The Devonian Nesna shear zone and adjacent gneiss-cored culminations, North-Central Norwegian Caledonides

The Early to Mid-Devonian strain distribution in the Norwegian Caledonides was characterized by large-magnitude extension, extension-normal shortening and sinistral strike-slip. In SW Norway, previous work has shown that the angle between the orogen and the Devonian maximum elongation trend decreases northward towards a zone of sinistral strike-slip. We provide evidence that: (1) late- to post-orogenic extension was much more important in North-Central Norway than recognized previously; (2) the Early to Mid-Devonian, roughly orogen-parallel, maximum elongation trend persisted over more than 200 km farther north than hitherto recognized; (3) a number of gneiss-cored culminations in North-Central Norway were affected by, and probably owe their present geometry to, extensional shearing and faulting. The Nesna shear zone is identified here as a major, late Early to Mid-Devonian, low-angle extensional shear zone in the North-Central Norwegian Caledonides. In the study area, the Nesna shear zone now constitutes the lower boundary of the Helgeland Nappe Complex, a terrane exotic to Baltica. The present situation of the Helgeland Nappe Complex is thus that of an extensional allochthon. The transport direction along the medium- to low-grade Nesna shear zone was top-to-the-WSW and thus roughly orogen-parallel. Extensional shearing in the Nesna shear zone may provide an explanation for a westward-younging, 40Ar/39Ar cooling pattern previously reported from the Ofoten-Lofoten area. Folding of the Nesna shear zone along axes parallel to the extensional transport direction indicates that extension-normal shortening continued at least into late Early and Mid-Devonian times. The WSW-ENE maximum elongation trend probably persisted in North-Central Norway through most of the Devonian period, and accompanied the transition from ductile to brittle deformation in the rocks of the Caledonian nappe-stack. Deformation in low-angle, initially medium-grade, extensional shear zones such as the Nesna shear zone was succeeded by shearing in steeper, low-grade, ductile-to-brittle shear zones that developed along the western margins of gneiss-cored culminations. Temporal overlap between activity in the Nesna shear zone and in the lower-grade shear zones that bound the culminations can be demonstrated. To the east, however, medium-grade extensional shear zones such as the present boundary between the Seve and Koli nappes were probably back-rotated as the culminations developed in the footwall of the low-grade shear zones. From the Mid-Late Devonian to Early Carboniferous, the low-grade, culmination-bounding shear zones constituted the eastern margin of a regional, transtensional system bounded in the south by the sinistral More-Trondelag Fault Complex.