Ductile and brittle extension in the southern Lofoten Archipelago, North Norway; implications for differences in tectonic style along an ancient collisional margin

Abstract

The Lofoten archipelago, north Norway, occupies the most internal position of the Caledonian belt in northern Scandinavia, and rocks and structures exposed there are crucial to understanding processes of how the Baltic basement and its cover allochthons responded to continental lithospheric subduction and subsequent continental separation. Relatively little is published about the structural and metamorphic development and especially the timing of these events; consequently, it is unknown how features exposed on these spatially 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-Caledonian basement exhumation. Tops-west extension is preferentially developed in meter to km-scale ductile shear zones containing west-dipping extensional shear bands, west-verging rootless folds, and asymmetric feldspar porphyroclasts. West-plunging, sinistral-oblique elongation lineations in the mylonitic foliation are interpreted to indicate the line of transport. Mesoscopic 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-west cataclastic normal faults, reflecting progressive unroofing of the shear zones to shallower crustal levels during late- and post-Caledonian extensional events. Northeast-striking, high-angle, brittle normal faults truncate all other fabrics and structures and juxtapose structurally deep undeformed Precambrian basement with the Caledonian nappe sequence. ^40^ Ar/ ^39^ Ar data indicate that ductile extension in Lofoten likely initiated soon after the Caledonian metamorphic peak (at approximately 430 Ma) and continued episodically until at least Permian time. Rocks passed through the brittle/ductile transition rapidly at approximately 275 Ma. Recent paleomagnetic data also point to later phases of brittle faulting in Lofoten, one in the Jurassic/Cretaceous and one related to Tertiary opening of the Norwegian Sea. The magnitude and style of late- to post-Caledonian extension and basement exhumation in Lofoten differs markedly from the formation of phenomenal Devonian basins and exposures of Caledonian ultra-high pressure assemblages in the Western Gneiss Region (WGR) of southern Norway. Differences in the styles of extension between northern and southern Norway are interpreted to reflect inherited differences in crustal architecture that may have been enhanced during the Caledonian orogeny.