Cratons, the ancient nuclei of continents that have been stable for billions of years, are underlain by keels of lithosphere with strongly melt-depleted compositions. These cratonic keels may have formed either from partial melting in a mantle-plume environment, or alternatively by melting at shallow depths in a subduction zone, during the successive accretion of slabs of oceanic lithosphere. The stability of cratonic keels has been attributed to a pervasive state of near-neutral buoyancy - isopycnicity - created by offsetting thermal and compositional effects on density. However, it is unclear how an isopycnic state can be sustained over geological time. Here we simulate the evolution of a simplified southern African cratonic keel, initiated in either a hot-plume or a cold-slab environment, over 3 billion years, using a numerical model that incorporates secular cooling of the mantle, coupled with gradual loss of radiogenic heating in the lithosphere. We find that the simulation that starts from a cold-slab environment best explains the subsidence history of the southern African craton. However, irrespective of how the cratonic keel formed, we find that the isopycnic state is inherently ephemeral: a cratonic keel that is approximately isopycnic under present conditions was more, or less, buoyant in the geologic past.
Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada; Centre de Recherche GEOTOP, Université du Québec À Montréal, CP 8888 succursale Centre-Ville, Montréal, QC H3C 3P8, Canada
Recommended Citation:
Eaton D.W.,Claire Perry H.K.. Ephemeral isopycnicity of cratonic mantle keels[J]. Nature Geoscience,2013-01-01,6(11)