STRUCTURAL EXPRESSIONS AND TECTONIC IMPLICATIONS OF GENERAL NONCOAXIAL FLOW IN THE MIDCRUST OF A COLLISIONAL OROGEN - THE NORTHERN SCANDINAVIAN CALEDONIDES

Mapping and strain analysis in the northern Scandinavian Caledonides i ndicate that one way in which contemporaneous upper crustal extension and deeper thrusting within collisional orogens may be linked in a kin ematically compatible manner is through the ductile flow of rocks in t he midcrust of these mountain belts. In the Ofoten-Efjorden region of north Norway, amphibolite facies metamorphism and strong penetrative s train 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 ind icates that the rock mass underwent heterogeneous noncoaxial flow, wit h components of foreland directed simple shear and subvertically short ened pure shear. General noncoaxial flow (simultaneous simple shear pure shear) can accomplish foreland-directed tectonic transport, subve rtical thinning, and transport-parallel elongation at deep levels of a nappe stack. This pattern of flow is significant tectonically for two reasons: first, it suggests spreading of the Caledonian allochthon du ring thrust emplacement, and second, it can form a kinematic link betw een normal faulting at shallower structural levels and thrusting at th e base of the nappes. By analogy with the Caledonides, general noncoax ial flow of midcrustal rocks may represent an important process in oth er collisional and contractional orogens, particularly in those that h ave undergone upper crustal extension while thrusting continued in the foreland.