Paleogene evolution of the Kodiak Islands, Alaska: Consequences of ridge‐trench interaction in a more southerly latitude

Abstract

The Kodiak Islands of Alaska are principally composed of a Late Cretaceous and Paleogene complex of accreted deep‐sea rocks. An early Paleocene magmatic event invaded the accretionary complex with mid‐ocean ridge basalt (MORB), andesite, and granodiorite. Geochemical evidence suggests both the andesite and granodiorite are mixtures of MORB and sediment. The volcanic rocks are interlayered with coarse clastic sediments and deformed with the latter; the granodioritic intrusions, with K‐Ar ages as old as 62 Ma postdate emplacement of the early Paleocene sedimentary and volcanic rocks. Interaction of the subduction complex with the Kula‐Farallon ridge most simply explains this early Paleocene magmatic episode. Paleomagnetic studies of the volcanic rocks indicate this interaction occurred at 40.3±6° north latitude, which is 25.3±9° south of the expected position of the Kodiak Islands relative to North America. The magmatic arc presently northwest of the Kodiak accretionary complex was active during the Late Cretaceous and early Paleocene, diminished after the ridge‐trench interaction, and resurged from late Eocene into Miocene time. The magmatic resurgence is coeval with the obductive offscraping of Eocene to Oligocene(?) submarine fan deposits in the Kodiak Islands. The petrology of these fan deposits indicates they are the most likely proximal equivalent for the Zodiac fan, thereby limiting its relative motion with respect to the Kodiak Islands. Possible models accounting for the northward motion of the Kodiak accretionary complex include (1) coupling the accretionary complex and possibly related arc to the northward‐moving Kula Plate and closing an ocean basin north of the arc in Paleogene time, (2) emplacing the Kodiak accretionary complex at the northward limit of the paleomagnetic data, with subsequent northward motion accomplished by closure of a small back arc basin and/or intracontinental shortening, and (3) transcurrent faulting and northward motion of the Kodiak accretionary complex without the presently adjacent arc.