The post-Caledonian history of the Røldal Shear Zone (SW Norway): an apatite fission track study

Abstract

The post-Caledonian tectonic history and landscape evolution of southwestern Norway is not very well understood and therefore the subject of an ongoing debate. The lack of onshore post-Devonian sediments makes the study of vertical movements in the upper few kilometers of the crust a challenge. At the present time, the only means for constraining upper crustal movements directly are thermochronological analyses. Apatite fission track analysis was applied with an aim to shed light on the post-Caledonian history of the Røldal Shear Zone area. The Røldal Shear Zone is a major shear zone running parallel to the Hardangerfjord Shear Zone located further north. The Røldal Shear Zone is, however, much less studied. The apatite fission track analyses yielded Permian to early Cretaceous ages with the majority of Jurassic ages. The ages are generally decreasing from the coast towards the inland, and are clearly offset across the Røldal Shear Zone. Inverse thermal history modeling was applied to four samples from the coast, representing the hangingwall and footwall, and the inland. The modeling suggested rapid Permo- Triassic cooling for the coastal samples, but slower and more constant cooling until the Cenozoic for the inland sample when it accelerated. This is suggested to reflect a rift related rift flank uplift in the coastal area, whereas the inland sample was much less affected by this uplift. The modeling for the coastal samples also suggested the samples to have been at (or close to) the surface in the Jurassic and was subsequently re-buried in the Cretaceous. This is not the case for the inland sample which reached the surface in the Cenozoic. The cooling rate accelerated dramatically in the Neogene. This is suggested to be a result of increased erosion rates due to the shift to a colder global climate. It is suggested that the evolution of the landscape of southwestern Norway is a result of sustained elevated topography shaped by rift-related uplift, extensional faulting and erosion since the Caledonian Orogeny.