CHANGES IN BASIN CONFIGURATION DURING ARC-CONTINENT COLLISIONS: AN EXAMPLE FROM MESOZOIC STRATA OF SOUTH-CENTRAL ALASKA

Upper Triassic to Upper Cretaceous strata exposed in the northwestern Talkeetna Mountains and central Alaska Range record four distinct stages of basin development associated with collision of an island arc to the continental margin of western North America.

Stage 1 - Continental margin/arc margin strata. Devonian to Upper Triassic sedimentary and volcanic strata, well exposed in the Chultina terrane (Alaska Range) represent part of the Paleozoic and Mesozoic continental margin of the Cordillera. U-Pb detrital zircons ages from these strata range from Mesozoic to Precambrian (Mz-Pz-Pc - 5%/65%/30%) with age clusters that suggest some affinity to North American. Upper Triassic strata in the northwestern Talkeetna Mountains, in contrast, represent part of the Mesozoic arc margin.

Stage 2 - Retroarc foreland basin. The Upper Jurassic-Lower Cretaceous part of the Kahiltna assemblage consists of marine strata that were deposited in a retroarc foreland basin that depositionally overly Upper Triassic arc margin rocks. These strata have northward paleoflow indicators, arc-derived sandstone composition, and detrital zircons that consist entirely of Mesozoic age. These strata were derived from uplifted parts of the island arc complex that were exposed in the hanging walls of south-dipping thrust faults.

Stage 3 - Remnant ocean basin. The Lower and Upper Cretaceous part of the Kahiltna assemblage consists of syncollisional strata derived from both arc and continental margin sources. These strata are interpreted to represent southwestward prograding submarine fan deposition in a remnant ocean basin setting. U-Pb detrital zircons ages from these strata range from Mesozoic to Precambrian (Mz-Pz-Pc - 70%-13%-17%). Source areas were uplifted in an along-strike suture zone related to oblique collision between the island arc and the continental margin.

Stage 4 - Peripheral Foreland basin. Overlying the Kahiltna assemblage (locally defined by an angular unconformity) are Upper Cretaceous or younger nonmarine fossil-leaf bearing strata. These rocks consist primarily of quartz-rich detritus and may represent deposition in a peripheral foreland basin setting associated with shortening and exhumation of continental margin strata during the final stages of arc accretion.