Dynamic Landscapes: The Fall and rise of a Small Continent
Inaugural Professorial Lecture delivered by: Professor Richard Norris MA DPhil FRSNZ
Castle Lecture Theatre, University of Otago, Wednesday October 13th, 1999.
Wild and dramatic landscapes are New Zealand's premier tourist attraction. Whether sailing on Milford Sound, skiing at Coronet Peak or trekking up the Franz Josef Glacier, the rugged landscape of southern New Zealand forms the basis of the country's tourist industry and provides the major marketing focus.
One of the first tourists to visit New Zealand remarked on sighting the South Island that it was "A great land uplifted high". Abel Tasman's comment showed considerable insight. He could have merely said "A mountainous land" or "A land of great height" or some other purely descriptive phrase which avoided any reference as to why the land was mountainous. Perhaps being a Dutchman and coming from a country where any land more than a few metres above sea-level is a major feature, made him think intuitively that the snowy peaks of the Southern Alps had not always been at their present height but had been uplifted from more humble elevations at some time in the past. Whether or not such implications were intentional, we can now confirm that Tasman was on the right track. Recent geological research, however, shows that not only is the South Island a land uplifted high, but that it is a land still actively being uplifted. The Earth's crust is being twisted, compressed and distorted as mountains are pushed up and collapse. The rates may be slow but they are inexorable. It is a sobering thought that the Southern Alps are higher now than when Tasman saw them and the western coastline has shifted some 8 metres further northeast. Since his visit, some 30 cubic kilometres of crust has been raised above sea-level and a similar amount eroded and washed down the rivers. The dynamic landscape and the geological processes responsible ultimately control the nature and evolution of the whole environment.
In this lecture I wish to explore how the recent geological evolution of New Zealand is responsible for the natural environment in which we live and for many of the processes which affect our interactions with it. Geology affects us in a variety of ways no matter where on Earth we live. Sometimes these influences may be indirect and not immediately apparent, for instance why a few countries appear to have a monopoly on the world's oil supplies. However, nowhere perhaps are the effects more dramatic than in New Zealand, where active volcanoes and frequent earthquakes remind us that dynamic landscapes carry with them an element of risk. Less obvious are the effects on climate, on drainage systems, on soils and sedimentation, on flora and fauna. Mountains may be uplifting but they are also falling down almost as fast. Deep-seated geological materials are being raised to the surface to be eroded and redistributed within the environment. Active Earth processes and the materials from which the Earth is formed are a fundamental influence on the flora and fauna which inhabit it and together constitute the natural environment.
New Zealand was born on the margin of the great southern continent of Gondwana. Gondwana was made up of Australia, Antarctica, South America, South Africa and India and occupied the southern hemisphere for over 200 million years until it began to break up some 120 million years (Ma) ago. The Tethys Ocean separated it from the northern supercontinent of Laurasia. The eastern coastline, formed by Australia and Antarctica, faced towards the great ocean of Panthalassa, the precursor of today's Pacific Ocean. It was an active continental margin, like the Pacific margins of today. It grew oceanward by the accretion of materials such as thick sedimentary deposits, volcanic arcs and slices of ocean floor. These additions were generally dismembered, deformed and metamorphosed as they were piled up against the continent by the westward underflow of the ocean floor.
Around 100 Ma, this westward underflow ceased and instead, the area was pulled apart as Australia, Antarctica and New Zealand began to go their different ways. By 85 Ma, the tensional forces had caused a large piece of the continental margin to split away, with new sea-floor being generated in the widening gap of the Tasman Sea and southern ocean. The New Zealand mini-continent had begun its independent existence and its long and continuing drift out into the Pacific. The New Zealand continental plateau covers approximately 3 million square kms, about ten times the present land area of the two islands. Most of it today is under water. The stretching of the crust that preceded the split from Gondwana meant that most of the mini-continent is substantially thinner than normal continental crust. As the newly independent New Zealand left the shores of Gondwana behind and moved off into the Pacific, its thinned crust cooled down and slowly sank beneath the waves. Only where areas of thicker crust remained, such as central Otago, did low lying islands project above sea-level. Elsewhere the sea extended across the mini-continent and deposited marine sediments such as the mudstones and limestones around Dunedin and Oamaru or the limestones of Waikato.
This early history may seem rather remote from the environment of today and to have little relevance. In fact, it is critical to much of the character of present day New Zealand. The rock units from which New Zealand is built were largely emplaced by this time. Most of New Zealand's deposits of minerals and coal had already formed. A thin layer of poorly consolidated marine clay was deposited over much of the country during this period. These clay layers overlying resistant basement rocks, such as the Abbotsford Mudstone near Dunedin, create easily eroded and unstable hillsides when uplifted above sea-level. Many of the geological fault lines and other zones of weakness in the crust were initiated during the stretching of the continental fragment, only to be reactivated again in later times. New Zealand's unique flora and fauna owes its character largely to this break from Gondwana and its subsequent history. Nevertheless, this early phase is essentially about the fall of a small continent. Had New Zealand remained as a series of low wind-swept islands in the South Pacific, similar to the Chatham Islands of today, it is doubtful whether its attraction as a tourist destination would have been notable. Fortunately for the tourist industry, events beginning around 45 Ma ago would lead to the rise of part of the continent from beneath the sea and eventually to "a great land uplifted high".
Sea-floor spreading in the Tasman Sea ceased around 55 Ma. Australia then parted company with Antarctica and followed New Zealand northwards, leaving behind a rapidly expanding southern ocean. Because Australia was travelling northwards faster than New Zealand, a plate boundary between them developed around 45 Ma. This boundary ran through western New Zealand and linked up with island chains in the western Pacific. The mini-continent was splitting in two. At first the plate boundary was a zone of slow oblique extension, with the Australian plate moving northwards and away from New Zealand. Small ocean basins such as the Emerald Basin formed south of Fiordland. Over time the motion became more of a NE-SW shear and around 25 Ma ago, the Alpine Fault developed through South Island as a discrete shear zone. Areas close to the fault were deformed and uplifted and the sea began to retreat. The rate of shear increased towards the present and, particularly over the last 7 Ma, developed a component of convergence. During this time, the tempo of deformation increased as the two halves of the mini-continent were driven together. The Pacific plate was thickened and uplifted to form the Southern Alps and the dynamic landscape of today was established.
The record of these events is preserved in New Zealand's sedimentary basins. Our petroleum and natural gas resources are a direct consequence. Work with my colleague Bob Carter in the 1970s on the sedimentary basins of southwest New Zealand reconstructed this tectonic history and showed that it closely matched predictions made from oceanographic studies in the southern oceans. Later, Rupert Sutherland was able, as part of his PhD at Otago, to produce a detailed quantitative analysis of the changing motion along the plate boundary during its 45 million year evolution. In order to understand New Zealand today, and to understand the present plate boundary, it is essential we understand its evolutionary history. The Earth has a long memory, and much of what we see today is the cumulative result of long periods of slow change.
In the final part of this lecture I want to examine the dynamic landscapes of today and the processes that contribute to their development. The major feature of the plate boundary through South Island is the Alpine Fault that runs down the West Coast and forms the northwestern boundary of the Southern Alps. To the east, the crust of the Pacific plate is thickened and crumpled, and thrust up and over the Australian plate to form the South Island ranges. Rates of spreading in the southern oceans allow the relative motion of the Pacific and Australian plates in New Zealand to be calculated. In the vicinity of Mount Cook, the total rate is some 38 mm/yr, with 36 mm/yr parallel to the Alpine Fault and 11 mm/yr perpendicular to it. Since significant convergence began around 7 Ma ago, over 70 km of crust has disappeared in central South Island. Much of this lost crust has been thickened to form a deep root to the Southern Alps. A recent seismic profile across the island clearly demonstrates that the thickened crust extends down to 45 km depth beneath the Alps, compared with a crustal thickness of only 27 km under the Canterbury plains. The rest has been uplifted and eroded. Uplift and erosion east of the Alpine Fault has exhumed rocks from deep within the crust. For instance, the rocks exposed in the glaciated valley below the Franz Josef glacier originated at more than 25 km depth within the crust and are now exposed at the surface.
One of the key questions faced by my colleague Alan Cooper and me when we began work on the Alpine Fault 20 years ago was: how much of this total displacement between the plates actually took place on the Alpine Fault and how much was distributed throughout the Southern Alps? By determining the age and displacement of geological features, such as gravel deposits, river channels and terrace surfaces, along the fault over the last 50ka, we have been able to provide some answers. The fault dips to the east so that it has the form of a ramp up which the Pacific plate is being driven to form the mountain front. The rate at which the Pacific plate is moving up the fault ramp varies along its length, reaching a maximum of 10 mm/yr near the glaciers but decreasing to zero south of Haast. Most of the movement on the fault however is due to the two plates slipping sideways, with the Australian plate moving northeastwards relative to the Pacific plate. This sideways rate of slip parallel to the Alpine Fault is fairly constant along its length and lies between 25 and 30 mm/yr. Thus the fault is currently accommodating between 70 and 80% of the fault-parallel component of plate motion, but a variable amount of the convergence between the two plates. Relative to sea-level, the Alps are rising at rates of 3-8 mm/yr, with a maximum in the area of the glaciers and Mount Cook.
Survey data obtained using GPS satellite measurements as part of a joint project between Otago University and the Institute of Geological and Nuclear Sciences, show that distortion of the South Island crust during the last 5-10 years is consistent with the rate of plate motion. Around 70% of the distortion is within the region around the Alpine Fault. Thus the long-term rates based on geological data and the short-term rates based on geodetic data are consistent. These two sets of data, however, measure different things. The geological data measure slip rates on the Alpine Fault itself whereas the survey data measure distortions of the crust. There has been no slip on the fault over at least the last 150 years. The question is, does the measured crustal distortion represent accumulating elastic strain, and if so how often is this strain released by slip events (i.e. earthquakes) on the Alpine Fault?
Detailed study of surface features along the fault trace, including excavation of trenches across it, has enabled researchers at Otago, IGNS and Canterbury University to recognise past earthquakes. When a major earthquake occurs, the ground surface is ruptured and displaced. In favourable circumstances, evidence of these ruptures is preserved and can be measured and dated. At Haast, we have recognised three major ground ruptures within the last 1000 years, each with around 8 m of horizontal displacement. North of the glaciers, we have found evidence for at least four, and possibly five events within the last 1000 years, with slightly smaller average displacements. Radiocarbon dating of material has relatively large errors, but more precise estimates of times of violent ground shaking can be obtained from the disturbance of tree growth along the fault trace, or from the wholesale destruction of stands of forest along the front of the Alps. Such studies have allowed more accurate dating of recent earthquake events.
Whether any of the fault ruptures broke along the whole of the fault is difficult to prove, but the data are consistent with some events affecting the whole fault. In particular, the last rupture is dated in the north at 1717 AD by tree-ring dating, and is of comparable age in the south. If this is a single break, as it appears, extending some 400 km along the fault and with a maximum offset of 8m, the resulting earthquake would have been around Magnitude 8. Such an event would affect the whole of the South Island. The last earthquake was over 280 years ago, since when, at 25 mm/yr, over seven metres of slip on the fault should have occurred. As no rupture has occurred in this time, this motion must have accumulated as elastic distortion in the crust. Eventually it must be released as slip on the fault. The probability of another similar large earthquake in the near future is high.
Clearly living in a dynamic landscape is hazardous! Not only is the Alpine Fault a major earthquake hazard, but so are the myriads of smaller faults east of the Main Divide which collectively accommodate the balance of the plate motion. These are responsible for the uplift of the Central Otago ranges and extend eastwards as far as the Otago coast, where the Akatore Fault south of Dunedin City has evidence for large earthquakes every few thousand years. Earthquake events are a major environmental process. Not only do they result in uplift of the mountains, they cause offset of rivers and changes to drainage systems. We have been able to demonstrate that the rise of the ranges in east Otago caused major changes in the river systems. Slope failures are an inevitable result of the ever-increasing steepness of the terrain. Earthquakes are an efficient trigger of extensive landslides, and probably contribute a large proportion of the debris filling the Alpine valleys which is then redistributed further down during each subsequent flood. Large-scale destruction of forest during major events leads to punctuated periods of re-afforestation, as demonstrated by the work in Westland of Andrew Wells of Lincoln University. The South Island is a crustal re-cycling plant and the whole natural environment ticks to its beat.
Even the climate is affected. The wall of the Southern Alps across the prevailing westerly air-stream results in up to 10m of annual rainfall on the western side and less than 5% of this in the valleys to the east. The intense erosion on the western side of the Alps has been shown by my colleague Peter Koons to unbalance the tectonic forces leading to a greater concentration of deformation along the Alpine Fault. Not only does the uplift of the Southern Alps affect the weather, but the weather, in its turn, affects the uplift of the mountains. The whole Earth System is a complex series of interrelated processes which must be integrated to understand how it works.
The South Island is one of the world's great natural laboratories for studying active Earth processes. It is also very beautiful and one of the world's great tourist destinations. And when the President of the United States stands on a golf course in Queenstown and admires the dramatic landscapes of Tasman's "great land uplifted high", he is seeing a dynamic, changing landscape powered by the inexorable motion of the tectonic plates and the constant attack of wind and rain. If we are to live in it, exploit it and be safe in it, we need to understand it.