How do sills become laccoliths? An answer from integrated laboratory and numerical modelling

Igneous intrusions in the upper brittle crust exhibit diverse shapes ranging from thin sheets (dykes, sills, cone sheets), to thick, massive intrusions (laccoliths, plutons, plugs). Presently, none of the established models of magma emplacement have the capability to simulate this diversity because they account for end member rheology of the host rocks (elastic, viscous or plastic), whereas natural rocks are complex elasto-plastic materials. We investigated the effects of host rock rheology on magma emplacement using scaled laboratory models. The model rocks were dry Coulomb granular materials of variable strength (cohesion). We show that strong (high-cohesion) host rock, results in the emplacement of thin, sheet intrusions (sills, cone sheets). Conversely, weak (low-cohesion) host rock results in the emplacement of massive intrusions (laccoliths, plugs). We integrate our laboratory results with numerical simulations to constrain the host rock deformation mechanism that accommodates magma emplacement in the experiments. Our results show how both sills and laccoliths result from initial thin sills that spread horizontally until triggering shear failure of the overburden at a critical radius. Two scenarios can then happen: (1) the overburden is cohesive enough and allows space opening in the sub-surface to accommodate viscous magma inflow along the failure planes, so sills evolve as sheets (saucer shape or cone sheets), or (2) the overburden is not cohesive enough and does not allow sub-surface space opening to accommodate viscous magma inflow along the failure planes, so the sill inflates and lifts up the overburden along shear zones to form a massive laccolith.