The Effect of Solidification on Sill Propagation: An Experimental Approach With Application to Visco-Elasto-Plastic Host Rocks

Abstract:

The dynamics of how solidification fronts grow at the margins of intruding magma bodies play an important role in controlling the transport and storage of magma in the Earth’s crust. Here, we investigate the role of solidification on the propagation and morphology of sills in a series of scaled laboratory experiments. Hot coconut oil (magma analogue), that solidifies during its emplacement, is injected as a sill into layered colder Laponite RD gel (visco-elasto-plastic host rock analogue). The injection flux and temperature are maintained constant during each experiment and are systematically varied between experiments. Sill intrusions with less pronounced solidification effects (high injection temperature) show the development of finger-like segments that exhibit an internal branching pattern with patchy solidified areas before the analogue magma erupts at the surface. These morphologies are similar to isothermal experiments. Conversely, when solidification effects are pronounced (low injection temperature), sill intrusions develop planar outer margins without internal branching and display lobes and time dependant, internal meandering channels that connect the central feeder to the lobes. These sills are discontinuous and develop discrete lobes, representing periods of momentary tip arrest during sill propagation. Since the injection rate is constant, the differences are due to thermal and solidification effects within the intruding magma analogue. Solidification is observed at the top and the bottom of intrusions, while their interiors remain partially solidified and channelized. For a constant injection rate, when the solidification effect is increased, the intrusion area decreases, while both its thickness and number of discrete lobes increases. Previous literature using solidifying magma analogues and visco-elastic gelatine host materials show similar discrete lobe-like morphologies during solidification, without internal branching and segmentation. The complex, non-planar propagating sill margins and segmentation observed in our experiments are similar to sills in nature, which indicates that host rock rheology, together with magma solidification, play a major role in determining the final morphology of sills and most likely other planar intrusions.