STRUCTURAL INVESTIGATIONS OF 'HEIER'S FAULT ZONE', LOFOTEN-VESTERÅLEN, AND IMPLICATIONS FOR CALEDONIAN TECTONICS

A long-standing problem concerning tectonic evolution of the Scandinavian Caledonides is how, or if rapid and large magnitude Devonian extension recognized in southwestern Norway is manifest in northern Norway. Recently, Wilson et al. (2006) reported that they have “observed” a tops-west, extensional “Devonian detachment” exposed in Lofoten-Vesterålen, a report that appears to conflict with our field and laboratory studies. Neither kinematic observations nor direct age dates are cited in support of their claim, and Devonian sediments are not unambiguously known to occur anywhere in Lofoten-Vesterålen. One of the structures, known as 'Heier's fault zone', is the subject of our ongoing structural and isotopic investigations. Our kinematic and geometric observations document Heier's zone to be a shallow, west-dipping, tops-east, crystal-plastic shear zone that was reactivated as a ductile-brittle fault, and then was orthogonally segmented by even later high-angle brittle faults. These observations require that Heier's zone either originated as i.) a tops-east thrust or ii.) as a tops-east normal fault that later was rotated to dip westward, neither being compatible with the interpretation for a tops-west “Devonian detachment”. Interpretation i.) is kinematically compatible with Caledonian thrust translation farther east, and ii.) is possible given that later faults could have rotated the structure. Timing of earliest movement along Heier's zone is not yet directly established but is one goal of our field and isotopic work. Heier's zone is cut by Jurassic age faults, related to rifting from Greenland, thus placing a minimum age on movement. Questions concerning Lofoten's role in Caledonian evolution (a sutured microcontinent or the western edge of Baltica or both?) leave an upper date for movement at 1.8 Ga, the age of the youngest rocks cut by Heier's zone. As this report was written, the significance of Heier's zone to Caledonian evolution was still unclear. We will present results of our ongoing work that helps to constrain the evolution of this notable structure.