Abstract

Sinuous overthrust belts at geosyncline-craton boundaries form a series of salients (convex cratonward) and re-entrants. Structural patterns suggest movements diverging across salients and converging across re-entrants with mass movement toward the craton. Orientation data on bedding, cleavage, thrust surfaces, striations, fold hinges, bedding-cleavage intersections, and striated subvertical fractures were collected at 22 stations on the western Wyoming salient and were plotted stereographically. For each structural element, π-circles, lineation girdles, or planes normal to point maxima were determined statistically, and azimuths of contained horizontal lines were averaged to get the station's mean movement direction. A map plot of the 22 mean movement directions indicates about 70° of divergence along the salient from Idaho, through Wyoming, and into Utah.

The statistical center of divergence is in northern Oneida County, Idaho, where uplift with erosion has beveled the rocks to approximately Precambrian age material. A total Paleozoic and Triassic-Jurassic isopach shows maximum thickness in a local basin in the miogeosynclinal trough to have been near the subsequent divergence center of Cretaceous and early Tertiary overthrust movements. The apex of uplift roughly coincides with the center of diverging movement and the estimated pre-existing depocenter; maximum uplift was greater than 45,000 ft. Present near isostatic balance, subsequent to 45,000+ ft of uplift, suggests isostasy was, in part, the mechanism of uplift and, in accord with isostatic principle, maximum uplift was in the area of greatest depression. Inasmuch as overthrust plates on the salient apparently do not involve crystalline basement rock, origin of the salient is best explained as a radial gravitational-gliding response of the sedimentary sequence to maximum local uplift at the center of diverging movement.

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