Exhuming Norwegian ultrahigh-pressure rocks: Overprinting extensional structures and the role of the Nordfjord-Sogn Detachment Zone

摘要

The Nordfjord-Sogn Detachment Zone (NSDZ) is widely cited as one of the primary structures responsible for the exhumation of Norwegian (ultra)high-pressure (UHP) rocks. Here we review data from the considerable volume of research describing this shear zone, and compile a strike-parallel cross section along the NSDZ from the Solund Basin in the south to the Sørøyane UHP domain in the north. This cross section highlights several previously unrecognized patterns, revealing a shear zone with top-to-the-west asymmetric fabrics that (1) initiated at amphibolite facies, (2) overprints metamorphic breaks and tectonostratigraphic contacts, and (3) has a gradational continuum of muscovite cooling ages. These patterns constrain the kinematic evolution of the NSDZ and suggest a new three-step model for the exhumation of Norwegian (U)HP rocks. The initial stages of exhumation were characterized by the rise of crustal rocks from (U)HP depths to the base of the crust by buoyancy-driven mechanisms not specified in this paper. Mantle exhumation was followed by top-to-the-west, normal-sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation. In the final stages of crustal exhumation, top-W brittle-ductile detachments soled into and partially excised this ductile shear zone, dropping the Devonian basins into contact with rocks of varying tectonostratigraphic levels. This new interpretation of the NSDZ is significant as it accounts for the extreme crustal excision observed in western Norway using three sequentially overprinting structures active at different stages of UHP rock exhumation.