Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology from innermost Hardangerfjord area
3, 4
pp. 243–260

The post-Caledonian structural and morphological evolution of the North Sea rift margin in southwestern Norway is largely unresolved. A comprehensive understanding of the importance of onshore fault reactivation and the magnitude of crustal uplift has been hindered by the near absence of post-Devonian sediments. This study aims to delineate the tectonic history of the passive margin hinterland from the Permian onwards by means of apatite fission-track (AFT) analysis and (U–Th)/He thermochronology. AFT analysis has been performed on 32 samples from the steep flanks of the innermost segments of the Hardangerfjord. The resulting cooling ages range from Late Triassic to Late Cretaceous and define a general positive age-elevation trend that is locally disturbed by large age offsets, suggestive of post-Mid Jurassic faulting on the order of several hundred metres. Four samples from the Eidfjord and Ulvik districts have been analysed by the (U–Th)/He method, giving primarily Cretaceous single-grain ages. Thermal history modelling reveals two distinct episodes of accelerated cooling (2–6°C/Myr-1), confined to the Permo–Triassic and the latest Cretaceous to Cenozoic.

The high Permo–Triassic cooling rates may be explained by flexural rift–flank uplift and increased tectonic activity onshore as a response to rifting in adjacent offshore areas. Rapid exhumation throughout the Cenozoic is consistent with sustained elevated topography and periodic rejuvenation of relief as a result of local fault activity. In combination with previously reported AFT data, the results presented in this contribution suggest that faulting has exerted a major control on the overall morphology of the passive margin. The Mesozoic–Cenozoic exhumation history reflects a complex interplay between tectonic activity, flexural uplift and erosion.

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