Detrital Zircon Dating of the Kodiak Accretionary Complex; South Alaska

Southern Alaska crustal exposures provide an excellent opportunity to study the growth of collisional continental margins through the processes of terrane accretion, magmatism, accretionary prism development, and subduction of oceanic spreading ridges. Kodiak Island is one of the world's best exposed accretionary complexes, and is characterized by transport along dominant large-magnitude strike-slip faults of west North America and South Alaska. In this study we focus on detrital zircon dating of the Kodiak accretionary complex on Kodiak Island Alaska. Accretionary complexes are an important fundamental element of convergent plate margins and are also important structures in continental crust formation. Their presence in the geologic record is an indicator of past subduction zones. Six graywacke detrital zircon samples from the Kodiak and the Ghost Rocks Formations were analyzed using U/Pb dating of single crystals via laser-ablation inductively coupled plasma mass spectroscopy. The maximum depositional age of the three major formations are as follow: Kodiak Formation 56.2-69.2 Ma, and Ghost Rock 58.5 Ma. The age for the small outcrop of Narrow Cape Formation is 54.1. Kodiak and Ghost Rock Formation detrital zircon age are consistent with fossil ages of these two formations, but the fossil content of Narrow Cape is younger than the detrital zircon age we measured for this formation. Some evidence in our data like resurrection of certain age zircon grains between the accreted sequences, age gaps between the accreted sequences, different rate of sedimentation and etc. are evidenc for episodical accretion in the complex and also displacement of the accreted units along the west North America and Alaska right lateral large magnitude strike slip faults.