Shear zones and boundary layers : deformation processes and phenomena in the Alpine FaultZzone, South Westland, New Zealand

Robertson, Jesse Craig.

The Alpine Fault Zone (AFZ) is the primary structure controlling the geological, geomorphologic and tectonic characteristics of the South Island. Significant Cenozoic displacements have occurred across the AFZ, both horizontally and vertically, due to the oblique The Alpine Fault Zone (AFZ) is the primary structure controlling the geological, geomorphologic and tectonic characteristics of the South Island. Significant Cenozoic displacements have occurred across the AFZ, both horizontally and vertically, due to the oblique motion of the Pacific Plate with respect to the Australian Plate. Uplift of the hanging wall of the AFZ has exposed sections of mylonite at the western range front of the Southern Alps.
A field examination of an exposed section through the AFZ at Doughboy Creek has shown that deformation within the zone is homogeneous on a large scale Deformation may be active in different parts of the mylonite belts at different times, and hence determining the uplift path for a section of mylonite is not a trivial problem, and is one which must be solved by a combination of intensive field observations and physical modelling.
Boundary layers occur in a flow when the conditions of flow (e.g. temperature, velocity fields) change rapidly over a small layer. These flows are characterised by their high length to width ratios, and this flow geometry permits a simplification of the governing fluid equations. Flow in a shear zone occurs as a very thin layer and it is suggested that the flow may be governed by boundary layer flow characteristics.
A fluid dynamic model for strain and temperature profiles within the mylonite belt has been developed based on boundary layer theory under Newtonian flow conditions with a temperaturedependent rheology determined from thermally activated crystal plastic deformation. It is shown that the boundary layer behaviour of large displacement structures such as the AFZ may be in part driven by the thermal profile across the shear zone, and hence that the shear zone will continue to grow at a rate determined by thermal diffusion from the hanging wall into the foot wall.

110 p. : ill. (some col.), map ; 30 cm. + 1 CD-ROM (4 3/4 in.) and 1 sheet.

2007Robertson

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