33 Geologic Maps, Digital Geologic Maps, and Derivatives from Geologic Maps (Posters)

Friday, 24 March 2006
8:00 AM - 12:00 PM

STRUCTURAL CHARACTER OF THE SMITH RIVER ALLOCHTHON IN CENTRAL AND SOUTH-CENTRAL VIRGINIA

Paper number: 33-14
Program: THEME SESSIONS
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The SRA is a large suspect terrane within the southern Appalachian Piedmont zone of VA. It has been emplaced on known Laurentian rocks along the Ridgeway fault (Rf). Rocks of the SRA are lithologically distinct from, and record a different tectonothermal history than, known Laurentian cover sequences of the Lynchburg Group (LG) immediately west of the SRA. The paleocontinental affinity of the SRA is currently unknown. Recent U-Pb monazite ages recovered in the SRA indicate that it experienced a c. 530 Ma tectonothermal event, which is consistent with an exotic, peri-Gondwanan, origin for the SRA. However, similarities between detrital zircon signatures in the SRA and the underlying LG, along with the lack of distinct peri-Gondwanan detrital zircon ages in the SRA suggest that the SRA is peri-Laurentian. This study examines the SRA's structural record for evidence that may correlate with the Early Cambrian monazite ages.

Mapping in the areas of Gretna, Altavista, and Bassett, VA indicate that structures across the SRA are homogeneously developed. A general stratigraphy has been established within the SRA consisting of the Bassett gneiss as the structurally lowest unit, cross-cut by and interlayered with the Bassett amphibolite. The schistose Fork Mt. Formation lies structurally above, and laterally interfingers with, the Bassett gneiss. The main structures in the SRA are a set of isoclinal recumbent folds with a prominent axial planar foliation. These folds are folded by a later set of open to closed, upright to inclined, non-cylindrical folds lacking a penetrative axial planar foliation. A later set of fold axes trend NNE/SSW and plunge gently to the NNE and SSW to form regional basins and domes in the SRA. Structural comparison of the SRA and the underlying LG reveals evidence of an earlier tectonothermal foliation that may indicate a history that is unshared with the LG, possibly the event recorded by the monazite crystallization age of c. 530 Ma.