Craig Magee , Christopher Aiden-Lee Jackson 

Dikes feed volcanic eruptions and drive crustal extension on Earth and other planetary bodies. Yet many dikes do not reach the surface, instead triggering normal faulting and graben formation in overlying rock. Whilst dike-induced faults provide a surficial and accessible record of active and ancient diking, unlocking these archives is difficult because we do not know how faults grow above or geometrically relate to dikes. We use seismic reflection data to quantify the 3D structure and kinematics of natural dike-induced faults, and test how their surface expression relates to dike geometry. We show dike-induced faults are non-planar, indicating fault dips measured at the surface cannot be projected downwards and used, along with graben half-width, to estimate dike depth. We also show multiple displacement maxima occur across individual dike-induced faults but never at their upper tips, suggesting the total extension accommodated by faulting, an assumed proxy for dike thickness, cannot be calculated by measuring fault heave at the surface. The observed displacement distribution is consistent with nucleation and linkage of isolated faults between the dike upper tip and surface, perhaps in response to cyclical stalling, thickening, and propagation of a laterally intruded dike. Our results demonstrate at-surface measurements of dike-induced faults cannot be used to estimate dike parameters without a priori knowledge of fault structure and kinematics. We show reflection seismology is a powerful tool for studying how faults grow above dikes, and anticipate future seismic-based studies will improve our understanding of dike emplacement and its translation into surface deformation.