Palaeomagnetic test of oroclinal rotation in the Dundas Trough, Tasmania

Abstract

Cambrian to Silurian rocks of the Dundas Trough, Tasmania, including the economically important Mount Read Volcanics, follow a curving trend around the northern margin of the Tyennan Block. This arcuate trend has recently been interpreted to be part of an orocline, the eastern limb of which is obscured below Jurassic dolerite. Other workers consider the arcuate trend of the Dundas Trough to be the result of Cambrian rifting, with the underlying basement continuing north below Bass Strait. We tested the orocline hypothesis by palaeomagnetic sampling of rocks from the Owen, Gordon and Eldon groups. Previous palaeomagnetic studies concluded that these rocks were overprinted. By use of modern demagnetisation protocols and principal component analysis of demagnetisation vectors we were able to isolate a pre-deformation characteristic remanence carried by magnetite and hematite from a normal polarity overprint carried only by hematite. The overprint resembles Cretaceous overprints recognised from the eastern Australian seaboard, and thought to be related to Tasman Sea rifting. The characteristic remanence passes the statistical orocline test, confirming the orocline hypothesis for the Dundas Trough. Rocks equivalent to the Mount Read Volcanics therefore continue under Jurassic dolerite cover below eastern Tasmania.