STRUCTURED AMORPHIZATION IN GOUGE – SEISMIC OR ASEISMIC SLIP?

Amorphous silicate phases formed during frictional sliding have been observed in a variety of experimental and natural situations. In known conditions of high-velocity slip of bare rock surfaces, amorphous material has been associated with formation of a gel phase during frictional displacement. Contrasting the latter are amorphous phases produced at aseismic slip rates. In this study we document the textures of amorphous material within precursor gouges of igneous material (Westerly granite, Mt. St. Helens dacite). The synthetic Westerly granite gouge is deformed experimentally at bulk displacement rates of 1-10 μm /s and the MSH gouge is formed during extrusion of crystallized volcanic domes at 1-6 m/day. Although both the experimental and natural gouges have undergone extensive normal cataclasis and grain size reduction, their most striking feature is the grain size zonation that forms multiple bands. In both cases, the textural zonation is defined by discrete, sharp surfaces at which amorphous to ultrafine-grained material is juxtaposed with coarser grained host gouge. The development of the amorphous material is established by analytical transmission electron microscopy of focussed ion-beam cross-sections that enable spatial association to be maintained. The sharp interfaces are interpreted as localized rapid slip surfaces formed during localizing within a bulk strain-hardening system; that is, transient seismic or near-seismic instabilities within aseismic deformation zones. Whereas the underlying kinematic symmetry is monoclinic, the grain size asymmetry may be linked to asymmetry of stresses at propagating crack tips. The various informal reports of similar textural zonation in natural and experimentally derived pseudotachylyte, although produced by melting as opposed to solid-state amorphization, points to this structure having a more common occurrence than formally reported and can serve as an indicator of stick-slip.