Laboratory Modelling of Sill Emplacement: Part 2 - sill segmentation

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.