MIOCENE INTRA-ARC BASIN DEVELOPMENT WITHIN THE WRANGELL VOLCANIC FIELD, FREDERIKA FORMATION AND LOWER WRANGELL LAVA, EASTERN WRANGELL MOUNTAINS, ALASKA

Analysis of the central Wrangell volcanic field (WVF) provides a framework for evaluating spatial and temporal variations in magmatism, basin development, and accretionary tectonics along the northern Pacific margin. Eighteen measured sections spanning the Frederika Formation (FF) and overlying lower Wrangell Lava (WL) document upsection transitions from mainly alluvial-fluvial-lacustrine sedimentation to emplacement of lava flows and volcaniclastic strata. Lacustrine and coal-bearing strata in the FF indicate active basin subsidence during sediment accumulation. However, expansion of intra-basinal volcanic centers prompted progradation of vent-proximal alluvial fans and volcanic centers across fluvial-lacustrine environments. Compositional and detrital zircon ages record upsection shifts from a mixture of intrabasinally- and extrabasinally-derived detritus to intrabasinally-derived volcanic detritus. U-Pb detrital zircon data from uppermost FF sandstone show three dominant peaks: 180-135, 97-70, and 12-9 Ma. The youngest peak matches the age of intrabasinal volcanic centers. Older peaks correspond with nearby plutons, but may also record recycling of Mesozoic sedimentary strata that underlie the FF. Tuffs from the middle of the FF yield 12-11 Ma U-Pb zircon ages. The 12-9 Ma zircon ages from the FF overlap published ages from volcanic successions that we interpret as intra-basinal eruptive centers, including Castle Mountain (10-9 Ma) to the east, White River (10-9 Ma) to the north, and Porphyry Mountain (15-7 Ma) to the south. Ongoing geochronology of FF/WL lavas should further constrain depositional ages. Sampled FF/WL lavas exhibit geochemical compositions typical of subduction-related arc volcanic suites. As a group, these lavas define elemental relationships indistinguishable from those observed for a subset of <5 Ma western WVF lavas interpreted to have been emplaced in an intra-arc extensional setting. Our new data, together with published studies, demonstrates that central WVF intra-arc basin development overlapped with cessation of strike-slip basin development and magmatism in the eastern WVF (ca. 10 Ma) and preceded arc magmatism in the western WVF (ca. 7-<1 Ma), consistent with diachronous collision of the Yakutat terrane against the northern Pacific margin.