Structural framework and the timing of landscape-forming faults - a study from Western Norway

<p>Brittle fracture and fault networks control the location of topographic features such as valleys and ridges and active faulting can lead to topographic rejuvenation. In Western Norway, however, it is debated how much faulting has contributed to rejuvenating of the topography during the late Mesozoic and Cenozoic. Geometric and temporal constraints on the brittle evolution are therefore important to obtain a comprehensive picture of the post-Caledonian topographic evolution of this region. In this study, we combine remote sensing, structural field measurements, paleo-stress analysis and isotopic dating to study the brittle evolution of a larger region of Western Norway. The region spans from the Sognefjord in the south to the Møre margin in the north. Lineament studies reveal important lineament sets trending N-S, NE-SW, E-W and NW-SE. Field observations show that these lineament sets correspond to both dip-slip and strike-slip faults, some of them parallel to ductile precursor structures and some cutting the ductile fabric. Epidote, chlorite, quartz and zeolite are the dominant mineralizations on fracture and fault surfaces. There is no clear correlation between the type of mineralization and fracture orientation in the region. Paleostress analysis on fault-slip data (n = 173), including faults reactivating older structures, show a good fit with a general E-W extensional regime. However, a considerable amount of faults (n = 115) formed under a strike-slip regime, which has so far not been documented in the region. We combine these findings with K-Ar fault gouge dating from six faults where five fractions (6-10 µm, 2-6 µm, 0.4-2 µm, 0.1-0.4 µm, <0.1µm) from each sample were analysed. These faults represent two of the four fracture sets observed, trending N-S and NE-SW, respectively, and show either strike-slip or dip-slip kinematics. XRD-data from these gouges show that K-feldspar and smectite are the main sources of potassium. The ages show a spread from the Triassic to the Cretaceous, where older ages can be affected by K-feldspar inherited from the host rock. Our results point to an important phase of Mesozoic strike-slip faulting in the region, with steep faults controlling the location of several major valleys. Extensional dip-slip faults might have contributed to the rejuvenation of the footwall topography.</p>

Fault gouge dating: history and evolution

ABSTRACTRadiometric dating of fault gouges has become a useful tool for regional tectonics studies and for exploring and understanding fault and earthquake processes. Methods to define the absolute age of faults achieved a solid scientific foundation almost 25 years ago when the development and application of illite age analysis for investigating sedimentary burial and thermal histories found a new potential application – defining the age of fold-and-thrust development. Since then, the methods have benefitted from further development and incorporation of the 40Ar/39Ar micro-encapsulation method and quantitative clay mineral evaluation to distinguish polytypes (Wildfire). These refinements to the methods have improved their application in fold-and-thrust terrains and have opened up applications in normal and strike-slip fault environments. Another important development is the use of absolute dating methods in retrograde clay gouges in which clays in a fault develop from igneous or metamorphic wall rocks that contain no clays. In addition, the method has also been shown to be useful at dating folds in fold-and-thrust belts. We think the method is now an established part of the geological toolkit, look forward to future fault structural and tectonic studies that incorporate fault ages and hope that researchers continue to probe and discover ways that the method can assist fault process studies, including earthquake fault studies.