Model Studies of the Displacement Transfer Associated with Overthrust Faulting

ABSTRACT The presence of displacement transfer zones within the Rocky Mountains of Canada has been discussed by a number of authors including Douglas (1958) and Dahlstrom (1970). Examination of displacement transfer zones through the medium of the scale model supports this concept. The two model types presented represent a simple fault-to-fold transition and a combination fault-to-fault-to-fold transition. They reveal that displacement can be transferred from a thrust fault to an adjacent concentric fold through a zone where the fold is conical in form. Fault-to-fault transitions do not require the presence of a fold structure provided the faults overlap sufficiently in their aerial extent. Trajectories of maximum and intermediate principal stress appear to fan out along the leading edge of a thrust sheet in order to remain perpendicular and parallel to the thrust fault, respectively. As a result, tear faults that form near the terminal zones of thrust faults appear to form at anomalously low angles to the transport direction. In fault-to-fault transitions, the upper, older thrust may become folded if the underlying thrust buckles into a simple fault-to-fold transition type of structure at its terminus. This process is a slight variation of one process proposed by Dahlstrom (1970) for the formation of folded thrusts. In the models the folding episode and faulting episode are concurrent, a fact which could support the syn-thrust hypothesis for the timing of the folding episode in the Rocky Mountains. However, the structural geometry of the fault-fold transition is the same as would be expected from the hypothesis of pre-thrust formation of low amplitude folds.