Abstract: Creating Restorable Models of Contractional Basement Structures Using Lithology-Specific Deformation Mechanisms

Peter H. Hennings

Existing quantitative models of the development of asymmetric, basement-cored, foreland structures either focus on the kinematic compatibility of specific parts of the structure or describe the deep fault shape in basement, without making the critical link to the geometry of the over-lying sediments. No current theory can produce a fully restorable model for the entire structure.

A restorable model can be produced if an appropriate deformation, algorithm is assigned to each lithotectonic component of the system. For competent stratigraphic units with internal layering it is appropriate to use flexural-slip, which is applicable except for areas with tectonic thickness changes. Ductile units, and regions of thickness changes, must be modeled using constant-volume general shear. The most appropriate algorithm for brittle basement rocks is simple shear. A model, combining lithology-specific deformation algorithms, will be restorable if there is constant shortening over the entire system.

In practice, the shortening is most easily determined, using seismic field, and well data, for the competent sediments. The angle of simple shear in basement, and the trajectory of the basement fault to depth, are uniquely determined using the shortening, and the geometry of the upper surface of basement in the hanging-wall. The geometry of ductile units is constrained by the shape of basement and the competent sediments.

Forward kinematic modeling using this technique shows that the angle of simple shear changes as the shortening increases. This suggests that shear strain in basement is heterogeneous in the deformed state but evolves in a predictable manner. The method has been used with success to modify and validate the interpretation of numerous foreland structures in Colorado and Wyoming.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana