An experimental investigation of sill formation and propagation in layered elastic media

A series of experiments are described where dyed water (a magma analogue) was intruded into solid gelatine (a crustal analogue) to investigate the formation of sills. We considered a layered gelatine system with contrasting adjacent layers. By varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer, with intrusion occurring in a plane directly below the interface. Experimental dykes were observed to propagate to the surface when the Young's Modulus ratio of upper to lower gelatine layers was less than one. Experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong. Two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak. The form of the intrusion depends on the balance of driving pressures and the Young's Modulus ratio of contrasting adjacent layers. When the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill. However, when the rigidity ratio and driving pressure are both close to one a dyke–sill hybrid forms. Under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer. During sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer. Experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill; causing acceleration with increasing length. Our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes. In ancient cratonic areas the Moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle. Mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts. The boundary between the upper and lower crust (Conrad discontinuity) may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration.