Syn-rift thermal structure and post-rift evolution of the Oslo Rift (southeast Norway): New constraints from fission track thermochronology

Abstract

The Permo-Carboniferous (240–305 Ma) Oslo Graben in southeast Norway is a classical magmatic continental rift. We present fission track (FT) data on apatites, zircons and sphenes from the rift and surrounding areas in order to clarify its syn- and post-rift thermal evolution. Zircons and sphenes within the rift record FT ages of between ∼ 270 and 180 Ma (i.e., syn-post-rifting). In contrast, the Precambrian basement surrounding the graben yields zircon FT/sphene FT ages of ∼ 500–650 Ma, indicating that heating associated with rifting was focused inside the rift as a result of advective heat flow. Syn-rift to early post-rift temperatures reached > 240°C at the present erosion level in the graben. Syn-rift advective heating ( ∼ 260–270 Ma) was probably a result of large-scale batholith intrusion; post-rift ( ∼ 180–220 Ma) heating seems to have occurred by hydrothermal circulation of high-temperature (100–300°C) fluids. The FT data, organic geochemical indicators and fluid inclusions suggest that hydrothermal circulation followed a highly complex pattern controlled by the tectonic and volcanic structure of the rift. Apatite FT ages and confined track length distributions reveal the post-rift exhumation history of the area. Apatite FT ages are continuous within and outside the graben and decrease from Triassic (200–240 Ma) in the southeast to Jurassic ( ∼ 160 Ma) in the northwest, indicating that post-rift exhumation took place on a much larger scale than Permian rifting. Modelling of apatite FT ages and track lengths suggests cooling/denudation events in the Triassic, Jurassic and Neogene, accounting for a total post-Permian erosion of 3–4 km. Triassic-Jurassic denudation coincides with the migration of rifting from the Oslo-Skagerrak area to the North Sea/Danish Basin, as revealed by offshore seismic data, and caused destruction of the hydrothermal regime. Neogene uplift and erosion of the area is also supported by seismic evidence. The inferred timing and amount of regional post-rift erosion suggests that post-rift subsidence and sedimentation within the Oslo Graben was minimal.