Contact relationships in the southeastern Trondheim Nappe complex, central-southern Norway: Implications for early Paleozoic tectonism in the Scandinavian Caledonides

Abstract

The Upper Allochthon, a composite thrust complex containing elements of an accreted terrane, has long been a controversial feature in the Scandinavian Caledonides. Although final emplacement of the Upper Allochthon occurred during the Siluro-Devonian Scandian orogeny, the complex experienced an intricate and still debatable early Paleozoic history. In the central Norwegian Caledonides, the Upper Allochthon comprises the Trondheim Nappe Complex, which contains an enigmatic axial unit of metasedimentary and meta-igneous rocks flanked by oceanic crustal sequences to the east and west. Determination of the nature of internal contacts and timing of assembly of the Trondheim Nappe Complex has been hampered by wide variations in lithology, strain and metamorphic grade, and scanty age constraints in some rock units. In the Lake Savalen area, southeastern Trondheim Nappe Complex, a polydeformed continental margin quartzite/schist sequence occurs in the core of the Einunnfjellet dome. These rocks, which can be linked to the Baltoscandian miogeocline, were overthrust by oceanic crust and pelagic sediment, then eroded and unconformably succeeded by polymict conglomerates and fine-grained clastic sediments. Early thrust and unconformable contacts were later overprinted by Scandian deformation and metamorphism. The garnet isograd cuts across lithologie contacts, and does not appear to be folded or interrupted by faulting within the 250-km 2 map area. Regional correlations indicate the widespread occurrence of tectonostratigraphic relationships similar to the Einunnfjellet-Savalen area, and imply that prior to the Scandian orogeny, portions of the Upper Allochthon were assembled into a composite mass consisting of: 1. (1) a lower Baltoscandian miogeoclinal sequence 2. (2) a thrust complex of fragmented oceanic terranes 3. (3) an unconformable succession that overstepped both (1) and (2). This mass was subsequently thrust eastward, imbricated, and metamorphosed during Scandian orogenesis. As a result of the pervasive Scandian overprint, pre-Scandian faults and unconformable contacts in the Upper Allochthon are commonly cryptic.