Subvolcanic magma pathways in transtensional regimes; AMS and structural evidence from SE Iceland

Vincent Twomey , William McCarthy, Craig Magee , Ben Latimer

This study presents an integrated analysis of magmatic and tectonic strain data recorded within and around the Reyðarártindur intrusion, southeast Iceland, combining anisotropy of magnetic susceptibility (AMS) with UAV-based fracture mapping. Thermomagnetic experiments and petrography confirm AMS fabrics capture magmatic-state strain, defining a consistent NNE–SSW magnetic lineation with predominantly sub-horizontal orientations that is here interpreted to record lateral SSW magma flow during emplacement. Localised steep fabrics at intrusion margins and within inclined sheets indicate vertical magma transfer into fracture-controlled conduits during syn- to late-stage evacuation. UAV-derived structural data reveal a network of brittle fractures in the basaltic host rocks forming a left-stepping sigmoidal pattern, consistent with a sinistral Riedel shear system. This fracture architecture closely matches regional palaeostress orientations for southeast Iceland, indicating that the host rocks preserve the contemporaneous tectonic strain field. Integration of these datasets demonstrates a spatial and kinematic correspondence between internal magmatic fabrics and external brittle structures: the dominant AMS fabric parallels the principal Riedel shear fracture set, indicating regional tectonic strain guided magma transport pathways. Fracture networks also acted as conduits for vertical magma migration, linking lateral flow within the intrusion to eruption-feeding sheets. Our results demonstrate that magma flow in the Reyðarártindur intrusion was strongly influenced by the interaction between internal magmatic processes and the surrounding regional structures;that may have been kinematically governed by an active Riedel shear system, linking subvolcanic transport directly to tectonic strain. Our findings demonstrate how integrating magnetic fabrics with field relationships reveal the interplay between tectonics and subvolcanic magma transport mechanisms.