|
Per Terje Osmundsen, Geological Survey of Norway (Norway)
Torgeir Bjørge Andersen, University of Oslo (Norway)
Alvar Braathen, University Centre in Svalbard (Norway)
David Roberts, Geological Survey of Norway (Norway)
|
|
The continental 'Old Red' basins of Norway formed during tectonic disintegration of the Scandinavian Caledonides. The excision of orogenic crust eventually led to exhumation of very deep levels in the orogen and to juxtaposition of the 'Old Red' basins with rocks exhumed from the middle and lower crust and from the upper mantle. The provenance of the 'Old Red' basins reflects their juxtaposition with Caledonian nappe rocks and, eventually, with gneiss-cored culminations, across large-magnitude normal and strike-slip faults such as the Nordfjord-Sogn and Høybakken detachments and strands of the Møre-Trøndelag Fault Complex. By Late Devonian-Early Carboniferous time, crustal thickness had probably returned to near normal under parts of the Scandinavian Caledonides.
The shingled alluvial, fluvial and lacustrine deposits in the 'Old Red' basins record deposition in migrating depocentres controlled by ramp-flat extensional and strike-slip faults. The faults controlled the migration of facies units in the direction(s) of maximum subsidence, resulting in dominantly retrogradational stacking patterns along the faulted basin margins. The stratigraphic architecture of the basins was affected by syn- and post-sedimentary normal and oblique faults as well as folds and thrusts that developed parallel to the principal stretch axis. These structures further affected the distribution of depositional environments and thus the sedimentary architecture in the 'Old Red'. The entire structure of the Western Gneiss Region with overlying mylonites, Caledonian nappes and 'Old Red' basins are folded around axes that are parallel to the principal stretch axis. This megastructure is in turn cut by undulating brittle low-angle normal faults. Commencing during basin sedimentation, the shortening of the basins eventually became associated with very low-grade metamorphism, indicating burial to 12-14 kilometres. Several phases of re-activation along the extensional detachment faults resulted in (10's of) metres-wide assemblages of fault rocks and in the final juxtaposition of the basins with their tectonic substrate. In the area of the Møre-Trøndelag Fault Complex (MTFC), the ENE-WSW-trending structural grain inherited from Devonian, extension-parallel folding and shearing was later reactivated in concert with a clockwise rotation of the regional stress field from the Permian into the Cenozoic. Some of the inherited Devonian structures became important in the Mesozoic-Cenozoic denudation of the NE Atlantic margin, as ENE-WSW inherited fold flanks and faults where re-used in the differential uplift of south Norway.
|
