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Frontal ramps constitute vital clues to the mechanics of low-angle thrusting, but their origins have not yet been established. Several critical elements of the kinematics, however, now have been deciphered for a classic locale of the Wyoming thrust belt. They demonstrate that the frontal ramp of the Hogsback thrust sheet in the Kemmerer region might have developed by shear fracture, and that the origin of the ramp is not related to the folding of the Lazeart syncline, even though this major fold is adjacent to the ramp.

Palinspastic analyses of highly constrained balanced cross sections demonstrate that the Lazaert syncline almost certainly originated during thrusting at the leading edge of the Absaroka thrust. The east limb was nearly horizontal when the west limb was overturned by Absaroka thrusting. The Lazeart syncline subsequently was carried piggyback as the upper plate of the Hogsback thrust moved over the frontal ramp. The east limb of the syncline then was rotated parallel to the frontal ramp by displacements along upper plate imbricates in the ramp region. Almost all of these imbricates formed in the break-back sequence; i.e., sequentially in the hinterland direction. They formed at the ramp when fault slip of the Hogsback plate ceased or decreased substantially along the flat east of the ramp; i.e., locking of the upper plate occurred within the ramp region itself after part of the plate had moved over the ramp. Locking at frontal ramps appears to be a potentially important phenomenon in the mechanics of low-angle thrusting.

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