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James C. Coogan1

(1) Anschutz Exploration Corporation, Denver, CO

ABSTRACT: Duplex Thrust Systems in the Wyoming-Utah-Idaho Thrust Belt: Prevalence, Hydrocarbon Potential, Kinematics, and Implications for Wedge Mechanics

Structural syntheses of the Wyoming-Utah-Idaho thrust belt usually emphasize large-scale fault bend folding as the primary structural style across the belt. However, seismic, deep drilling, and surface data demonstrate the prevalence of duplex structures in the east half of the belt.

Five examples of large-scale duplex zones are presented: (1) the northern Crawford thrust system near Meade Peak, Idaho, with basal Cambrian floor and Jurassic salt roof thrusts; (2) the southern Crawford thrust system near East Canyon, Utah, with basal Triassic floor and Jurassic salt roof thrusts; (3) the southern Absaroka thrust sheet near Anschutz Ranch, Cave Creek, and Anschutz Ranch East fields, Utah, with Lower Paleozoic floor and basal Triassic roof thrusts; (4) the central Absaroka thrust sheet beneath the Tunp Range, Wyoming, with basal Cambrian floor and upper Ordovician roof thrusts; and (5) the central Hogback thrust sheet east of Kemmerer, Wyoming, with basal Cambrian floor and upper Paleozoic roof thrusts.

These examples illustrate that simple, large-scale fault-bend fold interpretations do not adequately describe producing or prospective regional anticlinal trends, most notably in the southern Absaroka thrust sheet. In addition, the duplexed cores of these anticlinoria are often structurally compartmentalized into many small prospective closures, some of which occupy structurally low positions. Finally, most of these duplexes exhibit late-stage, break-back slip at the rear of major thrust sheets, which sometimes alternates with slip along the thrust fronts. Such alternating slip events may represent wedge taper adjustment at the scale of the individual thrust sheets, with uplift in the rear of a thrust sheet driving the local wedge to critical taper for basal sliding.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado