NEW CONSTRAINTS ON THE ORIGIN OF THE TIKCHIK TERRANE, SOUTHWESTERN ALASKA

New mapping, fossil ages, igneous geochemistry and sandstone detrital zircon (DZ) ages provide new constraints on the origin of the Tikchik terrane in SW Alaska, a structurally complex assemblage of lower Paleozoic to Lower Cretaceous chert, clastic rocks, volcanic rocks and limestone. The terrane comprises a broad SSW-plunging anticlinorium of 3 structural plates, bounded to the WNW by the Cenozoic dextral Togiak-Tikchik fault, and faulted over Upper Cretaceous Kuskokwim Group strata to the NE. The deepest structural level consists of pervasively boudinaged (NE-vergent) red ribbon chert (Ordovician? to Triassic) and undated sandstones with a continental DZ suite (3020-430 Ma), interpreted as dismembered sedimentary components of a distal Paleozoic continental margin-to-oceanic basin transition. The structurally overlying plate has a more coherent, imbricated stratigraphy with considerable strike exposures of 60-m-thick Permian limestone (on uncertain base) grading upward to Triassic deep-water argillaceous strata with carbonate turbidites. The highest structural package, in separate exposures on the west and east flanks of the anticlinorium, is characterized by coherent sections that include Permian chert- and slate-clast conglomerates and sandstones with a similar continental DZ suite (locally with Permian zircons) to that of the lowest structural plate. The west flank exposure consists of an andesite-dacite arc volcanic pile overlain by earliest Miss. turbiditic carbonates, upper Paleozoic conglomeratic turbidites, and Triassic mudstones. The east flank exposure consists of shallow-marine Permian pebbly strata unconformably overlying a deformed chert sequence as old as early Paleozoic. Locally, MORB and OIB basaltic blocks occur between structural packages. The Tikchik anticlinorium is overthrust on the south by a several km thick pile of Permian to Triassic submarine volcanic arc basalts with flat REE, the basal unit of the Permian-Early Cretaceous Togiak volcanic arc terrane. We infer the Tikchik terrane to represent imbricated components of a Permo-Triassic subduction complex (lowest plate) and its overlying/adjacent syn-deformational basin(s), offscraped during SW subduction (present coords.) that generated the Permo-Triassic section of the adjacent Togiak arc terrane.