ABSTRACT: Significance of Oblique Ramps in Fold-Thrust Belts: An Example from the Wyoming Salient

APOTRIA, TED G., Shell Development Company, Houston, TX

Thrust ramps oblique to the regional trend in fold-and-thrust belts differ from simple frontal ramps in that they localize displacement transfer and termination, out-of-plane strains, thrust sheet rotation, and modify or enhance fracture and cleavage development. Field and modeling studies are used to determine the 3-D displacements and strains associated with oblique ramps, and their potential economic significance.

The South Fork thrust, a hanging wall imbricate of the Absaroka thrust sheet, southwest Wyoming, has a steeply dipping (~70%), small displacement (~900 ft) oblique ramp in the hanging wall and footwall. The ramp lies along a N75%E trend with oblique structures in the adjacent Crawford and Hogsback thrust sheets. Three stages of deformation were recorded by folding, stylolitic cleavage, and calcite twinning strains. The first is attributed to early east-west layer-parallel shortening. Later stages locally deform both the hanging wall and footwall and suggest northeast-southwest-directed shortening, nearly perpendicular to oblique ramp strike. Near the western terminus of the ramp, obliquely oriented folds in the footwall and normal faulting in the hanging wall suggest out-of-plane flow at the frontal ramp-oblique ramp intersection. In addition, bedding and systematic fracture attitude changes indicate that the hanging wall underwent a quasi-rigid-body rotation as it was pinned near its western terminus.

Kinematic and mechanical modeling studies suggest that at an oblique ramp, principal shortening directions lie in a plane perpendicular to the strike of the ramp while local hanging wall displacement vectors are deflected out of the transport plane. These model predictions are consistent with the observed deformation.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)