Fold axis-parallel rotation within the Laramide Derby Dome Fold, Wind River Basin, Wyoming, USA

Abstract

Derby Dome, a doubly plunging anticline (7×3 km) on the eastern flank of the Wind River Range, Wyoming, trends NW–SE in response to the regional NE–SW directed shortening of the Cretaceous–Eocene Laramide orogeny. Mesozoic sediments are exposed around the fold hinge above an east-dipping thrust fault that offsets Archean crystalline rocks at depth. Stress and strain ellipsoidal data were determined through the measurement of mechanically twinned calcite in limestones (Triassic Alcova through J–K Morrison Formation rocks; 13 samples), calcite cements (5 samples), and synfolding calcite veins (16 samples) around the northern half of the fold. On the outer limbs of the fold the maximum shortening strain axis (−3.5%, 15% NEVs) in the limestones and cements is sub-horizontal, layer-parallel and normal (NE–SW) to the fold hinge reflecting regional Sevier–Laramide shortening. This regional layer-parallel strain fabric is rotated into a fold axis-parallel orientation (NW–SE) near the fold hinge indicating that significant rotations occurred during folding. Synfolding calcite veins, of varying orientations, also preserve a local sub-horizontal, hinge-parallel shortening strain (−4.0%, 17% NEVs), suggesting that the regional Laramide stress and strain field was locally rotated into parallelism with the fold during shortening and displacement on the underlying thrust fault. In both the country rock, cement and vein data sets, the strain overprint noise (NEVs) increases toward the fold hinge. Inferred differential stress magnitudes are also higher for the vein calcite than for the country rock limestones or cements, and there is no interpretable pattern around the fold (avg.=560 bars, range of 240–2000 bars). Fracture measurements ( n=74) in different lithologies have different orientations on each side of the adjacent Dallas Dome Fold suggesting layer-parallel rotation during folding, or active fracturing occurred uniquely on each fold limb.