Publications
The stiff layer excess area method for the analysis of variable thickness folds, GSA Abstracts with Programs (2000 Annual Meeting), P. Geiser
Restoration is a critical tool for the structural analysis of folds. Quantitative restoration techniques based on geometric and kinematic models for the deformation process, e.g., flexural flow and vertical/oblique slip are inadequate to describe the material redistribution in most variable thickness folds. This paper, utilizing three end member processes for variable thickness deformation: Internal Flow Folding (IFF), Differential Shortening Folds (DSF) and Volume Change Folds (VCF), describes a new technique for the restoration of such folds. The end member mechanisms are described by a set of structural elements: two "boundary" surfaces, two "stiff layers" of thickness t/2 where t is the minimum thickness between the boundary layers, and a medial "variable thickness layer" between the bounding "stiff layers". The basis of the analytic restoration technique is the combination of this fold geometry representation with the kinematics of the processes. Analyzing the folds individually and as part of fold trains provides additional constraints. IF and VC folds are modeled by two stiff layers with the medial variable thickness layer being extruded between them to form regions of "excess area". DSF folds are described by the two stiff layers with regions of tectonic thickening forming "excess area" between them. The method is illustrated using two sets of outcrop scale folds: 1] a multilayer consisting of relatively stiff calci-siltites interbedded with calcareous shales where the shales and calci-siltite layers have roughly the same thickness; and 2] intersecting kink bands formed in a sequence of thin to medium bedded calci-siltites separated by shale lamina. In 1, deformation occurred by both IFF and DSF with initial variation in layer thickness nucleating some of the finite amplitude folds. In 2, deformation occurred primarily by VCF with as much as 34% localized volume loss by pressure solution. Analysis of thickness gradient configurations of individual fold layers permits calculation of the deformed state area. Comparison of the predicted area change with the measured deformed state area allows determination of the initial layer gradient geometry.
