallevels outcrop from North to South. This area has not been affected by post-Archaean defonnation. We show that the disclosed strain field is the result of interfering gravitational diapiric bodies and regional shortening. At crustal scale, structures (diapirs and transcurrent shear zones) were developed within a single event that can be dated at around 2.5 Ga. They are synchronous with (i) an episode of juvenile continental crust formation (Closepet batholith) and (ii) regional metamorphism (migmatisation, granulitisation). Strain fields are not consistent with thrusts and stacking structures as usually observed in modern collision belts. The petrographie study reveals that sorne specifie diapiric zones, which underwent fast burial, have recorded a metamorphic prograde history associated with a pressure increase (from 3 to 7-8 kbar). Furthermore, these areas display sorne differences in the fluid phase that seem to be linked with the variations of permeability resulting from the type and the geometry of the metamorphic fabrics. In summary, the Archaean continental crust of D ar craton does not display any structure that could be responsible for the thickening of the crust. The regional strain results from body forces (diapirism) associated with boundary forces (transcurrent shear zones). The gravitational diapiric instabilities which concem large volumes of crust cannot be the result of crustal thickening and constitute a distinctive feature of Archaean tectonics., Le but de cette thèse est de caractériser l'évoiution tectono-métamorphique d'un segment de croûte continentale drchéenne (> 2.5 Ga). L'objet étudié est le craton de Oharwar (Inde du Sud) qui présente, du Nord vers le Sud, une transition continue de niveaux structuraux de plus en plus profonds. Cette région n'est affectée par aucune déformation post-archéenne. Nous montrons que les champs de déformation mis en évidence résultent de l'interférence entre le développement d'instabilités gravitaires diapiriques et un raccourcissement régional. Toutes les structures d'échelle crustale résultantes (diapirs et décrochements) sont générées au cours d'un seul et même événement que nous avons daté aux environs de 2.5 Ga. Elles sont contemporaines (i) d'un épisode de formation de croûte continentale juvénile (batholite du Closepet), (ii) d'un métamorphisme régional parfois intense (migmatisation, granulitisation). La cartographie des champs de déformation nous permet d'infirmer la présence de structures chevauchantes ou d'un empilement caractéristique des zones de collision continentale modernes. L'étude pétrographique révèle que seules certaines zones diapiriques dont la * subsidence a été la plus rapide ont enregistré une évolution prograde avec une augmentation de pression de 3 à 7-8 kbar. De plus, ces régions montlent une hétérogénéité dans la composition de la phase fluide qui semble être associée à une différence de perméabilité induite par la nature et l'orientatiôn des textures des roches métamorphiques. En conclusion, la croûte continentale archéenne du craton de Dharwar ne montre pas de structure tectonique susceptible d'avoir généré un épaississement crusta: important. Sa déformation est due principalement à des forces de volume (diapirisme) et, dans une moindre mesure, à des forces de surface (décrochements). Le développement d'instabilités gravitaires diapiriques, parce qu'il affecte des volumes de croûte considérables et qu'il ne peut être rapporté à aucune évidence structurale d'épaississement crustal, constitue une particularité de la tectonique de l'Archéen.

No abstract; L’étude de la croûte continentale ne saurait aujourd’hui avoir lieu sans l’aide des méthodes géophysiques, qui permettent l’exploration de domaines hors de portée de l’observation directe.Parmi ces techniques, la sismique réflexion verticale profonde a connu depuis quelques années un essor important, particulièrement grâce au développement des recherches pétrolières. De nombreux programmes de sondages sismiques profonds ont été mis en place sur tous les continents (COCORP, CALCRUST, LITHOPROBE en Amérique du Nord, ECORS, BIRPS, DEKORP, en Europe, ACORP en Australie), dans le but d’étudier des objets géologiques variés (marges, rifts, bassins). Les images obtenues ont permis de vérifier ou de renouveler bon nombre d’hypothèses sur la structure et la genèse de la croûte continentale (voire de la lithosphère continentale).

No abstract, L’étude de la croûte continentale ne saurait aujourd’hui avoir lieu sans l’aide des méthodes géophysiques, qui permettent l’exploration de domaines hors de portée de l’observation directe.Parmi ces techniques, la sismique réflexion verticale profonde a connu depuis quelques années un essor important, particulièrement grâce au développement des recherches pétrolières. De nombreux programmes de sondages sismiques profonds ont été mis en place sur tous les continents (COCORP, CALCRUST, LITHOPROBE en Amérique du Nord, ECORS, BIRPS, DEKORP, en Europe, ACORP en Australie), dans le but d’étudier des objets géologiques variés (marges, rifts, bassins). Les images obtenues ont permis de vérifier ou de renouveler bon nombre d’hypothèses sur la structure et la genèse de la croûte continentale (voire de la lithosphère continentale).