Dilatancy fabrics in conglomerates in the Awatere fault zone, New Zealand

Analysis of fabrics of faulted and unfaulted conglomerate within the Awatere fault zone of New Zealand shows that movement on individual faults was accompanied by dilatancy of the clast fabric. The fabric of unfaulted conglomerate is clast-supported. comprising 78% clasts by area. Individual fault zones studied range in width from 4 to 21 cm and have a clay matrix-supported fabric, comprising 33% clasts by area. The abundance of clay within fault zones indicates a highly plastic matrix intruded from the wall rocks into fault zones during localised dilation of the clast fabric. The shear thickening shear-stress-strain-rate relationship typical of dilatant materials suggests that the observed fabrics are more compatible with faults developing slowly as accommodation structures rather than forming rapidly during seismic movements. A rudimentary analysis of the transfer of the minimum lithostatic load through the conglomerate suggests that contact loads between clasts were significantly less than the strength of clasts determined by point load testing. The results provide a mechanism to support a previous interpretation of the fault pattern in terms of dilatant faulting within an oblique divergent tectonic setting.