CORA LAKE SHEAR ZONE PSEUDOTACHYLYTES: DEEP RUPTURE OF A INTRAPLATE STRIKE-SLIP CRUSTAL FAULT AND IMPLICATIONS FOR SEISMOLOGY OF EARTHQUAKES

Magnitude-area scaling relations of large strike-slip earthquakes have been a source of uncertainty and contention in the earthquake community. The depth to which fault slip may propagate is as fundamental to this issue as it is difficult to constrain in the modern and paleoseismic record, but pseudotachylytes (PST) cogenetic to deep shear zones could be key to providing an acceptable model.

The Cora Lake shear zone is a granulite to upper amphibolite-facies (1.0-0.8 GPa, 800-700 °C) structure that crops out in an exhumed section of early Proterozoic lower crust in northern Saskatchewan, Canada. The zone is characterized by 4-6 km of sinistral strike-slip mylonite with anastomosing ~20 meter thick ultramylonitic bands containing multiple generations of foliation-parallel PST. Pseudotachylyte generation surfaces locally display sinistral offsets and the vein networks are commonly proximal to kinematically compatible semi-brittle shear fractures. The fractures are decorated with a high-temperature mineral assemblage, including grain-size reduced orthopyroxene (1-15 µm), and the pseudotachylyte veins also locally contain microlitic garnet that is compositionally distinct from that in the host mylonite. Together these data suggest pseudotachylyte generation at conditions near 25 km depth and in excess of 700 °C.

Pseudotachylyte well below the onset of quartz plasticity has been documented in the rock record in other localities as well. However, lack of a widely accepted mechanism for pseudotachylyte generation in the ductile realm has prevented earth scientists from linking these paleoseismic markers to observations of modern seismic events. Several mechanisms have been proposed: stress amplification during ductile flow due to rheological contrasts; plastic instabilities due to shear heating; and downward propagation of fault tips. The latter appears to be most compatible with the pseudotachylyte systems observed in the Cora Lake shear zone, as field relationships suggest little correlation to potential rheological boundaries, and P-T conditions appear to have been unfavorable for the formation of plastic instabilities. Reasonable assumptions for the crustal column require propagation distances of coseismic fracture downwards from the thermally-defined schizosphere of on the order of 15 km.