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A traverse across the central Idaho-Wyoming fold-and-thrust belt from the Paris thrust (west) to the Green River Basin (east) has revealed the presence of three distinct regional rock strain patterns. Within the carbonates of the six major thrust sheets, mechanical twins in calcite have recorded an early, layer-parallel and thrust transport-parallel strain and a later, synthrusting non-layer-parallel strain. Synorogenic calcite veins, also twinned, preserve a third unique strain sequence. Calcite strain magnitudes range from 2 to 16% and are highest in vein material; differential stress magnitudes, inferred for the time of twinning, are near 100 MPa for each of the three strain fabrics.

The early, prethrusting, layer-parallel twinning fabric can be used to interpret the paleo-stress and strain fields at the time of initial oblique plate convergence along the western margin of North America. Continued oblique convergence from the western dextral margin (late Jurassic to Eocene) resulted in west-to-east thrust translation and the formation of a tectonic wedge. Using modern transpressive analogues, I estimate that the ratio of dextral fault displacement to thrust shortening was approximately 7:1 for the crustal shortening associated with this fold-and-thrust belt, and calculations can be made concerning general thrust sheet piggyback rotations along the margin.

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