Intrusions are sensitive, temporally constrained strain archives

As magma is transported and stored within the crust, it archives palaeostress regimes through mineral fabric development and the alignment of intrapluton structures. Sources of palaeostress include stress intrinsic to magma-chamber processes and tectonic stress, with the imparting multiple stress regimes forming composite or overprinting strain records. Our study investigates the strain sensitivity of intrusions to record successive palaeostress regimes during and after crystallization. Combining field and unmanned aerial vehicle mapping with anisotropy of magnetic susceptibility, we distinguish three phases of strain in the late Caledonian, Fanad Pluton in NW Ireland. The emplacement of subhorizontal and ring-like magmatic sheets records initial magma transport via viscous deformation of the host monzodiorite. Tectonic strain expressed as strain embrittlement of the monzodiorite mush is recorded in subvertical sheets, transitioning to fault and fracture development as the pluton cooled. Earlier tectonic fracture sets reflect NNE–SSW-orientated maximum shortening, followed by clockwise rotation to an NE–SW orientation. The recorded progressive rotation of maximum shortening direction is consistent with the transpressive–transtension transition recorded across the Irish and UK Caledonides. Our study shows that intrusions sited in deep-seated crustal structures offer important insight into both magma-chamber processes and evolving regional tectonic stress fields.