A NEW MODEL FOR CONTINENTAL FLOOD BASALTS

The cause of continental flood basalts remains a mystery. These eruptions, often producing millions of cubic kilometers in a million years, occur in a variety of settings. Probably the most puzzling of such locales are stable cratons, where seemingly strong, thick lithosphere is breached by these large basaltic outpourings. Conventionally, flood basalts have been interpreted as melting events produced by two processes: 1) adiabatic-decompression melting associated with lithospheric thinning, and/or 2) elevated temperatures associated with mantle plumes. However, there are severe problems with both of these mechanisms in cratonic environments. The substantial lithospheric thinning required for adiabatic decompression melting does not occur rapidly enough, nor is it consistent with xenolith evidence for the survival of thick lithosphere beneath flood basalt domains in many stable cratonic areas. High temperature plumes are capable of producing melt on a million year time-scale, but there are many occasions in cratonic environments where the presence of a plume can be excluded, such as basaltic outpourings that have been recently associated with collisional rifts in southern Africa.

We propose a two-stage model that interprets cratonic flood basalts not as melting events, but as rapid drainage events (stage two) that tap previously-created sublithospheric reservoirs of molten basalt. A drainage event may occur in response to an abrupt increase in the lithospheric permeability to melt, caused by a stress perturbation that induces lithospheric fractures. The southern African collisional rifts noted above, for example, were produced by stresses from nearby collisions. Reservoir creation/existence (stage one) may take place on a longer timescale. Long-term supersolidus conditions in the sublithospheric mantle could be maintained by an elevated equilibrium geotherm (appropriate for the Archean), a slow thermal perturbation (e.g. thermal blanketing or large-scale mantle upwelling), or a subduction-related increase in volatile content. Any such proposed condition should satisfy cratonic-geotherm constraints from mantle xenoliths and permit the survival of a refractory, melt/volatile-depleted cratonic lithosphere.