It is increasingly recognised that most sheet-like igneous intrusions such as sills and dykes have segmented, rather than planar margins. The geometry of these segments and their connectors can provide insights into magma propagation pathways and host-rock deformation mechanisms during their emplacement. Here we report the results of scaled laboratory experiments on the emplacement of shallow-crustal, saucer-shaped sills with a focus on their propagation and segmentation. Visco-elasto-plastic Laponite RD® (LRD) and Newtonian paraffin oil were used as analogues for layered upper crust rocks and magma, respectively. Our results indicate that: 1) experimental saucer-shaped intrusions are highly segmented with marginal lobes and fingers; 2) the evolution and geometry of marginal segments and their connectors are different within the horizontal inner sill and the inclined outer sill; and 3) the bimodal nature of segment aspect ratios is linked to propagation of the inner sill along a horizontal host-rock interface versus interaction of the inclined outer sill with a homogenous upper layer. Measurements of inlet magma pressure and structural analysis suggest that marginal finger and lobe segments propagate in a repetitive sequence that starts with segmentation, followed by merging of segments and new growth of fingers/lobes. Based on the 3D geometry of segments, we suggest that sill segmentation is linked to smaller scale visco-plastic instabilities that occur within the inner sill and large scale mixed mode (I+III) fracturing during the inclined sheet propagations.

We investigate the conditions under which saucer-shaped sills form through the upper crust and their geometries. 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 analogue. Both homogenous and layered analogue upper crust is considered. In homogenous 3 wt. % LRD, the injected oil formed a saucer-shaped intrusion with the shortest inner sill observed among all of the experiments. 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 characterised by non-planar margins. The experimental results show that: (1) the transition from an inner flat sill to outer inclined sheet occurs when the sill radius to overburden depth ratio (r/H) is between 0.5 and 2.5; (2) the inclined sheets propagate upwards with angles, θ = 15° to 25°; (3) the ratio of the Young’s modulus (E*) between the layers controls when the inner flat sill to outer inclined sheet occurs; and (4) irregular finger-like and/or lobe segment geometries form at the propagating tip of the intrusion. The results also suggest that there is no strict requirement for high horizontal stresses to form natural saucer-shaped sill geometries. We conclude that the layered visco-elastic-plastic crustal analogues better represent natural, complex saucer-shaped sill geometries. Furthermore, the observed sharp transitions between inner and outer sills are compatible with brittle-elastic fracture mechanisms operating at the intrusion scale.