DECRYPTING POLYPHASE DEFORMATION WITH QUARTZ CRYSTALLOGRAPHIC FABRICS IN THE WANN RIVER SHEAR ZONE, NORTHERN CANADIAN CORDILLERA

In the Yukon-Tanana terrane of northwestern British Columbia, the Wann River shear zone separates the Carboniferous and older metamorphosed passive margin rocks of the Florence Range suite from pre-Carboniferous arc-related meta-igneous and metasedimentary rocks of the Boundary Ranges suite. Based on the crystallization age of a sheared pluton and post-metamorphic cooling ages, the shear zone has been previously interpreted as a 185–170 Ma top-to-the-S thrust that was subsequently folded. We investigated the ~4 km thick Wann River shear zone in two locations: (1) a klippe on White Moose Mountain where it is sub-horizontal and (2) north of Llewellyn glacier ~40 km to the SSE where it is sub-vertical. Although our initial observations indicated a top-to-the-SE/sinistral sense of shear consistent with previous interpretations, quartz crystallographic <c> axis preferred orientations (CPO) analyses and SHRIMP U/Pb geochronology on variably deformed intrusive rocks reveal a significantly more complex polyphase deformation history. At White Moose Mountain, seven quartz-rich samples yield high-temperature (530–650 °C) top-to-the-SE CPO fabrics (D₁). A 191 ± 2 Ma tightly folded granitic dyke cross-cuts the D₁ shear foliation and contains a similar high-temperature top-to-the-SE CPO fabric, consistent with a late-D₁ emplacement. North of Llewellyn glacier, eleven samples yield moderate-temperature (415–545 °C) dextral CPO fabrics (D₂), which, after unfolding of the shear zone, indicate a top-to-the-NW sense of shear contrasting with that documented at White Moose Mountain. D₂ shearing at Llewellyn glacier is constrained between 184 ± 2 Ma and 180 ± 2 Ma based on the crystallization ages of pre- and post-kinematic orthogneisses. Moreover, the 180 ± 2 Ma orthogneiss and metapelite in this area record evidence of a low-temperature (~310 °C) sinistral overprint (D₃). Our study demonstrates that the Florence and Boundary Ranges suites must have been juxtaposed before 191 Ma along the Wann River shear zone, and that this tectonic boundary was reactivated at least twice at 184-180 Ma and after 180 Ma.