Abstract

The kinematics of detachment folds have been described by three different models: (1) hinge migration is responsible for fold amplification, (2) fold amplification is due to limb rotation, and (3) both hinge migration and limb rotation cause fold amplification. A numerical method is proposed to determine which of these mechanisms is responsible for the formation of natural detachment folds. This procedure consists of measuring and plotting geometric data collected from cross sections constructed across the termination of a fold where shortening dies out laterally, or in an area with a lateral shortening gradient. Assuming that observed spatial variations in fold geometry reflect temporal geometric evolution, the procedure allows determination of equations that govern the kinematics of the particular detachment fold analyzed. To validate the results obtained from the application of this technique to natural examples, they must be contrasted with other indicators of fold-amplification mechanisms such as microstructures, mesostructures, and syntectonic sediment patterns. This analysis is applied to an asymmetric growth fold, the Mediano anticline in the Southern Pyrenees of Spain, and shows that it grew due to limb rotation and minor hinge migration. These data, coupled with analysis of the growth stratal patterns using reverse and forward modeling techniques, are used to derive deformation rates and to display the kinematics of this fold.

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