FOLD RELATED FRACTURES IN THE WHITEROCK LIMESTONE: INDICATORS OF REGIONAL STRESS IN WHITEROCK, NORTH CANTERBURY, NEW ZEALAND

On New Zealand’s South Island, joints and faults are the surface expressions of brittle deformation at a transpressional plate boundary. Exposed as strike ridges, a folded sequence of Tertiary sediments is faulted and fractured in Whiterock, North Canterbury. The objectives of this study were to compare brittle structural data in the Miocene Whiterock Limestone to an ideal model for fractures in flexural slip folds in an attempt to determine if the fractures and faults result from the folding mechanism, from a later deformational phase or from a combination of both. Air photo analysis of the lineaments was conducted in the study area. Fractures, bedding parallel pressure solution seams and faults were measured and their crosscutting relationships were discerned. After a synoptic analysis of the data, the field area was divided into four structural domains, Hawke Syncline, MacIntosh Anticline, Milne Syncline and Carter Holt Harvey Anticline. Using two different six joint fracture models, one for the inner arc and one for the outer arc of a single layer in a flexural slip fold, the structures were evaluated within the context of their respective domains. The strong correlation between the predicted models and the measured joint sets, the relative age dating, and the directions of maximum compression indicate that the folds and fractures were formed in the same deformational phase. The azimuth of maximum compression for the folds and the fractures is 125°. This principal horizontal shortening orientation supports previous work that defines the present day stress regime in the region indicating that the folds are actively growing. Air photo analysis and measured faults were also consistent. Measured thrust faults were associated with local stress caused by the folding, while both dextral and sinistral strike slip faults, the youngest structural feature seen in the field area, were associated with the Porters Pass - Amberley Fault Zone, a young, dextral shear zone striking ENE to WSW. A rough estimate for the direction of maximum compressive regional stress from the fault orientations is ESE to WSW. The newly forming shear zone marks the initiation of stress evolution and migration as the stress regime advances towards the southwest initially accommodated by strike slip faults and later, thrust faults, as is seen through much of North Canterbury.