Novel application of the Anisotropy of Magnetic Susceptibility (AMS) in Cenozoic strata across tthe Central Italian Apennines as a paleogeodetic measure of strain and the eastward march of extension above a foreland dipping detachment

Abstract

We measured the anisotropy of magnetic susceptibility (AMS) along a SW-NE corridor across the transition from compression to extension in the central Italian Apennines. The Apennines are the subject of extensive geologic, geodetic, and geophysical investigations but the stresses responsible for historic faulting and the locus of the active extensional front remains enigmatic. Neogene-recent compressional deformation related to the subduction of Adria beneath Europe and the subsequent extensional deformation related to the retreat of the upper-plate drives a wave of extension above the foreland-dipping and seismogenic Alto-Tiberina low angle detachment. Here, we compare the AMS of 17 sites collected from Cenozoic strata exposed from the Monte Conero anticline in the foreland, west across the Umbria and Marche ridges, and extending past the drainage divide near Gubbio, in both unconsolidated sediments and in poly-deformed rocks. In many cases, tectonic deformation was superposed on compaction fabrics and samples retain an oblate magnetic ellipsoid. High frequency AMS produced more clustered principal susceptibility axes than standard AMS. Both Plio-Pleistocene and older strata SW of the Marche ridge record ~E-W k-max directions consistent with regional extension, while young sites NE of the ridge record NW-SE k-max directions consistent with continued contraction of the upper plate. AMS provides a novel way to distinguish these actively deforming regions. Also, there is a distinction in the AMS axes orientations as a function of site stratigraphic age. Similar to the AMS results in Plio-Pleistocene deposits around Sierra Nevada (Spain), the Italian AMS data records low-grade penetrative fabrics including compaction and recent tectonic deformation. These studies show that regardless of the ferromagnetic to paramagnetic ratio, as determined by hysteresis loops or from heating and cooling experiments, the AMS technique provides an effective way to identify the kinematics of active orogens. In both orogens, weak but well-organized AMS fabrics were recovered from young unconsolidated and unburied rocks and showed a great sensitivity in detecting penetrative fabrics from lithologies unable to be analyzed with more traditional methods.