ABSTRACT The results of new detrital zircon analyses of 15 (n = 1334) Sevier belt synorogenic (Jurassic–Eocene) conglomerates combined with U-Pb zircon ages from the literature (n = 2638) support the structurally dynamic role of the western Paris thrust sheet as the dominant high-standing, out-of-sequence portion of the Sevier belt. This result requires modification of the traditional structural view of the thin-skinned Sevier fold-and-thrust belt having formed by west-to-east shortening over an ~100-m.y. period (ca. 150–50 Ma) with episodic thrust motions that become younger toward the craton (east), as constrained by numerous synorogenic deposits shed to the east from each thrust hanging wall. Sevier thrusting was preceded by deposition of the Jurassic Stump Formation, which has a maximum depositional age of 149 Ma and a unique detrital zircon and heavy mineral (garnet, magnetite) provenance. The oldest thrust, the Paris (Willard) thrust, eroded and deposited the Jurassic–Cretaceous Ephraim Conglomerate as a synorogenic fan devoid of quartzite clasts and with a detrital zircon provenance consistent with reworked sediment from the fold belt, but not from the hinterland or the Sierra Nevada arc of the orogenic system. All subsequent synorogenic deposits from the mid-Cretaceous Echo Conglomerate (Meade-Crawford thrust) to a variety of more easterly Eocene deposits (Sevier belt, Green River, Absaroka, and Bighorn basins) are rich in quartzite clasts. All the quartzite clasts were eroded from the Paris thrust hanging wall, which reached its peak orogenic height at ca. 95 Ma, 50 m.y. after first motion, and the Proterozoic Brigham Group remained a quartzite clast source for ~40 m.y. The detrital zircon signatures of these samples require additional sources of sediment, reworked from the hinterland and the Sierra Nevada and Idaho Batholith arcs, thus implying that long-distance sediment fairway(s) were active during the Mesozoic–early Cenozoic. Based on the same detrital zircon data, variable sources of sediment are inferred between each of the thrust sheets; however, within each thrust system, the source of sediment remained the same. The Teton Range was thrust up at ca. 50 Ma, long after the Sevier belt formed, and it was not a buttress to thin-skinned Sevier deformation. Rather, Teton–Gros Ventre–Wind River Laramide uplifts deformed the older Sevier belt with numerous back and out-of-sequence thrusts and synorogenic deposits, including the Darby thrust, which records the youngest displacement.

The Hoback Basin of western Wyoming is in the zone of impingement between ranges of the Idaho-Wyoming thrust belt on the west and the Gros Ventre and Wind River foreland uplifts on the east. These ranges shed synorogenic clastic debris that filled the basin during Late Cretaceous and Tertiary time. Structural, stratigraphic, and sediment provenance studies provide detailed information on the chronologic and geometric relation between these two structural styles. Eastward thrusting in the thick sequence of miogeosynclinal rocks to the west began somewhat earlier than did reverse faulting, which uplifted blocks in the adjacent foreland. Subsequently, however, both areas underwent contemporaneous deformation, so that thrust belt and foreland structures overlap geographically and temporally. Furthermore, the last datable movements are the same age for both thrusts and major reverse faults. The Late Cretaceous rise of the ancestral Teton–Gros Ventre uplift and the subsequent rise of the Gros Ventre block provided a high-standing buttress of Precambrian rocks that deflected structures in the eastern part of the thrust belt. Late normal faulting and associated gravity sliding, possibly along a pre-existing, thrust-weakened zone, occurred in the thrust belt, and the Hoback and Gros Ventre Ranges acted as an integral block in spite of their previously distinct structural histories. Normal faulting also occurred in the foreland uplifts and, in some places, brought Precambrian rocks to the surface. Local tectonism and associated basin fill ceased by late Pliocene time in the Hoback Basin, and regional uplift initiated the dissection of both the lower Tertiary sediments and pediments of parts of adjacent ranges. Although the intimate spatial and temporal relations of the thrust belt and foreland structural styles do not require a single casual mechanism, they do suggest that thrusting in the thrust belt and uplift in the adjacent foreland were not entirely genetically separate.