The Uppermost Allochthon in the Scandinavian Caledonides: from a Laurentian birth through Taconian teenage traumas to Scandian adulthood... and demise

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author David, Roberts; Oystein, Nordgulen; Victor, Melezhik; Arne, Solli
Holding Date 04 October 2008

The highest nappe complexes in the Caledonides of Scandinavia, collectively termed the Uppermost Allochthon, are distinguished by particular lithological assemblages and magmatic units that, in many ways, are quite different from those in subjacent thrust sheets. Supracrustal successions derive mainly from platformal, shelf-edge and basinal slope environments and are characterised in particular by extensive developments of carbonate rock units that range in age from Cryogenian to Early Silurian. These include calcite and dolomite marbles, and carbonate conglomerates that can be followed for >600 km along strike, and spectacular, shelf-edge, carbonate breccias passing laterally into calcarenite turbidites which, in one area, attest to slope desposition in a basin deepening to the southeast. Metasedimentary and partly volcanogenic, iron oxide ore formations are also present in several areas. Another prominent feature is that of Ordovician to Silurian, arc-type, granitoid plutons and batholiths that dominate the geology in parts of the allochthon, particularly in the Helgeland Nappe Complex. There, the Bindal Batholith comprises plutons ranging in composition from gabbro to granite, and spanning an age range from 482 to 424 Ma. As well as these diverse lithological elements, the Uppermost Allochthon as a whole carries evidence of a complex Ordovician tectonothermal record and early-Caledonian, W- to NW-vergent thrust polarity that is unique in Norway. In the Helgeland district, high-grade metamorphism and thrust imbrication occurred at c. 480-475 Ma, in the Rödingsfjället Nappe Complex in Mid Ordovician time, whereas farther north, in Troms, the principal high-pressure metamorphic event is Late Ordovician (Katian) at c. 456-452 Ma. Taken together, these various features are indicative of a complex history of development and crustal growth along the eastern margin of Laurentia, involving an outboard magmatic arc, or arcs, and Taconian accretionary orogenesis. Moreover, it cannot be ruled out that the Early Ordovician Penebscotian arc and tectonothermal event may be represented in part of the Helgeland allochthon. In northern areas, metamorphism was followed by uplift, recycling and deposition in Late Ordovician to Early Silurian successor basins, prior to Laurentia-Baltica collision and the onset of the Scandian orogeny. The Taconian thrust sheets were then detached from their Laurentian roots and incorporated into the Siluro-Devonian, Scandian orogenic wedge on the Baltoscandian margin of Baltica. Taking into account the widely reported sinistral mega-shear arising from the Scandian, oblique collision and plate rotation, the rock units that now constitute the Uppermost Allochthon are likely to have originally been located closer to the northern Appalachian segment of the margin of Laurentia, in view of the strikingly similar lithostratigraphic, magmatic and tectonothermal histories of these two, now widely separated terranes.