Summary

Implementing trishear using a pseudo-3D algorithm can help predict the shape of beds that have been sheared, although not faulted, in a triangular prismatic volume ahead of a propagating fault tip. Applying trishear allows modelling of complex fault-related fold geometries that otherwise would not be captured using traditional geometrical restoration and forward modelling methods. As in 2D space, the expression of trishear in 3D is controlled by user-defined parameters including the magnitude of displacement, the propagation/slip ratio, the angle of the trishear zone and the initial position of the zone. Each of these parameters, which can be varied laterally along fault strike in Midland Valley’s Move software, will ultimately influence the fault–fold relationship.

Here we present a case study on the development of the East Kaibab monocline in the western United States that tests a range of parameters and considers the validity of the subsequent results. The kinematics of the monocline, which extends from northern Arizona into southern Utah and forms the eastern margin of the ca 200 km-long Kaibab Uplift, have been the subject of ongoing debate and the focus of a series of previous 2D and 3D trishear studies.

Introduction

The trishear method of fault-propagation folding was initially developed as a 2D alternative to the kink-band technique (Erslev 1991; Allmendinger, 1998) where a hanging wall moves at a constant velocity relative to a fixed footwall, while, within a triangular zone above the fault tip (or "trishear zone"), velocity decreases into the footwall (Fig. 1).

Trishear deformation can be applied in 3D space using two methods; true 3D or a pseudo-3D. True 3D trishear uses a local coordinate system attached to the location of the fault tip line (Cristallini and Allmendinger 2001), whereas pseudo-3D trishear is calculated along a series of parallel cross-sections that are orientated in the slip direction (Cristallini et al., 2004). Cardozo (2008) noted that due to increased processing speed the pseudo-3D approach was more versatile than a true-3D algorithm while sufficiently retaining volume in all structural settings. A pseudo-3D approach has been implemented in this study and allows forward modelling and restoration in geographical space using real, structurally complex fault and horizon geometries (Fig. 2).

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