Abstract

Archean and Proterozoic basement was involved in foreland thrust faulting during the Sevier orogeny in northern Utah. Mesozoic structures in the Farmington Canyon Complex provide evidence for the mechanics of crystalline thrust plate transport and emplacement. Structures formed during the last stage of transport of the Farmington Canyon Complex over folded but mostly unmetamorphosed Paleozoic strata occur in the upper plate of the Ogden thrust fault, which was emplaced by cataclastic flow. Intraplate strain was accommodated by brittle faulting, and new greenschist facies minerals formed during hydro-thermal alteration. The Paleozoic strata directly below the thrust fault were ductilely deformed.

The upper thrust plate was buckled into an anticline-syncline fold pair as it moved eastward along the Ogden thrust fault. The fold axes were oriented subparallel to the eastward direction of tectonic transport. Shortening at a high angle to the fault plane and eastward extension occurred in the cataclastic gneiss of the upper plate and the underlying sedimentary tectonites. These strains developed as the result of progressive simple shear during overthrusting. Strain due to buckling of the upper plate was superimposed on the simple shear strains in the cataclastic gneiss and caused lateral variations in the three-dimensional strain field within the thrust plate. Constrictional strain developed in the anticlinal hinge area of the thrust plate, whereas flattening strain occurred in the synclinal hinge zone.

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