The Formation of Continental Crust from a Physics Perspective
Résumé
The generation of crustal material and the formation of continental crust with a thickness of approximate to 40km involve different physical mechanisms operating over different time-scales and length-scales. This review focusses on the building of a thick crustal assemblage and on the vertical dimension where the consequences of gravity-driven processes are expressed most clearly. Continental crustal material is produced by a sequence of crust and mantle mlelting, fractionation of basaltic melts and sinking of dense mafic cumulates. The repeated operation of these mechanisms over tens of million years leads to a thick stably stratified crust. We evaluate the main mechanisms involved from a physics perspective and identify the key controls and constraints, with special attention to thermal requirements. To form magma reservoirs able to process significant magma volumes and to allow the foundering of mafic cumulates, melt must be fed locally at rates that are larger than that of average crustal growth. This requires the temporary focussing of magmatic activity in a few centers. In some cases, foundering of dense cumulates does not go to completion, leaving a deformed residual body bearing tell-tale traces of the process. Crust must be thicker than a threshold value in a 30-45 km range for mafic cumulates to sink into the mantle below the crust. Once that threshold thickness has been reached, further additions lead to increase the proportion of felsic material in the crust at the expense of mafic lithologies which disappear from the crust. This acts to enhance radiogenic heat production in the crust. One consequence is that crustal temperatures can be kept at high values in times of diminished melt input and also when magmatic activity stops altogether, which may lead to post-orogenic intracrustal melting and differentiation. Another consequence is that the crust becomes too weak mechanically to withstand the elevation difference with neighbouring terranes, which sets a limit on crustal thickening. The thermal structure of the evolving crust is a key constraint on the overall process and depends strongly on radiogenic heat production, which is surely one of the properties that make continental crust very distinctive. In the Archean Superior Province, Canada, the formation of juvenile continental crust and its thermal maturation 2.7 Gy ago can be tracked quite accurately and reproduced by calculations relying on the wealth of heat flow and heat production data available there. Physical models of magma ascent and storage favour the formation of magma reservoirs at shallow levels. This suggests that crustal growth proceeds mostly from the top down, with material that gets buried to increasingly large depths. Vertical growth is accompanied by lateral spreading in two different places. Within the crust, magma intrusions are bound to extend in the horizontal direction. Deeper down, lateral variations of Moho depth that develop due to the focussing of magmatic activity get relaxed by lower crustal flow. This review has not dealt with processes at the interface between the growing crust and the mantle, which may well be where dikes get initiated by mechanisms that have so far defied theoretical analyses. Research in this particular area is required to further our understanding of continental crust formation.