Uplift of deep crust during orogenic extensional collapse: A model based on field studies in the Sogn‐Sunnfjord Region of western Norway

Abstract

The Western Gneiss Region (WGR) in Norway experienced high‐pressure metamorphism during Silurian‐Devonian continent‐continent collision. The eclogite‐bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen; formation of the Devonian basins is related to the extension. The footwall of the detachment zones comprises three structural and metamorphic zones. The upper zone, zone 1, is characterized by penetrative homogeneous down‐to‐the‐west simple shear developed under retrograde greenschist‐facies metamorphism. Zone 2 suffered inhomogeneous simple shear of the same polarity. Petrography and mineral chemistry data from the lower zone, zone 3, show a record of initial eclogite facies metamorphism at 600°C and >16 kbar, which was decompressed almost isothermally to amphibolite‐facies conditions at 550°C and 10–12 kbar. Both the eclogite‐ and amphibolite‐facies metamorphism developed in a regime of pure shear with vertical shortening. The rapid decompression records an approach of approximately 20 km to the surface, related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere. The pure shear regime, which developed initially in the lower crust, was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels. Extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear. The shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust. A considerable amount of tectonic stripping of the orogenic welt predates deposition of the Devonian sedimentary basins.