Possible Mesozoic sediments in fault and breccation zones in Frøyfjorden, Mid Norway

Abstract

The 5.3 km long Frøya Tunnel beneath Frøyfjorden, Central Norway, links the islands of Hitra and Frøya and passes through a major zone of faulting and breccation which parallels the Møre-Trøndelag Fault Complex. Faults and fractures within the Frøyfjorden fault zone are concentrated in deformation zones that developed during Devonian to Tertiary crustal movements. Drillcores from these zones include segments comprising possible Upper Palaeozoic and Mesozoic sediments. These were probably deposited on a fractured peneplain, and subsequently incorporated into faults during their reactivation in Mid Jurrasic and later times. Extensive weathering of the fault rocks and the Early Palaeozoic igneous rocks within the fault zones mainly post-dates brittle deformation and breccation. Part of the weathering may have occurred in the Late Triassic-Early Jurassic, prior to the Middle Jurassic transgression and deposition of the sedimentary succession in Frohavet. The youngest deformation event is represented by cross-cutting, montmorillonite-filled fractures which we tentatively relate to the Tertiary thermal activity documented in nearby areas.