PROVENANCE SIGNATURE OF A FOREARC BASIN MODIFIED BY SPREADING RIDGE SUBDUCTION: DETRITAL ZIRCON GEOCHRONOLOGY AND DETRITAL MODES FROM THE PALEOGENE ARKOSE RIDGE FORMATION, SOUTHERN ALASKA

Upper Paleocene–Eocene fluvial-lacustrine strata (Arkose Ridge Fm.) exposed in the southern Talkeetna Mountains record deposition in a forearc basin modified by Paleogene spreading ridge subduction beneath southern Alaska. We obtained new U-Pb ages of detrital zircon grains and modal analyses from stratigraphic sections spanning the 2,000 m thick ARF in order to constrain sediment sources during ridge subduction.

U-Pb ages from >1400 detrital zircons in 15 sandstone samples reveal three main populations: late Paleocene–Eocene (60-48 Ma; 16% of all grains), Late Cretaceous–early Paleocene (85–60 Ma; 62%) and Jurassic–Early Cretaceous (200–100 Ma; 12%). Strata show enrichment in late Paleocene–Eocene detrital zircons from <2% in the west to >25% in the east. In eastern sections, this age population increases temporally from 0% in the lower 50 m of the section to >40% in samples collected >740 m above the base. Detrital modes from 31 conglomerate beds and 55 sandstone thin sections are dominated by plutonic and volcanic clasts and plagioclase feldspar with minor quartz, schist, hornblende, argillite, and metabasalt. Westernmost strata contain <5% volcanic clasts whereas easternmost strata contain 40 to >80% volcanic clasts. Temporally, eastern sandstones exhibit an upsection increase in volcanic detritus.

Our data show: (1) Detritus was eroded from igneous sources exposed directly north of the ARF strata, mainly Jurassic–Paleocene arc plutons and Paleocene–Eocene slab-window volcanic centers. (2) Eastern deposystems received higher proportions of juvenile volcanic detritus through time, consistent with construction of adjacent slab-window volcanic centers. (3) Western deposystems transported detritus from Jurassic–Paleocene arc plutons that flank the northwestern basin margin. (4) Metasedimentary strata of the Chugach accretionary prism did not contribute abundant detritus.

Conventional provenance models predict reduced input of volcanic detritus to forearc basins during exhumation of the volcanic edifice and exposure of subvolcanic plutons. However, ARF data show that forearc basins modified by ridge subduction may record upsection increases in non-arc, syndepositional volcanic detritus due to contemporaneous accumulation of thick volcanic sequences at slab-window volcanic centers.