Role of shear in fault-propagation folding

Abstract

The effect of layer-parallel shear on the shapes of fault-propagation folds is explored for the two theories of fault-propagation folding of Suppe & Medwedeff (1990): (1) constant layer thickness and (2) variable front-limb layer thickness; the range of possible fold shapes is significantly expanded relative to the case of no shear. In this analysis, a homogeneous differential layer-parallel shear is applied to the beds that are cut by the thrust fault in the cores fault-propagation folds. This shear may be applied in three ways: (1) self-similar fold growth during fault propagation with constant shear applied instantaneously along the bed of the fault tip, (2) progressively increasing shear during fault propagation, and (3) shear after the fault is locked modifying the existing fault-propagation fold. The final shape of the fold is independent of the history of shearing in relation to fault slip. The fold shapes are largely governed by the fault steepness and the amount of imposed shear. Strong differential shear profiles and shallow faults produce overturned or thickened front-limbs. Little differential shear and steep faults result in upright or thinned front-limbs.