Archive | Tectonics and Structural Geology RSS feed for this section

Imaggeo on Mondays: A rolling stone gathers no moss

31 Mar

Philippe Leloup brings us this week’s Imaggeo on Mondays, with tales from a mountain trail that show a geologist can never resist a good rock!

In reality, this shiny slab of rock is about 20 centimetres across. Polished to perfection, the layers of marble and amphibole are beautiful to behold. (Credit: Philippe Leloup via imaggeo.egu.eu)

In reality, this shiny slab of rock is about 20 centimetres across. Polished to perfection, the layers of marble and amphibole are beautiful to behold. (Credit: Philippe Leloup via imaggeo.egu.eu)

This image is that of a polished slab of a rock composed of interlayered marbles and amphibolites. The sample was once part of a small dry-stone wall bordering an outdoor kitchen along a trail along the Ailao Mountain Range in China (or Ailao Shan in Chinese).

As I passed by, a small black eye looked at me, and I couldn’t resist asking the owner to give me that stone – one that could easily be replaced by any other rock nearby, and he kindly agreed. The rock was special for me because I felt that its structure would be spectacular.

The Ailao Range is part of the Ailao Shan – Red River shear zone, a region that stretches for more than 1000 kilometres – from southeast Tibet to the Tonkin Gulf. During the Oligo-Miocene, the Indochina bock (encompassing Vietnam, Cambodia, Laos and Thailand) was pushed away from the collision between the Indian and Asian continents and moved several hundreds of kilometres towards the southeast along that ~10 kilometre-wide shear zone. Today, evidence that intense ductile deformation occurred are found in gneiss and marbles showing steep foliation, horizontal lineation, and numerous left-lateral shear features – a type of deformation that leaves rocks looking like this:

 A thin section microphotograph (total width ~0.5 mm) showing several feldspar crystals with bended tails. These tails show that they have slowly rotated counter-clockwise. These rolling structures are characteristic of left-lateral ductile deformation. (Credit: Philippe Leloup via imaggeo.egu.eu).

A thin section microphotograph (the total width is about 0.5 mm) showing several feldspar crystals with bended tails. These tails show that they have slowly rotated counter-clockwise. These rolling structures are characteristic of left-lateral ductile deformation. (Credit: Philippe Leloup via imaggeo.egu.eu).

When I cut the rock it turned out that the amphibole layer had a very special shape, like a Swiss roll, resulting from simple shear – something that revealed spectacular colours and a stunning shape when seen in section.

By Philippe Leloup, University of Lyon

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Sniffing out signs of an earthquake

28 Mar

Last year Kate Ravilious was awarded an EGU Science Journalism Fellowship to follow scientists studying continental faults. Now she’s out in Nepal alongside researchers who are working out when the county’s next big quake will be…

Sometimes the best rocks are found in the worst locations. Yesterday I was reminded of this as I watched Paul Tapponnier, from the Earth Observatory of Singapore, and his team tracing one of the most dangerous earthquake faults in the world, right next to a dusty, noisy, dirty and busy main road in southern Nepal. Hooters were blaring, bells ringing and people shouting. Clouds of fine orange dust (from quarrying down the road) coated us from head to toe and under our feet lay the rubbish chucked out of bus and car windows. Glamourous it was not.

But Tapponnier and his team are prepared to hold their noses and get on with the job in hand. They know that these rocks are likely to hold the answers to the puzzle they are trying to solve, and that studying them could eventually help to save many lives.

Chasing charcoal – the key to dating faults. (Credit: Kate Ravilious)

Chasing charcoal – the key to dating faults. (Credit: Kate Ravilious)

In Nepal earthquakes are a fact of life. India is slamming into Asia at a rate of 4 cm per year (pushing up the mighty Himalayan mountain range) and the strain that accumulates in the underlying tectonic plates releases itself periodically in the form of earthquakes. Tremors of magnitude 4 or 5 happen more than ten times every year, but the real worry is the ‘great’ earthquakes – magnitude 8 or more – which Nepal encounters once every few decades.

The last great earthquake – a magnitude 8.4 – occurred in 1934. It  razed around one quarter of Nepal’s capital, Kathmandu, to the ground and killed 17,000 people across India and Nepal. Since then the population of Kathmandu has grown sevenfold and dangerous multi-story concrete buildings have sprung up everywhere. Kathmandu (which happens to be built on jelly-like ancient lake sediments) has risen to an unenviable first position in the world earthquake risk list and scientists estimate that up to one million people could be killed when the next big quake shakes the Himalayan region.

But when and where will this next big  earthquake be? For decades no-one could even locate the fault that caused the 1934 quake, let alone estimate when it might move again. But Tapponnier has an uncanny nose for sniffing out earthquake ruptures, and in 2008 he found the culprit, hidden underneath layers of Nepalese jungle down on the edge of the hot Terai plains in southern Nepal. Since then he has returned every year, to uncover the extent of this massive fault and learn more about how it operates.

Paul Tapponier searching for signs of earthquakes past. (Credit: Kate Ravilious)

Paul Tapponier searching for signs of earthquakes past. (Credit: Kate Ravilious)

And so it was that I ended up standing next to the busy road, near the small town of Bardibas, thanks to a travel grant awarded by the EGU, to learn about the work that Tapponnier and his team are doing.

This year much of the focus of the field trip has been to map out the shape of the land, using a sophisticated LIDAR (Light Detection and Ranging) technology. This sleek silver canister rotates and sends out 150,000 pulses of laser light every second. The reflections are used to build up a high resolution three-dimensional picture of the surface.

Sorvigenaleon Ildefonso, a LIDAR technician  from the Earth Observatory of Singapore, along with Aurélie Coudurier-Curveur and Çagil Karakaş, both post-doctoral researchers at the Earth Observatory of Singapore, have spent the last two weeks gathering as many measurements as possible, from a range of locations (many of which are beautiful and tranquil spots), often working until the light fades and they can no longer see what they are doing. The day I arrive they are hauling the LIDAR and associated equipment from location to location, ignoring the energy-sapping heat, blaring of horns, stink of rubbish and clouds of dust. While I am constantly distracted by what is going on around me, they are all completely focused, setting up their equipment with precision and great care, and recording their measurements meticulously.

Sorvigenaleon Ildefonso setting things up for LIDAR. (Credit: Kate Ravilious)

Sorvigenaleon Ildefonso setting things up for LIDAR. (Credit: Kate Ravilious)

What they are searching for is changes in gradient, not always visible to the naked eye under the thicket of vegetation. Much of the hillside has a step-like appearance, and each of those steps may represent one upward thrust of the earthquake. By using the LIDAR to map out these steps in detail they can work out how many times the fault has moved, and how much land it thrust upwards each time.

Meanwhile, down at the bottom of the hillside Tapponnier and his Nepalese colleague, Som Sapkota, from the Department of Mines and Geology in Kathmandu, are standing in a ditch, searching for miniscule pieces of charcoal in amongst the sand and cobbles of a small outcrop of rock. These incredibly precious fragments (often no bigger than a sesame seed) are the key to dating the timings of the fault movement and working out how often the fault moves on average.

In the pit, picking out charcoal and peering into the past. (Credit: Kate Ravilious)

In the pit, picking out charcoal and peering into the past. (Credit: Kate Ravilious)

It is hot, tiring and often tedious work, but for this group of scientists it is puzzle they refuse to leave unsolved. “We have to study these things, and do it quickly, before the next big earthquake strikes,” says Tapponnier.

By Kate Ravilious, Science Journalist (http://www.kateravilious.net)

Geotalk: The mantle and models and measurements, oh my! Talking geophysics with Juan Carlos Afonso

27 Mar

This week in Geotalk, we’re talking to Juan Carlos Afonso, a geophysicist from Macquarie University, Sydney. He explains how a holistic approach is crucial to understanding tectonic processes and how a little “LitMod philosophy” can go a long way to achieving this…

First, could you introduce yourself and tell us a little about what you are currently working on?

My name is Juan Carlos Afonso and I’m a geophysicist currently working at Macquarie University in Sydney, Australia.  My research interests lie in the fields of geophysics and geodynamics, and span many different geophysical and geological processes. My current research integrates a lot of different disciplines, such as mineral physics, petrology, geodynamics, lithospheric modelling, nonlinear inversion, and physics of the mantle, to explore and improve our understanding of lithospheric evolution and plate tectonics.

More specifically, I am interested in the thermochemical structure and evolution of the lithospheric mantle, the mechanical and geochemical interactions between tectonic plates and the sublithospheric upper mantle, and their effects on small- and large-scale tectonic processes. The lithosphere is critical to humans because it is the reservoir of most of the natural resources on which modern society depends, as well as the locus of important geological and biological process such as seismic activity, CO2-recycling, mineralisation events, and volcanism!

Juan Carlos out in the field! (Credit: Juan Carlos Afonso)

Juan Carlos out in the field! (Credit: Juan Carlos Afonso)

During EGU 2012, you received a Division Outstanding Young Scientists Award for your research into the lithosphere and its properties. Could you tell us a bit more about your work in this area?

First of all, it was such a humbling experience to receive this award. I really admire the previous awardees and it is a real honour to have received this award.

I was selected for this award based mainly on the work I did on combining different geophysical and geochemical datasets into a single conceptual framework that has become known as the “LitMod approach”. This theoretical and computational framework fully integrates geochemistry, mineral physics, thermodynamics, and geophysics in an internally-consistent manner*. And allows researchers from different disciplines – seismology, geodynamics, petrology, mineral physics, etc. – to construct models of the Earth that not only satisfy one particular set of observations, but a multitude of observations. This is of primary importance because it guarantees consistency between theories and models (i.e. you can’t cheat!), and results in better and more robust data interrogation and interpretation. This approach is being applied to a wide range of geodynamic and geophysical problems, from studying the water content of the mantle to inferring the thermal structure of Venus.

More recently, my colleagues and I presented the idea of multi-observable probabilistic inversion, a technique that is similar to CAT-scanning in medicine, but that we used to study the thermochemical (or thermo-chemical-mechanical) structure of the lithosphere and upper mantle. We showed that it is a feasible, powerful and general method that makes the most out of available datasets and helps reconcile disparate observations and interpretations. This unifying framework brings researchers from diverse disciplines together under a unique holistic platform where everything is connected to everything else and it will hopefully help understand the workings of the Earth in a more complete manner. But there is a lot of work yet to be done to achieve this!!

…and off duty! (Credit: Juan Carlos Afonso)

…and off duty! (Credit: Juan Carlos Afonso)

How can programmes like LitMod help improve our understanding of plate tectonics?

A great scientist recently said “Each single discipline within the geosciences has progressed tremendously over the 20th century; the problems now lie at the interfaces between the sub-disciplines and ensuring that all geoscientific data are honoured in integrated models. We are well beyond the time when scientists can present their interpretations based on mono-discipline thinking. We absolutely must think of the Earth as a single physico-chemical system that we are all observing with different tools.”  These sentences capture very well the spirit of the LitMod approach, which forces you to think about and interpret geoscientific data in a manner that ensures consistency (as much as possible!). I think one of the reasons for the interest in such an approach is the need for robust and easy-to-use tools that researchers from different disciplines can apply to their individual datasets (seismic, gravity, magnetotelluric, etc.) and explore the connections to other related datasets and disciplines  it helps researchers have a better understanding of the broader implications of their own models. It is also useful to petrologists interested in testing the geophysical and geodynamic implications of their petrological and geochemical models.

LitMod provides a platform wherein chemistry and physics are married such that models of lithosphere and sub-lithospheric mantle must be consistent with petrology, heat flow, topography, gravity, geoid, and seismic and electromagnetic observations. Too often we see models of the Earth, derived from a single dataset, that are incompatible with other observations. Some are better, some are worse. To have a model that explains all observations does not imply that the model is correct, but it does minimise the chances of being wrong! Plate tectonics and science in general use this concept to advance our knowledge of the Earth.

An important (if not the most important!) factor to mention here is that, as with any other project of this magnitude, LitMod would not be possible without the contribution of many scientists who unselfishly helped me to put things together. I’d like to thank Javier Fullea, James Connolly, Nick Rawlinson, Yingjie Yang, Alan Jones, Bill Griffin, Sue O’Reilly and Manel Fernandez for all their help and crucial input to the “LitMod philosophy”.

Sussing out an outcrop. (Credit: Juan Carlos Afonso)

Sussing out an outcrop. (Credit: Juan Carlos Afonso)

And importantly, how does it work?

The main idea is actually quite simple:  a valid physicochemical model of the Earth has to explain all available data in a consistent manner. In essence, this is one of the main steps of the scientific method, right? The LitMod approach is simply a way of constructing Earth models (either by forward or inverse modelling) that satisfy basic physical principles and observations. In a nutshell, LitMod says “you cannot try to fit an observation by changing one parameter of your model without having to change all other parameters in a physically and thermodynamically consistent way, which in turn will affect the prediction of all the other observations”. This is a nice idea, and it should provide robust results as long as what one thinks is consistent, is actually correct. At this stage, we are confident with most of our choices, but there still is much work to do to get a complete understanding of how to model all available datasets simultaneously and how much we can believe our results.

The problem lies in the details, of course, because it is not easy to explain all data consistently when our understanding of each individual dataset is incomplete to different degrees. Moreover, the resolution and sensitivities of different datasets are markedly different too. This problem has a potential solution though. We just need to study the individual problems more carefully (e.g. more laboratory experiments, field case studies, etc.) until we obtain an understanding of them that is similar to the others. In practise this is not straightforward, and many gaps still exist in the description of some problems. A current example, but not the only one, is the discrepancy between results obtained by the magnetotelluric and seismic methods. But even in this case, an integrated modelling approach helps us to isolate the root causes of these discrepancies and to propose new studies to remediate them; something that could not be done by analysing the data separately.

And don’t forget the computational problems, which I find particularly fascinating and frustrating at the same time. Surprisingly, there is not much written about formal joint inversions of multiple datasets; we are learning as we go, but that is what keeps it entertaining!

Lastly, what are your research plans for the future?

I cannot know for sure what I’ll be doing in 10 years (probably geochemistry!), but I can tell you what I’m going to be doing in the next 5-6. Besides continuing working on regional scale inversions with LitMod, I am currently starting to work on two fronts that may appear disconnected at a first glance, but are actually intimately related. The first front is the construction of whole-Earth thermo-chemical-mechanical models, similar to what we are doing with LitMod, but at planetary scale. The other is modelling multiphase reactive flow in the Earth’s mantle with some new numerical techniques. In the end, 5-6 years from now, I think these two fronts will coalesce into a single thick wall… but noone knows whether the wall will stand solid or collapse like a castle of cards… we have to try though!

Want to know more about LitMod? Check out these resources:

Afonso, J. C. , Fullea, J., Griffin, W. L. , Yang, Y., Jones, A. G. , Connolly, J. A. D., O’Reilly, S. Y.: 3D multi-observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle. I: a priori petrological information and geophysical observables. J. Geophys. Res., 118, 2586–2617, 2013.

Afonso, J. C., Fullea, J., Yang, Y., Connolly, J. A. D., Jones, A. G.: 3D multi-observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle. II: General methodology and resolution analysis. J. Geophys. Res., 118, 1650–1676, 2013.

Fullea, J., Afonso, J. C., Connolly, J. A. D., Fernàndez, M., García-Castellanos, D., Zeyen, H.: LitMod3D: an interactive 3D software to model the thermal, compositional, density, rheological, and seismological structure of the lithosphere and sublithospheric mantle. Geochem. Geophys. Geosyst., 10, 2009.

*What is an internally consistent model?

By “internal consistency” I mean that all calculated parameters (e.g. thermal conductivity, bulk modulus, etc.) and observables (e.g. dispersion curves, travel times, et.c) are only and ultimately dependent on temperature, pressure, and composition (the fundamental independent variables), while being linked together by robust and sound (typically nonlinear) physical theories. This guarantees that a local change in properties (like density), which may be required to improve the fitting of a particular observable, will also be reflected in all other observables in a thermodynamically and physically consistent manner. It also implies that no linearity between observables needs to be assumed; each observable responds according to its own governing physical theory (e.g. sound propagation).

If you’d like to suggest a scientist for an interview, please contact Sara Mynott.

 

Dense rocks rise higher because isostasy says so

24 Dec

From space, the Brandberg Igneous Complex looks like a coffee-coloured birthmark set upon the bony complexion of the Namibian desert. Perfectly circular, its peaks soar in a ring of mighty topography, its massive granite cliffs etched with the muscular definition of spheroidal weathering. Its bulk seems to rise out of the barren landscape, driven upward by some unseen force.

In fact, granite intrusions like the Brandberg may actually be uplifted by powerful forces, unseen but not unfamiliar. New research, presented at the American Geophysical Union Fall Meeting last week in San Francisco, suggests that granite features like the Brandberg may owe their towering height to the elementary principle of isostasy.

Since Grove Gilbert first measured the formidable hardness and high density of granite in the 19th century, most geologists have attributed the striking topographic relief of many granite peaks to the former; they argued that granite was simply more resistant to erosion. But Jean Braun, of the Institute of Earth Sciences (ISTerre) at University Joseph Fourier in Grenoble, thinks the high density of granite paradoxically deserves the credit.

A Landsat 7 image of the Brandberg Igneous Complex in Namibia (Credit: NASA).

A Landsat 7 image of the Brandberg Igneous Complex in Namibia (Credit: NASA).

As most students learn in Geology 101, crustal rocks float on the asthenosphere like icebergs in the sea. The density contrast between the ice and the water dictates how much of the iceberg rises above the sea surface, and the same is true of rocks. Furthermore, when you melt an iceberg or passively erode a piece of crust, it will rebound to compensate. On average, the crust rebounds by about 700 metres for every kilometre of erosion. “That’s a well-accepted consequence of local isostasy,” Braun says. “The key is that this ratio can easily vary when you change the density of the surface rocks.”

For granite, whose density can exceed that of the rocks it intrudes by hundreds of kilograms per cubic metre, the rate of isostatic rebound can more than double. “There is nothing new about some granites being denser than the rocks they intrude and there is nothing new about the principle of isostasy,” Braun says. What is new is the realisation – albeit a counterintuitive one – that denser rocks will rebound faster than lighter rocks.

Inside the Brandberg Complex, whose peaks rise 2600 meters above sea level. (Credit: Julia Rosen)

Inside the Brandberg Complex, whose peaks rise 2600 meters above sea level. (Credit: Julia Rosen)

Braun and his colleagues demonstrated this by modelling the evolution of a hypothetical landscape using a surface process model that included isostatic effects. They varied the hardness and density of the surface rocks in the model and found that while resistance to erosion played a small role in preserving topographic high points, the enhanced isostatic rebound associated with density changes was the primary factor behind the high elevation of granite features.

However, there is some limit to the amount of isostatic rebound that can occur by this mechanism. “The principle of localised isostasy assumes that the Earth is made of vertical columns of lithosphere that can slide with respect to each other in any way,” Braun says. “However, we know this is not the case.” Instead, the intrusion must be of the same scale as the elastic thickness of the crust – a measure of its lateral strength. For example, in the thick, strong interior of continents, dense features must be about 100 kilometres long to rebound faster than the rocks around them while in weaker crustal regions, even small granite features can experience enhanced uplift.

After presenting these results at conferences like the AGU Fall Meeting, Braun’s idea has been met with nearly universal enthusiasm and support. “It’s too basic to not be true,” jokes Braun. Mostly, he and others just wonder, “how come we didn’t think of this before?”

The preliminary results of Braun’s modeling show that denser features form higher elevations (shown in red). (Courtesy of Jean Braun)

The preliminary results of Braun’s modeling show that denser features form higher elevations (shown in red). (Courtesy of Jean Braun)

On top of explaining the high elevation of granites, Braun’s theory may have broader applications for other places on Earth where surface rocks vary substantially in density. For instance, Braun cites rapidly uplifting mafic domes in a tectonically quiescent corner of the Tibetan Plateau. “These examples show that there is a simple isostatic component that has not been considered many places where you have large variations in rock density,” Braun says.

The next step is to test these theories against low-temperature thermochronology datasets, including apatite fission track and helium dates. These measures provide an estimate of the rate of cooling and uplift for large intrusions, and can be used to validate Braun’s hypothesis. Although the existing data support the idea that dense bodies experience enhanced isostatic rebound, none were collected with this purpose in mind. So next year, Braun and his colleagues will head to the Brandberg, where the granite bulges out of the Namibian desert, to see whether denser rocks really do rise higher.

 By Julia Rosen, freelance writer and PhD student at Oregon State University

Follow

Get every new post on this blog delivered to your Inbox.

Join other followers: