--> Abstract: New Excess-Area/Depth-to-Detachment Relationship for Fold-Thrust Structures, by R. H. Groshong, Jr. and J. L. Epard; #91012 (1992).

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ABSTRACT: New Excess-Area/Depth-to-Detachment Relationship for Fold-Thrust Structures

GROSHONG, R. H., Jr., University of Alabama, Tuscaloosa, AL, and J. L. EPARD, Universite de Lausanne, Lausanne, Switzerland

The excess area in a fold is the cross-sectional area between the original elevation of a horizon and its deformed position. The magnitude of the excess area is a function only of the depth to detachment and the displacement on the detachment. Previous depth-to-detachment calculations, based on the assumption of constant bed thickness and length, have been shown in some cases to give erroneous results or to imply kinematically impossible folds. The new relationship is a line on a graph of excess area versus depth to a reference horizon. Excess area from a minimum of two horizons is required to find the depth to detachment. Data from multiple horizons provide a least-squares estimate and a check on the consistency of a section. An area-balanced detachment fold is a straight line with s ope equal to the displacement on the lower detachment and a depth intercept at the true depth to detachment. Structures following this relationship include detachment, fault-propagation and lift-off folds, and some duplexes. A fault-bend fold is a straight line through the basal detachment level for beds below the upper detachment and a line of reduced slope for beds above the upper detachment. The slopes give the displacements on the lower and upper detachments respectively, and the break in slope locates the upper detachment horizon. Interpretations are illustrated with excess-area diagrams of modeled sections and with natural oil-field scale structures from the Appalachians and the Rocky Mountains.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)