TRACKING TERRANES IN VIRGINIA: SOUTHWESTWARD TRANSPORT ACROSS A TRANSPRESSIONAL OROGEN

The Virginia Blue Ridge and Piedmont are an amalgam of terranes with both Laurentian and peri-Gondwanan affinities. Marginal Laurentian terranes occur to the SE of the Blue Ridge and include the Potomac/Hardware terranes, the Smith River allochthon, and the Tugaloo terrane. The Chopawamsic terrane forms an exotic mid-Paleozoic arc terrane overlain by Siluro-Devonian successor basins. Peri-Gondwanan terranes include the Neoproterozoic Carolina, Raleigh, and Roanoke Rapids terranes and likely the enigmatic Goochland terrane.

Piedmont terranes are separated from one another by a series of anastomosing high-strain zones and faults, these include (from west to east) the Bowens Creek/Buckmarlson/Mountain Run, Brookneal/Shores, Spotsylvania, and Hylas zones. Individual high-strain zones range from 1-2 km in width up to zones 10-15 km wide with diffuse boundaries. Although some early faults are likely thrusts, the dominant penetrative fabrics in these terrane-bounding fault zones record general shear that occurred during dextral transpression, producing both orogen-parallel stretching and orogen-normal thinning. Terranes in the eastern Piedmont were translated to the southwest relative to more western terranes. Integrating ductile strain across individual fault zones yield minimum lateral displacements of 10s to 100s of km. Alleghanian deformation translated more northerly terranes to the southwest, approximately parallel to the old Laurentian margin. Although transpressional tectonics was first recognized in the Appalachians over three decades ago, the significant of transpressional deformation on both the ancient and modern architecture of the orogen is not fully appreciated.