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Tectonics and Structural Geology

Minds over Methods

Minds over Methods: studying dike propagation in the lab

Minds over Methods: studying dike propagation in the lab

Have you ever thought of using gelatin in the lab to simulate the brittle-elastic properties of the Earth’s crust? Stefano Urbani, PhD student at the university Roma Tre (Italy), uses it for his analogue experiments, in which he studies the controlling factors on dike propagation in the Earth’s crust. Although we share this topic with our sister division ‘Geochemistry, Mineralogy, Petrology & Volcanology (GMPV)’, we invited Stefano to contribute this post to ‘Minds over Methods’, in order to show you one of the many possibilities of analogue modelling. Enjoy!

 

dscn0024Using analogue models and field observations to study the controlling factors for dike propagation

Stefano Urbani, PhD student at Roma Tre University

The most efficient mechanism of magma transport in the cold lithosphere is flow through fractures in the elastic-brittle host rock. These fractures, or dikes, are commonly addressed as “sheet-like” intrusions as their thickness-length aspect ratio is in the range of 10-2 and 10-4 (fig.3).

Understanding their propagation and emplacement mechanisms is crucial to define how magma is transferred and erupted. Recent rifting events in Dabbahu (Afar, 2005-2010) and Bardarbunga (Iceland, 2014, fig.1) involved lateral dike propagation for tens of kilometers. This is not uncommon: eruptive vents can form far away from the magma chamber and can affect densely populated areas. Lateral dike propagation has also been observed in central volcanoes, like during the Etna 2001 eruption. Despite the fact that eruptive activity was mostly fed by a vertical dike to the summit of the volcano, several dikes propagated laterally from the central conduit and fed secondary eruptive fissures on the southern flank of the volcanic edifice (fig.2). Lateral propagation can hence occur at both local (i.e. central volcanoes) and regional (i.e. rift systems) scale, suggesting a common mechanism behind it.

fig-3mario-cipollini

Fig. 2 Lava flow near a provincial road, a few meters from hotels and souvenir shops, during the 2001 lateral eruption at Etna. Credit: Mario Cipollini

Therefore, it is of primary importance to evaluate the conditions that control dike propagation and/or arrest to try to better evaluate, and eventually reduce, the dike-induced volcanic risk. Our knowledge of magmatic systems is usually limited to surface observations, thus models are useful tools to better understand geological processes that cannot be observed directly. In particular, analogue modelling allows simulating natural processes using scaled materials that reproduce the rheological behavior (i.e ductile or brittle) of crust and mantle. In structural geology and tectonics analogue modelling is often used to understand the nature and mechanism of geological processes in a reasonable spatial and temporal scale.

d_grad_dike57_080Field evidence and theoretical models indicate that the direction of dike propagation is controlled by many factors including magma buoyancy and topographic loads. The relative weight of these factors in affecting vertical and lateral propagation of dikes is still unclear and poorly understood. My PhD project focuses on investigating the controlling factors on dike propagation by establishing a hierarchy among them and discriminating the conditions favoring vertical or lateral propagation of magma through dikes. I am applying my results to selected natural cases, like Bardarbunga (Iceland) and Etna (Italy). To achieve this goal, I performed analogue experiments on dike intrusion by injecting dyed water in a plexiglass box filled with pig-skin gelatin. The dyed water and the gelatin act as analogues for the magma and the crust, respectively. Pig-skin gelatin has been commonly used in the past to simulate the brittle crust, since at the high strain rates due to dike emplacement it shows brittle-elastic properties representative of the Earth’s crust. We record all the experiments with several cameras positioned at different angles, taking pictures every 10 seconds. This allows us to make a 3D reconstruction of the dike propagation during the experiment.

In order to have a complete understanding of the dike intrusion process it is essential to compare the laboratory results with natural examples. Hence, we went to the field and studied dikes outcropping in extinct and eroded volcanic areas, with the aim of reconstructing the magma flow direction (Fig. 3). This allows validating and interpreting correctly the observations made during the laboratory simulations of the natural process that we are investigating.

fig-1

Fig. 3 Outcrop of dikes intruding lava flows. Berufjordur eastern Iceland.

 

Minds over Methods: Experimental earthquakes

Minds over Methods: Experimental earthquakes

After our first edition of Minds over Methods, which was about Numerical Modelling, we now move to Rock Experiments! How can rock experiments be used to study processes within the Earth? We invited Giacomo Pozzi, PhD student at Durham University, to explain us how he uses rock experiments to study fault behaviour during earthquakes.

 

13072693_10207863372934990_7705005482414752149_oExperimental earthquakes to understand the weak behaviour of faults.

Giacomo Pozzi, PhD student at Durham University

As seismic slip along faults accommodates large deformations in the upper crust, the intriguing absence of significant heat flow anomalies (which are expected to be produced by intense energy dissipation during slip) along major geological bodies like the S. Andreas fault pushed the researchers to start conceiving a new, dynamic theory of friction, which eventually led to the concept of low frictional strength of faults during propagation of earthquakes.

rotary_apparatus

Fig 1. the Rotary apparatus

In the past two decades, the development of machines capable of shearing natural materials made it possible to achieve direct, experimental evidences of how friction in rocks (and gouges, when pulverised) drops from Byerlee’s values (μ=0.6-0.8) towards zero when approaching seismic velocities (>10 cm/s) and this independently of the rock composition.

However, even though a common bulk behaviour is witnessed, the weakening mechanisms that operate at the microscale are strongly dependent on the mineralogy and, despite a large amount of literature focused on this research, they are still poorly understood as their physic is an evergreen matter of debate.

My Ph.D. focuses on a weakening mechanism that has been recently proposed to occur in carbonate faults: viscous flow by grain boundary sliding, a diffusion creep dominated process particularly efficient in fine grained aggregates. In order to verify and characterise this hypothesis we try to reproduce coseismic shear conditions in pure calcite (CaCO3) gouges with a Low to High Velocity Rotary (LHVR) apparatus (Figure 1). This machine allows to simulate arbitrary amounts of slip in a thin volume of gouge, our experimental fault core, which is squeezed between two hollow cylinders. A piston located in the lower part of the apparatus lifts the lower cylinder producing an axial load (up to 25MPa) perpendicular to the plane of slip while the top cylinder spins at angular velocities up to 1500rpm (1.4 m/s tangential velocity at the reference radius).

rotary_lrDuring the experiments we record different mechanical parameters that can be processed to obtain: displacement, velocity, axial stress, shear stress, axial displacement and, with an opportune equation, the estimated temperature in the shear zone. The ratio between shear stress and axial stress gives the friction coefficient that produces a classic weakening profile when plotted against the displacement as in the graph of figure 2, where are evident two main stages: pre-weakening (μ>0.6) and weakening stage (μ<0.3).

At the end of each experiment we carefully remove the sheared sample in order to make microstructural analysis. We describe the architecture of the shear zone mainly by acquiring electron backscattered (EBS) images (figure 3) on polished sections of the samples using a scanning electron microscope. We are also planning to use cathodoluminescence and EBS diffraction to study in detail the distribution of strain, temperature and hidden geometries.

By coupling the mechanical data and the microstructural analysis of experiments stopped at different amounts of slip we are able to reconstruct the evolution of the shear zone, including the transition between a pre-weakening brittle behaviour to the steady state weakening stage where ductile-plastic processes are dominant. Understanding how the internal architecture of the shear zone changes with time and measuring its geometrical features is of paramount importance to achieve a quantitative description of the processes, which can lead to new physical laws.

With our experiments we are trying to link a qualitative description of complex natural processes and quantitative simulations based on the current physical knowledge. As a matter of fact, the obtained microstructures can be compared to natural equivalents while mechanical data and inferred laws can be implemented in numerical models.

weakening_profile

Fig 2. Weakening profile

sem_image

Fig 3. SEM BSE image of a cross section of the slip zone

Minds over Methods: Numerical modelling

Minds over Methods: Numerical modelling

Minds over Methods is the second category of our T&S blog and is created to give you some more insights in the various research methods used in tectonics and structural geology. As a numerical modeller you might wonder sometimes how analogue modellers scale their models to nature, or maybe you would like to know more about how people use the Earth’s magnetic field to study tectonic processes. For each blog we invite an early career scientist to share the advantages and challenges of their method with us. In this way we are able to learn about methods we are not familiar with, which topics you can study using these various methods and maybe even get inspired to use a multi-disciplinary approach! This first edition of Minds over Methods deals with Numerical Modelling and is written by Anouk Beniest, PhD-student at IFP Energies Nouvelles (Paris).

 

Approaching the non-measurable

Anouk Beniest, PhD-student at IFP Energies Nouvelles, Paris

‘So, what is it that you’re investigating?’ It’s a question every scientist receives from time to time. In geosciences, the art of answering this question is to explain the rather abstract projects in normal words to the interested layman. Try this for example: “A long time ago, the South American and African Plate were stuck together, forming a massive continent, called Pangea, for many millions of years. Due to all sorts of forces, the two plates started to break apart and became separated. During this separation hot material from deep down in the earth rose to the surface increasing the temperature of the margins of the two continents. How exactly did this temperature change over time, since the separation until present-day? How did this change affect the basins along continental margins?”

These are legitimate questions and not easy to answer, since we cannot measure temperature at great depth or back in time. In this first post on numerical methods, we will be balancing between geology and geophysics, highlighting the possibilities and limits of numerical modelling.

The migration of ‘temperature’ through the lithosphere is a process that takes time and depends heavily on the scale you look at. Surface processes that affect the surface temperature can be measured and monitored, yielding interesting results on the present-day state and variations of the temperature. The influence of mantle convection cycles and radiogenic heat production are already more difficult to identify, take much more time to evolve and might not even affect the surface processes that much. Going back in time to identify a past thermal state of the earth seems almost impossible. This is where numerical models can be of use, to improve, for example, our understanding on the long-term behaviour of ‘temperature’.

Temperature is a parameter that affects and is affected by a variety of processes. When enough physical principles are combined in a numerical model, we can simulate how the temperature has evolved over time. All kinds of different parameters need to be identified and, most importantly, they need to make sense and apply to the observation or process you try to reproduce. Some of these parameters can be identified in the lab, like the density or conductivity of different rock types. Others need to be extracted from physical or geological observations or even estimated.

Once the parameters have been set, the model will calculate the thermal evolution. It is not an easy task to decide if a simulation approaches the ‘real’ history and if we can answer the questions posed above. We should always realise that thermal model results at best approach the real world. We can learn about the different ways temperature changes over time, but we should always be on the hunt to find measurements and observations that confirm what we have learned from the simulations.

temperature_quick

 

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