The Southern Alps dominate the skyline of South Island, New Zealand and form a rugged, mountaineous spine extending some 500km, much of the length of South Island. The highest mountains (3724m) are very steep, snow-covered year-round, and flanked by glaciers up to 23km long. But what are the Southern Alps doing there? Unlike the Alpine-Himalayan belt they didn’t form due to the collision of two continents, and unlike the Andes they’re not a volcanic-dominated mountain belt sitting above a subduction zone.

The role of the Alpine Fault, a presently active right-lateral transform fault connecting subduction zones to the north and south of New Zealand, is key to understanding the origin of the Southern Alps. It marks the boundary between the Pacific Plate to the east and the Australian Plate. The Alpine Fault is seen easily on Google Earth, expressed as an almost ruler-straight cleft through the landscape along the western edge of the Southern Alps. On the ground it is generally more subtle, often defining the break of slope between the coastal hills and high mountains.

The Southern Alps record the very latest episode in the evolution of a continent whose geological record stretches back at least 500 Million years. Zealandia, the wider area of continental crust that includes both New Zealand and the much more extensive submerged plateaux (e.g. Campbell, Challenger, Chatham), comprises a patchwork of exotic terranes welded onto the east Australia-west Antarctic margins of Gondwana between 600-100 Million years ago. The latter stages of breakup of Gondwana were marked by the opening of the Tasman Sea and the rifting apart of Zealandia from Gondwana since 80 Million years ago (see palaeogeographies in Balance & Cotterall 2009).

The Alpine Fault developed more or less in its present configuration since 25 Million years ago. Initially it connected an oceanic trench to the north of New Zealand, the Kermadec Trench, where the Pacific Plate is subducted beneath the Australian Plate, to an oceanic spreading centre in the Pacific Plate, to the south. Transform faults are plate margins that initiate as small circles about poles of rotation of tectonic plates, accommodating ‘pure’ strike-slip motion i.e. plates each side of a transform move exactly parallel to the fault. But – and highly relevant to understanding the relationship between the Southern Alps and the Alpine Fault – once formed thereafter they are forever subject to changes in plate motions.

Southward migration of the Pacific Plate pole of rotation over the last 30 Million years has led to an increasing component of compression across the Alpine Fault manifested as i) the formation of an oblique subduction zone along its southern extent, the Macquarie Ridge, with Australian Plate lithosphere disappearing beneath the Pacific Plate, and ii) lithosphere thickening, uplift and mountain building along the onshore segment of the Alpine Fault in South Island.

The present day motion vector of the Pacific Plate, defining the eastern side of South Island, relative to the Australian Plate is 43mm per annum toward the WSW (DeMets et al. 1990), comprising 41mm p.a. fault-parallel movement and 14mm p.a. compression normal to the strike of the Alpine Fault. Compression causes thickening of the lithosphere and, just as thicker icebergs sit higher in the water, leads to uplift and the creation of high topography. The time-averaged rates of uplift – or strictly speaking the rates of exhumation, the bringing to the surface of formerly deeply buried rocks – in the Southern Alps are up to 11mm per annum, among the highest measured anywhere. What is driving such rapid uplift and erosion? The answer to this question provides one of the best examples in the wider earth sciences of the interrelations and feedbacks between natural processes that used to be treated entirely separately.

Travelling through the western side of the Southern Alps you can feel immersed in temperate rain forest unlike anything we get in Europe. It is misty and humid, comprising dense evergreen forest with thick mosses, ferns and epiphytes and, crucial to the uplift story, very wet. A few kilometres upstream of the Hokatika Gorge, a popular tourist trail, the Cropp River basin receives up to 16m rain per annum, and among the highest average rainfalls recorded anywhere occur on the western side of the Southern Alps.

This is a classic example of orographic precipitation, here due to condensation from warm, moist air coming off the Tasman Sea and forced up the steep west face of the Southern Alps. The contrast between west and east is similarly dramatic – at Arthur’s Pass, rainfall reduces from 5000mm to 1500mm per annum in only 15km, with the eastern flank of the Southern Alps being characterized by warm, dry Föhn winds.

The geomorphology of the Southern Alps reflects the climatic conditions with the western side characterized by steeply graded transverse rivers, steep interfluves and high discharge rates. Erosional processes are dominated by valley lowering and highly active landslides off the interfluves. On the dryer eastern flank the relief is relatively subdued with the rivers more influenced by rock type and geological structure. The cross-sectional form of the mountain belt is therefore highly asymmetric with longer and gentler sloping rivers flowing to the east and southeast (Willett 1999).

Collectively these observations combine in a model in which compression across the Alpine Fault led to the creation of the Southern Alps, a significant topographic barrier where continued uplift has been sustained by rapid erosion promoted by high rates of orographic rainfall. In terms of exhumation we can think of deeply buried rocks being continuously extruded at the surface due to a sort of ‘pull’ force exerted by the wet climate and rapid erosion.‍