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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.

Minds over Methods: Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

Minds over Methods: Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

For this first Minds over Methods of 2019, we invited Christopher Spencer, Senior Research Fellow at Curtin University in Australia, to tell us something about tectonochemistry. By applying geochemistry to tectonic processes, it is possible to get more insight into the different stages of the rock cycle. By combining fieldwork and geochemical analyses of the Oman/UAE ophiolite, Chris and his co-workers believe they found the first direct and in-situ evidence of sediment melting in the mantle.

 

Credit: Christopher Spencer

Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

Christopher Spencer, Senior Research Fellow, Curtin University, Australia

The rock cycle is the first thing we learn in Geology 101. Magma and lava cool to form igneous rocks. Igneous rocks then erode to form sediment, which forms sedimentary rocks as it is compacted. Increasing pressure and heat then create metamorphic rocks, which eventually will melt. In each of the transitions described in the rock cycle, tectonics is usually involved. Granite batholiths form in subduction zones and are uplifted and eroded in collision zones. The sediments derived therefrom are deposited along continental margins that are often then returned to subduction zones where they contribute to new magmatic systems. There is a wide array of tools that we can use to evaluate the role of tectonics in the rock cycle, of which geochemistry is able to provide insight into each stage of the process.

Applying geochemistry to tectonics is (unsurprisingly) referred to as tectonochemistry. Similar to tectonophysics, where geophysics is applied to address large-scale tectonic questions, tectonochemistry provides a unique view into geochemical proxies of tectonic processes. The melting of sediment along convergent margins is a classic tectonochemical problem, as the unique chemical signature of sediment found in a granite provides unequivocal evidence for the melting of a sedimentary rock. In collisional systems, like the Himalaya, tectonochemistry has been used to constrain the melting of meta-sedimentary rocks as crustal thickening and decompression drives dehydration of micas which leads to melting. Collisional systems provide clear and in situ evidence for sediment melting.

Figure 1: Clockwise from top left: tourmaline-bearing leucogranite from the Himalaya in NW India, leucogranite dykes intruding meta-sedimentary rocks exposed at 5000m altitude, in situ melting of meta-pelite and formation of leucogranite, incongruent melting of muscovite + plagioclase + quartz to form leucogranite but leaving the biotite behind. Credit: Christopher Spencer.

 

Sediments are also thought to melt in subduction systems, but given the difficulty of accessing the asthenosphere directly, it is more challenging to constrain the processes occurring deep in a subduction zone. The incorporation of sediment in subduction zones is often constrained using the geochemistry of the resulting magmatic rocks. The chemical signature of sediment provides a clear indication of its incorporation in the magma, but it is often unclear whether the contamination is occurring in the asthenospheric wedge or in the upper crust. For example, many granite batholiths contain zircon grains that are foreign to the host magma and whose age spectra match the detrital zircon age spectra of the adjacent sedimentary units. This relationship is a clear indication that sedimentary contamination occurred in the upper crust. Unfortunately, the geochemical proxies used to establish the sedimentary contamination only provide indirect evidence for the subduction of sedimentary material into the asthenospheric wedge. Such indirect evidence includes seismic stratigraphy showing sedimentary units being subducted beneath the forearc and whiffs of sedimentary geochemical signals in arc volcanics. Although these evidences point towards sediment being subducted deep into the asthenospheric wedge where it melts and contaminates the magmas coming off the subducting slab, they do not preserve direct evidence of sediment melting in the mantle.

To acquire direct evidence of processes happening deep in the mantle, I set my sights on the Oman/UAE ophiolite, where a thick succession of mantle peridotite is preserved beneath a complete stratigraphic section of oceanic crust. Previous work has shown that this ophiolite not only preserves an intact record of oceanic crustal stratigraphy, but also geochemical features of a subduction zone in the oceanic crust. This implies the ophiolite formed in a supra-subduction setting, where during the earliest phase of subduction, extension in the upper plate caused rifting and formation of oceanic crust above a subduction zone.

 

Figure 2: Oceanic crustal stratigraphy of the Oman/UAE ophiolite comprised of (clockwise from top left): pillow basalts, sheeted dykes, layered gabbros, and mantle peridotite. Credit: Christopher Spencer.

 

During fieldwork in the ophiolite, while traversing the 8-15 km thickness of the mantle peridotite, I encountered a number of granitoid dykes that cross cut the peridotite, but do not cross the petrologic Moho. Many of these dykes contained tourmaline, muscovite, biotite, and even andalusite, minerals that would be expected from the melting of sedimentary material. Finding these minerals in the mantle indicates these grantoid dykes formed from the melting of sedimentary material and here they were within the mantle! Subsequent analysis of zircon grains from these granitoid dykes revealed the age of these dykes was equivalent to the age of the overlying ophiolite providing bullet-proof evidence that they intruded while the ophiolite was forming above a subduction zone. To provide the nail in the coffin for a sedimentary origin, I performed oxygen isotope analysis of the zircon and quartz. Sedimentary material has a distinct oxygen isotopic composition and igneous rocks that are thought to have experienced sediment contamination have δ18O values that lie along mixing lines between a sediment end member and the mantle. The oxygen isotopic analyses of the sub-Moho granitoids of the Oman/UAE ophiolite revealed the highest δ18O values ever measured in igneous rocks, providing unequivocal evidence that these granitoids represent pure sediment melts. In a paper published in Geology (Spencer et al., 2017), my coauthors and I argue these igneous rocks represent the first direct and in situ evidence of sediment melting in the mantle. Lucky for us, we have just scratched the surface of the exciting things left to learn about these fascinating granitoids and I look forward to the opportunity to return to the Oman/UAE ophiolite.

Figure 3: Sub-Moho granitoids of the Oman/UAE ophiolite: A) Cathodoluminescence image of a zircon shown with location and result of δ18O analyses. B) Photograph of sub-Moho granitoids. C) Hand sample of granite with tourmaline and lepidolite (lithium-bearing mica). Credit: Christopher Spencer.

 

Minds over Methods: What controls the shape of oceanic ridges?

Minds over Methods: What controls the shape of oceanic ridges?

In this edition of Minds over Methods, Aurore Sibrant, postdoc at Bretagne Occidentale University (France) explains how she studies the shape of oceanic ridges, and which parameters are thought to control this shape. By using laboratory experiments combined with observations from nature, she gives new insights into how spreading rates and lithosphere thickness influence the development of oceanic ridges. 

 

Credit: Aurore Sibrant

What controls the shape of oceanic ridges? Constraints from analogue experiments

Aurore Sibrant, Post-doctoral fellow at Laboratoire Géosciences Océans, Bretagne Occidentale University, France

Mid-oceanic ridges with a total length > 70 000 km, are the locus of the most active and voluminous magmatic activity on Earth. This magmatism directly results from the passive upwelling of the mantle and decompression melting as plates separate along the ridge axis. Plate separation is taken up primarily by magmatic accretion (formation of the oceanic crust), but also by tectonic extension of the lithosphere near the mid-ocean ridge, which modifies the structure of the crust and morphology of the seafloor (Buck et al., 2005). Therefore, the morphology of the ridge is not continuous but dissected by a series of large transform faults (> 100 km) as well as smaller transform faults, overlapping spreading centres and non-transform offsets (Fig. 1). Altogether, those discontinuities form the global shape of mid-ocean ridges. While we understand many of the basic principles that govern ridges, we still lack a general framework for the governing parameters that control segmentation across all spreading rates and induce the global shape of ridges.

Geophysical (Schouten et al., 1985; Phipps Morgan and Chen, 1993; Carbotte and Macdonald, 1994) and model observations (Oldenburg and Brune, 1975, Dauteuil et al., 2002, Püthe and Gerya, 2014) suggest that segmentation of oceanic ridges reflects the effect of spreading rate on the mechanical properties and thermal structure of the lithosphere and on the melt supply to the ridge axis. To understand the conditions that control the large-scale shape of mid-ocean ridges, we perform laboratory experiments. By applying analogue results to observations made on Earth, we obtain new insight into the role of spreading velocity and the mechanical structure of the lithosphere on the shape of oceanic ridges.

 

Laboratory experiments

The analogue experiment is a lab-scale, simplified reproduction of mid-oceanic ridges system. Our set-up yields a tank filled from bottom to top by a viscous fluid (analogous to the asthenosphere) overlain by the experimental “lithosphere” that can adopt various rheologies and a thin surface layer of salted water. This analogue lithosphere is obtained using a suspension of silica nanoparticles which in contact with the salted water emplaced on the surface of the fluid causes formation of a skin or “plate” that grows by diffusion. This process is analogous to the formation of the oceanic lithosphere by cooling (Turcotte and Schubert, 1982). With increasing salinity, the rheology of the skin evolves from viscous to elastic and brittle behaviour (Di Giuseppe et al., 2012; Sibrant and Pauchard, 2016).

The plate is attached to two Plexiglas plates moving perpendicularly apart at a constant velocity. The applied extension nucleates fractures, which rapidly propagate and form a spreading axis. Underlying, less dense, fresh fluid responds by rising along the spreading axis, forming a new skin when it comes into contact with the saline solution. By separately changing the surface water salinity and the velocity of the plate separation, we independently examine the role of spreading velocity and axial lithosphere thickness on the evolution of the experimental ridges.

 

Figure 2. Close up observations of analogue mid-oceanic ridges and schematic interpretation for different spreading velocity. The grey region is a laser profile projected on the surface of the lithosphere: the laser remains straight as long as the surface is flat. Here, the large deviation from the left to centre of the image reveals the valley morphology of the axis. Credit: Aurore Sibrant.

 

Analogue mid-oceanic ridges

Over a large range of spreading rates and salinities (Sibrant et al., 2018), the morphology of the axis is different in shape. The ridge begins with a straight axis (initial condition). Then during the experiment, mechanical instabilities such as non-transform offset, overlapping spreading centres and transform faults develop (Fig. 2) and cause the spreading axis to have a non-linear geometry (Fig. 3). A key observation is the variation of the shape of the analogue ridges with the spreading rate and salinities. For similar salinity and relative slow spreading rates, each segment is offset by transform faults shaping a large tortuous ridge (i.e. non-linear geometry). In contrast, at a faster spreading rate, the ridge axis is still offset by mechanical instabilities but remains approximately linear.

Figure 3. Ridge axis morphology observed in the experiments and schematic structural interpretations of the ridge axis, transform faults (orange ellipsoids) and non-transform faults (purple ellipsoids). Measurements of lateral deviation (LD) correspond to the length of the arrows. For comparison, white squares represent the size of closeup shows in Fig 2. Credit: Aurore Sibrant.

We can quantify the ridge shape by measuring the total lateral deviation, which is the total accumulated offset of the axis, when the tortuosity amplitude becomes stable. For cases with similar salinities, the results indicate two trends. First, the lateral deviation is high at slow spreading ridges and decreases within increasing spreading rate until reaching a minimum lateral deviation value for a given critical spreading rate (Fig 4A). Then the lateral deviation remains constant despite the increasing spreading rate. Experiments with different salinities also present a transition between tortuous and linear ridges. These two trends reflect how the lithosphere deforms and fails. In the first regime, the axial lithosphere is thick and is predominantly elastic-brittle. In such cases, the plate failures occur from the surface downwards through the development of faults: it is a fault-dominated regime. In contrast, for faster spreading rate or smaller salinities, the axial lithosphere is thin and is predominantly plastic. Laboratory inspection indicates that fractures in plastic material develop from the base of the lithosphere upwards: it is a fluid-intrusion dominated regime.

 

 

Comparison with natural mid-oceanic ridge

In order to have a complete understanding of the mid-oceanic ridge system, it is essential to compare the laboratory results with natural examples. Hence, we measure the lateral deviation of nature oceanic ridges along the Atlantic, Pacific and Indian ridges. The measurements reveal the same two regimes as found in laboratory data. The remaining step consists of finding the appropriate scaling laws to superpose the natural and experiment data. This exercise requires dynamics similarity between analogue model and real-world phenomena which is demonstrated using dimensionless numbers (Sibrant et al., 2018). Particularly, the “axial failure parameter – πF” describes the predominant mechanical behaviour of the lithosphere relative to its thickness. Low-πF accretion is dominated by fractures in a predominantly elastic-brittle lithosphere: the lateral deviation of the ridges is tortuous, while at higher pF, accretion is dominated by intrusion in a predominantly plastic lithosphere: the shape of the mid oceanic ridges is mostly linear (Fig 4B).

 

Figure 4. (A) Lateral deviation values measured in the experiments in function of the spreading rate velocities and salinities. (B) Evolution of the lateral deviation of the ridge axis, normalized by the critical axial thickness (Zc) relative to the axial failure parameter. Dark grey is the laboratory experiments and the colored circles are the Earth data. Adapted from Sibrant et al., 2018.

 

Our experiments give insight into the role of axial failure mode (fault-dominated or intrusion-dominated) on the shape of mid-oceanic ridges. In the future, we want to use this experimental approach to investigate the origin of mechanical instabilities, such as transform faults or overlapping spreading centres. This experimental development and results are a collaborative work between Laboratoire FAST at Université Paris-Saclay and Department of Geological Sciences at the University of Idaho and involves E. Mittelstaedt, A. Davaille, L. Pauchard, A. Aubertin, L. Auffray and R. Pidoux.

 

 

References
Buck, W.R., Lavier, L.L., Poliakov, A.N.B., 2005. Modes of faulting at mid-ocean ridges. Nature 434, 719-723.
Schouten, H., Klitgord, K.D., Whitehead, J.A., 1985. Segmentation of mid-ocean ridges. Nature 317, 225-229.
Carbotte, S.M., Macdonald, K. C., 1994. Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns. J. Geophys. Res. 99, 13609-13631.
Phipps Morgan, J., Chen, J., 1993. Dependence of ridge-axis morphology on magma supply and spreading rate. Nature 364, 706-708.
Oldenburg, D.W., Brune, J.N., 1975. An explanation for the orthogonality of ocean ridges and transform faults. J. Geophys. Res. 80, 2575-2585.
Dauteuil, O., Bourgeois, O., Mauduit, T., 2002. Lithosphere strength controls oceanic transform zone structure: insights from analogue models. Geophys. J. Int. 150, 706-714.
Püthe, C., Gerya, T., 2014. Dependence of mid-ocean ridge morphology on spreading rate in numerical 3-D models. Gondwana Res. 25, 270-283.
Turcotte, D., Schubert, G., Geodynamics (Cambridge Univ. Press, New York, 1982).
Di Giuseppe, E., Davaille, A., Mittelstaedt, E., Francois, M., 2012. Rheological and mechanical properties of silica colloids: from Newtonian liquid to brittle behavior. Rheologica Acta 51, 451-465.
Sibrant, A.L.R., Pauchard, L., 2016. Effect of the particle interactions on the structuration and mechanical strength of particulate materials. European Physics Lett., 116, 4, 10.1209/0295-5075/116/49002.
Sibrant, A.L.R., Mittelstaedt, E., Davaille, A., Pauchard, L., Aubertin, A., Auffray, L., Pidoux, R., 2018. Accretion mode of oceanic ridges governed by axial mechanical strength. Nature Geoscience 11, 274-279.

 

Lisbon at the dawn of modern geosciences

Lisbon at the dawn of modern geosciences

Here, where the land ends and the sea begins...
Luís de Camões (Portuguese poet)

Lisbon. Spilled over the silver Tagus River, it is known by its beautiful low light, incredible food and friendly people. Here, cultures met, and poets dreamed, as navigators gathered to plan their journeys to old and new worlds. Fustigated by one of the greatest disasters the world has ever witnessed, Lisbon is intertwined with the course of Earth Sciences. For some, modern seismology was born here. For others, this might even have been the place where it all begun; what we now call geology.

On the morning of All Saints day of 1755, a giant earthquake struck the city of Lisbon. With a magnitude of ~8.7, the event was so powerful that it was felt simultaneously in Germany, as well as in the islands of Cape Verde. The main shock occurred around 9.40 am, when a significant portion of the population was attending the mass in churches. Lasting several minutes, many of the roofs collapsed and thousands of candles set fires that would last for days. While people were looking for safety at open areas near the river, three giant tsunami waves were on their way. Forty minutes after the main shock, the waves rose the Tagus River and flood the city’s downtown. The death toll in Lisbon reached up to 50,000 people, about one quarter of Lisbon’s population at the time. This event is known as the Great Lisbon Earthquake of 1755.

 

Painting depicting the day of the 1755 Great Lisbon Earthquake. Credit: Wikipedia.

 

The 1755 Lisbon Earthquake was a terrific natural disaster. A few years ago, the French magazine L´Histoire, considered this earthquake as one of the 10 crucial events that changed history. At the time, Lisbon was a maritime power in a maritime epoch. This was also the age of Enlightenment, when man started to realize that many events such as earthquakes, volcanoes and storms, had natural causes, and were not sent by gods.

Convento do Carmo, destroyed during the 1755 earthquake and kept as a ruin for memory. Credit: Flickr.

Lisbon was in the spotlight of the modern world and some of the most prominent philosophers like Kant, Voltaire and Rousseau focused on the destructive event of the 1st of November, 1755. In particular, Emmanuel Kant published in 1756 (yes, 1756!) three essays about a new theory of earthquakes (see Duarte et al., 2016 and the reference list below for two of the Kant’s essays). I recommend all geoscientists to read these documents. It is incredible how Kant understands and describes how earthquakes align along linear features that are parallel to mountain chains. Does this sound familiar? Moreover, he uses the then new physics of Newton to calculate the forces that were needed to set the seafloor off Lisbon in movement in order to generate the observed tsunami. He even refers to experiments with buckets full of water to explain how the tsunami formed (analogue modelling!?). And Kant was not alone…

The minister of the King of Portugal at the time, the Marquis of Pombal, sent an enquiry to all parishes in the country with several questions. While some of the questions were intended to evaluate the extent of the damage, it is now clear that the Marquis was also trying to gain (scientific) knowledge about the event (see Duarte et al., 2016 and references therein). For example, he asks if the ground movement was stronger in one direction than in other, or if the tide rose or fell just before the tsunami waves arrived. Today, we can reconstruct with rigor what happened that day because of the incredible vision of this man.

 

The center of Lisbon today. The statue of Marquis of Pombal facing the reconstructed downtown. Credit: Wikipedia.

 

Coming back to Lisbon. If you visit the old city by foot, you will realize that houses on the hills are closely packed, separated by narrow streets and passages, while in the flat downtown streets are wide and orthogonal. The hilly parts of Lisbon are an heritage of the Moorish and Medieval times. Mouraria and Alfama are the ideal neighborhoods to visit. The organized downtown was the area that was totally floored during the earthquake, due to ground liquefaction and the impact of the tsunami, and was rebuilt using a modern architecture (see Terreiro do Paço and the downtown area in the first figure in the top). The Grand Liberty Avenue is clearly inspired by the style of the Champs-Élysées. Going up the Liberty Avenue, from the downtown, you will find the statue of the Marquis of Pombal (see figure above). And if you are already planning to visit (or revisit) Lisbon, you should definitely stop by the Carmo Archeological Museum, a ruin left to remind us all of what happened on that day of 1755, and the Lisbon Story Centre.

The hills of Lisbon, with the Castle in the top left and the 25 de Abril bridge in the background. Credit: Flickr.

Rebuilding plan after the 1755 earthquake. Credit: Wikimedia Commons.

The 1755 Great Lisbon Earthquake was however not the only earthquake that hit the city. On the 28th of February 1969, another major quake, with a magnitude of 7.9, struck 200 km off the cost of Portugal, at 2 am in the morning. The earthquake generated a small tsunami but luckily, given the late hours, did not caused any casualties. This event also occurred in a particular point in history: The time of plate tectonics. The paper that inaugurated plate tectonics had been published only 4 years before, by Tuzo Wilson. And in 1969, geoscientists already realized that some continental margins were passive and did not generate major earthquakes, such as the margins of the Atlantic, while others were active and fustigated by major earthquakes, such as the margin of the Pacific (Dewey, 1969). It was somewhat strange that this Atlantic region was producing such big earthquakes, which therefore immediately resulted in scientists coming to study this area (see map below).

Fukao (1973), studied the focal mechanism of the 1969 earthquake and concluded that it was a thrust event. Purdy (1975), suggested that this could result from a transient consumption of the lithosphere, and Mckenzie (1977) proposed that a new subduction zone was initiating here, along the east-west Africa-Eurasia plate boundary (see the thinner segment of the dashed white line in the eastern termination of the Africa-Eurasia plate boundary, map below), SW of Iberia. Later on, in 1986, António Ribeiro, professor at the University of Lisbon, suggested that instead, a new north-south subduction zone was forming along the west margin of Portugal (yellow lines in the map), a passive margin transforming into an active margin. This could explain the high magnitude seismicity, such as the Great Lisbon Earthquake of 1755.

 

Map showing the main tectonic features in the SW Iberia margin. The Eurasia-Africa plate boundary spans from the Azores-Tripe Junction (on the left) until the Gibraltar Arc (on the right, with its accretionary wedge marked in grey). The yellow lines mark a new thrust front that is forming and migrating northwards away from the plate boundary and along the west Iberia margin. The smaller yellow line marks the approximate location of the 1969 earthquake. The 1755 Great Lisbon Earthquake might also have been generated in this region (see Duarte et al., 2013 for further reading on the tectonic setting of the region; the figure is adapted from this paper).

 

Today, we know that the SW Iberia margin is indeed being reactivated (Duarte et al., 2013). Whether this will lead to the nucleation of a new subduction zone is still a matter of debate, and we will probably never know for sure. Nevertheless, subduction initiation is one of the major unsolved problems in Earth Sciences, and the coasts off Lisbon might constitute a perfect natural laboratory to investigate this problem. It may be the only case where an Atlantic-type margin (actually located in the Atlantic) is just being reactivated, which is a fundamental step in the tectonic conceptual model that we know as the Wilson Cycle (see also Duarte et al., 2018 and this GeoTalk blog). In any case, we know that there are two other locations where subduction zones have developed in the Atlantic: in the Scotia Arc and in the Lesser Antilles Arc. How they originated is still being investigated; which is precisely what we are doing now in Lisbon. That is however a topic that deserves its own blog post.

 

Written by João Duarte

Researcher at Instituto Dom Luiz and Invited Professor at the Geology Department, Faculty of Sciences of the University of Lisbon. Adjunct Researcher at Monash University.

 

Edited by Elenora van Rijsingen

PhD candidate at the Laboratory of Experimental Tectonics, Roma Tre University and Geosciences Montpellier. Editor for the EGU Tectonics & Structural geology blog

 

For more information about the Great Lisbon Earthquake of 1755, check out these two video’s about the event: a reconstruction of the earthquake and a tsunami model animation

 

References:

Dewey, J.F., 1969. Continental margin: A model for conversion of Atlantic type to Andean type. Earth and Planetary Science Letters 6, 189-197.

Duarte, J.C., Schellart, W.P., Rosas, F.R., 2018. The future of Earth’s oceans: consequences of subduction initiation in the Atlantic and implications for supercontinent formation. Geological Magazine. https://doi.org/10.1017/S0016756816000716

Duarte, J.C., and Schellart, W.P., 2016. Introduction to Plate Boundaries and Natural Hazards. American Geophysical Union, Geophysical Monograph 219. (Duarte, J.C. and Schellart, W.P. eds., Plate Boudaries and Natural Hazards). DOI: 10.1002/9781119054146.ch1

Duarte, J.C., Rosas, F.M., Terrinha, P., Schellart, W.P., Boutelier, D., Gutscher, M.A., Ribeiro, A., 2013. Are subduction zones invading the Atlantic? Evidence from the SW Iberia margin. Geology 41, 839-842. https://doi.org/10.1130/G34100.1

Fukao, Y., 1973. Thrust faulting at a lithospheric plate boundary: The Portugal earthquake of 1969. Earth and Planetary Science Letters 18, 205–216. doi:10.1016/0012-821X(73)90058-7.

Kant, I., 1756a. On the causes of earthquakes on the occasion of the calamity that befell the western countries of Europe towards the end of last year. In, I. Kant, 2012. Natural Science (Cambridge Edition of the Works of Immanuel Kant Translated). Edited by David Eric Watkins. (Cambridge: Cambridge University Press, 2012).

Kant, I., 1756b. History and natural description of the most noteworthy occurrences of the earthquake that struck a large part of the Earth at the end of the year 1755. In, I. Kant, 2012. Natural Science (Cambridge Edition of the Works of Immanuel Kant Translated). Edited by David Eric Watkins. (Cambridge: Cambridge University Press, 2012).

McKenzie, D.P., 1977. The initiation of trenches: A finite amplitude instability, in Talwani, M., and Pitman W.C., III, eds., Island Arcs, Deep Sea Trenches and Back-Arc Basins. Maurice Ewing Series, American Geophysical Union 1, 57–61.

Purdy, G.M., 1975. The eastern end of the Azores–Gibraltar plate boundary. Geophysical Journal of the Royal Astronomical Society 43, 973–1000. doi:10.1111/j.1365-246X.1975.tb06206.x.

Ribeiro, A.R. and Cabral, J., 1986. The neotectonic regime of the west Iberia continental margin: transition from passive to active? Maleo 2, p38.

Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature 207, 343– 347

Minds over Methods: Linking microfossils to tectonics

Minds over Methods: Linking microfossils to tectonics

This edition of Minds over Methods article is written by Sarah Kachovich and discusses how tiny fossils can be used to address large scale tectonic questions. During her PhD at the University of Brisbane, Australia, she used radiolarian biostratigraphy to provide temporal constraints on the tectonic evolution of the Himalayan region – onshore and offshore on board IODP Expedition 362. Sarah explains why microfossils are so useful and how their assemblages can be used to understand the history of the Himalayas. And how are new technologies improving our understanding of microfossils, thus advancing them as a dating method?

 

                                                                          Linking microfossils to tectonics

Credit: Sarah Kachovich

Sarah Kachovich, Postdoctoral Researcher at the School of Earth and Environmental Sciences, The University of Queensland, Australia.

Radiolarians are single-celled marine organisms that have the ability to fix intricate, siliceous skeletons. This group of organism have captured the attention of artist and geologist alike due to their skeletal diversity and complexity that can be observed in rocks from the Cambrian to the present. As a virtue of their silica skeletons, small size and abundance, radiolarian skeletons can potentially exist in most fine-grained marine deposits as long as their preservation is good. This includes mudstones, hard shales, limestones and cherts. To recover radiolarians from a rock, acid digestion is commonly required. For cherts, 12-24 hours in 5 % hydrofluoric acid is needed to liberate radiolarians. Specimens are collected on a 63 µm sieve and prepared for transmitted light or scanning electron microscope analysis.

Animation of radiolarian diversity. Credit: Sarah Kachovich

Scale and diversity of modern radiolarians. Credit: Sarah Kachovich (radiolarians from IODP Expedition 362) and Adrianna Rajkumar (hair).

 

 

 

 

 

 

 

 

 

 

Improving the biostratigraphical potential of radiolarians

The radiolarian form has changed drastically through time and by figuratively “standing on the shoulders of giants”, we correlate forms from well-studied sections to determine an age of an unknown sample. A large effort of my PhD was aimed to progress, previously stagnant, research in radiolarian evolution and systematics in an effort to improve the biostratigraphical potential of spherical radiolarians, especially from the Early Palaeozoic. The end goal of this work is to improve the biostratigraphy method and its utility, thus increasing our understanding of the mountain building processes.

The main problem with older deposits is the typical states of preservation, where radiolarians partly or totally lose their transparency, which makes traditional illustration with simple transmitted light optics difficult. Micro-computed tomography (µ-CT) has been adopted in fields as diverse as the mineralogical, biological, biophysical and anatomical sciences. Although the implementation in palaeontology has been steady, µ-CT has not displaced more traditional imaging methods, despite its often superior performance.

Animation of an Ordovician radiolarian skeleton in 3D imaged through µ-CT. Credit: Sarah Kachovich

To study small complex radiolarian skeletons, you need to mount a single specimen and scan it at the highest resolution of the µ-CT. The µ-CT method is much like a CAT scan in a hospital, where X-rays are imaged at different orientations, then digitally stitched together to reconstruct a 3D model. The vital function of the internal structures provides new insights to early radiolarian morphologies and is a step towards creating a more robust biostratigraphy for radiolarians in the Early Paleozoic.

Linking radiolarian fossils to tectonics

Radiolarian chert is important to Himalayan geologists as it provides a robust tool to better document and interpret the age and consumption of oceanic lithosphere that once intervened India and Asia before their collision.The chert that directly overlies pillow basalt in the ophiolite sequence (remnant oceanic lithosphere) represents the minimum age constraint of its formation. In the Himalayas, over 2000 km of ocean has been consumed as India rifted from Western Australia and migrated north to collide with Asia. Only small slivers of ophiolite and overlying radiolarian cherts are preserved in the suture zone and it is our job to determine how these few ophiolite puzzle pieces fit together.

Another way I have been able to link microfossils to Himalayan tectonics is by studying the history and source of erosion from the Himalayas on board IODP Expedition 362. Sedimentation rates obtained from deep sea drilling can provide ages of various tectonic events related to the India-Asia collision. For example, we were able to date various events such as the collision of the Ninety East Ridge with the Sumatra subduction zone, which chocked off the sediment supply to the Nicobar basin around 2 Ma as the ridge collided with the subduction zone.

Left: Results from the McNeill et al. (2017) of the sedimentation history of Bengal Fan (green dots) and Nicobar Fan (red dots). Middle/right: Reconstruction of India and Asia for the past 9 million years showing the sediment source from the Himalayas to both basins on either side of the Ninety East Ridge.

 

 

 

 

 

 

 

 

 

 

 

 

Lastly, on board Expedition 362 we were able to use microfossils to understand how and why big earthquakes happen. We targeted the incoming sediments to the Sumatra subduction zone that were partly responsible for the globally 3rd largest recorded earthquake (Mw≈9.2). This earthquake occurred in 2004 and produced a tsunami that killed more than 250,000 people.

From the seismic profiles (see example below), we found that the seismic horizon where the pre-decollement formed coincided with a thick layer of biogenetically rich sediment (e.g. radiolarians, sponge spicules, etc.) found whilst drilling. Under the weight of the overlying Nicobar Fan sediments, this critical layer of biogenic silica is undergoing diagenesis and fresh water is being chemically released into the sediments. The fresh water within these sediments is moving into the subduction zone where it has implications to the physical properties of the sediment and the morphology of the forearc region.

The Sumatra subduction zone. The dark orange zone represents the rupture area of the 2004 earthquake. Also shown are the drill sites of IODP Expedition 362 and the location of seismic lines across the plate boundary.

Seismic profile: The fault that develops between the two tectonic plates (the plate boundary fault) forms at the red dotted line. Note the location of the drill site.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

Hüpers, A., Torres, M. E., Owari, S., McNeill, L. C., Dugan, & Expedition 362 Scientists, 2017. Release of mineral-bound water prior to subduction tied to shallow seismogenic slip. Science, 356: 841–844. doi:10.1126/science.aal3429

McNeill, L. C., Dugan, B., Backman, J., Pickering, K. T., & Expedition 362 Scientists 2017. Understanding Himalayan Erosion and the Significance of the Nicobar Fan. Earth and Planetary Science Letters, 475: 134–142. doi:10.1016/j.epsl.2017.07.019