Primary Rock Units:  The Southern Alps are composed of two primary rock types. They are a quartzo-feldspathic greywacke interbedded with an argillite-derived pelites. These units are represented by the light blue unit in the geologic map below. These units are primarily Tertiary in age. Close to the Alpine Fault the rock types change to a linear metamorphic band parallel to the trend of the Alpine Fault. These rocks are represented by the pink to purple units in the geologic map below. These rocks are Mid-Mesozoic in age. Metamorphic grade increases to the northwest from chlorite grade rocks represented by the color pink, to ganet to oligoclase grade rocks represented by a dark purple color.

The figure to the left shows vertical strain rates in the rocks adjacent to the Alpine Fault as a funciton of their distance from the fault. Higher values for vertical strain rate represent greater uplift in the rocks adjacent to the fault. This plot supports the idea that uplift and exhumation have been greatest near the Alpine Fault.

Approximately 480 km of dextural offset has been observed on the Alpine Fault. This offset can be seen by the conceptual reconstruction of the South Island based on the ages of rock units. Throughout much of the Alpine Fault's history it has been primarily a strike slip fault. Only recently, within the past 10-15 Ma, has there been a significant amount of shortening on the Alpine Fault (Walcot, 1979).

The Alpine Fault has changed character from a predominately strike slip fault to an obliquely convergent fault in the past 10-15 Ma. The relative plate velocities have also increased as can be seen in the chart to the left (Walcot, 1979). The evolution of the South Island then goes as follows, initially a slightly extensional environment as indicated by the negative perpendicular rates of convergence. Then the fault became predominately strike slip in nature, finally in recent times the fault has become oblique slip in nature.

Closer inspection of the Alpine Fault shows that in places it is actually a combination of smaller strike slip and thrust faults, as shown at right. Also shown at right is a schematic cross section through one of the smaller thrust faults (Walcot, 1979).

This is a schematic cartoon of the obliquely convergent margin between the Pacific and Australian Plates. Here you can see the subduction of the Australian Plate to the south of New Zealand, while to the north the Pacific Plate is being subducted. The South Island and the Southern Alps are located in a transition zone between these two subduction zones.