Constraints on kinematics and strain from feldspar porphyroclast populations
Published:January 01, 2004
Scott Giorgis, Basil Tikoff, 2004. "Constraints on kinematics and strain from feldspar porphyroclast populations", Flow Processes in Faults and Shear Zones, G. I. Alsop, R. E. Holdsworth, K. J. W. McCaffrey, M. Hand
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We develop a method for constraining the kinematics and finite strain of deformation in shear zones based on a three-dimensional numerical model of the rotation populations of rigid clasts. The results of the model are characterized in terms of a fabric ellipsoid, which is directly measurable from field data. Fabric ellipsoids measured from populations of prolate clasts have anisotropies that increase steadily and plateau; the shape of the fabric ellipsoid becomes increasingly more prolate with progressive deformation. The behaviour of populations of oblate clasts is much more complex because the stability of individual oblate clasts depends on their aspect ratio and the vorticity of deformation. Populations of oblate clasts may produce fabric ellipsoids with oscillating anisotropies and shapes if their aspect ratio is low enough for a continuous rotation. For either prolate or oblate clasts, the maximum anisotropy that a fabric ellipsoid will reach is governed by the aspect ratio of the individual clasts of that population. The theoretical maximum anisotropy is achieved when all of the clasts are perfectly aligned. The shape of the fabric ellipsoid, in conjuncture with the anisotropy, can be used to constrain the vorticity and finite strain of deformation.
The numerical model suggests that there is no consistent relationship between the asymmetrical orientation of a population of rigid markers and the simple shear component of deformation. Therefore, the asymmetrical alignment of a population of porphyroclasts is not a reliable shear sense indicator. Additionally, there is no direct correlation between the fabric ellipsoid and the strain ellipsoid.
Model results are applied to shape preferred orientation data collected from a feldspar megacrystic granite in the western Idaho shear zone (USA). Three-dimensional fabric ellipsoids are calculated from two-dimensional sectional measurements of oblate-shaped, unmantled, potassium feldspar porphyroclasts. Comparison of these data with the results of the numerical model suggests that transpressional deformation had an intermediate angle of oblique convergence (30°–60°). This implies that deformation in the western Idaho shear zone was characterized by a large component of convergent motion.
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Flow Processes in Faults and Shear Zones
Faults and their deeper level equivalents, shear zones, are localized regions of intense deformation within the Earth. They are recognized at all scales from micro to plate boundary, and are important examples of the nature of heterogeneous deformation in natural rocks. Faults and shear zones are significant as they profoundly influence the location, architecture and evolution of a broad range of geological phenomenao The topography and bathymetry of the Earth’s surface is marked by mountain belts and sedimentary basins that are controlled by faults and shear zoneso In addition, faults and shear zones control fluid migration and transport including hydrothermal and hydrocarbon systems. Once faults and shear zones are established, they are often long-lived features prone to multiple reactivation over very large time-scales. This collection of papers addresses lithospheric deformation and the rheology of shear zones, together with processes of partitioning and the unravelling of fault and shear zone histories.