Structural aureoles as proxies for magma emplacement in the brittle crust: Evidence for transtensional pluton formation at Slaufrudalur, Iceland

Shallow-crustal magma emplacement produces characteristic host rock deformation within a “structural aureole”. Two end-member mechanisms of magma emplacement dominate in the brittle crust: roof uplift, involving upward displacement of overlying rocks, and floor subsidence, involving downward displacement of the underlying rocks—either along tectonic faults or ring faults in cauldron subsidence. While these processes are fundamental to crustal growth, heat transfer, and ore formation, their identification in the field is often based on ambiguous criteria such as intrusion geometry or internal fabrics, leading to inconsistent classifications. We propose a robust set of structural criteria for distinguishing emplacement mechanisms of upper crustal (<5 km) intrusions. These are applied to the Slaufrudalur pluton, Iceland's largest exposed granitic intrusion, using structural mapping and photogrammetry. Previous interpretations framed Slaufrudalur as a classic cauldron subsidence pluton or a sill complex. Our results reveal that the pluton formed in a transtensional setting and is bound by strike-slip faults, fundamentally revising its geotectonic interpretation. This study underscores the value of detailed structural investigation of host-rock deformation for resolving emplacement processes and provides a transferable framework to link pluton architecture and tectonic setting in shallow crustal exposures, with implications for understanding magma transport, geothermal systems, and mineralisation. • Structural mapping and photogrammetry distinguish roof uplift vs. floor subsidence. • Slaufrudalur pluton formed as a pull-apart structure, bounded by strike-slip faults. • Results overturn its classic interpretation as cauldron subsidence or sill complex. • Structural aureole mapping links pluton architecture to tectonic setting and magma transport.