KINEMATICS, TIMING, AND MAGNITUDE OF SHORTENING WITHIN THE CRETACEOUS SEVIER-LARAMIDE FOLD-THRUST BELT OF IDAHO AND MONTANA

The Idaho-Montana fold-thrust belt is an understudied part of the North American Cordillera. We focus on a ~600 km long transect of this fold-thrust belt from the eastern Idaho batholith in central Idaho into southwestern Montana in a region of overlap between the Sevier and Laramide belts. We integrate: (1) results from structural studies from central Idaho to the Crazy Mountains Basin of western Montana; (2) U-Pb zircon provenance data from Cretaceous foreland basin strata of the Kootenai, Blackleaf, and Frontier formations, as well as the Beaverhead Group from southwestern Montana; and (3) complementary (U-Th)/He thermo- and U-Pb geochronometric data constraining the timing of shortening. Following early erosion of Mesozoic-upper Paleozoic strata, Neoproterozoic to middle Paleozoic strata were eroded between 135 Ma and ~110 Ma from bedding-parallel thrust sheets in central Idaho. This was followed at ~110-105 Ma by an apparent lull in exhumation related to thrusting as the fold-thrust belt approached the northeastern limit of Neoproterozoic and lower Cambrian rift strata in east-central Idaho. From ~105-95 Ma, the thrust front propagated northeastward and structurally upward into middle Paleozoic and younger passive margin strata, concomitant with emplacement of 97-91 Ma plutons that crosscut older folds and thrusts in central Idaho. By ~88 Ma, a double-decker thrust system developed: at deeper levels, above an ~25 km deep basal décollement, Mesoproterozoic quartzites in east-central Idaho were duplexed and crystalline basement rocks in southwestern Montana were exhumed during early shortening as thrust slip was transferred to the structurally overlying bedding-parallel thrust system. By 75-66 Ma, thrusts carrying mechanical basement rocks deformed overlying bedding-parallel thrusts and resulted in widespread exhumation of the deeper thrust system. Taken as a whole, these results suggest that the Sevier and Laramide belts within this portion of the Cordillera were kinematically linked and together accommodated 300 km of horizontal shortening. The change from a thin- to thick-skinned structural style was concomitant with propagation of the fold-thrust belt into the abrupt slope to shelf transition imposed by the Neoproterozoic and Paleozoic rift and passive margin succession.