Geology of western Ullsfjord, North Norway, with emphasis on the development of an inverted metamorphic gradient at the top of the Lyngen Nappe Complex

The Ullsfjord area of northern Troms comprises rocks from two allochthons: (1) a fragmented ophiolite complex and nonconformably overlying fossilifero us (Upper Ordovician-Lower Silurian) metasedimentary rocks (Balsfjord Group ) of the Lyngen Nappe Complex and (2) exotically derived metasedimentary an d meta-igneous rocks of the overlying Tromso Nappe Complex. Tromso Nappe Co mplex rocks are exposed as isolated klippen on the highest peaks in Ullsfjo rd. A synmetamorphic, inverted metamorphic gradient exists within the progr ade assemblages of the Balsfjord Group, such that chlorite-zone assemblages occur at the base of the sequence and sillimanite-zone assemblages occur i mmediately beneath the thrust contact with the overlying Tromso Nappe Compl ex. The entire Barrovian sequence of mineral zones, with the exception of k yanite, is present in the footwall. Complexity of rock Fabrics increases sy mpathetically with metamorphic grade progressing structurally upwards. Simp le 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 upr ight. Meso- and microscopic structures record two main deformational events : D-1 resulted in prograde assemblages, and D-2 formed post-metamorphic gen tle-folds. The combined structural and metamorphic data indicate thrust emp lacement of a hot Tromso 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 th e nappe boundary is attributed to thermal weakening during emplacement of t he Tromso Nappe Complex. Ar-40/Ar-39 mineral cooling dates indicate that me tamorphism and nappe emplacement occurred at ca. 432 Ma.