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

Abstract

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, 40 Ar/ 39 Ar 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 Köli 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 Møre–Trøndelag Fault Complex.