Geological evidence in favour of a Jotunheimen Caledonian suture

Abstract

EARLY plate-tectonic reconstructions of the proto-Atlantic by Wilson¹ and Dewey² placed a Caledonian suture through central, southern Norway. Later, more detailed interpretations generally favoured a single (proto-Atlantic, Iapetus) suture off the west coast^3–14, in agreement with the pre-plate-tectonic orthodoxy^15–18 of a western, eugeosynclinal source for the fartravelled (up to 1,000 km) (ref. 13), crystalline and supracrustal nappes of Norway and Sweden. In South Norway, however, not all interpretations fitted this pattern. Goldschmidt²¹ and Oftedahl²² proposed a local root zone in the Jotunheim for the crystalline Jotun Nappe south-east of its present northwesterly margin. More recently Battey and McRitchie^23,24 demonstrated a close fit between surface geology and the interpretations of gravity data by Smithson et al.²⁵ of the Jotun Nappe as an ‘Ivrea type’ flake with a local root possibly as deep as 16 km. Note that recent⁴³ aeromagnetic results suggest a maximum thickness of only 5 km for the Jotun Nappe). Evidence from the northwestern margin of the Jotunheim is clearly critical, for the southeastward transport of the Jotun Nappe deduced from areas to the south-east^14,18 is consistent with derivation from both the Iapetus suture and a possible Jotunheim suture. Some reports, however, apparently support the Jotunheim suture hypothesis. Between Fjaerlandsfjorden and Sogndalsdalen, Skjerli²⁶ found imbricated thrust slices of Basal gneisses and supracrustal rocks indicating transport to the north-west. A similar conclusion was reached by Battey and McRitchie²³ for Bøverdalen. Also, Elliott and Cowan²⁷ describe a sequence of tholeiitic greenstones with pillows and serpentinites from Holleindalen in Bøverdalen. Most recently, however, Roberts⁴² has concluded, using structural evidence from seven traverses across the northwestern margin, that the Jotun Nappe was transported to the south-east. The work described here does not support this conclusion.