KINEMATIC ANALYSES AND GEOMECHANICAL MODELING OF AN EXTENSIONAL FAULT-PROPAGATION FOLD

Mechanical stratigraphy exerts a major control on structural style at all scales. Field structural data from the Big Brushy Canyon monocline of west Texas are combined with finite element modeling (FEM) to bridge the gap between geometric, kinematic and mechanical analysis techniques. The monocline is developed in the Cretaceous Buda Limestone above the Del Rio Clay, which is thinned >90%. The underlying Santa Elena Limestone is offset vertically by approximately 74 m along a steep (80º) normal fault. The large fault displacement of the Santa Elena Limestone is not transferred upward to the Buda Limestone because of ductile flow in the intervening Del Rio Clay. Thinning of the Del Rio Clay and the resultant extreme displacement gradient at the tip of the fault have forced the Buda Limestone into a monocline with a maximum limb dip of 45º. The Buda Limestone is composed of two competent limestone packages (6 m and 2.7 m thick) separated by 10.5 m of less competent calcareous shale. Deformation features within the competent Buda beds include bed-perpendicular veins that accommodate bed-parallel extension, and bedding plane slip surfaces with up-dip shear that offset the veins. Deformation is concentrated in the monocline limb rather than the hinge. Finite element models were developed to replicate the monocline geometry and deformation distribution, and to assess the impact of material properties and boundary conditions on structural evolution. Stress and strain evolution were tracked throughout the model so that bed-parallel extension and shear strain could be compared directly to field observations. Iterative FEM runs revealed the importance of benchmarking the model results against both geometry and strain distribution in the natural example to provide a better understanding of the effects of material behavior and boundary conditions. For example, initial modeling using elastic-plastic materials with different stiffness and strength values for each mechanical layer could not match the observed thinning in the Del Rio Clay. To appropriately capture thinning in this unit it was necessary to employ a visco-elastic material model. With this modification, we successfully reproduced both monocline geometry and the distribution of strain and slip.