Bedrock geologic mapping of the Willow Creek area and deformational history of the Hatcher Pass schist, southern Talkeetna Mountains, Alaska

Abstract

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 regionally retrogressed chlorite-muscovite schist with plagioclase and garnet porphyroblasts in the Willow Creek area. I 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. I determined the following tectonic evolution for the HPs: subduction and incorporation into a subduction channel occurred no later than 75 Ma. The oldest known foliation (S₁) underwent isoclinal recumbent folding (F₂) that resulted in the development of the regionally dominant fabric (S₂). Fragmented 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. I interpret F₃₋₄ 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 the overlaying Arkose Ridge Fm. Based on my detrital zircon data (MDA = 80 Ma), the HPs is interpreted to be a subducted equivalent of the accretionary complex (Valdez Group) exhumed above a slab window formed from slab breakoff of the Kula Plate.