Deformation behavior during blind thrust translation as a function of fault strength

Deformation partitioning between translation and distortion for a thrust sheet and for the roof sequence above a moving thrust horse were analyzed in terms of the role of fault strength using finite element models. Greater frictional resistance on the thrust décollement decreased fault translation, increased distortion of the thrust sheet and roof sequence, and increased the ratio of backthrusting to forethrusting for the roof sequence. In all models with weak décollements (μ<0.35), the ratio of tip-line propagation to thrust displacement is very high during initial thrust movement, but decreases significantly during later fault motion. This observation supports interpretations in natural thrust systems where thrust faults propagate large distances before accumulating significant displacements. The tip lines for weak décollements also propagated into the foreland well in advance of the distortion front in the overlying thrust sheet or roof sequence. This result does not support interpretations from natural systems for the existence of `ductile beadsʹ in front of propagating thrusts, but rather suggests that recent models for fault-arrest or displacement-gradient folds may be mechanically valid. In addition, no model for roof sequences yielded a simple end member response of only backthrusting or forethrusting, suggesting that the many blind thrust belts where only one response has been interpreted, may need to be re-examined. Finally, in those cases where more than one thrust flat moves simultaneously in the models, partitioning of displacement and distortion in space and time changes significantly. This result supports interpretations from natural systems where more than one thrust is thought to move at once.