Testing the sensitivity of anisotropy of magnetic susceptibility (AMS) to the regional tectonic strain field in granite plutons: insights from two orogen-scale studies

Anisotropy of Magnetic Susceptibility (AMS) fabrics within many individual granite plutons have previously been interpreted as recording the regional syn-magmatic tectonic strain field. To test this hypothesis, we compiled a regional database of AMS data from multiple granite complexes across two orogens, the French Massif Central and the British and Irish Caledonides, and critically evaluated the degree to which the magnetic fabric of the granite plutons recorded the known, regional tectonic strain. AMS fabrics from nine plutons from the French Massif Central show that all intrusions recorded the syn-magmatic late Variscan extensional collapse, with the maximum susceptibility axes (i.e., magnetic lineation) aligned with the NW-SE regional stretching direction. AMS fabrics from ten late Caledonian ‘Newer Granite’ plutons appear to reliably record the changing tectonic regimes between 430 and 390 Ma, including the switch from transpression to transtension following Iapetus closure, and then the return to transpression following the onset of the Acadian Orogeny at 400 Ma. This study indicates that comparisons between AMS fabrics and regional tectonics is best achieved qualitatively by comparing the orientation of the susceptibility axes to the known strain field, and more quantitatively through Woodcock analysis. Overall, our results indicate that pluton-scale AMS fabrics from multiple complexes spaced across an orogen can record a complex and changing regional tectonic strain field. This indicates there is significant potential to utilise pluton-scale AMS studies, alongside precise geochronological ages, to refine the timings of an orogen’s tectonic evolution.