NEAR SIMULTANEOUS ACCRETIONARY PRISM GROWTH, FOREARC MAGMATISM, AND FAULTING: DETRITAL ZIRCON CONSTRAINTS FROM THE KODIAK ACCRETIONARY COMPLEX, KODIAK ISLAND, ALASKA

Kodiak Island is one of the world’s best localities for understanding accretionary complex processes, including prism growth, faulting and forearc magmatism. In this study, U/Pb detrital zircon age constraints are placed on the evolution of the Kodiak accretionary complex, Kodiak Island, Alaska. Detrital zircon samples from the Kodiak, Ghost Rocks, Sitkalidak and Narrow Cape Formations were analyzed using U/Pb dating of single crystals via laser-ablation ICP-MS. The age spectra from the units indicate source rocks in the Coast Plutonic Complex, the Talkeetna arc, and Laurentian North America. Minimum zircon ages from the Kodiak Formation range from 69-57 Ma and systematically decrease southward indicating sequential deposition and underplating. The youngest zircon ages from the Kodiak Formation are younger than crystallization ages for the Kodiak batholith, indicating that Kodiak Formation accretion overlapped with large volume forearc magmatism and partial melting of the accretionary complex itself. The youngest Ghost Rocks Formation zircons range from 64-58 Ma, and the youngest Sitkalidak Formation zircons are 54-53 Ma in age. The overlap in Kodiak and Ghost Rocks Formation minimum ages occurs adjacent to the Contact Fault and may indicate an accretionary hiatus related to a spreading ridge subduction event, structural complications or unit designation issues. Overall, southward decreasing minimum detrital zircon ages indicate linear Kodiak accretionary complex growth rates of 5-19 km/Ma, which is equivalent to rates at the fastest growing modern accretionary prisms such as the Mediterranean Ridge and Nanaki Trough. Finally, the gradual decrease in Proterozoic and Archean zircons with minimum zircon age suggest that the Kodiak accretionary complex moved away from continental North American sedimentary sources over its 16 Ma history.