MECHANICAL DIFFERENCES BETWEEN LIFT-OFF FOLDS AND DISHARMONIC FOLDS: EXAMPLES FROM THE SEVIER FOLD-THRUST BELT, WESTERN SAN RAFAEL SWELL, UT AND THE HUDSON VALLEY FOLD-THRUST BELT, NY

Detachment folds are one of the three types of fault-related folds that occur in fold-thrust belts. They are divided into two categories based on geometry: lift-off folds and disharmonic folds. This study compares strength measurements of layers within the folds from outcrops of both subtypes in an attempt to determine the influence of mechanical properties of the rocks on fold form. Massive gypsum beds with a lower detachment in gypsiferous clay in the Jurassic Carmel Formation from the foreland-most extent of the Sevier fold-thrust belt exposed along the western edge of the San Rafael Swell, UT show lift-off detachment folds with double-kink (box) fold geometry in down-plunge projection (DPP). In the Devonian Esopus Formation in the Hudson Valley fold-thrust belt, DPP of folds in quartz-rich sandstones with calcareous matrix and a lower detachment in shale show disharmonic detachment folds with concentric geometry. To address these differing geometries, strengths of folded units were measured. For competent units, strengths were measured with a Schmidt hammer and recorded as rebound numbers. In incompetent units, strengths were obtained with a pocket soil penetrometer and recorded in tons/ft². Values of compressive strength from both were converted to N/mm² for direct comparison. Results show that generally, the sedimentary section in UT is weaker than that in NY because of the lithological differences. Additional patterns emerge when looking at strength ratios. In both cases, there is a significant difference between the strongest and weakest layers. However, the disharmonic folds show a sharp competency contrast while the lift-off folds show more of a range of strengths between the strongest and weakest layers. This suggests that competency contrast plays a role in influencing fold form.