Origin of pseudotachylites in slow creep experiments

We conducted a series of experiments on granitoid cataclasites under mid-crustal conditions (Pc∼500 MPa, T=300 °C) and slow displacement rates (10−8 ms−1< d ̇ <10−7 ms−1) i.e. within an order of magnitude of plate tectonic velocities. The samples reach high peak strength (1250<τ<1375 MPa) then weaken slightly (by ∼40 MPa) and continue to deform without any abrupt stress drops up to a γ of ∼4. Microstructural observations show, that at peak strength nanocrystalline (mean grain size ∼34 nm, smallest crystalline particles ∼8 nm) to partly-amorphous material develops. Weakening occurs once this material is abundant enough to form through going anastomozing slip zones where the strain partitions. During higher strain deformation, deformation localizes further and 1 to 2 multiply connected layers of fragment loaded amorphous material develop. This material shares many similarities with naturally occurring pseudotachylites, however was formed at velocities up to 8 orders of magnitude slower than those calculated for earthquakes (∼1 ms−1). We conclude that the use of pseudotachylites as indicators for paleoseismic events should be reconsidered.