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 (R) (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 degrees to 25 degrees; (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. Plain Language Summary Magma migrates through the rocks of Earth's upper crust within cracks that propagate both upwards, as dikes, toward the surface and also laterally, and as sills, over potentially large distances. Laterally propagating sills often form saucer-shaped structures with a flat inner sheet and an outer inclined sheet. Here we present the results of scaled laboratory experiments that are designed to simulate and better understand how saucer-shaped sills form within layered upper crustal rocks. In our experiments, visco-elastic-plastic Laponite RD (R) (LRD) gels and Newtonian viscous paraffin oil represent layered host rocks and the ascending magma, respectively. Saucer-shaped sills with sharp transitions from inner flat to outer inclined sheets form when analog magma is injected along the interface between two analog host rock layers. Moreover, the leading edges of the experimental sills are irregular, comprising finger- and lobe-shaped margins that resemble the complex geometries observed in geophysical images of sills in sedimentary basins. The results suggest that the large-scale geometry of the experimental saucer-shaped sills reflects the brittle-elastic properties of LRD, making it a suitable material to model natural rock behavior during sill emplacement in nature. The complexity of the experimental sill margins likely reflects the operation of small-scale instabilities at propagating crack tips related to the complex rheology of the analog host material and its low fracture surface energy, both of which can be expected to apply to real upper-crustal rocks in nature.