GEOCHRONOLOGIC CONSTRAINTS ON ~30 MILLION YEARS OF DIACHRONOUS MAGMATISM ALONG AN ARC-TRANSFORM JUNCTION, WRANGELL ARC, SOUTHERN ALASKA AND CANADA

The Wrangell arc in southern Alaska provides a long-term record of interrelations between magmatism, flat-slab subduction of the Yakutat microplate, and transform slip along the Denali-Totschunda fault system. New geochronologic data provide improved spatial-temporal constraints on magmatism and deformation, including growth of some of the largest eruptive centers in the Pacific. U-Pb geochronology of detrital zircons and ⁴⁰Ar/³⁹Ar geochronology of sand- and gravel-sized igneous clasts from modern rivers encircling the arc, along with new ⁴⁰Ar/³⁹Ar bedrock ages, reveal sparse magmatism ~34–30 Ma (<1% of >2275 new ages) followed by continuous magmatism ~30 Ma to present. The oldest ages occur near the Alaska-Yukon Territory border. Watersheds along the north flank of the Wrangell Mountains in Alaska (Bond, Chisana, Cross, Nabesna, Rocker, Ptarmagin, White) yield abundant ~30 to 18 Ma ages, reflecting initial magmatism. Magmatism shifted outboard (southward) to volcanic centers presently exposed in the Yukon Territory based on previously reported bedrock ages (~18–10 Ma St. Clare, ~15–13 Ma Nines Creek, ~14–11 Ma Alsek fields). Magmatism then migrated progressively northwestward to volcanoes exposed in the St. Elias and Wrangell Mountains of Alaska. Southeastern watersheds (Chitistone, Hawkins, Nizina) yield chiefly ~13 to 4 Ma detrital ages. South-central watersheds (Kotsina, Kuskalana, Root) bear mostly ~8 to 2 Ma detrital ages. Western watersheds (Boulder, Copper, Dadina, Drop, Nadina, Sanford) yield mainly <2 Ma detrital ages. The Totschunda fault zone bisects the north flank of arc and was likely active during arc initiation given that fault gouge dikelets yield ~29 Ma ⁴⁰Ar/³⁹Ar ages. Companion geochemical analyses from igneous bedrock and detrital cobbles show that the same magma-generation processes active today (slab-edge melting producing adakites, mantle wedge melting and calc-alkaline arc volcanism, and tholeiitic intra-arc extension) shaped the arc throughout its >30 Ma evolution. The geochemical compositions, spatial-temporal patterns, and evidence for intra-arc dextral slip reflect progressive northwestward insertion of the Yakutat flat-slab along the outboard edge of the arc coeval with dextral translation of the upper plate along the Denali-Totschunda fault system.