No more blind dates with calcite: Fluid-flow vs. fault-slip along the Ockov thrust, Prague Basin

Dating of fracture-filling calcitewith U-Pb geochronology is becoming a rapidly adopted technique for determining the absolute timing of brittle deformation in the upper crust. Slickenfibre calcite is a desirable target, as it precipitates between individual fault slip displacement events, and provides additional kinematic information. Here we present a case study of slickenfibres formed on the Ockov thrust in the Lower Palaezoic Prague Basin, Bohemian Massif, utilising a combination of petrographic and in situ methods. We demonstrate that slickenfibre external textures can be preserved, whilst internally primary textures are removed by fluid infiltration and recrystallization, leading to variable U and Pb mobilisation. One slickenfibre yielded a date of ca. 250 Ma, which we interpret as recording fault slip along the Ockov thrust. Another cross-cutting slickenfibre yielded more scattered U-Pb data, with an imprecise apparent age around ca. 95 Ma. This slickenfibre is recrystallised, destroying the primary textures, and exhibits element mobility. The meaning of this younger apparent age is therefore questionable; whereas it likely reflects Cretaceous U and Pb mobility assisted by fluid-flow along the fault plane, it may not reflect a period of fault slip. Our results demonstrate that slickenfibre-based U-Pb dates do not unequivocally relate to fault motion, and that petrographic and elemental analyses are important requirements for interpreting calcite U-Pb data. (c) 2021 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.

Automated summary

Dating fracture-filling calcite with U-Pb geochronology is a useful technique for determining brittle deformation timing, but results may not always directly relate to fault motion. Petrographic and elemental analyses are crucial for interpreting U-Pb data from slickenfibre calcite, as internal textures can be altered by fluid infiltration, leading to variable U and Pb mobilisation. Specifically, the study of slickenfibres on the Ockov thrust in the Lower Palaezoic Prague Basin demonstrates the preservation of external textures and the destruction of primary textures, with potential uncertainties in the interpreted ages.