Roger Hanson: Exploring the Alpine Fault

Thursday, 9 February 2017

The majestic Southern Alps are the result of the upheaval of the Pacific Plate's rim.

The Alpine Fault is a huge 480km scar slicing through the western length of the South Island.

It extends from Fiordland to the Marlborough Fault system where it splits into several separate faults. If the South Island is cut along the Alpine Fault and the two parts are moved relative to one another then rock types present in Otago can be matched exactly with those in the Marlborough region.

The movement of the plates at the Alpine Fault occurs in stops and starts. Major ruptures, noted as strong earthquakes, occurred in 1460, 1630 and the last was in 1717.

Careful study by geologists reveals that this 480km offset, the separation of the rocks near Marlborough from those in Fiordland, has occurred in stages over the last 25 million years along the Alpine Fault.

How did the Alpine Fault form and what is making it move even now?

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The outermost 80 to 200kms shell of the Earth is called the lithosphere which comprises the Earth's crust and below that the upper section of the Earth's mantle.

The lithosphere is broken into seven large tectonic plates. New Zealand sits directly on the collision point of two of these huge plates, the Australian Plate on the west and the Pacific Plate on the east.

A layer of the Earth's crust, 25-70kms, carrying a landmass is called continental crust. A section of the crust submerged in the ocean and not carrying a surface landmass is called oceanic crust.

It is only about 10kms thick, but oceanic crust is rich in magnesium and iron and denser than continental crust which is rich in lighter feldspar and quartz.

When oceanic crust meets a section of continental crust, the heavier oceanic crust dives beneath the lighter continental crust.

But when continental plates collide head on, one or both crumple to form mountains. Rather than ram each other head on, the plates can slide past each other.

This is the case with the Alpine Fault.

The engines driving plate movement are called mid-ocean ridges. These are huge scars in the crust often thousands of kilometres long situated in the ocean bed. They are like elongated volcanoes which emit magma, driven by powerful convection currents from the Earth's hot interior.

The magma seeps through this scar, pushing the plates apart, solidifying and adding to the oceanic crust. This process is called sea-floor spreading and the Pacific Plate is moving because it is being pushed by sea-floor spreading which originates from a mid-ocean ridge far away in the Pacific Ocean.

One place where the giant oceanic Pacific Plate clashes with the continental Australian Plate is the Hikurangi Trench off the East Cape.

Here the Pacific Plate dives under the Australian Plate and is subducted beneath the North Island. Just north of Kaikoura the Hikurangi Trench peters out and from here south both plates become continental plates.

These two continental plates being of equal density, like two corks in water, slide against each other. The Pacific Plate sustains the most damage - the Southern Alps are the result of the upheaval of its rim.

The grinding of the plates against each other causes numerous cracks in the crust called faults, of which the biggest is the Alpine Fault.

The Pacific Plate is being pushed about 40mm per year westward by the sea-floor spreading generated by the distant mid-ocean ridge and then the plate movement is turned in a south westerly direction, moving at about 30mm per year.

The net effect is that the Pacific Plate moves around a pole of rotation which is located about 2,000kms south-south- east of New Zealand.

The southern end of the Alpine Fault leads off the coast of Fiordland to the Puysegar Trench. Here, unlike East Cape, it is the oceanic Australian Plate that is descending beneath the continental Pacific Plate.

The movement of the plates at the Alpine Fault occurs in stops and starts. Major ruptures, noted as strong earthquakes, occurred in 1460, 1630 and the last was in 1717.

At a rate of 30mm/year a total of 300 x 30mm which means 9,000mm or 9 metres of strain has built up in the fault since it last moved 300 years ago. Scientists agree that the next movement is overdue.

Thankfully most of the Alpine Fault is located in sparsely populated areas, so it is hoped the number of casualties, if any, will be low.