FRICTIONAL MELT BELOW THE BRITTLE-DUCTILE TRANSITION: TWO EXPLANATIONS FROM A SHEAR ZONE IN NORTHERN SASKATCHEWAN

Pseudotachylyte is amorphous vein material generated by highly localized strain considered to be a reliable indicator of seismic slip that is increasingly found to have formed in high-grade metamorphic environments dominated by ductile processes. Although there is no widely accepted mechanism for the formation of pseudotachylyte significantly below the estimated frictional-plastic transition, metamorphic and deformational overprints provide insight to likely mechanisms.

The Cora Lake shear zone in northern Saskatchewan, Canada, is a 4-6 km wide and >90 km long, deep crustal (ca. 1 GPa, 800-700 °C), sinistral strike-slip ductile shear zone that was active ~1.88 Ga in an intracontinental setting. The shear zone hosts multiple generations of sinistral pseudotachylyte, which have been variably overprinted by sinistral plastic structures and metamorphic mineral assemblages. The vein networks are commonly proximal to and inherit enigmatic but kinematically compatible semi-brittle shear fractures. Kinematic compatibility, a well-constrained regional exhumation history, mineral stability fields, and thermobarometry constrain the pseudotachylyte to forming contemporaneously with ductile shear >10 km below the paleo frictional-plastic transition.

The most commonly cited mechanisms for generating deep ruptures are: (1) shear heating positive feedback loops that may be amplified at sharp rheological gradients; and (2) propagation of slip from the brittle crust downwards into the nominally plastic regime. (1) allows brittle rupture to originate at conceivably any depth and may be seismically observable - i.e. earthquakes or tremor: minimum fault slip necessary to produce observed melt thicknesses in dozens of mapped veins is between 2 - 10 cm, corresponding to 0.5 - 1 Mw seismic events. (2) does not require seismic hypocenters below the brittle-ductile transition, instead suggesting that the middle and lower crust may not be as velocity-strengthening as previously considered. Assuming that ruptures generated at depth by (1) would not themselves propagate significantly into the plastically deforming volume, the proximal shear fractures may represent dynamic stress field effects from (2) rupture tips. Both mechanisms are considered viable candidates until more data can be collected.