SIMPLE GRAPHICAL TECHNIQUE FOR THE CONSTRUCTION AND RESTORATION OF GEOLOGIC CROSS SECTIONS OF FAULT-RELATED FOLDS

With this new graphical method, most fault-bend and fault-propagation folds can be drawn directly without having to first lookup any angular relationships or solve any equations. The resulting geologic cross sections are automatically balanced and sequentially restorable. This method can also be used to easily restore existing cross sections to an undeformed state. The general method is illustrated by application to a fault-bend fold consisting of a planar fault ramp that cuts up section from a horizontal, layer-parallel detachment horizon. The technique assumes rigid body translation and simple shear strain. Axial surfaces B and B' are fixed at the base of the footwall ramp and at the base of the hanging wall ramp, respectively and are parallel. If the fault-bend folds (B-B') are symmetric, then the fault-bend folding produces constant layer thickness and line length. Asymmetric folds with resulting thickness/line length changes are also easily accommodated. The hanging wall ramp is displaced an arbitrary amount along the footwall ramp. This rigid body translation of the hanging wall in a (hanging wall) ramp on (footwall) ramp geometry results in a gap between the hanging wall flat and the footwall ramp/flat. To close the gap, the hanging wall is deformed by distributed simple shear between axial surfaces B and B'. The simple shear is parallel to B-B' and increases linearly from zero at B' to a maximum at B. The final geometry between B and B' is the typical fault-bend fold geometry of a hanging wall flat on a footwall ramp. Concurrently, the hanging wall between B and the trailing edge translates as a rigid body resulting in a (hanging wall) flat on (footwall) flat geometry. Similar constructions are possible for other styles of fault-propagation and fault-bend folds. Fault-bend and fault-propagation folds are characterized by dip domains where parts of the fault-related folds have areas of constant dip bounded by axial surfaces and/or fault planes. Each dip domain is deformed by one or more episodes of simple shear parallel to a bounding axial surface or a fault and/or by rigid body translation(s). Dip domains of fault-propagation folds may be subdivided above and below the stratigraphic level of the tip line due to layer-parallel simple shear and/or pure shear of the layers below the stratigraphic level of the tip line.