Physical modeling of deformation patterns in monoclines above oblique-slip faults

Abstract

Many basement-involved fault-related folds are thought to form by reactivation of pre-existing faults. Although oblique slip is expected on such structures it is often difficult to identify because the underlying faults are blind, paleomagnetic records are ambiguous, and mesoscopic deformation patterns in the associated monoclines are often incompletely exposed or difficult to interpret. This study seeks to establish a relationship between oblique slip on a basement reverse fault and the deformation patterns in the overlying folded rock layers. Using scaled physical models of wet clay overlying a rigid basement, we create a suite of Laramide-style folds over basement faults with variable obliquity. Precise displacements and strains on the surface of the clay are recorded using close-range photogrammetry. We use these measurements to predict deformation patterns that we would expect to find in analogous natural structures. The results of our models show that there are three general strain zones that form in monoclines that form above oblique-slip faults. The upper-hinge region of the monocline is dominated by extension, the lower-hinge region by contraction, and the middle of the fold limb is dominated by shear strains. The boundaries of these three zones, as well as the magnitude of strain in each of the zones vary with the amount of oblique slip and fault throw. Deformation should be dominated by joints and extensional faults in the extensional zone, by contractional faults and cleavage in the contractional zone, and may contain extensional, contractional, or strike-slip faults along with joints and cleavage arranged in Riedel style geometries in the shear strain dominated zone. At fault obliquities above 45° there is significant overlap of the different strain zones and deformation patterns in these regions can consequently be very complex, involving reactivation or overprinting of earlier structures, or the formation of mixed-mode structures. Displacements on the surface of our model monoclines suggest that previous paleomagnetic interpretations of vertical axis rotations on natural monoclines may need to be re-evaluated. Although such rotations increase with increasing fault obliquity, the magnitude of these rotations will vary significantly with position across a monocline.