YM #1. Plume or impact? The 250km diameter Yilgarn Malgar Giant Ring Structure.
© Bob Bingley Watchorn Oct 2017
Summary
Plume?
This paper is the first in a series that will document research on the Malgar giant ring structure (GRS) in the Yilgarn Craton as observed in various geoscientific data, is it a plume? These papers are deliberately written in colloquial language for access to as many people as possible. The centre of the Malgar giant ring structure (GRS) is located at 30˚ 05’S and 120˚00’E about 100 km NNE of Southern Cross.
Landsat data shows that the centre of Malgar GRS is surrounded by greenstone of the most prominent appearance in the entire Yilgarn. The NNW Southern Cross Greenstones appear to have been overprinted by an ovoid triangle set of greenstones, sediments and ironstones. These form around the geographical centre of the Malgar GRS. There is a concentric magnetic feature at the centre of Malgar which is similar in dimensions and morphology to Chicxulub Impact structure. Were these greenstones intruded up around the median rim of the pre 2.9 Ga Malgar GRS as defined by the magnetic concentric feature? The giant ring structure cuts across all geology and there is no large-scale evidence that the Malgar GRS is a product of the regional geology.
Plume or impact?
Residual gravity highlights near surface greenstones that follow sections of the Malgar rings suggesting that they were intruded along these rings. Were these greenstones intruded following plume activity under Malgar or were the a result of Malgar resulting from an impact?
The overprinting relationships of the Malgar GRS suggests that it is the oldest of the GRS studied so far and is older than 2.9 Ga. The rings are therefore expected to be more geologically disturbed (and harder to observe) than the rings of Watchorn, Norseman, Cauden and Plumridge.
None of the GRS are perfectly circular but, even though distorted, they maintain their concentric ring structure while crosscutting all Yilgarn terranes, suggesting that the lower crust of the entire Yilgarn had cratonised before they formed. This also implies that these giant ring structures developed independently of the normal geological processes that formed the upper crust of the Yilgarn craton.
To read the full illustrated paper click the link below ……. If you find it useful comment and pass it on to our geoscience colleagues.
| YM #1 Malgar Giant Ring Structure introduction.pdf | 1 MB |