Focused flow during the formation and propagation of sills: Insights from analogue experiments

Abstract

Geophysical observations of sills suggest magma can be transported hundreds of kilometres within these thin (<100 m thick) horizontal sheet-like bodies. However, existing magma flow models struggle to explain how sills propagate so extensively. We conducted scaled analogue experiments to explore the dynamics of magma flow in sills. Two fluids (magma analogues) with different rheology (water, a Newtonian fluid, or xanthan gum, a shear-thinning fluid) were injected through the base of a clear-Perspex tank that contained two superposed gelatine layers with contrasting stiffness and a weakly bonded interface (crustal analogue). To visualise and map fluid flow over time, we applied particle image velocimetry to passive tracer particles that fluoresced under laser light and were suspended in the fluid. Flow within the feeder dyke varied with the injected fluid; fluid recirculation occurred around the margins in the Newtonian dyke, whereas fluid at the shear-thinning dyke margins was stagnant, and both formed a centralised vertical jet. A region of focused flow was observed within the shear-thinning sill and there was no evidence of focused flow within the Newtonian sill. These experiments suggest that focused flow may be common in shear-thinning magma intrusions and highlights the coupling of flow dynamics between the feeder dyke and sill. If such focused flow occurs in nature, enhanced by a non-Newtonian rheology, this could explain how larger sills develop with magma transport focused through a lower viscosity region localised within a larger volume of potentially eruptible magma mush.