Megadyke propagation down dynamic topography

Timothy Davis , Yuan Li, Adina E Pusok , Richard Katz

Magmatic dykes that align vertically and extend laterally for hundreds to thousands of kilometres are known as megadykes. Observations of solidified swarms of megadykes suggest the dykes propagate away from a common source. We hypothesize that megadyke propagation is driven by dynamic topography above a buoyant mantle plume. We develop a model describing lateral dyke propagation from a constant-pressure magma source. The model considers fluid-driven fracturing of elastic crust and solidification of magma. The dyke becomes confined at its level of neutral buoyancy, inhibiting further ascent. Dynamic topography warps this neutral-buoyancy level, giving the magma gravitational potential energy that drives lateral flow. Solidification eventually blocks the fracture perimeter, halting propagation when the speed of the fracture's lateral tip drops below a threshold. When assisted by dynamic topography, dykes in this model propagate to lengths approximately equal to the underlying plume head diameter. These results explain why megadyke swarms have characteristic lengths related to the plume-head size. Hence our model links a swarm's characteristic dyke length to the size of the ancient plume that created it. Furthermore, the model predicts a sensitivity to magma source pressure, explaining the existence of dykes that are far longer than others in the swarm.