Geometry and kinematics of fault-propagation folds with variable interlimb angle

Abstract

Several conceptual approaches have been proposed to account for the development of fault-propagation folds whose geometry and kinematics depend on the amount of displacement along a basal decollement level, the ramp angle and the slip to propagation ratio. Among these, the variable interlimb angle model of Mitra (1990) is able to explain open and close natural folds but its application is limited because the fold geometry and bed thickness evolution rely on imposed parameters that cannot be measured directly. Here, we use the ramp and the interlimb angles as input data to develop a forward fold model that accounts for thickness variations in the forelimb. The relationship between the fold amplitude and fold wavelength is subsequently applied to construct balanced geological cross-sections from surface parameters only and to propose a kinematic restoration of the folding through time. The model can catter for a wide variety of folds, reconstruct the deep architecture of anticlines and deduce the kinematic evolution of the folding with time. We consider three natural examples to validate the variable interlimb angle model. Along-strike thickness variation in the forelimb of the Turner Valley anticline in the Alberta foothills of Canada precisely corresponds to the theoretical values proposed by our model. Reconstruction at depth of the Alima anticline in the southern Tunisian Atlas implies that the decollement level is localised in the Triassic-Liassic series, as highlighted by seismic imaging. The kinematic reconstruction of the Ucero anticline in the Spanish Castilian mountains is also in agreement with the fold geometry derived from two cross-sections. The variable interlimb angle model predicts that the fault-propagation fold can be symmetric, normal asymmetric (with a greater dip value in the forelimb than in the backlimb), or reverse asymmetric (with greater dip in the backlimb) depending on the shortening amount.