FAULTED PHYLLITES: EXAMINING THE VARIABILITY IN CLAY MORPHOLOGIES WITHIN AN EXHUMED FAULT ZONE (MINAS FAULT ZONE, NOVA SCOTIA, CANADA)

Phyllosilicates are routinely argued to control the deformation mode within mature fault zones. Particular importance is assigned to reduced coefficients of friction and enhanced interphase diffusivity. Here we examine the micromechanical record of samples collected from the exhumed Minas Fault Zone, Nova Scotia, Canada (6-8km depth, ~240°C). Fault rocks exhibit mesoscopic features consistent with mixed-mode brittle/ductile deformation; additionally, the host lithologies and longevity of deformation has produced a broad, complex zone akin to accretionary wedge environments, but with a steep fault surface. Zoned phyllitic units (e.g., shear-band domain, ultrafine scaly mesocataclasite, wavy foliated phyllite) derived from the same sedimentary host have accommodated significant shear displacements. Perhaps unsurprisingly, significant variation in clay morphologies is observed over short distances due to the anastomosing nature of mature fault zones, while the clay mineralogy is consistently composed of muscovite and chlorite. Combined XRD of mineral separates, optical microscopy, and scanning and transmission electron microscopy were used to show the variations in morphology from the meso to nano-scale, and to elucidate the various deformation mechanisms. The shear-band domain contains dislocation-rich (sub-grain walls, dislocation lines) quartz and albite aggregates with precipitates of dislocation free quartz occurring at the edges, within an anastomosing system of muscovite and chlorite (grain size = 0.5-1µm). Precipitates of blocky quartz containing growth dislocations are entrained between phyllosilicate foliae, and are evidence for deformation via pressure solution accommodated creep. The mesocataclasite has a very fine grained (50-100nm) chlorite and muscovite rich, rutile bearing matrix which coats clasts of foliated phyllite and rounded disaggregated fluid rich quartz veins. Extreme comminution must have occurred to produce the very-fine-grained matrix, with associated chemo-mechanical attrition of vein fragments. The association of mesoscopically ductile and brittle fabrics, plus fluid pressure transients, in presumably weak, anisotropic phyllosilicate rich rocks raises the possibility that a transitional record from slow to fast earthquakes is preserved.