ABSTRACT Calcite twinning analysis across the central, unbuttressed portion of the Sevier thin-skin thrust belt, using Cambrian–Cretaceous limestones ( n = 121) and synorogenic calcite veins ( n = 31), records a complex strain history for the Sevier belt, Idaho and Wyoming, USA. Plots of fabric types (layer-parallel shortening, layer-normal shortening, etc.), shortening and extension axes for the Paris thrust (west, oldest, n = 11), Meade thrust ( n = 46), Crawford thrust ( n = 15), Absaroka thrust ( n = 55), Darby thrust ( n = 13), Lander Peak klippe ( n = 5), eastern Prospect thrust ( n = 6), and distal Cretaceous foreland ( n = 3) reveal a W-E layer-parallel shortening strain only in the Prospect thrust and distal foreland. Calcite twinning strains in all western, internal thrust sheets are complex mixes of layer-parallel (LPS), layer-normal (LNS), and non-plane strains in limestones and synorogenic calcite veins. This complex strain fabric is best interpreted as the result of oblique convergence to the west and repeated eastward overthrusting by the Paris thrust.

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.

ABSTRACT Strata preserved within the Sevier foreland basin of North America contain a suite of lithologic variations influenced by hinterland tectonic processes. Using U-Pb detrital zircon geochronology, we compared provenance signals of Upper Jurassic and Lower Cretaceous strata from a west-to-east, foredeep-to-forebulge-to-backbulge depozone transect across the state of Wyoming and evaluated major tectonic mechanisms operating during the early evolution of the Sevier orogeny. Our data included new and compiled U-Pb detrital zircon ages ( n = 6013) from 50 localities that were integrated into a revised chronostratigraphic framework and subsidence history for the basin. At the onset of the Sevier orogeny, we found evidence for uplift and erosion of early Mesozoic and late Paleozoic strata within the nascent Sevier fold-and-thrust belt. This event occurred prior to the Aptian Stage of the Early Cretaceous and is recorded by the coordinated progradation of coarse-grained fluvial systems across the overfilled foreland basin. Continued emplacement of thrust loads in the hinterland generated accommodation in excess of sediment supply, a condition likely exacerbated by a relative reduction of siliciclastic sediment supply due to greater unroofing of Paleozoic carbonates during the Aptian and Albian Stages of the Early Cretaceous. This led to an underfilled condition characterized by widespread calcareous lacustrine deposition across much of the foredeep depozone and condensed stratigraphic intervals in the forebulge and backbulge depozones. During the late Albian–earliest Cenomanian, fluvial systems sourced in the Appalachians invaded the foreland basin from the east, followed by the rapid incursion of the Western Interior Seaway, driven by accelerated thrust emplacement, flexural subsidence, and potentially the onset of dynamic subsidence.

ABSTRACT The highest-grade Barrovian-type metamorphic rocks of the North American Cordillera exposed today are Late Cretaceous in age and found within an orogen-parallel belt of metamorphic core complexes for which the tectonic histories remain controversial. Thermobarometric studies indicate that many of these Late Cretaceous metamorphic assemblages formed at pressures of >8 kbar, conventionally interpreted as >30 km depth by assuming lithostatic conditions. However, in the northern Basin and Range Province, detailed structural reconstructions and a growing body of contradictory geologic data in and around the metamorphic core complexes indicate these metamorphic rocks are unlikely to have ever been buried any deeper than ~15 km depth (~4 kbar, lithostatic). Recent models controversially interpret this discrepancy as the result of “tectonic overpressure,” whereby the high-grade mineral assemblages were formed under superlithostatic conditions without significant tectonic burial. We performed several detailed studies within the Snake Range metamorphic core complex to test the possibility that cryptic structures responsible for additional burial and exhumation might exist, which would refute such a model. Instead, our data highlight the continued discordance between paleodepth and paleopressure and suggest the latter may have reached nearly twice the lithostatic pressure in the Late Cretaceous. First, new detrital zircon U-Pb geochronology combined with finite-strain estimates show that prestrain thicknesses of the lower-plate units that host the high-pressure mineral assemblages correspond closely to the thicknesses of equivalent-age units in adjacent ranges rather than to those of the inferred, structurally overridden (para) autochthon, inconsistent with cross sections and interpretations that assume a lower plate with a deeper origin for these rocks. Second, new Raman spectroscopy of carbonaceous material of upper- and lower-plate units identified an ~200 °C difference in peak metamorphic temperatures across the northern Snake Range detachment but did not identify any intraplate discontinuities, thereby limiting the amount of structural excision to motion on the northern Snake Range detachment itself, and locally, to no more than 7–11 km. Third, mapped geology and field relationships indicate that a pre-Cenozoic fold truncated by the northern Snake Range detachment could have produced ~3–9 km of structural overburden above Precambrian units, on the order of that potentially excised by the northern Snake Range detachment but still far short of expected overburden based on lithostatic assumptions. Fourth, finite-strain measurements indicate a shortening (constrictional) strain regime favorable to superlithostatic conditions. Together, these observations suggest that pressures during peak metamorphism may have locally reached ~150%–200% lithostatic pressure. Such departures from lithostatic conditions are expected to have been most pronounced above regions of high heat flow and partial melting, and/or at the base of regional thrust-bounded allochthons, as is characteristic of the spatial distribution of Cordilleran metamorphic core complexes during the Late Cretaceous Sevier orogeny.

This Special Paper focuses on the evolution of the crust of the hinterland of the orogen during the orogenic cycle, and describes the evolution of the crust and basins at metamorphic core complexes. The volume includes a regional study of the Sevier-Laramide orogens in the Wyoming province, a regional seismic study, strain analysis of Sevier and Laramide deformation, and detrital zircon provenance from the Pacific Coast to the foreland between the Jurassic and the Eocene.