Criteria to discriminate between different models of thrust ramping in gravity-driven fold and thrust systems. (September 2021) - Alsop, G.I.
Abstract: Although most models of thrusting assume that the hangingwall is actively displaced up the thrust ramp while the footwall remains passive, it has been suggested that this could be an oversimplification and the footwall may also deform. Despite this, there are relatively few detailed investigations of thrusts where the footwall is deformed, perhaps reflecting issues with space and accommodation if the footwall actively moves downwards to deeper levels. Furthermore, such studies assume that the thrust is deeply buried, otherwise the hangingwall is more likely to rise and simply uplift the surface. Using examples from gravity-driven fold and thrust systems developed in unlithified late Pleistocene sediments around the Dead Sea Basin, we investigate pristine fold and thrust geometries unaffected by later compaction and deformation to establish two end-member models of overthrust and underthrust ramp development. During overthrusting, the hangingwall is uplifted and marker beds remain at or above regional elevation, whereas the footwall of underthrust ramps is depressed and marker beds are deflected below regional. The greatest displacement generally develops low down overthrust ramps and decreases upwards, whereas larger displacements form high up underthrust ramps and reduce downwards. The reduction in displacement in overthrust ramps is marked by decreasing dips, whereas displacement increases with decreasing dips up underthrust ramps. Fault propagation folding creates hangingwall antiforms above overthrust ramps, whereas footwall synforms develop below underthrust ramps. The effect of this folding is that hangingwall sequences and cut-offs are relatively thinned (stretch <1) in overthrust ramps, while footwall sequences and cut-offs are thinned in underthrust ramps (stretch>1). Not all ramps follow these end-member geometries and mixed 'wedge' ramps also develop in which the hangingwall and footwall to the ramp are both deformed to varying degrees. Underthrust ramps are generally developed where failure initiates in competent units higher up the deforming sequence, and then propagates downwards towards underlying potential detachments. Downward propagation is accommodated by footwall synforms and weak beds that absorb deformation by differential vertical compaction resulting in up to 50% thinning in some cases. A consequence of underthrusting is that the crests of hangingwall structures tend to remain at the same elevation and are therefore unable to build significant topography or bathymetry on the sediment-water interface, thereby rendering critical taper models of less relevance. Significant vertical compaction may facilitate expulsion of fluids that drive further deformation and may also complicate the use of area balancing techniques during restoration of thrust systems. Graphical abstract: Image 1 Highlights: Gravity-driven fold and thrust systems (FATS) in mass transport deposits (MTDs) are examined. Criteria to distinguish end-member models of overthrusting and underthrusting are established. We describe underthrusts for the first time in gravity-driven FATS within MTDs. Underthrusting is accommodated by footwall synforms and differential vertical compaction. Underthrusts lack of topography may render critical taper models less relevant in MTDs.