The influence of columnar joints and joint pattern on near-surface normal faulting: An analog modeling approach

Normal faults at mid-ocean ridges develop massively dilatant structures close to the surface. Situated close to plate boundaries, these faults form in volcanic rocks such as basalt, which are characterized by columnar joints. These joints cause an orthotropic mechanical behavior of the rock mass, characterized by contrasting strong intact rock and weak, low-cohesion joint networks. To date, the influence of these columnar joints on the geometry and evolution of near-surface normal faults remains poorly understood. We present an analog modeling approach utilizing new analog modeling materials: i) layers of desiccated cornstarch forming drying cracks and resembling layered columnar joints in basaltic lava flows, and ii) systematically stacked blocks representing different joint patterns with varying block overlaps above vertical cooling joints. The geometric scaling of these models allows to assess the near-surface structures in the upper 50-150 m. The models reveal that fault formation is dominated by opening mode displacement reactivating the pre-existing joints and the style of deformation changes with progressive localization of the main fault. Observed structural features such as a tilted block bound between a main and antithetic fault, relay ramps, vertical mass transfer, and open cavities transfer well to the natural prototypes. Our newly developed experiments form a basis for future studies exploring fault processes in jointed rock masses.