Structural and temporal evolution of a reactivated brittle-ductile fault - Part I: Fault architecture, strain localization mechanisms and deformation history

•Structural analysis of a multiply-reactivated brittle-ductile fault.•Fault initiated as brittle feature, and evolved into a weak fault, thus widening.•Dolomite decarbonation and metabasalt carbonation were the weakening mechanisms.•Faults' seismogenic behavior depends also on their structural-evolutionary stage.•Mechanical models need thus to be calibrated against the dynamic nature of faults.

Faults are by nature dynamic, as their architecture and composition evolve progressively in space and through time steered by the interplay between strain weakening and hardening mechanisms. This study combines structural analysis, geochemistry and chlorite geothermometry to investigate deformation and strain localization mechanisms of the Kvenklubben fault, a Paleozoic brittle-ductile thrust in northern Norway, with the goal to constrain their temporal variations and the consequences thereof on fault architecture development and rheological behavior. The fault evolved from an initially discrete brittle feature slipping mainly by seismogenic ruptures to a wide brittle-ductile phyllonite deforming by aseismic creep. The formation of mechanically weak phyllosilicates by decarbonation of footwall dolostones and carbonation of hanging wall metabasalts was the main weakening mechanism, whereas partitioning of fluid flow and fracture sealing following transient high pore pressure-driven embrittlement caused episodic and localized strain hardening. The interplay between strain weakening and hardening mechanisms caused the fault core to widen. We suggest that the ability for carbonate-hosted faults to slip by seismogenic rupture is also a function of the faults' structural-evolutionary stage, and that it decreases progressively with fault maturity. This study demonstrates the importance of calibrating the present-day fault anatomy against the dynamic character of faults, which evolve geometrically, compositionally and mechanically in space and through time.