SIGNIFICANT SOUTHWESTWARD TRANSPORT OF THE GOOCHLAND TERRANE ALONG THE SPOTSYLVANIA HIGH-STRAIN ZONE, VIRGINIA PIEDMONT

An unresolved controversy in southern Appalachian tectonics concerns the origin of the Goochland terrane in the Virginia Piedmont. Some models suggest the Goochland terrane is of Laurentian affinity, whereas others suggest it is an accreted peri-Gondwanan fragment. In central Virginia, the Goochland terrane is bounded to the west by the Spotsylvania Zone, a ~15 km wide zone of predominantly mylonitic rocks. The western boundary of the Spotsylvania Zone is marked by a prominent geophysical lineament, abundant fault breccia, and modern seismicity. Previous workers have interpreted this structure to be a significant Alleghanian thrust fault, a suture, or a minor zone of brittle en-echelon faulting. Upper greenschist to amphibolite facies mylonitic rocks from the Spotsylvania Zone strike northeast-southwest, dip moderately to gently southeast, and are characterized by gently plunging strike-parallel elongation lineations. Asymmetric structures consistently record dextral shear with a weakly triclinic geometry. Strain and vorticity analyses indicate general shear with predominantly constrictional strains and sectional strain ratios of ~20:1. Strain and vorticity data were used to retrodeform the Spotsylvania Zone and ‘restore’ the Goochland terrane to its paleogeographic position prior to Alleghanian dextral transpression. Minimum displacement estimates indicate that the Goochland terrane was transported 80 to 300 km southwestward relative to the Chopawamsic terrane. The Goochland terrane is currently separated from the Laurentian Virginia Blue Ridge by the dextrally transpressive Mountain Run, Brookneal/Shores, and Spotsylvania high-strain zones. Retrodeformation across all three zones places the Goochland terrane at least 500 km northeast of the Virginia Blue Ridge prior to the Alleghanian orogen. Tectonic models directly linking the Goochland terrane to the Virginia Blue Ridge are untenable because of significant dextral translation. If the Goochland terrane is Laurentian, it originated near or north of the New York promontory, not outboard of the Virginia Blue Ridge.