Geology of New Zealand Field Trip Guidebook - ResearchGate

In contrast, the Axial Tectonic Belt (Fig. 1) is by far New Zealand’s most active
region. It extends across both islands, from Fiordland in the southwest through the Southern
Alps, Marlborough, and the eastern half of the North Island. It includes the most important
fault systems in New Zealand, particularly the Alpine Fault bounding the Southern Alps to
the west, the Hope Fault in Marlborough, and the North Island Shear Belt (Aitken 1996).
Because the collision between the Pacific Plate and the Australian Plate is oblique here, a
tremendous amount of shear stress is generated along with compression. Thus, almost all of
the faults active in this region produce shear (strike slip motion) as well as compression
(reverse motion). This combination of forces along this system can produce intense seismic
activity. Movement along the Alpine fault averages 35 mm/year; it produces major quakes
every 250-400 years with average movements of 8 m horizontally and 12 m vertically
(Aitken 1996). Based on mapping of rocks on either side of the Alpine Fault, geologists
estimate that it has moved 480 km in the last 20 Ma (McSaveney and Nathan 2007).
Movement along the Marlborough fault system has a slightly lower rate of 15-25 mm/year.
On the North Island, rates of movement are more difficult to measure because the fractured
rocks are buried beneath a thick blanket of more recent volcanic debris. However,
observations of the topography show that horizontal, rather than vertical, movement
dominates here, owing to the angle of the plate collision.
The Taupo Volcanic Zone (Fig. 1) is a region of crustal thinning caused by heating
and extension associated with subduction under this region (Aitken 1996). Where continental
crust generally averages about 35 km thick, the crust under the Taupo region is only about 15
km thick. This reflects significant heat generated by the rapidly subducting Pacific Plate. As
this heat rises through the crust, it causes the crust to expand, rise, thin and pull apart to a
small degree. This, topped by the volcanoes themselves, produces the spectacular
topography found in the Taupo Volcanic Zone.
The Western North Island and Canterbury-Chathams Platform (Fig. 1) are
relatively quiet compared to other regions. Fault movement in the Canterbury-Chathams
region averages less than 1 mm/year (Aitken 1996), while few recently active faults are
known from the Western North Island.
Tectonic History
The oldest rocks exposed in New Zealand today were formed on an ancient
continental shelf adjacent to present-day Australia and Antarctica (McSaveney and Nathan
2007). These rocks began their lives as sediments washed down from mainland Gondwana
during the Cambrian through Devonian periods (refer to the geologic time scale at the end of
this guide for age dates). During the Late Devonian and Carboniferous, subduction
developed to the east of the Gondwanan passive margin, compressing, metamorphosing, and
folding these sediments into the near vertical orientations observed today (McSaveney and
Nathan 2007). What started as clastic deposits are now schists and gneisses. In some places,
metamorphosing temperatures grew so high that the ancestral sediments actually melted,
recrystallizing as the granites and diorites exposed along the west coast of the South Island
from Fiordland to Nelson. These rocks are hard and resist weathering and erosion, producing
Fiordland’s spectacular steep-walled valleys.
At about 200 Ma, subduction ceased and a passive margin redeveloped along the
eastern coast of Gondwana (Fig. 2). Sediments once again began to stream off the
Gondwanan highlands, producing the Torlesse greywackes (dirty sandstones), which are
thousands of meters thick and cover more than half of the New Zealand land mass
(McSaveney and Nathan 2007). In most places, the sandstone alternates with darker
mudstone, indicating that these were produced in a deep-water continental shelf and slope
environment where submarine landslides producing turbidity currents deposited most of the
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