Strike-slip faults in the Cambro-Silurian rocks of the Oslo Rift - Caledonian compression or Permian transtension?

The Oslo Region provides the opportunity to study both the development of the late Silurian Caledonian fold-and-thrust belt and the formation of the Permo-Carboniferous continental Oslo Rift. The complex structural evolution of the region is reflected in the Lower Palaeozoic sediments that have been downfaulted and preserved in the Oslo Rift. This study investigates, through detailed mapping and the construction of balanced cross-sections, the characteristics and origin of sub-vertical strike-slip faults found within the Oslo Region. Previous investigations of the tectonic evolution of the region have regarded such faults as representative of several Permo-Carboniferous strike-slip regimes related to the formation of the Oslo Rift. On a larger scale, it has been proposed that the Oslo Rift is connected to the Variscan orogenic front and the Sorgenfrei-Tornquist Zone through the Permo-Carboniferous Skagerrak and Horn grabens, and the strike-slip faults have been proposed to reflect Variscan compressional tectonics and related transpression and dextral strike-slip on the Sorgenfrei- Tornquist Zone. Approximately 3 months of detailed mapping at Huk and Slemmestad in the Oslo Region, deemed representative of the regional geology, revealed small scale tear faults terminating in overlying and underlying Caledonian thrust faults. On a larger scale, adjacent domains of differing compressional structures were observed, where the lateral break in geology can only be explained by Caledonian tear faults. These observations demonstrate that some steep strike-slip faults were formed as part of Caledonian thrusting, rather than post-dating it. The prevalence and impact on the surrounding geology make the Caledonian tear faults an integral part of the Caledonian structural style. This study therefore suggests that sub-vertical strikeslip faults in the Oslo Region should be individually mapped and checked against a set of criteria proposed in this thesis to distinguish between Caledonian tear faults and strike-slip faults of a Permo-Carboniferous age. This study further suggests that Caledonian tear faults likely have been misinterpreted in previous studies, and included in the analyses of some of the tectonic phases relating to the Oslo Rift. Furthermore, investigation of the sills in the study areas suggests, in contrast to some earlier studies, that a late Carboniferous compressional phase is not required to explain sill emplacement. Consequently, the Variscan orogeny probably had less of an impact on the initiation of rifting than previously assumed. Since early compression and strike-slip regimes have been used as arguments for passive rifting in the Oslo Rift, this weakens the argument from field observations for the passive rifting model.