Extensional shear zones of Devonian age are superimposed on the Caledonian nappe stack in northern Central Norway, where they constitute signs of extensional denudation of the orogen. They divide in to either broad detachment zones, or narrow, normal-sinistral shear ro nes, all showing a consistent, WSW-ENE bulk stretching axis, highly oblique to the recorded Scandian thrust direction (mainly SE-directed). Characteristically, mediumgrade and ductile, low-angle detachment zones truncate the nappes at a low angle, and sole out in a basal shear zone ( decollement) above Fennoscandian basement which, based on metamorphic grade, was located at a middle crustal level. This dècollement may have partitioned dominantly NW-SE contractional strain of the lower crust from mainly extensional tectonics in the middle-upper crust. Medium-grade detachments are succeeded by narrow, low-grade, ductile to brittle, steeper shear zones that cut deep into the parautochthonous(?) Fennoscandian basement. Later events include slight, ENE-directed thrust-inversion of some detachrnent and shear zones, and succeeding to p-WNW and top-SW extensional faulting. The overall extension pattern probably reflects Early Devonian, near orogen-parallel extension and upper-crust extension and block extrusion, followed by deeper-seated, Late Devonian to Early Carboniferous oblique extension. Major, top-WSW, latest-Scandian extension can be attributed to plate-driven, sinistral transpression during waning stages of collision between Baltica and Laurentia. Later, to p-WNW extension may reflect a postorogenic transtensional setting of Late Devonian - Carboniferous(?) age.
Orogen-parallel extension of the Caledonides in northern central Norway: an overview