Cataclastic rocks along the Nesodden Fault, Oslo Region, Norway: a reactivated Precambrian shear zone

Abstract

The Nesodden Fault Segment (NFS) is the northern segment of the Oslofjorden Fault (OF), which separates the Precambrian rocks in the east from Palaeozoic rocks in the west. The NFS is characterized by two groups of fault rocks. Group 1 comprises a series of crush breccias to ultracataclasites containing material derived from the Precambrian basement. These fault rocks were formed at a depth greater than 5 km, by attrition brecciation and distributed crush brecciation, at some time between the Sveconorwegian orogeny (950 Ma) and the deposition of the Cambrian Alum shale (540-505 Ma). Group 2 comprises five crush breccias with a similar quartz matrix containing fragments of older rocks and Permian intrusive rocks. They were formed at a depth of less than 3.5 km by the interaction of cataclasis and hydrothermal percolation (attrition- and implosion-brecciation), during the formation of the Permo-Carbonifereous Oslo Rift (300-230 Ma). Evidence of repeated hydrothermal fluid flow and fluidization has been recognized in the youngest breccias. The seven different fault rocks emphasize the importance of fault reactivation and show that old fault zones commonly act as planes of weakness along which repeated deformation takes place and act as pathways for hydrothermal fluids.