Use of anisotropy of magnetic susceptibility (AMS) measurements for orogenic studies: examples from the Betic Cordillera, Spain and Central Apennines, Italy

Abstract

Largely unmeasured penetrative deformation in unconsolidated and unburied surficial deposits contribute to orogenic energy budgets and may record deep Earth processes. The AMS technique provides an effective paleogeodetic tool to quantify strain for kinematic and dynamic interpretation of actively deforming orogens. We collected AMS data from Plio-Pleistocene age fluvial and alluvial fan sediments surrounding the Sierra Nevada massif, Spain, and Cenozoic sediments and sedimentary rocks across the central Apennine foreland, Italy, to explore the deformation fabrics in these regions. Sierra Nevada is a regional massif in the hinterland of the Betic Cordillera consisting of folded and stacked thrust sheets. We recovered similar AMS axes orientations regardless of specimen magnetic minerology, structural position around the massif, or age. The AMS ellipsoids all have a low degree of anisotropy and oblate shapes and everywhere record a compaction fabric with superimposed tectonic strain. The AMS maximum principal axes are oriented NE-SW in agreement with mineral lineations in the Sierra Nevada infrastructure and with regional GPS geodesy and seismicity results. These results are consistent with the convergence history of the Africa-Eurasia plate boundary. We also determined the AMS measured fabric of specimens collected from sites along a NE-SW corridor that spans the transition from crustal shortening to extension in the central Italian Apennine foreland. Similar to the Betics, we found that specimens have oblate AMS ellipsoids that are consistent with tectonic deformation superposed on compaction fabrics. These specimens have AMS measured fabrics compatible with shortening in the Apennine wedge and have locked-in penetrative contractional fabrics, even for those specimens from the actively extending domain. Collectively, these studies demonstrate the novel ways that AMS can be combined with structural, seismic, and GPS geodetic data to resolve orogenesis in space and time.