THE SEVIER-LARAMIDE HINTERLAND: INITIAL LATE JURASSIC-EARLY CRETACEOUS SHORTENING, RENEWED EARLY LATE CRETACEOUS SHORTENING AND THE INFLUENCE OF A DELAMINATION CYCLE ON WEDGE MECHANICS

Deeply exhumed Barrovian metamorphic rocks in the Sevier-Laramide hinterland record substantial tectonic burial and a complex Late Jurassic to Paleogene tectono-metamorphic history of alternations in contraction and extension that reflect the evolving mechanics of the retroarc orogenic wedge. This history both pre-dates and is synchronous with protracted shortening in the Sevier fold-thrust belt and Laramide foreland province. Early and Middle Jurassic shortening – localized along the eastern arc fringe and along the shelf-slope margin of a Triassic backarc basin and probably driven by arc collision/accretion ­– together with the westward-thickening passive margin sediments, provided initial wedge taper that was amplified during the Late Jurassic to Paleogene non-collisional Sevier orogeny. Results from coupled Lu-Hf garnet dating, ⁴⁰Ar/³⁹Ar thermochronology, and petrologic modeling of garnet growth to determine PT paths illuminate the following: (1) Late Jurassic thrust-burial metamorphism (158.2 ± 2.6 Ma) in the Funeral Mountains documents some of the earliest recognized shortening within the integrated Sevier orogen; (2) Valanginian (132.1 ± 2.4 to 138.7 ± 0.7 Ma) tectonic burial in the northern Albion Mountains and Latest Jurassic (149.9 ± 1.2 Ma) tectonic burial in the eastern Raft River Mountains are consistent with an eastward progression of initial deformation at this latitude during development of the Sevier orogenic wedge; (3) Coniacian-Santonian tectonic burial in the Grouse Creek Mountains (85.5 ± 1.9 Ma) and Wood Hills (82.8 ± 1.1 Ma) is consistent with renewed crustal shortening in the wedge interior; (4) Late Cretaceous thrust burial forms the initial stage of a 3-part Late Cretaceous to Early Eocene history of burial, exhumation and heating, and burial that we interpret to record the dynamic responses of a delamination cycle on the orogenic wedge. This tectono-metamorphic sequence may also be consistent with recent geodynamic predictions of dynamic topographic adjustments resulting from passage of a subterranean oceanic plateau embedded in the Farallon plate; (5) Ongoing studies of PT paths for synkinematic garnets that yielded ages of 51.0 ± 2.0 and 53.3 ± 2.2 Ma from the Grouse Creek Mountains will help to resolve the age of the transition from final contraction to extension.