DEFINITION OF NEW ZEALAND; ALPINE FAULT AND OTAGO SCHIST

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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