NJG79-1-02
1999
Basement-cover relations and late- to post-Caledonian extension in the Leknes group, west-central Vestvågøy,Lofoten,north Norway
79
1
pp. 19-31

Rocks underlying the Lofoten islands of north Norway have long been recognized as presenting significant problems for models proposed for Caledonian orogenic evolution. Lofoten occupies the most internal position of exposed rocks in north Norway, yet, remarkably, earlier workers report little or no Caledonian reworking of Precambrian basement rocks. Furthermore, a critical question prevails as to whether or not Lofoten experienced extreme magnitudes of extension like its Western Gneiss Region counterparts in southwest Norway. Geologic mapping and kinematic analysis of rocks exposed on west-central Vestvågøy, Lofoten, were done to address these problems. Results indicate progressive ductile sinistral shearing, ductile-brittle extension, and uplift of the region concurrent with or immediately following Caledonian (ca. 425 Ma) contraction. Slivers of tectonized Baltic basement granite overlain by allochthonous metasedimentary and metavolcanic/ metavolcaniclastic units of uncertain origin (the Leknes group) were thrust eastward over pre-Caledonian Baltic crystalline basement of the northern Western Gneiss Region. The emplacement of these allochthons onto the Lofoten basement (during D1-D2) was contemporaneous with partial subduction of the Baltic craton beneath Laurentia during the Caledonian orogeny. Following nappe emplacement, low-angle sinistral-oblique and normal plastic shear zones (D3-D4) formed parallel to the basement-allochthon contact and overprinted earlier Caledonian thrust fabrics. These late ductile shear zones record a top-west sense of shear and are locally cross-cut by top-west cataclastic normal faults (D5). reflecting progressive unroofing of the shear zones to shallower crustal levels during late- and post-Caledonian extensional events. The normal faults and fabrics are interpreted to have episodically reactivated a Caledonian thrust during Devonian-Permian extensional collapse of the orogen. Northeast-trending, high-angle, brittle normal faults (D5) that occur throughout Lofoten-Vesterålen are interpreted to have formed during Permian or Jurassic-Cretaceous faulting events. Results of this study suggest that rocks exposed in Lofoten experienced extension that was roughly contemporaneous with phenomenal extension in southwest Norway, but of a lesser magnitude. Furthermore, extension in Lofoten is interpreted to have been accommodated by many discrete shear zones rather than along one or two master detachments, as was the case in southern Norway.

Andre C. Klein, Department of Geology, Auburn University, Auburn, Alabama, USA 36849 and Rice University, Department of Geology and Geophysics, MS-126 Houston, Texas, USA 77005-1892;
Mark G. Steltenpohl, Department of Geology, Auburn University, Auburn, Alabama, USA 36849

0029-196X