A MID-CRETACEOUS AGE FOR SYNCONVERGENT OROGEN-PARALLEL EXTENSION IN THE INTERIOR OF THE SEVIER OROGEN CONSTRAINED BY LASERPROBE ⁴⁰AR/³⁹AR DATING OF STRAIN FRINGES

UV and CO₂ laserprobe ⁴⁰Ar/³⁹Ar in situ analyses of phlogopite and muscovite in fibrous strain fringes from greenschist-facies metamorphic rocks document mica growth ages at temperatures lower than their closure temperatures, and therefore directly date deformation. The new dates resolve the age of the earliest ductile fabric recorded in the Raft River-Albion-Grouse Creek metamorphic core complex of Utah and Idaho. Phlogopite was dated in quartz-calcite-phlogopite strain fringes around pyrite in Pennsylvanian-Permian rocks from the Grouse Creek Mountains (Utah) using both the UV and CO₂ laserprobe; muscovite was dated in quartz-muscovite strain fringes around pyrite in deformed Jurassic sills from the Black Pine Mountains (Idaho) using the CO₂ laserprobe. Phlogopite ⁴⁰Ar/³⁹Ar ages for individual strain fringes (Grouse Creek Mountains) range from 92 Ma to 110 Ma, with the most reliable ages ranging from 101 Ma to 110 Ma (mean age, 105.0 ± 5.8 Ma). Muscovite ⁴⁰Ar/³⁹Ar ages for individual strain fringes (Black Pine Mountains) range from 97 Ma to 112 Ma (mean age, 104.7 ± 5.8 Ma). Strain fringes are associated with a sub-horizontal foliation and a generally N-trending elongation lineation exhibiting components of top-to-the-north simple shear and coaxial strain accommodating N-S extension and sub-vertical shortening. Mid-crustal northward flow at 105 (± 6) Ma within the interior of the Sevier orogen, coeval with east-directed shortening in the foreland and with plate convergence, records orogen-parallel synconvergent extension. We favor gravitational relaxation of structural culminations resulting from focused crustal shortening as a driving mechanism for orogen-parallel flow.