Geometry and kinematics of a passive-roof duplex in the interior of the Idaho-Wyoming-northern Utah thrust belt
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Detailed geologic mapping reveals the presence of a passive-roof duplex in the interior of the Absaroka thrust sheet, central Salt River Range, Wyoming. The passive-roof duplex occurs beneath the foreland-directed Stewart Thrust in the core of the Stewart Anticline. The roof thrust of the duplex occurs along a single bedding plane in a prominent dolomite horizon. Growth of the Stewart Anticline occurred by forward displacement on the Stewart Thrust and delamination of the passive-roof sequence during tectonic wedging. Growth of the tectonic wedge occurred by forward propagation of the backthrust and footwall imbrication. A change in vergence of the Stewart Anticline along strike to the south correlates to a decrease in displacement on the Stewart Thrust relative to that on the backthrust. The Stewart passive-roof duplex constitutes a triangle zone that is different than the triangle zones commonly formed at the foreland margins of thin-skinned fold-and-thrust belts. Differences include: 1) possible late-stage formation in the interior of the fold-and-thrust belt; 2) a passive-roof sequence that is of similar strength and ductility as the intercutaneous wedge; and 3) formation of a roof thrust in dolomite. The identification of the Stewart passive-roof duplex raises the possibility that similar structures may be more common in the Idaho-Wyoming-northern Utah thrust belt than previously envisioned.
