In situ mica Rb–Sr geochronology and microstructural analysis to evaluate the timing and sense of shear zone movement: Shannon and d'Abbadie Faults, Yukon, Canada

This study examines the microstructural and geochronological record in specimens collected along transects away from the Shannon fault and d'Abbadie fault, in the northern Canadian Cordillera. Microstructural analysis for both shear zones indicate dextral shear and an increase in paleostress and inferred strain magnitude toward each fault. In situ mica Rb–Sr geochronology from across a strain gradient developed by a single phase of deformation within the Shannon pluton, adjacent to the Shannon fault, dominantly records syn-emplacement deformation and/or rapid cooling at ca. 100 Ma with sparse analyses ranging to as young as ca. 63 Ma. U–Pb geochronology in apatite from the same specimens returns three ca. 100 Ma dates and two ca. 85 Ma dates. In contrast, lithologically heterogenous specimens that have experienced multiple deformational events, collected in proximity to the d'Abbadie fault, exhibit more variation in Rb–Sr dates, ranging from presumed regional cooling at ca. 195 Ma, a possible thermo-tectonic event at ca. 150 Ma, syn-pluton emplacement deformation and/or rapid cooling at ca. 100 Ma, and local partial resetting of Rb–Sr in mica most likely by brittle fault movement at ca. 80 Ma. U–Pb in apatite geochronology in the same specimens also records the ca. 100 Ma event. The results of this study demonstrate that lithological differences and multi-phase deformational histories create complexity in patterns of in situ Rb–Sr geochronology, and require interpretation within thermal and structural contexts, to quantify the timing of deformation in ductile high-strain shear zones.