EVALUATING THE RELATIVE CONTRIBUTIONS OF MECHANICAL AND SURFACE CONDITIONS ON THE DEVELOPMENT OF SHORTENING STRUCTURES

The individual structures that accommodate shortening in fold-and-thrust belts exhibit a wide variety of styles that reflect the mechanical behavior and external conditions of the rocks that are being deformed. Understanding the factors that determine the variability in structural styles that occur naturally during crustal shortening is essential to many practical applications, including petroleum geology, earthquake hazard assessment, and regional tectonic studies. The role and relative contributions of different aspects of the mechanical strength, layering, and boundary conditions of a shortening system are investigated through a series of discrete element models of fold and thrust belts. Modeling emergent contractional structures with this method demonstrates that a.) the major different styles of shortening structures can all be reproduced under different mechanical circumstances within the range of realistic mechanical conditions, and b.) different aspects of the mechanics of the deforming rock units, such as peak strength, strain weakening, layer strength anisotropy, and conditions at the free surface, such as erosion and syntectonic sedimentation, exert various degrees of control on the styles of structures that develop in the models as shortening progresses. Observations of the distribution of stress and strain throughout the resultant structures demonstrate the degree to which flexural slip, second-order structures, and localized shear contribute to the development of different overall structural styles. These analyses inform our understanding of the relative importance of these different factors in determining the primary and secondary controls on structural styles in fold and thrust belts.