Pierre Debat

A Paleoproterozoic ultramafic-mafic assemblage and associated volcanic rocks of the Boromo greenstone belt: fractionates originating from island-arc volcanic activity in the West African craton

Abstract The Loraboue Birimian ultramafic-mafic assemblage, located in the Boromo greenstone belt (Burkina Faso), is interpreted as the remains of a magma chamber that crystallized at the base of an island arc. The ultramafic rocks exhibit an heteradcumulate texture, being generally made up of wehrlites and more rarely dunites. The crystallization sequence inferred from the cumulates is olivine+chromite followed by clinopyroxene+amphibole±orthopyroxene±biotite. The gabbroic rocks are mainly layered and grade into more differentiated facies with sub-pegmatitic texture, containing up to 70% modal plagioclase including zircon and, more commonly, apatite crystals. Textural relationships and mineral phase chemistry are indicative of crystallization at elevated pressures (>8 kbar), the parental magma being generated by a moderate to high degree of partial melting of a mantle source affected by previous metasomatic events above a subducted oceanic slab. The Loraboue volcanic formations exhibit a range of geochemical features. They consist dominantly of calc-alkaline basalts, pyroclastites and rhyolite and, more rarely, of basalt, dolerite and gabbro of tholeiitic affinity. These different types of basalt, as well as the dolerite and the isolated massive gabbro, show the classic features of arc magmatic suites, namely LILE and Pb enrichment, depleted HFSE patterns and high Ce/Nb and Th/Nb ratios. Thus, the calc-alkaline plutonic and volcanic assemblages of Loraboue could represent the roots of an island arc and the associated coeval volcanic rocks. The Paleoproterozoic crust of the West African craton was heterogeneous and was not the consequence of a single process of genesis. As some modern igneous province, the Birimian crust was generated by both volcanic arc accretion and oceanic plateau accretion.

Deformation and dynamic recrystallization of K feldspar augen in orthogneiss from Montagne Noire, Occitania, Southern France

Abstract From previous optical studies on natural deformation microstructures in feldspars from gneisses and pegmatites, reported in a preceding paper, it would appear that recrystallization is a major process accompanying deformation of K feldspars under mesozonal conditions. In the present paper this process has been documented in augen gneisses from the Cammazes formation, Montagne Noire, Southern France, using optical, X-ray and microprobe analyses, and a TEM descriptive study. The augen studied are typically composed of a central part with one or several megacrysts surrounded by a recrystallized zone with small polygonal microcline grains. The recrystallized zone was found to consist of neograins made up of subgrains in various deformation states, which characterize a d ynamic recrystallization. This dynamic recrystallization was found to have proceeded together with chemical changes, i.e. migration of a sodic phase within a K feldspar augen.

Optical studies of natural deformation microstructures in feldspars (gneiss and pegmatites from Occitania, southern France)

Abstract Natural deformation microstructures of feldspars have been investigated optically in augen gneisses and pegmatites from southern France that exhibit microstructures due to polyphase deformation. This deformation was produced in both cases under a pressure of about 2 Kb and a temperature decreasing from about 550° to 200–300°C. In microcline, recrystallization, tension gashes and shear planes are the most important microstructures observed. In plagioclase, shear planes and kink-bands prevail. The development of these structures is controlled primarily by three parameters: temperature, influence of the surrounding material and amount of bulk strain. The orientation of the planes of anisotropy of the minerals with respect to the compression axis is also important. Some perthitic patterns are ascribed to deformation.

A dynamic model of the structural evolution of the Hercynian Pyrenees

Abstract The structural history of the Hercynian Pyrenees is considered with respect to the relationships between the metamorphism and the successive deformations, the characteristics and mode of emplacement of the gneissic and anatectic cores of the metamorphic domes, the migmatization and the intrusions of plutonic and basic and ultrabasic rocks. The metamorphism can be shown to have progressed upwards with time, attaining the presently exposed levels just before, during and a short time after the major penetrative regional deformation. The intrusion of major plutonic, anatectic and gneissic massifs, which has determined the metamorphic domes in country rocks, was roughly synchronous with the progression of the metamorphism, but seems to have continued later in the uppermost levels. The basic and ultrabasic rocks uplifted by the intrusion of the anatectic cores of the metamorphic domes have been deformed and foliated by the major regional penetrative deformation, thus showing that their intrusion predated this deformation in the lowermost levels. These results, together with the presently available sedimentological data, lead us to propose a dynamic model of the tectonic evolution of the Hercynian Pyrenees. The first step was a crustal extension and thinning, attaining its maximum during Frasnian time, marked in the upper levels by a “ horst and graben” controlled sedimentation and in the lower crust by intrusions of basic magmas and the onset of the very low-pressure metamorphism and anatexis characteristic of the Hercynian Pyrenees. The second step was a crustal shortening marked by large-scale crustal thrusting followed by widespread regional penetrative deformation. The flysch troughs formed in front of the progressing and emerging thrusts. In lower levels, the thrusts are believed to have controlled the progressive ascent of the plutonic and anatectic magmas which continued their uplift and emplacement during major penetrative regional deformation in the actual mesozone and probably after it in the uppermost levels.

Thrust-related, diapiric, and extensional doming in a frontal orogenic wedge: example of the Montagne Noire, Southern French Hercynian Belt

Abstract The Montagne Noire, which is situated at the toe of the orogenic wedge of the French Massif Central South European Variscides, appears to be a well-suited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins. The data reported in the present paper show that the Montagne Noire dome is a particular type of basement-involved frontal culmination in an orogenic wedge and foreland basin system. This frontal culmination is characterized by a syn-contractional HT decompression recorded by clockwise PTt paths and widespread strata overturning in thrust and fold structures, which controlled the sedimentation in the adjacent foreland basin. These unusual characteristics are interpreted to be a result of the succession of thrusting, diapirism and extensional collapse. Antiformal stacking of syn-metamorphic thrust sheets controlled the first stages of the foreland basin development. Diapirism was essentially responsible for the HT decompression and widespread strata overturning. Extensional doming was a result of late- to post-metamorphic collapse acting on the pre-existing high-amplitude dome. Diapirism and associated isothermal decompression metamorphism, which constitute the essential difference between the Montagne Noire and ‘ordinary’ frontal ridges in orogenic wedges, were probably enhanced by a local partial melting of the upper to middle crust. It is suggested that the occurrence of these phenomena in front of an orogenic wedge was related to local over-thickening due to the superposition of an upper crustal antiformal stack on top of a lower crustal ramp anticline.

Nucleation of ductile shear zones in a granodiorite under greenschist facies conditions, Néouvielle massif, Pyrenees, France

Abstract The problem of ductile shear zone nucleation under greenschist facies conditions is approached from the example of small-scale shear zones developed in the Neouvielle granodioritic pluton (Pyrenees, France) by means of field and microstructural observations and chemical analyses. These shear zones are not related to pre-existing fractures and exhibit networks of numerous conjugate, fairly parallel and regularly spaced centimetre-scale brittle–ductile shear zones involving diffuse localization mechanisms. Although the mode of deformation depends on the minerals present, deformation in the shear zones is basically controlled by hydration processes. Hydration and consequent fluid-controlled alteration and deformation are related to fluid migration towards developing cracks. All these cracks are extensional and formed on the scale of at most a few grains and frequently single grains. A comparison of chemical compositions of undeformed and sheared granodiorite shows that the shear zones can be interpreted as isochemical and isovolumetric systems. We propose a sequence of mechanisms by which the nucleation of a small-scale brittle–ductile shear zones spreading out within a granodioritic rock may occur. In these mechanisms shear zone nucleation occurs independently of pre-existing fractures and results from the heterogeneous character of the polymineralic rock. In the studied granodiorite the mineral heterogeneities favour focusing of locally derived fluids by processes involving grain-scale hydraulic fracturing. Fluid focusing generates instabilities by local softening and subsequently shear zone nucleation.

Paleodeformations d'un massif orthogneissique: Massif des Cammazes, Montagne Noire occidentale, France

Abstract The Cammazes orthogneissic massif in the western Montagne Noire (France) has been affected by numerous tectonic phases during the Hercynian orogeny. The principal deformation, synchronous with a mesozonal metamorphism, is represented by different structural features such as foliation, ‘augen’ structures and deformation of megacrystals. These structural features are essentially varied with the concentration of feldspar megacrystals in the gneisses. The structural studies of these gneisses have allowed the recognition of the nature and the orientation of the principal deformation. To determine the intensity of this deformation, large undeformed feldspar crystals have been used as markers. These crystals have been assimilated to rigid ellipsoidal elements disposed in a random fashion in the initial stage of the mesostase which is considered as a viscous Newtonian fluid. The statistical studies of the orientation of these crystals with reference to the plane of flattening (foliation) has caused the appearance of an anisotropy in the final deformed state. This anisotropy, when compared to that obtained theoretically from a population of rigid elements included in a Newtonian fluid flowing at a slow speed, could evaluate the rate of the finite strain.

The Mylonite zones in the Pyrenees and their place in the Alpine tectonic evolution

Abstract In the Pyrenees, the development of mylonites zones is one of the most striking structural features. Two sets of mylonites of regional extent have been recognized: large longitudinal E-W to N110°E trending zones (e.g. Merens fault and North Pyrenean fault) and oblique NW-SE trending zones cross-cutting both the Hercynian and the post-Hercynian terrains. The longitudinal zones limit the major structural zones of the Pyrenees and are associated with NW-SE “en echelons” folds in the Mesozoic terrains and rotations of rootless plutonic or gneissic massifs, acting as competent inclusions in a more ductile matrix, in the Hercynian basement. The oblique mylonite zones limit map-scale fold-bands and appear as the sheared limbs of these folds. The age of the oblique zones and of the major movements along the longitudinal zones is clearly Alpine and the “en echelons” folds seem to have controlled the sedimentation during the Upper Albian and possibly during the Upper Cretaceous. Early movements along the longitudinal zones may have been Hercynian. The analysis of the structures at all scales leads us to interpret these mylonite zones and associated structures as the ultimate result of a transcurrent simple shear acting during the whole Mesozoic period. This strike-slip shearing was probably associated with an extension perpendicular to it from the Permian to the Upper Cretaceous and then to a shortening component also perpendicular to it from the Late Cretaceous to the Eocene. The development of the mylonite zones appears to have predated the major Alpine thrusting but to have been reactivated during this thrusting, acting as initiation sites for the thrusts or as oblique ramps in the case of the oblique mylonite zones.

Conjugate ductile shear zones

Abstract The geometric character of conjugate ductile shear zones has been defined from the structures developed in a undeformed granitoid corresponding to an homogeneous and isotropic material. In the plane perpendicular to the intersection line of conjugate zones (plane σ 1 σ 3 ) one notes that: (1) the principal direction of finite shortening determinated from the orientations of internal structures bisects the acute angle of the conjugate sets; (2) the zones cross along a sub-rectangular intersection zone lengthened perpendicularly to the direction of finite shortening and thickened in this direction; and (3) in the intersection zone the foliation has a crenulated shape. Conjugate ductile shear zones seem to have the same evolution as conjugate brittle shear zones and are characterized by the morphology of the intersection zone, showing a local more complex strain. Since geometrical patterns of conjugate shear depend on the observation planes, the angular relation between the conjugate sets appears insufficient to determine the bulk strain directions.

Rheological properties of rock inferred from the geometry and microstructures in two natural shear zones

Abstract We describe the geometric and kinematic characteristics of natural isolated shear zones developed in two different rock-types (granodiorite and limestone) under the same low temperature conditions. Profiles of shear strain (γ) across the shear zones have a gaussian or trapezoidal shape. From these shapes, we infer theological changes during the progressive strain, with strain softening in the limestone (a monomineralic rock) and strain hardening in the granodiorite (a polymineralic rock).