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.

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!

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
Skip to toolbar