TS
Tectonics and Structural Geology

Analog modelling

Minds over Methods: Reconstruction of salt tectonic features

Minds over Methods: Reconstruction of salt tectonic features

What is the influence of salt tectonics on the evolution of sedimentary basins and how can we reconstruct such salt features? Michael Warsitzka, PhD student at the Friedrich Schiller University of Jena, explains which complementary methods he uses to better understand salt structures and their relation to sedimentary basins. Enjoy!

 

Credit: Michael Warsitzka

Reconstruction of salt tectonic features from analogue models and geological cross-sections

Michael Warsitzka, PhD student, Institute of Geosciences, Friedrich Schiller University Jena

Salt tectonics, as a sub-discipline of structural geology, describe deformation structures developing due to the special deformation behaviour of salt (as synonym for a sequence of evaporitic rocks). Salt behaves like a viscous fluid over geological time scales and, therefore, it may flow due to lateral differences in thickness and density of the supra-salt layers. This influences the structural evolution of sedimentary basins, because salt flow can modify the amount of regional subsidence of the basin. Local sinks (“minibasins”) develop in regions from where salt is squeezed out and salt structure uplifts, e.g. diapirs or pillows evolve in regions of salt influx. Unfortunately, temporal changes of salt flow patterns are often difficult to reconstruct owing to enigmatic ductile deformation structures in salt layers. Understanding the evolution of salt-related structures requires either forward modelling techniques (e.g. physically scaled sandbox experiments) or restoration of sedimentary and tectonic structures of the supra-salt strata.

In my PhD thesis, I tried to integrate both, analogue modelling and restoration, to investigate salt structures and related minibasins developed in the realm of extensional basins. The sandbox model is a lab-scale, simplified representative of natural salt-bearing grabens, e.g. the Glückstadt Graben located in the North German Basin (Fig. 1). A viscous silicone putty and dry, granular sand were used to simulate ductile salt and brittle overburden sediments. Cross sections were cut through the model at the end of each experiment to conduct reconstruction of the final experimental structures. The material movements were monitored with a particle tracking velocimetry (PIV) technique at the sidewalls of the experimental box.

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Fig 1: 2D restoration of the supra-salt (post-Permian) strata in the Glückstadt Graben (Northern Germany). Credit: Michael Warsitzka

Using experimental and geological cross sections, structures in the overburden of the ductile layer can be reconstructed, if present-day layer geometries and lithologies of the overburden strata can be identified. From natural clastic and carbonatic sediments we know that they compact with burial, reducing the layer thickness. Therefore, the reconstruction procedure sequentially removes the uppermost layer and layers beneath are decompacted and shifted upwards to a horizontal surface (Fig. 2). The sequence of decompaction and upward shifting is then repeated until the earliest, post-salt stage is reached (Fig. 1). It intends to restore the initial position, shape and thickness of each reconstructed layer.

In analogue experiments, no decompaction is necessary, because the compressibility of the granular material is insignificant for depths of a few centimetre. Restoration can be directly applied to coloured granular layers revealing detailed layer geometries for each experimental period (Fig. 2a). The PIV technique displays coeval material movement and strain patterns occurring during the subsidence of the experimental minibasins (Fig. 2b). Based on the observation that the experimental structures resemble those reconstructed from the natural example (Glückstadt Graben during the Early Triassic, Fig. 1), it can be inferred that strain patterns observed in the experiments took place in a similar manner during the early stage of extensional basins. This demonstrates the advantage of applying both methods. First, original geometries of basin structures can be determined from the restoration and then reproduced in the model. If the restored geometries are suitably validated by the models, the kinematics observed in the model can be translated back to nature and help to understand the effect of salt flow on the regional subsidence pattern.

Fig 2: Result of an analogue model showing (a) reconstructed sand layers restored from a central cross section, and (b) monitored displacement and strain patterns in the viscous layer above the left basal normal fault. Credit: Michael Warsitzka

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.

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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.

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