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New geological map of central South Island - 23/01/2008

Eight years of work by a team of geologists has come to fruition with the publication of a new geological map of the Aoraki area in central South Island. The map, which shows the region’s geology in more detail than ever before, has been generated from a computer database using Geographic Information Systems (GIS) technology, which means it can be updated regularly as new information comes to hand.

David and Simon

The map replaces existing geological maps of the area, published in the 1960s, and combines a vast amount of published and unpublished material, plus new research, to produce a valuable resource about central South Island geology.

The large full-colour 1:250,000-scale map is tucked into a sleeve in the back cover of a 71-page companion book. This is illustrated throughout, and includes some stunning colour diagrams and landscape photographs.

Geology of the Aoraki Area covers 24,000sqkm of South Westland and central parts of the Canterbury region, from Castle Hill basin through the Rakaia and Rangitata Rivers to MacKenzie basin. The map also extends offshore past the Canterbury and Westland coasts. The main towns - Franz Josef, Twizel, Fairlie, Pleasant Point, Geraldine, Temuka, Methven and Ashburton - lie on the coastal plains and downlands.

It is the latest of a series of geological maps covering the whole of New Zealand, published by GNS Science. Started in 1996, the series of 21 maps is scheduled for completion in 2010, with QMAP Aoraki being the 14th to be completed. QMAP stands for quarter-million scale map.

Aoraki map

The map text summarises the geology and tectonic development of the Aoraki region, which is crossed by the major Alpine Fault on which most of the ongoing movement between the Australian Plate (to the west) and Pacific Plate (to the east) occurs.

The fault brings together rock sequences with very different geological histories and results in the formation of the Southern Alps, which are being pushed up by 4mm to 6mm a year. The map text describes all the rocks and deposits, and also highlights aspects of economic geology and potential geological hazards.

The most significant advances arising from this recent mapping is in the wealth of detail in the mountains, which are dominated by greywacke rocks. The map shows, for the first time, the degree to which these rocks have been folded and faulted across the region.

Subsurface structure to depths of 5km is depicted on cross-sections, using seismic experiments carried out during the late 1990’s together with the new mapping, and shows the buckling of the earth’s crust during formation of the mountains. The map compilers have also given considerable attention to the effects of past glaciation. The new map records more than 70 types of gravel deposits resulting from glacial advances and retreats, many of which are used as a source of aggregate - the region’s most valuable geological resource.

Relative to other parts of New Zealand, the Aoraki region has had a low historic level of earthquake activity since records have been kept. However, shaking from both local and distant earthquakes has been recorded, and a very large earthquake is still expected on the Alpine Fault. Many faults are shown as “active”, meaning that movement has occurred along them in the recent geological past. These faults are considered to have potential to rupture gain in the future, say map compilers Simon Cox and David Barrell.

Aoraki mountain

As well as the Alpine Fault, which traverses the western base of the Southern Alps, active fault systems extend in the east through the MacKenzie and Fairlie basins, the foot hills, and along the edge of the Canterbury Plains. In addition, many more “inactive” faults are shown. Other geological hazards are landslides, rock avalanches, tsunami, erosion, flooding and sedimentation, which affect many parts of this geologically active region.

Dr Cox said producing the map and GIS dataset has been a mammoth effort, compiling previously existing work and integrating it with new mapping throughout the Southern Alps. It includes vast areas that had not previously been visited by geologists.

The eight year government-funded project involved many geologists and field assistants, and support from over 20 organisations. Dr Cox would particularly like to pay tribute to the many landowners and Department of Conservation, who allowed access and use of huts for accommodation, plus local helicopter companies, who provided transport services.

Digital data from which the map was produced is already being used extensively in a number of applied and scientific projects. End users expected to benefit include regional councils, engineers, developers, scientists, and mineral exploration companies. People with an interest in geology and the Southern Alps would also find the map useful.

The map is available from GNS Science sales for $30.