Subduction of the Iapetus Ocean crust beneath the Møre Gneiss Region, southern Norway

Abstract

Basement–cover relations between Caledonian nappes and underlying Precambrian basement along the western coastline of southern Norway have long intrigued geologists and considerable progress has recently been made in understanding the Caledonian geology1–3, geochronological studies proving particularly useful. For example, the Precambrian gneisses along the coast from Bergen to Namsos yield ages up to 1,700 Myr, supposedly the age both of the formation of the rocks and the main metamorphism and deformation phases4. The Lofoten–Tromso archipelago is a presumed continuation of this basement block once sandwiched between the Caledonides of Norway and Greenland. An important phase of rock formation and metamorphism affected the gneisses here at around 1,800 Myr, whereas subsequent orogenies at 1,100 and 400 Myr (Grenville–Sveco-Norwegian or Dalslandian, and late Caledonian orogenies, respectively) had little effect. COCORP reflection profiling in the southern Appalachians5 has demonstrated that crystalline ‘basement’ has been thrust for a large distance over autochthonous early Palaeozoic sediments of the proto-Atlantic continental margin. Data from a seismic profile6 across the More Gneiss Region (often called the northwestern basal gneisses of southern Norway) show evidence of P-wave velocity reversal at a depth of about 14 km, and the associated low-velocity layer (LVL) has a thickness of 4 km. As there is no excess of heat flow to account for the LVL, we consider here the hypothesis that oceanic crust and accompanying sediments have been subducted under the gneisses.