Tracking Fault History in Sandbox Models
Dan Davis, Stony Brook University
Saad Haq, Purdue University
Christopher Grady, Stony Brook University
In recent years, the non-Coulomb behavior of sand has been mapped out (e.g., Lohrmann et al., 2003), with initial failure governed by a peak stress and subsequent shear taking place at a lower, stable stress. Similar behavior is observed in natural sedimentary rocks such as those found in fold and thrust belts. We have used particle image velocimetry (PIV) analysis of imaged analog modeling experiments map the strain rate field throughout a series of modeling experiments in the Haq modeling lab at Purdue. These data allow measurements of the dip angles of the series of thrust faults, including occasional out-of sequence thrusts and backthrusts.
In this poster we describe the results from one such experiment, for which we determined the orientation history of each forward thrust that formed. We find that successive forward thrusts in the experiment experience similar evolutionary cycles: initiation at a steep angle followed by shallowing with time until a new active forward thrust is generated at the front of the wedge. At that point slip ceases and the fault (shear) ceases to slip, steepens, and undergoes repeated periods of reactivation.
Prior to a distinct point in the experiment, a large portion of the nascent wedge is not near failure. During this early stage, each new thrust experiences a similar, stable evolution with a dip at the time when slip ceases within 10° of the dip at the time of fault initiation. Starting at a distinct point in the experiment, however, a critical wedge grows, with each thrust forms at a near-uniform steep dip prescribed by the internal friction before experiencing a steady decline to about 7°. This dip range appears to be defined by the nearly ideal Coulomb slip criteria of the wedge and its base. Other, related work demonstrates that the initial formation of pop-up structures at the model deformation front are consistent with the Coulomb criterion, but that the basal boundary condition becomes important only after a small but finite amount of shortening in the structure.
Preliminary results are consistent with fault evolution governed by the constraints on the stress field of simultaneous failure on the thrusts and the basal detachment. It is also consistent with mechanical behavior of the deforming sand that is not simply Mohr-Coulomb, but rather one that is initially strain-weakening.