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Present-day exposures of ancient faults represent only the end result of the faults' often protracted and heterogeneous histories. Here we apply K–Ar dating of synkinematic illite/muscovite to constrain the timing of the complete temporal evolution of a complex, multiply-reactivated brittle–ductile fault, the Kvenklubben Fault in northern Norway. All obtained ages vary as a function of grain size. Geologically significant events are identified principally on the basis of detailed structural analysis presented in a companion paper ( Torgersen and Viola, 2014 ). Faulting initiated at 531 ± 11 Ma , but most strain was accommodated during Caledonian compression at 445 ± 9 Ma . The fault was reactivated extensionally at 121 ± 5 Ma . C and O isotopic composition of carbonates and silicates in the fault rocks demonstrates that mineral authigenesis was linked to wall–rock disintegration through dolomite decarbonation and metabasalt carbonation. We suggest that the commonly observed case of age decreasing with grain size in K–Ar and 40 Ar/ 39 Ar dating of brittle fault rocks can be interpreted as a consequence of mixing between two end-member illite/muscovite generations: an authigenic and a protolithic, in which the finest authigenic grains constrain the timing of the last faulting increment. Integrating detailed structural analysis with age dating is the key towards a better understanding of fault architecture development and the temporal evolution of strain localization and deformation mechanisms. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Present-day exposures of ancient faults represent only the end result of the faults' often protracted and heterogeneous histories. Here we apply K–Ar dating of synkinematic illite/muscovite to constrain the timing of the complete temporal evolution of a complex, multiply-reactivated brittle–ductile fault, the Kvenklubben Fault in northern Norway. All obtained ages vary as a function of grain size. Geologically significant events are identified principally on the basis of detailed structural analysis presented in a companion paper ( Torgersen and Viola, 2014 ). Faulting initiated at 531 ± 11 Ma , but most strain was accommodated during Caledonian compression at 445 ± 9 Ma . The fault was reactivated extensionally at 121 ± 5 Ma . C and O isotopic composition of carbonates and silicates in the fault rocks demonstrates that mineral authigenesis was linked to wall–rock disintegration through dolomite decarbonation and metabasalt carbonation. We suggest that the commonly observed case of age decreasing with grain size in K–Ar and 40 Ar/ 39 Ar dating of brittle fault rocks can be interpreted as a consequence of mixing between two end-member illite/muscovite generations: an authigenic and a protolithic, in which the finest authigenic grains constrain the timing of the last faulting increment. Integrating detailed structural analysis with age dating is the key towards a better understanding of fault architecture development and the temporal evolution of strain localization and deformation mechanisms. [ABSTRACT FROM AUTHOR]