MAGNETIC FABRICS AND STRAIN IN MUDROCKS: AMS OF PENCIL STRUCTURES

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 Fm. 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 ellipsoids axes is consistent with the incipient tectonic fabric of the pencil slates. The maximum axes of susceptibility 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. A 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). The T parameter (T=[lnF-lnL]/[lnL+lnF]) describes the shape of the magnetic susceptibility ellipsoid, which appears more sensitive to low strain then 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, it determines the magnitude and directions of tectonic strain in weakly deformed clay-rich rocks, where strain indicators are usually lacking or poorly developed.