Structural and temporal evolution of a reactivated brittle-ductile fault - Part II: Timing of fault initiation and reactivation by K-Ar dating of synkinematic illite/muscovite

• Published in: Earth and Planetary Science Letters
• Main Authors: Torgersen, E., Zwingmann, H., Harris, C., VIOLA, GIULIO
• Other Authors: Viola, Giulio
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: Brittle-ductile faulting; Dolomite decarbonation; K-Ar dating; Mineral authigenesi; Stable isotope; Geochemistry and Petrology; Geophysic; Earth and Planetary Sciences (miscellaneous); Space and Planetary Science; Kvenklubben; Norway; Torgersen; Northern Norway
• Online Access: http://hdl.handle.net/11585/569750; https://doi.org/10.1016/j.epsl.2014.09.031; http://www.sciencedirect.com/science/journal/0012821X/321-322

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±11Ma, but most strain was accommodated during Caledonian compression at 445±9Ma. The fault was reactivated extensionally at 121±5Ma. 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 40Ar/39Ar 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.