Document Type

Thesis

Abstract

New geologic mapping and structural analysis of the boundary between the Blue Ridge and Piedmont geologic provinces in the central Appalachians provides critical data regarding the poorly understood kinematic history of the tectonic boundary. In the Howardsville 7.5’ quadrangle, a ~5km transition zone of steeply dipping and strongly deformed rocks separates Paleozoic units of the eastern Blue Ridge (EBR) and western Piedmont (WP). Two Triassic half-grabens cut the Paleozoic structures and a suite of Nto NNW-striking Jurassic dikes intrudes older rocks and structures. Rocks in both the EBR and WP are cut by a steeply dipping, WNW- to NWstriking joint set absent in the Mesozoic rocks. This major fracture set formed with the maximum principle stress oriented NW-SE, and likely developed during late Alleghanian (~280 to 300 Ma) regional shortening. This hypothesis is supported by cleavage refraction of fracture propagation across (345-320 Ma) foliation interfaces. The youngest brittle deformation features occur as sub-vertical joints in Mesozoic rocks, and include a NE-striking set and a younger NNW-striking set parallel to Jurassic diabase dikes. This change in primary fracture set orientations requires a 30° to 50° rotation of the principal stress direction between the late Triassic and Jurassic. Linear features (lineaments) visible in the landscape from aerial photographs and Digital Elevation Models (DEM) are commonly thought to portray dominant brittle fracture features. In the study area, aerial photographs of the bedrock in the James River and a regional DEM of central Virginia do not correlate well to bedrock outcrop fracture orientations and instead likely represent foliation fabrics.

Date Awarded

2016

Department

Geology

Advisor 1

Christopher M. Bailey

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