Paradoxical pseudotachylyte – Fault melt outside the seismogenic zone

Fault generated melt, pseudotachylyte, is an established indicator of palaeoseismic faulting. The existing consensus that frictionally induced melting occurs within the classic seismogenic zone contrast the contention over how pseudotachylyte forms within the ductile regime. Central to this issue is whether all pseudotachylyte originates as pressure-dependent frictional melt along slip surfaces, or if pressure-independent processes have roles in its formation. Propagation of high-velocity slip into deeper crustal levels provides a satisfactory explanation for pseudotachylyte at depth, but does not of itself rationalize earthquake nucleation outside the classic seismogenic zone. Pseudotachylyte from the Minas Fault Zone, Nova Scotia, Canada is used to demonstrate the formation and preservation of fault-related melt under lower crustal conditions. Microstructures retain evidence of intense dislocation glide with minimal climb, and ductile disaggregation of the host; the latter are consistent with intracrystalline deformation in the Peierls stress-controlled glide regime. It remains unclear whether the crystal plasticity serves only as a precursory stage to rupture and high-velocity slip or is itself responsible for both instability and the thermal transient. There are similarities between accelerating plastic slip leading to rupture and aseismic creep bursts (tremor) that emphasize the mechanistic complexity of deep faulting, and the need to extend consideration beyond that of a simple brittle-ductile response. The occurrence of tremor bursts fall within the depth range of “paradoxical” pseudotachylyte and provides a circumstantial link between active tectonics and the geologic record that merits examination.