Shear angle and amount of extension calculations for normal faults emanating from a detachment: Implications on mechanisms to generate rollovers

Abstract

The reconstruction/restoration/modelling of normal faults (both listric and planar) emanating from a detachment at depth and their associated rollover folds, using the vertical or inclined shear method is widely utilized because its simplicity and the information it can provide. However, it has a rather serious issue derived from the uncertainty about the shear angle, the type of shear and the amount of extension that should be employed in each situation. Here we describe a new methodology that, using easily acquired input data, allows estimation of whether the shear was vertical, antithetic or synthetic and the values for both the shear dip and the amount of extension. These calculations rely on the use of graphs of throw versus heave for different horizons affected by the normal fault and the associated rollover, and are checked using an area-based method which permits the determination of whether these values are correct. These graphs may be used as a predictive tool or as a guide to show how the assumptions deviate, such as distinguishing quickly whether other mechanisms apart from vertical/inclined shear took place. The effects of syn-extension sedimentation and reverse fault reactivation on the proposed method are also examined. The analysis of experimental and natural examples shows that the initiation of some rollovers with a component of fault-propagation and/or drag folding, and/or development of a crestal collapse graben cause the estimated shear dips to be smaller than the actual values and the amounts of extension to be greater. In addition, these analyses show that the shear dip may increase with increasing extension.