Kinematic analysis of a large-scale leading edge fold, Lost River Range, Idaho

Abstract

The Mahogany Creek-Buck Creek structure within the Lost River Range, Idaho, is a large-scale, leading edge fold which developed in late Paleozoic outer shelf stratigraphy during the Sevier orogeny. Incremental strain histories determined from antitaxial fibrous pressure shadows are used to quantify temporal variations in the magnitude and orientation of elongation as a function of structural position around the leading edge of this northward and eastward propagating blind thrust. Plane strain is indicated by coaxial, up-dip extension on cleavage planes. On the backlimb of the structure, the Bluebird Mountain Formation, a calcareous sandstone, has deformed by flexural-flow whereas the forelimb of the structure records spin through a fixed steeply plunging incremental extension direction. The flat limb ahead of the anticline exhibits top-to-the foreland simple shear. In contrast, the underlying Surrett Canyon Formation, a thick-bedded limestone, has experienced flexural-flow localized within thin zones on both limbs of the anticline. Samples from the anticlinal hinge zone suggests kinematic partitioning with pin lines which were variably distributed within each unit and which were temporally transient. Finite elongations are greatest in the hinge and above the foreland flat in the Bluebird Mountain Formation but are negligible in the Surrett Canyon Formation, except within the mechanically active interbeds of the hanging wall and within the footwall adjacent to the fault along the northernmost exposures. Strain histories in the hanging wall do not vary significantly along strike, while the footwall varies from undeformed in the south to penetratively deformed in the north. Thus, the strain data are consistent with self similar along-strike fold development in the hanging wall during strike-parallel ramp propagation. Forward thrust propagation may have been blunted by a combination of fold-accommodated shortening and footwall deformation to the north. Once the fold had tightened and fold shortening required greater differential stress, forward fault propagation resumed, transporting the anticline onto the upper flat with little change in hanging wall fold geometry or strain distribution.