Cataclastic Processes within the Alpine Fault Zone

Abstract

Cataclasis, associated with slip on a fault, results in the formation of fine grained fault rocks which have undergone fragmentation and shape change. This is accompanied by propagation of micro-fractures through the fault core and into the damage zone surrounding the fault principal slip surface. Previous studies have defined a relationship between the damage zone and (1) confining pressure and temperature during fault slip, (2) fluid move- ment, and (3) fault displacement. In the cataclastic rocks, fragmentation of grains by different mechanism results in the development of distinct particle size distributions. Particle shapes, which have previously been examined in more detail in sedimento- logical and volcanological studies, may also provide insights into formation mechanisms of cataclasites. Both particle size and shape influence the per- meability of the cataclasite zone, while macro- and micro-fractures within the damage zone will also provide off-fault permeability. New Zealand’s Alpine Fault Zone contains a well-developed cataclasite zone that ranges in thickness from metres to tens of metres and is exposed on thrust segments of the fault. Two particularly extensive exposures of the cataclasite zone are found along the Waikukupa River and at Gaunt Creek. Microstructural observations of quartz fragments indicate a number of dif- ferent fragmentation processes operated during cataclasis. Open, shallow, concave cavities are present on the margins of quartz grains, surrounded by a fine matrix of particles. This is inferred to relate to the processes of chipping and spalling of grains during cataclasis. Elongate grains are also observed within the cataclasite. These occur when a trans-granular fracture cuts through a clast. Grains have been recemented after an initial comminu- tion processes, then refractured, to form clasts that characteristically have fractures along the boundaries of the original grains. The methods used to obtain quantitative measurements of Particle Size Distributions (PSD) and particle shape will be fully examined an the effects that cause variation will be discussed. Particle size distribution and shape information was obtained using auto- mated image analysis techniques applied to reflected light images obtained from polished thin sections. PSDs display fractal dimensions that are not simply related to those predicted by common comminution theories, be- cause of the complex range of processes that occur during fragmentation.