Cooling and exhumation history of the Kodiak accretionary prism, southwest Alaska

Abstract

In this paper, we combine zircon and apatite fission track dating with 4 0 Ar/ 3 9 Ar mineral dating to establish the thermal history of accreted units from the Kodiak Accretionary Complex in southwest Alaska. The Kodiak archipelago exposes a series of northeast-trending subduction-related units that range in age from Jurassic on the northwest to Eocene in the southeast. Thus, this complex was assembled and has resided along a margin with a long-lived history of convergence. This analysis focuses on the thermal history of the Late Triassic through Late Cretaceous units: the Early Jurassic Afognak pluton, Jurassic Raspberry schist, the Late Cretaceous Uyak Complex and the early Maastrichtian Kodiak Formation. This tectonic assemblage straddles the arc-accretionary prism boundary (marked by the Border Ranges fault) and constitutes over 60% of the exposed accretionary complex. 4 0 Ar/ 3 9 Ar data of feldspar and fission track data from the Afognak pluton suggest a relatively simple history of slow cooling through Ar closure from 150 to 120 Ma. Apatite and zircon fission track analyses for these units show that the pluton and schist underwent similar thermal histories through the closure of zircon and apatite fission tracks, suggesting that these two diverse units may have been in proximity to one another since at least the Late Jurassic. These results also suggest that the Border Ranges fault, which separates these units on Kodiak Island, has not experienced any vertical throw that has impacted the post-Jurassic cooling history. Thermal modeling based on fission track lengths in apatite suggests rapid cooling of the pluton, schist, and the Uyak Complex in the Late Cretaceous-early Tertiary. In the highest structural levels of the Kodiak Formation (the landward and seaward belts), zircon fission track dates are consistently ca. 50 Ma. The lowest structural level (the central belt) records 44 Ma ages for both zircon and apatite, indicating rapid cooling in the Eocene within the core of a regional-scale antiform. Apatite fission track ages for the landward and seaward belts range from 44 to 25 Ma from northwest to southeast, suggesting a seaward wave of cooling within the higher structural levels. The antiformal structure and its associated cooling history could be a response to underplating, with seaward propagation of a forearc high leading to erosional unroofing. The two rapid cooling events in the latest Cretaceous-early Tertiary and Eocene-Oligocene correspond with known events of rapid accretion. Thus, rapid accretion may lead to rapid cooling of the prism, probably due to uplift and exhumation of the accretionary complex. The prism can experience extremely slow cooling in the absence of fluxes of sediment into and off the top of the wedge during accretion/unroofing events.