A theory for mega-dyke propagation as driven by hotspot topography.

How can mega-dykes propagate hundreds of kilometres laterally through the crust? These blade-shaped dykes are enormous geological structures characterised by widths up to 100 metres. Ernst and Baragar (1992) showed that mega-dykes propagate away from a point at the centre of the dyke swarm. The magma for such dykes is believed to originate from a hotspot impinging on the base of the lithosphere, and this process typically precedes rifting events (Ernst, 2001; Srivastava et al., 2019). Current models do not adequately explain the mechanisms driving the propagation and termination of mega-dykes. We hypothesise that mega-dyke propagation is driven by the gradient in gravitational potential energy associated with the topography of a hotspot swell.

We present an analytical model linking the length of mega-dykes to the dimensions of a topographic swell above a hotspot. Our model accounts for various energy sources, including magma-source pressure and gravitational potential energy, and energy sinks such as viscous dissipation, elastic wall-rock deformation, and fracturing at the dyke tip. We define the ground surface deformation above a hotspot using an analytical model (Morgan, 1965) and demonstrate, in this context, that the dyke width scales with distance from the magma source. The final dyke length is computed by finding the point at which the sum of energy sources becomes less than the energy sinks. Furthermore, we explore the trade-offs between parameters controlling the swell size and the final length of a mega-dyke. We tentatively apply our findings to observed mega-dyke swarms and investigate the hot-spot sizes required to produce the observed lengths of these structures.

References

Ernst, R.E. and Baragar, W.R.A., 1992. Evidence from magnetic fabric for the flow pattern of magma in the Mackenzie giant radiating dyke swarm. Nature, 356(6369), pp.511-513. doi:10.1038/356511a0

Ernst, R.E., 2001. The use of mafic dike swarms in identifying and locating mantle plumes. Geological Society of America Special Papers, 352, p.247-265. doi:10.1130/0-8137-2352-3.247

Morgan, W.J., 1965. Gravity anomalies and convection currents: 1. A sphere and cylinder sinking beneath the surface of a viscous fluid. Journal of Geophysical Research, 70(24), pp.6175-6187. doi:10.1029/JZ070i024p06175

Srivastava, R.K., Ernst, R.E. and Peng, P. eds., 2019. Dyke swarms of the world: A modern perspective. Springer Geology. doi:10.1007/978-981-13-1666-1