Abstract The Blue Ridge and Piedmont provinces in the central Virginia Appalachians are underlain by Proterozoic and Paleozoic rocks that record multiple episodes of continental collision and rifting. This trip focuses on rocks and structures formed at the southeastern margin of Laurentia during: (1) the Mesoproterozoic assembly of Rodinia, (2) the Cryogenian to Ediacaran rifting that ultimately created the Iapetus Ocean, and (3) the Paleozoic deformation and metamorphism associated with the closure of the Iapetus Ocean and Appalachian orogenesis. A Neoproterozoic to Early Cambrian cover sequence records the transition from continental rifting to a passive margin, but the character of this sequence is vastly different on the eastern and western limbs of the Blue Ridge anticlinorium, reflecting spatial differences in both the timing and tectonics of the Iapetan rift. Blue Ridge rocks experienced NW-directed contractional deformation during the Neo-Acadian (355-330 Ma), whereas low-grade metasedimentary rocks in the western Piedmont were deformed and cooled prior to ca. 400 Ma. In central Virginia, the boundary between the eastern Blue Ridge and western Piedmont is a 3- to 5-km-wide zone of distributed dextral transpression.

Abstract This field guide covers a two-day east-to-west transect of the Blue Ridge and Valley and Ridge provinces of northwestern Virginia and eastern West Virginia, in the context of an integrated approach to teaching stratigraphy, structural analysis, and regional tectonics. Holistic, systems-based approaches to these topics incorporate both deductive (stratigraphic, structural, and tectonic theoretical models) and inductive (field observations and data collection) perspectives. Discussions of these pedagogic approaches are integral to this field trip. Day 1 of the field trip focuses on Mesoproterozoic granitoid basement (associated with the Grenville orogeny) and overlying Neoproterozoic to Early Cambrian cover rocks (Iapetan rifting) of the greater Blue Ridge province. These units collectively form a basement-cored anticlinorium that was thrust over Paleozoic strata of the Valley and Ridge province during Alleghanian contractional tectonics. Day 2 traverses a foreland thrust belt that consists of Cambrian to Ordovician carbonates (Iapetan divergent continental margin), Middle to Upper Ordovician immature clastics (associated with the Taconic orogeny), Silurian to Lower Devonian quartz arenites and carbonates (inter-orogenic tectonic calm), and Upper Devonian to Lower Mississippian clastic rocks (associated with the Acadian orogeny). Alleghanian structural features include the Little North Mountain thrust, Cacapon Mountain anticlinorium, Broad Top synclinorium, and Wills Mountain anticlinorium. Within the road log of this field guide we include both planned and optional stops, so that readers can explore the pedagogic concepts discussed herein in more detail, if desired.

High-strain zones are common in basement terranes, and understanding their tectonic significance requires quantitative knowledge of deformation kinematics. We report on strained rocks from different tectonic settings that record pure shear dominated (W m =≤0.4) deformations. Mylonitic rocks derived from Mesoproterozoic basement granitoids are exposed in the Lawhorne Mill high-strain zone in the Virginia Blue Ridge. Chemical and mineralogical differences between the leucogranitoid protolith and mylonite are consistent with ∼50% volume loss during deformation. Minimum finite strains in XZ sections range from 4:1 to 7:1, and three-dimensional strains plot in the field of apparent flattening; however, with volume loss these rocks likely experienced bulk plane strain. The R s /Θ and quartz c -axis vorticity gauges yield W m values of 0.0–0.6. Fabric asymmetries normal to both foliation and lineation are consistent with modest triclinic deformation symmetry. Mylonitic rocks from the Lawhorne Mill high-strain zone record a pure shear dominated deformation that produced ∼70% contraction across the zone with only minimal displacement parallel to the zone (<0.5 km). Pure shear dominated high-strain zones occur in a variety of mid-crustal settings. Ultramylonites from metamorphic core complexes in Arizona record very low vorticity values (W m < 0.4). Well-foliated, steeply dipping, upper amphibolite facies rocks from the Coast shear zone in British Columbia are characterized by orthorhombic fabrics formed during pure shear dominated deformation that accommodated crustal contraction. These zones differ from simple and general shear zones because displacement across these zones is minimal relative to the overall finite strain. However, zone-normal shortening and zone-parallel stretching are significant in pure shear dominated zones. Steeply dipping zones formed in contractional settings serve to effectively shorten and thicken the crust across basement massifs, whereas gently dipping zones formed in extensional settings thin the crust.