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The relative motion of the two plates, westward
for the Pacific plate, and northward for the Australian plate,
resulted in collision and compression of the plate edges. In
the north the contact zone is located at sea east of the North
Island, and forms the Hikurangi Trench, where the Pacific Plate
is dragged underneath the Australian Plate (subduction movement).
As a result the North Island geomorphology displays the classic
sequence of:
- the forearc region that includes the North Island axial
ranges (from Wellington through Tararua and Ruahine Ranges
to the Raukumara Range), which absorbs the greatest part of
the compression,
- the volcanic arc, with a clear line of active volcanoes
from Ruapehu to White Island,
- the backarc region which includes the western part of the
Taupo Volcanic Zone, and where the earth's crust is being
thinned and subsides (sinks).
In the South Island the contact between the
two plates is different and forms the Alpine Fault. There the
two plates move sideways relative to each other, with a total
displacement so far of 480 km (300 miles). The motion has also
been more compressional in the last 10 million years, giving
rise to the Southern Alps.
South of Fiordland the relative movement of
the plates changes again: there the Australian Plate subducts
below the Pacific Plate, formin the Puysegur Trench.
Current
rates of uplift and displacement
Pacific Plate:
| Location |
Direction of motion |
Rate of motion |
| North Island |
West |
50-60 mm/year |
| South Island |
South-west |
30-40 mm/year |
Alpine Fault: displacement of 30-40
mm/year
Southern Alps: uplift of up to 10 mm/year
Kaikoura: uplift of 5 mm/year
North Island axial ranges: uplift of up to 3 mm/year
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