Geology of the Canterbury Region
To the west of the Southern Alps lies the Alpine Fault, a major fault boundary, that passes through the South Island from Fiordland in the south, to the Marlborough Region in the north, where it divides into multiple faults. The Pacific Plate lies to the east of the Alpine Fault and the Australian Plate lies to the west. The Pacific Plate is sliding SSW at about 35mm/yr, relative to the Australian Plate, and rising up 10mm/yr, generating the Southern Alps.
The Alpine Fault did not develop until early Miocene times (23 Ma). Ten million years ago the Southern Alps were low hills, and they only became mountainous as recently as 5 million years ago.
To the east of the Southern Alps are the Canterbury Plains, formed by the sediment eroded from the Southern Alps. On the coast, just southeast of Christchurch, is Banks Peninsula, composed of two large mainly basaltic Miocene volcanoes.
All basement rocks beneath the Canterbury Region belong to the Torlesse Composite (Rakaia and Pahau) Terrane. They are largely composed of greywacke (hardened sandstone and mudstone), that accumulated in a deep marine environment, and were accreted to the Gondwana continent, before the Tasman sea opened up in late Cretaceous times (80 Ma). They outcrop in the Southern Alps and the foothills. The Rakaia Terrane rocks, of Permian to late Triassic age (300-200 Ma), occur south of Rangiora. The Pahau Terrane rocks, of late Jurassic to early Cretaceous age (160-100 Ma), occur to the north, and are probably derived from the Rakaia Terrane. At the boundary between these two terranes, is the Esk Head Belt, an 11 km wide melange of broken and deformed rocks.
The Southern Alps began as sedimentary deposits between 230 and 170 million years ago. The predominantly greywacke formations were uplifted into mountain ranges between 140 and 120 million years ago. From about 26 million years ago, a second continuing period of uplift formed the present-day Southern Alps, driven by the collision of the Pacific tectonic plate with the Australian Plate. The plate boundary is represented by the Alpine Fault which runs slightly west of the main divide of the Southern Alps.
Mount Somers Volcanics
Volcanic activity occurred around the Mount Somers area, and nearby foothills, in late Cretaceous times (100 - 66 Ma). Remnants still exist in the foothills from the Malvern Hills and the Rangitata River.
Sandstone, mudstone and some limestone were deposited throughout many areas from late Cretaceous to Pliocene times. Oligocene (34-24 Ma) limestone outcrops occur in South Canterbury around the Opihi River area, and in North Canterbury around Omihi, and further north near Waiau.
Glacial Lakes and Moraines
Much of the Canterbury Plains are covered by alluvial deposits from the many large rivers coming out of the Southern Alps.
Generally, there are few earthquakes in the Canterbury Region. However, the Alpine Fault last ruptured in 1717 CE, and a major earthquake on the Alpine Fault can be expected to occur every 200 to 300 years. Liquefaction of the ground can occur up to 150 km from the epicentre for a large earthquake.
Notable earthquakes include:
- The 1888 North Canterbury earthquake
- The 2010 Canterbury earthquake, 7.1 magnitude, causing widespread damage
- The 2011 Canterbury earthquake, 6.3 magnitude, causing widespread damage and deaths
Geological sites of interest
- Drive around Banks Peninsula. Look for Castle Rock, above Heathcote Valley, The Remarkable Dikes near Kaituna Pass, etc.
- Drive from Christchurch to Arthurs Pass, and over the Southern Alps, to Greymouth. Look for the change from greywacke to schist, as you near the Alpine Fault.
- Drive up to Mount Cook, and visit the Hooker and Tasman Glaciers.
- Drive from Christchurch to Hanmer Springs, for a hot swim, and see the limestone at Weka Pass on the way.
Geological maps of New Zealand can be obtained from the New Zealand Institute of Geological and Nuclear Science (GNS Science), a New Zealand Government Research Institute. GNS publishes a Map of New Zealand's Geological Foundations.
The main maps are the 1 : 250 000 QMap series, which will be completed as a series of 21 maps and booklets in 2010.[dated info] Low resolution versions of these maps (without the associated booklet) can be downloaded from the GNS site for free. The map for the Christchurch Area was published in 2008, and the map for the Aoraki Area was published in 2007.
- John Wilson (2009-03-09). "Canterbury region - Geology". Te Ara - the Encyclopedia of New Zealand. Ministry for Culture and Heritage / Te Manatu- Taonga. Retrieved 2011-02-23.
- New Zealand Institute of Geological and Nuclear Sciences (GNS Science) - a New Zealand government Crown Research Institute.
- GNS Map of New Zealand's Geological Foundations
- GNS New Zealand Geological Maps.
- Geology of the Christchurch Area, Forsyth, P.J., Barrell, D.J.A, Jongens, R., GNS Science, 2008. (1 : 250 000 map).
- Geology of the Aoraki Area, Cox, S.C., Barrell, D.J.A, GNS Science, 2007. (1 : 250 000 map).