Computerized tomographic analysis of displacement trajectories and three-dimensional fold geometry above oblique thrust ramps

Computerized tomographic analysis of physical models of fold-thrust structures permits nondestructive, systematic analysis of variations in displacement trajectories and variations in three-dimensional fold geometry as a function of thrust-fault geometry. We studied models in which sand layers with embedded transport-parallel marker lines were shortened incrementally up to a maximum of 24% over rigid, footwall fault templates. These templates simulated a right- stepping oblique ramp connecting two ramps perpendicular to the shortening direction. In response to movement over the oblique-ramp templates, overturned folds formed and marker-fine deflections indicative of out-of-plane strains developed. Marker lines within layers originally truncated by fault templates were deflected leftward away from the right-stepping oblique ramp during deformation, whereas marker lines within sand layers that originally extended across the upper flat of the fault template were deflected in a more radial pattern. Differences in marker-line deflections imply that displacement trajectories within natural thrust sheets may vary as a function of overburden and structural position. Furthermore, our tomographic studies graphically demonstrate that out-of-plane strain and differential rotation of layers within thrust sheets occur in the vicinity of oblique ramps.