Stokes Magnetic Anomaly

The Stokes Magnetic Anomaly (also known as the Stokes Magnetic Anomaly System, SMAS,  New Zealand Junction Magnetic Anomaly, JMA, great Nelson magnetic disturbance, Junction Anomaly, Campbell Magnetic Anomaly System, CMAS) is a magnetic anomaly on the Earth's surface that extends from New Caledonia to the Chatham Rise with complexity consistent with the theory of plate tectonics.

History
It is named after Captain (later Admiral) John Lort Stokes by G. C. Farr in 1916 as he described it first although such naming has proved controversial, hence many of the alternative names. The magnetic declinations were observed by Captain Stokes when captaining HMS Acheron and Commander (later Admiral) Byron Drury in HMS Pandora between 1851 and 1853.

Geology
The Stokes Magnetic Anomaly has been characterised for over 3000 km and was essential for understanding the geology of Zealandia as a mainly underwater continent. It extends from 700 km south of New Caledonia to almost the eastern edge of the Campbell Plateau. Over much of its length it has peaks about 30 km to 50 km apart, although this is not the case for much of its New Zealand west coast course. Where the anomaly crosses New Zealand it is displaced by approximately right angle changes in direction for a total of 1000 km running down the western side of New Zealand from the Northland Peninsula in the North Island to Fiordland but then exiting New Zealand's South Island on its Otago east coast.

The Stokes Magnetic Anomaly has been related to magnetic anomaly extending in Australia as the east Lachlan Fold Belt or New England Fold Belt as an extension of where it commences near the western Challenger Plateau and Lord Howe Rise. This gives an age of up to 83 million years before present in its formation but alternatively, it may be extended to represent the earliest ocean crust formed between New Zealand and Marie Byrd Land in Antarctica so could be even older.