The formation of steeply-plunging folds in fold-and-thrust belts

The Selwyn fold-and-thrust belt between the Stewart and Nadaleen rivers, Yukon, Canada, has several features that are rare in thrust belts, such as steep oppositely-dipping thrusts and steeply-plunging folds, the latter of which are important controls on Au-mineralization in the area. This paper describes fold and fault geometries and cross-cutting relationships to determine how the steeply-plunging folds formed. Fold analysis and field cross-cutting relationships indicate that the folds formed through buckling and pre-date thrusts. Earlier buckle folds with hinge lines oblique to the bulk shortening direction back-rotated on later thrust imbricates to their steep geometry. We suggest that the geometry of early folds was partly controlled by vertical axis rotations in the fold train caused by shortening against an irregular basin margin. The basin margin also acted as a backstop against which the thrust system shortened. This shortening resulted in over-steepening of the thrusts until they were too misoriented for further slip, at which point back thrusts formed and cut the fore thrusts. Continued shortening steepened both fore and back thrusts.