Cretaceous to Oligocene magmatic and tectonic evolution of the western Alaska Range: Insights from U-Pb and 40Ar/39Ar geochronology

New U-Pb and 40Ar/39Ar ages integrated with geologic mapping and observations across the western Alaska Range constrain the distribution and tectonic setting of Cretaceous to Oligocene magmatism along an evolving accretionary plate margin in south-central Alaska. These rocks were emplaced across basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Triassic and Jurassic plutonic and volcanic rocks of the Peninsular terrane, and Jurassic and Cretaceous siliciclastic strata of the Kahiltna assemblage. Plutonic rocks of different ages also host economic mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related rare-earth elements. The oldest intrusive suites were emplaced ca. 104–80 Ma into the Peninsular terrane only prior to final accretion. Deformation of the northern Kahiltna succession and underlying Farewell terrane occurred at ca. 97 Ma, and more widespread deformation ca. 80 Ma involved south-ver­gent folding and thrusting of the Kahiltna assemblage that records collisional accretion of the Peninsular-Wrangellia terrane and juxtaposition of sediment wedges formed on the inboard and outboard terranes. More widespread mag­matism ca. 75–55 Ma occurred in two general pulses, each having distinct styles of localized deformation. Circa 75–65 Ma plutons were emplaced in a transpressional setting and stitch the accreted Peninsular and Wrangellia terranes to the Farewell terrane. Circa 65–55 Ma magmatism occurred across the entire range and extends for more than 200 km inboard from the inferred position of the continental margin. The Paleocene plutonic suite generally reflects shallower emplacement depths relative to older suites and is associ­ated with more abundant andesitic to rhyolitic volcanic rocks. Deformation ca. 58–56 Ma was concentrated along two high-strain zones, the most prominent of which is 1 km wide, strikes east-northeast, and accommodated dextral oblique motion. Emplacement of widespread intermediate to mafic dikes ca. 59–51 Ma occurred before a notable magmatic lull from ca. 51–44 Ma reflect­ing a late Paleocene to early Eocene slab window. Magmatism resumed ca. 44 Ma, recording the transition from slab window to renewed subduction that formed the Aleutian-Meshik arc to the southwest. In the western Alaska Range, Eocene magmatism included emplacement of the elongate north-south Mer­rill Pass pluton and large volumes of ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, a latest Eocene to Oligocene magmatic pulse involved emplacement of a compositionally variable but spatially concentrated suite of magmas ranging from gabbro to peralkaline granite ca. 35–26 Ma, followed by waning magmatism that coincided with initiation of Yakutat shallow-slab subduction. Cretaceous to Oligocene magmatism throughout the western Alaska Range collectively records terrane accretion, translation, and integration together with evolving subduction dynamics that have shaped the southern Alaska margin since the middle Mesozoic.

A new genus and species of polychelid lobster (Crustacea, Decapoda, Eryonidae) from the Early Jurassic (Hettangian) of British Columbia

A single specimen of decapod crustacean, preserved in ventral view and compressed, represents a new genus and species of eryonid lobster, Wrangelleryon perates. The discovery in Lower Jurassic (Hettangian) sediments of the Sandilands Formation in British Columbia represents the first occurrence of Eryonidae in North America and reinforces a global distribution of the family in the Jurassic. The occurrence in British Columbia on the Wrangellia terrane supports the lower latitude setting in which the species lived.

Globally significant Early Permian crinoids from the Mount Mark Formation in Strathcona Provincial Park, Vancouver Island, British Columbia — preliminary analysis of a disappearing fauna

Strata of the Mount Mark Formation, Buttle Lake Group, exposed in the vicinity of Marble Peak in Strathcona Provincial Park, central Vancouver Island, contain a diverse Early Permian crinoid fauna. This is the first Permian fauna containing crowns and cups recognized from Wrangellia terrane. The fauna contains representatives of each of the major Paleozoic crinoid subclasses: Camerata, Disparida, and Cladida. Specimens were observed and photographed between 2004 and 2008. No specimens were collected in adherence to regulations of Stratcona Provincial Park. Preliminary identifications recognize several new genera and species within the fauna, but they are not named or described lacking specimens for repository. A minimum of 24 species are judged to be in the fauna, making it the second most diverse Permian fauna known from North America. Identified genera suggest a greater relationship to North American faunas than to Paleotethyan faunas, suggesting Wrangellia was closer to North America than to the Paleotethyan realm during Early Permian time. Exposures of the Mount Mark Formation in the vicinity of Marble Peak are undergoing karstification, and specimens are being lost under the harsh weathering conditions. Observations of individual specimens over a five-year interval found that morphologic details critical for identification are being lost at an alarming rate. It is recommended that sufficient specimens in the fauna should be collected as soon as possible for identification and analysis. These specimens should be preserved for future reference and could form an educational display in the Strathcona Provincial Park headquarters.