EFFECT OF DEFORMATION RATE ON FAULT POPULATIONS

We used scaled clay models to investigate how deformation rate affects the geometry, size, and spacing of faults. In the models, a layer of wet clay covered a rectangular latex sheet. Displacing one of the long edges of the latex sheet deformed the sheet and the overlying clay layer. We varied the angle (alpha) between the deformed-zone trend and the displacement direction in 15-degree increments from 90 degrees (regional orthogonal extension) to 0 degrees (regional shearing). Independently, we varied the displacement rate from 3 to 36 cm/hr. For all alpha angles, as the displacement rate increased, the number of faults cutting the top surface of the clay generally increased, the maximum displacement on these faults generally decreased, and the fault spacing generally decreased. Regardless of displacement rate, normal faults developed in models with alpha greater than 30 degrees, and two sets of oblique- to strike-slip faults developed in the other models. In these latter models where two sets of faults developed, one set of faults was subparallel to and had the same sense of shear as the deformed zone, and the other set of faults was at a high angle to and had the opposite sense of shear as the deformed zone. For alpha angles of 30 degrees and less, the displacement rate also affected the geometry of the faults. Specifically, oblique- to strike-slip faults that were subparallel to and had the same sense of shear as the deformed zone became more dominant as the deformation rate increased.

The modeling results, if applicable to nature, suggest that deformation rate affects fault-population characteristics. In the Triassic Danville rift basin of eastern North America, lacustrine strata contain two populations of normal faults. Set 1 consists of widely spaced meter-scale faults; Set 2 consists of much more closely spaced centimeter-scale faults. Set 2 faults are ubiquitous except for the stress-reduction zones surrounding Set 1 faults. Thus, Set 2 faults formed after Set 1 faults. Because both sets of faults cut the same rocks and formed after lithification but before tilting, we infer that the deformation rate was lower for Set 1 faults and higher for Set 2 faults.