Saucers, Fingers, and Lobes: New Insights on Sill Emplacement From Scaled Laboratory Experiments

We investigate the conditions under which saucer-shaped sills form and segment in the upper crust. We performed a series of scaled laboratory experiments that employ visco-elastic-plastic Laponite RD® (LRD) gels to model upper crustal rocks, and Newtonian paraffin oil as the magma analog. Saucer-shaped sills always formed in experiments with a two-layer upper crust. These experiments show sharp transitions from an inner flat sill to outer inclined sheets, which are characterized by non-planar margins. The results show that: (a) the transition from an inner flat sill to an outer inclined sheet occurs when the sill radius to overburden depth ratio is between 0.5 and 2.5; (b) outer inclined sheets dip angles vary from 15° to 25°; (c) this transition is controlled by the ratio of the Young's modulus between the host material layers; (d) irregular finger-like and/or lobate segment geometries form at the propagating tip of the experimental sills; and (e) the evolution and geometry of marginal segments and their connectors are different within the inner and outer sill. The results also suggest that there is no strict requirement for high horizontal stresses (>5 MPa) to form natural saucer-shaped sill geometries. We conclude that analog experiments of magma emplacement into layered visco-elastic-plastic upper crustal analogs reproduce the complexity of natural saucer-shaped sills and their marginal segmentation. The behavior of the experimental sills is compatible with brittle-elastic fracture mechanisms operating at the intrusion scale, while marginal lobes and finger-like segments are most likely linked to small-scale visco-plastic instabilities occurring at the crack tip scale, possibly aided by the low fracture surface energy of the host material.