GEOCHRONOLOGICAL EVIDENCE FOR RAPID FOREARC SUBSIDENCE AND SEDIMENTATION DURING PALEOGENE SPREADING RIDGE SUBDUCTION ALONG THE SOUTHERN ALASKA CONVERGENT MARGIN

The southern Alaska active margin is believed to have been modified during Paleogene time by spreading ridge subduction, as evidenced by the ages of the Sanak-Baranof near-trench plutons and the eruption of the highly depleted Caribou Creek volcanics (CCV) beginning at ~59 Ma (Cole et al., 2006). Proximal terrestrial sedimentary strata, tuffs, and mafic lavas of the Paleogene Arkose Ridge Formation (ARF) are exposed in sections up to 2 km thick in the southern Talkeetna Mountains north of the Castle Mountain fault. In the eastern portion of their outcrop belt these forearc basin strata lie adjacent to and are overlain by the CCV. The eastern ARF is enriched in volcanic clasts and has detrital zircon populations dominated by ~58-60 Ma ages, reflecting major volcanic contributions to ARF sedimentation. These observations suggest a link between ridge subduction, extension, volcanism, and forearc sedimentation. However, previously available age data for the ARF were too imprecise to validate this link.

We report new isotopic ages that precisely constrain the age of the ARF as mid- to late Paleocene. Fourteen U-Pb zircon ages ranging from 56.5 to 60.4 Ma were determined for felsic tuffs by LA-ICPMS. The tuffs show minor contamination by zircons derived from adjacent Jurassic-Cretaceous arc plutons. The youngest 12 to 33 zircon crystals from each sample define statistically coherent eruptive ages. These zircons have Th/U >0.1, suggestive of a magmatic origin. Uranium contents from spot analyses vary widely from <200 to >1000 ppm. A few samples contain minor amounts of deeply colored zircons with >3000 ppm U, but they exhibit no correlation between U content and age or degree of concordance attributable to Pb loss from radiation damage. The U-Pb results are corroborated by preliminary whole-rock ⁴⁰Ar/³⁹Ar ages of 55-59 Ma obtained from three basaltic andesite flows in the ARF.

The new ages define a span of less than 4 m.y. for deposition of the ARF and require average deposition rates of >500 m/m.y. They demonstrate a strong temporal link between ARF deposition, the onset of slab-window volcanism (CCV), and near-trench plutonism. Our integrated stratigraphic and geochronologic data show that spreading ridge subduction may prompt more rapid accumulation of thicker sections of forearc strata than subduction of typical oceanic crust.