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Meso-Scale Patterns in Gravity Data for Eastern Australia and Their Relationship

to Fossil Impact Craters

All images and text are Copyright © 2005 by Larry Robinson

PowerPoint 6-10

Prepared as part of:

The Spatial and Temporal Distribution of theMetal Mineralisation in Eastern Australia

and the Relationship of the Observed Patternsto Giant Ore Deposits

byLarry Robinson

A thesis submitted for the degree of

PhD

2006

We begin with a Grey-Scale Gravity Image

Image supplied by Geoscience AustraliaA.G.S.O Gravity Data, P42 Gravity Grid of Australia 2001, CD-ROM

Note:Grey-scale images have 256 gradations of ‘grey’; 0 being black and 255 being white.

We then create a Filtered Median Image.

Note:With median filtering the value of an output pixel is determined by the median of the neighbourhood pixels, rather than the mean.

The median is much less sensitive than the mean to extreme values (called outliers). Median filtering is therefore better able to remove these outliers without reducing the sharpness of the image.

(MATLAB Help, 2002)

The median value for each cell has been determined using 60 cells (60 pixels) in each direction, which are the neighbourhood cells mentioned previously. Each cell is 0.9 km on a side.

The Filtered Median Image

This means that the median value for each cell was determined from 14,400 neighbouring cells.

There are 15,000,000 cells in this image.

The Fine-Scale features in the Gravity Data are revealed when the two images are subtracted from one another.

MINUS = Fine Scale

Features

Grey Scale Image Filtered Median Image

The Fine-Scale features in the Gravity Data for Eastern Australia

The dominant wavelength is 50 to 80 km.1

The dominant wavelength of the Golden Network in eastern Australia is ~80 km.

The highlighted area is presentedin the following slide.

This area includes most of the Lachlan Fold Belt in New South Wales and Victoria as well as all of Tasmania.

The Location of the Cross Section Lines in the Fine-Scale features in the Gravity Data for Southeastern Australia

Note the symmetry in

thenorth-south

cross section.

Note the lesser

symmetry in the

east-west cross

section.

Known faults causing the

lack of symmetry in

the east-west cross section.

Proposed Deniliquin Impact Site2

with Gold Deposits

Likely Age of Impact is4.2–3.8 Ga

From Condie (2001) - Figure 3.26 Corona on Venus from Mantle Plumes and Their Record in Earth History, p. 93

The Coronal features on Venus are similar in geometry, symmetry and scale to the circular features of eastern Australia.3 & 4

Coronae are impact craters.3

Venus, Mercury, the Moon, Mars, and the Earth, underwent meteor bombardments at (4.2–3.8 Ga), and (3.2 Ga).5 & 7

It is probable that these symmetrical, circular, annular features in Australia are fossil impacts.

Gravity data reveals the near perfect symmetry in this relatively recent impact (65 Ma).

The Chicxulub Impact Site, the

Yucatan Peninsula,

Mexico7

Meteor Impacts

Only meteors above a certain size (>10 kilometres in diameter) are likely to have an impact on the mantle.6

Lesser meteors would have superficial, crustal impact and eventually would be eroded away here on Earth.7

Reflection Seismic Profile of the Chicxulub Structure8

It seems likely that the structures shown here that are above the Moho extend into the mantle leaving a ‘permanent’ template long after the surficial evidence has eroded away.

This permanent ‘accidental template’9 in the mantle, consisting of concentric and possibly radial fractures, would impose those structures on any rocks that were subsequently laid down or emplaced over the mantle.

The End

Thank You

References

1. Wellman, P., 1976, The gravity field of the Australian basement: BMR Journal of Australian Geology & Geophysics, v. 1 (1976), p. 287-290.

2. Yeates, T. N., Meixner, T. J., and Gunn, P. J., 2000, An interpreted approximately 1240 km-diameter multi-ring structure, of possible impact origin, centred beneath the Deniliquin region, southeastern Australia, in Anonymous, ed., Exploration beyond 2000; conference handbook., Australian Society of Exploration Geophysicists. Spring Hill, Queensland, Australia. 2000.

3. Vita-Finzi, C., R. J. Howarth, S. Tapper & C. Robinson, 2004, Venusian Craters and the Origin of Coronae, 35th Lunar and Planetary Science Conference in Houston, TX, in March 2004. http://www.mantleplumes.org/Venus_Craters.html

4. Condie, Kent C., 2001, Mantle Plumes and Their Record in Earth History, Cambridge University Press

5. Glikson, A. Y., 2001,The astronomical connection of terrestrial evolution: crustal effects of post-3.8 Ga mega-impact clusters and evidence for major 3.2+or-0.1 Ga bombardment of the Earth-Moon system, Journal of Geodynamics, 32, 1-2, p. 205-229

6. Grieve, R., and Cintala, M., 1997, Planetary Differences in Impact Melting: Advanced Space Research, v. 20, p. 1551-1560.

7. Grieve, R., 1997, Extraterrestrial impact events: The record in the rocks and the stratigraphic column, Palaeogeography Palaeoclimatology Palaeoecology, 132, 1-4, p. 5-23

8. Grieve, R., and Therriault, A., 2000, Vredefort, Sudbury, Chicxulub: Three of a Kind?: Annu. Rev. Earth Planet. Sci., v. 28, p. 305-338.

9. Ortoleva, P. J., 1994, Geochemical Self-Organization, Oxford University Press, 411 p.