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!


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.

 


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

tectonophysics-10-2016

 

http://www.sciencedirect.com/science/article/pii/S0040195116304735

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.

 

fig4

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).

 

fig5

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.

 

fig-9

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.

 

fig10

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).

 

 

fig11

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: https://skfb.ly/ToHY Check it out!

 

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Rock plateform Potato Point

 

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Kink fold – Mystery bay

 

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Microtectonics – Mystery Bay

 

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View from the cabin at the Beachcomber caravan park

 

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

 

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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. http://www.files.ethz.ch/structuralgeology/jpb/files/english/8folds.pdf

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

IMG_9264_stitch-s


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.


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