The Silurian to Permian history of a metamorphic core complex in Lofoten, northern Scandinavian Caledonides

Abstract

[1] The Lofoten archipelago exposes Precambrian Baltic basement and Caledonian allochthonous sequences within a 1000 km long chain of gravity and magnetic highs and structural culminations along the extended, British and Norwegian continental shelf. Previous regional geophysical studies indicate that post-Caledonian extension and development of the northern Norwegian shelf occurred during broadly defined Carboniferous-Permian, Cretaceous-Jurassic, and early Tertiary events. Structures related to these events are known to young westward. We report field, structural, and Ar-40/Ar-39 thermochronologic data from rocks exposed in Lofoten that further define the history and style of post-Caledonian extension. The islands of southwest Lofoten also represent the most outboard exposures of Caledonian basement in northern Norway that presumably formed the middle to deep crustal core of the orogen. Metasedimentary rocks and penetratively deformed basement in Lofoten record high-grade Silurian-Devonian metamorphism and top-to-the-east ( hereinafter tops-east) thrusting followed by episodes of Late Devonian to Early Carboniferous, tops-west, ductile extension which progressed into oblique left-slip movements. The structural style and timing of Silurian contraction in this area are remarkably similar to that determined for the more forelandward areas on the mainland, similar to 120 km to the east, supporting the inference that distal parts of the Baltic continental margin that were once deeply subducted are presently exposed in Lofoten. The timings of post-Devonian structural events that affected rocks in Lofoten are partially constrained by the ages of unconformities and strata known to be preserved in graben flanking the Lofoten culmination. The radiometric age and structural data presented in this study, in combination with stratigraphic constraints, suggest a westward progression through time of extensional deformation over a protracted interval of Silurian to Permian time. The latest, Permian extension in Lofoten is largely characterized by brittle structures that formed at conditions substantially less than 300 degreesC. Compared to the exhumation history of the southern Western Gneiss Region, the depth of Caledonian, continental (A-type) subduction and subsequent unroofing of Lofoten are of lesser magnitude, and the present erosional level remained in the middle crust for a much longer interval of time. The Permian Ar-40/Ar-39 mineral ages documented in this study are the youngest such ages yet identified in Scandinavia. These ages relate to episodes of deformation and cooling in response to extensional tectonic events that occurred roughly 100 m. y. after comparable effects identified on the Caledonian mainland. Our preferred explanation for the Carboniferous-Permian radiometric ages, structural evolution, and stratigraphic data for Lofoten is that they all developed in the context of a long-lived Cordilleran-style metamorphic core complex.