Basement-cover relationships in the central Scandinavian Caledonides

Abstract

Within the central part of the Scandinavian Caledonides, autochthonous Proterozoic crystalline basement rocks, with a thin veneer of Lower Palaeozoic sediments, are exposed in the thrust front in Sweden. Similar basement and cover units reappear further west in windows where they are treated as parautochthonous; they can be followed a distance of ca. 200 km westwards into the interior of the orogen. Upper Cambrian radioactive black shales, well-known on the stable Baltoscandian Platform for their high contents of trace elements such as U, V and Mo, retain their geochemical signature within the orogen in this sedimentary veneer despite the increase in metamorphic grade from sub-greenschist facies in the east to amphibolite facies in the west. The Baltoscandian Platform in the Late Cambrian extended at least as far west as the present Norwegian coast. In the front of the orogen, a décollement style dominates the structure, the cover-shortening occurring over a passive basement. Further west, towards the border between Norway and Sweden, the sole thrust passes beneath an allochthon composed of basement rocks, penetratively foliated but otherwise closely comparable with those in the underlying parautochthon. Basement-shortening of at least 70 km is implied by these relationships and may be substantially greater if the basement cores in the Swedish windows (treated here as parautochthonous) prove to be allochthonous. The major allochthonous units of the Swedish Caledonides, the Offerdal and Särv Nappes and Seve-Köli Nappe Complex, overlying the tectonic units referred to above, thin westwards, but reoccur in Norway beneath the Trondheim Nappe. This tectono-stratigraphy is treated here with particular reference to the Grong-Olden, Tømmerås and Trollheimen areas.

Key Words:

• Basement
• sedimentary cover
• autochthon
• parautochthon
• allochthon
• black shale
• trace elements
• gamma radiation
• tectonics
• lithostratigraphy
• crustal shortening
• areal geology
• Offerdal Nappe
• Särv Nappe
• Seve-Köli Nappe Complex
• Grong-Olden Culmination
• Tømmerås Window
• Tømmerås Antiform
• Trollheimen Antiform
• Surnadal Synform
• Caledonian Front
• Vestranden
• Baltoscandia
• Gjevilvatnet Group
• Hemre Kam formation
• Midlre Kam formation
• Indre Kam formation
• Svarthammer formation
• Blåhø group
• Okla conglomerate
• Okla sandstone
• Proterozoic
• Cambrian
• Lower Palaeozoic
• east central Norway
• west central Sweden
• N6150 N6410 E1600 E0900