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The eastern Mediterranean: What’s in a name?

The eastern Mediterranean: What’s in a name?

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. To kick off this series, Anne Glerum introduces us to the eastern Mediterranean, which has been a natural laboratory for generations of scientists.

The name of our Remarkable Region is quite descriptive: it designates the region around and including the eastern part of the Mediterranean Sea. From the Latin word mediterraneus, meaning in the middle of land (Wikipedia), this Sea is a large body of water surrounded by land: the African, European and Asian continents. In turn, the convergence of these continents is what helped shape the region. Such a meeting of continents is in itself a promise of scientific treasure.

McKenzie phrased the cause of scientific interest in Mediterranean deformation a little more prosaically in 1972: “it is an accessible and reasonably well-studied area where the motion between the major plates involved is well known”. These reasons have only become more valid today. The first point will be readily agreed upon; perhaps you are even reading this blog post while stretched out on one of the Mediterranean’s beautiful beaches (or, more in line with my view of geo-people on vacation, after a week-long hike along the tops of an Alpine mountain chain).

McKenzie’s second point is related to both the first and the last: the more readily accessible a region is, the more easily data can be collected and hypotheses tested. At the same time, knowledge of the major plate motions provides boundary conditions to the region under investigation. The major plates involved in the Mediterranean (Fig. 1) are the Nubian and Arabian plates presently converging at about 0.6 and 1.5 cm/yr, respectively, with Eurasia (Nocquet 2012).

The interaction of these major plates was part of the evolution of the larger Tethys region with its Alpine-Himalayan orogenic belt now running from the Mediterranean to Indonesia (Hafkenscheid 2004). The Tethys region is named after the Proto-, Paleo- and Neo-Tethys oceanic domains (Berra and Angiolini, 2014), whose opening and closing resulted in the continental collisions forming this mountain chain. For a clearer mental picture, watch for example these reconstructions by Zahirovic et al. 2012, 2016.

Figure 1: The Mediterranean region and its three major plates (Nubia, Arabia and Eurasia). Brown shaded areas indicate Alpine fold and thrust belts. Plate motions are indicated w.r.t. Eurasia by black arrows. Image credit: Modified by Anne Glerum from Woudloper (Own work) [CC BY-SA 1.0 (http://creativecommons.org/licenses/by-sa/1.0)], via Wikimedia Commons.

Consumption of the Tethyan oceanic domains occurred through mostly northward subduction underneath Eurasia (the northern Nubia-Arabia margin is a passive margin). Back-arc spreading related to roll-back of the subducting plates created smaller oceanic basins; the subsequent closure of the smaller and larger basins resulted in the accretion of continental fragments to Eurasia (Hafkenscheid 2004).

Within our remarkable region, the approximately Oligocene closure of the Neo-Tethys (e.g. Hafkenscheid 2004; Agard et al. 2011; Berra and Angiolini 2014) resulted in the continental collision of Arabia and Eurasia. Remnants of this Neo-Tethys subduction are the Bitlis and Zagros suture zones (Hafkenscheid 2004). To the west, in the Aegean region, the Nubian plate is still subducting, as it has been continuously for at least the last 100 My (e.g. van Hinsbergen et al. 2005; Jolivet and Brun 2010). This continuous subduction included oceanic domains as well as continental fragments of about 300-500 km (Facenna et al. 2003; van Hinsbergen et al. 2005; Jolivet and Brun 2010), of which the upper crust was scraped off and accreted as nappe stacks (van Hinsbergen et al. 2005).

While these nappe stacks are mostly preserved on mainland Greece (Jolivet and Brun 2010), back-arc extension has thinned the Aegean-west Anatolia region (van Hinsbergen and Schmid 2012; Faccenna et al. 2014; Menant et al. 2016;) after the Paleocene compressional phase that resulted in a.o. the Dinarides and Hellenides mountain belts (Faccenna et al. 2014, see Fig. 1). Due to the slab-retreat related extension, high-temperature methamorphic domes were exhumed (van Hinsbergen and Schmid 2012; Facenna et al. 2014). The speed of extension of the Aegean-west Anatolian region increased significantly around 15 Ma (Faccenna et al. 2003; van Hinsbergen and Schmid 2012; Menant et al. 2016), coincident with a bending of the subduction zone, possibly facilitated by tearing of the Aegean slab below western Anatolia (Jolivet et al. 2015).

At the present-day, Nubian subduction and trench retreat are still ongoing. GPS velocity fields illustrate how the motion of the Aegean and Anatolian plates differs from the overall Nubia-Eurasia convergence: their counter-clockwise rotation is facilitated by the strike-slip North Anatolian Fault and Trough and the East Anatolian Fault and increases towards the Hellenic trench (Le Pichon and Kreemer 2010; Nocquet 2012). These motions result from the interplay –in various proportions according to different authors- of the continental escape of Anatolia, Hellenic trench retreat, gravitational potential energy variations and asthenospheric flow (e.g. Le Pichon and Kreemer 2010; Faccenna and Becker 2010; England et al. 2016; Menant et al. 2016).

All in all, the present eastern Mediterranean has a complex geological history that has sparked and continues to spark the interest of many geo-scientists. Faccenna et al. (2014) neatly summarize the new concepts that were coined and/or tested based on the accessibility, wealth of data and known boundary conditions of the Mediterranean region, such as oroclinal bending and the opening of back-arc basins, extensional and strike-slip tectonics in an overall convergent setting, continental escape, trench rollback and slab tearing. A remarkable region indeed!

References:

Agard, P. et al. (2011), Zagros orogeny: a subduction-dominated process, Geological Magazine, Cambridge University Press, 148 (5—6), 692—725.
Berra, F. and Angiolini, L. (2014), The Evolution of the Tethys Region throughout the Phanerozoic: A Brief Tectonic Reconstruction in AAPG Memoir 106: Petroleum Systems of the Tethyan Region, 1—27.
England, P., Houseman, G. and Nocquet, J.-M. (2016), Constraints from GPS measurements on the dynamics of deformation in Anatolia and the Aegean, J. Geophys. Res.: Solid Earth, 121.
Faccenna, C., Jolivet, L., Piromallo, C. and Morelli, A. (2003), Subduction and depth of convection in the Mediterranean mantle, J. Geophys. Res., 108, B2, 2099.
Faccenna, C. and Becker, T. W. (2010), Shaping mobile belts by small-scale convection, Nature, 465.
Faccenna, C. et al. (2014), Mantle dynamics in the Mediterranean, Rev. Geophys., 52.
Hafkenscheid, E. (2004), Subduction of the Tethys Ocean reconstructed from plate kinematics and mantle tomography. PhD thesis, Utrecht University.
Jolivet, L. and Brun, J.-P. (2010), Cenozoic geodynamic evolution of the Aegean, Int. J. Earth Sci., 99, 109—138.
Jolivet, L. et al. (2013), Aegean tectonics: Strain localization, slab tearing and trench retreat, Tectonophysics, 597—598, 1—33.
Jolivet, L. et al. (2015), The geological signature of a slab tear below the Aegean, Tectonophysics, 659, 166—182.
Le Pichon, X. and Kreemer, C. (2010), Kinematic Evolution of the
 Eastern Mediterranean and Middle East and Its Implications for Dynamics, Annu. Rev. Earth Pl. Sc., 38, 323—351.
McKenzie, D. (1972), Active Tectonics of the Mediterranean Region. Geophys. J. R. astr. Soc., 30, 109—185.
Menant, A., Jolivet, L. and Vrielynck, B. (2016), Kinematic reconstructions and magmatic evolution illuminating crustal and mantle dynamics of the eastern Mediterranean region since the late Cretaceous, Tectonophysics, 675, 103—140.
Nocquet, J.-M. (2012), Present-day kinematics of the Mediterranean: A comprehensive overview of GPS results, Tectonophysics, 579, 220—242.
Van Hinsbergen, D. J. J., Hafkenscheid, E., Spakman, W., Meulenkamp, J. E. and Wortel, R. (2005), Nappe stacking resulting from subduction of oceanic and continental lithosphere, Geology, 33, 325—328.

The world’s largest magnet

The world’s largest magnet

The Geodynamics 101 series serves to show the diversity of topics and methods in the geodynamics community in an understandable manner for every geodynamicist. PhD’s, postdocs, full professors, and everyone in between can introduce their field of expertise in a lighthearted, entertaining manner and touch on some of the outstanding questions and problems related to their method of choice.
This week Maurits Metman, PhD student at the Deep Earth Research group at the University of Leeds in the United Kingdom, explains the dynamics of the core. Do you want to talk about your research area? Contact us!

Rock bottom

Approximately 3,000 km below our relatively minuscule feet lies the Earth’s core. It is our planet’s innermost and therefore most secluded region. It is also the primary source of Earth’s magnetic field that we observe here at the surface. With its dynamics, composition, magnetic field generation, and thermal history not yet completely understood, the core remains amongst the most enigmatic parts of the Earth. It has been established that the core can be partitioned in an inner and outer region, which have distinct physical and chemical properties. For example, the two regions are in a different state of matter: the inner core being solid and the outer core liquid. Therefore, it is the outer core that is of particular geodynamical interest – here we will touch upon some important aspects of the dynamics that take place within the outer core.

The outer core consists of an electrically conducting iron alloy liquid, which circulates throughout the outer core volume. In terms of the forces that drive these motions, there are similarities to the dynamics of other terrestrial systems such as the mantle, oceans, and atmosphere. For example, in all cases gravitational forces are overcome by the process of convection, through which relatively hot and buoyant material at the base of the system rises towards the surface, while elsewhere cold material sinks. Additionally, the flows in these systems are subject to forces due to pressure differences and those associated with the deformation of the material.

Figure 1: An impression of convection in the outer core (not to scale), which is aligned along columnar rolls, and flow in- and outside the tangent cylinder is separated (Credit: United States Geological Survey).

Nevertheless, the dynamics in the outer core are certainly different to other geophysical flows. For one, it is estimated that a typical velocity for outer core flow U ∼ 10-1 mm s-1, which is relatively high for the solid Earth. In fact, recent work has shown that these velocities may locally be as high as roughly 1 mm s-1 (Livermore et al., 2016). Additionally, rotational effects (e.g. centrifugal force, Coriolis effect) have a tremendous impact on the style of convection. This is for example not the case in the mantle, due to the fact that flow velocity is comparatively low there and viscosity is large. In this respect, the so-called Taylor-Proudman theorem provides an important constraint on the style of core motions, and states that for rapidly rotating systems flow is two-dimensional: it can not change parallel to the axis of rotation. More generally, the style of convection inside of the outer core is strongly cylindrical, in the sense that flow is aligned in ‘columnar rolls’ aligned with the axis of rotation (Fig. 1).

Magnetic soup

With its ability to generate a magnetic field, the outer core further distinguishes itself from other parts of the Earth. That this field must indeed be generated somewhere inside the Earth was already demonstrated by Gilbert (1600), but the fact that it is linked to core fluid flow remained unknown for centuries. We now know that the convective motion of the electrically conducting outer core liquid generates such a magnetic field. This conversion of kinetic to magnetic energy is a process that has fittingly been coined the geodynamo.

What clues do we have that this field must be generated internally? A relatively simple argument can be made from the age of the magnetic field, which paleomagnetic observations have shown to be over 109 year. However, if we were to assume there would be no field generation, the present-day field would decay through simple diffusion (or equivalently due to Joule heating of the fluid) on a timescale of 105 year, inconsistent with these observations. Therefore, it is required that some field generation in the outer core acts to sustain the magnetic field against diffusion, which can be accomplished with a specific core fluid flow there.

Initially, some rejected the existence of such a flow. One well known so-called anti-dynamo theorem is Cowling’s (1933), who showed that a steady and axisymmetric flow field can never maintain a magnetic field indefinitely, which led to the general consensus that sustained dynamo action through fluid flow would not be possible.

Figure 2: A schematic of magnetic field generation through the α-effect at different timesteps ti. Here, u, B and j represent fluid velocity, magnetic field and electric current density.

The development of the mean-field theory, which describes how small-scale flow perturbations can on average create a large-scale magnetic field, changed this. An example of such field generation is through the α-effect (Parker, 1955). In this case there is a rising and rotating flow (imagine a corkscrew-shaped motion) moving and twisting a magnetic field line (Fig. 2). The magnetic loops created this way induce an electric current parallel to the the field, which in turn generate a secondary magnetic field that is perpendicular to the initial field. A similar conversion the other way around is also possible, and a planetary dynamo that relies on these two processes is considered to be of the α2-type. Another source of field generation that follows from mean field theory is the ω-effect. This process is the bending of magnetic field lines, due to the differences in rotation rate, also creating magnetic field in a direction opposite to the initial direction (Fig. 3). A dynamo that generates a magnetic field through the α- and ω-effect is referred to as an αω-dynamo.

 

Therefore, it is quite clear that core flow and Earth’s internal magnetic field are deeply intertwined. As mentioned earlier, field generation through fluid motion and field diffusion are two competing processes that control time variations in the field. The magnetic Reynolds number is the ratio of the magnitudes of these contributions, and is defined as:

Figure 3: A schematic of the ω-effect which converts the magnetic field from the initial direction (aligned South-North) to a secondary direction (West-East and vice versa), at different timesteps ti. The solid and dashed curve represent the magnetic field and rotation axis, respectively.

Rm = UL / η

where η is the magnetic diffusivity and L is a length scale for the magnetic field (Roberts and Scott, 1965). For the outer core it is estimated that Rm ∼ 102, and therefore the diffusion term is often considered negligible (at least for relatively large length scales). This is referred to as the frozen-flux approximation. As the name suggests, magnetic field lines are then dynamically ‘frozen’ into the liquid, so that they evolve as though they were material line elements. How realistic is this particular scenario? From the above equation it should be clear that the frozen-flux approximation can break down if the typical length scale decreases. This may for example be the case for flux expulsion, i.e. when a radially expelled field is concentrated below the core-mantle boundary (Bloxham, 1986). This concentration increases the gradient of the field locally, enhancing radial diffusion. However, to what extent this process is realistic remains a subject for debate.

Forecast: reversals?

For the last two decades, advances in computing power have allowed numerical models to reproduce certain properties of the Earth’s magnetic field. For example, such models have been shown to exhibit magnetic polarity reversals (Glatzmaier & Roberts, 1995) and Rm that are similar to the outer core’s. Despite this numerical success and despite the fact that reversals have been documented extensively within the field of paleomagnetism, it remains unknown what physical process underlies these phenomena. This is particularly interesting as the most recent reversal occurred around 0.78 Myr years ago, which has led to speculation that a future reversal is imminent. Future numerical work, increases in computing power, better theoretical understanding of the internal dynamics of the core, and more geomagnetic observations may in time provide a physical explanation for these events.

References
Bloxham, J. (1986). The expulsion of magnetic flux from the Earth’s core. Geophysical Journal International, 87(2):669-678.
Cowling T. G. (1933) The magnetic field of sunspots. Monthly Notices of the Royal Astronomical Society 94: 39-48.
Gilbert W. (1600) De Magnete. London: P. Short.
Glatzmaier G. and Roberts P. (1995) A three-dimensional self-consistent computer simulation of a geomagnetic field reversal. Nature 337: 203-209.
Livermore, P. W., Hollerbach, R. and Finlay C. C. (2016). An accelerating high-latitude jet in Earth's core. Nature Geoscience 10: 62-68.
Parker E. N. (1955) Hydrodynamic dynamo models. Astrophysical Journal 122: 293-314.
Roberts, P. H. and Scott, S. (1965). On Analysis of the Secular Variation. 1. A Hydromagnetic Constraint: Theory. Journal of Geomagnetism and Geoelectricity, 17(2):137-151.

A Geodynamicist and an Early Career Scientist

A Geodynamicist and an Early Career Scientist

This week Adina Pusok, postdoctoral fellow at the Institute of Geophysics and Planetary Physics (IGPP) at Scripps Institution of Oceanography, University of California San Diego, USA, discusses what it is like to be an Early Career Scientist within the EGU Geodynamics division.

The terms “Early Career Scientist” (ECS) or “Young Scientist” (YS) are now so widely used in the scientific community, that certain meetings, sessions, awards, and social events are entirely dedicated to promote this group of scientists. But what are ECS and YS? These terms were created to describe scientists in the early stages of their career. Because the term Young Scientist might have an age connotation and make some scientists feel excluded, the EGU has recently adopted the ECS term instead. The definition of ECS on the EGU page says “an Early Career Scientists (ECS) is an undergraduate or postgraduate (Masters/PhD) student or a scientist who has received his or her highest degree (BSc, MSc, or PhD) within the past seven years*. (*with additional year(s) of parental leave time per child, where appropriate)”.

Yet, it is still not clear why there is a need to create a subgroup and organize specific activities for this group, when the research community is considered open, where everyone can bring contributions in an equal manner? To answer this, I look back at the first meetings I attended as a PhD student. I remember I felt intimidated by the experience and scientific eloquence of the more established scientists (especially the “big names” in the field), only because I knew there was still a lot for me to learn and experience to gain. And I am sure I was not the only PhD student experiencing this. The truth is, ECS have different needs compared to established scientists. The list of challenges for ECS is long (see this special issue in Nature). Besides doing research, ECS need to promote their work, socialize more to expand their network, enter the very competitive job market, and not to mention present and defend their scientific achievements to an unknown audience. For many established scientists, these are stages past gone. Everyone knows them, everyone talks about their work.

As an ECS, I value the interactions with established scientists, but I also welcome events organized for ECS (i.e. career development or social events, such as grant writing and academic presentation courses). However, I feel the best outcome is achieved when both established and early career scientists support and participate in these events. Which is why I agreed to be the ECS representative for the EGU Geodynamics division (GD) – to bring together and improve visibility of ECS in the GD community. And for the occasional beer and karaoke special sessions (see last week’s blog post) with other ECS Geodynamicists.

What does EGU do to support ECS?

A large proportion of the Union’s members consists of scientists in the early stages of their career. For this reason, EGU wants to offer support to this group, by providing reduced conference fees, recognizing outstanding students, awarding travel grants, organizing short courses, arranging networking possibilities and more. Besides, the EGU encourages further support at the division level, such as outreach activities (e.g. blogs and social media), social events, mentoring programs or short courses at the General Assembly. All ECS need to do is pay attention to the opportunities provided.

Probably a lot more could be done, but small contributions can already make a huge difference. For example, the ability to attend a conference due to an awarded travel grant can be very important to meet other scientists and create exposure for your research, create future collaborations or even sign up for that future job.

What can ECS do for themselves?

Other than help create the environment they want to be in? It might sound idealistic or as too much work involved, but again, small steps can go a long way. Anything from organizing to participating, from being informed to inform others, from taking part in outreach to supporting outreach activities, from mentoring to being mentored, the ECS can contribute in various ways to their own and their community’s development.

In Geodynamics, there are many enthusiastic and fun ECS that get together at meetings and workshops, create friendships and collaborations across national and academic borders. I’ve met enough of them to believe that GD ECS can create a more coherent structure under the EGU umbrella. For example, within less than 1 year, the GD ECS have managed to launch the EGU GD Facebook page, organize social events at major conferences (AGU Fall Meeting 2016, EGU General Assembly 2017), and recently, launch the EGU Geodynamics Blog.

Adapted from www.xkcd.com and www.soest.hawaii.edu

More things can be done though. There is currently a strong need for scientists to become more actively involved in science public outreach worldwide, and this is one of the directions where GD ECS can contribute easily. Geodynamics is a field known for “beautiful pictures” that show the complexity and dynamics of natural processes. Thus, highlighting the diversity of Geodynamics studies (numerical simulations, laboratory experiments, or data compilations) to a wider audience can benefit the research community at large.

Because all these initiatives are run by members, there is always a need for motivated people with refreshing ideas. This is why, I encourage other ECS to bring forward new ideas that we can develop within the GD division (see how to get involved). Along with the rest of GD ECS volunteers, I look forward to working with you!

By Adina Pusok
As the ECS GD representative, I am the link between the EGU and the ECS GD community. I provide EGU with feedback from students and early career researchers, so that the union can take action to improve the ECS activities at the EGU General Assembly and maintain the support for early career scientists throughout the year. I am also involved in public and community outreach of the Geodynamics division.

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