Features of Giant radiating dyke swarms – Yuri Fialko

The most remarkable feature of giant dike swarms and their associated flood basalts is the enormous supply of magma from an upper mantle source to the Earth's crust. For example, a single dike of the Proterozoic Mackenzie swarm in Canada probably transported a volume of magma equivalent to a century of melt production at the present-day global mid-ocean ridge system. The large volumes of magma, as well as high magma fluxes (individual giant dikes may have been emplaced on a time scale of weeks to months, and an entire magmatic episode might have occurred on a time scale of millions of years or less) imply a major but short-term thermal and/or compositional perturbation in the upper mantle. The inferred high rates of magma production are suggestive of robust advection of hot material into the upper mantle.

Thermo-mechanical arguments alone do not constrain the depth from which the hot/fertile material was supplied to the base of the lithosphere. The association of giant dike swarms and flood basalts with mantle plumes (e.g., Morgan, 1981) is based on the observation that many flood-basalt provinces seem to initiate hotspot tracks (e.g., Duncan & Richards, 1991), and on the isotopic signatures of flood-basalt lavas (Carlson, 1991). The radial pattern of giant dike swarms is often attributed to uplift caused by mantle plume heads (Ernst et al., 1995). However, the amplitude of the predicted uplift is insufficient to accommodate large dike swarms such as the Mackenzie swarm by simple magma fracture. Given the large magma volumes in the source region, and high fluxes during dike emplacement, a large percentage of the observed dike thicknesses may have formed by non-dilatant mechanisms (e.g., via melting of the dike walls). If this is the case, it follows that extensional stress in the crust after dike emplacement may have been larger than before. If so, the radial or fanning patterns of large dike swarms might be due to a self-induced stress field. The main effect of a large-scale uplift centered on the magma source was probably to produce a gravitational driving force that resulted in long-range lateral magma transport.

The large volumes of magma involved in flood basalt events pose several mechanical problems. If the giant dike swarms were fed from an intermediate storage region in the crust, there is a space problem for a large (thousands of cubic kilometers) shallow magma chamber. If dikes ascended directly from a mantle source, and spread horizontally upon reaching a level of neutral buoyancy, it is unclear how the required large volumes of gravitationally unstable melt might have accumulated in the source region. One possibility is that most of the melting took place while the lithosphere was under horizontal compression, so that the melt could not readily escape upwards. Regional uplift due to mantle upwelling generates horizontal compression in the lower lithosphere that may result in magmatic underplating. Subsequent sub-vertical transport of ponded magmas may be possible if the horizontal compression is relieved, e.g., by thermally-activated creep, gravitational collapse or tectonic extension. Regional uplift is nevertheless not a pre-requisite for trapping of large volumes of melt at the base of the lithosphere, which may, in principle, be accomplished by any process that causes horizontal compression in the lower lithosphere.

References

• Carlson, R.W., Physical and chemical evidence on the cause and source characteristics of flood basalt volcanism, Austral. J. Earth Sci., 38, 525-544, 1991.
• Duncan, R.A. and Richards, M.A., Hotspots, mantle plumes, flood basalts, and true polar wander, Rev. Geophys., 29, 31-50, 1991.
• Ernst, R.E., Head, W.J., Parfitt, E., Grosfils, E. and Wilson, L., Giant radiating dike swarms on Earth and Venus, Earth Sci. Rev., 39, 1-58, 1995.
• Morgan, W.J., Hotspot tracks and the opening of the Atlantic and Indian oceans, in The Sea, 7, 443-487, 1981.

29th January, 2004