The palaeomagnetism of outcropping oceanic crust on Macquarie Island
Titel:
The palaeomagnetism of outcropping oceanic crust on Macquarie Island
Auteur:
Williamson, P.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 25 (1978) nr. 7-8 pagina's 387-394
Jaar:
1978-12
Inhoud:
Macquarie Island offers a rare opportunity to investigate outcropping ocean crust. In this study, palaeomagnetic samples were collected from 32 sites. After frequency demagnetization, 15 of these sites were found to be stable. Stable sites were from lavas, dykes, and gabbros, representing different depths of formation in the oceanic crust. A successful bedding correction, assuming a simple model for the oceanic crust, was applied to the directions of magnetization. The former assumed dykes were injected vertically, lavas laid down horizontally, and gabbros layered horizontally. However, the palaeomagnetic poles from the centre and south of the island were not conformable at their 95 percent confidence levels. To remedy this, a structural correction was then applied to sites from the south of the island, where the average strike of dykes is 55° different from that in the centre of the island, which parallels regional seafloor-spreading anomalies. This correction involved rotation around a vertical axis, which could not be accounted for in a simple bedding correction. When the average strike of the dykes was brought into parallelism with that of the regional seafloor-spreading anomalies (and presumably that of the fossil spreading ridge), the result was to bring the palaeomagnetic poles from the centre and the south of the island into conformity. The final pole for Macquarie Island was found to be consistent with a pole of similar age from the Australian Continent. This is consistent with Macquarie Island being formed as part of the Indian plate. The magnetic properties of the rocks are consistent with seafloor-spreading anomalies originating from the lavas, with a possible deeper contribution from the gabbros. The dolerite dykes which are palaeomagnetically stable appear to have natural remanent magnetizations (NRM) which are too low to contribute significantly to seafloor-spreading anomalies.
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