KINEMATIC SIGNIFICANCE OF MONOCLINIC AND TRICLINIC HIGH-STRAIN ZONES IN THE VIRGINIA BLUE RIDGE PROVINCE

A family of kinematically related high-strain zones cut granitoid rocks of Blue Ridge basement complex in central and northern Virginia. Individual high-strain zones form anastomosing, northeast-southwest striking belts of mylonitic rock, 0.5 to 3 km thick, that dip moderately to the southeast. From north to south these zones include the Sperryville, Champlain Valley, Quaker Run, White Hall, and Rockfish Valley zones. Collectively, these zones display an en-echelon map pattern with individual zones extending 30-100 km and tipping out along strike. Blue Ridge high-strain zones are characterized by both monoclinic and triclinic deformation symmetries. Triclinic symmetries are revealed by the geometry of fabric elements with respect to high-strain zone boundaries and fabric asymmetries on planes both parallel and normal to elongation lineations. Elongation lineations plunge to the southeast and kinematic indicators on XZ sections record a northwest-directed (reverse) sense of shear. Mylonitic rocks with a triclinic symmetry also record a component of strike-parallel sinistral shear. Strain ratios, measured with quartz grain shapes and boudinaged feldspars, range from 4-20 in XZ sections. Three-dimensional strains are moderately to strongly oblate (K=0.8-0.0). Vorticity analysis indicates that these high-strain zones experienced bulk general shear deformation (W_m=0.6-0.9). The total displacement across individual high-strain zones, estimated by integrating shear strains over a zone’s thickness, range from 1-4 km. Total displacement estimates are in accord with field relations demonstrating that at many locations the same basement unit occurs in both the footwall and hanging wall of the high-strain zone. These modest offsets are incompatible with tectonic models that suggest Blue Ridge high-strain zones separate distinctly different Grenvillian massifs. The kinematic significance of high-strain zones in the Virginia Blue Ridge basement complex is threefold: 1) these zones record northwest-directed reverse movement that accommodated crustal shortening during the Paleozoic, 2) displacement on these zones is on the order of a few kilometers, and 3) widespread flattening strains require significant strike-parallel (orogen-parallel) material movement.