“BRITTLE STRUCTURAL FACIES” ANALYSIS TO UNRAVEL AND DATE MULTIPLE SLIP EVENTS OF LONG-LIVED FAULTS: THE LÆRDAL-GJENDE FAULT CASE STUDY (SW NORWAY)

Regional-scale faults typically experience multiple reactivations during their lives. They contain multiscalar domains characterized by varying (micro)-structure and mineralogical composition, which result from strain partitioning during the recorded faulting stages. A detailed structural analysis coupled with K-Ar dating is necessary to identify these tightly juxtaposed domains, which we refer to as “Brittle Structural Facies” (BSF). BSF are important in that they may preserve the isotopic and kinematic signature of different slip episodes. Their unravelling may thus lead to a more comprehensive understanding of faulting histories and processes, including seismogenic rupturing.

We present a multidisciplinary study based upon meso- and microstructural analysis, chemical characterisation and K-Ar fault rock dating to unravel the evolution of the Lærdal-Gjende Fault (LGF) in southwestern Norway. The LGF is a long-lived, top-to-the-NW extensional fault. In its 1 m thick fault core, we recognized, characterized and successfully dated five BSF: I) Indurated red 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 (<0.1 to 6-10 μm) and analyzed by XRD, SEM, TEM and K-Ar geochronology.

The c. 180 Ma age cluster of the coarsest 2-10 μm size fractions likely documents fault nucleation during Jurassic rifting in the North Sea. The ages of the finest fractions, enriched in synkinematic illite, smectite and K-feldspar, constrain faulting at c. 121, 87, 78 and 57 Ma. Ages indicate that the LGF accommodated strain in response to hyperextension of the Norwegian margin down to the Late Cretaceous and then slipped again during the Paleogene. Alternating widening and narrowing of the active fault zone core in response to varying deformation mechanisms (including coseismic rupturing) led to the present complex fault architecture.

BSF characterization as part of a multidisciplinary workflow is key (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.