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WESTERN
AUSTRALIAN DIVISION West Australian Geologist (WAG) Bi-monthly newsletter of the Western
Australian Division of the Geological Society of Australia Inc. Number 493: February ̶
March 2012 (4.3 Mb PDF file) Monthly Meetings Time:
5.30 pm for 6.00 pm
formal start (bar open upstairs before talk) Venue: Irish Club of WA, Download a map showing the
location, or check out the venue with Google
Maps. FEBRUARY MEETING Wednesday 1st
February, 2012 Talk
title: Impact structures in Western Australia Speaker: Alex
Bevan (Earth & Planetary Sciences, Western Australian Museum) Abstract: Fourteen
structures in Western Australia have been recognised to varying degrees of
certainty as of impact origin; a further five enigmatic structures known only
from geophysical anomalies have been suggested as possible impact structures
but remain unproven. The structures range in size from a few tens of metres
in diameter, to many tens of kilometres, and in age from a few thousand years
to more than a billion years. Three well-preserved bowl-shaped craters are associated with surviving
fragments of the impactor. The recovered projectile fragments are mainly iron
meteorites (Wolfe Creek, Veevers), but also include a mesosiderite
(Dalgaranga). The Hickman crater lacks projectile fragments. Impact-generated
thermomechanical damage in the surviving projectile
fragments ranges from simple fractures, through shockhardening, to plastic
and shear deformation, severe reheating, recrystallization and melting.
Compression and viscous drag between projectile and target appears sufficient
to melt and possibly vaporize significant volumes of projectiles, which could
account for the paucity of surviving meteoritic materials. Mechanical fracture
is an important mechanism for the disruption of the projectile, allowing
survival, whereas shear-related ‘jetting’ may be responsible for
vaporization. Suspected impactite glass,
representing melted target rocks, has been recovered from Wolfe Creek Crater. Ten large, deeply weathered, complex or transitional structures are
either exposed at the surface (e.g. Yarrabubba, Spider Structure, Goat
Paddock), or buried beneath post-impact sediments (e.g. Yallalie, Woodleigh).
The Spider structure may be one of the few known examples of an oblique
impact. The target rocks vary from sedimentary terrains (e.g., Yallalie,
Spider Structure, Goat Paddock) to
granite–greenstone terrain (Yarrabubba) and mixed targets (e.g., Shoemaker). In the absence of projectile fragments, or of an unequivocal chemical
signature of the impactor, diagnostic shock-metamorphic evidence of an impact
origin includes multiple sets of Planar Deformation Features (PDFs) in quartz
and feldspar (e.g., Woodleigh, Yarrabubba, Shoemaker), and shatter cones
(e.g., Yarrabubba, Spider Structure, Glikson,
Shoemaker). Other corroborating, although not diagnostic, evidence of impact
includes breccias (e.g., Yallalie), pseudotachylites
(e.g., Yarrabubba, Shoemaker), melt sheets (e.g., Yarrabubba) and geophysics
(e.g., Yallalie). An unusually large central uplift at Yallalie indicates
impact into volatile-rich target rocks, possibly sub-aqueously. Post-impact hydrothermal alteration renders the generally open
architecture of ancient impact structures susceptible to mineralization,
possibly of economic importance. About
the speaker: After graduating from University College London
in 1972, Dr Alex Bevan worked briefly at the Geological Society in London,
and then for 13 years at the British Museum (Natural History) before emigrating
to Australia to take up his current position. At the BMNH, Dr Bevan completed
a PhD on the metallurgy of meteorites under the joint University of
London/Government Laboratories scheme. The work involved periods of research
at Manchester University, Lehigh University in Pennsylvania, and the Indian
Museum in Calcutta. At the WA Museum he established a programme of meteorite
recovery and study (WAMET) from the Nullarbor Region. The new material has
expanded knowledge of early Solar System materials, and opened up many new
lines of research, including the innovative use of ancient meteorite falls to
interpret past climatic change. In addition, Dr Bevan has developed the
State’s mineralogical collections, concentrating on documenting and
interpreting some of WA’s world famous mineral deposits. Dr Bevan’s research
focuses on the effects of catastrophic collisions between minor planets
during the early history of the Solar System. His wider research interests
extend to metallurgy and materials science, with special subjects including
mineralogy, meteoritics (including impact studies),
petrology, geochemistry, metallurgy. MARCH MEETING Wednesday 7th
March, 2011 Talk
title: The redox budget of subduction zones – and
implications for the ore-deposit source zones Speaker: Katy
Evans Abstract: Published
in the February – March 2012 WAG. Gibb Maitland Medal Guidelines
download: MS Word
doc Website maintained
by: Peter Haines |
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