Jean-Claude Soula

Characteristics and mode of emplacement of gneiss domes and plutonic domes in central-eastern Pyrenees☆

Abstract Gneiss domes and plutonic granitoid domes make up almost 50% of the pre-Hercynian terrains in the Central and Eastern Pyrenees. From a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks, it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism. The study also shows that the internal structure, the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes. This appears to be in good agreement with H. Rambergs (1967, Gravity Deformation and the Earths Crust . Academic Press, London; 1970, Model studies in relation to intrusion of plutonic bodies. In: Mechanisms of Igneous Intrusion (edited by Newall, G. & Rast, N.) Geol. J. Spec. Issue 2 , 261–286.) model studies showing that dome or mushroom-like structures, similar to those observed, develop when there is a small viscosity ratio between the rising body and its enclosing medium. This implies a high crystal content for the granitoid magma. This crystal content has been estimated by (i) calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension, using the model and program of J. P. Carron et al. (1978 Bull Soc. geol. Fr. 20 , 739–744.); (ii) using the recent results of experimental deformation of partially melted granites of I. van der Molen & M. S. Paterson (1979, Contr. Miner. Petrol. 70 , 299–318.) and (ii) comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes. The minimum crystal content for the development of a dome-like structure has been, thus, estimated to about 70%, i.e. a value very close to that estimated by van der Molen & Paterson (1979) to be the critical value separating the granular framework flow from suspension-like behaviour. The effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments. These small variations appear to exert a strong control on the shape and rate of rise of the domes. They are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes. Finally, more complex experiments, with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data, are reported. Their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed.

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.

Controls on timing of exhumation and deformation in the northern Peruvian eastern Andean wedge as inferred from low‐temperature thermochronology and balanced cross section

In northern Peru, a 500 km long regional balanced section has been constructed across the eastern Andean wedge, using fieldwork, industrial seismic sections, and wells. The structure is characterized by a thin-skinned thrust system involving the Eastern Cordillera (EC), the sub-Andean zone (SAZ), and the Maranon foredeep. In the SAZ and the easternmost foredeep the development of the thrust system has been driven by the combination of two structural events. Permian thrust faults had been reactivated to form a basement duplex underlying the SAZ and the foredeep. At the same time a Triassic-Jurassic extensional basin has been transported as a crustal ramp anticline on to the duplex roof fault, giving rise to the EC. The impingement of the EC was responsible for the deformation of the SAZ and the propagation of the thrust wedge. The minimum shortening calculated is 142 km, representing a shortening strain of ~ −28%. A sequential restoration calibrated by (U-Th)/He and Fission Track dating on apatites and vitrinite reflectance values shows that shortening rates vary from 7.1 mm yr−1 between 17 and 8 Ma to 3.6 mm yr−1 between 8 Ma and today and suggests that the thrust wedge commenced propagation between 30 and 24 Ma. When compared with other Andean thrust wedges, we suggest that the timing of the thrust wedge propagation is not a simple function of the distance to the hinge of the Bolivian orocline and the propagation is not controlled by the precipitation regime. We rather suggest that reactivated basement faults favored thrust wedge propagation.

Genesis and evolution of Permian and Triassic basins in the Pyrenees by regional simple shear acting on older Variscan structures: field evidence and experimental models

Abstract A sedimentological and structural study of Permian and Triassic basins, Pyrenees (France, Spain), and their underlying Variscan substrate has been made in conjunction with model experiments. We conclude that opening of the basins occured during a regional deformation by simple shear along preexisting basement faults initially situated in first and third quadrants. The opening was developed by block rotation, with accompanying shortening and extension, respectively parallel and normal to the faults. Deposition structures, similar to sedimentary structures observed in the field, were duplicated by experimental models. These models show also that normal faulting or development of a vertical lateral slip conjugate fault system with acute angle bisectrix parallel to the major axis of the basins are not necessarily related to a regional non-ro tational deformation, but can and will occur in the cover if the opening of the basins is due to a regional simple shear, even so if in the underlying basement faults act as strike slip faults.