STRUCTURAL GEOMETRY AND DEFORMATION HISTORY OF THE SPOTSYLVANIA HIGH-STRAIN ZONE, VIRGINIA

The Virginia Piedmont is composed of distinctive terranes that vary in lithology, age, and are bounded by faults or high-strain zones. Many of these terranes are suspect or exotic, with a poorly understood provenance. The Spotsylvania High-Strain Zone (SHSZ) is a wide (~10 km) belt of mylonitic rock that separates the Chopawamsic Arc Terrane from the enigmatic Goochland Terrane in the central Piedmont. Previous workers have mapped much of the western Goochland Terrane as the Maidens Gneiss, traditionally interpreted to be Proterozoic. In this study we use new mapping, structural analysis, and U/Pb zircon geochronology to better understand the temporal history of deformation in the SHSZ and the Goochland Terrane.

We mapped at two sites located on either side of the SHSZ. At Hidden Rock Park, biotite-garnet bearing mylonitic orthogneiss is crosscut by variably deformed pegmatitic dikes. The mylonitic gneisses experienced high-strain (X/Z >20:1) with a consistent dextral asymmetry in kinematic indicators. U/Pb zircon ages in the mylonitic gneiss range from 330 to 425 Ma, but we interpret a crystallization age for this rock between 385 to 405 Ma. The deformed pegmatite also yields a range of U/Pb zircon ages with peaks at ~320 and ~400 Ma. Collectively, we interpret these data to indicate early Devonian plutonism followed by partial melting, pegmatite generation, and deformation between 300 and 330 Ma during the early Alleghanian.

Along the James River near Cartersville, at the western edge of the SHSZ, we collected samples of strongly deformed biotite-rich paragneiss and cross-cutting mylonitic granitic gneiss. These rocks are strongly foliated and gently dipping with a SW plunging strike-parallel elongation lineation. In the paragneiss, U/Pb zircon core ages are dominated by Ediacaran grains with a few young cores between 460 and 475 Ma, while rim ages most range between 330 and 355 Ma. The granitic gneiss had only a few concordant zircons that yielded ages between 330 and 355 Ma. At Cartersville, the sedimentary protolith can be no older than Ordovician, with magmatism and metamorphism occurring between 330 and 355 Ma, consistent with Neoacadian ductile deformation. Our work demonstrates that the Maidens Gneiss is not a Proterozoic unit and that deformation in the SHSZ was protracted from the Neoacadian to the Alleghanian.