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Patrick Monie - One of the best experts on this subject based on the ideXlab platform.
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shear band formation and strain localization on a regional scale evidence from anisotropic rocks below a major detachment betic cordilleras spain
Journal of Structural Geology, 2011Co-Authors: Philippe Agard, Romain Augier, Patrick MonieAbstract:Regional-scale deformation taking place in a strongly anisotropic, yet homogeneous metapelitic protolith during an apparently single tectonic event was systematically investigated as a function of the distance to the main tectonic contact (i.e., the Filabres shear zone, a major detachment in the Betic Cordilleras, Spain). The density of C3' shear bands (or extensional Crenulation Cleavage) reworking the earlier S2 schistosity increases exponentially towards the contact, in parallel with the decrease in the size of the shear domains. Systematic variations in angles and shape ratios are also reported. Deformation and age patterns, however, suggest that this spectacular trend at least partly results from a progressive localization of the deformation through time. This fossilized shear strain gradient was thus produced somewhat diachronously. Such shear strain patterns nevertheless provide a mean to constrain the rheological properties for such weak lithologies and a mean to better understand crustal deformation.
S E Johnson - One of the best experts on this subject based on the ideXlab platform.
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the influence of Crenulation Cleavage development on the bulk elastic and seismic properties of phyllosilicate rich rocks
Earth and Planetary Science Letters, 2011Co-Authors: Felice M J Nausthijssen, S E Johnson, Andrew J Goupee, Senthil S Vel, Christopher GerbiAbstract:Abstract The anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks. Phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust. Crenulation Cleavage is one of the most common microstructural fabrics found in multiply-deformed, phyllosilicate-rich, crustal rocks. We calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of Crenulation Cleavage development: an initial planar foliation, a moderately-developed Crenulation Cleavage, and a well-developed Crenulation Cleavage. Mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method. The difficulties involved with sample preparation and data acquisition of phyllosilicate-rich rock samples are also discussed. We compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map, namely Voigt and Reuss averages. These averages do not account for grain-scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization. Our results show that the rocks characterized by a planar foliation and a moderately developed Crenulation Cleavage are highly anisotropic, with P-wave anisotropies up to 30.9% and S-wave anisotropies up to 34.2%, whereas the rock characterized by a well developed Crenulation Cleavage is only mildly anisotropic, with a P-wave anisotropy of 15.5% and S-wave anisotropy of 10.7%. Progressive development of the fabric also causes the orientations of P- and S-wave velocity maxima and S-wave polarization directions to change markedly. Despite the high anisotropy imparted by a planar schistosity, the variety of folds and fabrics typically found in phyllosilicate-rich rocks within larger-scale crustal volumes will tend to mute the anisotropy, possibly to the point of appearing nearly isotropic.
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numerical modeling of Crenulation Cleavage development a polymineralic approach
Journal of Structural Geology, 2010Co-Authors: Felice M J Nausthijssen, S E Johnson, P O KoonsAbstract:Abstract The finite element method was used to investigate how the elastic interactions of quartz and muscovite minerals affect grain-scale stress and strain distributions at different stages of Crenulation Cleavage development. The polymineralic structure comprises individual grains that were each assigned their own 3D stiffness tensor and orientation. Gradients in mean stress and volumetric strain within quartz grains develop between the limbs and hinges of microfolds at the earliest stages of Crenulation development, with higher values in the microfold limbs. These gradients decrease with development of the Crenulation Cleavage, as the microfold limbs become phyllosilicate-rich (P) domains and the hinges become quartz- and feldspar-rich (QF) domains. Crystallographic orientations of the quartz grains have a relatively minor effect on the mean stress and volumetric strain distributions. Our findings are broadly consistent with both pressure solution and strain-driven dissolution models for Crenulation Cleavage development. However, because Crenulation Cleavage development typically involves metamorphic reactions, we favor a model in which dissolution is driven by those reactions, and mass transfer leading to development of the mineralogically segregated fabric is driven by pore fluid pressure gradients that follow gradients in volumetric strain. Local concentrations of stress and strain across mineral interfaces may identify sites of enhanced reaction.
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porphyroblast rotation during Crenulation Cleavage development an example from the aureole of the mooselookmeguntic pluton maine usa
Journal of Metamorphic Geology, 2006Co-Authors: S E Johnson, M E Dupee, Charles V GuidottiAbstract:In the low-pressure, high-temperature metamorphic rocks of western Maine, USA, staurolite porphyroblasts grew at c. 400 Ma, very late during the regional orogenesis. These porphyroblasts, which preserve straight inclusion trails with small thin-section-scale variation in pitch, were subsequently involved in the strain and metamorphic aureole of the c. 370 Ma Mooselookmeguntic pluton. The aureole shows a progressive fabric intensity gradient from effectively zero emplacement-related deformation at the outer edge of the aureole ! 2900 m (map distance) from the pluton margin to the development of a pervasive emplacement-related foliation adjacent to the pluton. The development of this pervasive foliation spanned all stages of Crenulation Cleavage development, which are preserved at different distances from the pluton. The spread of inclusion-trail pitches in the staurolite porphyroblasts, as measured in two-dimensional (2-D) thin sections, increases nonlinearly from ! 16! to 75! with increasing strain in the aureole. These data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing Crenulation Cleavage. The data spread is qualitatively modelled for both pure and simple shear, and both solutions match the data reasonably well. The spread of inclusion-trail orientations (40-75! ) in the moderately to highly strained rocks is similar to the spread reported in several previous studies. We consider it likely that the sample-scale spread in these previous studies is also the result of porphyroblast rotation relative to one another. However, the average inclusion-trail orientation for a single sample may, in at least some instances, reflect the original orientation of the overgrown foliation.
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back rotation during Crenulation Cleavage development implications for structural facing and Cleavage forming processes
Journal of Structural Geology, 1999Co-Authors: S E JohnsonAbstract:Abstract Structural facing can be a useful tool for understanding macroscale structural geometries, particularly where poor outcrop inhibits the mapping of fold closures. However, in some situations facing must be determined with considerable care. In graded metaturbidites, where bedding and a near-parallel foliation have been overprinted by a Crenulation Cleavage, the earlier foliation in the metapelitic layers can be substantially `back-rotated' in the hinges of the overprinting Crenulation Cleavage. Thus, the rotation in the Crenulation hinges is opposite to the rotation in the Crenulation limbs. When viewed in the metapelitic layers, relative to a bedding surface, back-rotation can cause an apparent reversal in the structural facing (and vergence) on the rotated foliation. To avoid such misinterpretation, structural facing on the earlier foliation should be determined in the metapsammitic layers, where the effects of the overprinting Crenulation Cleavage are minimal. Because foliations that intersect bedding at a low angle are commonly hard to identify in metapsammitic outcrops, microstructural analysis may be required. The back-rotation process provides important constraints on mechanisms and kinematics of Crenulation Cleavage development, and may also have important implications for porphyroblast rotation, folding mechanisms and issues of strain compatibility in compositionally interlayered rocks.
Chris J. Beaumont-smith - One of the best experts on this subject based on the ideXlab platform.
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The role of conjugate Crenulation Cleavage in the development of ‘millipede’ microstructures
Journal of Structural Geology, 2001Co-Authors: Chris J. Beaumont-smithAbstract:Abstract Millipede microstructures are described from Archean metaturbidites surrounding the Back River volcanic complex, eastern Slave Structural Province. They are locally developed within andalusite porphyroblasts that grew in response to the intrusion of late syn-kinematic granitoids. The porphyroblasts acted as a mechanical heterogeneity and resulted in the nucleation of conjugate Crenulations, which were then helicitically overgrown by the porphyroblasts. This process produced the millipede morphology through a simple, consistent mechanism. The model presented here for the development of millipede microstructures represents an alternative to other models requiring complex deformation partitioning and restricted porphyroblast growth.
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Abstract: The role of conjugate Crenulation Cleavage and stepwise porphyroblast growth in the development of “Millipede” microstructures
1998Co-Authors: Chris J. Beaumont-smith, Paul F. WilliamsAbstract:This paper describes the results of a microstructural study into the development of 'Millipede' microstructures found in Archean metatwbiditic rocks sunoundingthe Back Rivervolcanic complex, eastern Slave structural province. Millipede microstructures are locally developed within andalusite porphyroblasts which developed late in the deformation history of the study area in response to the intrusion oflate kinematic granitoids. The mechanism for the development of the millipedes involves the stepwise porphyroblast growth with andalusite growth occurring after initial D 3 buckle folding. These initial andalusite porphyroblasts represent a rheological heterogeneity resulting in the development of conjugate Crenulation Cleavage along the porphyroblast margin. Subsequent porphyroblast growth included the Crenulation Cleavage with the inclusion trail within _the porphyroblasts defined by the trace of the quartz-rich Crenulation Cleavage microlithon. The classical reversal in inclusion trail vergence is the result of the porphyroblasts overgrowing conjugate pairs formed along the porphyroblast margin.
L K Stewart - One of the best experts on this subject based on the ideXlab platform.
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Crenulation Cleavage development by partitioning of deformation into zones of progressive shearing combined shearing shortening and volume loss and progressive shortening no volume loss quantification of solution shortening and intermicrolithon movement
Tectonophysics, 1997Co-Authors: L K StewartAbstract:Abstract An analytical method for determining amounts of Cleavage-normal dissolution and Cleavage-parallel shear movement that occurred between adjacent microlithons during Crenulation Cleavage seam formation within a deformed slate is developed for the progressive bulk inhomogeneous shortening (PBIS) mechanism of Crenulation Cleavage formation. The method utilises structural information obtained from samples where a diverging bed and vein are offset by a Crenulation Cleavage seam. Several samples analysed using this method produced ratios of relative, Cleavage-parallel movement of microlithons to the material thickness removed by dissolution typically in the range of 1.1–3.4:1. The mean amount of solution shortening attributed to the formation of the Cleavage seams examined is 24%. The results indicate that a relationship may exist between the width of microlithons and the amount of Cleavage-parallel intermicrolithon-movement. The method presented here has the potential to help determine whether Crenulation Cleavage seams formed by the progressive bulk inhomogeneous shortening mechanism or by that involving Cleavage-normal pressure solution alone.
Philippe Agard - One of the best experts on this subject based on the ideXlab platform.
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shear band formation and strain localization on a regional scale evidence from anisotropic rocks below a major detachment betic cordilleras spain
Journal of Structural Geology, 2011Co-Authors: Philippe Agard, Romain Augier, Patrick MonieAbstract:Regional-scale deformation taking place in a strongly anisotropic, yet homogeneous metapelitic protolith during an apparently single tectonic event was systematically investigated as a function of the distance to the main tectonic contact (i.e., the Filabres shear zone, a major detachment in the Betic Cordilleras, Spain). The density of C3' shear bands (or extensional Crenulation Cleavage) reworking the earlier S2 schistosity increases exponentially towards the contact, in parallel with the decrease in the size of the shear domains. Systematic variations in angles and shape ratios are also reported. Deformation and age patterns, however, suggest that this spectacular trend at least partly results from a progressive localization of the deformation through time. This fossilized shear strain gradient was thus produced somewhat diachronously. Such shear strain patterns nevertheless provide a mean to constrain the rheological properties for such weak lithologies and a mean to better understand crustal deformation.