2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 5
Presentation Time: 9:15 AM


BAILEY, Christopher M., FRANCIS, Barbara and FAHRNEY, Eleanor E., Dept. of Geology, College of William & Mary, Box 8795, Williamsburg, VA 23187, cmbail@wm.edu

High-strain zones are common in a variety of tectonic settings and structural geologists have long sought to understand the their kinematic significance. Understanding the kinematic significance of high-strain zones requires knowledge of their finite deformation symmetry, rotational strain (vorticity), three-dimensional strain, and volume change. Kinematic information must be integrated with geographic coordinates such as strike-parallel, oblique, or dip-parallel transport to understand the overall tectonic significance. We have quantified the finite deformation components in several high-strain zones from contractional, extensional, and transpressional settings. Paleozoic contractional high-strain zones in the Virginia Blue Ridge province record monoclinic to triclinic deformation symmetries, general shear (Wm=0.4 - 0.8), moderate to strong flattening strains, and modest volume changes (<10%). Mylonitic rocks from Cenozoic extensional metamorphic core complexes in Arizona record monoclinic symmetries, general shear (Wm=0.4 - 0.9), bulk plane strain, and no significant volume changes. High-strain zones in the central Appalachian Piedmont experienced dextral transpressive movement in the late Paleozoic. Vorticity and strain analysis of mylonitic rocks from these zones indicates general shear (Wm=0.2 - 0.95), and three-dimensional strains that range from flattening to constrictional. All of these high-strain zones experienced general shear deformation with a component of shortening normal to the high-strain zone. Although general shear was originally used to describe a two-dimensional deformation that combined pure and simple shear, we feel general shear is a useful term for describing three-dimensional strains with both a rotational and shortening component. Quantifying the finite history of mylonitic rock is valuable, but the next steps require refining the tools and techniques to understand the progressive deformation path experienced by high-strain zones from different tectonic environments.