Dextral strike-slip faulting history of the Mid-Cretaceous d'Abbadie fault zone, Pelly Mountains, south-central Yukon, Canada

The d'Abbadie fault zone is located in south-central Yukon, northeast of the Teslin fault. It is a north-striking, steeply dipping strike-slip fault with a strike length of 70 km. There is a complex deformation and metamorphic history that predated the d'Abbadie fault zone where Paleozoic supracrustal rocks of the Last Peak succession, in the Yukon-Tanana terrane, were polydeformed and metamorphosed under greenschist facies conditions in the Permo-Triassic, and cooled in the Early Jurassic. This study focuses on history of a transect of the d'Abbadie fault zone, in the northeast portion of Livingstone Creek area (NTS 105E/8).;The ca. 96 Ma Last Peak granite is a NW-SE striking, steeply dipping, variably foliated and mylonitic leucogranite that lies within the central part of the study area. Petrological and kinematic studies of the S-C mylonites show that the development and discrete distribution of the mylonites to the granite was facilitated by the heat of the pluton itself while it was intruding. Such heat made the rocks weak, which therefore made them susceptible to deform in a ductile manner under cold upper crustal conditions.;The Last Peak granite was syntectonic with the displacement of the d'Abbadie fault zone. Evidence includes the discrete distribution of the mylonitic rocks to within the pluton, cross cutting contacts, contact aureole, and the presence of an eastward ductile strain gradient within the Last Peak granite towards the centre of the d'Abbadie fault zone.;Sense of shear indicators that are preserved in both the cataclastic and mylonitic fault rocks provide the foundation for a kinematic analysis of the fault movement of the d'Abbadie fault zone. Structures and microstructures in mylonitic rocks in the syntectonic Last Peak granite and the relative motion on brittle shear fracture planes throughout the fault zone indicate that the d'Abbadie fault zone is a dextral strike-slip fault.;Geological mapping and structural analysis indicate that the d'Abbadie fault zone constitutes a 2 km-wide zone of primarily anastomosing brittle dextral strike-slip faults, and a localized 200-500 m-wide strand of dextral S-C mylonites in the Last Peak granite. The brittle faults are filled with 2-30 m-wide cataclasites and 1 mm to 40 cm-wide cataclasite zones that are distributed in variably sized strands along the faults. The cataclastic rocks form three domains: the Cataclasite, Mixed and Fracture domains. The S-C mylonites form three domains: the Protomylonite, Mylonite and Ultramylonite domains.;Timing of the deformation along the d'Abbadie fault is in part constrained by syntectonic emplacement of the ca. 96 Ma Last Peak granite within the fault zone. Localized cataclasite zones, slickenlines on fault polished surfaces, and fractures overprint the S-C mylonites in the Last Peak granite. These crosscutting relationships suggest that brittle faulting outlasted cooling of the granite and ductile deformation. Metamorphic conditions of the brittle and ductile rocks are confined to the greenschist facies. This is based on chlorite along fault surfaces and in strain shadows of K-feldspar porphyroclasts in mylonites in the syntectonic Last Peak granite.;The major deformation mechanism responsible for the formation of the cataclastic and brittle rocks include: cataclastic flow in and between grains, frictional grain-boundary sliding, and stress-induced solution transfer. Such mechanisms are consistent with a brittle deformation regime and are indicative of the rheology of a brittle crust. Therefore, the syntectonic depth-temperature conditions for the d'Abbadie fault zone are best estimated between 5-10 km and < 250 °C.;The d'Abbadie fault zone is made of coherent fault rocks that are consistently distributed. This suggests that it has not undergone any significant reactivation and that it was a short-lived structure whose motion was restricted to the mid-Cretaceous, at ca. 96 Ma.