An Experimental Investigation of Sill Formation in Layered Elastic Media: Rigidity Contrasts and the Strength of an Interface

A key issue for magma transport is constraining the conditions for dike arrest and magma storage in the continental crust that comprises multiple layers with contrasting mechanical properties. We present results from a series of experiments that inject dyed water (a magma analogue) into solidified layers of gelatine (a crustal analogue) to give insight into magma ascent processes in the brittle elastic crust. The experimental intrusions are pressure-driven and are injected under initially hydrostatic conditions into a gelatine solid comprising multiple layers with contrasting rigidities. In previous experiments we investigated a two-layered gelatine system, where the layers had equal thickness but contrasting rigidities. We found that sill formation occurred when the upper layer was more rigid than the lower layer, where a vertical experimental dike turned horizontal and intruded the interface between the layers to propagate as a sill (Kavanagh et al., 2006, EPSL, v. 245, p. 799-813). We build on these results to address key questions regarding the rigidity contrast: does the thickness of the more rigid layer affect its ability to act as a barrier to dike propagation? We present results from experimental intrusions formed in a three-layered gelatine host solid, where the middle layer is more rigid than the upper and lower layers. By varying the volume of the intermediate layer we can constrain if there is a threshold thickness that needs to be met in order for the experimental dike to arrest at the interface between the middle more rigid and lower less rigid layers. Once dike arrest is instigated, an experimental sill is formed if the interface is sufficiently weak and can be intruded (Figure 1A). However, if the interface is strong it is not intruded. In this case the dike propagated as a lateral dike underneath and perpendicular to the interface, before resuming vertical propagation through the middle layer and continuing to the surface (Figure 1B,C). These results have important implications for magma storage mechanisms in the crust, and for the development and evolution of magma chambers and plutonic bodies. Figure 1. Pressure-driven dyed water injected into solidified gelatine comprising three layers. The middle layer has half the volume of lower and upper layers. Rigidities E1 and E2 are the same in both experiments shown, however E1 < E2. A) An experimental dike has converted to an experimental sill, intruding the interface between the lower and middle layers. The interface is weak. B) An experimental dike crosses the interface between lower and middle layers, intrudes the middle and upper layers to erupt at the surface. The interface is strong. C) Side view of B).