Jacques Ingles

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.

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.

Theoretical strain patterns in ductile zones simultaneously undergoing heterogeneous simple shear and bulk shortening

Abstract Structures observed in ductile shear zones show that deformation is continuous and strain is heterogeneous. This leads us to define a simple model for shear strain rate γ. When a shear zone simultaneously undergoes simple shearing and pure shearing (contraction or extension), the condition of continuity of deformation shows that γ must be a function of time. Integration of the deformation-rate equations yields the particle paths in the deforming body. From these we can determine, at any time and at any point in the body, the finite strain tensor (lengths and orientations of the axes of the finite strain ellipsoid); we can therefore define the finite strain trajectories and compare them with orientations of internal structures (schistosity, tension gashes). The particle paths also allow us to study the progressive deformation of passive strain markers.

Terminations of ductile shear zones

Abstract We propose a plane strain model for ductile shear zone terminations in rocks that otherwise record no visible shear strain. This model may explain the strain distribution patterns in naturally occurring terminations of shear zone which show, in the XZ plane of finite strain, (1) a decrease in the area of finite strain and in strain intensity (ellipsoidal or lens-like feature) and (2) an increase in the strained area as strain intensity decreases (splay-like or horse-tail feature).

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).

Theoretical and natural strain patterns in ductile simple shear zones

Abstract A simple empirical model representing the variation of shear strain throughout a simple shear zone allows us to determine the evolution of finite strain as well as the progressive shape changes of passive markers. Theoretical strain patterns (intensity and orientation of finite strain trajectories, deformed shapes of initially planar, equidimensional and non-equidimensional passive markers) compare remarkably well with patterns observed in natural and experimental zones of ductile simple shear (intensity and orientation of schistosity, shape changes of markers, foliation developed by deformation of markers). The deformed shapes of initially equidimensional and non-equidimensional passive markers is controlled by a coefficient P , the product of 1. (1) the ratio between marker size and shear zone thickness 2. (2) the shear gradient across the zone. For small values of P (approximately P P lead to “ retort” shaped markers. This theoretical study also allows us to predict, throughout a simple shear zone, various relationships between the principal finite strain trajectory, planar passive markers and foliations developed by deformation of initially equidimensional passive markers.

Determination of finite strain from passive initially planar markers deflected in ductile rocks undergoing simultaneous superposition of heterogeneous strains

Abstract From an empirical model of strain in ductile heterogeneous shear zones simultaneously undergoing a bulk pure shear, we study the influence of the parameters of the deformation on the shape changes of passive initially planar markers and describe a method for determining the finite strain, and even sometimes the instantaneous strain, from these deflected markers.