PIECING TOGETHER THE DISTRIBUTED BITS OF THE TALKEETNA ARC, SOUTHERN ALASKA

The origins of the Mesozoic accreted Talkeetna Arc of southern Alaska have long perplexed many geologists. Generally, exposed sections of mafic to intermediate plutonic rocks of ages spanning ca. 230 to 145 Ma have been attributed to a single, disrupted arc or multiple accreted terranes of oceanic and continental affinity. We compiled and mapped age, geochemical, isotopic, and P–T data from all available sources in order to spatially analyze these various data sets. Based on age and geochemical trends, we separated the arc into different regions, including northwest, central, and southeastern Talkeetna mountains; Alaska Peninsula; Chugach Mountains; and Afognak/Kodiak. We then identified outliers to the general trend of northwest-younging igneous ages, and analyzed four different models of arc assemblage. Based on available data, we conclude that the Talkeetna arc is formed of at least two parts: (1) a Talkeetna arc with a separate or trailing Wrangellia arc as proposed by Rioux et al., 2007, and (2) a Triassic–Jurassic mafic–ultramafic sequence that was built atop a second subduction zone to the south (the proposed Nanwalek Arc of Wilson, written commun., 2020) or an ophiolitic forearc basement sequence (Kusky et al., 2007). In either case, <160 Ma trondhjemite sutured the significantly older northwestern and southeastern portions of the Talkeetna Mountains, and detrital-zircon studies suggest that accretion terminated by early Cretaceous. This complex array of igneous rocks demands more detailed mapping and structural, chemical, and age analyses in order to fully understand its origins and implications for complex ocean–continent interactions.