NEW TECHNIQUES IN PHYSICAL MODELING OF BASEMENT-INVOLVED FAULT-RELATED FOLDS

We have recently designed and constructed a unique physical modeling apparatus that allows us to investigate numerous variables associated with the three-dimensional growth of basement-involved fault-related folds. The apparatus has a 1.5 square meter modeling area, can utilize a variety of modeling materials to simulate the sedimentary cover rocks, and is driven by an electronically controlled, programmable actuator whose advance rate can be varied from roughly 5 - 100 mm per hour. The novel part of the machine lies in the use of 5 cm tall by 6 mm thick strips of rigid plastic as the "basement" material. During an experiment, clay is placed over a series of plastic strips placed side-by-side on their edges to simulate the rigid crystalline basement in nature.

Pre-existing basement faults of different dips are simulated by cutting each plastic basement strip at a specific angle. Using dozens of identical strips laid adjacent to one another, we can create simple faults with constant along-strike geometries. By simply cutting adjacent strips at slightly different lengths or different angles, we can similarly create complex fault geometries whose strikes and dips may change substantially along strike.

The basement fault in a model can be subjected to a specific displacement profile by using specially shaped indentors to apply different amounts of horizontal displacement to each individual basement strip. By doing this, we can model the effects of lateral fault growth on the deformation of the cover rocks.

By using basement strips of different lengths we can simulate different amounts of backlimb rotation that occur as a basement block is transported along the model fault plane. This type of rotation has often been recognized as an important component of the basement-involved faulting process, but the influence of different backlimb rotation styles on cover deformation has never been systematically investigated.

We are presently conducting a variety of experiments on basement-involved fault-related folding. Our long-term goal is to determine what basement fault parameters are significant in controlling the pattern of minor faulting and fracturing that occurs in the sedimentary cover.