The collapse of the Caledonian orogen in SW Norway: Insights from quartz textures

Abstract

Extensional collapse is a common late-to post-collisional feature of orogens. It is particularly prominent in the SW Scandinavian Caledonides, where extensional detachments formed progressively from the initial reactivation of the basal thrust zone to the formation of hinterland-dipping extensional shear zones. A mature stage, documented here, involves the development of bivergent (both hinterland- and foreland-dipping) shear zones and associated vertical basement mobilization. The main foreland-facing extensional shear zone in the study area is the Bergen Detachment – an until recently overlooked or misinterpreted structure. This detachment overprints top-to-W mylonitic fabrics related to the earlier Devonian extension stages and developed in response to the updoming of the Baltica basement west of Bergen (the Øygarden Complex) into a late core complex. Our microstructural and textural examinations suggest that for both the Hardangerfjord Shear Zone and the Bergen Detachment, strain was localized by activation of dislocation creep in quartz through the operation of multiple slip systems in the <a> direction, predominantly prism <a> and rhomb <a>. These examinations and existing radiometric age constraints suggest that the progressive shear zone development occurred over maybe as little as 5 million years, under upper to middle greenschist facies conditions. Synkinematic cooling brought both the Bergen Detachment and Hardangerfjord Shear Zone through the ductile-brittle transition zone. The main explanation for this prolonged collapse development is 1) that the early low-angle detachment became too low-angle for continued shearing, giving rise to the first hinterland-dipping set of shear zones, and 2) that the basement weakened rheologically and mobilized gravitationally with the formation of large upright folds with new detachments along their flanks (the bivergent stage), including the Bergen Detachment.