David Wiltschko, Daniel Eastman, 1983. "Role of basement warps and faults in localizing thrust fault ramps", Contributions to the Tectonics and Geophysics of Mountain Chains, Robert D. Hatcher, Jr., Harold Williams, Isidore Zietz
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Foreland thrust belts are regions where the stratigraphic section has failed. The close association in the field of thrust ramps and other structures suggests that this failure may not be haphazard. In this paper, we investigate two of these field associations—pre-existing basement warps and faults—and find that they both play a dual role in controlling the locations of thrust ramps. Two-layer, two-dimensional, photoelastic models show that pre-existing gentle basement warps primarily deflect faults by offering a rigid surface which causes the regional principal stress trajectories to bend; the highest stress concentration is about 1½. While also disturbing the regional stress trajectories, the main effect of pre-existing basement faults is to concentrate stress and, thereby, facilitate failure. The strength of the stress concentration above basement faults depends upon the fault angle and may be as high as 2½. Although not directly investigated, these results have three-dimensional implications. First, since failure takes place somewhere within the section above basement faults, faults must propagate down to join the sole fault as well as up to the surface for significant motion to take place, an intuition first arrived at by Kehle (written communication). Second, these results suggest that, once started, a thrust fault may propagate along strike away from these stress concentrations into regions where there is no apparent mechanical cause for the initial break. Consequently, only a few stress concentrations may control the gross architecture of an entire thrust belt since, once started, thrust faults presumably propagate on the strength of their own fault-tip singularity.