MESOZOIC BASIN DEVELOPMENT AND DEFORMATION ALONG THE INBOARD MARGIN OF THE WRANGELLIA COMPOSITE TERRANE

Mesozoic strata in the Alaska and Talkeetna Ranges of southern Alaska provide a long-term record of sedimentary basin development and collisional tectonics along the inboard margin of the Wrangellia composite terrane. Analysis of basinal strata, metamorphic rocks, and major faults documents the progressive collision of an island arc assemblage (the Wrangellia composite terrane) with the former North American continental margin. New basin analysis, structural, and geochronologic data indicate the following stages of collision. (1) Deposition of 3 to 5 km of Upper Jurassic-Upper Cretaceous strata (the Kahiltna assemblage) recorded the initial collision of the island arc assemblage with the continental margin. The Kahiltna assemblage exposed in the northern Talkeetna Range represents a Kimmeridgian-Valanginian backarc basin that was filled by northwestward-flowing submarine fan systems that transported sediment derived from Mesozoic strata of the island arc assemblage. The Kahiltna assemblage exposed in the southern Alaska Range represents a Valanginian-Cenomanian remnant ocean basin filled by west-southwestward-flowing submarine fan systems that transported sediment derived from Paleozoic continental margin strata uplifted along strike in the suture zone. (2) Metamorphism of submarine fan deposits of the Kahiltna basin occurred at ~ 74 Ma as determined from a new U-Pb zircon age for a synkinematic sill. Coeval with metamorphism in the southern part of the suture zone was development of a thrust-top basin (the lower Cantwell Formation) in the northern part of the suture zone. Geologic mapping and compositional data suggest that the 4 km of Late Cretaceous nonmarine and marginal marine sedimentary strata in this basin record regional subaerial uplift of the suture zone. (3) Shortening and exhumation of the suture zone peaked from 65 to 60 Ma based on geochronologic data and an unconformity in the Cantwell basin.