Magnetic fabrics and strain in pencil structures of the Knobs Formation, Valley and Ridge Province, US Appalachians

Abstract

Pencil structures, which are found in weakly deformed mudrocks, reflect the bedding–cleavage intersection in weak- to moderately-cleaved rocks. Their presence indicates conditions where bedding and cleavage fabrics are approximately equal in intensity. We have determined the anisotropy of magnetic susceptibility (AMS) in pencil structures from a sequence of mudstones of the Ordovician Knobs Formation in the Valley and Ridge Province of the US Appalachians. Magnetic mineralogy was determined by X-ray analysis and low-temperature susceptibility measurements. Distribution of the magnetic ellipsoid axes is consistent with the incipient tectonic fabric of the pencil mudstones. The maximum susceptibility axes parallel the pencils’ long axes, while the minimum axes of susceptibility are normal to the primary sedimentary fabric. Independent strain quantification permits a correlation between magnetic fabric and tectonic strain. An exponential relationship between the AMS shape parameter T and tectonic shortening has been found for the interval of 10–25% shortening: shortening(%)=17∗exp( T). This relationship appears to be supported by tectonic strains up to 40%. The T parameter ( T=[ln F−ln L]/[ln L+ln F]; where L, lineation is K max/ K int and F, foliation is K int/ K min) describes the shape of the magnetic susceptibility ellipsoid, which appears more sensitive to strain than past correlation attempts with the magnetic intensity parameter P (or P′). Whereas this correlation between strain and AMS is only valid within a restricted window of strain (10–40% shortening), it establishes the magnitude and directions of tectonic strain in weakly deformed clay-rich rocks, where strain indicators are otherwise lacking or are poorly developed.