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

Analysis of fabrics of faulted and unfaulted conglomerate within the Awater e fault zone of New Zealand shows that movement on individual faults was ac companied by dilatancy of the clast fabric. The fabric of unfaulted conglom erate is clast-supported, comprising 78% clasts by area. Individual fault z ones studied range in width from 4 to 21 cm and have a clay matrix-supporte d 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 thicke ning shear-stress-strain-rate relationship typical of dilatant materials su ggests that the observed fabrics are more compatible with faults developing slowly as accommodation structures rather than forming rapidly during seis mic movements. A rudimentary analysis of the transfer of the minimum lithos tatic load through the conglomerate suggests that contact loads between cla sts were significantly less than the strength of clasts determined by point load testing. The results provide a mechanism to support a previous interp retation of the fault pattern in terms of dilatant faulting within an obliq ue divergent tectonic setting. (C) 1998 Elsevier Science Ltd. All rights re served.