Very low-grade metamorphism of the Dezadeash Formation (Jura-Cretaceous): Constraints on the tectonometamorphic history of the Dezadeash flysch basin and implications regarding the tectonic evolution of the Northern Cordillera of Alaska and Yukon

Mesozoic convergence of the Wrangellia composite terrane with the western margin of North America resulted in the collapse of intervening flysch basins. One of these basins, the Jurassic-Cretaceous Gravina-Nuzotin belt, comprises from south to north, the Gravina sequence and Gravina belt in southeastern Alaska, the Dezadeash Formation in Yukon, and the Nutzotin Mountains sequence in eastern Alaska. Previous work shows that the Gravina sequence and Gravina belt were underthrust > 20 km beneath the margin of North America in mid-Cretaceous time, culminating in amphibolite facies metamorphism. This tectonometamorphic scenario was subsequently applied to the entire Gravina-Nutzotin belt, despite any detailed studies pertaining to the tectonometamorphic evolution of the Dezadeash Formation. The present analysis of the Dezadeash Formation reveals that metamorphic mineral assemblages in sandstone and tuff document subgreenschist, high temperature zeolite facies metamorphism; Kübler indices of illite and Árkai indices of chlorite in mudstone record diagenetic to high anchizone metapelitic conditions; and the color of organic matter (i.e., the Thermal Alteration Index of palynomorphs and the Conodont Alteration Index) and pyrolysis of organic matter in mudstone and hemipelagite beds document thermal maturation at catagenesis to mesogenesis stages. Collectively, the mineralogic and organic thermal indicators in the Dezadeash Formation suggest that strata experienced maximum pressure-temperature conditions of 2.5 ± 0.5 kbar and 250 ± 25 ℃ in the Early Cretaceous. The inferred tectonometamorphic evolution of the Dezadeash Formation does not support the northern part of the Gravina-Nutzotin belt being underthrust > 20 km beneath the western margin of North America in mid-Cretaceous time, thus contrasting sharply with the Gravina sequence and Gravina belt in the southern part of the Gravina-Nutzotin belt. The diverse tectonometamorphic histories recorded by the southern and northern parts of the Gravina-Nutzotin belt may be a manifestation of oblique collision and diachronous south-to-north accretion of the Wrangellia composite terrane to North America.