VERTICAL CRUSTAL TECTONICS IN THE ARCHEAN? ANALOG MODELING

One of the principal outstanding questions regarding tectonics on the early Earth is whether large-scale vertical displacements, such as solid-state and magmatic diapirism were important processes in the evolution of the Archean continental crust. Archean cratons are believed to be composed of rocks formed by tectonometamorphic pulses, between 3.6 Ga and 2.6 Ga. Geological structures documented in pre-3.0 Ga terranes, such as the Dharwhar craton (India) and the Pilbara craton (Australia) suggest that solid-state and magmatic diapirism were important processes, that may have led to large scale crustal recycling. Indeed, published finite element models also suggest that vertical tectonic displacemements may have been important in the Archean. In contrast, extensive structural and tectonic analyses of ca. 2.7 Ga terranes in the southern Superior Province (Canada) have led to models that invoke mainly modern style plate tectonics to explain accretion of that Archean craton. This suggests that styles of crustal tectonics may have changed significantly from the Early to Late Archean. Our approach, in trying to understand the evolution of the continental Archean crust, is to use analogue modeling experiments. Analogue models are used to study how the crust may have evolved with the addition of heat by magma underplating or in response to mantle plume activity. We investigate whether the change in tectonic styles may have resulted from changes in such paramaters such as geothermal gradient, densities, and thicknesses of crustal layers from Early to Late Archean time. By introducing a heat source at the base of the experimental model we are able to set up precise thermal gradients which correspond to the supposed Archean thermal gradients for Early and Late Archean times. The resulting analogue crustal structures will be investigated and compared to the documented profiles from Early to Late Archean terranes. They are discussed with respect to the tectonic evolutions of the different terranes.