Drawing 3D geological structures with sketchup

I discovered this week-end that it is possible to script sketchup, an application to draw objects in 3D. Here is a first test with two semi-transparent inclined planes, and their intersection lineation. The possibility to use scripts instead of the mouse to draw opens up the possibility of multiple 3D drawings of geological structures.

 

 


Another way to look at strain

Here is another way to look at strain from PIV analysis of analogue models. After computing all the spatial derivatives of the velocity or incremental displacements field, the 2D small strain tensor can be assembled.

Below I calculated the principal strain direction from the small strain tensor. This is the direction of one of the principal strain, measured from the x-axis.

PrincipalSmallStrainsThetap

And then I calculated the values of the maximum principal strain and minimum principal strain. Two horizontal bands appear in the plot. The white cells in the top right and bottom left corners show that this experiment is a sinistral horizontal shear.

The data presented here is not interpolated. Each cell is a data point obtained from image cross-correlation. The maps could be interpolated and be made smoother.

PrincipalSmallStrainsEmax

PrincipalSmallStrainsEmin

and then we can claculate the maximum shear. We can see where the shear is occuring, where the difference between the maximum and minimum principal strains is largest.

PrincipalSmallStrainsGmax

Finally, since we have the directions and values of the principal small strains, we can plot them as crosses. The length of the line is linearly proportional to the magnitude of the principal strain, the orientation of the line is the orientation of the principal strain, and red indicates contraction while blue indicates extension.

PrincipalSmallStrainsCrossesCrop

A zoom-in shows the directions of S1 and S2 around the developping plastic shear zone fitting what we expect for a sinistral horizontal shear.

PrincipalSmallStrainsCrossesZoomCrop

I will integrate this into TecPIV rapidly.


Exhumation of (U)HP/LT rocks

Crustal rocks metamorphosed at ultra-high pressure (UHP) record burial to 100–150 km depths and subsequent return to the surface. Although it is well accepted that UHP rocks are formed by deep subduction of continental passive margin rocks, the mechanisms by which these rocks are exhumed remain debated.

Here, three-dimensional thermo-mechanical analogue models investigate how diachronous slab breakoff may lead to the exhumation of subducted continental crust. Slab breakoff initiates spontaneously in one location and migrates laterally along the plate boundary, causing a transient excess downward pull force on the plate boundary in front of the propagating slab tear. This pull force locally reduces the pressure between the plates, which promotes buoyancy-driven exhumation of subducted crust.

However, both the surface area undergoing the pressure reduction and its duration are limited. Our experiments show that the rate of slab breakoff propagation controls both the duration of the pull force and the magnitude of pressure reduction. Our results further demonstrate that exhumation occurs where the slab breakoff propagation rate is lowest, rather than where the pull force is strongest, corresponding to where the slab tear initiates or terminates.

Here is the link to the JSG paper.

Analogue modelling of diachronous slab break-off causing exhumation of subucted crust

 

Illustration of proposed dual-mechanism exhumation of (U)HP rocks associated with propagating breakoff. 1: Horizontal propagation of detachment in the subducted lithosphere; 2: Excess slab pull generated ahead of the propagating tear; 3: Normal pull is produced on interplate zone causing reduction of pressure; 4: Pressure reduction allows buoyancy-driven exhumation of subducted crust; 5: After passage of tear, the lower plate bounces upward causing normal push on plate boundary and increase in interplate pressure; 6: Increased pressure terminates and crustal units are squeezed further upward.


CRC MinEx funded

The Assistant Minister for Science, Jobs and Innovation, Zed Seselja, and Minister for Resources and Northern Australia, Matt Canavan, announced $50 million of Australian Government funding for MinEx CRC.

MinEx CRC is a major endeavour comprising:

  • $50M cash from the CRC Programme
  • $41M cash from geological surveys and from industry
  • $49M non-staff in-kind
  • $78M or 311FTE staff in-kind
  • TOTAL $218M

MinEx CRC’s research will include:

  • Developing more productive, safer and environmentally-friendly drilling methods to discover and drill-out deposits, including coiled tubing drilling technology.
  • Developing new technologies for collecting data while drilling, bringing forward mine production.
  • Implementation of a National Drilling Initiative (NDI) – a world-first collaboration of Geological Surveys, researchers and industry that will undertake drilling in under-explored areas of potential mineral wealth in Australia.

Congratulations to all involved with the Bid.  We are delighted at this success and its scale. MinEx CRC will, at its commencement, be approximately twice the size that DET CRC was at its commencement. MinEx CRC’s 34 current participants are listed below.  Additional sponsors may apply to join MinEx CRC. If you are interested please refer to the attached and contact Andrew Bailey or www.minexcrc.com.au.

The MinEx CRC announcement was followed by a National Press Club speech by Minister Canavan which announced the commissioning of a ‘National Resources Statement’ to address the challenges facing the sector including the need to make new mineral and energy discoveries. The National Resources Statement is to be delivered by a very well credentialed ‘Resources 2030 Taskforce’ within six months.

The MinEx CRC’s current participants are: Anglo American, Barrick Gold, BHP, South32, Atlas Copco, Geotec Boyles, HiSeis, Imdex, LKAB Wassara, McKay, Olympus, Sandvik, Geoscience Australia, Geological Surveys of NSW, SA and WA, Curtin University, Universities of Adelaide, Newcastle, South Australia and Western Australia, MRIWA and CSIRO.

Current MinEx CRC Affiliates are Investigator, Minotaur, DataCode, Minalyze, Mudlogic, Southern Geoscience, Geological Surveys of NT, Queensland and Victoria, Mineral Resources Tasmania and the SA Department of State Development.

this is a re-post from : https://www.aseg.org.au/news/minex-crc-funded


Combined excursions GEOS3170-GEOS3330

This year GEOS3170 – Resource and Exploration Geology provided an excursion to Cobar NSW to explore how the mineralizations are structurally controlled in the Cobar basin. The excursion included mapping of the folded and faulted host rock as well as visits of the CSA and Peak Gold mines to demonstrate how the geology is explored from exploration and production cores. We are very gratefull to both companies to have provided such window into the real work of a mine geologist.

We combined the excursion to Cobar with our usual field trip to Broken Hill for GEOS3330 where the effects of multiple phases of metamorphism and deformation are unravelled. We had the opportunity to get a lecture by Prof. Ian Plimer on the Broken Hill ore body formation. Thank you very much Ian.

 


Bending orogens into oroclines

Structural geologists have long debated the tectonic significance of the sinuous map patterns of mountain belt trend lines. The term orocline was originally defined by Carey (1955) to denote map-view curves that developed by bending of an existing linear orogenic belt about a vertical axis of rotation. Although considerable evidence has thus been reported for oroclines, the mechanisms by which these belts acquired their arcuate shape remains disputed.

An arcuate shape can be produced by bending or buckling of a linear object. Bending (flexure) characterises the deformation of a linear object subjected to an external load applied perpendicularly to its long axis, while buckling is the deflection caused by an external load applied parallel to the long axis. A common view is that oroclines develop in response to an along-strike gradient of tectonic forces oriented at a high-angle to the long axis of the orogen. Such bending about a vertical axis can be generated in response to a horizontal pull produced by a sinking, negatively buoyant, lithosphere, or in response to a horizontal push (compression) due to the arrival of an obstacle or indenter in a subduction zone, or a combination of pushing and pulling (e.g., Rosenbaum and Lister, 2004).

An alternative proposition is that oroclines develop by horizontal buckling in response to a tectonic force oriented parallel or sub-parallel to the long axis of an orogen. Suggested tectonic scenarios for such buckling about a vertical axis include escape or extrusion out of a collisional orogen, attempted subduction of a
continental ribbon or orogen oriented at a high angle to the subduction zone, or by margin-parallel drag (e.g., Johnston, 2001; Offler and Foster, 2008; Cawood et al., 2011).

Here we employ three-dimensional analogue laboratory experiments to explore how such buckling may produce an orocline and the geodynamic conditions required for it to occur.

A first series of experiments demonstrates that a crustal ribbon carried by a subducting plate cannot buckle and detach from its mantle root because it weakens and deforms when entering the subduction zone, such that little compressive stress is transferred through the ribbon.

A second series of experiments shows that the aspect ratio of the ribbon impacts the wavelength of buckling and that the experimental tank employed is too small (maximum equivalent length is < 1500 km) to generate multiple buckles.

Finally, a third series of experiments shows that if the plate boundaries surrounding the ribbon resist its horizontal lateral motion, thrusts or strike-slip fault systems may be generated in the ribbon thereby preventing buckling.

We conclude that oroclinal buckling is favoured when a crustal ribbon is pulled by subduction, causing backarc extension. Hence, buckling and bending models for orocline formation are not mutually exclusive but reinforce each other.

 

You can find the paper here

 

Experimental results of crustal buckling (top), and lithospheric buckling (bottom) with (bottom right) or without (bottom left) side plates

 

 

Continuum between bending and buckling and associated sense of movement on strike-slip faults through the orogen.


GEOS2080 – 2018 – Excursion to South Coast NSW

The semester has started with another excursion to the South Coast for GEOS2080 – Field Geology. Rain on day one at the Bombo Quarry and big swell at Bingie point on the last day. This year the tide was high around mid-day making most of the outcrops less accessible.

 

Flow or Sill? – Bombo Quarry

 

Making of a stratigraphic log at Pebbly beach

 

Pebbly beach – top of the log

 

View at Pebbly Beach

 

Ripples on the platform

 

Zoom-in on the ripples

 

Dikes at Bingie Point

 

Large swell at Bingie


Geometry of the Gilmore Fault Zone

Deepika Venkataramani successfully completed her M.Phil and her first publication has been out for a while now.

Deepika used a joint inversion of geophysical potential fields to assess the geometry of the Gilmore Fault Zone, a key fault to unlock the story of the Lachlan Fold Belt and the assembly of eastern Australia. Here are the key findings:

  • In this new work we interpret the GFZ to be a west-dipping, crustal penetrating thrust fault that is distinct from the shallow, east- dipping fault that should be separately classified as the Barmedman Fault.
  • The models presented herein show that the Macquarie Arc is thrust under the WMB along a separate major thrust which does not reach the surface.
  • The GFZ separates the ~20km deep Siluro-Devonian Tumut Trough to the east from the Ordovician–Early Silurian Macquarie Arc (thus the GFZ may have initiated as a normal fault!)
  • however, it is not a crustal suture (as defined by Scheibner and Basden, 1998) as there are slices of the same aged rocks (Ordovician–Early Silurian and Siluro-Devonian) bound by the Barmedman Fault further west.

The paper can be obtained here.




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