STRUCTURAL EVOLUTION OF THE HATCHER PASS SCHIST, SOUTHERN TALKEETNA MOUNTAINS, ALASKA

The Willow Creek area in the southernmost Talkeetna Mountains of south-central Alaska is the southern extent of the Wrangellia composite terrane (WCT). Most of south-central Alaska represents a subduction-accretion complex built upon the southern WCT. The purpose of this project is to better constrain the origin and evolution of the Hatcher Pass schist (HPs), a variably retrogressed chlorite-muscovite schist with plagioclase and garnet porphyroblasts in the Willow Creek area. Based on preliminary detrital zircon data, the HPs is interpreted to be a subducted equivalent of the accretionary complex (Valdez Group) exhumed along the southern margin of the WCT. We conducted bedrock mapping of the Willow Creek area along with structural and petrographic analyses to better constrain the petrogenetic and structural history of the HPs, its contact with forearc sediments that lie structurally above (Arkose Ridge Fm), and mid-Cretaceous Willow Creek plutonism to the north. We determined the following tectonic evolution for the HPs: subduction and incorporation into a subduction channel along the WCT back stop occurred sometime between 75 and 57 Ma, based on preliminary detrital zircon geochronology and ⁴⁰Ar/³⁹Ar theromchronology. The oldest known foliation (S₁) underwent isoclinal recumbent folding (F₂) that resulted in the development of the regionally dominant fabric (S₂). Dismembered boudins, porphyroblast asymmetry, and rare S-C geometries indicate top-to-the-east shearing during D₂. Two sets of open and upright folds, with largely SW-NE and NW-SE trending fold axes, postdate the development of S₂ and define a type 1 interference pattern. We interpret F_3-4 to have formed during doming (F₃) and continued tectonic exhumation (corrugations; F₄) along a south-vergent detachment responsible for the juxtaposition of the HPs with overlaying Arkose Ridge Fm. This tectonic history may have resulted from subduction processes such as spreading-ridge subduction and/or slab breakoff. We plan to further constrain the nature and timing of the structural and metamorphic history of the schist with U-Pb geochronology of cross cutting porphyritic dikes, thermobarometry, and Raman spectroscopy.