INFRASTRUCTURAL MIGMATITIC UPWELLING IN EAST GREENLAND INTERPRETED AS THERMAL CONVECTIVE STRUCTURES

The migmatitic gneiss upwellings in East Greenland between 72° 40′-73° 30′ N and 26°-28° W described by Haller (1971) and Henriksen and Higgins (1976) are compared and contrasted with gneiss-cored nappes and domes in other orogens. The patterns of the metamorphic isograds in vertical exposures suggest that such upwellings had a thermal convective effect. The possibility that these orogenic structures were the result of thermal convection is tested by ignoring any primary density contrast which may have been present and calculating an approximate Rayleigh number for the crustal layer which Haller referred to as the infrastructure. The non-dimensional Rayleigh number is an indication of how likely convection is in a fluid system and is encouragingly high for the rocks in question. The various implications of such a model are discussed and some of Haller's original sections are reinterpreted in ways which suggest that true cyclic convection may have occurred in rock sections where appropriate thermal gradients persisted. The movement pattern within the infrastructure is found to have a cellular pattern similar to that of convection produced in fluid layers in the laboratory. The mutual exclusion of late and post-orogenic granites and infrastructural upwellings is accounted for. It is possible that other extensive orogenic areas characterised by horizontal foliation may owe their origin to solid-state thermal convection in the sialic crust. © 1979.