MESOZOIC-CENOZOIC CHANGES IN THE TECTONIC SETTING OF A SEDIMENTARY BASIN ALONG AN ACCRETIONARY CONVERGENT MARGIN: WRANGELL MOUNTAINS BASIN, SOUTH-CENTRAL ALASKA

The ~7,000 m of Triassic-Miocene sedimentary strata of the Wrangell Mountains basin record distinct changes in the tectonic setting of a sedimentary basin through time. Temporal variations in basin architecture, sediment composition, and structural controls on sedimentation record northward migration of the basin and underlying Wrangellia terrane from a tropical intraoceanic setting to the continental margin of southern Alaska. Translation and collisional orogenesis prompted the following stages of basin development: (1) An Upper Triassic-Lower Jurassic carbonate platform developed on an intraoceanic plateau at low paleolatitudes. Subsidence prompted transgression from a supratidal-subtidal carbonate shelf to a siliceous basin floor. (2) A subduction zone formed along the southern margin of Wrangellia during Middle to Late Jurassic time, forming an intraoceanic arc and backarc basin characterized by deposition of fine-grainedstrata with volcanic petrofacies. (3) Latest Jurassic-Early Cretaceous oblique collision of Wrangellia prompted development of a narrow thrust belt and retroarc foreland basin along the inboard margin of the Jurassic arc. Coarse-grained siliciclastic strata were deposited on a north-dipping basin floor influenced by syndepositional thrust faults. (4) Volcanism was re-established along the northern basin margin during late Early Cretaceous time, placing the basin in a continental forearc setting and prompting paleodrainage reversal. Cretaceous marine siliciclastic strata were deposited on a southward-dipping basin floor influenced by intrabasinal normal faults that record reactivation of accretion-related thrusts. (5) The basin arrived at its current position by dextral translation along orogen-parallel strike-slip faults. Neogene accretion of the Yakutat terrane to southern Alaska prompted construction of a new volcanic field along the northern margin of the Wrangell Mountains basin. Neogene nonmarine strata deposited in extensional intra-arc basins unconformably overlie remnant marine forearc basin deposits. The evolution of the Wrangell Mountains basin underscores the importance of defining a sedimentary basin in terms of its plate tectonic setting during deposition rather than its present plate tectonic position.