GRAVITY OVERTURNS, EXTENSION, AND BASEMENT FAULT ACTIVATION

Rayleigh-Taylor instabilities occur when low-density layers of viscous fluids are overlain by denser layers. Gravity overturn of such instabilities leads to the rise of the unstable light (source) fluid as diapirs (e.g. of salt) through the denser (overburden) layers (e.g. of clastic sediments). Lateral extension or movements of faults in the bottom boundary at any stage during such gravity overturns are expected to have a great effect on the geometry, growth rate and location of any salt structures. Two groups of material models, each consisting of three series, were used to study the effect of uniform extension, and the non-uniform extension due to reactivation of pre-existing basement faults, on gravity-driven overturns at different stages. The overburden in a third group model consisted of a stiffer non-Newtonian fluid. All the models were loosely scaled to an early stage of the North Sea. All the diapirs were upbuilt, because all the overburden was in place from the start. Pre-extension/prefaulting model diapirs are essentially vertical, symmetrical and finger-like with circular planforms and with isotropic lateral spacings predictable by Ramberg theory. When reactivated and deformed by later extension, these diapirs become inclined in profile, and develop elliptical planforms elongate in the direction of extension. Diapirs which rise in an unstable sequence already thinned uniformly are only smaller, closer and slower. Postfaulting model diapirs (in group 2 models) rise as asymmetric walls or rows of fingers, some above the fault-scarps in the basement, others above the fault blocks. Synextension model diapirs rise from a thinning source layer; some are inclined and asymmetric in profile; some have circular planforms, while others are elliptical and elongate in the extension direction. Synfaulting diapirs, which form during reactivation of extensional basement faults, develop as inclined and asymmetric walls overhanging the fault scarps. Finger-like diapirs separated from synfaulting walls occur in rows parallel to the faults, and have elliptical planforms elongate in the extension direction or, if located in local regimes of shallow compression, perpendicular to it. When the overburden was non-Newtonian with a power law component n = 8.1, diapirs of the source surfaced only along extensional faults in the overburden. The model results are used to comment on recent interpretations of how halokinesis and extension interacted in the North Sea.