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Role of Side-Burden Strength in the Shaping of Active Salt Diapirs

Abstract

Rates of salt supply, sedimentation, tectonic extension and shortening are considered some of the prominent controls on the shaping of salt diapirs. The lateral mechanical resistance offered by the side-burden and its role in shaping the salt diapirs was ignored in the ‘fluid era’ of salt tectonics when the sediments were modeled as viscous materials. Although results from analogue models clearly show the role of frictional side-burden in resisting the lateral growth of salt diapirs, most conceptual models continue to use the diapir shapes to infer relative rates of salt rise and sedimentation and treat the side- and over-burden sediments as rigid materials without explicitly considering their mechanical impact on the shape of the diapir. In this paper, we present 2D plane strain finite element simulations of active diapirism in a basal halite layer with varying frictional strength of the rock layers in the side- and over-burden. We consider cases when the side-burden is comprised of predominantly frictional-plastic sediments with finite strength and cases when it is comprised predominantly of weak salt-dominated layered evaporite sequences. Our model results indicate that in neutral settings, the strength of the side-burden plays a prominent role in the shaping of the diapir. When the side-burden is comprised of weak sediments or evaporites, the structural relief on the salt diapir is significantly suppressed and the diapirs tend to develop wider stalks than when the side-burden is comprised of sediments with finite strength. If the pressure in the diapir salt can overcome the strength of the side-burden at a given depth, then the diapir can develop mushroom geometries at those depths. All other parameters being identical, flaring or tapering of the salt diapir at different depths is therefore strongly dependent on the distribution of weak (or over-pressured) intervals within the side-burden. In tectonic settings, undergoing active shortening or extension, however, the shape of the diapirs evolve in a much more complex manner depending on the distribution of weak intervals within the side-burden in addition to the rates of extension or shortening vis-à-vis salt mobility. A first order analysis of the geomechanical response of the side-burden, under a given tectonic loading, is therefore recommended before using the diapir shapes to infer the relative rates of salt rise and sedimentation or when interpreting the timing of prospects adjacent to salt diapirs from seismic data.