MODELING PLASTIC DEFORMATION AROUND A THRUST FAULT ASPERITY USING A MODIFIED SQUEEZEBOX DESIGN AND A WAX ANALOG

We have designed and built a new analog modeling rig in order to replicate the kinematics of a plastically deforming body of rock as it encounters a deep fault asperity within a thrust zone by modifying the traditional squeezebox design. While the typical classic squeezebox utilizes sand to demonstrate brittle, Mohr-Coulomb style deformations, our modified squeezebox utilizes a viscoplastic wax analog, consisting of white spherical strain markers “cemented” into a lower melting temperature black matrix, in order to model the ductile behavior of rock within the high temperature and pressure environments of deep thrust fault settings.

Our squeezebox rig is composed of a reservoir, made from 0.25-inch aluminum plating with a molded polyurethane push-plate at one end. A trailer jack, equipped with a stepper motor to ensure a constant displacement rate, drives the push-plate forward. Plastic deformation is facilitated by the addition of heating elements lining the underside and exterior walls of the aluminum reservoir. An asperity, fashioned out of aluminum, is secured to the floor of the squeezebox reservoir. The additional overburden of rock is simulated using water filled bladders resting on the upper surface of the fused wax block.

A complete dataset consists of three separate experimental runs where each of the blocks of wax are sectioned along one of three mutually perpendicular planes. Three-dimensional strain analysis can then employed by tracing a large number of the deformed wax strain markers. These two-dimensional datasets will be combined to yield a high resolution distribution of the three-dimensional strain ellipsoids.

As expected, changing the strain rate and temperature conditions has significant impact on the material properties. Further experimentations are anticipated to document the amount of material flow perpendicular to the compression direction in the vicinity of the asperity, and how exactly that local non-plane strain deformation is ultimately balanced within the larger scale flowing body in a system with nonmoving lateral boundaries. We hypothesize that the major factor contributing to whether slip continues on an existing fault versus the initiation of a new fault is associated with the local strains associated with fault surface irregularities.