TECTONIC EVENTS LEADING TO ESTABLISHMENT OF YUKON FLATS BASIN, ALASKA

The vast Yukon Flats basin in east-central Alaska is a product of latest Cretaceous(?) and early Tertiary extension and right-lateral movement along the Tintina fault system.

The basin is underlain by the extensive Tozitna terrane, a Devonian to Jurassic oceanic assemblage obducted onto the Arctic Alaska margin during the Jurassic and remobilized at least once during the mid-Cretaceous. Southeast of Yukon Flats, Tozitna terrane rocks occupy the highest structural position in the Kandik thrust belt, a southeast-directed contractional belt that was emplaced onto the North American margin during mid-Cretaceous time. Yukon Flats Basin lies west of the Kandik thrust belt, where one would expect to find the thrust belt's root zone.

A transition to extensional tectonics during the latest Cretaceous, at least locally, is indicated by the Maastrichtian Nation River Basin, which occupies a half-graben above the remnants of the Kandik thrust belt. Cheyenne Creek Basin, southwest of Yukon Flats, is also Maastrichtian, but its original tectonic setting and location have been obscured by movement on the Tintina fault system. Seismic profiles show an extensional basin in southern Yukon Flats that also may have started forming as early as the Maastrichtian.

Early Tertiary bimodal magmatism affected a broad area on the southern and western borders of Yukon Flats and may have involved rocks now buried in the basin. In the southwestern corner of Yukon Flats, rapid exhumation of kyanite-bearing metamorphic rocks, generation of strong ductile fabrics, and creation of small basins accompanied emplacement of 60-Ma bimodal plutonic and volcanic rocks. This crustal extension event also may have affected rocks now underlying the basin.

Apatite fission-track data record multiple episodes of movement on the Tintina system after the extensional event. These movements resulted in uplift of rocks on the southern boundary of the basin. At ~23 Ma, uplift related to reverse faulting occurred along NE-SW oriented splays of the Tintina fault southwest of Yukon Flats Basin. This event effectively dammed the Yukon River, isolating Yukon Flats from the lower Yukon Basin. Widely distributed horizontal reflectors imaged by seismic reflection may represent Miocene lacustrine and fluvial strata deposited in response to the change in hydrologic profile.