Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: Stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat–North America collision zone

摘要

New geochronologic, geochemical, sedimentologic,nand compositional data fromnthe central Wrangell volcanic belt (WVB)ndocument basin development and vol canismnlinked to subduction of overthickened oceanicncrust to the northern Pacifi c plate margin.nThe Frederika Formation and overlyingnWrangell Lavas comprise >3 km of sedimentarynand volcanic strata exposed in thenWrangell Mountains of south-central Alaskan(United States). Measured stratigraphic sectionsnand lithofacies analyses documentnlithofacies associations that refl ect depositionnin alluvial-fl uvial-lacustrine environmentsnroutinely infl uenced by volcanic eruptions.nExpansion of intrabasinal volcanic centersnprompted progradation of vent-proximalnvolcanic aprons across basinal environments.nCoal deposits, lacustrine strata, and verticalnjuxtaposition of basinal to proximal lithofaciesnindicate active basin subsidence thatnis attributable to heat fl ow associated withnintrabasinal volcanic centers and extensionnalong intrabasinal normal faults. Thenorientation of intrabasinal normal faultsnis consistent with transtensional deformationnalong the Totschunda-Fairweather faultnsystem. Paleocurrents, compositional provenance,nand detrital geochronologic ages linknsediment accumulation to erosion of activenintrabasinal volcanoes and to a lesser extentnMesozoic igneous sources. Geochemical compositionsnof interbedded lavas are dominantlyncalc-alkaline, range from basaltic andesite tonrhyolite in composition, and share geochemicalncharacteristics with Pliocene–Quaternarynphases of the western WVB linked to subduction-nrelated magmatism. The U/Pb agesnof tuffs and 40Ar/39Ar ages of lavas indicatenthat basin development and volcanism commencednby 12.5–11.0 Ma and persisted untilnat least ca. 5.3 Ma. Eastern sections yieldnolder ages (12.5–9.3 Ma) than western sectionsn(9.6–8.3 Ma). Samples from two westernnsections yield even younger ages of 5.3 Ma.nIntegration of new and published stratigraphic,ngeochronologic, and geochemicalndata from the entire WVB permits a comprehensiveninterpretation of basin developmentnand volcanism within a regional tectonicncontext. We propose a model in which diachronousnvolcanism and transtensional basinndevelopment refl ect progressive insertion of anthickened oceanic crustal slab of the Yakutatnmicroplate into the arcuate continental marginnof southern Alaska coeval with reportednchanges in plate motions. Oblique northwestwardnsubduction of a thickened oceanic crustalnslab during Oligocene to Middle Miocene timenproduced transtensional basins and volcanismnalong the eastern edge of the slab along thenDuke River fault in Canada and subductionrelatednvolcanism along the northern edge ofnthe slab near the Yukon-Alaska border. Volcanismnand basin development migrated progressivelynnorthwestward into eastern Alaskanduring Middle Miocene through Holocenentime, concomitant with a northwestward shiftnin plate convergence direction and subductionncollision of progressively thicker crust againstnthe syntaxial plate margin.