ALONG-STRIKE VARIATION IN FOLD-THRUST BELTS CAUSED BY PRE-EXISTING FAULTS: INSIGHTS FROM ANALOG MODELS

Along-strike changes in fold-thrust belts can be expressed by the clustering of branch lines between the thrust sheets. This study presents results from analog models, where pre-existing heterogeneity, simulating existing faults in the lower cover, is introduced into an otherwise homogeneous sand pack. The sand pack is scaled to represent bulk upper crustal properties, and compressed. Varying obliquities and geometries of the pre-existing heterogeneities were used, relative to the imposed compression. A final model in the sequence used a ductile layer to separate the faulted and un-faulted sand-packs. During analysis, the resultant development of thrust fault traces and branch lines in map view was tracked. After completion, cross-sections were also analyzed. The models show that faults confined to the base layer promote the clustering of branch lines in the surface thrusts. Strong clustering in branch lines is also noted where several faults are in close proximity or cross. Complexity in the surface thrust sheets is also noted where the faulted and un-faulted sections are decoupled by a ductile layer. Slight reverse-sense reactivation of faults cut through the sedimentary sequence is noted in cross-section, where the fault and the subsequent thrust belt interact. An example is presented for the southern Sawtooth Range in Montana, where the fold-thrust belt interacts with the Scapegoat-Bannatyne trend of the Great Falls Tectonic Zone. Model results and observations of thrust geometry based on map analysis were combined to predict the location of discrete fault strands within the Scapegoat-Bannatyne trend. The discrete deep-seated faults partition the overlying fold-thrust belt into zones. Thrust sheets in each zone have similar characteristics and are often separated from thrust sheets in neighboring zones by branch lines. In this area, the deep-seated faults are more likely to be reactivated as strike-slip features in nature, given the small obliquity between the compression direction and the faults. More generally, these results can be used to govern evaluation of thrust belts deforming over faulted basement, and to predict the locations of specific fault strands and fold-thrust belt zones in a region where this information is unknown.