ANISOTROPIC MAGNETIC SUSCEPTIBILITY (AMS) FABRIC AS A PALEOGEODETIC MEASURE OF TECTONIC DEFORMATION IN QUATERNARY DEPOSITS: AN EXAMPLE FROM THE ITALIAN APENNINES

The anisotropy of magnetic susceptibility (AMS) is a proven measure of tectonic strain for rocks that have reached temperatures > 50°C. We pursue the hypothesis that AMS tectonic fabrics can also be encoded in rocks or sediment that have experienced shallow burial (< 250 m) over Quaternary time scales in a neotectonic stress field. We use the well-known shortening to extension Quaternary strain history in the Italian Apennines as a natural experiment to explore AMS fabrics in surficial deposits as a paleogeodetic measure of strain histories. We sampled Quaternary deposits from ten sites in central Italy representing intermontane alluvial and lacustrine facies from active extensional, former wedge-top, and active wedge-top basins. At each site we sampled nine oriented specimens and recorded their AMS fabrics using fifteen position measurements on a Kappabridge KLY-3s. The total strain in all samples is low, typically 1-3%, the sample fabric is mostly oblate, and K₁ axis plunges are shallow, typically < 10°. No AMS fabric is recorded in samples buried less than 10 m. A compaction fabric emerges in most of the samples buried more than 10 m, but only a subset of these samples has a K₁ direction consistent with a tectonic fabric. Samples buried deeply in the Gubbio and Colfiorito extensional basins have a NE-SW oriented K₁ direction consistent with the dominant stretching direction. In contrast, surficial samples in the former wedge top basins typically have no tectonic fabric, but at least one sample has a weak compressional fabric consistent with previously published results. The ability of a specimen to acquire a tectonic fabric is linked to its magnetic mineralogy with magnetite particles displaying a range of maximum susceptibility orientations that contrast with more platy paramagnetic clays that typically are oriented near to the bedding plane yielding better defined compaction fabrics. Experiments are ongoing to determine the dominance of ferromagnetic and paramagnetic carriers. Despite the considerable noise of the nine specimens that constitute a sample, non-random K₁ directions are always either consistent with extension or shortening, leading us to conclude that neotectonic fabrics can be encoded in these young deposits, with obvious implications for geodetic interpretations of accordant seismic hazards.