Low-temperature thermochronological and structural study of the inner Hardangerfjord area, southern Norway
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The origin of the mountainous topography in southern Norway is at present unresolved. Post-Devonian sediments are absent onshore, making quantification of crustal uplift and fault displacements difficult. Low-temperature thermochronological techniques can be employed to constrain the timing of vertical movements through the uppermost few kilometres of the crust and are at present the most effective means of obtaining information about the topographic evolution prior to the Quaternary glaciations. This study aims to delineate the post-Caledonian morphotectonic evolution of the inner Hardangerfjord region by the means of apatite fission track and (U-Th)/He thermochronology, in combination with inverse thermal history modelling. Thirty-two samples derived from the steep flanks of the inner segments of the Hardangerfjord were analysed by the apatite fission track method. The resulting cooling ages range from Late Triassic to Late Cretaceous. A general positive age-elevation trend is evident, with abundant Early Cretaceous ages close to sea level and Jurassic ages on the Hardangervidda plateau. Four samples from the Eidfjord and Ulvik districts were analysed by the (U-Th)/He method, giving dominantly Cretaceous single grain ages. Fisson track age-elevation gradients and combined data from the apatite fission track and (U-Th)/He thermochronometers reveal low Jurassic-Cretaceous cooling rates in the order of ~1 °C/Ma. Large age jumps over limited horizontal distances suggest post-Middle Jurassic offset across both small-scale faults and regional structures. Offset age-elevation gradients indicate local displacements in the order of several 100 to more than 1000 metres. Thermal history modelling reveals two distinct episodes of accelerated cooling, which can be linked to documented pulses of tectonic activity onshore southern Norway and in adjacent offshore areas. Rapid cooling (2-6 °C) is inferred for the Permo-Triassic and is consistent with rift flank uplift and accelerated denudation in connection to the development of the North Sea Basin. The Jurassic and Cretaceous periods were characterised by low cooling rates (<- 1 °C/Ma) and relatively minor regional exhumation, suggesting that the effects of the second North Sea rift phase were not pronounced in inland areas. Localized, periodically increased exhumation rates associated with fault displacement and footwall uplift are inferred from the fission track age distribution, but are not resolved by the thermal history models. The second episode of rapid cooling (~2 °C/Ma) is constrained to the late Cretaceous-Eocene and may have been attributed to tectonic activity in relation to the North Atlantic breakup or enhanced topographic relief following thermally induced uplift triggered by the Iceland mantle plume. Distinctly different Palaeogene cooling paths for adjacent structural blocks suggest that fault activity may have continued into the Cenozoic. Considering the fission track age distribution patterns and thermal history models reported from southwestern Norway in general, it is suggested that extensive fault activity has exerted a significant control on the overall morphology of the passive margin. Pre-Eocene peneplanation and domal tectonic uplift, as has been proposed in previous studies, cannot fully account for the thermochronological data obtained in the current work.
PublisherThe University of Bergen
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