Thursday 11 October 2012

How to emigrate from the Northern Rivers

Most people will be surprised that once, the Northern Rivers area was not east of the Great Dividing Range. It seems likely that there was a range which Ollier & Pain (1994) refer to as the Tasman Divide. This divide was originally speculated by Jones & Veevers (1983). This divide probably meant that rivers such as the modern day Clarence actually flowed to the west, indeed, if you had of stood on Cape Byron or looked out from the headlands of Port Macquarie you’d not see the wonderful blue ocean but land and possibly hills with the sea located possibly many hundreds of kilometres further away than today. I guess a good question to ask is where did all that land go?

Looking out to the Tasman Sea may have
been looking out to another mountain range
The short answer is the sea floor around one of Australia’s offshore territories, Lord Howe Island. Lord Howe Island was actually formed during the Cenozoic from volcanoes but these volcanoes were situated on what we call the Lord Howe Rise which despite being submerged in the ocean (where you’d expect to find oceanic crust) is actually made from continental crust, like the Australian landmass. This is a mostly huge submerged continent called Zealandia which extends from New Zealand to New Caledonia. Some of this old continental crust is visible in the North of the South Island of New Zealand for which the rocks are related to the Lachlan Fold Belt in Southern New South Wales and Victoria.

In short, the Australian continent was much bigger than it currently is with Zealandia being the eastern edge. Approximately greater than 80 million years ago (beginning in the Cretaceous period) something happened deep below the crust under the Tasman Divide, it seems that the convecting mantle was pulling the east coast of Australia in two different directions. The area of the future Australian Continent seemed to remain fairly stable but the west, the future Zealandia, was dragged to the east. This process split the continent in two and created a mid ocean spreading ridge. The Lord Howe Rise part of Zelandia was dragged and stretched, creating huge Horst and Graben fault systems and consequently many basins. The effect of stretching and faulting thinned the Lord Howe Rises Continental Crust which meant that it began to sink below sea level.

It is unknown whether the action of the convecting mantle may have been directly associated with the hotspot/hotspots associated with the Cenozoic volcanics such as those of the Tweed Volcano and Ebor Volcano as well as Lord Howe Island itself. One thing is clear though, is that as crust thinned it would increase the ability for molten rock to approach the surface and create volcanoes. Many of the areas in the Northern Rivers such as the Central Volcanic Province, Alstonville Basalt, Maybole Volcanics etc are seem to be in some way related to this episode of rifting instead of the later hotspot volcanoes. Indeed even the latest research such as Sutherland et al (2012) can't easily fit the ages of this volcanism into a traditional hotspot model.

If you look at a bathymetric map of our coastline you will see that the continental shelf is very thin in comparison to the rest of the world. It is also very abrupt, and this structure also points to the process of rifting that occurred during the late Cretaceous and early Cenozoic. The Tasman sea was the result of all this rifting and turmoil. It seems that the Zealandia just wanted to emigrate from the Australian continent. Sometimes I feel the same with our region including the wonderful New England region, I often think that we’d be better off if we were not part of New South Wales but a separate state but hopefully we don’t need to the extreme of rifting this part of the continent away to do it. A new state is, of course, politics and so I should probably end there.


*Jones, J.G. & Veevers, J.J. 1983. Mesozoic origins & antecedents of Australias eastern highlands. Journal of the Geological Society of Australia V30.
*Ollier, C.D. & Pain, C.F. 1994. Landscape evolution and tectonics in southeastern Australia. AGSO Journal of Geology & Geophysics V15.
*Sutherland, F.L., Graham, I.T., Meffre, S., Zwingmann, H. & Pogson, R.E. 2012. Passive-margin prolonged volcanism, Eastern Australian Plate: Outbursts, progressions, plate controls and suggested causes. Australian Journal of Earth Sciences V59.

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