Early proterozoic basement in Ny Friesland—implications for the Caledonian tectonics of Svalbard

Abstract

Recent isotope age-determination studies in western parts of the Svalbard's Eastern Terrane in northern Ny Friesland have provided evidence that early Palaeozoic and Late Proterozoic successions, characteristic of the Laurentian margin, are underlain by an amphibolite facies complex composed, at least in part, of Early Proterozoic granites (ca. 1750 Ma) and metasedimentary rocks. New work on the structural relationships, reported here, indicate that these granites compose extensive thrust sheets, tectonically intercalated with metasediments. Some of the latter were originally intruded by the granites; others probably composed a younger cover, inferred to be of mid- or even Late Proterozoic age. Caledonian penetrative strains dominate the lower structural levels in Ny Friesland and most of the rock units are concordant, strongly foliated and display a range of tight to isoclinal folds. The granitic rocks, most of which have been previously interpreted as metavolcanites, are generally gneissic and often mylonitic. Early folds, regional foliations and thrusts are all arched by a major N-S-trending fold (the Atomfjella Antiform) that runs the 150-km length of western Ny Friesland. Caledonian thrusting of the basement is inferred to have been directed westwards. The new structural and isotopic evidence enhances comparisons of Svalbard's Eastern Terrane with the Caledonides of East Greenland. Terranes further west on Spitsbergen have little in common with the Eastern Terrane, supporting the hypothesis that terrane accretion in this part of the Caledonides involved very substantial transcurrent displacements.