Strain and vorticity analysis of transpressional high-strain zones from the Virginia Piedmont, USA
Published:January 01, 2004
Christopher M. Bailey, Barbara E. Francis, Eleanor E. Fahrney, 2004. "Strain and vorticity analysis of transpressional high-strain zones from the Virginia Piedmont, USA", Flow Processes in Faults and Shear Zones, G. I. Alsop, R. E. Holdsworth, K. J. W. McCaffrey, M. Hand
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Strain and vorticity analysis of two Late Palaeozoic high-strain zones from the southern Appalachian Piedmont indicates that these zones experienced general shear transpression with a monoclinic to triclinic symmetry. Granitic rocks in the Brookneal high-strain zone from the southwestern Virginia Piedmont were transformed into mylonites under greenschist facies conditions. Sectional strains, estimated from quartz grain shapes, in mylonites range from three to ten and three-dimensional fabrics record flattening strains. The mean vorticity number (Wm) estimated with the Rs/θ method ranges from 0.3 to 0.95. In the central Virginia Piedmont, lower amphibolite facies deformation in the Spotsylvania high-strain zone affected biotite gneisses, amphibolites, and granitic pegmatites. Minimum sectional strains, estimated from folded and boudinaged pegmatite dykes, of 8–20 are common and three-dimensional strains are dominantly constrictional. Porphyroclast hyperbolic distribution analysis of ultramylonites yields Wn values from 0.4 to 0.8. The kinematic significance of these transpressional high-strain zones is threefold: they record tens to hundreds of kilometres of strike-slip offset; 40 to 70% contraction normal to the zone; and significant orogen-parallel material elongation.
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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.