ABSTRACT Eocene Eurekan deformation has proven to be an enigmatic sequence of tectonic episodes dominated by tectonic plate compression and translation in the circum-Arctic region. Prins Karls Forland on western Spitsbergen is composed of Neoproterozoic siliciclastic metasediments of Laurentian affinity regionally metamorphosed to greenschist facies conditions. A crustal-scale ductile to brittle deformation zone, here named the Bouréefjellet fault zone, contains the amphibolite facies Pinkiefjellet Unit exposed between the lower metamorphic grade, upper structural unit of the Grampianfjella Group and the Scotiafjellet Group in the footwall. A preliminary age for the amphibolite facies metamorphism (ca. 360–355 Ma) indicates Ellesmerian tectonism, unlike other higher-grade basement rocks on Svalbard. Ten metasedimentary rocks from within the fault zone were collected for multiple single-grain fusion 40 Ar/ 39 Ar geochronology, with up to ten muscovite crystals dated per sample. High strain in the rocks is evinced by mylonitic structure, mica fish, and C’ shear zones, and dynamically recrystallized quartz with significant grain bulging and subgrain rotation, indicative of >350 °C temperatures. There is notable dispersion in the 40 Ar/ 39 Ar ages between samples, with single muscovite dates ranging from ca. 300 Ma to as young as 42 Ma, reflecting recrystallization and resetting of the muscovite. Younger, reproducible ages were obtained from samples that possess chemically homogeneous muscovite, yielding dates of 55–44 Ma for the Eurekan deformation on Prins Karls Forland. We suggest that Ellesmerian structures on Prins Karls Forland were reactivated during the Eocene (commencing as early as 55 Ma) progressing under warm, yet brittle, conditions that continued to 44 Ma. These 40 Ar/ 39 Ar muscovite dates are the first documented Eurekan deformation ages from Svalbard and enable a better understanding of the stages of Eurekan deformation in the Eocene to improve correlations across the circum-Arctic region.

Abstract The Norwegian Arctic Svalbard archipelago is famous for its prolific record of Early–Middle Triassic vertebrate fossils. These represent mainly marine amniotes, together with sharks, bony fishes and temnospondyl amphibians, the latter providing an important faunal correlate with coeval assemblages from the Danish autonomous region of Greenland. However, substantial biostratigraphical gaps exist in the Upper Triassic strata of Svalbard, which are marked by pronounced facies shifts from marine to deltaic systems and intermittent depositional hiatuses. These are accompanied by a dearth of documented vertebrate remains, a notable exception being the partial skull of the capitosaurian Capitosaurus polaris and a few isolated stereospondylian intercentra probably from the middle–late Carnian De Geerdalen Formation of Spitsbergen. Reassessment of this material, which incorporates the only undisputed capitosaurian fossil from Svalbard, indicates affinity with Cyclotosaurus , known elsewhere from the late Norian–early Rhaetian Fleming Fjord Formation of Greenland. The Scandinavian Arctic temnospondyls constituted components of sympatric assemblages that inhabited the Boreal margin of Pangaea throughout the Triassic.