Shear criteria and structural symmetry (original) (raw)
Related papers
Strain can be retrieved from rocks through a range of different methods.
The development of flanking folds during simple shear and their use as kinematic indicators
Journal of Structural Geology, 2001
We used a numerical ®nite element simulation to model the formation of¯anking folds around rotating planar structures (e.g. veins, faults or dykes) in a non-linear viscous medium during ideal simple shear. If the planar structure is much more viscous than its host it does not deform and¯anking folds with no displacement along the structure develop. Their vergence is consistent with the overall sense of shear. However, if the planar structure is much less viscous than its host, strain is concentrated within the structure and a secondary shear zone is developed in which slip is opposite to the overall sense of shear. Then,¯anking folds develop that have a vergence, which is incompatible with the drag on structure. If the de¯ection of markers is not clearly preserved, then such¯anking folds can be easily misinterpreted as shear bands indicating a wrong shear sense. The de¯ection of the foliation in¯anking folds is very similar to deformed asymmetric pull-aparts and can therefore help to interpret these otherwise ambiguous shear sense criteria. Because our model (ideal simple shear boundary conditions) fails to describe back-rotation of planar structures in rocks we speculate that shear band geometries are indicative for general shear. q
Strain partitioning in banded and/or anisotropic rocks: Implications for inferring tectonic regimes
Journal of Structural Geology, 2013
Among the many concepts that have contributed to the development of structural geology in the last half-century, two are particularly important. These are: (i) the link between the symmetry of the structure, the vorticity of the associated deformation process and the tectonic regime, and (ii) the widespread partitioning of strain. However, a clear understanding of these concepts and the links between them are not always apparent in structural analyses carried out to determine the prevailing tectonic regime of an area through the use of structural symmetries and kinematic indicators. This contribution, based on field studies from Cap de Creus, experimental data and theoretical concepts, highlights some of the problems encountered when attempting to deduce the type of strain and associated regional tectonic regime from field structures. The relation between the symmetry of a structure and its associated kinematics is not unique. For example, it is demonstrated that in mechanically anisotropic materials, symmetry variations do not necessarily reflect marked vorticity variations, but rather the angular variations between the kinematic framework and the anisotropy planes. Because of widespread strain partitioning, local and general kinematics do not generally match. This mismatch is especially accentuated in rheologically heterogeneous rocks. Nevertheless, correct interpretations are possible by performing continuous multi-scale structural analyses in which the impact of strain partitioning and of the presence of material anisotropies and heterogeneities are considered.
An outline of shear-sense analysis in high-grade rocks
2006
Ductile shear zones are important in tectonic reconstructions as a source of information on the relative motion of large crustal blocks or plates in the geological past. Methods to interpret fabric in ductile shear zones were mostly developed for low grade rocks where overprinting relations are usually well preserved. However, high grade shear zones are common and dominate in many Precambrian terrains. High grade shear zones should be analysed in a different way from low grade zones.
Reversible deformation phenomena of a high stressed rock samples
The search for reliable methods of forecasting macrofailure is one of the basic problems of modern geodynamics. This problem depends to alarge extent on the possibility of determining reliably defined precursors of failure (Sagiya 2011). Till now, these have been considered to be a rise in the acoustical activity of rocks and mass and large anomalous deformations of source areas and connected with formation of the area site for. However, restrictingsearches for precursors to field sites only for an area sharply narrows their nomenclature, and so also the quantity of independent information sources. There is a need tosearch for additional precursors that are closely connected with the formation of the sourceof the macrofailure. Such precursors can be found in the area near the source which are directly bordering on the area site for. Attention to the reversible character of linear deformations of rock samples was presented, apparently, for the first time in . The authors did not research the mechanisms of the origination of the deformation anomalies, but already in subsequent works such attempts have begun to be undertaken. So the reversible character of the deformations of rocks was contacted with a barrel-shaped straining of samples at uniaxial compression . In the works of other researchers, residual stresses were proposed in the capacity of reasons for deformation anomalies of various types . However, these hypotheses are not supported by critics on closer examination . In this paper, based on specially developed complex research methods, including on deformation, acoustical and mathematical methods, the authors analyse deformation anomalies of reversible types in samples of rocks at uniaxial compression, define the mechanism of their origin, and develop a mathematical model of the phenomenon.
The analogue shear zone: From rheology to associated geometry
Journal of Structural Geology, 2008
The geometry of ductile strain localization phenomena is related to the rheology of the deformed rocks. Both qualitative and quantitative rheological properties of natural rocks have been estimated from finite field structures such as folds and shear zones. We apply physical modelling to investigate the relationship between rheology and the temporal evolution of the width and transversal strain distribution in shear zones, both of which have been used previously as rheological proxies. Geologically relevant materials with well-characterized rheological properties (Newtonian, strain hardening, strain softening, MohreCoulomb) are deformed in a shear box and observed with Particle Imaging Velocimetry (PIV). It is shown that the width and strain distribution histories in model shear zones display characteristic finite responses related to material properties as predicted by previous studies. Application of the results to natural shear zones in the field is discussed. An investigation of the impact of 3D boundary conditions in the experiments demonstrates that quantitative methods for estimating rheology from finite natural structures must take these into account carefully.
Tectonophysics, 2016
We study pinch-and-swell structures in order to uncover the onset of strain localization and the change of deformation mechanisms in layered ductile rocks. To this end, boudinaged monomineralic veins embedded in an ultramylonitic matrix are analyzed quantitatively. The swells are built up by relatively undeformed original calcite grains, showing twinning and minor subgrain rotation recrystallization (SGR). Combined with progressive formation of high-angle misorientations between grains, indicative of SGR, severe grain size reduction defines the transition to the pinches. Accordingly, dynamically recrystallized grains have a strong crystallographic preferred orientation (CPO). Towards the necks, further grain size reduction, increasingly random misorientations, nucleation of new grains and a loss of the CPO occur. We postulate that this microstructure marks the transition from dislocation to diffusion creep induced by strain localization. We confirm that the development of boudins is insensitive to original grain sizes and single-crystal orientations. In order to test these microstructural interpretations, a self-consistent numerical grain size evolution is implemented, based on thermo-mechanical principles, end-member flow laws and microphysical processes. Applying constant velocity and isothermal boundary conditions to a 3-layer finite element pure shear box, pinch-and-swell structures emerge out of the homogeneous layer through grain size softening at a critical state. Viscosity weakening due to elevated strain rates and dissipated heat from grain size reduction promotes strain rate weakening until a critical grain size is reached. At this point, a switch from dislocation to diffusion creep occurs. This state locks in at local steady states and is microstructurally expressed in pinches and swells, respectively. Thus, boudinage is
Journal of Petrology, 2002
Acknowledgements ix Symbols, Abbreviations and Units xi 6.5.2 Intracrystalline plasticity 62 6.5.3 Diffusive mass transfer 63 6.5.4 Cataclasis 63 6.6 Other microstructures 63 6.7 Non-magmatic deformation 63 7 Microstructural Shear Sense Criteria 65 7.1 Introduction 65 7.2 Curved foliation 66 7.3 Oblique foliations and shape preferred orientations 66 7.4 Porphyroclast systems 67 7.4.1 Characteristics and classification 67 7.4.2 Mechanisms of formation 68 7.4.3 Stair-step direction: o-and S-type tails 69 7.4.4 Faces of aa-type tail 69 7.4.5 Deflection and embayments of S-type tails 69 7.5 S-, C-and C'-fabrics 70 7.5.1 Characteristics and classification 70 7.5.2 Formation and evolution 72 7.5.3 Curvature of S-foliation 73 7.5.4 Shear on C-or C'-surfaces 73 7.6 Pressure shadows and fringes 73 7.6.1 Kinematics of pressure shadows and fringes in shear zones 73 7.6.2 Geometry of the last increment of growth. . .
Postseismic deformation and the strength of ductile shear zones
Earth, Planets and Space, 2004
A good understanding of the rheology and strength of the whole crust is needed to obtain a physics-based earthquake prediction models. However, geodynamics-based and laboratory-based strength estimates disagree. Geodynamics tend to indicate that the actual strength of the plastic crust is less than deduced from laboratory experiments. Here, I evaluate lower crust strength from observations of transient postseismic deformation. Fault motion during an earthquake produces only a small stress perturbation, but that perturbation is sufficient to significantly affect the deformation rate of the aseismic levels of the crust, as observed by space geodesy. Even considering the non-linearity of plastic flow in geological materials, one cannot escape the conclusion that the pre-earthquake stress on the region where transient postseismic deformation occurs is not more than an order of magnitude larger than earthquake-induced stress perturbations. Using a simple shear zone model and assuming wet quartzite rheology, I show that such stress levels are not compatible with a km-scale shear zone, in spite of the geological evidence for localized deformation in the plastic crust. This implies that in plastic shear zones, rock strength is reduced. Possible explanations for the strength reduction include structural effects such as reduced grain size and/or a localized thermal anomaly associated with the shear zone.
Journal of Structural Geology, 1991
The effect of varying strain ratio (Ra) on initially uniform and non-uniform orientation distributions of passive marker lines is calculated. The frequency graphs for uniform distributions have a unimodal form (Gaussian-like distribution),centered in the finite extension direction (X) of the strain ellipse. The maximum frequency is related to the strain. Using non-uniform distributions the frequency graph has a different unimodal pattern. The difference between the measured strain ratio (R) with the maximum frequency and the true strain ratio (Ra) to the initial distribution is quantified for different types of distribution. The method is applied to samples of gneiss from southern Spain, using the orientation of tourmaline and feldspar lying in the schistosity plane, with respect to the stretching lineation. The strain of the marker distribution has been quantified, and after distinguishing the initial type of distribution, the true strain ratio of the whole rock in the schistosity plane can be estimated.