The structural geometry and evolution of foreland thrust systems, northern Virginia

Abstract

Seismic reflection data reveal that the structural geometry of the central Appalachians of northern Virginia consists of three distinct thrust systems. Each thrust system is characterized by a unique internal geometry.The Blue Ridge thrust sheet is a composite thrust sheet composed primarily of imbricated Precambrian crystalline rocks. It over-rode Cambrian-Ordovician carbonates and formed a sheared, basement-cored fault-bend fold. Thrusts within the sheet may be Taconic and earliest Alleghanian, whereas final thrusting and emplacement of the sheet were probably slightly younger but still early Alleghanian. The North Mountain thrust sheet is characterized by imbricated Cambrian-Ordovician carbonates that are deformed into large-amplitude mode II fault-bend folds and fault-propagation folds. Rocks of this sheet were transported more than 60 km across a similar section of carbonates. The leading edge of the North Mountain thrust sheet was deformed into a fold with a mode II fault-bend fold geometry and was juxtaposed against middle Paleozoic rocks. The middle Paleozoic rocks occur in a ramp across which displacement along the North Mountain thrust was transferred to a higher detachment. More than 60 km of cover rocks displaced during the emplacement of the North Mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western Valley and Ridge province or were backthrust above the sheet. The timing for imbrication and emplacement of the North Mountain thrust sheet is probably Main Phase Alleghanian. The Lower Carbonate duplex extends from beneath the Blue Ridge and North Mountain thrust sheets, westward across the western Valley and Ridge province. The Lower Carbonate duplex is characterized by imbricated Cambrian-Ordovician carbonates that form low-amplitude fault-bend folds. This thrust system also probably formed during Main Phase Alleghanian deformation.