Structural expressions and tectonic implications of general noncoaxial flow in the midcrust of a collisional orogen: The northern Scandinavian Caledonides

Abstract

Mapping and strain analysis in the northern Scandinavian Caledonides indicate that one way in which contemporaneous upper crustal extension and deeper thrusting within collisional orogens may be linked in a kinematically compatible manner is through the ductile flow of rocks in the midcrust of these mountain belts. In the Ofoten‐Efjorden region of north Norway, amphibolite facies metamorphism and strong penetrative strain occurred at deep levels of the Caledonian nappe stack as it was thrust eastward onto the western margin of Baltica. Kinematic analysis of deformation in the lower portion of the Caledonian nappe stack indicates that the rock mass underwent heterogeneous noncoaxial flow, with components of foreland directed simple shear and subvertically shortened pure shear. General noncoaxial flow (simultaneous simple shear + pure shear) can accomplish foreland‐directed tectonic transport, subvertical thinning, and transport‐parallel elongation at deep levels of a nappe stack. This pattern of flow is significant technically for two reasons: first, it suggests spreading of the Caledonian allochthon during thrust emplacement, and second, it can form a kinematic link between normal faulting at shallower structural levels and thrusting at the base of the nappes. By analogy with the Caledonides, general noncoaxial flow of midcrustal rocks may represent an important process in other collisional and contractional orogens, particularly in those that have undergone upper crustal extension while thrusting continued in the foreland.