GEOS2190 – Excursion to South Coast

Here are some photos of our structural geology excursion to Potato Point, South Coast NSW. It’s been great. Thanks everyone!


GEOS1050 – Excursion to New England

Here are some photos from the 2017 excursion to New England.


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


Controls on sill and dyke-sill hybrid geometry and propagation in the crust: The role of fracture toughness

 

Analogue experiments using gelatine were carried out to investigate the role of the mechanical properties of rock layers and their bonded interfaces on the formation and propagation of magma-filled fractures in the crust.

Water was injected at controlled flux through the base of a clear-Perspex tank into superposed and variably bonded layers of solidified gelatine. Experimental dykes and sills were formed, as well as dyke-sill hybrid structures where the ascending dyke crosses the interface between layers but also intrudes it to form a sill.

Stress evolution in the gelatine was visualised using polarised light as the intrusions grew, and its evolving strain was measured using digital image correlation (DIC).
During the formation of dyke-sill hybrids there are notable decreases in stress and strain near the dyke as sills form, which is attributed to a pressure decrease within the intrusive network. Additional fluid is extracted from the open dykes to help grow the sills, causing the dyke protrusion in the overlying layer to be almost completely drained.

Scaling laws and the geometry of the propagating sill suggest sill growth into the interface was toughness-dominated rather than viscosity-dominated. We define KIc* as the fracture toughness of the interface between layers relative to the lower gelatine layer (KIcInt / KIcG). Our results show that KIc* influences the type of intrusion formed (dyke, sill or hybrid), and the magnitude of KIcInt impacted the growth rate of the sills. KIcInt was determined during setup of the experiment by controlling the temperature of the upper layer Tm when it was poured into place, with Tm < 24 °C resulting in an interface with relatively low fracture toughness that is favourable for sill or dyke-sill hybrid formation. The experiments help to explain the dominance of dykes and sills in the rock record, compared to intermediate hybrid structures.

Tectonophysics 698 (2017) 109–120
http://dx.doi.org/10.1016/j.tecto.2016.12.027

 

Dyke-sill hybrid formation, with fluorescent particles in the gelatine illuminated by a thin vertical laser sheet. The intrusion is viewed perpendicular to the dyke strike direction

 

Photo of dyke-sill hybrid formation. The intrusion is viewed with polarised light, approximately perpendicular to the strike direction of the dyke. Interference colours indicate the evolving distribution and intensity of stress within the gelatine host.

 

Dyke-sill hybrid formation. The intrusion is viewed looking down and from the side, onto the interface between the gelatine layers. The position of the interface against the tank wall is indicated by the dashed line.
A) A penny-shaped dyke has propagated through the lower gelatine layer and slightly protruded into the upper layer, with two small sills intruding the horizontal interface where it is intercepted by the dyke margins.
B) The dyke protrusion in the upper layer quickly became arrested as the sills grew.
C) The sills joined together within the interface, continued to grow and then coalesced with one margin of the dyke to create the final dyke-sill hybrid structure.


The material or views expressed on this Blog are those of the author and do not represent those of the University.  Please report any offensive or improper use of this Blog to RPS@newcastle.edu.au.
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