Testing volcano deformation models against 3D seismic reflection imagery of ancient intrusions

Magma intrusion often drives uplift of the overburden and free surface. Analytical modelling of such surface uplift at active volcanoes allows us to estimate intrusion geometries, positions, and volume and pressure changes; these insights have proven critical to forecasting volcanic unrest and eruptions. However, it is rarely possible to compare geodetic source parameters retrieved from elastic half space inversions to known intrusion geometries. Seismic reflection data offer an opportunity to image and quantify ancient, buried intrusions geometries and their overburden deformation (i.e., a forced fold) in 3D. Here, we use 3D seismic reflection data offshore NW Australia to investigate an Early Cretaceous forced fold developed above a laccolith emplaced at ~0.6–1 km depth. We remove the effects of post-emplacement, burial-related compaction and derive possible surface displacement patterns for the forced fold. Analytical modelling of these surface displacements, using both thin plate bending and elastic half-space solutions, suggest source (intrusion) estimates of position and lateral dimensions are similar to those of the actual laccolith. There are some differences between measurements of the laccolith and modelled source estimates, which we attribute these to syn-intrusion space-making mechanisms (e.g., compaction). We particularly find penny shaped crack and rectangular dislocation elastic half-space solutions underestimate source emplacement depth by ~0.22–0.89 km, probably reflecting a lack of heterogeneity (layering) in our models. Our novel approach highlights seismic reflection data is a powerful tool for understanding and testing how magma emplacement translates into surface deformation at active volcanoes.