Fracture geometry of pseudotachylyte generation zones: a study of shear fractures formed during seismic events

The orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the Ikertôq shear belt, West Greenland are described, and the fracturing sequence is inferred. The generation zones occur in strongly foliated gneisses, and consist of concordant, usually paired, principal displacement shear fractures (paired shears), on which most melt was generated, and a system of minor shear fractures. Minor shear fracturing was most intense between principal displacement shear fractures, and most melt intruded from these paired shears inwards with respect to the zone. Intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures, is believed to be contemporaneous with minor shear fracturing in the generation zones.Generation zones affect intact rocks, and individual principal displacement shear fractures have a maximum displacement of 4.6 m. Paired principal displacement shear fractures are up to 3 m apart, and have a maximum observed length of 1 km. In all the zones described, the fracturing appears to be the result of a single episode of slip. Both dextral and sinistral generation zones occur within the study area, and have different geometries. In each case the displacement is strike-slip.