Friction or fiction: seismic indicators along the Okanagan Valley Shear Zone (Western Canada)

Paleo-seismic indicators, such as pseudotachylytes and ultracataclasites, provide direct insight into the weakening mechanisms that promote deformation along shear zones during the co- and interseismic cycles. These structures are particularly relevant along low-angle normal faults, where the fault geometry necessitates efficient weakening mechanisms to enable seismic slip. The Okanagan Valley Shear Zone (OVSZ) is a crustal-scale low-angle normal fault that facilitated Eocene exhumation of the Shuswap metamorphic core complex, the largest core complex in North America. Although previous work has reported the presence of pseudotachylyte veins along the detachment, their origin and significance have yet to be investigated. We conducted fieldwork at a central exposure of the OVSZ footwall in the southern Okanagan Valley (British Columbia, Canada), where amphibolite and quartzofeldspathic gneisses display a strong WNW-trending stretching lineation and S-C and S-C-C' fabrics indicating top-to-W kinematics. Apparent pseudotachylyte veins hosted within the gneiss were identified and sampled for 2D microstructural and geochemical characterization. These analyses provide the foundation for evaluating the deformation mechanisms governing strain localization along the OVSZ. In outcrop, the "pseudotachylytes" occur as thick (5 mm–16 cm), laterally continuous layers of black, glassy material that are sub-parallel to the host-rock foliation (~069/14) and lack offshoot injection veins or vein networks typical of pseudotachylytes. Preliminary scanning electron microscopy (SEM) imaging and energy-dispersive spectroscopy (EDS) of the veins reveal an ultra-fine-grained matrix (<0.5µm–5µm) primarily composed of anhedral biotite (~65%), quartz (~22%), and plagioclase (~13%) grains, with curved to irregular grain boundaries. The vein matrix exhibits a strong foliation defined by aligned biotite grains. This foliation wraps around heavily rounded, equant to elongated host-rock porphyroclasts (30 µm–2.2 mm) of plagioclase and quartz, with minor apatite and monazite. Many plagioclase porphyroclasts display δ- and σ-type mantles that record a top-to-the-west sense of shear. Bands of dynamically recrystallized quartz (grain diameters of <10–120 µm; band widths of 45–220 µm) commonly form quarter fold structures around feldspar porphyroclasts. Contacts between the veins and host rock range from sharp boundaries, locally marked by recrystallized quartz bands (up to 0.74 mm), to transitional zones characterized by progressive grain-size reduction from ~1.8 mm to ~2 µm toward the vein boundary. Collectively, these preliminary microstructural observations are more consistent with ultramylonitic layers produced by intense localized ductile deformation than with frictional melts. These results suggest that slip along low-angle normal faults may involve limited frictional heating and instead be accommodated predominantly by solid-state processes, producing seismic indicators that differ from classical pseudotachylyte structures associated with steeper faults.