New Zealand lies along the junction between the Indo-Australian
plate and the Pacific plate in the southern Pacific Ocean. The
plate boundary is curved and the two plates have various
relationships to each other along the boundary. The Pacific
plate is being subducted toward the west beneath the North
Island, forming the overlying Taupo volcanic zone. On South
Island, the Pacific and Indo-Australian plates are moving
roughly parallel to each other and have formed a dextral continental
transform fault known as the Alpine fault. South of
New Zealand the plate margin curves again so there is contraction
across the boundary, but here the Indo-Australian
plate is being subducted beneath the Campbell plateau on the
Pacific plate. The Alpine fault has a component of contraction
across the plate boundary, causing the main part of the
South Island to be thrust over the Australian plate, and
uplifting the mountains of the Southern Alps by 0.3 inches
per year (7 mm/yr).
The South Island of New Zealand can be divided into a
number of different belts, cut by the active Alpine fault.
Western Fiordland is underlain by high temperature/high
pressure crystalline metamorphic rocks, while Southland is
underlain by volcanosedimentary rocks of the Hokonui
facies. A scattered belt of ultramafic rocks separates the
Hokonui facies and high-grade metamorphic rocks from
rocks in the east that include non-schistose and schistose
rocks of the Torlesse facies. About 12,000 square miles
(19,500 km2) of the Southern Alps are underlain by a structurally
complex rock unit known as the Otago Schist, part of
the Torlesse facies. These rocks form a strongly curved outcrop
belt that is deflected and offset by the Alpine fault and
occupy rugged terrain with peaks reaching 5,000 feet (1,500
m), the upper part of which is above the tree line. The curved
outcrop pattern, strong strain gradient toward the Alpine
fault, and well-preserved structures in the schist led to many
structural interpretations of the area, elevating the region to a
classic area for structural geology. The schists are bounded
on the northeast and southwest by Permian-Triassic
graywackes, on the west by the Livingstone fault, and on the
northwest by the Alpine fault. The schists include a group of
quartzofeldspathic graywackes and argillites of a turbidite
sequence. They have been metamorphosed to the chlorite
grade, with biotite, garnet, and oligoclase zones. The
graywacke layers in the schists show a wide variety of fold
morphologies that vary according to metamorphic grade,
amount of strain, and proximity to the Alpine fault. The
regional structure of the belt includes a large stack of recumbent
fold nappes that are overturned toward the northeast.
Late Tertiary faulting and uplift on South Island formed
many of the visible landforms including valleys and plateaus.
The western part of the island was heavily glaciated in the
Pleistocene, while parts of the eastern region are covered in
loess deposits.
See also STRUCTURAL GEOLOGY; TRANSFORM PLATE MARGIN
PROCESSES.
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