PHYSICAL MODELING TO ESTABLISH CONTROLS ON FAULT DEVELOPMENT IN LARAMIDE STYLE FAULT-RELATED FOLDS

This study uses scaled physical models of clay overlying rigid plastic strips, to simulate less competent sedimentary rocks folding and fracturing over a reverse fault in the underlying rigid, crystalline basement. The purpose of the study is to establish relationships between the 3-D geometry and slip distribution of the basement fault, and the architecture of minor fault systems that develop in the sedimentary cover rocks. Our experiments specifically examine how minor fault system architecture evolves during the 3-D growth of the fault-fold system. Establishing the evolution of fault system architecture is important because as the fault-fold system evolves, different regions of the fold experience varying amounts, orientations, and sequences of stress and strain. Consequently, the orientation of minor faults and the overall intensity of faulting are likely to vary in space and time during fold growth.

The modeling apparatus allows us to vary four different fault parameters: fault dip, fault displacement profile along strike, fault shape, and fault obliquity. By holding any three parameters constant, and varying the fourth, we can determine the effects of each variable on the minor fault pattern in the overlying clay. We used a 3600 cm², 3 cm thick, homogenous clay layer, with a density of 1.8 gm cm^-3 for each experiment. Horizontal displacement rates ranged from 20 - 70 mm hr^-1. Deformation styles for different fault parameters are determined through analysis of surface faulting and strain markers in the clay throughout the evolution of the fault-fold structure.

Preliminary experiments have been performed using variable fault obliquity, and displacement profiles. Experiments with variable obliquity show maximum faulting intensity occurring from 30 to 45 degrees of obliquity. The difference in orientation of minor faulting in relation to the strike of the basement fault is also greatest at 30 to 45 degrees obliquity. For the experiments with variable displacement profiles, a second set of faults develop in response to the lateral growth of the basement fault. These faults form at the sides of the developing fold in the footwall, and are eventually overlapped by the primary fault set that forms subparallel to the strike of the basement fault during further evolution of the fold.