Factors controlling Giant radiating dyke swarms – Don L. Anderson

The parameters controlling dike or sill intrusion include magma density and viscosity, magma pressure, stress magnitude and orientation in the lithosphere and yield stress and density of the lithosphere. The history of the local area is also important. A long history of melt ponding in the lower crust or upper mantle may be required for the formation of long radiating dikes. Mantle and lithosphere properties also change with time. The Archean world was a world of giant radiating dykes and komatiites; the present world is not.

Dikes form as blade-like fingers of magma moving laterally in the shallow crust. Simple gravitational stresses on elevated features such as Hawaiian volcanoes (Fiske & Jackson, 1972), or neutral buoyancy (Ryan, 1993), can control the depth of lateral dike injection, but either way, the condition of formation of radial dikes is that the intermediate and least principal stresses in the crust be equal. Dikes are shallow, and as volcanoes grow, regions of lateral dike injection migrate upward to stay at about the same depth below summits.

Volcanic loads themselves can create the conditions for dike formation (Hieronymus & Bercovici, 1999). Bora Bora in the Societies is an example of a high, free-standing volcano with radial dikes. The length of dike propagation is not an issue. The underlying stress field controls whether dikes are radial or parallel. Length has mainly to do with magma supply and perhaps elevation of the source volcano or magma chamber. Lithospheric stress measurements show that stress can be uniform over large distances.

Some so-called radial dike swarms are actually subparallel to or sub-perpendicular to rifted margins (see also CAMP page). In Scotland, later-generation dikes cross-cut earlier dikes. It is also not always clear whether the various dikes in a single set are simultaneous, as implied by the plume hypothesis, or sequential. Uplift is not one of the requirements for either LIPs or radiating dikes (see also Siberia page) which is inconsistent with a thermal plume origin..

For long-distance lateral injection the main requirements are for a supply of magma at either high pressure or elevated compared to the surroundings, and suitable lithospheric stress. The age of the giant radiating dike swarms may be telling us that the mantle was hotter in the Precambrian or that crustal stress conditions were more favorable, or both. Archean plate tectonics may have involved heat sheets or pipes rather than plate tectonics (see also Energetics page).

References

• Fiske, R.F, and Jackson, E.D., 1972. Orientation and growth of Hawaiian volcanic rifts: the effect of regional structure and gravitational stresses. Proc. Roy. Soc. Lond., Series A, 329, 299-326.
• Hieronymus, C.F., and D. Bercovici, Discrete alternating hotspot islands formed by interaction of magma transport and lithospheric flexure, Nature, 397, 604-607, 1999.
• Ryan, M.P., The mechanics and three-dimensional internal structure of active magmatic systems: Kilauea volcano, Hawaii. J. Geophys. Res., 93, 4213-4248, 1988.
• Ryan, M.P., Neutral buoyancy and the structure of mid-ocean ridge magma reservoirs. J. Geophys. Res., 98, 22,321-22,338, 1993.

25th January, 2004