Ductile and brittle extension in the southern Lofoten archipelago, north Norway: Implications for differences in tectonic style along an ancient collisional margin

The Lofoten archipelago, north Norway, occupies the most internal position of the Caledonian belt in northern Scandinavia, and rocks and structures ex posed there are crucial to understanding processes of how the Baltic baseme nt and its cover allochthons responded to continental lithospheric subducti on and subsequent continental separation. Relatively little is published ab out the structural and metamorphic development and especially the timing of these events; consequently, it is unknown how features exposed on these sp atially isolated islands relate to those of the adjacent mainland. Rocks in Lofoten were affected by Caledonian regional metamorphism, and structures record tops-east contraction and later extension related to late- to post-C aledonian basement exhumation, Tops-west extension is preferentially develo ped in meter to km-scale ductile shear zones containing west-dipping extens ional hear bands, west-verging rootless folds, and asymmetric feldspar porp hyroclasts. West-plunging? sinistral-oblique elongation lineations in the m ylonitic foliation are interpreted to indicate the line of transport. Mesos copic backfolds are locally developed within the tops-west shear zones and further document west-directed transport of structurally higher rocks, The ductile extensional shear zones locally are cross cut by low-angle, tops-we st cataclastic normal faults, reflecting progressive unroofing of the shear zones to shallower crustal levels during late- and post-Caledonian extensi onal events. Northeast-striking; high-angle, brittle normal faults truncate all other fabrics and structures and juxtapose structurally deep undeforme d Precambrian basement with the Caledonian nappe sequence. Ar-40/Ar-39 data indicate that ductile extension in Lofoten likely initiated soon after the Caledonian metamorphic peak (at similar to 430 Ma) and continued episodica lly until at least Permian time. Rocks passed through the brittle/ductile t ransition rapidly at similar to 275 Ma. Recent paleomagnetic data also poin t to later phases of brittle faulting in Lofoten, one in the Jurassic/Creta ceous and one related to Tertiary opening of the Norwegian Sea.-The magnitu de and style of late- to post-Caledonian extension and basement exhumation in Lofoten differs markedly from the formation of phenomenal Devonian basin s and exposures of Caledonian ultra-high pressure assemblages in the Wester n Gneiss Region (WGR) of southern Norway. Differences in the styles of exte nsion between northern and southern Norway are interpreted to reflect inher ited differences in crustal architecture that may have been enhanced during the Caledonian orogeny.