Viscous flow during salt welding

Abstract

Salt can be partially removed by viscous flow from between wall rocks to form a salt weld. Welds in autochthonous and allochthonous salt can form significant structures in evaporite basins, where petroleum and mineral discovery can hinge on whether salt welds act as seals or windows for migrating hydrocarbons or brines containing dissolved metals. Despite the importance of welds, little is known about salt evacuation during welding. We investigate viscous flow during welding using analytical and numerical models, based on exact solutions to the Navier–Stokes equations for idealized geometries and boundary conditions. We explore two questions: how does salt thin during evacuation, and what are the limits of viscous flow during salt welding? Hydraulic-gradient and displacement boundary conditions are shown to drive salt evacuation, which is rate-limited by drag along the boundaries of a salt layer. Where salt flow is restricted, for example beneath a broad, prograding sediment wedge, up to ~ 50 m salt can remain in an incomplete weld. Where salt flow is unrestricted, for example beneath a subsiding minibasin, viscous flow can remove all but a vanishingly thin (<<1 m) salt layer. In both cases, any remaining salt must be dissolved to leave a weld containing no remnant salt. Evacuation rate increases with increasing differential stress and decreasing flow length and dynamic viscosity of the salt. Translation of wall rock parallel to bedding may result in a fault weld but may also inhibit evacuation if the displacement counteracts flow driven by a hydraulic gradient. Evacuation of multilayered evaporites is controlled by the distribution of layer thickness and viscosity. Multilayered evaporites can be compositionally modified during evacuation.