“Brittle structural facies” analysis: A diagnostic method to unravel and date multiple slip events of long-lived faults

Regional-scale faults typically experience complex, long-lasting histories, commonly recording evidence of multiple reactivation events. Therefore, they contain multiscalar structural domains characterised by varying microstructures, mineralogical compositions and kinematics. These domains result from differential strain partitioning during the recorded faulting stages, and, as a result, can preserve the isotopic and kinematic signature of the different slip periods. Their detailed structural analysis integrated with K-Ar dating of the fault rock assemblage can help to identify these commonly tightly juxtaposed, although not coeval, domains, which we refer herein to as “Brittle Structural Facies” (BSF). BSF analysis is pivotal (i) to understand the structural heterogeneity of fault zones, (ii) the diachronic formation of geometrically and kinematically complex fault cores and (iii) to reconstruct faults' evolution in time and through space. Following this approach, this study relies on meso- and microstructural analysis, chemical characterisation and K-Ar dating to unravel the evolution of the Lærdal-Gjende Fault (LGF, southwestern Norway). The LGF is a multiply reactivated top-to-the-NW extensional fault with a 1 m thick poorly consolidated core. We recognised, sampled and characterised five BSF: I) Indurated dark reddish gouge, (II) Poorly consolidated cataclasite, (III) Weakly foliated greenish gouge, (IV) Clay-rich gouge and (V) A few mm-thick clay smear decorating the principal slip surface. Samples were separated into five grain size fractions (from <0.1 to 6-10 μm) and analysed by X-Ray Diffraction, Transmission Electron Microscopy and K-Ar geochronology. A c. 180 Ma age cluster defined by 10 ages of the coarsest grain size fractions (2-10 μm) likely documents fault nucleation during Jurassic rifting in the North Sea. The ages of the finest fractions, enriched in synkinematic K-bearing minerals (illite, smectite and K-feldspar), constrain four periods of faulting at c. 121 ± 3, 87 ± 2, 78 ± 2 and 57 ± 1 Ma. Ages indicate that the LGF accommodated strain due to hyperextension of the Mid-Norwegian margin down to the Late Cretaceous and finally slipped again during the Paleogene. The alternating widening and narrowing of the active fault zone in response to varying deformation mechanisms, including coseismic rupturing, formed the present complex fault architecture. This study highlights the importance of BSF characterisation as part of a multidisciplinary workflow to derive structural and temporal datasets of complex fault zones. BSF analysis, moreover, is demonstrated to be key for investigating the diachronic evolution of fault cores and to resolve multiple slip events of faults.