The Rigid Grain Net (RGN): An alternative method for estimating mean kinematic vorticity number (Wm) (original) (raw)
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Empirical paths of poles to planes (eppps) constrain the kinematics of geological shear zones
Ductile shear zones are tabular bodies of deformed rocks bound by less deformed wall rocks. This work introduces a simple empirical approach to analysing the 3D kinematics of shear zones. The orientations of pre-shear planar markers distorted across natural shear zones by local strains are systematically measured and plotted as poles on lower hemisphere equal area projections that constrain smooth empirical paths of poles to planes (eppps). Such eppps recording local strain gradients are used to fix a reference frame to the plane of greatest shear in any homogeneous bulk strain. Assuming that space can be taken as a proxy for time, the curvatures of pre-shear planar markers across shear zones are interpreted as the records of the 3D bulk strain histories of shear zones. The sig- or zig-moidal symmetries of sheared markers record different amounts of the same general strain within the same overall movement pattern (i.e. in a constant flow field) whatever its geometry or history. In effect eppps represent the strain memories of shear zones with successively inward readings recording successively younger shearing. In planes other than the bulk XY, great circle eppps indicate simple shear while hyperbolic eppps indicate pure shear. Eppps for suites of shear zones in Proterozoic gneisses in Sweden exhibit the parabolic shapes indicative of pure rather than simple shear.
Arabian Journal of Geosciences, Vol. 5, No. 1, 159–167, 2012
New structural, metamorphic, finite strain, and kinematic vorticity data for mylonitic granitic rocks from northern thrust in Wadi Mubarak reveal a history of deformation reflecting different tectonic regimes. The vorticity analysis of porphyroclasts was determined in high temperature mylonites. The kinematic vorticity number for the mylonitic granitic samples in the northern thrust in Wadi Mubarak range from 0.66 to 0.90, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. The accumulation of ductile strain during thrusting was not by simple shear and involved a component of vertical shortening, which caused the subhorizontal foliation in the northern thrust in Wadi Mubarak and adjacent units.
Determining vorticity axes from grain-scale dispersion of crystallographic
2016
Kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones. The appropriate plane in which to interpret shear sense indicators is the vorticity normal surface, which is often inferred using fabric (foliation, lineation) orientation. Strain modeling, however, suggests that such fabrics can be unreliable for determining a kinematic framework. We demonstrate the application of a new quantitative method, crystallographic vorticity axis (CVA) analysis, that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale. We apply CVA analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries. In all cases, the aggregate of calculated grain-scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis. This new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information, and without assumptions about the kinematics of deformation.
Determining vorticity axes from grain-scale dispersion of crystallographic orientations
Geology, 2015
Kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones. The appropriate plane in which to interpret shear sense indicators is the vorticity normal surface, which is often inferred using fabric (foliation, lineation) orientation. Strain modeling, however, suggests that such fabrics can be unreliable for determining a kinematic framework. We demonstrate the application of a new quantitative method, crystallographic vorticity axis (CVA) analysis, that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale. We apply CVA analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries. In all cases, the aggregate of calculated grain-scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis. This new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information, and without assumptions about the kinematics of deformation.
Bulk kinematics from shear zone patterns: some field examples
Journal of Structural Geology, 1987
Geological deformations which are statistically homogeneous at bulk scale (e.g. the macroscale) are often.localized into arrays of narrow shear zones at a smaller scale (e.g. the mesoscale). This paper shows that shear zone patterns can be used to estimate both a bulk finite strain ellipsoid and aspects of the bulk deformation history. We describe examples of heterogeneously deformed granitic rocks which reveal the following features.
Geological Society, London, Special Publications, 2006
Recent fieldwork in western Bhutan, dedicated to unravelling the tectonic structure of the mid-crustal rocks, indicates a complex deformation pattern in the Greater Himalayan Slab (GHS). A system of normal shear zones, striking NE-SW and steeply to moderately dipping to the SE, has been recognized within this extruding slab or wedge of crystalline rocks. The zones are characterized by well developed shear-sense indicators pointing to a top-down-to-SE sense of shear. The main Barrovian metamorphic minerals are bent and stretched by extensional shear bands and associated deformation mechanisms indicate a range of brittle-ductile deformation conditions. Normal shear zones are concentrated in the middle-upper part of the GHS and indicate a thrust-transport-parallel lengthening of the core itself. Vorticity analysis highlights a non-coaxial flow with pure and simple shear acting together during deformation (mean vorticity number bracketed between 0.63 and 0.76). These data, when compared...
Geophysical Journal International, 2005
Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses depend on the mean velocity (Ū ), viscosity (µ) and channel thickness (h), uniquely through the term µŪ /h 2 . These stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 10 18 Pa s flowing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr −1 .
Kinematic and vorticity analyses of the western Idaho shear zone, USA
Lithosphere, 2016
The western Idaho shear zone (WISZ) is a Late Cretaceous, mid-crustal exposure of intense shear localized in the Cordillera of western North America. This shear zone is characterized by transpressional fabrics, i.e., downdip stretching lineations and vertical foliations. Folded and boudinaged late-stage dikes indicate a dextral sense of shear. The vorticity-normal section is identified by examining the three-dimensional shape preferred orientation of feldspar populations and the intragranular lattice rotation in quartz grains in deformed quartzites. The short axes of the shape preferred orientation ellipsoid gather on a plane perpendicular to the vorticity vector. In western Idaho this plane dips gently to the west, suggesting a vertical vorticity vector. Similarly, sample-scale crystallographic vorticity axis analysis of quartzite tectonites provides an independent assessment of vorticity and also indicates a subvertical vorticity vector. Constraints on the magnitude of vorticity are provided by field fabrics and porphyroclasts with strain shadows. Together these data indicate that the McCall segment of the WISZ displays dextral transpression with a vertical vorticity vector and an angle of oblique convergence ≥60°. North and south of McCall, movement is coeval on the Owyhee segment of the WISZ and the Ahsahka shear zone. Together, the kinematics of these shear zones are consistent with northeast-southwest-directed convergence. Plate motion in this orientation acting on a curved plate boundary could have produced pure shear-dominated transpression in the Owyhee (a = 40°) and McCall (a = 60°) segments of the WISZ, while causing reverse-sense shearing (a = 90°) in the Ahsahka shear zone.
Flow in natural shear zones—the consequences of spinning flow regimes
1986
Abstract The ancient flow regime in natural shear zones is often considered to have followed a deformation path comparable to that in theoretical shear zones, ie progressive simple shear between rigid wall rocks with a persistent flow plane orientation parallel to the edges of the zone. This is often based on the presence of monoclinic fabric elements in the zones which indicate a dominantly non-coaxial flow regime, though not necessarily persistent simple shear.
International Journal of Earth Sciences, 2012
Constraining magnitudes of mechanical and thermo-mechanical parameters of rocks and shear zones are the important goals in structural geology and tectonics (Talbot in J Struct Geol 21:949-957, 1999). Such parameters aid dynamic scaling of analogue tectonic models (Ramberg in Gravity, deformation and the Earth's crust in theory, experiments and geological applications, 2nd edn. Academic Press, London, 1981), which are useful to unravel tectonics in further details (Schultz-Ela and Walsh in J Struct Geol 24:247-275, 2002). The channel flow extrusion of the Higher Himalayan Shear Zone (HHSZ, = Higher Himalaya) can be explained by a top-to-S/SW simple shear (i.e. the D 2 deformation) in combination with a pressure gradient induced flow against gravity. Presuming its Newtonian incompressible rheology with parallel inclined boundaries, the viscosity (l) of this shear zone along a part of the Himalayan chain through India, Nepal and Bhutan is estimated to vary widely between *10 16 and 10 23 Pa s, and its Prandtl number (P r ) within *10 21 -10 28 . The estimates utilized ranges of known thickness (6-58 km) of the HHSZ, that of its top subzone of ductile shear of normal shear sense (STDS U : 0.35-9.4 km), total rate of slip of its two boundaries (0.7-131 mm year -1 ), pressure gradient (0.02-6 kb km -1 ), density (2.2-3.1 g cm -3 ) and thermal diffusivity (0.5 9 10 -6 -2.1 9 10 -6 m s -2 ) along the orogenic trend. Considering most of the parameters specifically for the Sutlej section (India), the calculated viscosity (l) and the Prandtl number (P r ) of the HHSZ are deduced to be l: *10 17 -10 23 Pa s and P r * 10 22 -10 28 . The upper limits of the estimated viscosity ranges are broadly in conformity with a strong Tibetan mid-crust from where a part of the HHSZ rocks extruded. On the other hand, their complete ranges match with those for its constituent main rock types and partly with those for the superstructure and the infrastructure. The estimated mechanical and thermo-mechanical parameters of the HHSZ will help to build dynamically scaled analogue models for the Himalayan deformation of the D 2 -phase.