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Approximate theory of substratum creep and associated overburden deformation in salt basins and deltas

Chapter

Abstract

Lubricating squeeze flow of a ductile substratum under varying overburden is a characteristic element of the tectonics of salt basins and many deltas. Large-scale mass movements of salt or overpressured shales can occur in this manner, accompanied by deformation of a sedimentary overburden. The theory outlined in this paper deals with large-scale extrusive flow in mobile substrata that is driven by differential loads and buoyancy forces. The theory assumes slowly varying overburden and substratum thicknesses. It treats the salt (or shale) substratum as a viscous ‘lubricating layer’ and the overburden as a dead load that offers no significant resistance to shear along vertical planes, but will support horizontal stresses within the limits of ‘active’ and ‘passive’ Coulomb plastic states. Assuming the sea floor (sedimentation boundary) and basement elevations given, the theory yields a single differential equation in the layer thickness h of the salt (or shale) layer. When buoyancy effects are negligible, squeeze flow in a substratum of varying thickness will propagate substratum isopachs as ‘kinematic waves’ with a speed proportional to (tan α) n h n +1, where tan α is the overburden slope and n the power law exponent governing the creep response of the substratum. Buoyancy introduces nonlinear unstable (‘backward’) diffusion effects leading to localized flow reversal and suggesting a mechanism for the generation of ‘pinch-and-swell’ structures; it also enters as the principal driving force for the large-scale updip extrusion of salt in areas affected by differential subsidence. The theory also sheds light on a frequently observed form of gravity-induced slope failure, which is characterized by the simultaneous occurrence of extensional and compressive faulting in distinct sections of the slope.

Keywords

Horizontal Stress Substratum Thickness Kinematic Wave Squeeze Flow Viscous Substratum 
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© Springer-Verlag Berlin Heidelberg 2000

Authors and Affiliations

  1. 1.Geodynamics - Physics of the LithosphereUniversity of BonnGermany

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