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Geothermal Energy and Structural Geology?

Geothermal Energy and Structural Geology?

Fieldwork is a necessity to expand the brain, to kick-start 3D thinking. Field studies with a specific application in mind have – until now – usually been geared towards hydrocarbon reservoirs. However, with the increasing use of the subsurface, for example for CO2 storage and geothermal energy, alternative field studies gain importance. Here, we will focus on geothermal energy, which is in many countries a prime target of the energy transition. Around half of our energy consumption is simply the need for heat for houses, industry and other applications. Of course, some tectonic settings lend themselves easier to obtaining heat than others. In settings with volcanoes (rift and subduction zones), countries such as Iceland and Italy are frontrunners in developing high temperature (>100⁰C) geothermal energy. No matter which geographic location and which heat source is targeted, all geothermal energy reservoirs consist of the same components. This blog is about the holy trinity of a usable geothermal system: the heat source, the fluid, and the fluid pathways. With this blog, we will take you along on a yearly field trip to discover how outcrops of intrusions can be used to teach BSc, MSc and PhD students about geothermal energy. We go to the Larderello area and Elba. Tuscany, due to the extensional setting, is characterized by a high heat flux, as much as 150 mW/mm2 (compared to 40 mW/mm2 in most regions). This high heat flux means that it is a prime area for geothermal energy. This trip is organized by Prof. David Bruhn (TU Delft, the Netherlands, and GFZ, Germany), together with the invaluable aid of Prof. Domenico Liotta (University of Bari Aldo Moro, Italy).

Left: Fumaroles and geothermal power plants (with crosscutting pipelines) dominate the view in the Larderello area. Credit: Anne Pluymakers.

Discussions on top of fumaroles, where steam escapes through fracture networks. Credit: Anne Pluymakers.

 

To better understand the technological background, we first explore what a currently active geothermal system looks like. For this, we go to the Larderello area, a beautiful region near the Tuscan coast, on western edge of the Apennines. Here, geothermal energy is visible both through the local presence of fumaroles, as well as by the omnipresent pipelines and cooling towers. The pipelines are sometimes colored green to be less conspicuous. One could find them ugly, but given that they provide 3% of Italy’s energy plus employment throughout the region, they serve a great purpose! In this area abundant heat is present, which is tapped by in total 300 extraction wells and 34 power plants (run by Enel Greenpower). There are another 100 re-injector wells, to ensure the extracted steam and water are re-injected into the aquifers at depth. An example: the geothermal powerplant we visited is fed by 20 boreholes, spread over 7 km2.

An active drill site in the Larderello area. Credit: Anne Pluymakers

Tourist well: 200°C steam, at 2 bars, and 10 tonnes of steam per hour. Credit: Anne Pluymakers

The two main geothermal reservoirs in this region are (1) a shallow reservoir, with 200°C and 2 bar pressure, and (2) a deep (2 km) reservoir, with the same temperature but then at high pressure, 7 bar (and higher). The heat is carried by steam. The pressures aren’t very high, but the flow rates are enormous. This is demonstrated by the ‘tourist well’, that is now no longer in use. It contains 200°C steam, at just 2 bars – an average biketire carries between 2 and 5 bars. However, this tourist well produces 10 tonnes of steam per hour – so standing on the 85 decibel line it clearly shows the power of the earth! In this region, the power plants function on higher pressure, which renders this specific well not economically viable. Economic interest is an interplay between offer and demand, and as such is location-dependent. In other words, also here it would be technically possible to convert the heat from the tourist well into energy. To go from steam to electricity a turbine and generator are necessary. However, the steam is contaminated with fluids and other gases, which need to be separated before the steam can power the turbine and generator. Precipitation of salts in the water can lead to one of the main production issues associated with geothermal wells: scaling.

 

Scaling, or the build-up of precipitate inside pipelines, can be a big issue in geothermal power plants. Credit: Anne Pluymakers.

Scale model of the first “hot pond under cover” for boric acid extraction, where the boilers were powered by steam (display in the Geothermic Museum of Larderello, photos by Anne Pluymakers)

 

Fun fact: the first use of geothermal waters in the region was to produce boron salts. Production of boric acid and its sodium salt, borax, started as early as 1818, though mass production initiated in 1827. Francesco Larderel built a covered hot pond to gather natural steam, needed to feed the boric water evaporation boilers. By the start of the 20th century, the Larderello region produced electricity for local use.

 

Geological map of Elba island (from Rocchi, S., Westerman, D. S., Dini, A., & Farina, F. (2010). Intrusive sheets and sheeted intrusions at Elba Island, Italy. Geosphere, 6(3), 225–236. https://doi.org/10.1130/GES00551.1)

 

After this demonstration of a working geothermal system at the earth’s surface, the excursion moves on to the geological analogue. Analogue field studies are a common way of imagining what the subsurface could look like in the targeted region. Therefore, we move to the island of Elba, just off the coast of Tuscany. The equivalent heat source is the Monte Capanne pluton, on the west of the island. The first evidence of geothermal activity constitutes of precursor dike and sill intrusion into an ophiolite sequence. Large scale melting happened in the Miocene, at a depth of 6 to 7 km. To generate these volumes, water had to be present – this geological evidence then represents the ‘mother’ of the heat flux, and where the first geothermal fluids on Elba have come from. In the outcrops of the Monte Capanne pluton xenolites and large feldspar crystals are visible. The feldspar crystals indicate large convection patterns in what must have been viscous, cooling magma. After cooling, this pluton was fractured, and meter-sized mafic intrusions with fine-grained crystals share orientations with these fractures. Analysis of fluid inclusions in the outcrop indicate that the fluids are all relatively local. That is not surprising, since upon cooling, felsic magmas release large amounts of water, and also the surrounding meta-sediments (i.e. former ocean floor) were full of water.

Where there is a magma, there are interesting percolating fluids, with all kinds of rare and precious minerals. As can already be seen in the geological map, the evidence of fluids is obvious throughout the island, in the form of several mines, especially on the east side. In the east, the people have mined for iron for decades, specifically for hematite, limonite, pyrite, magnetite and ilvaite. Of these, ilvaite is a mineral that is quite specific to Elba. On the west, closer to the pluton, we can find tourmaline, beryllium, orthoclase and quartz.

Mine on Elba island. The elongated shape follows the fault path. Credit: Anne Pluymakers

Many of these mines follow distinct pathways: those of fractures and faults, demonstrating also in this fossil system the importance of fractures and faults as fluid pathways (i.e. compare the photo of the small scale fracture network on Elba to the photo of the fracture networks associated with the fumaroles). Elba hosts a famous shear zone: the Zuccale fault. This outcrop contains many textbook structures, from brittle to ductile deformation.

The less weathered mineral veins provide clear evidence of past fluid pathways. A key message for geothermal energy recovery that can be obtained from outcrops like these, is that permeability is directional and local. The fracture zone itself is asymmetric along the shear zone. The shear zone is impermeable across fault, but not necessarily as impermeable along the fault. As an engineer responsible to determine the best location to drill a well (approximately one million euro per km!), where would you go?

 

Brittle (left) and ductile (right) deformation at the Zuccale fault outcrop. Credit: Anne Pluymakers.

 

An earlier version of this blog appeared on the Focus on vips blog. This is a slightly modified version.

Trieste, where the word Karst originates

Trieste, where the word Karst originates

The city of Trieste lies in north-eastern Italy along the border with Slovenia. It is positioned at  the corner point between the Romance, Germanic and Slavic worlds and serves as an important seaport in the region. It is fascinating for both its history and geology. My relationship with Italy’s town of Science, as Trieste is often referred to, started about a year ago. I got the opportunity to start a PhD focused on imaging evaporates, at the University of Trieste and the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS). Walking around the city, two elements caught my attention. First, I was impressed by the topography of the area (Figure 1), the city itself is bordered by beautiful abrupt cliffs and steep Carsic hills plunging into the Adriatic Sea.  Second, I found myself surprised I was in Italy, as the Austro-Viennese architecture dominates the streets. While I was marvelling at the beauty and size of the buildings, which nowadays testify of a thriving and multicultural past, I have started to wonder why I had never heard about this city before. The history of Trieste is out of topic of this article, but I invite you to check it out, as there are many lessons that can be learned regarding Identity politics. So, why should Trieste appear in the ‘Geology in the city’ series? First let’s zoom out and review the regional geological context.

 

Figure 1. The carbonate cliffs of the Strada Napoleonica at sunset, an idyllic spot for climbing and wandering. The subvertical strata represent the southwestern flank of the Trieste – Komen anticlinorium developed above the NW-SE Karst Thrust. The strata visible here are the latest limestone units, Paleocene to Eocene in age. The lower part of the slope covered by vegetation is made of Eocene Flysch. Credit: Simon Blondel.

 

A natural fortress

Trieste is built along the north-eastern Adriatic coast, where it lies on top of Eocene flysch, which consist of silty marls interbedded with sandstones. They don’t outcrop much, but you can see evidence of these all around the city. The sandstone interbeds were used as building blocks, regionally called “masegni”, to build the city. For example, the Molo Audace (an iconic pier at the harbour) and the pavement of the city’s main square: the breath-taking Piazza dell’Unità d’Italia, are made of these “masegni” (Figure 2).

 

Figure 2. View of Città nuova from Molo Audace, with Piazza dell’Unità d’Italia at the centre. The pier is built above the wreck of ship that sank in the port of Trieste in 1740. It is made of “masegni” stones extracted from the sandstone units of the Eocene Flysch. Credit: Simon Blondel.

 

As you go inland, the topography abruptly steepens toward the Karst plateau. It rises 400 m above the Gulf of Trieste, overhanging the city and confining it with cliffs and walls, to the delight of many climbers. When I say Karst I mean the geographical name given to this area and not the geological term, but I’ll come back to that a bit later. After this exhausting ascent, especially if you decide to do it by bike (my first excursion with a single-geared Dutch bike was quite gruelling), you can relax and wander around the flat “Altopiano Triestino”. It consists of limestone and dolomitic limestone formations, Valanginian to early Eocene in age. They are assimilated to a succession of carbonate platforms, whose nomenclature was recently unified within the framework of the HYDROKARST European project in 2015 (http://book.hydrokarst-project.eu/files/assets/basic-html/index.html#1). They were brought to the surface with the collision between the Eurasian and African plates, and nowadays form a wide NW-SE fold – the Trieste – Komen anticlinorium- assimilated to the external Dinaric Imbricated Belt (Figure 3).

 

Figure 3. Simplified geological map of the Gulf of Trieste. The A-B line represents the position of the cross-section in Figure 4. The green part corresponds to the seismic data, the purple part corresponds to the schematic cross-section. Trieste lies above Eocene flysch, at the front of the NW-SE Karst Thrust that delimit the Karst plateau. Offshore, the Karst’s carbonate platform is repeatedly thrusted and folded until the margin of the platform. Courtesy of Martina Busetti.

 

The anticlinorium overthrusts the Eocene flysch that deposited at its foreland (the Gulf of Trieste), along the NW – SE Karst and Palmanova thrusts (Figure 4). The Karst’s carbonates were also widely used in construction and there are many quarries in the surroundings. One of the most famous is near Villaggio del Pescatore, where a perfectly preserved skeleton of Tethyshadros insularis (Antonio to his friends) was discovered in 1994 (Figure 5).

 

Figure 4. Seismic (data acquired in the gulf) and geological schematic cross-section (of the Karst Plateau) perpendicular to the Karst Thrust, as shown on Figure 3. The Carbonate platform is folded and thrusted along the Karst thrust, overlapping the younger Eocene Flysch along the coast. Offshore the Flysch are covered by Pliocene and Pleistocene sediments, and onlap the folded Carbonate platform. Courtesy of Martina Busetti.

 

Figure 5. The reconstructed skeleton of Antonio in a former limestone quarry located between Duino and Villaggio del Pescatore). The original fossil was moved to the Museum of Natural History in Trieste. Credit: www.dinosauroantonio.it

From Kras to Karst

As stated previously, the word Karst is both a geographical and geological term in the region. At the beginning, this was just the name given to the hinterland of the Gulf of Trieste. It is supposed to be of pre-Indoeuropean origin from the root kar-, meaning rock, to describe these unusual and barren lands that were considered hostile for travellers. This term is still preserved in the Italian Carso, Slovenian Kras and German Karst. On the surface, the Karst plateau is strewed by numerous dolines and caves, which are the most dominant features that characterize it. There are many other geomorphological features that can be observed, such as the campi solcati (limestone pavement) or the kamenitze (natural ponds) in Val Rosandra (Figure 6). These natural ponds result from millions of years of erosion and dissolution by meteoritic waters of pre-existing carbonates.

 

Figure 6. From left to right, waterfall emerging from the Karst in Val Rosandra, The Val Rosandra valley with numerous Kamenitze and typical Carsic vegetation, the Škocjan caves where the Reka river plunges beneath the surface for a 40 km journey underground. Credit: Simon Blondel.

 

The use of the word Karst, as a general term to describe areas exhibiting various geomorphological features linked to erosion and dissolution, became widely accepted by the scientific community during the 19th century. Back then, Trieste was one of the most dynamic and multicultural city of the world under the Austro-Hungarian empire. Most of the population was Slovenian-speaking in the hinterland, therefore the region was referred as Kras by the locals. However, the geoscientists who started to investigate the origin of its features in more details were German-speaking, hence explaining why the German version has been retained for the geological term. What triggered these studies was principally the need for water.  As the population grew, so did the need for freshwater, which led the city to launch investigations of the nearby Karst. Unfortunately for them, the water was too deep (at about 300 m below the surface, close to the sea level) to be easily pumped for the city. But in their quest for groundwater, the scientists discovered a dense and sinuous network of cavities and tunnels dug by the groundwater, close to the current sea-level. They discovered that a clear majority of the Karst’s underground water is captured by the Reka river that plunges underground through the ponor (natural sinkhole where surface water enters) of Škocjan, a UNESCO Natural Heritage site in Slovenia (Figure 7). It reappears out from the underworld 40 km away from Škocjan, at the Timavo springs, where it nourishes the 2 km long river that outflows into the Gulf of Trieste.

The Karst Plateau hosts numerous caves and I will only mention one to conclude: the Grotta Gigante. It was discovered in 1840 and currently is both a tourist attraction and an important location for scientific studies. It hosts a seismological station, a horizontal pendulum and a clinometer to calculate the movements of the earth’s (Figure 7), as well as a measuring station for water percolation and concretion, a monitoring system for Radon gas, and finally a probe sensor measuring the temperature and the electric conductivity of groundwater. Some of these are operated by the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale and the Department of Mathematics and Geosciences of the University of Trieste.

 

Figure 7. The Reka river flowing underground in the Škocjan caves (left) and the Grotta Gigante where we can notably see the two giant tubes protecting the horizontal pendulum used to calculate the movements of the Earth’s crust. Credit: Simon Blondel.

 

After reading this introduction to Trieste and its geology, it is time to come to visit us. The city is currently preparing to host the EuroScience Open Forum – ESOF 2020, so you’ll hear the name again. In the meantime, stay tuned to our ETN SALTGIANT’s research, a project funded by the European Union’s Horizon 2020 research and innovation grant, and follow us on LinkedIn and Twitter.

I would like to thank Martina Busetti and Michela Dal Cin (OGS), who reviewed and contributed to the writing of this article.

Minds over Methods: Dating deformation with U-Pb carbonate geochronology

Minds over Methods: Dating deformation with U-Pb carbonate geochronology

Credit: Nick Roberts

For this edition of Minds over Methods, we have invited Nick Roberts, a research scientist at the British Geological Survey, working within the Geochronology and Tracers Facility (GTF) running a LA-ICP-MS laboratory. Nick has a background in ‘hard-rock’ geology, incorporating geochemistry, geochronology, and magmatic and metamorphic petrology across a wide range of tectonic settings, and is now involved in projects covering a breadth of earth system science, from Archean ore deposits to early hominid evolution. Nick is experienced in developing LA-ICP-MS U-Pb methods. He was involved in the first characterisation of a natural carbonate for use as a reference material, and in demonstrating the applicability of LA-ICP-MS U-Pb carbonate geochronology to a number of key applications, such as dating brittle deformation, ocean crust alteration, and paleohydrology.

 

Faults and fractures are ubiquitous in the Earth’s upper crust. As well as providing deformation histories of basins and orogens, they are critical for understanding the formation, migration and storage of natural resources. Determining the absolute timing of fault slip and fracture opening has lacked readily available techniques. Most existing methods require specific fault gouge mineralogy that is not always present, e.g. K-Ar illite dating. Other methods require a specific composition of fault-hosted mineralisation, e.g. U-Th/He dating of hematite, Th-Pb dating of hydrothermal monazite, U-Pb/U-Th dating of opal, and U-Pb/U-Th dating of carbonate. The latter is the most widely applicable, since carbonate minerals (e.g. calcite, dolomite) are common to many fault and fracture systems throughout a wide range of geological settings. The ability to date carbonate mineralisation with the popular method of U-Pb Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS), is opening up new doors in tectonics and structural geology.

 

The method

The key to the LA-ICP-MS method is its high spatial resolution (~20-200 microns); the method involves sampling of material with a laser and measuring the composition of that material with an ICP-MS. Uranium concentrations in carbonate are low when compared to most other U-Pb chronometers, typically 10 ppb to 10 ppm, which is one or two orders less than a typical zircon. Uranium concentration, particularly in vein-filling calcite, can also be highly variable within in a single sample, spanning orders of magnitude over length-scales of 10s of microns or less. A major benefit of LA-ICP-MS dating, is that this variation in composition can be targeted and sampled with the laser, which can lead to precise age estimates through measurement of wide-ranging U/Pb ratios (see example in Figure 1). High uranium zones can also be rather elusive and searching for a needle in a haystack is often an appropriate analogy. Another benefit of LA-ICP-MS is that many samples can be screened in a single session, to select those of the most favourable composition for dating, and to find within those samples the regions with highest U. The method can be utilised on polished thick sections or blocks, or chips or grains mounted in epoxy-resin blocks. This in situ technique makes it highly versatile and allows for complementary analysis that are crucial for providing context to the dates, such as cathodoluminescence, optical microscopy, trace element mapping, and in-situ Sr isotope analyses.

 

Figure 1. Example of vein-filling calcite with LA-ICP-MS elemental map of uranium, and corresponding U-Pb concordia (Tera-Wasserburg) plot from LA-ICP-MS U-Pb spot analyses across the same region. Credit: Nick Roberts

 

Dating deformation

Carbonate, primarily calcite, occurs as vein-filling mineralisation in fractures and along fault planes (see Figure 2 for examples). Such occurrences of carbonate have precipitated from a fluid, and thus, provide a record of past fluid-flow. Carbonate vein-fill can also be used to provide absolute timing constraints on brittle deformation. Carbonate that infills a fracture provides a minimum age for the timing of fracture opening. Many fractures and fault planes exhibit multiple opening and/or slip events that lead to multiple episodes of carbonate precipitation. From these, carbonate dates can be used to bracket the timing of fault slip, and potentially provide constraints on the longevity and periodicity of fault slip events.

Figure 2. Examples of calcite vein-fill in various faults and fracture types. Credit: Nick Roberts and Jonny Imber

 

Good dates need good petrography

The key to dating fault slip and fracture opening is detailed petrography. Vein mineralisation takes on a wide variety of growth textures, and these can used to infer the history of mineral precipitation as well as fault kinematics. For example, veins may exhibit syntaxial (growth from the margin towards the centre), antitaxial (growth from the centre towards the margin) or with no preferred growth direction. Calcite morphologies are wide-ranging, from flattened, to blocky, to elongate, to fibrous (see example in Figure 3). Calcite can also exhibit deformation twinning which can be used to infer the strain regime. These textures and morphologies, along with features such as inclusion trails, shear bands and healed fractures, allow us to interpret the combined history of fracture opening, displacement and fluid precipitation.

Figure 3. Example of a complex calcite fracture-fill from a mudstone-hosted normal fault, showing cathodoluminescence and reflected light imaging, schematic textural interpretation, LA-ICP-MS trace element mapping, and a simple model of vein evolution. Credit: Nick Roberts

 

Carbonate – a treasure trove of geochemical proxies

One of the key benefits to dating carbonate mineralisation, is that its elemental and isotopic composition provides a wide-ranging archive of fluid chemistry and temperature. Traditional methods include stable carbon and oxygen isotopes and radiogenic Sr isotopes; these provide information on the source of fluids and allow the characterisation of subsurface fluid pathways. Trace elements provide further information on fluid source and rock-fluid interaction and are particularly useful for characterising proxies such as the reducing potential of precipitating fluids. Clumped isotopes are a novel method that has been explored over the last decade and provide a proxy for the fluid temperature during carbonate precipitation. In combination with U-Pb dating, clumped isotopes can characterise ancient hydrothermal fluid-flow in the upper crust (MacDonald et al., 2019). A benefit of LA-ICP-MS geochronology, is that various in-situ methods can be combined, allowing fluid composition and dating to be analysed from the same domain. For example, carbon and oxygen isotopes can be measured in situ using an ion microprobe, and Sr isotopes and trace elements can be measured using LA-ICP-MS.

 

Application to tectonics

To date, LA-ICP-MS carbonate geochronology has been applied successfully to a range of tectonic settings to constrain the timing of brittle deformation. These include the far-field effects of the Pyrenean orogeny in southern England (Parrish et al., 2018) and Alpine orogeny in Sweden (Goodfellow et al., 2018), rift-related faulting during opening of the North Atlantic in the Faroe Islands (Roberts & Walker, 2016), graben formation in the Alpine orogen (Ring & Gerdes, 2016), the timing of the Dead Sea and North Anatolian transform fault zones (Nuriel et al., 2017 and 2019, respectively), nappe stacking in the Arabian Peninsula (Hansman et al., 2018), foreland deformation of Sevier-Laramide orogenesis (Beaudoin et al., 2018) and brittle deformation within an accretionary wedge (Smeraglia et al., 2019).

 

Edited by Derya Gürer

References

Beaudoin, N., Lacombe, O., Roberts, N.M. and Koehn, D., 2018. U-Pb dating of calcite veins reveals complex stress evolution and thrust sequence in the Bighorn Basin, Wyoming, USA. Geology46(11), pp.1015-1018.

Goodfellow, B.W., Viola, G., Bingen, B., Nuriel, P. and Kylander‐Clark, A.R., 2017. Palaeocene faulting in SE Sweden from U–Pb dating of slickenfibre calcite. Terra Nova29(5), pp.321-328.

Hansman, R.J., Albert, R., Gerdes, A. and Ring, U., 2018. Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite. Geology46(3), pp.207-210.

MacDonald, J.M., Faithfull, J.W., Roberts, N.M.W., Davies, A.J., Holdsworth, C.M., Newton, M., Williamson, S., Boyce, A. and John, C.M., 2019. Clumped-isotope palaeothermometry and LA-ICP-MS U–Pb dating of lava-pile hydrothermal calcite veins. Contributions to Mineralogy and Petrology174(7), p.63.

Nuriel, P., Craddock, J., Kylander-Clark, A.R., Uysal, I.T., Karabacak, V., Dirik, R.K., Hacker, B.R. and Weinberger, R., 2019. Reactivation history of the North Anatolian fault zone based on calcite age-strain analyses. Geology47(5), pp.465-469.

Nuriel, P., Weinberger, R., Kylander-Clark, A.R.C., Hacker, B.R. and Craddock, J.P., 2017. The onset of the Dead Sea transform based on calcite age-strain analyses. Geology45(7), pp.587-590.

Parrish, R.R., Parrish, C.M. and Lasalle, S., 2018. Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England. Journal of the Geological Society175(3), pp.425-442.

Ring, U. and Gerdes, A., 2016. Kinematics of the Alpenrhein‐Bodensee graben system in the Central Alps: Oligocene/Miocene transtension due to formation of the Western Alps arc. Tectonics35(6), pp.1367-1391.

Roberts, N.M. and Walker, R.J., 2016. U-Pb geochronology of calcite-mineralized faults: Absolute timing of rift-related fault events on the northeast Atlantic margin. Geology44(7), pp.531-534.

Smeraglia, L., Aldega, L., Billi, A., Carminati, E., Di Fiore, F., Gerdes, A., Albert, R., Rossetti, F. and Vignaroli, G., 2019. Development of an Intrawedge Tectonic Mélange by Out‐of‐Sequence Thrusting, Buttressing, and Intraformational Rheological Contrast, Mt. Massico Ridge, Apennines, Italy. Tectonics38(4), pp.1223-1249.

Minds over Methods: Virtual Microscopy for Geosciences

Minds over Methods: Virtual Microscopy for Geosciences
The next “Minds over Methods” blogpost is a group effort of Liene Spruženiece (left) – postdoctoral researcher at RWTH Aachen and her colleagues Joyce Schmatz, Simon Virgo and Janos L. Urai.

Credit: Liene Spruženiece

The Virtual Microscope is a collaborative project between RWTH Aachen University and Fraunhofer Institute for Applied Information Technology (Schmatz et al., 2010; Virgo et al., 2016).

In the multitude of tools to analyze rocks, the optical microscope is still one of the first go-to methods for rock characterization. A look on a petrographic thin section under an optical microscope gives a quick overview of the mineralogy and fabric of a rock and helps to determine the areas of interest for more detailed analysis with other methods. It is also a crucial part for teaching geology classes.

Most geoscience institutions already have a petrographic microscope that is equipped with a camera and maybe an automated stage for capturing image mosaics. However, these images make only limited use of the vast amount of information available, such as the overview of the sample fabric at different magnifications or the change of optical mineral properties (pleochroism and extinction behavior) with the rotation of polarizers. Although there have been recent developments in the field of virtual microscopy (NASA, 2007; The Open University; 2010; Tetley and Daczko, 2014), to our knowledge, none of the existing systems has been able to fully emulate a user experience in a virtual environment that is equal to using a traditional analogue microscope.

 

Figure 1. The setup of the PetroScan microscope. Credit: Fraunhofer FIT

How does it work?

The idea behind the virtual microscope is to capture the full information of entire rock thin-sections in a digital format, including the possibility to switch between plain light and crossed polarizers, rotate the polarizers and zoom everywhere in the sample at high magnifications.

The hardware part of our system consists of a fully automated petrographic microscope, connected to a computer (Figure 1). The software contains three modules; a PetroScan launcher for setting up the image acquisition, optimizer for combining and interpolating the scanned images and a viewer program (Figure 3).

 

Figure 2. Automated sample stage allows capturing high resolution images of entire thin sections by combining thousands of images taken sequentially along a predefined grid. Credit: Jochen Hürtgen

High-resolution mosaics (Figure 2) containing up to a million images can be acquired in a few hours. With crossed polarizers, the scans are performed for several rotation angles. The movement accuracy of the sample stage is about 10 nm. This ensures precise overlap of the scanned mosaics at different rotation angles, where each corresponding pixel has the same x-y coordinates. Thus, the images of each rotation angle are precisely overlapping and allow to interpolate the change in brightness values for every individual pixel in the mosaics, producing smooth curves (Figure 3) that reflect extinction behavior in minerals (Heilbronner and Pauli, 1993).  Each attribute of these interpolated curves (phase, amplitude) of the sample can be viewed at any rotation angle.

 

Quantification of optical images

Figure 3. Screenshot of the PetroScan TileViewer window showing a thin section that is scanned under crossed polarizers. On the right side the scanned image is overlain by the phase map, where each pixel is color-coded according to the mineral extinction angles. Credit: Liene Spruženiece, Joyce Schmatz, Simon Virgo and Janos L. Urai

Virtual microscopy offers powerful means for quantitative analysis of giga pixel images to extract multi-scale information. It has a resolution of up to few micrometers for areas in sizes of 10 square centimeters. In its current form the viewer software that we developed contains a built-in toolbox for displaying and tresholding the intensity, saturation and hue values of the scanned images. This can be used for a quick estimation of sample porosity or proportions of different minerals. In addition, the scans with interpolated crossed-polarizer rotation also contain information of the mineral extinction behavior that is used to produce a “phase map” (Figure 3). The “phase map” displays the variations in the mineral optical axis orientations. Although it cannot provide absolute values for the crystallographic orientations, it allows a clear distinction between differently oriented mineral grains and allows easy visualization of qualitative lattice misorientations inside individual mineral grains. Furthermore, import and export functions are incorporated in the image viewing software. Thus more advanced image analysis can be carried out in specifically designated external softwares (e.g. Fiji/ImageJ, GiS, Matlab, Python), then inserted back in the PetroScan viewer software.

 

Applications in teaching

Figure 4. Mineralogy and fabrics of the scanned samples can be segmented and quantified using built-in toolboxes. The recognition of features is greatly improved by the possibility to zoom in the samples, switch between the plain light and crossed polarizers and rotate the polarizers. Credit: Liene Spruženiece, Joyce Schmatz, Simon Virgo and Janos L. Urai

At RWTH Aachen University, virtual petrography has been used as a teaching tool for several years. For example, it is extensively used in the microtectonics course. Each class consists of a short introduction in some of the common rock microstructures, such as veins, cataclasites, mylonites, dissolution-precipitation features and others. This is followed by an hour-long presentation by a student group, where they describe and discuss the respective microstructure in a scanned thin-section, projected to a screen with a high-resolution beamer. The students use laser pointers for characterizing the features and origins of the microstructure, switch between views in plain-polarized or crossed-polarized light, zoom in and out across the sample, adjust illumination settings, rotate polarizers and do basic image analysis (Figure 4). This has proven to be a highly engaging teaching method. The students in the audience question the presenting group asking to provide a closer look or more details on any interesting microstructural feature. Often hypotheses are formed by the listeners and immediately tested by the presenters, allowing to perform an investigation and agree on a reasonable explanation at the end of the class. Virtual petrography can be especially important in teaching institutions that do not own well-equipped microscopy labs. It only requires a computer and a projector. The thin section scans and PetroScan viewing software will be available for download online. Thus, such a method will not incur additional costs to the institutions, at the same time will provide a large collection of a high variety of geological samples.

 

The vision

We imagine a world-wide community built around the platform of virtual microscopy, where the information from different analytical methods can be exchanged between users and stored in open databases, available for teaching or research purposes. Many advantages arise from such a transformation. Work can be carried out anywhere without a need to access microscopes, several users can simultaneously view the same samples, data can be easily quantified and integrated between different methods, and thin-section libraries can be shared and exchanged by the user community.

Further plans for developing this method include collaboration with the computer vision department at RWTH Aachen University in order to create deep learning algorithms that allows quick and precise segmentation of rock microfabrics, such as mineral content and distribution, grain boundaries, grain sizes, porosity, etc. This has been made possible by recently obtained funding from the RWTH Aachen University “Exploratory Research Space – ERS”. The final data set will be public and shared with communities.

 

Edited by Derya Gürer

 

References

Heilbronner, R.P., Pauli, C. (1993). Integrated spatial and orientation analysis of quartz c-axes by computer-aided microscopy. Journal of Structural Geology, 15, 369-382.

Tetley, M.G., Daczko, N.R. (2014). Virtual Petrographic Microscope: a multi-platform education and research software tool to analyse rock thin-sections. Australian Journal of Earth Sciences 61, 631-637.

NASA (2007). Virtual Microscope. Available at: http://virtual.itg.uiuc.edu/

Schmatz J., Urai J.L., Bublat, M., Berlage, T. (2010). PetroScan – Virtual microscopy. EGU General Assembly, EGU2010-10061.

The Open University (2010). The Virtual Microscope for Earth Sciences Project. Available at: http://www.virtualmicroscope.org/.

Virgo S., Heup, T., Urai J.L., Berlage, T. (2016). Virtual Petrography (ViP) – A virtual microscope for the geosciences. EGU General Assembly, EGU2016-14669.

From Mountains to Modernists: the geological foundations and inspirations of Barcelona

From Mountains to Modernists: the geological foundations and inspirations of Barcelona

Barcelona is a vibrant city on the Mediterranean coast, nested snugly between the sea and the Collserola Ridge of the Catalan Coastal ranges. The story of Barcelona starts around 2000 years ago as an Iberian settlement, owing to its strategic location on the coastal route connecting Iberia and Europe. The combination of easily defendable ground and the fertile soils of the Besos and Llobregat deltas have helped shape its history as an important trade hub and place of varying political importance. However, the rocks on (and of) which the city is built tell a much longer and intriguing story. Today we take a close look at the rocks that built Barcelona, and the fascinating history that produced them.

Our story begins in the Paleozoicum, when the oldest rocks outcropping near Barcelona were formed. Iberia collided with the remains of Gondwana and became part of the supercontinent Pangea. During this period, the Hercynian orogeny, intense metamorphism produced  part of the rich mineral assemblage found in the Collserola Ridge today (although it should be noted that collecting rocks is forbidden, as the ridge enjoys a Natural Park status). During the Mesozoic, when Pangea broke apart and Iberia developed into a separate plate, the area that is now Catalunya was mostly submerged under a sea. Most of the sediments deposited during this stage have however been eroded away. This was due to the uplift of the Coastal Ranges during the Pyrenean orogeny in the Paleogene, as Iberia reconnected with Europe between 55 and 25 million years ago. The Catalan Coastal Range was rotated anticlockwise, making it run oblique to the N-S contraction in the Pyrenees. As a result of this uplift, the Collserola Ridge still has an elevation of up to 500 meters, providing sweeping vistas over the city and Mediterranean. During the middle Miocene (around 14 million years ago) the current Barcelona was a coastal area, where the erosional products of the Coastal Ranges were deposited under varying sea levels. Extension started in this period, and continues up to today along reactivated normal faults associated with the Valencia Trough.

This geological map shows the modern city and the distribution of bedrock types underlying it. Credit: Google Earth, Geological Map adjusted from Sergisai.

 

Montjuïc – foundations and façades

So how has this geological history affected the city of Barcelona? The most obvious influence is in the very materials used for constructing it, leaving clues to the geological history of Catalonia all over the city. Situated to the south of Barcelona’s centre is Montjuïc, a ~200 m high block of Tertiary deltaic sediments, product of erosion from the Collserola mountains. The block was tilted due to recent extension, and the surrounding materials eroded, leaving it as an isolated hill surrounded by the alluvial plain filled with modern sediments.

Left: Study of Montjuïc Quarry Artists: Enric Galwey, Barcelona, 1864-1931. Credit: Museu Nacional. Right: View of Montjuïc castle. Credit: barcelonaconnect.com

On top of the Montjuïc hill sits an imposing castle with 360views over the city, built in the seventeenth century. It was used for defensive purposes, for example as a base by Catalan forces fighting a Spanish army during the Battle of Montjuïc in 1641, but also to control the city itself, being used for bombardments and imprisonment at various occasions since the fall of Barcelona during the War of the Spanish Succession (1701-1714). Its bloody history continued into the Spanish Civil War (1936-1939). Montjuïcs most important mark on the city however has been the sandstone quarried there and used in many of Barcelona’s most iconic structures. The very short distance from the source area has caused a rich and poorly sorted mineral assemblage, with the metamorphic minerals of the Collserola Ridge somewhat preserved. The sediments are a sequence of conglomerates, sandstones, mudstones and marls due to the changing sedimentary conditions with variation of sea levels. Although the main mineral assemblage is quartz and feldspars, it includes small amounts of biotite, muscovite, zircon, chlorite and tourmaline, adding to the vibrant colours of the sediments. Montjuïc stone’s high silica content (making it exceptionally durable) and vibrant colours made it a cherished resource, therefore being quarried from Iberian times well into the 20th century. Amongst others, the Palau de la Genaralitat, City hall, Catedral de Barcelona and Barcelona University were constructed from Montjuïc stone. A notable example is the Santa Maria del Mar church, of which the construction formed the backdrop for the 2006 novel (and 2018 Netflix drama) by Idefonso Falcones, La Catedral del Mar, telling the story of a stoneworker and illuminating the hardships and triumphs of medieval life in Barcelona.

 

Antoni Gaudí – the modern face of Barcelona

House Casa Mila (La Pedrera) in Barcelona building by the great Spain architect Antonio Gaudi. Credit: Adobe Stock.

Apart from the materials found in Barcelona’s buildings, their design has also been influenced by the impressive geology found on the Iberian peninsula. The most obvious example of this is found in the works of Antoni Gaudí, the architect who has had perhaps the biggest single-handed influence on the face of the city.

Besides from his most famous works, the (still unfinished) Sagrada Família Cathedral, Casa Battló, Casa Milà and Parc Güell, he designed about ten more iconic buildings, and hundreds of other objects. Part of his architectural genius came from the inspiration he pulled from landscapes surrounding Barcelona, leading him to become one of the founders of the school of organic architecture. The Prades Mountains in the region of Terragona bear a striking resemblance to some of the design element used in Casa Battló and La Pedrera,while the beautiful l´Argenteria in the Collegats gorge in the Pyrenees has been rumoured to have inspired the Nativity façade of the Sagrada Familia (see the photos below). The gorge was incised into Tertiary conglomerates deposited on the interface between the (then endorheic) Ebro Basin and the Pyrenees, subsequently affected by karstification and finally covered in travertine precipitated from natural spring water, resulting in this spectacular geological feature. Gaudi’s works give the city an unique and organic feel, and form part of the main tourist attractions.

 

Left: L’Argenteria, Colgats gorge. Right: Nativity Façade of the Sagrada Familia. It is easy to see how Gaudi could have drawn inspiration from the beautiful cliff for some of the elements of the façade. Credit: Chris Kuijper.

 

As with most modern cities, the geological features visible in Barcelona are only a tiny fraction of the richness of rocks and structures hidden underground and as the city rapidly expanded in the 20th century, very little of this information was properly recorded. But, a curious eye and wandering feet can discover many clues to the rich history of the region, and any travelling geologist would be more than satisfied with what Barcelona has to offer.

The Netherlands: In search of the oldest rocks of a muddy country

The Netherlands: In search of the oldest rocks of a muddy country

Technically speaking, the Netherlands isn’t really a city, even though it is the most densely populated country in the European Union and one of the most densely populated countries in the world, with 488 people/km­2. It is a delta, and for many people geology is not the first thing that springs to mind when thinking about the flat countryside. Large parts of the country have been more often below sea level than above.

The Netherlands, aka the flat country, seen from the air…

…and on the ground. Credit: Anne Pluymakers.

 

 

 

 

 

 

 

However, below the centuries of accumulated sand and clay, there are real rocks – rocks that form extensive reservoirs, for hydrocarbons (such as the Groningen reservoir, one of the world’s largest onshore gas reservoirs), potential CO2 storage reservoirs, as well as reservoirs now of interest to recover geothermal energy. It is the interest in these reservoirs that inspires many companies and the Dutch Geological Survey to figure out what lies below our feet. There is a publicly available database, the DINO database, which provides a view of the Dutch subsurface. It has been constructed using more than 135 3D seismic surveys and over 577.000 km of 2D seismic lines, as well as 1305 wells (www.dinoloket.nl).

Left, a map of the Netherlands, with the approximate location and length of our cross sections indicated (Based on Netherlands with Provinces – Multicolor, by FreeVectorMaps.com). On the top right, a cross section through Amsterdam, with the formations as mentioned in the text. The same formations can be found in the south, but then at the surface. The Heimansgroeve, with the oldest outcropping rocks of the country, are part of the Geul subgroup, indicated with a red dot. (Cross sections from https://www.dinoloket.nl/ondergrondmodellen)

 

Everyone who visited the Netherlands will most likely remember the narrow merchant houses lining the many canals of Amsterdam. These houses are actually built on wooden piles, which are grounded in a sand layer at about 12 meters depth. Drawing a N-S profile through Amsterdam, we dig through many different deltas, swamps and bogs, showing thousands of years of rising and falling sea level. It is then no surprise that many of the mid-19th century houses are now tilted. Digging down in Amsterdam, it takes about 1000 meters until we find the first solid rock, the Upper and Lower Germanic Trias Group (RB, RN), consisting of clay-, sand- and siltstones. Below, there are relatively thin layers of the Zechstein and Rotliegend groups (red and pink), which contain the many on- and offshore hydrocarbon reservoirs found in Dutch territory. The lowest formation of interest (light and dark grey in the cross-sections above) is the Limburg group, in Amsterdam at 2 to 5 km depth. This is where we are heading! But instead of digging up Amsterdam, we travel south, to Limburg. This is the most southern province in the Netherlands, and as the map shows, it is wedged between the Belgian and German border. It is a region with a lot of tourism, and a great place to practice cycling uphill. This gently sloping countryside is the Dutch version of the Belgium Ardennes nearby, which are influenced by the Hercynian orogeny.

The Heimansgroeve was a quarry more than 100 years ago. Today, it is a geological monument. Credit: Anne Pluymakers.

The ancient castle of Valkenburg, a touristic town in Limburg. It was almost entirely built with the yellow marls from the region. Credit: Wikipedia.

The most famous rocks of Limburg are the yellow marls, remnants of a tropical sea and of Upper Cretaceous age. Even though they are soft, they have been used for centuries as building stones, where in the centre of the touristic little town of Valkenburg an ancient castle is almost completely made up out of marl. In the hills around this town hundreds of kilometres of tunnels have been excavated.  When excavated 100 to 150 years ago, one cubic meter of marl used to cost 7 cents – though now a staggering €800,000 euro. Since the rocks are so soft, the only purpose for mining them nowadays is restoration.

It is in this region that we find the oldest outcropping rocks of the Netherlands. They surface in the Heimansgroeve, and are from the Geul Subgroup (dark grey in the cross section) and Carboniferous in age. The depositional setting of these rocks, consisting mainly of sandstones, coal layers and shales, ranged from deep basin, via prodelta, to delta front. Planning a visit to the outcrop with the oldest rocks of the Dutch mainland is surprisingly easy. From the parking lot of campsite Cottesserhoeve (paid parking) it is a leisurely walk towards the river Geul, and then a short walk along the river before we see the outcrop. This outcrop is part of the Heimansgroeve, a former quarry, northwest of Cottessen. In the early 20th century, this quarry was used to mine road-building material, and in 1910 it was discovered by the geologist Eli Heimans.


Fun fact: this region led to one of the first Dutch popular science books on geology, ‘Uit ons Krijtland’, published in 1911. It describes the countryside near this quarry, and encourages people to visit. In 1936, the Heimans quarry was enlarged by the geologist Willem Jongmans for research purposes. Nowadays it hosts a seismological station, to register the occasional earthquakes.


Tilted layers, slickensides and precipitation in open veins. All the features one would look for in the field are present, also in this outcrop in the flatlands! Credit: Anne Pluymakers.

Even though structurally it is not the most exciting place, it still shares many of the features a geologist looks for in the field. The quarry comes with a signpost explaining the geology, for any amateur- or potential future  geologists passing by on a sunny day. Scurrying around, some other small details can be seen; all the features a wandering geologist is hoping for when out rock-hunting. Several places contain slickensides, an indicator for fault movement. For the mineralogist, there are plenty of tiny crystals to be found, showing fluid movement and precipitation in what must’ve been open fractures.

This outcrop shows you don’t have to dig that deep to witness real rocks in a country that is mainly known for being a Delta. So, next time you travel to the Netherlands, why not trade a touristy and overcrowded Amsterdam for a rock-hunting visit in the gentle sloping hills of the South?

 

Minds over Methods: The faults of a rift

Minds over Methods: The faults of a rift

Do ancient structures control present earthquakes in the East African Rift? 

Åke Fagereng, Reader in Structural Geology, School of Earth and Ocean Sciences, Cardiff University

For this edition of Minds over Methods, we have invited Åke Fagereng, reader in Structural Geology at the School of Earth and Ocean Sciences, Cardiff University. Åke writes about faults in the Malawi rift, and the seismic hazard they may represent. With his co-workers from the Universities of Cardiff, Bristol, and Cape Town, the Malawi Geological Survey Department, Chancellor’s College, and the Malawi University of Science and Technology, he has studied these faults in the field and in satellite imagery. Their methods span scales of observation from outcrop to rift valley, allowing insights on how rifts and rift-related faults grow.

 

Åke Fagereng looking at rocks by Lake Malawi. Credit: Johann Diener.

Introduction

East Africa hosts some of the longest faults on the continents, yet it is not a well-known location for significant seismic hazard. There is, however, historical record of large earthquakes (> M7; Ambraseys, 1991), and topographic evidence for major scarps (Jackson and Blenkinsop, 1997), so we ask where, and of what magnitude, could future earthquakes occur?

The instrumental record of East African earthquakes is short, and sparse instrumentation only detects moderate to large magnitude events. Recent, seismic deployments have, however, made great progress in understanding active seismicity in the southern East African rift (e.g. Gaherty et al., 2019; Lavayssière et al., 2019). Such studies have demonstrated that the seismogenic thickness – the depth range where earthquakes nucleate – spans the entire crust (up to 40 km depth). This large seismogenic thickness is important; it theoretically means that ~ 100 km long border faults could rupture along their entire length and throughout the thickness of the crust, creating > M8 earthquakes. Such events would have severe impact, particularly given rapid population growth, urbanization, and high vulnerability of the East African building stock (Goda et al., 2016).

There is a scale dependence to fault observations in rifts. On the rift valley scale, active extension broadly follows ancient plate boundaries marked by deformed, metamorphic rocks separating ancient continental building blocks of cratons and shields. However, does the same observation apply to individual faults, and at what depths? Is there evidence for large earthquake events? A hypothesis to test is whether ancient structures control the location and geometry of major rift border faults, and allow them to be long and continuous.

 

The Bilila-Mtakataka fault in Malawi (black), broadly following but locally cross-cutting foliations (red). Map reproduced after Hodge et al. (2018).

Methods and Results

In the PREPARE project, we focus on Malawi as a case study in the southern, non-volcanic, portion of the rift. Although we have established a network of campaign GPS stations, slow extension rates (< 3.5 mm/yr; Saria et al., 2014) imply that reliable geodetic data will take several more years to collect. Similarly, long earthquake recurrence times lead to a question of how representative the instrumental record can be. Therefore, we have explored other methods for understanding fault scarps and the likely behavior of their faults.

In his PhD work, Michael Hodge used a combination of data sets, including satellite imagery and digital elevation models, photogrammetry, and field observations to analyse the major Bilila-Mtakataka fault from outcrop to rift scale. This fault sits within an ancient ‘mobile belt’ and follows these structures in some places, but cross cuts them in others (Hodge et al., 2018). These observations show that the fault only exploited near-surface ancient structures where these were very well oriented for failure, while elsewhere the fault would take another orientation.

 

The Bilila-Mtakataka fault, illustrated by its ~ 10 m high scarp. Credit: Åke Fagereng.

 

To better understand the controls on fault reactivation and how our surface observations relate to faulting at depth, we place our field and satellite observations in a framework of fault mechanics. It is not uncommon that normal fault segments take two strike orientations: one parallel to pre-existing weaknesses, the other perpendicular to the extension direction (e.g. McClay and Khalil, 1998). However, although there is uncertainty around local extension directions, where fault segments in Malawi cross-cut ancient structures they do not seem to have a consistent, extension-perpendicular, strike (Hodge et al., 2018).

Hodge et al. (2018) tested whether a mid- to lower crustal ancient structure could control the average orientation of the Bilila-Mtakataka surface scarp. This test was based on whether model predictions fitted detailed measurements of scarp height and orientation using a digital elevation model. Such measurements are time consuming, but using a semi-automated method, can also be applied efficiently at the scale of multiple faults (Hodge et al., 2019). Here, the model supports a conclusion where long fault scarps form above localized, deep crustal structures, that are locally deflected near the surface. This model allows the long scarps to have formed in multiple smaller earthquakes rather than a single very big one.

 

Field observations of the Bilila-Mtakataka fault, which locally parallels (A) and locally cross cuts foliation (B). Map from Hodge et al. (2018), Michael Hodge and Hassan Mdala study the fault in A (credit Åke Fagereng) and Åke Fagereng measures a fracture in B (credit Johann Diener).

 

Model, simplified from Hodge et al. (2018), showing how upward propagation of a deep structure is consistent with a segmented surface trace, if the fault is deflected by well-oriented foliation locally and near –surface.

Future Work

Newly discovered fault scarps within the Zomba graben, further south in Malawi, furthermore show that fault orientations vary on the scale of multiple faults within a graben. Here, strike variations also lack a clear fit to expected orientations inferred from the rift-wide extension direction. Ongoing studies are now employing similar methods to analyze the relation between fault geometry and pre-existing structures in the field, on the scale of a complete graben, further bridging the outcrop and rift scales.

Geophysical and remote sensing approaches have been and continue to be invaluable to understand the regional and subsurface structure of the East African and other continental rifts. We emphasise, however, that fault scale structural observations aid interpretation of these data and allow deduction of fault growth mechanisms. Our methods in future work will therefore continue to span scales, and root interpretation in detailed fault-scale field studies linked to rift-scale satellite data.

 

Edited by Elenora van Rijsingen

 

References

  • Ambraseys, N.N., 1991. The Rukwa earthquake of 13 December 1910 in East Africa. Terra Nova 3, 202-211.
  • Gaherty, J.B., Zheng, W., Shillington, D.J., Pritchard, M.E., Henderson, S.T., Chindandali, P.R.N., Mdala, H., Shuler, A., Lindsey, N., Oliva, S.J. and Nooner, S., 2019. Faulting processes during early-stage rifting: seismic and geodetic analysis of the 2009–2010 Northern Malawi earthquake sequence. Geophysical Journal International 217, 1767-1782.
  • Goda, K., Gibson, E. D., Smith, H. R., Biggs, J., & Hodge, M. (2016). Seismic risk assessment of urban and rural settlements around Lake Malawi. Frontiers in Built Environment, 2, 30.
  • Hodge, M., Fagereng, Å., Biggs, J. and Mdala, H., 2018. Controls on early‐rift geometry: new perspectives from the Bilila‐Mtakataka fault, Malawi. Geophysical Research Letters 45, 3896-3905.
  • Hodge, M., Biggs, J., Fagereng, Å., Elliott, A., Mdala, H. and Mphepo, F., 2019. A semi-automated algorithm to quantify scarp morphology (SPARTA): application to normal faults in southern Malawi. Solid Earth 10, 27-57.
  • Jackson, J. and Blenkinsop, T., 1997. The Bilila‐Mtakataka fault in Malaŵi: An active, 100‐km long, normal fault segment in thick seismogenic crust. Tectonics 16, 137-150.
  • Lavayssière, A., Drooff, C., Ebinger, C., Gallacher, R., Illsley‐Kemp, F., Oliva, S.J. and Keir, D., 2019. Depth extent and kinematics of faulting in the southern Tanganyika Rift, Africa. Tectonics 38, 842– 862.
  • McClay, K. and Khalil, S..1998. Extensional hard linkages, eastern Gulf of Suez, Egypt. Geology 26, 563–566
  • Saria, E., Calais, E., Stamps, D.S., Delvaux, D. and Hartnady, C.J.H., 2014. Present‐day kinematics of the East African Rift. Journal of Geophysical Research 119, 3584-3600.

Features from the Field: Boudinage

Features from the Field: Boudinage

The Features from the Field series is back! In our previous posts, we have shown how rocks can deform during ductile deformation, producing folds. Folds very commonly develop in rocks when rock layers are shortened by tectonic forces in a specific direction. On the other hand, when layers are extended, we develop boudins.

Saucisson is a dry cured sausage (boudin) from France. Did you know that geology is full of food analogies? Indeed, we love barbecues. Photo credits © Nate Grey (Flickr)

Boudins – the term comes from the French word for ‘sausage’ – are fragments of original layers that have been stretched and segmented. They develop in layers that are stronger and more resistant to deformation (i.e. more competent) than the surrounding rocks. In the example shown at the top of the page, the boudinated layer is made up of ‘strong’ amphibolites that are surrounded by relatively weaker quartzites. As you can see, the layering of the quartzites is deflected in the ‘pinches’, as if they where flowing in the gaps of the boudins during the extension. That’s it! During ductile deformation, rocks flow over millions of years in a plastic way.

Different rock types are characterized by a different strength during deformation, which is significantly influenced by temperature, pressure or -very important- presence of water. Boudinage is a very common structure which helps  the geologist understand ‘who is stronger than who?’ and helps them guess the physical conditions at which deformation took place.

Boudinage style can also vary enormously. For example, the eye-shaped boudins shown at the top of the page are called ‘pinch-and-swell’ structures. Indeed, you can notice that the boudins are not entirely separated and are connected by a very thin amphibolite layer, as if they were ‘pinched’ by a finger. This structure suggest that both the boudinated layer and the surrounding rocks are deforming in a ductile way.

However, fracturing can also play an important role in boudinage. In the example shown below, the top layer consists of strong quartz-metaconglomerate that has been boudinated within very weak phyllites (the bottom layer). The quartz-metaconglomerate is fragmented along symmetric (dextral and sinistral) fractures with an offset of several centimeters. The white material filling the gaps are vein of quartz that were deposited during the boudinage process. Note how the fractures are restricted to the metaconglomerate.

 

Symmetric boudinage of a quartz metaconglomerate in phyllites. The gaps of the boudins are filled with white quartz veins (Punta Bianca, La Spezia, Italy). Photo credits © Samuele Papeschi

 

Boudins can be symmetric, as in the example above, or asymmetric, as in the example below, where the boudinated layer is an amphibolite surrounded by weaker micaschists and quartzites. The boudins are separated by small scale sinistral shear fractures and systematically rotated clockwise. In this case, the geologist can obtain information about rock strength during deformation, but also on the sense of shear – which here is top to the right.

 

Asymmetric boudinage of amphibolite (the blackish layer) in dark grey biotite-micaschists and white quartzites (Elba Island, Italy). The amphibolite layers is fragmented by several, sinistral shear fractures. Photo credits © Samuele Papeschi

 

Rocks are not stretched in a single direction. Layers can also be flattened and stretched along 2 directions. When this occurs, you get fragmented boudins surrounded by 2 sets of fractures, as in the last example below. You already know that geologists like food analogies, so are you able to guess the name of this last structure? Yes, it is a chocolate tablet boudinage.

 

Chocolate tablet boudinage: a grey dolomite vein has been stretched along two, nearly-perpendicular, directions. The gaps between the boudins are filled by calcite deposited by fluids (Punta Bianca, La Spezia, Italy). Photo credits © Samuele Papeschi.

 

To sum it up, boudinage is a very important structure when studying rocks in the field, which gives us important insights about deformation, rock strength, pressure and temperature conditions and the sense of shear. Together with folds, lineations and foliations, it represents one of the most important features that can be described in the field.

Minds over Methods: Mineral reactions in the lab

Minds over Methods: Mineral reactions in the lab

 

Mineral reactions in the lab

André Niemeijer, Assistant Professor, Department of Earth Sciences at Utrecht University, the Netherlands

In this blogpost we will go on a tour of the High Pressure and Temperature (HPT) Laboratory at Utrecht University and learn about some of the interesting science done there.

André Niemeijer next to a striated fault surface. Credit: André Niemeijer.

André’s main interest is fault friction and all the various processes that are involved in the seismic cycle. This includes the evolution of fault strength over long and short timescales, the evolution of fault permeability and the effects of fluids. His current research is aimed at understanding earthquake nucleation and propagation by obtaining a better understanding of the microphysical processes that control friction of fault rocks under in-situ conditions of pressure, temperature and fluid pressure.

Most of the deformation in the Earth’s brittle crust occurs on and along faults. Fault movement produces fine-grained wear material or gouge, which is very prone to fluid-rock interactions and mineral reactions (Wintsch, 1995). It has long been recognized that the presence of a fluid allows for deformation to occur at much lower differential stresses than without.

Pressure solution

One of the mechanisms by which this deformation occurs is pressure solution (alternatively termed “solution-transfer creep” or “dissolution-precipitation creep”). This mechanism operates through the dissolution of materials at sites of elevated stress, diffusion along grain boundaries and re-precipitation at low stress sites (e.g. pores). Pressure solution is an important diagenetic process in sandstones and carbonates as evidenced by the presence of stylolites in many carbonate rocks, which are often used as counter tops and floors (particularly in banks, I noticed). In addition, it has been suggested that pressure solution plays an important role in the accommodation of (slow) shear deformation of faults (Rutter & Mainprice, 1979) and possibly in controlling the recurrence interval of earthquakes (Angevine, 1982).

Fluid-rock interactions in the lab

Experimentally, it is challenging to activate pressure solution or mineral reactions in the laboratory, because they are typically slow processes. Moreover, it is difficult to find evidence of their operation. We have used a unique hydrothermal rotary shear apparatus, which is capable of temperatures up to 700 °C to activate pressure solution in fine-grained quartz gouges. We were able to prove that new material was precipitated by using a combination of state-of-art electron microscopy techniques that involve cathodoluminescence (CL).

The hydrothermal rotary shear apparatus at the HPT laboratory at Utrecht University, the Netherlands. Credit: André Niemeijer.

Signature of pressure solution

The CL signal of a mineral depends on the type and level of impurities and defects that are present. We used quartz derived from a single crystal which showed relatively uniform CL. Because our apparatus has various metal alloy parts, small amounts of aluminium are present in the fluid. Aluminium can be incorporated in newly precipitated quartz, which gives a different CL signal. This allows us to map the locations where quartz has newly formed and link this to the experimental data. Taken together, we can use these to derive and constrain microphysical models for fault slip that can be used to extrapolate to natural conditions (e.g. Chen & Spiers 2016, van den Ende et al., 2018).

RGB overlay of secondary electron and cathodoluminescence signals in a deformed quartz sample. Newly precipitated quartz shows up in a blue colour. Credit: Maartje Hamers.

Mineral reactions

Outcrops of natural faults often show evidence for enhanced mineral reactions with increasing shear strain. For instance, the Zuccale fault (Isle of Elba, Italy) has a high content of talc in the highest strained portion of the fault (Collettini & Holdsworth, 2004). Talc is a frictionally weak mineral and its presence in the Zuccale fault provides an explanation for the possibility of slip along this low-angle normal fault. We were able to produce talc experimentally from mixtures of dolomite and quartz in only 3-5 days of shearing at low velocity. This shearing was accompanied by major weakening, with friction dropping from 0.8 to as low as 0.3. The reaction to talc is sensitive to temperature and fluid composition. At slightly higher temperature, we produced diopside and forsterite which are frictionally unstable and generated audible laboratory earthquakes.

Identifying reaction products

We tried a whole range of different analytical techniques to identify the reaction products. Despite the obvious frictional weakening that we observed, talc was only observed in two samples with x-ray diffraction (XRD). Fourier-transform Infrared analysis, on the other hand, proved to be very sensitive to talc and has the big advantage that only a small amount of material is needed (~70 mg). Electron microscopy with EDS-analysis (Energy Dispersive X-ray Spectroscopy) proved helpful to some extent, because it shows the phase distribution. However, the small size of reaction products gives a mixed chemistry, which complicates the identification of reaction products. Finally, to positively identify the various phases in the different samples, we employed Raman mapping.

RGB overlays of EDS analyses of samples deformed at 300 °C (left) and 500 °C (right). Dolomite appears in yellow, quartz in blue, calcite in red, talc in cyan in the left image, while dolomite is orange, calcite is red, diopside is purple and forsterite is cyan in the right image. Credit: André Niemeijer.

Outlook

Our studies have shown that reactions can be quite rapid in fine-grained fault gouges. These reactions can have a profound effect on both fault strength and stability but are typically ignored in large-scale models of the seismic cycle. Incorporating reactions requires models that can account for the effect of stress and grain size reduction on the development of faults, which is not an easy task, but is a necessary ingredient to understand the long-term behavior of faults.

Edited by Derya Gürer

References

  • Angevine, C. L., Turcotte DL, Furnish MD. (1982) Pressure solution lithification as a mechanism for the stick-slip behavior of faults. Tectonics 1 (2), 151-160 doi:10.1029/TC001i002p00151.
  • Chen, J. and Spiers CJ. (2016) Rate and state frictional and healing behavior of carbonate fault gouge explained using microphysical model. Journal of Geophysical Research: Solid Earth 121 (12), 8642-8665 doi:10.1002/2016JB013470.
  • Collettini, C. and Holdsworth RE. (2004) Fault zone weakening and character of slip along low-angle normal faults: Insights from the Zuccale fault, Elba, Italy. Journal of the Geological Society 161 (6), 1039-1051 doi:10.1144/0016-764903-179.
  • E H Rutter, D H Mainprice (1979)On the possibility of slow fault slip controlled by a diffusive mass transfer process. Gerlands Beitr. Geophysik, Leipzig 88 (1979) 2, S. 154-162.
  • van den Ende, M. P. A., Chen J, Ampuero J., Niemeijer AR. (2018) A comparison between rate-and-state friction and microphysical models, based on numerical simulations of fault slip. Tectonophysics 733, 273-295 doi:10.1016/j.tecto.2017.11.040.
  • Wintsch, R. P., Christoffersen R, Kronenberg AK. (1995) Fluid-rock reaction weakening of fault zones. Journal of Geophysical Research: Solid Earth 100 (B7), 13021-13032 doi:10.1029/94JB02622.

Minds over Methods: Massively dilatant faults in Iceland – from surface to subsurface structures

Minds over Methods: Massively dilatant faults in Iceland – from surface to subsurface structures
In this Minds over Methods we don’t have one, but two scientists talking about their research! Michael Kettermann and Christopher Weismüller, both from Aachen University, explain us about the multidisciplinary approach they use to understand more about massively dilatant faults. How do they form and what do they look like at depth?

Massively dilatant faults in Iceland – from surface to subsurface structures

Michael Kettermann & Christopher Weismüller, RWTH Aachen University

Michael (left) and Christopher (right) in the field. Credit: Michael Kettermann and Marianne Sophie Hollinetz.

Iceland is a volcanic island in a unique setting on the Mid-Atlantic Ridge, separating the Eurasian and North American plates. A deep mantle plume lies beneath Iceland, and the combination of rift and plume leads to very active basaltic volcanism. Ubiquitous features along the rift zone are normal faults, often exquisitely exposed at the surface. Normal faults in basalts are also common in many volcanic provinces, like Hawaii, the East African Rift, and along mid ocean ridges. These faults often form as massively dilatant faults (MDF), which show apertures up to tens of meters at the surface and supposedly have large volumes of open voids in the subsurface.

Figure 1. View along-strike a massively dilatant fault in layered basalt. The geometry of the vertical fault faces is prescribed by the cooling joints. Several basalt columns have been loosened and dropped into the fault, now being stuck in the fault (top) or filling the cavity (bottom). Opening width < 3 m. Credit: Michael Kettermann.

These openings form pathways for fluids like magma or hydrothermal waters and consequently are of importance for volcanic plumbing systems, mineralization and geothermal energy supply.

Iceland provides a perfect natural laboratory to study MDF. Due to its position on the Mid-Atlantic Ridge, Iceland is cut by extensional fault systems roughly from southwest to north. A wide range of oblique extensional to pure extensional faults can be observed mostly in flood basalts, but also in sub-glacially formed hyaloclastites (weaker volcanic sediments), pillow lavas and occasionally sediment layers formed in warmer times. Outcropping rocks in Iceland are younger than 20 Ma distal from the rift (eastern and western Iceland), while tectonic and corresponding volcanic activity at the ridge (central Iceland) constantly causes the formation of new rocks. The rough climate hinders soil formation and vegetation to overgrow faults, providing unique outcrop conditions.

While it is relatively easy to access and study the faults at surface level, investigations into the subsurface are much more challenging. Direct observations are only possible down to depths of some tens of meters by climbing into the fractures. Cavities are often filled with rubble, sediments, water or snow (Fig. 1). Steep, open fractures with meter-scale aperture are hard to detect with geophysical methods (seismic reflection/refraction, ground penetrating radar, electrical resistivity tomography) at depths greater than some meters.

We therefore started the massively dilatant fault project, a multidisciplinary, integrated project bringing together remote sensing, fieldwork, analogue modelling and numerical simulations. In essence, we utilize a modelling approach to recreate the structure and evolution of MDF at depth, using real 3D surface data as input and comparison data set.

 

Drone mapping and photogrammetry

In a first step, we capture and analyse the surface expressions of MDF at a number of representative fault areas in Iceland. To this end, we flew 27 drone surveys during our five weeks long field season in summer 2017, covering a total length of more than 42 km of faults. Luckily, in the Icelandic summer the days are very long, so the National Park Service allowed us to fly the drones early in the morning and late in the evening outside of tourist hours. For each area, we took several hundred to thousands of overlapping photographs (e.g. Fig. 2). We processed these sets with photogrammetry software, applying the Structure from Motion (SfM) technique. SfM is an increasingly popular, fast and cheap technique to reconstruct high-resolution 3D information from 2D images. This allows us to recreate digital elevation models and ortho-rectified photo-mosaics of the faults in resolutions better than 15 cm per pixel (Fig. 3). The largest area at the famous Thingvellir fissure swarm covers a length of almost 7 km with an average resolution of 11 cm per pixel.

We use these digital elevation models and ortho-photos to retrieve a wide range of structural data. Mapping the fault traces in a GIS software allows for the measurement of fault opening width, throw, orientation and length. From throw and aperture, we can then estimate the fault dip at depth. Digital elevation models further provide surface dip data that we then compare with observations from analogue models.

Figure 2. A drone photograph facing South of the Almannagjá fault in Thingvellir, where the Thingvallavegur road crosses the fault. The Almannagjá fault resembles the western shoulder of the Thingvellir graben system with locally > 50 m opening width and 40 m vertical offset. Credit: Christopher Weismüller. .

Figure 3. Digital elevation model created from drone photographs using photogrammetry software. It contains the faults at Sandvik on the Reykjanes Peninsula (SW Iceland). The detail panes (red square) show the DEM (right) at a higher zoom level and the corresponding ortho-rectified photograph (left). The bridge crossing the fault depicted in the detail panes is a famous touristic spot, known as „The bridge between the continents“, since the fault symbolically divides the North American and Eurasion plates. Credit: Michael Kettermann.

 

 

 

 

 

 

 

Figure 4. Sideview of an analogue model showing three timesteps of the development of a massivley dilatant fault and associated fractures in hemihydrate (Bücken, 2017). Note the tilted block developing at the surface of the model and the dilatant jogs and voids in the subsurface. The opening at the surface is not directly linked to the fault at depth, but caused by the rotation of the tilted block. Credit: Daniel Bücken..

Modelling approach

For the analogue modelling approach on the hundreds to thousand meter scale, we use cohesive powders as modelling material (Bücken, 2017). Especially hemihydrate powder has been proven suited to model dilatant fractures (Holland et al., 2006; Kettermann et al., 2016; van Gent et al., 2010) as it has a well characterized true cohesion and tensile strength. Faults in Iceland transform from opening mode fractures to shear mode faults at depth when overburden stress is high enough. As we are interested in the upper dilatant parts of the faults, i.e. above the shear mode faulting, we chose a basement-fault controlled approach, where a rigid basement represents the shear mode fault. It moves down-dip along a predefined surface, deforming the powder sieved on top. The basement fault dip follows the data we derived from the field and is set to 60° – 65°. The scale of the models calculates from strength and weight of the natural prototype and the modelling material. 1 cm of powder equals about 50 m of basalt.

 

Comparison of models and nature

Results show a close similarity between field and experiment at the surface structures. Open fractures form with large apertures at the surface and often we observe the formation of tilted blocks (Fig. 4). The existence and scaled dimensional similarity of fractures and tilted blocks in the field and using a scaled material suggest a validity of other observations in the models. Glass sidewalls in the analogue models provide the opportunity to examine how the faults evolve at depth. We observe that large caves form underneath these blocks and we predict that these must exist in the field as well, albeit potentially filled with rubble. Our models corroborate earlier predictions that extensional faults are open down to 800 – 1000 m (Gudmundsson and Bäckström, 1991). We also learned that below that, a hybrid failure zone exists where dilational jogs, open extensional fractures between shear mode faults, provide lateral pathways for magma or water, even at depths where the overburden stress prevents the formation of purely extensional faults.

 

Outlook

The previously shown experiments investigated MDF at a larger scale in purely dip-slip kinematics. However, faults at rifts often have strike-slip components, forming normal faults with oblique kinematics. In further experiments, we therefore explored the effect of varying basement fault obliquities, i.e. the range between dip-slip normal faults and strike-slip faults (Bitsch, 2017). As expected, early phases of faulting are dominated by Riedel shears. Surprisingly, the surface structure of mature faults, however, does not change distinctly up to obliquities of 60°, but the subsurface connectivity decreases with increasing obliquity.

Figure 5. Analogue model resembling successive layers of lava flows with cooling joints created by carefully stacking several layers of dried corn starch slurry (Winhausen, 2018). The dip of the basement fault is prescribed by the apparatus. The fault geometry generated in the model is very similar to the ones observed in Iceland. Large cavities develop and are partially refilled by loosened columns, as shown in figure 1. Tensile fractures develop on the surface of the footwall, similar to the hemihydrate model and the field. Credit: Lisa Winhausen.

Zooming in on the faults, an inherent mechanical anisotropy (orthotropy) of basalts gains more influence on the macroscale structure of faults. Due to the shrinking during cooling of flood basalts, polygonal to blocky columns form and present regular weak zones in the rockmass. Introducing mechanical anisotropy into a stronger modelling material (dried corn-starch slurry) beautifully illustrates how the small-scale structure of the faults is affected by the layering of flood basalts and cooling fractures therein (Fig. 5; Winhausen, 2018). Close to the surface the strong basalt does not fracture, but propagating faults rather localize at the pre-existing cooling joints. This causes a jagged structure of the fault, formation of caves, and eroded basalt columns filling the opening fractures.

We are currently working on implementing all these learning points into discrete element simulations, where we can adjust material properties in a way that allows for modelling deeper parts of the faults with better mechanical control.

 

 

References

Bitsch, N.D., 2017. Massively dilatant faults in oblique rift settings – an analogue modeling study (MSc Thesis). RWTH Aachen University, Germany, Aachen.

Bücken, D.H., 2017. Effect of mechanical stratigraphy on normal fault evolution – Insights from analogue models and natural examples in Iceland (MSc Thesis). RWTH Aachen University.

Gudmundsson, A., Bäckström, K., 1991. Structure and development of the Sveinagja graben, Northeast Iceland. Tectonophysics 200, 111–125. https://doi.org/10.1016/0040-1951(91)90009-H

Holland, M., Urai, J.L., Martel, S., 2006. The internal structure of fault zones in basaltic sequences. Earth Planet. Sci. Lett. 248, 301–315. https://doi.org/10.1016/j.epsl.2006.05.035

Kettermann, M., von Hagke, C., van Gent, H.W., Grützner, C., Urai, J.L., 2016. Dilatant normal faulting in jointed cohesive rocks: a physical model study. Solid Earth 7, 843–856. https://doi.org/10.5194/se-7-843-2016

van Gent, H.W., Holland, M., Urai, J.L., Loosveld, R., 2010. Evolution of fault zones in carbonates with mechanical stratigraphy – Insights from scale models using layered cohesive powder. J. Struct. Geol. 32, 1375–1391. https://doi.org/10.1016/j.jsg.2009.05.006

Winhausen, L., 2018. Influence of columnar joints on normal fault geometry and evolution An analog modeling study Master Thesis (MSc thesis). RWTH Aachen University.