FAULT-RELATED FOLDING: A SIMPLE UNIFYING MODEL FOR FAULT-BEND AND FAULT-PROPAGATION FOLDING

Analysis of existing geometric models of fault-related folding suggest a more intimate relationship between all styles of fault-related folding than has been previously suggested. Specifically, all styles of fault-propagation folding form part of a continuum that reduces to fault-bend folding. Fault-propagation folds consist of a fault-cored anticline-syncline fold pair with a fault tip line where the displacement vanishes. The three general types of fault-propagation fold models consist of a foreland verging anticline-syncline fold pair where: (1.) both axial surfaces dip more steeply than the fault with an undeformed footwall, (2.) both axial surfaces dip less than the fault with a footwall syncline, or (3.) a fixed hinge model where the anticlinal axial surface dips more steeply and the synclinal axial surface less steeply than the fault, which also produces a footwall syncline. Depending upon the initial geometry, the trailing edge of the hanging wall may remain straight or in some cases it is deformed by layer-parallel simple shear (e.g. bedding plane slip?) at stratigraphic levels below the tip line. As the dips on the axial surface approach the dip of the fault plane, the common limb of the fold pair becomes very narrow, the layers are overturned and are both highly elongated and thinned. At some stage, this narrow, highly deformed fold limb would resemble a fault zone, and it may be mechanically easier to develop a fault-bend fold along the overturned bedding, which is nearly parallel to the fault, than for the fault-propagation fold to continue. When the axial surfaces are exactly parallel to the fault, all of the fault-propagation fold models reduce to the simple case of a fault-bend fold with an undeformed trailing edge. For example, an initial fault-propagation fold may later develop into a fault-bend fold merely due to changes in fold geometry rather than having to first lock up and later fault through as a younger fault-bend fold as has been previously suggested. The presence of a narrow footwall syncline, including large overturned blocks, beneath a large scale, overthrust fault, such as the Pine Mountain thrust, suggests an early history of fault-propagation folding. The exact manner by which the Pine Mountain thrust ultimately turned into a large scale fault-bend fold is not clear.