The Ullsfjord area of northern Troms comprises rocks from two allochthons: (l) a fragmented ophiolite complex and non- conformably overlying fossiliferous (Upper Ordovician-Lower Silurian) metasedimentary rocks (Balsfjord Group) of the Lyngen Nappe Complex and (2) exotically derived metasedimentary and meta-igneous rocks of the overlying Tromsø Nappe Complex. Tromsø Nappe Complex rocks are exposed as isolated klippen on the highest peaks in Ullsfjord. A synmetamorphic, inverted metamorphic gradient exists within the prograde assemblages of the Balsfjord Group, such that chlorite-zone assemblages occur at the base of the sequence and sillimanite-zone assemblages occur immediately beneath the thrust contact with the overlying Tromsø Nappe Complex. The entire Barrovian sequence of mineral zones, with the exception of kyanite, is present in the footwall. Complexity of rock fabrics increases sympathetically with metamorphic grade progressing structurally upwards. Simple bedding-cleavage relations at the base of the section give way upward to crenulation cleavage, transposition fabrics, schistosity and gneissosity. Relict cross-beds, graded beds and pillow basalts are stratigraphically upright. Meso- and microscopic structures record two main deformational events; D1 resulted in prograde assemblages, and D2 formed post-metamorphic gentle-folds. The combined sructural and metamorphic data indicate thrust emplacement of a hot Tromsø Nappe Complex upon the cooler Lyngen Nappe Complex. The inverted metamorphic gradient is due to originally inverted isotherms although minor syn-emplacement reshuffling along foliation planes may have aided in the vertical stacking. An increase in shear strain approaching the nappe boundary is attributed to thermal weakening during emplacement of the Tromsø Nappe Complex. 40Ar/39Ar mineral cooling dates indicate that metamorphism and nappe emplacement occurred at ca. 432 Ma.
Mark G. Steltenpohl, Department of Geology, 210 Petrie Hall, Aubum University, Aubum, Alabama, 36849-5305, USA;
Arild Andresen, Department of Geology, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway