Reliability of magnetic fabric of weakly deformed mudrocks as a palaeostress indicator in compressive settings

In this work we compare the results of anisotropy of magnetic susceptibility (AMS) with palaeostress analysis at local scale. The AMS data refer to 14 sites from weakly deformed mudrocks (Lower and Middle Miocene lacustrine sediments) from two areas of the internal part of the Ebro foreland basin, the Bardenas Reales and Monegros areas. Nine sites display a magnetic fabric related to a subtle tectonic overprint showing a roughly E–W magnetic lineation, whereas the rest of sites (most of them located at the Monegros area) show a sedimentary fabric without a well-defined magnetic lineation. Palaeostress data result from fault populations and joint sets exposed in the limestone beds interbedded with the mudrocks at the nearest possible outcrop from the AMS site. The origin of the magnetic lineation is related to a N–S compression coeval with the earliest diagenesis of the studied sediments prior to consolidation. The comparison between magnetic fabric and palaeostress data demonstrates the coaxiality between the AMS and palaeostress ellipsoids both at local and regional scales, thereby validating the reliability of AMS as palaeostress indicator in compressive settings.