PEEKING THROUGH THE SLAB WINDOW: EXHUMATION OF ACCRETIONARY SEDIMENTS OVER A SLAB WINDOW IN THE WILLOW CREEK AREA, SOUTHERN TALKEETNA MOUNTAINS, ALASKA

Most of south-central Alaska is interpreted as a subduction-accretion complex built on the southern margin of the Wrangellia composite terrane (WCT). The Hatcher Pass schist (HPs) of the WCT is an enigmatic chlorite-muscovite schist with plagioclase and garnet porphyroblasts located in the Willow Creek area of the southern Talkeetna Mountains. Preliminary detrital zircon (DZ) data suggests that the HPs may be correlative to accretionary complex rocks (Valdez Group). We conducted 1:24,000 scale bedrock geologic mapping, microstructural and petrographic analysis, thermobarometry, forward petrologic modeling, and DZ analysis of the HPs to test the hypothesis that the HPs is a subducted equivalent of the Valdez Group. Structural mapping shows that the oldest known foliation (S₁) underwent isoclinal recumbent folding (F₂) that resulted in the development of a regionally pervasive fabric (S₂). Fragmented boudins, porphyroblast asymmetry, and rare S-C geometries indicate top-to-the-east shearing. Two sets of open, upright folds with SW-NE and NW-SE trending 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 juxtaposing the HPs against the overlying Arkose Ridge Formation. Forward modeling of garnet and plagioclase microchemical data with Theriak-Domino indicate that the HPs reached peak metamorphic conditions of 500-650 ℃ and 10 kbar in a fluid-oversaturated system prior to exhumation. DZ data indicates a maximum depositional age of ca. 81 Ma and supports correlation with the Valdez Group. Based on these data, we propose the following tectonic evolution for the HPs: 1) incorporation into a subduction channel after ca. 81 Ma; 2) burial to >30 km paleodepths; 3) structural exhumation ca. 61-50 Ma during deposition of the Arkose Ridge Formation. Exhumation may have been driven by formation of a slab window via break-off of the Kula Plate.