NJG93-3/4-04
2013
Polyphasal brecciation history revealing the nature and depth of Caledonian contractional deformation in the outboard terranes of West Central Norway
93
3, 4
pp. 215–228
978-82-92-39488-5

This study focuses on two minor outcrops (<4000 m2 ) of breccias in an Early Silurian, Caledonian granite located at two headlands on the northeasternmost part of the island of Frøya, Mid Norway. The Skaget breccia has previously been interpreted as a result of a fluidisation process formed during post-Cretaceous crustal extension, whereas the Rottingen breccia has not previously been studied in detail. The breccias are sandwiched between host granite at the base and an unconformably overlying sedimentary breccia at the top. The breccias display a complex texture and bear evidence of repeated events of cataclasis and brecciation. Discrete contractional faults and the overall orientation of the breccia units correspond well with the known orientation of Caledonian thrusts in the area. Following consideration of alternative hypotheses, we conclude that the breccias are of tectonic origin, and probably related to Caledonian contraction. The breccias were later eroded and overlain by Devonian(?) sedimentary breccias and conglomerates and cross-cut by brittle Mesozoic structures related to the Møre–Trøndelag Fault Complex. Our findings suggest that cataclastic brittle faulting occurred during Caledonian thrusting, a feature which has not previously been reported in this area. We attribute this inferred shallow depth of deformation to the reported differential uplift in the area, where progressively deeper crustal levels were exhumed inboard (to the east), and where outboard areas (such as the study area at Frøya) were generated at shallow crustal palaeo-depths which have undergone far less uplift.

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