David G. Gee

Grenvillian basement and a major unconformity within the Caledonides of Nordaustlandet, Svalbard

Abstract Variously deformed granites and migmatites underlie the Neoproterozoic successions of Nordaustlandet, Svalbard. New U Pb zircon investigations of two types of intrusions from Lapponiahalvoya, northwestern Nordaustlandet, have yielded ages of 939 ± 8 Ma (Kontaktberget granite) and 961 ± 17 Ma (Lappoinafjellet granite), confirming previous, less precise multi-grain, Pb Pb evidence of Grenvillian igneous activity. The Kontaktberget granite cuts metasediments (Brennevinsfjorden Group), that are tightly folded and are separated from overlying volcanic and volcaniclastic rocks (Kapp Hansteen Group) by a major discordance (the Botniahalvoya Unconformity). The latter has been mapped in northwestern Nordaustlandet and is inferred to exist in central areas between Vestfonna and Austfonna. The Kapp Hansteen Group, with associated intrusive quartz porphyries, are probably a late orogenic expression of the Grenvillian igneous activity. The lack of igneous rocks in the overlying Neoproterozoic succession (Murchisonfjorden Supergroup) suggests that the base of this major unit is also a significant unconformity. This new evidence emphasizes the importance of the belt of Grenvillian tectonothermal activity, referred to here as the Nordauslandet Orogeny, that reaches from the classical provinces of North America and southern Scandinavia, via the East Greenland Caledonides, into the high Arctic.

Early Proterozoic basement in Ny Friesland ― implications for the Caledonian tectonics of Svalbard

Abstract Recent isotope age-determination studies in western parts of the Svalbards 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 Svalbards 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.

Late Caledonian extension in the scandinavian Caledonides—the Røragen Detachment revisited

Abstract This paper concerns the interplay of compression and extension during Palaeozoic collisional orogeny in Scandinavia, and focuses on a small Old Red Sandstone basin in the central Scandes at Roragen. Early Devonian sediments are preserved there in a half-graben located in the western limb of a major, late, upright antiform. The latter, the Vigelen Antiform, exposes Proterozoic crystalline rocks in the core and folds the entire Caledonian tectonostratigraphy of the Lower, Middle and Upper Allochthons. Mylonites related to the eastward-directed thrust assembly of these allochthons have been mapped over the antiform; they occur in the western limb of the fold in the footwall of a high-angle W-dipping normal fault, the latter separating the Old Red Sandstones from the Caledonian nappes. These thrust-generated mylonites exhibit a suite of superimposed down-to-NW shear sense indicators in the footwall of the fault. Displacement on this Roragen Detachment is inferred to be about ten kilometers. Reflection seismic profiling across the northern extension of the Vigelen Antiform, in the Storlien area, indicates that this major longitudinal fold developed in the hanging wall of a sole thrust that dips very gently westwards beneath this part of the mountain belt. It is argued here that the Early Devonian Roragen half-graben may have developed during thrust stacking of the Vigelen Antiform, providing evidence of Scandian upper crustal extension accompanying the emplacement of the thrust sheets onto the Baltoscandian Platform. In the southern Scandes, full crustal extension would have occurred thereafter, leading to the development of the mid-Devonian basins in the hinterland of westernmost Norway.