ARE BUOYANCY FORCES IMPORTANT DURING THE FORMATION OF RIFTED MARGINS?

Abstract

Profiles of crustal thickness across rifted continental margins are examined in an attempt to understand the key observations and controlling parameters. Crustal stretching factor profiles from rifted continental margins supplemented by isochron data for early seafloor spreading have been used to determine a correlation between strain-rate (&formmu0;) and stretching factor (β). Despite the different methods, assumptions and data sources, our &formmu1;-β relationship for rifted margins is consistent with that observed by Newman and White for intracontinental rift basins. The &formmu2;-β relationship we derive is also consistent with the dynamic models of Newman and White which include thermorheological strain-hardening and strain-softening, but omit crustal buoyancy forces generated by lateral crustal thickness variations. Whilst crustal buoyancy forces are not included in the above dynamic models, the &formmu3;-β data do not necessarily preclude their importance. Simple numerical models of buoyancy force evolution show that for the first ~30 Myr after rifting the thermally-derived buoyancy forces within the lithosphere that assist extension are larger than the crustal buoyancy forces that oppose extension. This 'rift push' force acts as a positive feedback mechanism, is of the order of 3 × 1012 N m-1 and dominates over the opposing crustal buoyancy forces immediately after rifting. It is therefore clear that the delocalising effects of the crustal buoyancy force are dominant over a restricted range of conditions, namely at low strain-rate and at long times after rifting. Histograms of the lateral pressure gradients derived from crustal thinning factors along rifted margins show a dominant peak at 0.05 ± 0.3 kPa m-1 and a significant secondary peak at 0.8 ± 0.3 kPa m-1. The lower lateral pressure gradient peak corresponds to thinned parts of the continental crust which is adjacent to unstretched continental crust and may define the edge of a zone of thermal strain-softening. Independent observations show that narrow margins are associated with rapid strain-rates and are consistent with thermal strain-softening predicted by thermorheological models. However the dominant near-zero pressure gradient peak is consistent with the operation of crustal buoyancy force processes during rifting, which attempt to remove variations in crustal thickness.