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Beetroot with your Peanut Butter and Marmite? The basic geological structure of New Zealand

New Zealand must be one of the best places on the planet for geology. We are famous among tourists for having so much variety of landscape in such a small area – you don’t have to drive far to see something totally different. This goes for the geology as well.

The basic geological structure became clear in the 19th century – New Zealand was composed of a collage of sharply defined blocks, of differing lithologies and ages. In New Zealand we have a granite-dominated complex of old rocks in the west of the South Island. This was clearly some sort of ‘continental’ core. To simplify things (quite a bit) east of that there is a strip of volcanic island arc rocks (Takitimu Mountains area), and east of that again, a band of sediments deposited in shallow-water, often richly fossiliferous (the Murihiku), and east of that again, a group of mainly deep-water, mostly unfossiliferous rocks (the Torlesse). This could be (and was: Fleming 1970) interpreted as a simple eastward deepening geosyncline, alongside the volcanic arc.

Along with the rest of the world, the paradigm of plate tectonics took hold by the early 1970s and New Zealand’s construction began to be interpreted in that light (Landis and Bishop 1972). Plate tectonics allowed the idea that some bodies of rock in New Zealand had arrived from somewhere else. Coombs et al. (1976) proposed this was the case for the ‘Torlesse’. This, and the other distinct bodies of roick that made up New Zealand were termed ‘terranes’. Perhaps it is safe to say that  the full potential of this mechanism was just starting to be grasped.

One of the earliest (the earliest?) syntheses of New Zealands Mesozoic geology in terms of plate tectonics (Coombs et al. 1976). The Torlesse 'terrane' (blue) is shown as coming from somewhere to the east.

One of the earliest (the earliest?) syntheses of New Zealands Mesozoic geology in terms of plate tectonics (Coombs et al. 1976). The Torlesse ‘terrane’ (blue) is shown as coming from somewhere to the east.

Enter a very simple tool – the QFL ternary diagram. This is a way to quantify the relative proportions of the grains that make up most sandstones: Q=quartz, F=feldspar, L=lithics (you get the basic data for these by counting a thin section of rock under a microscope). By plotting up sandstone composition on one of these it was apparent that the Torlesse was rich in quartz. This showed it was not simply a deep-water equivalent of the of the volcanic-rich sediment adjacent the volcanic island arc. The sediment, in fact the entire body of Torlesse rock, seemed to have come from somewhere else (Mackinnon 1983), as had been foreseen by Coombs et al. (1976).

A QFL diagram showing the different compositions of New Zealand's Mesozoic terranes (Mackinnon 1983).

A QFL diagram showing the different compositions of New Zealand’s Mesozoic terranes (Mackinnon 1983).

I remember a seminar at the Geology Department in the University of Otago, when everything seemed to change. It would have been early 1980s – I cant recall who gave it, a visiting American guru I think. The topic was ‘terranes’. The basic point was: In the past, when explanations were sought to explain how fault-bounded, but adjacent bodies of rock originated, there was a basic assumption that their relative locations had remained the same. At least without strong evidence otherwise.

The new ‘Terrane-concept’ asked for a 180 degree change in mental framework – don’t assume the bodies of rock were always neighbours. Instead, assume they may have nothing at all to do with each other. For a place with such a rich, complex geology as New Zealand, this was powerful stuff. With a bit of leg-work many of our distinct bodies of rock could be considered as ‘suspect terrane’s. Then, with suspicions more or less confirmed, it could be regarded as a full-blown ‘tectonostratigraphic terrane’ (to give it the correct term). These are fault-bounded package of rocks of regional extent that have a distinct geological history from neighbouring terranes. With the known mechanism of plate tectonics, they could have been juxtaposed after travelling from very different locations. Of course, defining the problematic rocks as terranes didn’t solve anything. But it did give geologists the freedom to think more …. laterally. It was clear by this time that the faults the bounded terranes represented a lot of movement – hundreds of kilometres, if not thousands. Very soon, the basic outlines of New Zealand’s tectonostratigraphic terranes were mapped (Bishop et al. 1985), and except for some local, but important additions since then, the map has stood the test of time.

New Zealand's major terranes and a (too) simplistic genetic interpretation. Modified from Bishop et al. (1985).

New Zealand’s major terranes and a (too) simplistic genetic interpretation. Modified from Bishop et al. (1985).

The perfect example of a terrane to have travelled a long way is the Permian Akatarawa Formation in South Canterbury. This is a tiny exposure of limestone, volcanic and sedimentary rocks, within a broader expanse of more typical Torlesse. The limestone contains fusulinids – an extinct group that lived in warmer, more equatorial seas, while the fossils in the surrounding Torlesse indicate distinctly cooler water (Hada and Landis 1995). The limestone is thought to have accumulated in shallow water on top of an isolated volcanic sea-mount in the tropics or subtropics. It was then rafted south on its oceanic plate, until it was sheared off as it encountered a deep sea trench in cooler, higher latitude waters. As it was sliced into the Torlesse sediments – it became another, albeit small, terrane (Bishop et al. 1985). The same history may apply to one of New Zealand’s very few occurrences of Carboniferous rock – a melange with conodont-bearing marble in the Kakahu region of South Canterbury (Jenkins and Jenkins, 1971; Hitching,1979).

Schematic explanation of the origin of the Akatarewa Terrane - as a 'sea mount' that accumulated a carbonate cap in tropical waters, before being rafted into cooler waters and being subducted into the Torlesse Terrane.

Schematic explanation of the origin of the Akatarewa Terrane – as a ‘sea mount’ that accumulated a carbonate cap in tropical waters, before being rafted into cooler waters and being subducted into the Torlesse Terrane.

In the early days fossils were the main clues to  the ages of the terranes. They could show, for instance, that there was not a simple progression of age from east to west. But they were often rare, and when present at all, sometimes enigmatic. The next major breakthrough was the ability to precisely date zircons. These form inside rocks like granite, and from that point they are virtually indestructible. The rest of the rock can decay to clay or dissolve away, but the zircons remain. They can then be ‘recycled’ into a sandstone, and when that erodes, into another one. The immense power here is that if a sandstone is found to contain three ‘populations’ of zircons, say of 360, 210 and 200 million years old – this puts tight limits on whereabouts in the world these could have come from. For the Torlesse, this seems to have been somewhere off the coast of Queensland (Adams 1998, 2010; Adams et al. 1998).

The possible Late Triassic (c. 210 million years ago) locations of what are now New Zealand's main Mesozoic terranes (after Adams et al.1998).

The possible Late Triassic (c. 210 million years ago) locations of what are now New Zealand’s main Mesozoic terranes (after Adams et al.1998).

So there you have it: you could think of New Zealand as a slice of bread with a smear of butter, marmite, peanut butter, a sprinkling of peppercorns and then some cheese, and not to forget the obligatory beetroot.

References

Adams, C. J. 1998: A provenance in northeast Australia and south China for New Zealand Palaeozoic terrane sediments. Journal of African Earth Sciences 27: 218-220.

Adams, C. J. 2010: Lost Terranes of Zealandia: possible development of late Paleozoic and early Mesozoic sedimentary basins at the southwest Pacific margin of Gondwanaland, and their destination as terranes in southern South America. Andean Geology 37: 442-454.

Adams, C. J., Campbell, H. J., Graham, I. J. & Mortimer, N. 1998: Torlesse, Waipapa and Caples suspect terranes of New Zealand: Integrated studies of their geological history in relation to neighbouring terranes. Episodes 21: 235-240.

Bishop, D. G., Bradshaw, J. D. & Landis, C. A. 1985: Provisional terrane map of South Island, New Zealand. In. Howell, D. G.  (Ed.)  Tectonostratigraphic terranes. Provisional terrane map of South Island, New Zealand, Circum-Pacific Council for Energy and Mineral Resources Earth Science Series 1: 515-521.

Coombs, D. S., Landis, C. A., Norris, R. J., Stinton, J. M., Borns, D. J. & Craw, D. 1976: The Dun Mountain ophiolite belt, New Zealand, it setting, consitution and origin, with special reference to the southern portion. Americal Journal of Science 276: 561-603.

Fleming, C. A. 1970: The Mesozoic of New Zealand : chapters in the history of the circum-Pacific mobile belt (22nd William Smith lecture.).  Q. J. Geogr. Soc. Land 125: 125-170.

Hada, S. & Landis, C. A. 1995: Te Akatarawa Formation – an exotic oceanic-continental margin terrane within the Torlesse-Haast Schist transition zone. NZ Journal of Geology and Geophysics 38: 349-359.

Hitching, K. D. 1979: Torlesse geology of Kakahu, South Canterbury. New Zealand Journal of Geology and Geophysics 22: 191-197, DOI: 10.1080/00288306.1979.10424218.

Jenkins, D. G. & Jenkins, T. B. H. 1971: First diagnostic Carboniferous fossils from New Zealand. Nature 233: 117-118.

Landis, C. A. & Bishop, D. G. 1972: Plate Tectonics and Regional Stratigraphic-Metamorphic Relations in the Southern Part of the New Zealand Geosyncline. Geological Society of America Bulletin 83: 2267-2284.

Mackinnon, T. C. 1983: Origin of the Torlesse Terrane and coeval rocks, South Island, New Zealand. Geological Society of America Bulletin 94: 967-985.

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