The Thermal Evolution of Western Norway Based on Multi‐Sample Models of an Elevation Transect: Implications for the Formation of High‐Elevation Low‐Relief Surfaces on an Elevated Rifted Continental Margin

Abstract

The post‐Caledonian thermal and geomorphological evolution of onshore Western Norway is poorly understood, including the formation and age of the high‐elevation low‐relief surfaces seen across the Norwegian landscape. We present new apatite fission track (AFT) and (U‐Th‐Sm)/He analyses from an elevation transect (ET) covering ∼1,800 m vertical distance below a high‐elevation low‐relief surface in the inner Nordfjord. The AFT ages increase with elevation from 159 ± 11 Ma to 256 ± 21 Ma and apatite (U‐Th‐Sm)/He ages increase with elevation from 80 ± 4 Ma to 277 ± 15 Ma. In order to test different possible thermal evolutions, we present the first multi‐sample thermal history models from Norway using HeFTy combining both AFT and (U‐Th‐Sm)/He ages along the ET, refining available thermal history models for the area considerably. The best modeling results are found for a thermal evolution with slow cooling throughout the Mesozoic and increased cooling rates from the Late Cretaceous until present, indicating a Cenozoic age for the low‐relief surface at the top of the transect. The models also allow for cooling to surface conditions in the Late Jurassic, but such an evolution must have been followed by rapid burial by 1.5–3 km Cretaceous sediments, and by re‐exhumation in the Cenozoic, indicating that the low‐relief surface cannot represent a simply uplifted Jurassic or Cretaceous peneplain. We compare our results with multi‐sample models from the wider North Atlantic region, supporting previous findings of Cenozoic exhumation and landscape forming processes within that region.