Palaeomagnetic Constraints on the Early History of the Møre-Trøndelag Fault Zone, Central Norway

Two principal remanences, SMA and SMB, have been identified from Ordovician and Devonian rocks from Smola, Central Norway. The proposed youngest, SMB, compares with Late Devonian (Solundian) palaeomagnetic data from Western Norway. SMA, however, is plainly anomalous and it is necessary to infer a tectonic rotation of Smola to accommodate the palaeomagnetic data. Pre-ORS rocks in the elongate area embracing Smola, Hitra and Orlandet are characterized by a phase of Mid to Late Ordovician deformation and intrusive activity, and are scarcely affected by the Silurian Scandian orogeny. It is proposed that this “Smola Terrene” docked against the Western Gneiss Region in Late Devonian time. This collisional docking involved emplacement on a major detachment, and suturing along the More-Trondelag Fault Zone (MTFZ). The nature of this suture was essentially one of a reverse-fault, and the rotation of Smola is thought to have been accommodated by strike-slip faulting between Smola and Hitra. The syn-to post-collisional, Late Devonian history of the MTFZ could have been one of sinistral-strike slip, dependent on the orientation of the collisional stress-vector.The MTFZ is an example of a long-lived fault zone, and important phases of post-Devonian reactivation with the formation of a variety of faultrocks and hydrothermal alteration occurred in Permian, Late Jurassic/Early Cretaceous and Early Tertiary times.