The Nordfjord-Sogn detachment Zone in the Naustdal region, Norway

Abstract

Late Scandian (400 – 425 Ma) exhumation in the Scandinavian Caledonides involved extensive displacement along several shear zones. The Western Gneiss Region (WGR) hosts one of these detachments also called the Nordfjord-Sogn detachment shear zone (NSDZ). It is still unclear to what extent this shear zone was involved in the exhumation of Baltica. Furthermore, whether this shear zone formed at mantle or crustal depths and how much of the displacement during exhumation was accommodated by the NSDZ. In this study these issues were addressed and researched through field research along cross-sections at several locations perpendicular to the NSDZ. Furthermore, monazite geochronology was used for determining apparent ages of monazites in the Precambrian basement and overlying nappes. As such it was possible to indentify that both the basement and nappes have been affected by the NSDZ in a up to at least 2 km wide zone which is characterized by amphibole and chlorite metamorphism. Even though the NSDZ does not runs along the basement-nappe contact continuously it is thought that because this is the only major detachment in the WGR it must have had some influence on the relative basement-nappe positioning. Monazite have an apparent age of ~ 500 Ma in both the Preacambrian basement and overlying the nappe. These Finmarkian monazites implying that basement and overlying nappe went through the same metamorphic cycle in a single episode.