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Very low-grade metamorphism of the Dezadeash flysch (Jura-Cretaceous): Constraints on the burial history of the Nutzotin-Dezadeash basin and implications regarding the tectonic evolution of the Northern Cordillera of Alaska and Yukon
  • Grant Lowey
Grant Lowey

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Secondary mineral assemblages in sandstone and tuff indicate high temperature zeolite facies metamorphism; Kübler indicies of illite and Árkai indicies of chlorite in mudstone record diagenetic to high anchizone metapelitic conditions; and pyrolysis of organic matter and the color of organic matter (i.e., the Thermal Alteration Index of palynomorphs and the Conodont Alteration Index) in mudstone and hemipelagite beds suggest thermal maturation reached catagenesis to mesogenesis stages. Collectively, the mineralogic and organic thermal indicators suggest the Dezadesh Formation was subject to pressure-temperature (P-T) conditions of 2.5 kbar and 250 °C. The estimated P-T conditions, together with published thermochronometric data, shows that the Dezadeash Formation underwent rapid, short-term heating followed by gradual, long-term cooling. Moreover, a calculated tectonic subsidence curve indicates rapid, short-term subsidence, followed by gradual, long-term uplift. Secondary clay minerals associated with heating and subsidence are characterized by a restricted assemblage dominated by 2M1 illite and chlorite. The thermal history, subsidence history, and secondary clay mineral assemblage are not supportive of deposition in peripheral foreland, backarc, strike-slip, and rift basins; nor are the results corroborative with previous deformation and crustal-scale reconstructions depicting the Dezadeash Formation being underthrust >20 km beneath the Blanchard River assemblage, Kluane Schist, and Yukon composite terrane (YCT). The Dezadeash-Nutzotin basin contrasts sharply with the contemporaneous Gravina belt and Gravina sequence in southeastern Alaska that were apparently underthrust >20 km beneath the YCT. The contrasting tectono-metamorphic histories may be a manifestation of oblique collision and diachronous, south-to-north accretion of the Chitina arc and WCT to YTC.