TS
Tectonics and Structural Geology

Tectonics and Structural Geology

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.

Beyond tectonics: The present-day tides are the biggest they have been since the formation of Pangea

Beyond tectonics: The present-day tides are the biggest they have been since the formation of Pangea
“Beyond tectonics” is a blog series which aims to highlight the connections between tectonics and other aspects of the Earth system. In this iteration of the “Beyond tectonics” series we talk about how plate tectonics have affected the tides on Earth over geological timescales. We will talk about tectonics on the Earth since the formation of Pangea to the present day, and into the future, ending with the formation of the next Supercontinent in around 200 – 250 Million years from now.

 

Tides of the Planet Earth

The Earth’s tides are predominantly caused by the gravitational pull of the Moon, and the centripetal force of the Earth due to its rotation. The force of the Moon and the spin of the Earth cause two tidal bulges to form, one that follows the Moon, and one on the opposite side of the planet. These two tidal bulges move around the Earth with a period of 12.5 hours. When the buldge moves over a coast, a high tide occurs, and when a bulge is not over a coast, a low tide occurs. This is why there are two low and high tides each day. These tides vary in strength around the world because ocean and coastal morphology plays a large role in how the tidal energy is distributed.

 

How do tectonic plates affect the tide?

The surface of the Earth is broken up into pieces like the shell of an egg. These pieces, or plates, can be divided into oceanic and continental plate. The main difference between the two types of plate is their buoyancy. Both types of plate are buoyant, however, after it is formed at a mid ocean ridge, ocean plate becomes less and less buoyant with age. Eventually, after around 30 million years, ocean plate is less buoyant than the underlying mantle meaning it could sink if it reached a subduction zone. In the present-day oceans, almost all plate older than 180 million years old has sank back into the mantle at a subduction zone. This recycling of ocean crust is what causes the oceans to change shape. The continents are pulled together by the closing oceans (sinking ocean plate), and pushed apart by the opening ones (creation of ocean plate). In the present day, the Pacific is closing and the Atlantic is opening, causing the plate drift map to look like this:

The major subduction zones on Earth. Black arrows illustrate trench migration vectors, while open arrows illustrate plate velocity (credit – Schellart et al., 2007)

As the oceans grow and shrink, their width changes. This means the tidal wave in those oceans has either too little, too much, or just the right amount of space to flow in the ocean. In the present-day the Pacific is too big, the Indian ocean too small, but the Atlantic ocean is just the right size to make the tide resonant.

 

What is resonance?

To explain resonance, imagine the tidal wave moving across the Atlantic ocean and back, like a child on a swing. If you apply the force on the swing when it is at the highest point, you are applying a force at one of the natural frequencies of the system, so the energy you input will make the swing go higher. If you push the swing before or after it reaches its peak, then you might input some energy, but it won’t be as efficient. The Atlantic ocean is just the right width to allow the wave to “swing” back and forth. The input of energy is applied at just the right point, one of the natural frequencies of the tide, to allow resonance.

It is possible for this to happen in any ocean basin. An ocean basin can house resonant tides when the width of the basin (L) is equal to a multiple of half wavelengths of the tide  (𝜆 = √gHT),  where (T) is the tidal period, (g) is gravity, and (H) is water depth. Essentially, when the ocean basin has a width that intersects with a natural frequency of the tide, it will become resonant.

 

The Super-tidal cycle

The reason why the present day tides are the biggest since the formation of Pangea is because the Atlantic is currently resonant with the tide, i.e. it is in a Super-tidal period. Looking at the energy of the tides from when Pangea existed to the present-day (Green et al., 2017), we can see that during Pangea’s life the tides were weak. That status quo continued from 180 to 1 million years ago, when the Atlantic suddenly developed large tides. The Atlantic had been growing all that time, and had finally reached a width where the tide became resonant.

M2 tidal amplitudes since the breakup of Pangea (credit – Green et al., 2017)

The large present-day Atlantic tide is unlikely to last. Green et al., (2018) predict that this super-tidal period will last around 20 million years, and Davies et al., (2019) predict another won’t occur for tens of millions of years, depending on how the future Earth develops.

Therefore, what are the implications of the present-day Atlantic having such large tides? Further testing of the implications of the Super-tidal cycle is needed before any conclusions can be made on how it may affect the Earth system. However, the larger energy input into the oceans during a Super-tidal period may enhance tidal mixing, which means the ocean will have a better distribution of nutrients and oxygen, i.e. it is less likely for it to become stratified. What we do know right now, is that Supercontinents generally have very weak tides (Green et al., 2018), and periods of continent divergence similar to the present day, have larger, or sometimes Super-tides (Balbus 2014; Davies et al., 2019).

 

References

  • Balbus, S. A. 2014, Dynamical, biological and anthropic consequences of equal lunar and solar angular radii. Proc. R. Soc. A, 470. Available at: http://doi.org/10.1098/rspa.2014.0263
  • Davies, H. S., Green, J. A. M., Duarte, J. C., 2018, Back to the future: testing different scenarios for the next supercontinent gathering. Global and planetary change, 169, 133 – 144. Available at: https://doi.org/10.1016/j.gloplacha.2018.07.015
  • Davies, H. S., Green, J. A. M., Duarte, J. C., 2019. Back to the future 2: Tidal modelling of four potential scenarios for the next Supercontinent gathering. Presented at EGU 2019. Available at: https://meetingorganizer.copernicus.org/EGU2019/EGU2019-1004-1.pdf
  • Green, J.A.M., Molloy, J.L., Davies, H.S., Duarte, J.C., 2018. Is there a tectonically driven super-tidal cycle? Geophys. Res. Lett. 45 (8). Available at: https://doi.org/10.1002/2017GL076695.
  • Schellart, W.P., Freeman, J., Stegman, D.R., Moresi, L., May, D. 2007. Evolution and diversity of subduction zones controlled by slab width. Letters to Nature, 446, 308 – 311. Available at: doi:10.1038/nature05615

Meeting Plate Tectonics – Barbara Romanowicz

Meeting Plate Tectonics – Barbara Romanowicz

These blogposts present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Get to know them, learn from their experience, discover the pieces of advice they share and find out where the newest challenges lie!


Meeting Barbara Romanowicz


Barbara Romanowicz studied mathematics and applied physics and did two PhDs, one in astronomy from Pierre and Marie Curie University and one in geophysics from Paris Diderot University. After her postdoctoral studies at the Massachusetts Institute of Technology, she researched at the Centre national de la Recherche Scientifique (CNRS), where she developed a global network of seismic stations known as GEOSCOPE to study earthquakes and the interior structure of the earth. She currently splits her time between a professorship at UC Berkeley, California, where she does research, and a teaching position as the Chair in Physics of the Earth’s interior at Collège de France, in Paris, where she teaches to the public.

I go between theory and observations, back and forth.

What is your main research interest and which approach do you use in your research?

Barbara Romanowicz in class. Credit: Barbara Romanowicz

My main research interest is the Earth’s interior: figuring out the dynamics and the evolution of the Earth by providing constraints from seismic imaging at the global and continental scale, from the lithosphere to the inner core of the Earth. The methodology that we use is primarily tomography. In my team, we develop new techniques in tomography, so we can achieve higher resolution. But also other types of seismic waveform modelling.

What would you say is the favorite aspect of your research?

What I find most exciting is that I go between theory and observations, back and forth. This brings different types of excitements. For example, developing a method that works is exciting, and so is finding something new in the data. Making progress and discovering something new, basically through a lot of attempts at modelling, and commonly after a lot of time, is very rewarding.

If we do not contribute to it, we will not have any more data.

Why is your research relevant? What are the possible real world applications?

The research is relevant because we are trying to understand the driving mechanisms of plate tectonics. And plate tectonics is what causes earthquakes, volcanoes, tsunamis, and all other natural disasters related to the solid Earth. It is not directly relevant, of course, because of the different timescales; the dynamics of the interior of the Earth are in millions of years, and people are interested in timescales of decades, maybe hundreds of years. So this is a bit of a challenge, but if we do not understand the causes of natural disasters, it is not possible to mitigate them.

Depth cross-sections through model SEMUCB_WM1 (French and Romanowicz, Nature – 2015, doi:https://doi.org/10.1038/nature14876) highlighting broad low velocity “plume-like” conduits beneath major hotspot volcanoes in the central Pacific.

What do you consider to be your biggest academic achievement?

I was asked this question recently, and I did not hesitate to say that I was able to make some impact with my research, but also to contribute to the infrastructure of research. I have been involved since very early in my career, in the development of seismic networks at a global and later regional scale, or trying to put stations in the oceans… Developing the infrastructure to collect data for research is a very recurrent issue that people should keep in mind: if we do not contribute to it, we will not have any more data. If the younger generation of researchers keeps on considering that the data is granted, and do not take up this challenge, the good situation that we’re at will not last.

I thought it is kind of cool that we could show that.

What would you say is the main problem that you solved during your most recent project?

In a fairly recent project, we were able to not only to confirm that there is an ultra slow velocity zone at the base of the Iceland plume near the core-mantle boundary, but also to determine that it is circular in shape. This required being able to illuminate it from different sides, and showing that the same model works for whichever way you look at it. I think that the fact that we can show that is kind of cool, as it combined modelling of seismic waveforms, as well as some imagination in 3D geometry.

Seasonal changes in the dominant locations of the sources of the earth’s low frequency “hum” (top) as inferred from seismic data, compared to the distribution of significant ocean wave height (bottom).

We are not doing enough to raise funds [to build a seismic network infrastructure].

What would you change to improve how science in your field is done?

In my field, which is global seismology, we really rely on a large network of stations, and we need a lot of instruments. Ideally, we would like to cover the entire Earth with instruments, which is not only logistically difficult but also very expensive. I think we are not doing enough to raise funds to build this better infrastructure. The astronomical community, for example, develop decadal plans to build the next generation instruments. In a way, it is easier for them because they need perhaps only a small number of telescopes, whereas our systems are completely distributed, so it is harder for us to join forces. Nevertheless, we are not doing enough of that.

3D rendering of a portion of upper mantle shear velocity model SEMum2 (French, Lekic and Romanowicz, 2013 – Science, doi:10.1126/science.1241514) showing interaction of mantle plume conduits with the asthenosphere beneath the south Pacific superswell (A) and the presence of quasi-periodic low velocity “fingers” aligned in the direction of absolute plate motion extending below the oceanic low velocity zone (B).

What do you think are the biggest challenges right now in your field?

There are several computational challenges, in the sense that we are moving increasingly towards modelling the complete seismic wavefield using numerical methods that are computationally very expensive. One has to think about how big the computer is that you can use, and balance that by finding smart ways to speed up computations in a way that doesn’t rely too much on big computers.

Another really big challenge is to reach the ocean floor and to cover the oceans with broadband seismic observatories. We don’t have enough such stations, and two-thirds of the Earth is covered by oceans. We have less resolution in the southern hemisphere and in the middle of the ocean just because we do not have enough seismic stations on the ocean floor. This is a problem for research on ocean basin structure and deeper upper mantle structure beneath the oceans, but also for research on the very deep Earth, including the inner core. Ocean Bottom Seismometers are great, but we really need very broadband recording, with good coupling to the ground and for long enough times (several years), as well as really large aperture arrays to be able to catch seismic waves over a large azimuth and depth range.

I never really worried about my career.

Barbara Romanowicz. Credit: Barbara Romanowicz

When you were in the early stages of our career, what were your expectations? Did you always see yourself staying in academia?

I think times have changed a lot. When I was doing my Ph.D., I really didn’t have any expectations. I never worried about my career. I simply did not think about it. Probably because I was naive, but also because there was less of a concern at that time… maybe it was easier to find jobs. The landscape was quite different.

Primarily thinking about their [ECS] research will get them where they want to be.

What is the best advice you ever received?

I think the best advice I received is to be daring, to think broadly and about the big picture. So, my best advice to Earth Career Scientists (ECSs) is the same. I would recommend ECSs not to worry too much about their immediate results or about their citation index, but to really think about their research. Primarily thinking about their research will lead them where they want to be. Otherwise, their thinking can be polluted by practical worries. Also, you will always get into situations where you cannot do all the work that you need to do for your research because you have other demands on your time. So my other advice to ECSs is to always keep a couple of hours (the best ones) during the day to completely isolate yourself and work on your research. It is very important. Everything else is easier, but the research itself is the hardest, and if you get distracted you will end up frustrated by not being able to accomplish much.

 

Barbara Romanowicz. Credit: Barbara Romanowicz

 

Interview conducted by David Fernández-Blanco

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?

 

Introducing our new blog team!

Introducing our new blog team!

After three succesful first years of the Tectonics and Structural Geology blog, it is time to bring our platform to the next level! To provide you more frequent content over a wide range of topics, we invited some new people to join our team. We are still always on the lookout for new guest authors and/or team members, so let us know if you want to contribute! So, what will you be reading on our blog? We will continue the successful Minds over Methods and Geology in the City series, but we have some great new things coming up as well. Since our Meeting Plate Tectonics series is coming to an end, we plan on including new interviews with scientists from the TS field, that will hopefully continue you to inspire you. And we are very happy to have brought back to life our Features from the Field series, where we discuss common structures in the field in an accessible way. Curious to know about the other new content of the blog? Stay tuned!

So, who are those people behind the Tectonics and Structural Geology blog?

 

Elenora van Rijsingen

I am a postdoc in geophysics at École Normale Supérieure in Paris, France. In my research I focus on seismotectonics, using various methods to understand more about earthquakes in subduction zones. I am fascinated by nature, and especially the power of it. Thinking of what nature is capable of reminds me of how small us humans actually are, and helps me to put things in perspective. I have been part of the TS blog since the beginning, trying to create continuous content and to bring in new people. I love editing blog posts, such as the Minds over Methods blogs, interacting with other scientists about their work and learning many new things. I occasionally write blogs myself as well, such as the Mind Your Head series about mental health in academia, a topic I believe deserves more attention. Please contact me via e-mail if you have any questions or ideas about the blog.

 

David Fernández-Blanco

I’m an early career tectonicist that likes to mingle with other disciplines… and making lists. My big goal in life is to understand how the Earth’s vertical motions evolve in time. To answer this question, I do 3 things that I love. [1] Fieldwork, especially in islands and other areas where I might get burned and I have to hand-speak for anything I might need; [2] Mingle things around using bits and pieces from other disciplines, especially geomorphology, stratigraphy, geodynamics. [3] Take up the challenge of geo-communication and translate our geeky, scienc-y, sometimes complicated geo-knowledge to a more general audience. That’s maybe my favourite 3-item list! (I said I like lists, remember?). I also like music, movies, travelling and drinking with my friends, just like every Tom, Dick and Harry. I’m currently the ECS TS Representative, so get in contact via e-mail or reach me at @_GeoDa_ or visit my webpage. Rock on!

 

Derya Guerer

I am a Lecturer in Earth Sciences at the School of Earth and Environmental Sciences, University of Queensland, Brisbane, Australia. My research evolves around tectonics and the evolution of Earth’s lithosphere at various spatio-temporal scales. I combine field-based observations (structural geology, stratigraphy) with laboratory analysis (U-Pb geochronology, paleomagnetism, micro-structural analyses) to build kinematic reconstructions and compare those to the structure of the underlying mantle imaged by seismic tomography. My current research projects focus on subduction-dominated records in the Tethyan region (with focus on Turkey, Iran and Ladakh Himalaya) and recently the SW Pacific realm. As part of the blog team I am happy to edit blogs, particularly for the ‘Minds over Methods’ series and also to occasionally contribute myself. In my free time, I enjoy the outdoors, travel, food and a good cup of coffee with friends. You can reach me via e-mail.

 

 

Anne Pluymakers

I am a post-doctoral researcher in the Rock Mechanics Lab at TU Delft, in the Netherlands. My work-related hobby is figuring out how rocks break, and how fluids affect fracture dynamics. My main rock of interest is limestone at the moment, but I also happily work on related topics, such as what do fractures in natural rocks look like, and imaging projects on fluid flow. I  mostly do laboratory work, and any associated microstructural investigations. But there are also the occasional field excursions, to not lose touch with geology. In the blogteam I am happy to edit blogs, and also to occasionally contribute myself. Outside of work, I love sitting in the sun with a decent cup of coffee and a good book, or to organize dinners for my friends. You can reach me via e-mail.

 
Samuele Papeschi

I have recently gained my PhD at the University of Florence (Italy) and -before that- I graduated in geology at the University of Pisa. My research is focused on understanding deformation of rocks, investigated in the field and in the lab. I combine classic structural geology techniques, like field surveying, with powerful analytical tools such as electron back scatter diffraction and electron microprobe. I like to share everything from what I am researching to observations from the field. I believe, indeed, that research is not done if it’s not shared! I also run one of the most useless geoscience facebook pages: Geology is the Way. The field is my natural habitat and I will share snapshots from it in the ‘Features from the Field’ series, hoping that you will enjoy looking at it through my eyes. You can reach me via e-mail.

 

Hannah Davies

I am a PhD student at Lisbon University, Portugal. Using numerical models and GPlates, I am investigating the link between plate tectonics and tides in the deep future and past. It was recently discovered that tides change over geological time scales as ocean basins change shape due to plate tectonics. As an editor and writer of the TS blog I want to bring this newly discovered link between tides and tectonics to an audience which may not have heard of it yet. When I am not doing real science, I am devouring science fiction. I spend a good amount of my free time in coffee shops reading. When I don’t read, I like to explore Lisbon. It is a very old place with a lot of history from the past two millennia so there is always somewhere interesting to find. You can reach me via e-mail.

 

Silvia Brizzi

I am a postdoctoral researcher in the Natural and Experimental Tectonic group at the University of Parma, Italy. My research is aimed at understanding the relationship between geodynamic parameters and the seismogenic behaviour of the subduction megathrust. More specifically, by combining observational and analog modelling approaches, I try to understand if specific (geodynamic) conditions can favour the occurrence of very large earthquakes in subduction zones. At present, I am working (really hard!) to measure and calibrate the rheological properties of innovative materials with complex rheologies to better mimic Earth’s behavior in the lab (yes, The Sassy Scientist, I am actually spending months on this!!). I am really excited to be part of the blog team as an editor. I will also be in charge of sharing our activities on Twitter. In my spare time, I love to read books and binge-watching TV series. Oh, and I also love aperitivo with friends. You can reach me via e-mail.

Meeting Plate Tectonics – Jean-Philippe Avouac

Meeting Plate Tectonics – Jean-Philippe Avouac

These blogposts present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Get to know them, learn from their experience, discover the pieces of advice they share and find out where the newest challenges lie!


Meeting Jean-Philippe Avouac


Prof. Jean-Philippe Avouac initially studied mathematics and physics during his undergraduate and graduate degrees. Later he got more inclined towards geophysics and then he discovered Earth Sciences. During his Ph.D. at the Institut de Physique du Globe de Paris, advised by Paul Tapponnier, he immersed himself in geology and tectonic geomorphology. Currently, Jean-Philippe Avouac is a Professor of Geology at the California Institute of Technology.

Like living organisms, earthquakes have a life cycle: they nucleate, grow and arrest. There can be some lineage but each earthquake is a different being.

Fieldwork along the Kali Gandaki (Nepal) in 1999. Credit: Barbara Avouac

Where lies your main research interest?

I study crustal dynamics: How the crust is deforming as a result of earthquakes, but also as a result of viscous processes. I study the process of stress accumulation on faults, the release of this stress by earthquakes, as well as how earthquakes and other mechanisms of deformation are contributing to building the topography and geological structures in the long run.

 

How would you describe your approach and methodology?

In my group, we develop techniques to measure crustal deformation using in particular remote sensing and seismology. We were using radar images initially, and we have moved toward using more optical images with time and also GPS data… We try to reproduce the observations (geodetic deformation, kinematic models of seismic ruptures, gravity field…) using dynamic models to determine what are the forces and rheologies needed.

 

What would you say is the favorite aspect of your research?

What I like most about my research is mentoring Ph.D. students and postdocs. I love matching their skills with good problems, problems that will be attractive to them and that will resonate with the currently hot questions in Earth Sciences. I really love doing that.

The other thing I love is to use what I learned as I student (maths and physics) to answer science questions arising from natural observations. I love that part when you look at nature, you observe something and try to measure it quantitatively and then you try to explain the observation with dynamic models. I really enjoy going back and forth between observations and modelling. And the field! I really like being in the field… This is an aspect of the job that really attracted me initially.

We built from what other researchers had done before, but we reached quite different conclusions […] that’s exciting!

Jean-Phillipe Avouac leading a field excursion in the Dzungar basin, 2006. Credit: Aurelia Hubert-Ferrari

 

Why is your research relevant? What are the possible real-world applications?

A significant fraction of my research is relevant to seismic hazards. After my Ph.D., I worked for the Commissariat à l’Energie Atomique (CEA) for 10 years. I was conducting seismic hazard assessment studies for nuclear facilities. So, I have been exposed to the applied side of earthquake science and I like that some of the research we do in my group can help to improve the way we do seismic hazard assessments.

But what I really want to say is that I do not think relevance should drive academic research. In that regard, I should say that I don’t like much the way the funding system works today. I think there is too much emphasis on relevance to society. The idea that you start from stating problems of societal relevance, and only then see what kind of research we can do to solve this problem is not a good approach, in my opinion. I don’t think this is the way important scientific discoveries are made. You make discoveries by being curious, by observing nature with an open mind, by exploring new ideas and coming up with new concepts, or by observing something that is not explained in the current theoretical framework that we have and then you make use of the knowledge that you build after looking at these problems. There is no way you can clearly anticipate where the joyful exploration of an intriguing idea or observation can lead but we know from experience that the society benefits from curious scientific exploration. So, although I think there is relevance in what I am doing, I do not think that, in general, relevance to society should be driving academic research.

 

An outcome of Jean-Phillipe Ph.D Thesis, later published in Kinematic model of active deformation in Central Asia (Avouac and Tapponnier, GRL – 1993; doi: https://doi.org/10.1029/93GL00128).

I do not think relevance to society should drive academic research

What would you say is the main problem that you solved during your most recent project?

People in my group work on many different projects that are all very exciting to me. I’m going to mention just one project though because I can not possibly list them all.

We have done a lot of work in the past to develop techniques to invert geodetic measurements for fault slip at depth. A postdoc and a graduate students in my group have moved on to improve the technique and use it to document slow slip events in Cascadia over the last 15 years. That was a daunting work but their hard work and perseverance have really paid back. The end result is amazing! We see how the slow slip event initiate, propagate, arrest, trigger one another… We built from what other researchers had done before us, but we reached quite different conclusions given that we now have a more complete view of the behaviour of the system –that’s exciting! I anticipate that we are going to learn a lot about the dynamics of slow-slip events, and maybe it will have important implications for regular earthquakes!

What do you consider to be your biggest academic achievement?

The research for which my group is probably best known is that we have done in the Himalaya. In particular, we have built a model of the seismic cycle that explains the observations that we have from seismology, geodesy, geomorphology and geology. We worked a lot on the Himalaya, in part because I love mountains, but also because it is a very unique setting to study orogenic processes which are still active today. There is really no better place where you can get geological constraints on the thermal and structural evolution of the range. There is a lot of erosion and it has been going on for a long time, so the rocks that have been brought to the surface have recorded the thermal and deformation history over tens of million years. Our research has helped understand how the Himalaya has formed as a result of seismic and aseismic deformation, and I think it has yielded important insight on orogenic processes and the seismic cycle in general.

By the way, I don’t mean that earthquakes are periodic. Like living organisms, earthquakes have a life cycle: they nucleate, grow and arrest. There can be some lineage but each earthquake is a different being.

Animation showing the process of stress build up and release associated to earthquakes along the Main Himalayan Thrust fault, along which India is thrust beneath the Himalaya and Tibet. Credit: Jean-Philippe Avouac, Tim Pyle and Kristel Chanard.

We tend to build walls between disciplines […] We would not have been able to discover plate tectonics without a deep cross-disciplinary dialogue

What do you think are the biggest challenges right now in your field?

As I mentioned before, the funding system is an issue. Funding agencies are clearly making a big mistake in prioritizing social relevance as a criterion to evaluate proposals. Aside from that, the challenge that we have in the Earth Sciences is that we tend to build walls between disciplines. Specialization is a natural drift, and you can make a very successful career in a particular field pushing further a particular analytical or modelling technique. Also, it is easier to get funding for what you are known to be good at. As a result, walls between disciplines are building with time. The vocabulary is evolving in each individual discipline and it is increasingly difficult to make major advancements that can bridge different disciplines. In my research, I try to navigate from one discipline to the other… but it is a challenge –while it can be key to make significant discoveries, it takes time and effort. There are fewer and fewer people making a carrier this way. It can be dangerous because of a dilution effect, but at some point, it is needed. Look at plate tectonics for example: it happened because of advances in different disciplines but most importantly because some scientists were aware of these advances and were able to connect them and derive a coherent global framework. We would not have been able to discover plate tectonics without a deep cross-disciplinary dialogue.

Another challenge is that nowadays we have a lot more data than we used to have. This is both an opportunity and a threat. There is a trend to produce more and more publications, that look very solid because they use a lot of data, but that are in fact very incremental. More of the same is not necessarily advancing knowledge at a fundamental level. We have to be imaginative with regard to how to process the increasing flux of data, but it should not come at the cost of being imaginative with regard to what they mean.

I do not like the way the funding system works today

When you were in the early stages of our career, what were your expectations? Did you always see yourself staying in academia?

After my Ph.D. I did not stay in academia. But even when outside academia, I kept doing research, because I had an appetite for it and was working in an environment where scientific curiosity was valued, even if science was not the main objective. Although I was not unhappy at all outside academia, I decided to go back to it since I found it more exciting for myself: I like to solve scientific questions but there is not so much I could solve without the help of students and postdocs. I didn’t consider staying in academia after my PhD because there were sides of the academic life I did not feel comfortable with… I was finding people in academia to be a bit… difficult sometimes, with big egos and not so open minded. Also, we are a very conservative community. There’s a reason for that, for we as scientists have to be sceptical and to push back new ideas and new observations. I guess I have now become now one of those crazy and conservative academic guys (laughs)!

 

Mapping and sampling Holocene terraces abandoned by rapid climate-driven incision in the Tianshan. Credit: Luca Malatesta

If you have a new idea… you will probably have a hard time

What advice would you like to share with Early Career Students?

My first advice is to be aware of the important questions that we should try to solve. Not because they are relevant but because they are interesting and because they are timely, given the tools and data that we have access to. Being aware of the really big questions is important because we tend to forget them sometimes as we become more specialized. And be also aware of the new techniques available, especially those that you could draw from other fields; computer science or medical imagery for example… It is important to be curious and see what is happening in other fields so that you can transfer new ideas and new techniques to your own field and give a try at answering big science questions.

Be curious, be adventurous. Take risks. Try things that might not work. You might be losing your time but it is also an opportunity to make real fundamental advancements. You can make a career by increments, but I think it is not as rewarding as taking risks and really solving a difficult problem.

Follow your own dreams and don’t be intimidated by peer pressure. If you put a new idea on the table, a really new one, first, you will probably have a hard time expressing it clearly… And second, peers will most probably push back, as they should. So do not be intimidated, believe in your ideas, and keep adjusting and pushing them forward. I see too many times students or postdocs who meltdown and get discouraged if they receive a negative comment after a presentation… – I would say, that could, in fact, be a good sign! You may be doing something different and maybe people are not understanding because there is something disturbing and really new!

 

Jean-Phillipe Avouac. Credit: Trish Reda.

 

Interview conducted by David Fernández-Blanco

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.

Meeting Plate Tectonics – Nicolas Coltice

Meeting Plate Tectonics – Nicolas Coltice

These blogposts present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Get to know them, learn from their experience, discover the pieces of advice they share and find out where the newest challenges lie!


Meeting Nicolas Coltice


Nicolas Coltice graduated with a PhD from the École Normale Supérieure of Lyon, France. He then became assistant professor at the Université Claude Bernard in Lyon, and ultimately, full professor. As of last year, he also holds a professorship position at ENS Paris, France. He has received an ERC grant for the project AUGURY and he is one of the co-founders of the manifesto ’Did this really happen?’, which addresses sexual harassment and inequality issues within sciences.

 

Nicolas Coltice. Credit: Eric Le Roux / Université Claude Bernard Lyon 1.

I think it is extremely important that models are supported by evidence or data.

Hi Nicolas, could you tell us about your research interests and the methods you use to solve your problems?

Sure! My research interest is focussed on mantle convection and geochemistry. The research I do is strongly directed to combine models and observations to understand, for example, the geochemical cycle. I also combine observations and inverse models to build tectonic reconstructions and 3D spherical models. I work a bit with geologists and so I sometimes go into the field. I think it is extremely important that models are supported by evidence (or data) and so I try to combine this as much as I can in my research.

You have been active on different topics. What achievement in your carer are you most proud of?

The one thing I’m most proud of is setting up an ERC team for the project ‘AUGURY’, which happened to have more women than men, which is quite rare in our field. I feel we made quite some progress on undermining the patriarchy within sciences with this ERC project. I’m very proud to work with my team. One of the good things that came out of ‘AUGURY’ is our manifesto ’Did this really happen?’. It is a website where we tell the stories on sexual harassment and gender inequality that women in sciences using comics. Besides advocating gender equality science I also teach, which I find very fulfilling and my teaching is well-received.

Good research needs time.

Did this really happen?. Courtesy of www.didthisreallyhappen.net.

It’s fantastic that you are making the community aware of these more social issues. In terms of research, how does that benefit society?

The application of my research to society is first of all doing the job by itself. Every day that scientists invest in understanding parts of our planet is beneficial to society, just by the very act of it. Publishing my work might give a breach to society and offer perspectives that were not thought of before. I guess a more concrete way my research benefits society would be in the reserve or resources industry, where we always like to understand better where resources form and why they form under certain condition. This will eventually help to actually find them and exploit them and the better we understand that, the less impact it will eventually have on the environment.

Every day that scientists invest in understanding parts of our planet is beneficial to society, just by the very act of it.

 

How do you see the future in geoscience?

In my opinion, good research needs time. Currently we are given very little time to do good research. If we want to change the publishing-focussed mentality, we need to start at the bottom. We do not necessarily have to create a big revolution, but from the inside we can collaborate and slowly change the system. For example, if you publish, public money is used to pay for your publication. This public money then often goes to stakeholders, which is not good! We can change this by publishing in different journals with different ethics. This way, we can slowly lower the pressure we feel on publishing nowadays. So in terms of future, I think we need to get back to the core, do good research.

Selected 3-D view state of the model. Continental material is highlighted in yellow. Figure from Coltice & Shephard, 2018 “Tectonic predictions with mantle convection models”, Geophysical Journal International, 10.1093/gji/ggx531.

When you feel it gets rough, stick with your plan and keep your relationship with your colleagues positive.

One last question, what advice would you like to give to Early Career Scientists?

When I was hired 15 years ago, times were different. If recruiters had the choice, they would always go for the youngest person, not necessarily the best. Nowadays productivity is the factor that counts most and is imposed on people which makes it very difficult to maintain an interesting profile at an early stage in your career. I would advise to find time and space to feel good and let go of the pressure you might feel in your work. I believe there is room for everyone, just keep the spirit up. When you feel it gets rough, stick with your plan and keep your relationship with your colleagues positive.

Nicolas Coltice. Credit: Nicolas Coltice.

 

Interview conducted by Anouk Beniest