| dc.description.abstract | The topographic and tectonic post-Caledonian evolution of western Norway is still not fully understood. Improving resolution in the low-temperature thermochronological database, this study focuses on a constrained area around the Nordfjord in western Norway, where little previous data is available. Aim of the study is to understand the development of the Nordfjord region in the frame of the post-Caledonian evolution of western Norway, studying vertical movements, fault activity and the tectonic architecture. 8 samples at elevations from 3 to 56 masl were analysed by LA-ICP-MS apatite fission track dating. The obtained ages range from 133 ± 9 to 325 ± 41 Ma (Middle Carboniferous to Early Cretaceous). Most ages are Late Triassic to Late Jurassic. The ages are strongly offset, suggesting fault activity during Triassic-Cretaceous between samples. A reliable number of track lengths could be measured for 3 samples. Mean track lengths are rather short, ranging from 11.91± 1.60 to 12.29±1.67 μm and suggesting comparably slow cooling through the PAZ. Thermal history modelling of these 3 samples suggests a period of faster cooling (1.5-2°C/Ma) driven first by exhumation due to Devonian extension and then Permian-Triassic flexural rift plank uplift, followed by a period of slower cooling (0.2-0.4°C/Ma) from 250-150 Ma until present. Differences between the models have been explained by localized faulting during exhumation. Structural geological field work provided an extensive database of foliation, fracture and slicken fibre lineation measurements, accompanied with interpretation of fault kinematics based on slicken fibres. Essentially two main types of fractures have been observed in the: 1) ∼ N-S-striking fractures, with a steep dip to the west and mainly normal kinematics, closely followed by sinistral kinematics. 2) E-W to NE-SW striking, less steeply dipping fractures with mainly sinistral kinematics and some normal kinematics in the south, often parallel ductile precursors. Whereas the N-S trending faults are interpreted to have formed possibly as early as Late Devonian-Carboniferous, but mainly in relation to rifting in the North Sea, NE-SW and E-W trending were most likely formed during Devonian extension. The latter are suggested to have been reactivated as normal or strike-slip faults. Based on the kinematics a transtensional regime is suggested for the area. A model of localized normal faulting along preferential oriented structures and sinistral strike-slip kinematics along less preferential oriented precursor structures, the latter balancing and releasing stress built up by normal faulting and differential extension, is suggested for the study area during post-Caledonian. The study stresses the importance of structural inheritance, influencing fault orientations and perturbing the regional stressfield locally, as has been suggested for the North Sea. | |
