RETHINKING THRUST PROPAGATION IN CARBONATE ROCKS

Thrusts commonly propagate step-wise through sedimentary sequences to produce three classes of folds: 1) hanging wall ramp anticlines (fault-bend folds), 2) fault-cored (fault-propagation) folds, and 3) upright detachment (lift-off) folds. However, complex fault-fold geometries in many thrust belts do not fit these simple models. Among the observed geometries not accommodated by commonly applied models are 1) horses of overturned strata entrained along thrust planes, 2) recumbent, isoclinal, anticline-syncline fold pairs, and 3) duplexes of upright to recumbent isoclinal folds. Paleozoic carbonate rocks of the Sawtooths and adjacent ranges, Montana, display all these anomalous structures.

The spatial variation of observed geometries suggests the following stages in the evolution of at least some major thrusts in the Sawtooths. 1) Deformation in the Cambrian and Mississippian carbonate sequences began as isolated detachment or fault-propagation folds. 2) As shortening continued, each anticline rolled forward in a manner analogous to a tank or caterpillar tread, overriding underlying strata to produce a pair of isoclinal recumbent folds, anticline lying upon syncline. For each fold pair, a thrust terminated in the core of the anticline and a thrust propagated upward from the core of the underlying syncline. 3) Eventually, the thrust in the core of the recumbent anticline cut upsection along the fold’s axial plane to merge with the thrust emanating from underlying syncline. Simultaneously, the synclinal thrust cut down section hindward to merge with the thrust coring the anticline. Thus, the two thrusts merged to enclose a slice (horse) of overturned strata. 4) As thrust displacement increased this envelope of overturned strata was diced into horses strung out along the thrust surface. 5) As these horses were diced finer, most evidence of the overturned limb was destroyed, leaving hanging wall and footwall cutoffs that misleadingly fit a simple ramp-flat thrust-fold model.

Although the ramp-flat model seems to work well to explain many thrust-fold geometries, structural variation in Paleozoic carbonate rocks of western Montana thrust belts suggests that many thrusts evolved from upright to recumbent isoclinal fold nappes.