What About the Slip? Examining the Influence of Frictional Layer-Parallel Slip on Fault-Related Fold Geometry

Hodge, Austin M.; Johnson, Kaj; Douglas, Bruce

The complex relationship between folding and faulting in global fold-and-thrust belts has led to the development of a number of models, both two and three dimensional, aiming to accurately pinpoint the nature of faulting and the origins of various fold geometries. These fold geometries range from very broad, rounded fold forms to extremely localized, sharp-hinged, flat-topped folds.

Previous studies have used different modeling styles to characterize the varying types of fault-related folds. The two main categories of modeling, kinematic/geometric reconstructions and mechanical models, both have their respective advantages and disadvantages. Previous work has been focused more towards kinematic/geometric constructions, with less attention paid to the mechanical modeling methods that take into account the physical conditions that form fault-related folds. However, minimal work has explored what effect flexural layer-parallel slip has on mechanical models of fault-related folds.

For the purposes of this study a code, Boundary Element Analysis of Flexural Slip (BEAFS), was developed as a way to recreate multiple fold form geometries by varying a range of physical conditions, thereby helping to shed light on the role of layer-parallel slip. BEAFS is a code that allows for the deformation of an elastic layered medium undergoing frictional slip. A dipping fault is embedded within the medium with a lower detachment and/or an upper detachment. Initial work with BEAFS has been aimed towards defining the physical conditions responsible for the formation of end-member typed fold geometries. The physical conditions that can be manipulated are the thickness and number of layers, coefficient of friction (range 0-0.8), and initial uniform differential stress (horizontal-vertical) before folding (range 0-10MPa). The study has determined that sharp-hinged, flat-topped folds tend to form when the elastic layers are thin, under frictional layer slip, and have high initial uniform differential stress (>5MPa). Broad, round folds form when the layers in the medium are thicker and/or are under frictionless slip between the layers. When layers are allowed to slip, localized folds tend to form, but when layers are bonded together broader folds form. Continuing work will be focused on using BEAFS to try and recreate folds seen in seismic reflection data to better understand the role of layer-parallel slip.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013