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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 10-15øC/km higher than the present value of 20øC/km. During the Triassic and Jurassic, a total of 1.3-3.5 km of overburden was removed from the study area, assuming a 30øC/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 combina-Copyright 1995 by the American Geophysical Union. Paper number 95TC00088. 0278-7407/95/95TC-00088510.00 tion of induced mantle convection, resulting in thermal erosion of the lithosphere, and the operation of intraplate stresses. Introduction It has become apparent in recent years that rifted margins may record significant postrift (late stage) uplift events, in addition to better understood synrift flank uplift [e.g., Cloetingh and Kooi, 1992; Sales, 1992]. For instance, Cloetingh et al. [1990; 1992] stress the importance of accelerated postrift subsidence, in conjunction with margin upl...

We present a numerical pure‐shear stretching model to study the effect of the “depth of lithospheric necking” on the state of flexure at extensional basins. This model avoids the need for low flexural rigidities during synrift basin subsidence. The model also accounts for long‐standing rift flank uplifts in the absence of significant thermal anomalies or underplating beneath rift shoulders, and it predicts low β factors beneath basin centers. Numerical modeling of synthetic continental rift zones and rifted continental margins together with their gravity characteristics demonstrates that isostatic residual anomalies are sensitive indicators of the state of flexure at extensional basins, particularly at basin margins. Isostatic residual anomalies are particularly useful to study the state of lithospheric flexure because the results are not biased by assumptions on the loads that have to be made in a forward modeling approach. Previous studies of lithospheric flexure and gravity modeling might have underestimated flexural rigidities at extensional basins. Comparison of predicted gravity anomaly signatures with gravity data suggests that at narrow rift basins and young rifted continental margins, upward flexure occurs more commonly than does downward flexure. This can be explained by intermediate or deep levels of necking (>15 km). For rifted continental margins, the apparent transition in the state of flexure during basin evolution from upward to downward flexure points to intermediate levels of necking (15–20 km). The occurrence of intermediate to deep levels of necking at narrow rift basins and intermediate levels of necking at rifted margins is in accordance with brittle‐ductile rheological models of the lithosphere. An intermediate depth range for the level of lithospheric necking at rifted continental margins is consistent with a scenario in which rifting preferentially occurs in an area where the lithosphère is weakened by a thickened crust Rifting in areas of stronger lithosphere, as for example cratons, is probably more often associated with a deep level of necking, explaining the widespread occurrence of rift shoulders at failed rift basins. We have tested several depths of necking and a model of local isostasy for the Gulf of Lions margin in the northwestern Mediterranean. At positions of deep synrift grabens, models with depths of necking of 25–35 km predict crustal thicknesses that are more in accordance with observations than are models with a shallower level of necking. It appears that local isostasy cannot account for the present‐day basin configuration and at the same time reproduce the observed strongly laterally varying bathymetry at the end of rifting, supporting a significant postrift flexural rigidity.

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[1] We present a comparison between numerical and analogue models focusing on the role of inherited lithospheric structures in influencing the process of continental break-up. Our results highlight that the presence of pre-existing anisotropies localizes strain and favors continental break-up and formation of a new ocean. For a fixed strain rate, the pre-rift lithosphere configuration influences rift duration, melt production and width and symmetry of the continental margin pair. Model results show a mainly two-phase tectonic history from continental extension to oceanization. In the first phase extension affects contemporaneously the whole rift structure, while in the second phase asthenosphere upwelling occurs into punctiform regularly-spaced spots sequentially propagating in an extension-orthogonal direction.