Abstract

Micrites of the Jurassic Twin Creek Formation in the Idaho-Utah-Wyoming thrust belt show spaced cleavage developed during folding and thrusting. Cleavage in each thrust sheet forms mainly by pressure solution, aided by elevated temperatures and pressures caused by increased overburden as a result of emplacement of earlier overriding thrust sheets. Within each thrust sheet, cleavage develops early and is contemporaneous with microscopic to mesoscopic buckle folds and faults. Large-scale ramp folds, formed during major motion on the thrust, postdate the cleavage and passively rotate it into major fans.

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