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

<abstract> <p>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 &gt; 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 &gt; 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.</p> </abstract>

Provenance analysis of the Dezadeash Formation (Jurassic–Cretaceous), Yukon, Canada: implications regarding a linkage between the Wrangellia composite terrane and the western margin of Laurasia

The Mesozoic convergence of the allochthonous Wrangellia composite terrane (WCT) with the western margin of Laurasia coincided with the construction of the Chitina magmatic arc on the WCT, and the dispersal of volcanic flows and sediment gravity flows into an adjacent flysch basin. The basin, preserved as the Gravina–Nutzotin belt, includes the Dezadeash Formation in southwest Yukon, the Nutzotin Mountains sequence in southern Alaska, and the Gravina belt in southeastern Alaska. The Dezadeash Formation is a submarine fan system comprising stacked channel-lobe transition and lobe deposits interposed with overbank deposits. Conglomerate pebble-counts, sandstone point-counts, detrital zircon ages, and major element, trace element, rare earth element, and Sm–Nd isotopic geochemistry of sandstone, mudstone, and hemipelagite beds suggests that the deposits consist mainly of first-cycle volcanogenic detritus shed from the undissected Chitina arc, in addition to material eroded from the WCT. The arc was constructed of undifferentiated magma sourced from the depleted mantel, as well as older crustal material attributed to the WCT proxying for continental crust. The compositional provenance results, together with published paleocurrent data for the Dezadeash Formation and compositional and directional provenance indicators from the Nutzotin Mountains sequence and Gravina belt, does not require a sediment source from Laurasia. The provenance record is compatible with deposition of the Gravina–Nutzotin belt in a convergent plate margin setting.

U–Pb and Hf isotopic analyses of detrital zircons from the Taku terrane, southeast Alaska

The Taku terrane consists of metamorphosed Carboniferous through Triassic marine clastic strata, volcanic rocks, and limestone that occur along the western margin of the Coast Mountains in southeastern Alaska. These rocks are juxtaposed along mid-Cretaceous thrust faults over Jura-Cretaceous basinal strata of the Gravina belt to the west and beneath Proterozoic through Carboniferous metamorphic rocks of the Yukon–Tanana terrane to the east. This paper presents U–Pb ages and Hf isotope analyses of detrital zircons from the Taku terrane, and compares these values with information from the adjacent Wrangellia, Alexander, and northern and southern portions of the Yukon–Tanana terrane (YTTn and YTTs). These comparisons suggest that (i) Carboniferous strata of the Taku terrane were shed mainly from mid-Paleozoic igneous rocks of YTTs, (ii) Permian strata of the Taku terrane were shed from mid-Paleozoic igneous rocks and intraformational Lower Permian volcanic rocks of YTTs as well as Upper Permian volcanic rocks exposed in YTTn, and (iii) Triassic sandstones were shed from mid-Paleozoic igneous rocks of YTTs, whereas conglomerates were shed mainly from mid-Paleozoic arc rocks in YTTn. Hf isotope analyses of Paleozoic zircons record increasing continental input during Silurian–Devonian and Permian phases of magmatism. Similarities in isotopic characteristics, combined with stratigraphic and geochemical information presented by previous workers, suggest that strata of the Taku terrane accumulated on (and partly as lateral equivalents of) rocks of YTTs, and that the combined assemblages formed outboard or along strike of YTTn.

Emplacement, rapid burial, and exhumation of 90-Ma plutons in southeastern Alaska

In southeastern Alaska, granodiorite–tonalite plutons of the Admiralty–Revillagigedo belt intruded the Jurassic–Cretaceous Gravina belt along the eastern side of the Alexander terrane around 90 Ma. These plutons postdate some deformation related to a major contractional event between the previously amalgamated Wrangellia and Alexander terranes and the previously accreted terranes of the North American margin. We studied the aureole mineral assemblages of these plutons near Petersburg, Alaska, determined pressure and temperature of equilibration, and examined structures that developed within and adjacent to these plutons. Parallelism of magmatic and submagmatic fabrics with fabrics in the country rock indicates synchroneity of pluton emplacement with regional deformation and suggests that magma transport to higher crustal levels was assisted by regional deformation. Replacement of andalusite by kyanite or sillimanite indicates crustal thickening soon after pluton emplacement. Regional structural analysis indicates the crustal thickening was accomplished by thrust burial. Thermobarometric analyses indicate the aureoles reached near-peak temperatures of 525 to 635 °C at pressures of 570 to 630 MPa. Consideration of the rate of thermal decay of the aureoles suggests that burial was rapid and occurred at rates around 5 to 8 mm/year. Structural observations indicate there was contractional deformation before, during, and after emplacement of the 90-Ma plutons. Initial exhumation of the Admiralty–Revillagigedo belt in the Petersburg area may have occurred along a thrust west of the pluton belt within the Gravina belt.

Detrital zircon constraints on the tectonic evolution of the Gravina belt, southeastern Alaska

Upper Jurassic - Lower Cretaceous marine clastic strata and mafic to intermediate volcanic rocks of the Gravina belt are part of a complex suture zone separating the Alexander and Wrangellia terranes on the west from the Yukon-Tanana and Stikine terranes to the east. U-Pb ages have been determined on 118 single detrital zircon grains from Gravina strata in an effort to determine the tectonic setting of the Gravina belt and the paleoposition of outboard terranes prior to their Late Cretaceous juxtaposition against inboard terranes. Samples from five stratigraphic units yield ages of 105-120 (n = 5), 140-165 (n = 56), 310-380 (n = 17), 400-450 (n = 19), 520-560 (n = 5), 920-1310 (n = 5), and 1755-1955 Ma (n = 5). The 105-120 and 140-165 Ma grains were shed primarily from arc-related plutons that lie outboard of the Gravina belt. The lack of 120-140 Ma ages coincides with a lull in magmatism in the outboard arc and in the western United States, which suggests that Gravina strata accumulated during major changes in plate motion along the Cordilleran margin. The 400-560 Ma zircons were derived from rocks of the Alexander terrane which also lie to the west. In contrast, the 310-380 and >900 Ma grains were apparently shed from inboard regions. Likely sources include the Yukon-Tanana and Stikine terranes in the northern Cordillera and assemblages in the northern California region which contain igneous rocks and detrital zircons of the appropriate ages. Our data accordingly support models in which the Gravina basin formed in narrow rift or transtensional basins, whereas the outboard Alexander and Wrangellia terranes were located along the California - Oregon - Washington - British Columbia - Alaska margin. Our data are less supportive of models in which the Gravina strata and underlying Alexander and Wrangellia terranes were separated from western North America by a large ocean basin, or were located along the coast of Mexico.