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!


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


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.

UON students at HEDG meeting

Our Honours and MPhil students have presented their projects at the HEDG (Hunter Earth Sciences Discussion Group).

  • Sean Melehan, Unravelling long-distance facies associations and sequence stratigraphy of the northern Sydney Basin
  • Sebastian Wong, Provenance and structural evolution of the Yancannia Formation, southern Thomson Orogen
  • Ryan Dwyer, Age and tectonic significance of the Louth volcanics: implications for the evolution of the Tasmanides
  • Deepika Venkataramani, Understanding the subsurface structure of the Gilmore fault Zone through geophysical modeling: implications for Lachlan tectonic reconstructions

It was a great opportunity for our students to take their work beyond the uni. Thanks to Phil Gilmore from NSW Geological Survey for maintaining the HEDG group.


HEDG talk at the Newcastle museum. Under the globe.

HEDG talk at the Newcastle museum. Under the globe.


Our students

Our students (left to right: Deepika, Ryan, Sebastian and Sean)



Sebastian’s structural analysis of the Yancannia



Slab breakoff: insights from 3D thermo-mechanical analogue modelling experiments


The detachment or breakoff of subducted lithosphere is investigated using scaled three-dimensional thermo-mechanical analogue experiments in which forces are measured and deformation is monitored using high-speed particle imaging velocimetry (PIV). The experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs. Slow subduction experiments (with scaled convergence rates ∼1 cm yr −1) have lower Peclet numbers and are characterised by lower tensile strength subducted lithosphere, causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low. When continental collision is preceded by slow oceanic subduction, the subducted lithosphere therefore need not be very long or extremely negatively buoyant to cause detachment because the subducted oceanic lithosphere is hot and weak. Under such conditions detachment may occur sooner after the onset of continental subduction than previously predicted. In contrast, if a collision is preceded by rapid subduction (∼10 cm yr −1), breakoff will be delayed and occur only when the convergence rate slowed sufficiently to thermally weaken the slab and cause its eventual failure. The analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary. Stereoscopic PIV reveals a characteristic strain pattern that accompanies the detachment. Horizontal contraction and subsidence (with scaled values up to 1200 m) in the trench and forearc area preceeds the passage of the detachment, which is followed by horizontal extension and uplift (up to 900 m). High-frequency monitoring captures rapid propagation of the detachment along the plate boundary at rates of up to 100 cm yr −1. However rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether.



Successive side views of the models in Experiment 1 and 2. Experiment 1 (a-e), the subducting lithosphere is pushed by the piston at the constant velocity of 2.5 × 10 −4 m s −1 (equivalent to ∼10 cm yr −1 in nature). The slab becomes vertical due to the negative buoyancy but does not break. It folds when hitting the rigid plate that models the impenetrable lower mantle. Experiment 2 (f–j), The model is identical to Experiment 1 but it is the upper plate that is pushed instead of the lower plate. The model evolution is similar to Experiment 1 until the slab touches the lower mantle. The slab angle reduces in the late stages (dashed line in panel j).



Successive side views of the models in Experiment 3. The model is identical to that employed in Experiment 1 (Fig. 4), but the imposed rate is one order of magnitude lower, 2.5 × 10 −5 m s −1 (equivalent to ∼1 cm yr −1 in nature). Very slow subduction leads to multiple slab detachments at 2283, 4266 and 6420 s. We note that the repeated detachment caused extension in the trailing edge of the upper plate, and a slab graveyard sitting on top of the rigid upper mantle.



Sketch of propagating slab detachment with distribution of surface deformation and uplift. Horizontal contraction and surface subsidence is generated ahead of the breakoff tip, while horizontal extension and uplift follow.



Maps of earthquakes hypocenters along the Aleutian subduction zone (a), and Java-Sumatra-Andaman subduction zone (b), with profiles showing the Wadati-Benioff zone. Earthquakes hypocenters are represented by circles with diameter proportional to magnitude, and color indicating depth (see profiles for color scale). Hypocenters are from EHB catalogue (Engdahl et al., 1998). White arrows represent the convergence vectors calculated using the MORVEL global kinematic model (DeMets et al., 2010). V is the convergence rate (in mm yr −1), θ is the obliquity (angle between the normal to the trench and convergence vector), and Vn is the convergence in the direction of the profile (i.e. convergence corrected from obliquity, in mm yr −1). Topography/bathymetry from Smith and Sandwell (1997).




Slow oblique subduction along the northern branch of the Caribbean subduction zone. Map shows the topography of the trench characterized by a deep through between 65 and 67°W. Convergence from MORVEL global kinematic model (DeMets et al., 2010), is only 19 mm/yr at 64°W (white arrow) and the obliquity is approximately 67°, which yields about 7 mm/yr of normal convergence in the subduction west of 64°W. 3 North-South profiles are plotted showing that this area is also characterized by a deep negative free air anomaly (Sandwell et al., 2014), the peak of which is located the forearc. Based on our experimental results we propose that both the topography and gravity anomalies are caused by an excess downward pull in the subducted lithosphere due to ongoing slab detachment. Topography/bathymetry from Smith and Sandwell (1997), gravity from Sandwell et al. (2014).


GEOS2190 – Potato Point – South Coast NSW

GEOS2190 – Structural and Field Geology changed its location in 2016!

Instead a mapping on km-scale in the Hill-End trough, we travelled to Potato Point and Mystery Bay in South Coast NSW. The objective was the mapping of bedding and younging in simply folded Ordovician turbidites of Potato Point to build a cross-section, and stereonet analysis. This exercise really pushes the idea that structure and stratigraphy must be worked out together in folded areas.

Then a short day at Mystery Bay introduced the notion that many areas have been folded several times. Next step, the multiply folded and metamorphosed meta-turbidites of Broken Hill.

#UonGeos #GEOS2190

My friend Stefan Vollgger from Monash Uni made the basemap used by the students with his UAV. It worked great. A 3D model is also available here: Check it out!



Rock plateform Potato Point



Kink fold – Mystery bay



Microtectonics – Mystery Bay



View from the cabin at the Beachcomber caravan park



Students setting up their tents at night because our bus broke down and we arrived late



Locals at Beachcomber caravan park

UAV map

I have performed a mapping of a headland with my friend Stefan Vollgger from Monash University. I will bring the students there for my second year structural geology course (GEOS2190). We did fly the UAV some 40m above the platform and were able to get a high resolution map of the headland to be used as a base-map.

UON Earth Science will get its UAV (drone) very soon. But we will need to sort out the licences as well before we can continue.


Making of UAV map

Making of UAV map

Ptygmatic folds

Ptygmatic folds (from πτύσσω, to buckle in ancient Greek) involve an irregularly folded, isolated “layer”, typically a quartzo-feldspathic vein in a much more ductile schistose or gneissic matrix. They occur in high-grade rocks, mostly migmatites as trains of rounded and near-parallel, commonly concentric folds in which the amplitude is large (>10) and the wavelength small with respect to the almost constant layer/vein thickness (meander-like pattern). They have a lobate, tortuous to squiggled appearance (for example, limbs fold back on themselves and the interlimb angle is negative) and tend to be polyclinal; however, they have no axial plane foliation

J.-P. Burg.

You can also see in the photo below that the folds have multiple wavelengths: a short wavelength (couple cm) on top of a larger (couple meters). There is a second, thinner vein with shorter small wavelength and larger long wavelength (bottom left corner).

The image is a mosaic of several high resolution images. I must figure out how to publish an HD image. Location: Broken Hill, NSW


Particle Imaging Velocimetry

I have spent some time last year creating a Particle Imaging Velocimetry system to be employed to monitor analogue modelling experiments of tectonics. Basically an image correlation technique is used together with a calibration function to calculate physical displacements and deformation in the models.

My PIV software has been published and is available now. I will post resources soon to help getting started. But first here are some videos of the software. The software has successfully been used last year by my students David Beckett and Maxime Henriquet. It allowed them to quickly become able to quantify their experiments. The example of exported result in the video below is from Beckett’s work. The shear zone setup in the first video is from Maxime’s work.

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