STRAIN, VORTICITY, AND DEFORMATION PATH MODELLING OF MYLONITIC ROCKS IN THE HYLAS ZONE, EASTERN PIEDMONT, VIRGINIA

The Hylas zone forms a major tectonic boundary in the eastern Piedmont of Virginia and separates high-grade Proterozoic to mid-Paleozoic rocks of the Goochland terrane from the ~300 Ma Petersburg batholith and the Richmond/Taylorsville Mesozoic rift basins. Previous workers have mapped a 1-3 km wide NE-striking zone of heterogeneous mylonitic and metamorphic rocks from the James River north to the Coastal Plain onlap near the North Anna River. The Hylas zone has been interpreted as a late Paleozoic dextral transpressional high-strain zone, but the observed foliation and lineation pattern differs markedly from the classic model for a transpressional high-strain zone. In this study we report on strain and vorticity estimates obtained from mylonitic rocks in the Hylas zone between the Newfound and North Anna rivers in Hanover and Caroline counties. In addition, we’ve developed a dynamic three-dimensional model to better visualize the complex strain and deformation path followed by these rocks.

Mylonites, derived primarily from intermediate to mafic gneisses, are L-S tectonites with bulk rock strain ratios of >10:1 in XZ sections. Back-rotated porphyroclasts are common and consistent with general shear (W_m <0.7). Ultramylonites are typically S-tectonites and may be the result of a non-steady state deformation path during strain softening. The three-dimensional deformation model is developed using a 4x4 matrix with a medley of JavaScript web technologies. The result is a real-time, interactive visualization tool that can be sheared or stretched in any direction. The diagonal elements of the matrix represent the volumetric extensions and the off-diagonal elements represent the shear strains.