Part 2 of New Zealand Tectonics
Part 1: The Pacific Plate
Part 2: The Alpine Fault Sliding between Australia and the Pacific along a
single fault line.
Part 2a:Central North Island Faults:
Wellington Earthquakes along many fault lines
Part 3: Euler poles: how
the Pacific Plate rotates relative to the Australian plate.
Part 4: Accretion wedge:
East Coast grew from the Pacific sea floor - the Napier Earthquake.
Part 5: Twisted New Zealand and
wasted Australia.
The alpine fault is easily seen from space. Note how it splits into several faults in the North East (top right). Image is modified from NASA visible earth web site, click to see the original, click on the picture for a big version, modified by nzphoto and suitable for computer wall-paper. |
|
Sliding between continental rocks (trans-current fault) began 25mya (million years ago) At the start of sliding, New Zealand was barely above sea level and there were no mountains. Collision started 10mya (trans-pressional fault). Probably the relative motion between Australia and the Pacific Plate changed when collision became important. |
Graeme Steven's
map --> Metamorphic means rocks 345-167 million years old changed by heat and pressure deep in the earth's crust.
|
|
Southern Alps of New Zealand |
||
|
Sliding along the alpine fault was first pointed out by H.W. Wellman (1952) and here is his original map:
Terrane Map of South Island (Links to the University of Otago modern version of Wellman's map) The alpine fault runs to the west of the Southern alps, which are pushed up by the fault movements.The central part of the alpine fault has very thick crust,
about 45km, and this "crustal root" is thickest under the high peaks of
Tasman and Cook. What happens in the middle of the alpine fault, under the high peaks, is not certain. Slow earthquakesJapanese faults were first identified to have "slow earthquakes" lubricated by ground water. Now similar events have been recorded from Canada, USA and New Zealand.
A slow earthquake has been recorded near Gisborne (East Coast, New Zealand) sliding at 2mm per day and located 20km under the sea floor. (It started October 31 2004 and is currently still moving. A similar slide occured here in 2002). The movement is 10 times faster than normal for this region. It has been detected by continuous GPS recording instruments which send the data back to GeoNet every 30 seconds. While slow 'quakes are normally recorded by GPS, sensitive seismic recorders have detected them deep to Washington state and British Columbia, between the Juan de Fuca and North American tectonic plates. Strain guages in bore holes detected them earlier in San Francisco region along the San Andreas fault. Slow earthquakes are an aseismic release of strain (which makes the "earthquake" part of the term a bit problematic) and they may just be a common feature of sliding between tectonic plates that has finally been detected by sensitive measuring instruments. Deep earthquakesThese are rare under the Southern Alps, but common at either end of the alpine fault. Deep earthquakes occur along a subducting plate as it sinks into the earth, and the alpine fault does not subduct. |
Creating the Southern Alps from flat land.Mountain building indicates sliding is not the only motion at the alpine fault line, there is also an element of collision. The Pacific Plate rides up the roughly 40 degree slope at the edge of the Australian plate. The Southern alps are asymmetrical:
Steep on the west,
where the summits are within 15km from the narrow coastal plain, close to
where the fault reaches the surface. Elevation of the alps is accompanied by their destruction through erosion. The rocks here are weak (and difficult for rock-climbers, whose anchors break out easily). If the rocks were stronger, the mountains would be much higher. SIGHTThe recent SIGHT study showed high pressure "fluid" characteristics in the thick crust under the big central mountains. Sliding movement along the alpine fault is not constant, despite the central lubricating fluid, but happens in big, 8 metre jumps, causing earthquakes about 8 on the Richter scale. There is a long, quiet gap between jumps, which are said to have happened around 1100, 1450, 1620 (North end of fault) and 1717 (South, Milford Sound end). That means the next big jump is due about now, and Richter 8 is a huge quake... There have been recent significant shallow earthquakes along the fault. These cause slight fault movements, land-slides and human consternation, but are not the major way the fault moves. We await the big one... (University of Otago has prepared possible scenarios for this big earthquake) The hot ground water in the alpine fault comes to the surface at several hot springs along the northern part of the fault. Maruia springs, Hamner springs and Copeland valley for example. SIGHT = South Island Geophysical Transect
|
The fluid in the alpine fault under the central peaks is high pressure water, not molten rock.It was identified from its sound reflection and transmission properties. The alpine fault comes to the surface at the
north and south ends. Cross sections through it revealed
the movement dates
to experts by: The thick root of crustal rocks under the alps is needed to hold the mountains up, just as an iceberg floats with much of the ice under water. This root is found by measuring gravitational anomalies. The mass of the mountains above the surrounding plains should produce a gravitational pull. But the force is bigger than computed, showing there is more mountain than meets the eye.. Pacific Plate direction of movement has changed often. One change happened when the ridge reaching out to the Chatham Islands (East) was broken off the Pacific plate and added to "Zealandia" |
Any casual tourist finds the rocks in eastern river beds and beaches near Christchurch are rather boring greywacke. Then they travel over the Southern alps to find interesting and pretty schist on the West Coast beaches. Nobody takes eastern pebbles home, but pretty western pebbles are treasured. Earlier, gold could be washed from western rivers, but miners have collected most of that now. Greenstone was valued by the Maori to make tiki and mere, and now this New Zealand jade is made into jewels at Hokitika. But the lovely deep green stone is only found in the west. So what happened?
Mudstone forms from compacted mud on the bottom of the sea and much of this mud came from erosion of a continent near ancient New Zealand. (Australia and Antarctica are candidates). Compressed mud mildly heated forms papa rock, which is hardly rock at all. North Island has plenty of papa rock, which gets a surface of slippery mud after heavy rain. More heat and pressure makes greywacke, which is a featureless grey stone with no fossils or structure.
When greywacke is pushed deep into the earth it is further compressed and heated to transform into schist. This is a prettier rock with alternating layers. Other rocks are cooked in the depths, including greenstone and gold gets concentrated in quartz seems oozing into cracks in the rock. But how do these treasures come back to the surface?
The Pacific Plate at the Alpine Fault not only slides along but is also pushed up to over-ride the Australian Plate. So there is a "gentle" rise to the Southern Alp Peaks from the East, but where the plate rides over at the Alpine Fault it breaks away to make steeper Western slopes. In the process the deep rocks, which changed into schist, are exposed on the western face of the Southern Alps. So greywacke from the top surface of the plate is carried away by eastern rivers, but schist and other metamorphic rocks from the depths of the plate are washed down western rivers.
Geologists are excited by wandering down the west face of the Alps. At the top they find half cooked rocks. Gradually these change until at the base of the slope are rocks which were transformed more extensively by heat and compression. The west slope of the Southern Alps is a wonderful place to study rock metamorphosis, without any digging, since the Alpine Fault has done all the work.
|
|
The Alpine Fault is the only major fault in central South Island. In central New Zealand, movement between the Pacific Plate and the Indo- Australian Plate is split over many parallel faults. These movements are discussed |
Go to Part 3: Euler Poles
Part 2a: Wellington Faults and Earthquakes
Part 4: Napier Faults and Earthquakes:
The Eastern Accretion Wedge.
