EXHUMATION OF UHP ROCKS IN WESTERN NORWAY - THE ROLE OF THE NORDFJORD-SOGN DETACHMENT ZONE

The ultrahigh pressure (UHP) province of western Norway has long been considered to represent the lowest structural level of the Western Gneiss Region, forming the footwall to the major late-Caledonian extensional detachment in western Norway – the Nordfjord-Sogn Detachment Zone (NSDZ). We carried out a detailed structural, metamorphic and thermochronological analysis of the Nordfjord area, examining the transition from lower pressure rocks westward to the UHP province. Our findings show, in contrast to the widely held view, that (1) the UHP rocks are located within an allochthonous gneiss complex above the autochthonous Baltica basement, and (2) the NSDZ bifurcates from a single detachment into a wide zone of symmetrical shearing (7–10 km) with roof and floor detachments, which bounds the UHP rocks. The floor detachment follows an early–orogenic thrust surface.

This new understanding of the geometry of the NSDZ has important implications for the exhumation mechanics of the UHP province. If the top–W extensional displacement along the NSDZ is removed, the UHP rocks are restored to a structurally higher position ~100 km farther inland, overlying the Baltica basement. However, this juxtaposes the UHP rocks (2.5–3.0 GPa), against basement eclogites showing similar peak pressures (2.2–2.5 GPa). Therefore, movement on the NSDZ was intra-crustal only, post-dating the return of the high–pressure rocks to the base of the orogenic crust; the processes and structures that exhumed the UHP rocks through the mantle remain enigmatic. 40Ar/39Ar muscovite ages of ~393–390 Ma throughout the NSDZ, indicate that this structure became inactive once it reached the mid-crust, and denudation was then transferred to the structurally higher normal faults.