The Tectono‐Thermal Evolution of Western Norway – New Insights From Low‐Temperature Thermochronology

Abstract The tectono‐thermal evolution of Western Norway's onshore rifted margin is debated. Previous studies suggest it does not exhibit a typical rifted margin cooling pattern. We present 29 new apatite fission track (AFT) ages and 54 new apatite single grain (U‐Th)/He ages from 14 samples, covering the onshore transition from the northern North Sea to the Norwegian Sea margin. AFT ages range from 323 ± 27 Ma to 140 ± 4 Ma, and (U‐Th)/He ages range from 228 ± 12 Ma to 57 ± 3 Ma. The samples show no clear age‐elevation correlation and large age offsets over short distances. Inverse thermal history modeling using HeFTy on 13 samples together with other geological constraints identifies four tectonic domains with distinct thermal and structural histories: (a) The inner Nordfjord subregion is dominated by N‐S striking faults, cooled to upper crustal levels (<8 km) in the Permian, remained stable until the Late Cretaceous, and experienced increased Cenozoic cooling. (b) The Sognefjord subregion cooled to upper crustal levels in the Carboniferous and offsets of AFT ages across fjords indicates Triassic/Jurassic reactivation of various faults. (c) The Hornelen subregion is dominated by prominent E‐W faults inherited from ductile precursors and shows the oldest AFT and (U‐Th)/He ages, reaching surface conditions already in the Jurassic with limited later reheating. (d) The Møre subregion shows rapid cooling in the Jurassic, likely due to footwall uplift accompanied by considerable topography. The new data set indicates a complex fragmentation of the onshore margin influenced by ductile precursor structures.