The Geological History of New Zealand
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River deposits, central Canterbury
Introduction

New Zealand is a continent. Or rather, New Zealand is (most of) the emergent part of a continent - a submerged one, called Zealandia. Zealandia has been formed over 500 million years by cycles of deposition and uplift (see Rock Cycle if you need to review this).
NZ Geological history can be divided up into periods of erosion and deposition separated by periods of 'mountain building', or orogenies. There are three such orogenies recognised in NZ's geological history: the Tuhua Orogeny the Rangitata Orogeny and the Kaikoura Orogeny We will examine them in detail later.

What are orogenies? Before modern plate tectonic theory was developed, it was thought that areas underwent cycles of 'geosynclyinal' activity when depositional processes were dominant, alternating with 'orogenic activity' when mountain building activity was dominant. Today, we recognize that 'orogenies' represent times when convergent plate boundary activity occurs in a particular area. Plate boundaries move around over time, so the amount of orogenic activity varies at different times in different places. We say an orogeny is occurring in a particular place when certain tectonic activities come to dominate: volcanism, intrusion, regional metamorphism and uplift
Since we now recognise that these activities are actually going on all the time, just in different places, the concept of orogeny is not being used so much; many geologists prefer to use terms such as 'orogenic pulse'. For example, Dr. Hamish Campbell's book 'Unearthing Ancient New Zealand' doesn't refer to the three orogenies. However, I will use them in this page because they are in the Achievement Standard notes.
Between orogenies: These are the times when NZ lay was away from a plate boundary (like Australia is today, with few volcanoes or earthquakes) and the dominant processes were erosion and deposition. In this tectonic environment, mountains wear away in a process called peneplanation. Australia is doing this at the moment, although the process is quite slow because Australia is so dry.
If this process goes on for long enough the land can become swampy and be invaded by the sea. This is called a marine transgression, and occurred in NZ history in the lower part of the Tertiary period ( approximately from 50 - 24 million years ago, before the Kaikoura orogeny).The Gulf of Carpentaria in Australia is a modern example of where this is happening.
The diagram below shows a timeline of the main events of NZ's geological history. It is colour coded; red where orogenic activity is dominant, greens where erosional and depositional activity are dominant. This does not mean there was no deposition during orogenic periods or vice-versa, it is just that one or other process was dominant in the area that is now NZ. Click to enlarge the view.
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Click to see full size. Red areas are orogenies.

Note: background on plate tectonics has been movedhere


Phase 1. The Tuhua depositional phase and Tuhua orogeny
The oldest rock in NZ were laid down in a sedimentary basin off the coast of Gondwana about 550 to 370 million years ago, a period of time known as the Tuhua depositional phase. Geologists using the geological time scale call this time the lower Paleozoic: the Cambrian, Ordovician and Silurian eras.
The situation then was something like what is shown in the diagram below. The buff coloured land in the picture is the older Gondwana area - what is now part of Western Australia and inland Antarctica. Off the coast of this was an extensive area of deposition, shown in the map below the picture. Today that depositional area makes up much of eastern Australia and coastal Antarctica as well as part of Zealandia. The rocks of these neighbours of ours are, to a geologist, noticeably similar to those in parts of the South Island.
The diagram is much simplified because features such as the location of subduction and different depositional zones moved around over time.
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West-East section through Tuhua depositional basin approx 410 my ago

Note: two important depositional zones are the continental shelf, the relatively flat and shallow area immediately off the coast (depth 50 to about 500 m) and the continental slope. The continental slope is where the shelf slopes down to the abyssal plain, the deep ocean which is usually about 5000 m deep. The abyssal plain is not an area of major deposition for sedimentary rocks: it is made of ocean-floor basalt and the underlying low-silica intrusive rocks about 5km thick (oceanic crust). This oceanic crust usually doesn't survive - it gets subducted. However, it is sometimes overlain by a thin layer of sedimentary rock (mudstone and cherts) which can get 'scraped off' during subduction and mixed up with the sediments of the continental slope. On the map below, the outlines of NZ on the light blue colour are only imaginary - that is the abyssal plain material which has been subducted.
The dominant depositional process on both the shelf and slope is by turbidity current. If you haven't already heard of these, follow the link or go to the Year 11 sedimentary page and scroll down to the section on water.
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Land area and sedimentary basin in Tuhua times

Note: Diagrams like these were originally in SVG format but I have converted to PNG to upload. Non-IE browsers may allow you to right click to view at greater size. Contact me to get higher resolution versions if you need them.
Where are the older rocks? When NZ broke off from Gondwana, we left them behind: in Australia (central and Western Australia) and Antarctica, shown in the map to the left in the brick colour. These are the rocks that were eroded more than 380 million years ago to form the sediment of the Tuhua depositional phase. These older rocks still exist in the places we left behind - you can find them in Western Australia and they include some of the oldest rocks on Earth.
Some of the sediments that were laid down to make the rocks of the Tuhua depositional phase, in what is now NZ, are also found in parts of eastern Australia - they also were left behind when we split off.
Note on the map on the left that the outline of the North Island and east coastof the South Island that is shown is for reference only - these areas did not exist at all at that time. The ocean floor that is coloured light blue on the map has long since been subducted back into the mantle. It's gone.

Types of rock in the Tuhua depositional phase
The rocks of the Tuhua depositional period include metamorphic and some sedimentary rocks. Metamorphism resulted from the Tuhua Orogeny and from the intrusion of the granites. The area that is magenta on the map was uplifted to become land about 370 million years ago in a period of mountain building called the Tuhua orogeny. Before this orogeny, there was no ‘land’ in New Zealand.
Note: there had been convergent plate boundary activity before the main uplift stage of the Tuhua Orogeny, but it resulted mostly in subduction and volcanism. The 'orogeny' was probably the result of a change in the direction of plage movement, resulting in compression rather than subduction. This is similar to the way that subduction on the East Coast of the North Island becomes compression in the northern South Island, uplifting the Kaikoura Ranges.

Trilobite
Trilobite
Sedimentary rocks: Sedimentary rocks of the Tuhua Depositional Period are dominated by sandstones and mudstondes, often called greywacke. These are typical of the continental shelf depositional environment. However, limestone was deposited towards the end of the period. This indicates that the sediment input dwindled, probably due to a loss of relief in the landmass to the relief causing less erosion. The limestone has since been metamorphosed to marble (see below).
The sedimentary rocks of the Tuhua depositional phase contain some fossils e.g. trilobites (right) in the Cobb Valley and graptolites on the West Coast. Some of these fossils are similar to those found in southeastern Australia, and the sedimentary rocks are noticeably similar to those from Queensland to Tasmania. This is part of the evidence that we were once connected to that landmass, then part of Gondwana.
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Marble quarry, Takaka Hill
Igneous rocks: There are a few special igneous rocks called ultramafics in this area. These are probably the remains of a volcanic island arc that got squeezed up in the Tuhua orogeny. They have been metamorphosed into serpentine (a type of greenstone) and asbestos. Metamorphosed ultramafics also form pounamu on the Alpine Fault.There are also some related volcanic rocks (see this map)
Metamorphic rocks: In Fiordland and parts of north-west Nelson the sedimentary rocks have been metamorphosed through to schist and gneiss. These rocks have been through several orogenies, so it is unsurprising that these are some of the most metamorphosed rocks in NZ. Some of them have po
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Gneiss, Manapouri underground hydro
ssibly melted to form some of the granites found in the area. Gneiss is the highest grade of metamorphic rock, formed by burial to depths of over 30 km and subsequent uplift.
In Abel Tasman National Park there is are well known rock formations of marble, known as the Takaka/Mt Arthur marble. This also dates from this time and would once have been seashells forming limestone, which has undergone later metamorphism. It contains extensive caves including Harwood's Hole (see a short video of this here). The marble is also quarried (picture above left), for use as a building stone and to make agricultural lime (picture on right).
The marble dates from towards the end of the Tuhua external image moz-screenshot.jpgdepositional phase and suggests that before the Tuhua Orogeny the sediment input from land dwindled a lot. This is likely to be due to a loss of relief in the adjacent landmass.

Uplift: The Tuhua Orogeny:

About 370 million years ago the amount of tectonic activity increased. The sediments and rocks described above were pushed onto Gondwana by the plate motion from the Pacific, and the Pacific plate was subducted under eastern Gonwana. The sediments and rocks were squeezed up, raising some of them out of the sea and causing related activity such as metamorphism. This is known as the Tuhua Orogeny in New Zealand, but there is extensive evidence for the same orogeny in eastern Australia where it is known by other names.

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The uplift actually happened over a period of time, at different rates in different places. There was some intrusion of granites associated with it, particularly in Australia. There was also intrusion, volcanism and metamorphism. The plate boundary moved out to sea over time, as sometimes happens when the lithosphere sinks faster than the sea-floor spreading pushes it forward. This causes the subduction zone to move towards the spreading ridge.
An important intrusive body formed at this time is known as the Karamea Batholith. It forms a striking 'pegmatite' granite (granite with large crystals) found near Karamea and elsewhere in the western foreland.
There are not many places where there is stratigraphic contact between the Tuhua rocks and the next phase, but everywhere it is unconformable. A regional unconformity is a another common sign of orogeny (if you don't know what an unconformity is, please follow the link).
The uplift which happened during this orogeny gave New Zealand some land area for the first time in our geological history. The area of North-west Nelson and Fiordland became land (along with eastern Australia and parts of Antarctica), and sediments eroded from these areas and elsewhere contributed to the next stage of NZ geological history. However, the arrangement of the landmasses was very different from now and the sediment source for the eastern South Island was different than that for the north and west.

Phase 2: After the Tuhua Orogeny - Rangitata depositional period
The Tuhua Orogeny uplifted the oldest sediments to become land, in what is now Fiordland, Golden Bay and also parts of eastern Australia and parts of Antarctica. The land eroded away into a sedimentary basin (called in older literature the "New Zealand Geosyncline") off the east coast of Gondwana, on the continental shelf and continental slope. We will refer to this period of time as the Rangitata Depositional period (in geolgists terms, late Permian to lowermost Cretaceous; see here for geological timescale, but it is not examinable). None of the erosional surface formed on the older rocks during this time has survived in New Zealand.
Eastern and Western blocks: The rocks of the Rangitata Depositional Phase fall intwo two quite distinct 'blocks', of roughly similar age, - the Eastern block is the dark blue part on the cross section and map and is separated from the Western block by a narrow belt of ultramafic intrusives. A possible cross section, (looking north, i.e. west on the left) of the depositional enviroment at the time is shown below: I have based this on a section roughly through Nelson. Note that the eastern block was probably further away than the cross section suggests.
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The map below right shows a very simplified view of the geography at the time. Note that the eastern South Island is down towards Antarctica. Western block rocks are light blue and eastern darker blue. However, I have conflated (grouped together) different eastern units. For a more detailed view, look at this map (note that it is a 6 Mb download as a PDF). The red is the ultramafic rocks (Dun Mt Belt) and the pink and magenta represent the parts of NZ above water at the time (Tuhua rocks).
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Western block strata in Southland.
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Depositional zones pre Tasman Sea

Western block (Murihiku and Brook Street terranes): The Muruhiku terrane rocks are well layered, fossil rich greywackes and found in Southland, Nelson and Kawhia to Port Waikato. On high altitude photos the layering of these rocks can distinctly be seen because they are folded into
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Jurassic fossil log, Curio Bay

a large syncline, as seen in the image above left (looking west towards Gore from a bit south of Balclutha). The parallel hills and valleys are caused by dipping layers of resistant sandstone (geologists call these hog back ridges).
The Brook Street terrane rocks are related volcanics, probably from an island arc of volcanoes. They are named after Brook Street in Nelson, but similar volcanics occur in Southland e.g. the Takitimu Hills. They lie landward of the Muruhiku rocks and the volcanics are probably a major source of the sediment.
Western block type rocks occur in Southland. Nelson, and Kawhia to Port Waikato. Similar rocks and fossils are also known from New Caledonia and Malaysia.
In some places, plant fossils are found e.g. the fossil forest at Curio Bay in Southland, and the fossil ferns at Port Waikato. This indicates that the coast was nearby. The abundance of fossils indicates fairly shallow water - continental shelf. The photo right is of fossil tree trunks at Curio Bay, Southland.

Do you remember your Yr11 sedimentary rock notes? if not brush up here.

Note - Greywacke:
If you haven't already come across this term, it is quite an important one. Greywacke is a hard, muddy sandstone. It is often layered with mudstone in New Zealand (the hard mudstone is called argillite). Greywackes are deposited by submarine currents, usually turbidity currents, which are underwater currents of muddy water (follow the link if you have never heard of them). They therefore indicate a marine environment: basin, or continental shelf, or continental slope. They differ from very 'pure' sandstones in the large variation of grain size; the original sediment would be a mixture of sand and mud. The grains that make up the sediment contain quite a large range of minerals. By comparison, a beach type sand or river sand will have well sorted grain size and fragments that consist largely of one mineral, usually quartz. Greywackes are found in both the Tuhua and Rangitata depositional period rocks, and dominate the Rangitata period. By contrast, the post-Rangitata is dominated by siltstones, mudstones, marl, limestone and coal.


Eastern block:
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Torlesse Greywackes, Mesopotamia Station, from Rangitata River
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Volcanic material in greywacke, Mesopotamia

To the east lie deeper water sediments These now make up the greywacke rocks of the main ranges of both islands. They are poorly layered, usually have few fossils and sometimes contain pillow lavas and related things from the deep ocean (photo above right) . They are very hard greywackes, on the verge of being metamorphic, indicating deeper burial than the western block and are in places metamorphosed to schist.
There is considerable evidence that these sediments were sourced from different rocks than those in the west, and within them different sources in the north and south.
These eastern sediments make up a number of 'terranes' - the Torlesse, Caples, and Waipapa terranes. These were orginally in different places with different sedment sources. They have been 'assembled' as pieces of a jigsaw into their present positions by plate motion. The schists (see below) are a metamorphic 'overprint' on the more deeply buried parts of the Torlesse and Caples terranes.

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Dunite (green) enclosed in basalt
Maitai/Dun MountainTerrane (sometimes called the Dun Mountain "Belt") : The eastern and western blocks are separated in many places by a belt of intrusive rocks of a type called ultramafics. This belt and its associated volcanics was known as the Dun Mountain Belt (after Mt Dun in Nelson) and is made of a rock called dunite. This association of rocks is now termed the Maitai Terrane, after the Maitai River in the west of Nelson City. The Maitai Terrane was formed when the Eastern and Western areas, which were quite far apart (see map) were pushed closer together by plate motion, squeezing up bits of ocean floor basalt and their underlying ultramafic rocks from the mantle between them. This seems to have happened about the time of the Rangitata orogeny.
Assemblages of such rocks (sea-floor basalts, gabbros and ultramafics) are called ophiolites. There are much younger ophiolites in Northland, emplaced at the beginning of the Kaikoura orogeny, by a slightly different process.
Mt Dun in Nelson and Red Mountain in western Southland have identical rocks, and lie on either side of the Alpine Fault, separated by over 400 km. The idea that the two were originally together and were pulled apart by the movement of the Alpine Fault was proposed by Wellman in the 1940s. At the time, it was a radical suggestion because most geologists thought faults could only move a few kilometres at most - which was their reason for rejecting the idea of continental drift.
The rocks of the Dun Mountain/Matai terrane are very rich in magnetic minerals, and even where they are buried it is possible to detect them at depth because of the effect they have on the earth's magnetic field at the surface.

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Otago schist

Metamorphism:
Review your metamorphic rock knowledge here
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Schist, Franz Josef (Haast Schist)
Schist: parts of the eastern block sedimentary rocks have such large thicknesses of sediment that they have been metamorphosed through to schist. This indicates a great deal of uplift - over 14 km, so that we are today looking at rocks from much deeper down in the 'pile'. Because fragments of ths schist exist in rocks only just younger than the Rangitata Orogeny, geologists think that the uplift occurred during this orogeny (see the next section). It also tells us that the 'pile' of sedimentary rocks that were deposited in the NZ Geosyncline was very thick - over 25 km. (see diagram here; a diagram for this level course is in preparation)
These schists are the deeper equivalent of the greywackes, and if you drive around central Otago and look carefully at the road cuttings you can notice the transition between greywacke and schist. In the photo on the right, the 'foliation' of the schist dips steeply to the left. this is not necessarily the direction of bedding in the original sedimentary rock. The schist also contains some metamorphosed volcanic and related materials, such as the manganese nodules you sometmes see in the greywacke (it forms a distinctive pink schist because of the manganese mineral piemontite). Before the Rangitata Orogeny, all the schist rocks were very deep - at least 14 km.
Note that as implied in the diagram below, the schists were uplifted more extensively in the south-west than in the north-east, possibly because this part was closer to land and has a greater thickness of sediment. This means that as you travel across Otago, the metamorphic rocks are from deeper and deeper down. This results in a change in various characteristics of the rocks, particularly the minerals present. They are all schist, but different 'grades' of schist. However, the very high grades of metamorphism (biotite schists and gneiss) near the Alpine Fault are likely to have been brought to the surface more recently, in the Kaikoura Orogeny.. I am in the process of preparing a map and some diagrams, and a bit more explanation of this, as it came up in the 2009 exam.

Phase 3: The Rangitata Orogeny and post-Rangitata erosion
Convergent margin volcanic activity increased towards the end of the Jurassic period (140 my). In rocks of the lower Cretaceous (130 my) there are a number of interesting features:
  • There is an extensive unconformity which covers the whole of NZ. Nowhere in NZ is there a continuous conformable sequence of sediments from the late Jurassic to the mid-Cretaceous, there is always a break (unconformity) in the lower Cretaceous.
  • Large volumes of plutonic rocks which were intruded into the older rocks of the Tuhua sequence at this time
  • There are areas containing island-arc style volcanic rocks (which are always found near a subduction zone); for instance, in Nelson City there is a formation called the Brook Street Volcanics. Nearby are rocks from the mantle (ultramafics) which seem to have been squeezed to the surface as a result of colliding plates.
  • There is extensive regonal metamorphism (Otago, Haast and Marlborough Schists).

Geologists interpret this as being evidence for a major orogenic pulse. About 130 my ago the sediments of the NZ geosyncline were extensively uplifted. This probably happened because the eastern Pacific plate boundary once again moved into the area that is now New Zealand.
Volcanic activity and uplift gradually decreased, and by 115 my ago had mostly ceased. The period of uplift and volcanism is termed the Rangitata orogeny.
Fragments of Otago schist are found in sedimentary rocks from the mid-Cretaceous (about 100 my ago), so the metamporphism and uplift that produced the Otago Schist must have occurred prior to this. This is another piece of evidence for extensive uplift at this time. However, the higher grades of metamorphic rock found near the Alpine Fault are not in evidence in these deposits. This indicates that the schists have been subjected to two episodes of uplift (one at the Rangitata orogeny, and further uplift at the Kaikoura orogeny).
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Map showing present day location of Tuhua and Rangitata rocks, with younger overlying rocks removed
Summary: the map on the rightshows the present day location of the main rock groups deposited before the opening of the Tasman Sea. It is a much simplified version of a map from GNS Science, available here, (note this is a 6MB pdf download). All rocks younger than 100 million years have been 'stripped off' the top. The map below shows roughly how they might have been arranged before everything was moved around by plate movement associated with the Kaikoura Orogeny, particularly movement of the Alpine Fault. I have grouped into one the different western terranes (Muruhiku, Maitai, Brook Street) and eastern terranes (Hunua, Caples, Torlesse, Rakaia, Pahau and the Waipa Supergroup) and omitted the metamorphism (although I may redo this and show the metamorphism).
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Simplified NZ geology, pre-Tasman

















Note: Basement: The term 'basement' is used in geology to describe the oldest rocks you can usually drill to in an area. The map on the right is a simplified map of the 'basement' rocks in NZ. All these rocks are older than 100 million years or so. The magenta colour area is the rocks of the Tuhua depositional phase. They are separated from from the rocks of the Rangitata depositional phase (light and dark blue) by a series of granites that were intruded over a very long period of time - from Tuhua times to the Rangitata Orogeny. These granites are coloured pink on the map (and sometimes in real life as well!).
The eastern and western terranes of the Rangitata depositional period (dark and light blue) are separated by the Dun Mountain belt (mentioned above) - dark red on the map. This is probably the remains of an island arc chain associated with activity at the beginning of the orogeny.

After the Rangitata Orogeny

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Possible peneplain surface on granite, Heaphy Track area

Peneplanation
Peneplantion is the term used to describe the wearing away of a landscape to a fairly level
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Possible peneplain remnant, Lake Cobb
erosional surface. Such surfaces can be buried and later exposed, and possibly eroded again after being uplifted. The can show up as a set of mountains with very even crests, or a flat but partly dissected surface.
The picture above lshows a probable peneplain surface in Kahurangi National Park. The flat, slightly tilted surface is cut by much younger valleys (this is called dissection). If this were sedimentary rocks, we would suspect that this might be a depositional surface e.g a lake bed or ocean floor. However, the rock is granite so it must be erosional.
The process of wearing down that occurred after the Rangitata orogeny resulted in the formation such a peneplain. Remnants of this peneplain can be found in Otago and Northwest Nelson. The wearing down led to a landscape with very low relief (few hills) and therefore poor drainage. This later on led to extensive swamps, which today make up NZ's coal deposits. Eventually, most or all of this landscape sank below the sea in a marine transgression.
The hills around Cobb reservoir in Kahurangi National Park (picture on right) show very even summits, which are possibly remains of the post-Rangitata peneplain and part of the more extensive flat surface seen in the satellite image above.
Note that not all geologists accept that this peneplanation was universal across the whole of Zealandia. The flat surface above may be the remains of an extensive wave-cut bench, much like the coastal plains of south-eastern Queensland today. If you look at the cross section titled 'submergence' further down the page, you will see that I have inferred this.
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(right click on image and open in new window to see full size)

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Light blue is Zealandia.
Break up of Gondwanaland and opening of the Tasman Sea - 80 to 60 my

Do you understand sea floor spreading and plate tectonics? If not, revise it here. It is very important for this section.
Following period of peneplanation discussed above, another important event occurred. Gondwanaland began to break up. The separation of NZ began 80 to 90 million years ago, with the formation of a rift valley (plate movement may have started before this). Evidence for this includes certain sorts of igneous rocks rocks which are known to be found in modern rifts; there are upper Cretaceous volcanic and intrusive rocks of this sort found in several places in NZ. The extensive intrusion of granites to form the 'median batholith' (pink on map above) was also possibly related to the tectonic events leading to the formaton of this rift valley. These granites are found in Australia as well as NZ.
Tectonic warping associated with the beginning of the rifting led to formation of some very active local sedimentary basins with steep margins; the Hawks Crag Breccia on the Buller River was formed by such activity (not by the Rangitata Orogeny as implied in the 2007 exam).
Over a few million years, this rift valley would have widened, becoming a narrow sea like the modern Red Sea. It is possible that some of our oil and gas fields were formed in this environment, which is why the exploration for these is concentrating in the area that was once the 'proto-Tasman Sea'. Over the next million years or so this narrow sea formed a spreading ridge and over the next 20 million years it widened by sea-floor spreading to form the modern Tasman Sea. The remains of this spreading ridge can still be seen on images of the Tasman Sea floor. In the false-colour image of the Zealandia continent on the left you can see the remains of the spreading ridge - it has been traced in red to make it easier to see (the yellow lines are where it has been cut by transform faults) The light blue area in the picture is Zealandia, the part of Gondwana that broke off 80 million years ago. It has been considerably reshaped by intersection with varous plate boundaries in the last 50 million year, as is implied by its complex topography. As you can see, this Gondwana fragment includes New Caledonia.

Note: Granites and intrusive rock - convergent margin or spreading?
New Zealand has an extensive belt of granites and other intrusive rocks (gabbro, norite etc.) found today at Stewart Island, Fiordland and NW Nelson. They are also known from offshore drilling, such as in Taranaki, and granite fragments occur in Murihiku rocks at Awakino (where they were transported from land which at that time lay to the west). These intrusives make up part of the "Gondwana" foreland. They seem to have been intruded over a very long period of time, starting before the Tuhua orogeny and finishing up at about the time of the opening of the Tasman. Some of them (e.g. the Karamea Batholith) seem to be assoicated with convergent margin activity and their age coincides with orogenic pulses - the Tuhua and Rangitata orogenies. However, others seem to be associated with the rifting that eventually led to the opening of the Tasman - particularly the more eastern ones (e.g. Separation Point Granite).
Riftng and convergent margin activity are not as distinct as simple plate tectonics would seem to suggest. There is quite often a 'back-arc rift' associated with a convergent margin, and the modern Havre Trough and Taupo Volcanic Zone in the North Island seem to be results of this. Granitic magmas are presently being intruded at depth below Taupo. It is likely that the convergent margin activity from Tuhua to Rangitata times had such 'back-arc' rifting associated with it, and following the Rangitata Orogeny this developed into full sea-floor spreading as part of the wider break up of Gondwana. Not all rift valleys develop in this way; there is no evidence of convergent margin activity preceding the modern East Africa Rift. It is also unlikely that the rifting in the TVZ today will develop into sea floor spreading.

Other events in post-Rangitata times:

Death of the dinosaurs: Towards the end of the time during which NZ was drifting away from Australia, at 65 my ago, the cataclysm occurred which caused the extinction of the dinosaurs and about 2/3 of other life.It is generally thought to be due to a meteor which struck the Earth in present day Mexico (for a discussion on this, link to K-T boundary). The band of rare-earth rich clay which marks this boundary is found in parts of NZ (e.g. in Marlborough) and certain key fossils disappear above it.
End of the sea-floor spreading: Five million years after the death of the dinosaurs, about 60 my ago, the Tasman reached its present width and sea-floor spreading stopped. By this stage a mini-continent called Zealandia had formed, whch includes much area that is now underwater and extends north to New Caledonia. Much of Zealandia is today underwater. Some of it was underwater then, too, but not necessarily the same places as today. Sixty million years ago, NZ was at last a continent of our own.


Marine transgression/Submergence:
The wearing away of Zealandia continued, and it became flat and swampy and gradually sank below the sea. This is known as a marine transgression. Rock types associated with this include mudstone, siltstone, marl (a carbonate-rich mudstone), coal and limestone.
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Coal inter bedded sith siltstone, south of Westport
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Kahikatea swamp


Coal deposition: From 60-30 million years ago, Zealandia continued to wear away. The relief was already low after the Cretaceous 'peneplanation'. Swampy lowlands, like the kahikatea swamp in the picture, formed because the low relief meant poor drainage. The swamp forests built up dead organic matter, which became peat and was later converted to coal. Rivers flowing across these swampy plains often flooded or changed course. The silt borne by these rivers is interspersed with the coal, as you can see in the thin coal beds seen in road cuttings south of Westport. The amount of land area shrank over time as it was invaded by the sea.
Coal was actually formed in a variety of situations, not all in land that was being invaded by the sea. Some was formed in estuaries that became blocked from the sea, filled in with sediment and became swamps. Such coals are much higher in sulfur and are not widely used as yet in NZ. The coal deposits are therefore not continuous, but form local 'coal measures'.

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Possible cross section through Zealandia 38 million years ago (early Oligocene)


The hypothetical cross section (west on left, east on right) above shows the situation just before the last true land sank. Note that the limestone overlies the coal (shown in black), as this was formerly swampy land which has now sunk. However, coal was far less continuous than this diagram implies.
Extreme temperature: Close to the boundary between the Paleocene and Eocene times (56 my ago), there was a sudden increase in the amount of CO2 in the atmosphere. This was almost certainly due to release of methane from clathrates on the sea floor. Geochemical evidence suggests atmospheric CO2 reached about 500ppm (as opposed to 350 ppm now) and global average temperatures increased by about 6ºC. In NZ, this event is marked by a layer of muddy limestone found in parts of Marlborough. Quite a few species of plankton went suddently extinct at that time, but we don't know the effect on land in NZ (elswhere, many mammals went extinct). The fact that a different type of rock was deposited for a period suggests some change in weathering processes here. This is a very 'inconvenient truth' for those who deny that CO2 has an effect on global temperature. Read more...

Warraber Island, a modern cay
Warraber Island, a modern cay
Limestone:
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Limestone pinnacles, Punakaiki
The land sank further and, by about 35 my ago, very little land area was left. It is possible that the only land was in the form of shellbank islands, such as the one shown left (known as cays). These could not contribute any non-carbonate sediment, and in rocks of the late Oligocene in New Zealand there have been no such (rocky land derived) sediments found to date. As the amount of land reduced, it meant a reduction in rock-derived sediment (sand and mud); so increasingly the only thing to form new rocks were the shells of living things -shellfish, plankton and so on (this is called a sediment-starved environment). From about 45 million years ago onwards, the sediments develop an increasing calcium carbonate (lime) content, becoming lime-rich mudstones and marls until, by about 30 my ago, most of the rocks laid down in NZ were limestone (right). The coal measures and limestones in NZ are together known as the Te Kuiti Group. They are found as far north as the Bay of Islands and as far south as Te Anau, but probably once covered most of Zealandia (some is buried, some has been removed by later erosion).
These limestones make up the caves at Waitomo and elsewhere, and the famous rock formations at Punakaiki. The Punakaiki and other limestones often have a 'flaggy' (like stacked flagstones) appearence caused by the slightly different weathering rates of depositional and lithification structures in the sedimentary rock.
There is some evidence of convergent margin activity at this time, north of present day NZ in the area known as the Norfolk Rise. As this activity moved south, the pace of tectonc activity picked up, setting the scene for the most recent 'orogeny:

Phase 4. The Kaikoura orogeny and modern times
About 22 million years ago tectonic activity moved into the parts of Zealandia that are now land. Although there had been some plate boundary activity in Zealandia prior to this, it was well away from the present land area of New Zealand, so we see little evidence of it on land. An animation of the plate movement at the time can be seen on the IGNS website here. Signs of this new convergent boundary activity include:
  • Various parts of NZ were either uplifted or downfaulted. The downfaulted areas formed new sedimentary basins, such as the one in which the Waitemata Formation was laid down around Auckland.
  • New plate boundary volcanoes appeared in Northland - these are now located at Cape Reinga, Whangaroa Harbour, the Waipoua forest and off the Dargaville to Kaitaia coasts, and at Whangarei heads. Over time, the volcanic activity extended down the West Coast of the North Island (Waitakeres and southwards) and south to Great Barrier and the Coromandels in the east.
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Pillow lava, Cape Reinga
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Extinct volcano, Whangaroa, Northland
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Island arc volcanics, Karekare
Whangarei heads - extinct volcano
Whangarei heads - extinct volcano

  • a huge chunk of material slid into place over Northland in a kind of giant landslide.This formed a series of sediments called the Onerahi formation, which are actually older than the rocks underneath them. The term for sediments 'out of place' like this is an allochthon.
  • Chunks of ocean crust called ophiolites were also pushed onto land and make up some of the flat topped volcanic hills you can see in parts of Northland, such as the Tangihua hills.

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Chunks of ocean floor make flat-topped hills, Northland
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Ocean floor basalt being quarried, Pawarenga

All of these things are due to an increase in tectonic activity which signalled the start of a new orogeny called the Kaikoura Orogeny, which is still going on today.
An interpretation of the geological relationships at this time is given in the map below (modified after Stagpoole and Wood). The approximate locations of present day land is given on the diagram, and you can see that parts of NZ have moved around considerably as a result of plate motion in the Kaikoura orogenic pulse. Some areas have 'disappeared' , for example by overthrusting fault activity, and some new land areas have been created by uplift. The representation of the eastern South Island and northern Southland is very approximate; if I were to try to draw it more accurately you wouldn't be able to recognise it (also, it was quite difficult to draw with the software I used).

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The Kaikoura orogeny is still going on today. It is a result of gradual and quite complex shifts in the locations of the plate boundaries in the south-west Pacific; it is a story which is still being unravelled. Both the plates and the plate boundary itself are continuing to move.Over time, the exact location and shape of the subduction zone seems to have changed, and therefore the type of tectonic activity in different parts of NZ has changed.

The focus of volcanic and mountain building activity has moved south as time has gone on. The present volcanic area runs from Mt Ruapehu through White Island and the Kermadecs to Tonga (see map here), but at the start of the Kaikoura Orgeny run up the Coromandels and east coast of Northland. Another chain of volcanoes runs down the west coast of the northern North Island, and there are submerged volcanic remnnats off the west coast. The Taranaki volcanoes are possibly the youngest continuation of these. The reason for the two parallel chains is not fully understood.
At various times in this period, several large, mostly basalt, volcanic complexes were developed in the South Island, e.g.Banks Peninsula, Otago Peninsula, Auckland Islands, Timaru. These are intra-plate volcanoes and are not directly related to the plate boundary. More recent intra-plate activty has occured in Auckland and Northland.

Other Kaikoura Orogeny activity: Alpine Fault and Rise of the Southern Alps
The Southern Alps began their uplift about 5 million years ago, due to vertical movement on the Alpine Fault . The fault had already undergone extensive 'sideways' movement - such faults are termed 'transcurrent' or 'strike-slip'. The amount of sideways movementwas huge - hundreds of kilometres, as shown by matching rocks in Nelson and Southland (discovered by Harold Wellman). This is why NZ geologists were among the earliest believers in continental drift. It extensively rearranged Zealandia.
The nature of movement changed over time, with a lot of the sideways movement occurring before the uplft. The Alpine Fault was probably not originally part of the plate boundary, but was an extension of a transform fault on a spreading ridge (see map above). There was no subduction to the south of NZ at this time. When the Indo-Australian plate began subducting beneath the Pacific Plate to the south of the South Island, the sideways movement of the Alpine Fault accomodated the change in direction between the two subduction zones, and it became a 'transform plate boundary'. Along a section of this, the plates converge but do not subduct. This causes the uplift of the Southern Alps, which created new habitats, and led to the evolution of new species of plants and animals in NZ.

Ice Ages
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Milford Sound is a drowned glacial valley
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Gouges caused by rocks in ice, valley wall, Milford

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Tillites - glacial lake deposits, Erewhon.
From a time somewhat before the Kaikoura Orogeny, there had been some cooling of the Earth's climate. This was probably due to changes in ocean circulation, particularly the development of the circum-Antarctic current when tectonic movement opened up a gap between the Antarctic Peninsular and South America. Ice began to move out from the Trans-Antarctic mountains, reflecting more of the Earth's heat into space and reducing the planet's 'heat budget'.
About 2 my ago things reached a critical point. Snow which fell in the northern winter did not completely melt in summer. The new snowy ground reflected even more of the Sun's heat back into space, causing further cooling (the reverse of what is happening now). Superimposed on the cold-warm cycles (Milankovitch cycles) already happening, this caused a rapid, catastrophic cooling. Ice built up, moving south from the North Pole and out from mountains until it covered the north of North America and Eurasia, and the major mountain chains of the Southern Hemisphere. The loss of all this water to ice on land caused sea level to drop, exposing much of the continental shelves. The weathering of the newly exposed land produced calcium ions, which sucked CO
2
from the atmosphere in the form of bicarbonate ions and dissolved it in the sea, producing further cooling.
In NZ, icecaps developed over the Alps, Fiordland and the Tongariro areas.Small glaciers developed even in the axial ranges of the North Island.
There were a number of ice ages (or glaciations) and sea level rose and fell multiple times. At various times it has been higher than present (during warmer interglacials) and lower than present (during glaciations). Aucklanders may not realise that some of the relatively flat ridges on which many main roads are built are actually wave-cut benches from interglacial sea level highs. The highest of these can be seen on the road to Piha, just before you start descending to the beach.

Ice-Age Landscape features:

One way that the evidence for glaciation can be seen is in the large, U-shped valleys carved by the glacier. Milford Sound is an example of such a valley, but many other examples are found e.g. Cobb Valley (photo). At Milford, you can even see the gouges on the valley wall formed where rocks containied in the ice scratched at the hard granite country rock, as seen above.
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Ice polished granite, Cobb Valley
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Terminal moraines, Cobb Valley

Hard rocks such as granite can be polished by the ice, as seen in the photo above. Material bulldozed into place by glaciers can leave piles of rock called moraines which persist long after the glaciers have melted. These are found along the side of the former glacier (lateral moraines) or at their furthest terminus (terminal moraines). In the picture below left the lake is dammed by the terminal moraine
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Tamsan Laker with moraines
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Erratic boulder, Molesworth



Other glacial landscape features include erratic boulders, (above right hand picture) - these are large isolated boulders pushed into place by glaciers which have since disappeared.
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Wind blown dust, Tasman Glacier
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Loess near Christchurch

The piles of material pushed into place are prone to dust storms. The photo above left is a dust cloud above the lateral moraine from the Tasman Glacier. The dust eventually settles elsewhere to form loess , a term for wind-blown 'rock flour' ground up by glacial action. There are deposits around the Port Hills in Christchurch and elsewhere in the South Island - the picture above left is on the hills above Sumner in Christchurch. Loess is very prone to erosion and can be problematical in earthquakes, though it doesn't seem to have been a major factor in the September 2010 Christchurch quake.
Tillites are glacial lake sediments - many glaciers have lakes at their end which receive huge amounts of sediment and form thick flat sedimentary deposits on the floor. The photo from Erewhon station above (near one of the filming sites for Lofd of the Rings: The Two Towers) is of such deposits in a former lake. The modern lakes Tasman and Tekapo would be accumulating similar deposits.
Even in the North Island there are some examples of glacial features such as eroded 'ice basins' (cirques) in the Ruahine Ranges.

Sea level change:
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Above are two views from Sacred Heart College across Tamaki River to the Coromandel Range. The first view is the landscape as it appears today; the second is a reconstruction of what it may have looked like 30,000 years ago during the last ice age. The estuary is gone - sea level was lower. On a fine winters day, snow may have briefly capped the nearly 900 m high Mt Moehau.

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Marlborough Sounds - drowned river valleys
Sea-level change:
Sea level fell and rose a number of times during glaciations and interglacials. During the lowest sea-level times the North and South Islands were joined by a land bridge between Wanganui and Golden Bay (see map below). Most of our harbours and areas such as the Hauraki Gulf were dry land. There were two quite large offshore islands off the Kaikoura coast and other island groups, such as Three Kings, were considerably larger. Many of our harbours, such as the Waitemata in Auckland, are river valley/basin systems which were flooded by the sea as it rose. The Marlborough Sounds are also drowned valleys. The west coast harbours of the North Island are valley systems which were not only drowned, but were partly blocked from the sea by extensive sandspits with very large dunes on them. These dunes were formed when sea level fell; exposed sand was picked up by the prevailing westerlies and blown inland, but trapped by the bay behind. The constant renewal of sand supply by the north-flowing current on the west coast carrying eroded material from the Taranaki volcanoes and the Waikato ensured that the sandspits were able to grow to huge sizes.
northheadprehuman.jpgSea level reached a maximum a few metres above present 6000-3000 years ago and until the end of the 19th century was falling; this is responsible for many of our beaches (which form more readily during falling sea level because the wind picks up exposed sand and blows it up the beach). For example, North Head would have been an island joined to the mainland by a tidal sandspit as pictured on the left.
Sea level is now rising quite fast, and we can expect beach erosion to become a much more dominant coastal process.
The map below shows the approximate coastline during the lowest sea level of the last ice age, about 30,000 years ago. Sea level began to rise towards present about 20,000 years ago but rose in stages. Some of the more recent rise is possibly the source of flood myths around the world. However, NZ was not inhabited at that time.

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NZ in the ice ages. Blue line is the then coastline

Vegetation change: Much of what is now forest in the SI was tussock or tundra, as was the NI high country. Kauri forest retreated to Northland only. Today, kauri do not naturally occur south of a line from Raglan to Tauranga, even though you could plant a kauri in Dunedin and it would grow. Kauri seeds are 'helicopter seeds' and don't spread far; together with the slow growth (at least 100 years to sexual maturity) their movement south has been very slow.
Forest types changed; for example, although the modern lowland forest around Auckland is podocarp, the pollen from fossil swamp deposits at Pt England indicates that there was extensive beech forest in the immediate area. Today, beech is only found above 600m in the upper NI and not at all north of Auckland. The shifting ecosystems had a considerable effect on the evolution of NZ's flora and fauna, as discussed in the biology standard of this course. The map above is based on the one in Sir Charles Fleming's book. The green line gives his inference on the southern limit of woody vegetation - basically, forest and scrub. This huge reduction of protected, forest habitat would have led to the one bottlenecks discussed on the evolution page.


Further Reading:

Thornton, Jocelyn: Field guide to New Zealand Geology. Should be in your school library, part of it is online at Google Books here. A good description of the geology of various areas of New Zealand. Some of the interpretations have been updated somewhat in the 20 years since this was first published. Intended as an introduction to geology for the enthusiast as well as a description of NZ geology.
Graham, Ian J (chief editor: A continent on the move. 2008. A comprehensive book which covers the developments in NZ earth science of the last quarter-century. It is written as a series of thematic articles, so synthesising a chronological historical approach (as I have tried to do on this page) from this source is not for the faint-hearted and it would be fairly heavy going for those without some tertiary-level education in earth science. However, for breadth and depth of coverage it is the best available and school libraries should get a copy if they don't have one.
Mortimer, Nick and Smith Lyttle, Belinda: New Zealand's Geological Foundations. A3 map and text download here (6 Mb PDF). This is reproduced in the book above.
Campbell, Hamish and Hutching,Gerard In search of ancient New Zealand (Penguin). more about this book. Written for lay people but probably quite heavy going for students. Dr Campbell has some fairly strong personal views on the interpretation of NZ geology and the book does reflect this at times.
Fleming, C. A (Sir Charles): The Geological History of New Zealand and its Life (Auckland University Press) 1979. City libraries may have this. Fleming was an iconic worker, particularly in NZ paleontology, but this book is now mainly of historical interest.
McSaveney, Eileen and Sutherland, Rupert: New Zealand Adrift. Institute of Geological and Nuclear Sciences series no. 69 2005 A brochure which covers some of the main points for this unit.
Suggate, P (chief editor): The Geology of New Zealand. 1978. 2 Volume set with comprehensive descriptions of NZ Geology, but rather out of date in terms of tectonic interpretation. A good source of information about local geology, particulalry lithology (the rocks) and paleontology. Large city libraries and University libraries will have this. You would probably need tertiary-level geological knowledge to cope with this book.h
Searle, E.J: City of Volcanoes. The geology of Auckland, focusing on the volcanic cones but with general background.

Beneath New Zealand - documentary from NZ On Air