Palinspastic reto-deformation of regional scale aeromagnetic data
Dr Ross Cayley
Senior Geologist, Geological Survey of Victoria
The Stavely Arc developed above a continent-dipping subduction zone active along the eastern margin of Gondwana in the Cambrian (Foden et al., 2006). Initially Japan-style, the arc-complex transitioned to an Andean-like convergent scenario at the end of the Cambrian, shortened and accreted to the margin. This is important, as the modern Andes host giant metals systems, and the geological setting suggests similar prospectivity may exist in the Stavely Arc in western Victoria.
Understanding Stavely Arc geology is challenging. The region has a complex history. The arc is disrupted into an array of offset and variably-rotated fault-segments. Exposure is poor, most arc-segments buried beneath Ordovician-Silurian Grampians Group, Devonian Rocklands Volcanics, Murray Basin sediments and young basalt lava flows. Fortunately, exposed arc segments at Mount Stavely, Mount Dryden, and Black Range/Rocklands have characteristic magnetic and gravity character, allowing extensions to these belts to be interpreted beneath cover. Geophysical data, in combination with other geological constraints (especially drilling and the complex structural histories mapped in enclosing Cambrian metasediments and in the overlying Grampians Group), has allowed for the recognition of many additional arc fragments.
Explaining the full context of the complex present-day arc-fragment configuration has required advancing understanding of the tectonics of the Tasmanides. A key breakthrough is the realisation that a dextral transtensional deformation and magmatism regime active in the adjacent Lachlan Fold Belt in the Early Devonian (Cayley & Musgrave, in review) can be applied as a template to constrain retro-deformation solutions for Stavely Arc aeromagnetic data. Geophysical data allows large dextral strike-slip faults and associated vertically-plunging drag-folds and kinks with tens-of-km amplitudes mapped in Cambrian bedrock to be traced beneath the Grampians Ranges. The bedrock structures coincide in position with similar-style drag-folds (e.g., Mafeking Orocline) and strike-slip faults developed in this cover succession. The recognition of large, spectacular scissor-fault detachments within the Grampians, hiding in plain view, is a world-class breakthrough that finally explains the complex distribution of Grampians Group within the main ranges. All these faults appear to link east at depth into the footwall of the Moyston Fault, implicating transtensional reactivation of this fault as the driver of this Siluro-Devonian deformation.
Overprinting criteria tightly constrains the Early Devonian age of this second generation of bedrock structures, and permits cover and bedrock structures to be retrodeformed together, using aeromagnetic data as a template. Palinspastic restoration reveals that up to 160 km right-lateral displacement occurred along the Moyston Fault and footwall splay structures in the Early Devonian. The Stavely Arc restores to three sub-parallel, mostly west-dipping and mostly west-facing fault slices of arc volcanics and related rocks. These fault belts of Stavely Arc rocks formed during the Cambrian Delamerian Orogeny. Most lie subparallel to the Yarramyljup Fault to the west and possibly represent successive footwall splays from this structure. The restored fault-belts are quite continuous along-strike and so provide a greatly simplified template for correlation of now-disrupted belt segments, for mineral exploration and for target selection. This restoration represents a start-point for understanding the nature of the Cambrian deformation that originally accreted the Stavely Arc and probably controlled the distribution of the metallic mineralisation developed within it.
160km of right-lateral deformation across the position of the Moyston Fault in the Early Devonian cannot be explained by closure of the Wentworth and Menindee Troughs north along-strike alone. Such a large translation is hard to reconcile with the presence of the Curnamona Craton farther north in western NSW. The results of the Stavely Project therefore dictate a re-think of the timing of formation of the Curnamona Craton, the associated Nackara Arc, and deformation events within the Flinders Ranges (Cayley, 2016).
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Student sponsorship
Jan Tympel
The University of Melbourne
Thanks to the Student Research Scholarship that was generously provided by the GSAV I was able to attend the 2016 Goldschmidt geochemistry conference located in the scenic harbor area of Yokohama, Japan. The six-day international conference was well attended (by over 3,000 delegates) and featured 18 themed sessions, two of which were of particular interest to me: Early Earth Processes and the Continental Crust. Personal highlights of the conference include participating as a student helper, a keynote talk given by Richard Carlson (Carnegie DTM) on formation controls on Earth’s chemical composition and a plenary talk delivered by Vickie Bennett (ANU) on Eoarchaean rocks from Greenland, which was followed by a ‘meet-the-plenary’ lunch.
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I was given the opportunity to present some of my findings on titanites that are hosted in ~3.5 Ga old metabasalts from the Pilbara Craton (NW Australia) in an oral session on the accessory mineral record of the continental crust. Talking in front of a diverse group of researchers who work on related topics has been a very helpful experience. The daily poster-sessions in the afternoon provided ample opportunities to mingle, discuss and connect with other researchers in a relaxed environment. I greatly appreciate the financial assistance from the Geological Society of Australia, Victoria Division, which made this trip possible.
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Cleifden Heritage Issues
Support is needed for the proposed heritage listing. Submission period closes on 14 March 2017.
See information below and at
http://www.environment.nsw.gov.au/heritageapp/ViewHeritageItemDetails.aspx
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Speleo2017
The International Union of Speleology (UIS) conducts the International Congress of Speleology (ICS) every 4 years. The next ICS is being held in Sydney in July 2017. This is the first time it has been held in Australia and will be a great opportunity to find out about the geology of caves as well as the more exciting exploration activities from around the world.
Although the ICS attracts a lot of recreational cavers, serious geological papers are presented by geologists and other related scientists. Some session topics include: Speleogenesis, palaeontology, archaeology, biospeleology, microbiology in extreme environments, cave and karst in carbonate rocks, pseudokarst, volcanic caves as well as exploration and cave diving sessions.
Registration is still open and limited spaces on some field trips are still available.
For details see the website Speleo2017 at https://www.speleo2017.com/index.html.
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The internationally renowned monthly social get-together
for explorers, miners & other geoscientists
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Fellow Geoscientists,
Welcome to the inaugural GeoPub Melbourne. This monthly event provides an opportunity for members of the Melbourne exploration community to catch up on the industry “goss”, have a few drinks, talk technical, reminisce, and/or generally socialise. Occurring on the second Friday of the month, GeoPub Melbourne aims to become a regular event on any Geologist’s social calendar.
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INVITATION TO GEOPUB MELBOURNE
Join Us: At 5:30pm, every 2nd Friday of the month as your work-spouse-budget-health allows.
Why: Meet other people working / interested in geology, mining & exploration.
And Do? Catch up on "goss", have a few drinks, talk technical, reminisce, and/or generally socialise.
Where: Little Mule Cafe, 19 Somerset Place, Melbourne.
Contact: geopubmelb@gmail.com
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Forthcoming events
Unless otherwise noted, all 2017 talks will be held at the Fritz Loewe Theatre, School of Earth Sciences, University of Melbourne.
February 23rd: Monthly meeting:
Talk at Monash University. Details above.
Ross Cayley
Palinspastic retro-deformation of regional scale aeromagnetic data
March 30th: Monthly meeting:
Kieran Iles
Granite formation and the origins of isotopic heterogeneity
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Student Scholarships
The GSAV are pleased to offer scholarships for honours and postgraduate students in geological sciences for assistance with travel costs associated with attending conferences (fieldwork excluded). The number and value of the scholarships awarded each year is made at the discretion of the GSA Victoria committee. Up to $500 for travel within Australia and between Australia and New Zealand and $700 for travel elsewhere is available, paid half before and half after the conference. More information, including the eligibility criteria and application form, is available at http://www.gsavic.org/scholarship.html.
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Contributions to The Victorian Geologist
If there are any events, happenings, news, or views that would be of interest to the membership, please send your details and information to Kieran Iles at kiles@student.unimelb.edu.au
Newsletter deadline: First Friday of the month, except for December and January.
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Contribute to TAG
It is member contributions which make TAG (The Australian Geologist) a member magazine – please keep the contributions coming and assist with informing all of the membership (not just your Division) about your activities.
Please send your news to: tag@gsa.org.au
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About the GSA Victoria Division
General information about the Geological Society of Australia and GSA Victoria Division can be found at www.gsa.org.au and www.gsavic.org.
Contact details for the GSAV Committee can be found at www.gsavic.org/committee.html.
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