TECTONIC EVOLUTION OF THE AEFJORD-SITAS AREA, NORWAY-SWEDEN by Kip

The Aefjord-Sitas area (approximately 68*15'N;16*45'E) includes three of the principal tectonic elements of the northern Scandinavian Caledonides: a) a nappe complex, representing the main Caledonian allochthon and consisting of metasedimentary and metaigneous rock sequences that are probably in part of oceanic provenance; b) the Rombak window, a fenster through the nappe complex revealing Baltic Shield basement gneisses below; and c) the Aefjord culmination, part of the Archean-Proterozoic "Western Gneiss Region" of Scandinavia. Similarities in mineralogy and age between principal rock types in the Aefjord and Rombak gneissic terrains suggests that the Aefjord culmination represents the westward extension of the Baltic Shield to the Norwegian coast. Emplacement of the nappe complex onto the Aefjord and Rombak terrains is considered to have occurred during underthrusting of the Baltic craton beneath the Greenland craton in late Ordovician to Silurian time. The rock sequences in the area have been subdivided into ten lithotectonic units. This tectonic stratigraphy enables the recognition of six deformational events. D1 represents stacking of individual nappes within the allochthon. D2 resulted in emplacement of the main Caledonian allochthon onto the Aefjord basement terrain along the Forsa thrust, and in large-scale recumbent folding of the nappe pile. D3 and D4 led to the development of N-S (?) recumbent and E-W upright folds, respectively. D5 resulted in imbrication of the Aefjord-Rombak basement terrain and further cratonward translation of the main Caledonian allochthon along the Frostisen-Mereftasfjell thrust complex. D6 is represented by ~N-S trending, west vergent back-folds. The minimum age of synmetamorphic D2 deformation is constrained by K-Ar hornblende cooling ages which average 449±19 my. Through the use of discordant hornblende and biotite K-Ar ages and currently available experimental data for Ar diffusion in these minerals, closure temperatures for the two mineral-isotopic systems and the average cooling rate over the discordancy interval can be independently determined. The calculated closure temperature for Aefjord hornblendes is 473(+11/-7)*C, that for biotite is 263(+10/-5)*C, and the cooling rate is estimated as 3.0(+2.9/-1.O)*C/my. Rocks of the main Caledonian allochthon and subjacent Aefjord basement terrain were metamorphosed at uniform kyanite-grade conditions. The D5 Mereftasfjell thrust, which forms the eastern margin of the nappe complex, marks a metamorphic discontinuity separating amphibolite facies nappe terrain rocks in the upper plate from greenschist facies lower plate rocks which constitute a D5 schuppenzone (the Storriten complex). Low variance mineral assemblages are common in certain pelitic horizons within the nappe complex, and these permit the use of mineral equilibria geothermometers and geobarometers to constrain metamorphic conditions. Samples which show textural evidence of retrogradation yield a linear array on a pressure-temperature diagram. This array is thought to represent a portion of the P-T trajectory of the Aefjord area during uplift and cooling. Thesis Supervisor: Title: Dr. B.C. Burchfiel Professor of Geology