Tectonics and Geology

The mega-scale geomorphology of South Island is controlled by regional structural processes. These are plate tectonics, which are a result of continental drift. Specifically, South Island is a zone of collision between the Australian continental plate and the Pacific oceanic plate.
The major expression of this collision is the rise of the Southern Alps, and the Alpine Fault. The Southern Alps have a mean transform velocity of ~ 40 mm a-1, and a mean vertical velocity of up to 10 mm a-1. However, sudden variations in uplift can occur. The largest recorded in South Island was a single displacement of 5m during the 1929 earthquake (Table 1). The Alpine fault links two subduction zones, one on North Island where numerous active volcanoes occur.

The New Zealand landmass comprises a small proportion of submerged continental crust and rocks of marine heritage that have only become exposed due to events in the last 25 million years. ‘Basement rocks’ occur at depth and are older metamorphic and volcanic rocks. Superimposed upon these are ‘cover rocks’ with particular lithologies and deformation styles (Figure 2). Two broad orogens (mountain building periods) are recognised; Tuhua (Precambrian to Devonian), and Rangitata (late Palaeozoic to Mesozoic). Cover rocks are mostly sedimentary and volcanic terranes with many landforms developed upon them. Overall there are two primary rock types; Quartzo-feldspathic greywacke, which is located east of the main divide; and metamorphic schist which runs parallel to and along the Alpine Fault and outcrops extensively in central Otago. This variety gives New Zealand a rich number of resources, as well as an incredibly diverse landscape, that brings a range of limitations and opportunities for its inhabitants.