Identification of Fold Hinge Migration in Natural Deformation: A New Technique Using Grain Shape Fabric Analysis
Rose, Kelly Kathleen
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Partitioning of finite strains in different domains within the limb and hinge regions of a fold can be used to understand the deformation processes operative during fold formation. Samples taken from the limb and hinge regions of a gently plunging, asymmetric, tight, mesoscale fold in the Erwin formation of the Blue Ridge in North Carolina were analyzed to determine the deformation mechanisms and strains associated with the folding event. Rf/phi grain shape fabric analysis was conducted for each sample and used to calculate the orientation and magnitude of the final grain shape fabric ellipsoids. Flexural folding and passive-shear folding models predict that the highest finite strains will be recorded in the hinge of a fold. The highest grain shape magnitudes recorded in the North Carolina fold, however, lie along the overturned fold limb. The final geometry of many folds indicates that hinge plane migration processes are active during compressive deformation events. Numeric, conceptual, and analogue based studies have demonstrated the migration of fold hinges during deformation. However, documentation of these processes in field based studies is rare and limited to techniques that are frequently site specific. Methods proven successful in natural studies include the analysis of superposed folding; the migration of earlier hinge-related features such as fractures, cleavage planes, and boudinaged bedding planes; and the kinematic analysis of syntectonic pressure shadows. The magnitude and orientation of the grain shape ellipsoids calculated for the North Carolina fold indicate that rocks in the overturned limb were once located in the hinge of the fold. Subsequent noncoaxial deformation processes operative during folding resulted in the migration of the hinge to its present orientation and position. This relationship indicates that it is possible to use strain/shape fabric analysis as a test for hinge migration in folds, and that this technique may be more generally applicable in natural settings than previously proposed tests.
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