Orogen-parallel transport and vertical partitioning of strain during oblique collision, Efjorden, north Norway

Thrust sheets in the Caledonian allochthon in northern Scandinavia were emplaced generally toward the ESE, an orientation nearly perpendicular to the trend of the Caledonian orogen. In the Efjord area of north Norway, however, orogenesis also produced top-to-the-SSW motion on N-dipping thrust faults in the cratonal basement beneath the allochthon. The leading edges of some SSW-directed thrust-sheets moved upward into the overlying metasedimentary cover and formed the cores of S-verging recumbent fold-nappes at the base of the Caledonian allochthon. As deformation continued, these fold nappes became entrained in the top-to-the-ESE transport of the overlying allochthon. Mutually cross-cutting relationships suggest contemporaneous SSW-directed thrusting in the cratonal basement and top-to-the-ESE movement of the allochthon. Thus, structures in the Efjord study area reflect vertical partitioning of strain into components with nearly orthogonal displacement directions at different structural levels. Collectively, the top-to-the-SSW (orogen-parallel) and top-to-the-ESE (orogen perpendicular) components of transport are consistent with overall sinistral-oblique, post-collisional convergence between Baltica and Laurentia. Implications include: (1) in some parts of the orogen, recumbent folds with hinges sub-parallel to the transport direction of the allochthon were produced by orogen-parallel transport rather than orogen perpendicular sheath-folding; and (2) orogen-parallel components of structural transport make two-dimensional palinspastic restoration in planes perpendicular to the orogen difficult or inappropriate in some locations.