INVESTIGATING THE LOST ARC: GEOLOGIC CONSTRAINTS ON ~29 MILLION YEARS OF CONTINUOUS MAGMATISM ALONG AN ARC-TRANSFORM JUNCTION, WRANGELL ARC, ALASKA

The Wrangell arc in southern Alaska records tectonic processes associated with Neogene flat-slab subduction of the Yakutat microplate. Although the arc provides a long-term geologic record of the interrelations between subduction, transform slip, and magmatism, expansive segments of the arc have not been studied. Our new geochemical and geochronologic datasets from previously unsampled bedrock and major rivers provide improved spatial-temporal constraints on magmatism and deformation of >15,000 km² of this enigmatic arc. U-Pb geochronology of detrital zircons and ⁴⁰Ar/³⁹Ar geochronology of sand- and gravel-sized volcanic clasts from modern rivers encircling the arc, together with new bedrock ages, reveals very sparse magmatism from ~34 to ~29 Ma (<1% of >1600 new ages) followed by continuous magmatism from ~29 Ma to present. Magmatism initially extended over ~50 km along the north flank of the Wrangell-St. Elias Range before stepping outboard (southward) and then migrating northwestward at ~ 6 Ma. Drainages with abundant <3 Ma volcanic clasts and detrital zircons occur in the western Wrangell Mountains, including the Boulder, Chetaslina, Copper, Dadina, Drop, Kuskalana, Nadina, and Sanford Rivers. New bulk rock geochemistry from 129 volcanic-plutonic bedrock samples and 208 detrital volcanic clasts, show that the same magma-generation processes that are active today (possible slab edge melting producing adakites, mantle wedge melting and “typical” calc-alkaline intermediate arc volcanism, and tholeiitic intra-arc extension) have occurred over the last ~29 Ma across the extent of the arc. The dating of a ~29 Ma felsic dikelet within Totschunda fault zone gauge indicates the Totschunda fault was active before ~29 Ma and likely active during the initiation of the Wrangell arc. Our integrated datasets document potentially offset volcanic centers with similar ~3-2 Ma ages and geochemical compositions along the seismically active Totschunda fault between Lime Creek, Eucher Mountain, and Skookum Creek, indicating tens of kilometers of dextral displacement since ~3 Ma. We relate the spatial-temporal changes in arc magmatism and deformation to flat-slab subduction of the Yakutat microplate along the outboard edge of the arc coeval with dextral translation of the upperplate along the Denali-Totschunda fault system.