Meso‐Cenozoic morphotectonic evolution of southern Norway: Neogene domal uplift inferred from apatite fission track thermochronology

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Abstract
Apatite fission track (AFT) thermochronology of Precambrian and Paleozoic basement samples from southern Norway reveals a post‐Paleozoic exhumation history, related to offshore Mesozoic and Cenozoic extensional basin development. The data indicate two major phases of rapid exhumation. A first Mesozoic phase started in the Triassic (∼220 Ma) in the east and south of the study area and migrated to the west where Jurassic (∼160 Ma) ages of exhumation predominate. A second event is indicated by thermal history modeling of AFT ages and track length distributions. It is inferred to be Neogene in age, initiated at about 30 Ma, and it produced a domal pattern of AFT isochrons which follow present‐day topographic elevation. Youngest AFT ages (∼100 Ma) are encountered at sealevel in the inner fjords near the areas of highest topography; ages increase radially outward to the mountain peaks and the coastlines. Forward modeling of age‐elevation patterns suggests that Mesozoic geothermal gradients were 10–15°C/km higher than the present value of 20°C/km. During the Triassic and Jurassic, a total of 1.3–3.5 km of overburden was removed from the study area, assuming a 30°C/km geothermal gradient for that period. We attribute this to rift margin erosion as a result of erosional base level lowering and flank uplift, as evidenced by thick continental clastic sequences deposited in Triassic‐Jurassic half grabens in the North Sea basins. We propose that 1.5–2.5 km of Neogene exhumation were a result of late stage domal uplift. This is supported by basinward dipping pre‐Neogene strata in the basins surrounding southern Norway and the infill of a 1‐ to 2‐km‐thick Neogene sediment wedge containing various internal unconformities. Domal uplift probably started in the Late Oligocene, may have been amplified in the Pliocene, and was overprinted by Plio‐Pleistocene glacial erosion. Maximum Neogene tectonic uplift is estimated at approximately 1–1.5 km, radially decreasing outward to a value <500 m near the shoreline. Neogene domal uplift is coincident with Oligocene and Pliocene plate reorganizations in the North Atlantic; similar Neogene domes are found around the Norwegian‐Greenland Sea (i.e., Svalbard and the Barents Sea, northern Norway, east Greenland), suggesting a regional tectonic cause. The onset of Neogene uplift postdates major volcanism and continental breakup by ∼25 m.y. and predates Plio‐Pleistocene glaciations. Its origin is possibly a combination of induced mantle convection, resulting in thermal erosion of the lithosphere, and the operation of intraplate stresses.

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