Modes of oblique compression: Late Cenozoic tectonics of the south island of New Zealand
Authors: Walcott R. I. 
Published: February 2004

When continental crust is consumed at a plate boundary, it is not subducted into the asthenosphere like oceanic crust but either is pushed up and eroded or is moved sideways away from the collision to spread the deformation over adjacent and in some cases, far distant, regions. Both modes occur in the South Island of New Zealand, where the Australian and Pacific plates move obliquely relative to each other so that there is motion both parallel and perpendicular to the plate boundary. The perpendicular component has resulted in the rise and erosion of the Southern Alps. Because compression is continuing, the early stages of continental collision can be studied in one of the most simple and accessible of plate boundaries. Since 6.4 Ma, 90 km of shortening in the continental crust of the central South Island has occurred (somewhat less in the south and more in the north), together with 230 km of dextral strike‐slip. Prior to 6.4 Ma, relative plate motion was strike‐slip with only a small compressive component. Hence a strip of continental lithosphere some 90 km wide has been consumed in Pliocene‐Quaternary time. It has been accommodated in different ways in different parts of the plate boundary. In the south, that part of the Pliocene‐Quaternary convergence not involved in subduction of oceanic lithosphere of the Australian plate under Fiordland is accommodated by low‐intensity deformation distributed up to 200 km from the plate boundary, resulting in some crustal thickening and minor erosion. In the north, continental crust has moved away from the collision zone along transforms to be placed over adjacent subducted oceanic lithosphere of the same plate, and the major process accommodating excess continental lithosphere is overthrusting of continental crust, accompanied by subduction of the lower part of the lithosphere. In the central South Island, accommodation of excess continental crust is by crustal thickening and erosion. Rock uplift at rates of about 11 mm/yr at the Alpine fault is balanced by erosion producing some 0.35 × 106 km³ of sediment deposited mainly over the Challenger Plateau immediately west of the South Island. Some of the excess continental crust in this part of the plate boundary is accommodated as a root to the Southern Alps. On the basis of a number of observations it is argued that a 5‐km‐thick tabular body of high‐grade schist near the middle of the Alpine fault is being extruded from the lower crust at a higher velocity than the plate convergence rate because (1) ductile lower crust outcrops in a highly condensed section, (2) fluid inclusion studies indicate temperatures as high as 360°C at depths of around 6 km, (3) stretching lineations in young mylonites show that motion normal to the Alpine fault is equal to, or faster than, the strike‐slip component, and (4) a normal fault bounds the upper surface of the body, and the reverse Alpine fault bounds the lower surface. A similar process occurs in other convergent continental zones. While large earthquakes are known to have occurred on those parts of the Alpine fault north and south of the central segment of the plate boundary, it is unclear whether great earthquakes have been, or can be, generated on middle reaches of the fault, where the structure of the Alpine fault differs markedly and high temperatures are believed to exist close to the surface.