Abstract

The Charleston transverse zone of north-central Utah is an east-west–trending corridor of faults and folds that forms the boundary between the Provo salient of the Sevier fold-thrust belt and the Uinta-Cottonwood arch. The zone trends nearly 90° to the regional structural grain of the fold-thrust belt. Structural analysis and mapping in American Fork Canyon, near the western margin of the Charleston transverse zone, demonstrates that the zone contains an array of low-angle thrust faults and high-angle reverse and normal faults. Crosscutting relations indicate that the low-angle thrust faults formed first, followed by the reverse faults and possibly selected normal faults, and then by the majority of normal faults. Locally, normal faults reactivate reverse faults. The overall pattern of faulting and folding in the Charleston transverse zone suggests that the zone is a left-lateral strike-slip flower structure superimposed on an imbricate thrust fan. Trends of normal faults in the transverse zone suggest that many formed during a later phase of oblique extension that had a component of right-lateral shear. Our results support a tectonic model in which the Charleston transverse zone initiated during the Sevier orogeny to accommodate along-strike changes in the geometry of the thrust belt, caused in turn by along-strike changes in thickness of the stratigraphic sequence involved in thrusting. Thrust sheets in the Provo salient to the south of the Charleston transverse zone moved farther toward the foreland than did thrust sheets to the north of the zone, so the Charleston transverse zone evolved into a left-lateral strike-slip accommodation zone. Preliminary paleomagnetic data support this proposal: they suggest that fault blocks within the Charleston transverse zone locally rotated counterclockwise about a vertical axis. Laramide uplift of the Uinta-Cottonwood arch tilted structures of the Charleston transverse zone to the south and may have locally reactivated faults within the zone. Subsequent extensional tectonism reactivated the Charleston transverse zone with an oblique component of right-lateral shear.

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