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.




Viscoelasticity at large strains

The Maxwell body of a linear-elastic Hookean spring in series with a Newtonian dashpot is the simplest rheological model for geological deformation. It has been employed to describe many deformation processes.

However current Maxwell models display well-known errors when the associated strains and, importantly, rotations are large. Such conditions are often met in Earth sciences. Large rotations pose a mathematical challenge when elasticity is considered in the rheology of highly transformed materials as one requires an objective formulation of the stress rate (time derivative of stress).

In a new publication, Schrank et al. introduce a new large-strain model for Maxwell viscoelasticity with a logarithmic co-rotational stress rate (the ‘FT model’). An analysis of homogeneous isothermal simple shear with the FT model compared
to a classic small-strain formulation (the ‘SS model’) and a model using the classic Jaumann stress rate (the ‘MJ model’) leads to the following key conclusions:

  • At W  ≤ 0.1, all models yield essentially identical results.
  • At larger W, the models show increasing differences for γ > 0.5. The SS model overestimates shear stresses compared to the FT model while the MJ model exhibits an oscillatory response underestimating the FT model.
  • The MJ model violates the self-consistency condition resulting in stress oscillations and should be disregarded. It does not deliver truly elastic behaviour.
  • In the intermediate-W regime, the shear-stress overestimates of the SS model may constitute acceptable errors if energy consistency is not important. If energy consistency is desirable, the SS model should not be used at W ≥ 0.3.
  • In the high-W regime, stresses in the SS model become unacceptably
    large. The FT model should be used in this domain.

The FT model constitutes a physically consistent Maxwell model for large non-coaxial deformations, even at high Weissenberg numbers (W). It overcomes the conceptual limitations of the SS model, which is limited to small transformations, not objective and not self-consistent. It also solves the problem of the energetically aberrant oscillations of the MJ model.

 


Broken Hill 2017

GEOS3330 excursion to Broken Hill provided a new opportunity this year. For the first time since I started doing this excursion, it rained on the field area during the day. Not enough to make us stop, or worry too much about the dirt road and the creek… Also, with the rain some foliations became a lot easier to spot while other almost disappear.

 

Broken Hill panoramas

Panoramas from Double Schistosity


Updates on TecPIV

TecPIV is my MATLAB GUI package for calibration, correction of images of analogue models, correlation of images to get incremental and cumulative displacements and spatial derivatives. The code is explained in the paper:

Boutelier D. TecPIV – A MATLAB-based application for PIV-analysis of experimental tectonics. Comput Geosci 2016;89:186–99. doi:10.1016/j.cageo.2016.02.002.

A new version of TecPIV has been posted. You can get the files from bitbucket here in the dev branch. I will soon post a zip file of the whole new version on this page.

I suggest if you have a version of TecPIV already installed, you make a zip file of it so the new TecPIV folder and its content can be uniquely defined in your MATLAB path.

Here is the change log:

  • Use MATLAB vectorization instead of parfor for parallelisation of correlation. (faster)
  • Allows window deformation in multipass. (better for narrow features)
  • Window overlap fixed at 50% (better for multipass)
  • Eulerian and Lagrangian sums in the postprocessing menu. Lagrangian output shown as deformed grid instead of vectors.
  • Closing the main window using the button does the same as using the save and close function.
  • Closing the second window makes its content, and the window itself invisible instead of deleting the object.
  • Less warning messages.

Exploring the diffusion equation

While preparing a lab on heat diffusion, I thought it would be interesting to compute the diffusion of topographic relief using the same forward Euler finite difference in 2D (explicit method with central difference in space and forward in time, see here).

I downloaded a dem of largest topographic reflief on the planet, the Himalayas and roughly converted the lat/long into meters before applying the finite difference method. As expected, diffusion acts on the small scale features first, and especially those with large gradients… So the board topography remains but the details are progressively vanishing. Although it is incorrect for a landscape simulation, it is valuable to show the characteristics of diffusion.

In the second figure you can see the difference between the start and end stages and a few zoom-ins showing how material is diffused from the small (sic!), steep peaks to fill in the valleys.


Digital thin sections

With the acquisition of a computer-controlled stage we are now able to digitise thin sections under the microscope. There are many issues to fix, and questions to answer before we can put a web page online but that’s our goal. This would not replace the student’s experience at UON Earth Sciences with microscopes but complement it.

I have been working on assembling the images from the microscope into large images which can then be broken into tiny tiles to build a responsive web page. This is key for microstructural analysis where the fabric needs to be observed at multiple scales. Watch this space!



#uongeos

Following our GEOS2080 excursion to South Coast I posted some photos on social media which can be found using the hashtag #uongeos. If you have photos to share from our excursions please do post and tag. You can be specific and use #uongeos2080 or #uongeos2190 as well.

 

Melville Point, South Coast

 

Melville Point, South Coast NSW

 

Bingie Bingie, South Coast NSW

 

Best classroom


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