Strain and vorticity analysis of transpressional high-strain zones from the Virginia Piedmont, USA

Abstract

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 highstrain 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/O 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.

Vorticity of flow in ductile thrust zones: examples from the Attico-Cycladic Massif (Internal Hellenides, Greece)

“…Klepeis et al 1999;Xypolias & Koukouvelas 2001;Law et al 2004;Bailey et al 2004;Jessup et al 2006;Iacopini et al 2008) to estimate W m in natural high strain zones. All methods rely on Jeffery's (1922) (Passchier 1987), where M x is the long axis of the clast and M n is the short axis.…”

“…This necessity is also expressly recognized by quantitative studies in shear zones from different structural settings (e.g. Klepeis et al 1999;Holcombe & Little 2001;Bailey & Eyster 2003;Bailey et al 2004;Iacopini et al 2008).…”

The Grenville orogenic cycle of southern Laurentia: Unraveling sutures, rifts, and shear zones as potential piercing points for Amazonia

“…2) are primarily associated with accreted Appalachian terranes and were also thrust westward. Many of these internal massifs, however, originated as fragments of the Grenville orogen that were rifted and translated laterally from Laurentia during or after the opening of the Iapetus ocean at 565 Ma (e.g., Bartholomew and Tollo, 2004;Bailey et al, 2004) during the breakup of Rodinia (e.g., Bond et al, 1984;Cawood et al, 2001). Many also experienced lateral translation during subsequent amalgamation of accreted terranes to (1980)).…”

Implications of strain and vorticity of flow analyses to interpret the kinematics of an oblique convergence event (Zagros Mountains, Iran)

“…The relation between these parameters is a function of the mean kinematic vorticity of deformation (Fossen and Tikoff, 1993;Bailey et al, 1999). Bailey et al (2004) argued that this method does not require the assumption of steady-state deformation. However, vorticity number estimates using strain-ratio/quartz c-axis-fabric (R xz /b) method are very sensitive to small changes in the evaluated angle b (Grasemann et al, 1999).…”