Topography and bathymetry in the region of the northwestern Pacific. This area characterized by multiple subduction zones as indicated by a deep trench (darkest blue), such as that along Japan. Credit: NOAA
Subduction zones host one of the most complex and fascinating tectonic systems on the planet. Numerical models by Király and colleagues recently published in Earth and Planetary Science Letters reveal that the strength of the toroidal flow depends on the mantle viscosity and the magnitude of the slab pull force while the characteristic size of the toroidal cells mainly depends on the size of the convecting mantle.
The motion of Earth’s tectonic plates—the lithosphere—is driven by the subduction of relatively cold and dense oceanic plates into the mantle. Subduction zones are some of the most striking features on Earth. They represent one of the two types of convergent plate boundaries, in which one tectonic plate sinks underneath another one into the Earth’s mantle. The resulting forces induce mantle flow around the subducting plate, but the manner in which this happens is still a matter of debate.
Below the tectonic plates, the mantle moves in a slow and unseen manner—as cold slabs sink, hot upwellings rise, and convection slowly rids the Earth’s interior of its primordial heat. Much of our knowledge of mantle flow patterns is indirect, inferred from geodynamical models or seismic tomography images.
Seismological and geochemical investigations suggest 3D subduction-induced mantle flow around lateral slab edges from the sub-slab zone towards the mantle wedge. Such flow has also been observed in laboratory experiments and numerical models of subduction. In particular, geodynamic numerical models of subduction demonstrate that back-arc extension at narrow subduction zones is driven by rollback-induced mantle flow.
Observations of seismic anisotropy—the directional dependence of seismic wave speed—provide us with tantalizingly direct information about mantle flow direction. For example, crystals of olivine, which is the most common upper-mantle mineral, tend to become aligned by the mantle flow, and seismic anisotropy is an indicator of this alignment.
To apply the laboratory-derived viscosity laws to nature, they must be extrapolated over 10 orders of magnitude, which introduces uncertainty. Moreover, it is unclear to what extent experiments on centimeter-scale samples are representative of the crust and lithosphere.
The viscosity distribution of the lithosphere and the surrounding mantle is therefore one of the least certain parameters in geodynamics. Alternative ways to determine viscosity on geological time scales are thus needed.
Schematic diagram of a subduction zone, showing the dominance of 3D flow beneath the slab and the competing influence of 2D and 3D flow fields in the mantle wedge. Credit: Long and Silver, 2008, Science.
Writing in Earth and Planetary Science Letters, Király and colleagues present a cutting-edge numerical model that shows how the strength and length scale of the toroidal flow vary with the mantle viscosity and the magnitude of the slab pull force. Király and co-workers highlight some remarkable implications of these effects: around subducting plates, the characteristic length, axis, and shape of the toroidal cell are almost independent of the slab’s properties and mainly depend on the thickness of the convecting mantle. The independence of the shape of the toroidal cell on the slab width can appear controversial with respect to previous studies[6–7] that showed that the overall mantle flow is dependent on the slab width. However, as Király et al. point out, these differences can be explained with the model setup adopted and the range of slab widths investigated.
In order to characterize the flow—in terms of its geometry and strength—Király and co-workers analysed the vertical component of the mantle vorticity as well as the ratio between the vertical and the trench parallel component of the vorticity vector. With a series of numerical experiments, they find that subduction-induced mantle flow is highly three-dimensional, and that the toroidal component is sub-horizontal with some vertical flow components. According to the authors, this vertical flow around the slab edges has significant implications, as it can be responsible for the presence of the off-arc volcanism around several laterally confined subduction zones.
The numerical experiments by Király et al. represent another step forward in our understanding of how mantle circulation plays a relevant role in the shaping of tectonic features around subduction zones, and they provide evidence that the slab properties only impact the vigour of the flow around subducting slabs. The extension of these numerical experiments to a range of different parameters will allow for a fuller characterization of the olivine fabric, which in turn will allow seismologists to relate their measurements of seismic anisotropy to flow directions and, ultimately, to mantle processes.
1. Long, M. D., and P. G. Silver (2008), The subduction zone flow field from seismic anisotropy: A global view, Science, 319, 315-318.
2. Funiciello, F., C. Faccenna, and D. Giardini (2004), Role of lateral mantle flow in the evolution of subduction systems: insights from laboratory experiments, Geophy. J. Int., 157, 1393-1406.
3. Jadamec, M. A., and M. I. Billen (2010), Reconciling surface plate motions with rapid three-dimensional mantle flow around a slab edge, Nature, 465, 338-342.
4. Sternai, P., L. Jolivet, A. Menant, and T. Gerya (2014), Driving the upper plate surface deformation by slab rollback and mantle flow, Earth Planet. Sci. Lett., 405, 110-118.
5. Király, A. Capitanio, F.A., Funiciello, F., Faccenna, C. (2017). Subduction induced mantle flow: Length-scales and orientation of the toroidal cell. Earth Planet. Sci. Lett., 479, 284–297.
6. Funiciello, F., Moroni, M., Piromallo, C., Faccenna, C., Cenedese, A., Bui, H.A., 2006. Mapping mantle flow during retreating subduction: laboratory models analyzed by feature tracking. J. Geophys. Res., Solid Earth 111, 1–16.
7. Stegman, D.R., Freeman, J., Schellart, W.P., Moresi, L., May, D., 2006. Influence of trench width on subduction hinge retreat rates in 3-D models of slab rollback. Geochem. Geophys. Geosyst. 7.
I tried to google 'weak seed' for relevant figures, but that did not result in anything I could use.
Credit: B Jana, Flickr, and Max Pixel.
The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to Professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. This month we continue the conversation that was started at NetherMod 2017 by discussing how we initiate deformation in numerical models of the lithosphere and more importantly, does it matter how we start such models? Do you want to talk about your research? Contact us!
Geodynamic modelling often concerns itself with the study of localized deformation in the crust and lithosphere. The models generally start with an initial geometry, boundary conditions and a prescribed set of initial conditions that can be either thermal or mechanical. This initial setup is usually a standard representation of the lithosphere and asthenosphere, as inferred from geological and geophysical observations and laboratory rock experiments. The boundary conditions usually drive the deformation in the system. However, as a synthetic (computer) model is pristine at the start, the localization of deformation can take a long model time of up to millions of years, as numerical disturbances need to accumulate. Besides that, the deformation will likely localise near or at the boundary of the system because of the boundary conditions. In order to avoid this long starting phase, and to exert some control on the location of the initial deformation, several approaches are widely used to initiate and localize deformation. Examples of these are the S-point velocity discontinuity at the bottom of the system (e.g. Braun and Beaumont, 1995; Ellis et al., 1995; Willett, 1999; Beaumont et al., 2000; Buiter et al., 2006; Thieulot et al., 2008; Braun and Yamato, 2009) and the use of a weak seed. The latter are usually small zones that are weaker than the surrounding crust or lithosphere. As the lithosphere and crust are never homogeneous, using weak seeds in a model can be easily justified. They could represent regions of different material properties (e.g., heat production), inherited faults, inherited crustal thickness changes and/or plumes impacting the lithosphere. The popular NetherMod 2017 potatoes (yep, still referring back to that one. If you don’t know what this is about, check out this post) are an extreme case in which multiple weak seeds are used to reflect local geology. Traditionally, simpler, single weak seeds are used.
Here, I will give a brief (and by no means comprehensive) overview of the different weak seed methods used to initiate deformation in models of continental extension, and I will conclude with a discussion on how these different initial conditions affect the model evolution.
Modified figure from Burg & Podladchikov (1999): a thermal perturbation is used in the middle of their model to localise deformation there.
Seeding through thermal effects
The weak zone could be implemented as a temperature anomaly which could reflect a region in the crust of higher radiogenic heat production or a region in the lithosphere of locally reduced viscosity. They can be implemented by elevating the temperature or basal heat flux at the crust-lithosphere boundary or lithosphere-asthenosphere boundary with a certain amplitude over a finite region. Examples of studies using thermal weak seeds are Burg and Podladchikov (1999), Frederiksen and Braun (2001), Hansen and Nielsen (2003), and Brune and Autin (2013).
Seeding by mechanical inhomogeneity
Modified figure from Kaus (2009): Model setup with a rectangular, viscous inclusion in the middle of the domain to initiate and localise deformation. Dimensions of the weak zone were varied, but maintained the aspect ratio 2:1.
A weak seed could be composed of a material with a lower rheological strength than its surroundings. There are multiple ways of achieving this, but often used methods include:
• A weak seed with a lower viscosity (e.g., Gray and Pysklywec, 2010; Gray and Pysklywec, 2012b; Gray and Pysklywec, 2012a; Kaus, 2009, and Mishin, 2011)
• A weak seed with a different angle of internal friction (e.g., Pysklywec et al., 2002; Kaus and Podlachikov, 2006; Thieulot, 2011; Gray and Pysklywec, 2013; Chenin and Beaumont, 2013)
• A weak seed with a different density (e.g., Tirel et al., 2008)
• A weak seed consisting of a different material (e.g., Pysklywec et al., 2000; Huismans and Beaumont, 2007)
• A weak seed with more accumulated strain than its surroundings (e.g., Lavier et al., 2000; Huismans et al., 2005; Warren et al., 2008a; Warren et al., 2008b; Petrunin and Sobolev, 2008; Beaumont et al., 2009; Allken et al., 2011; Kneller et al., 2013; Allken et al., 2013)
Apart from these different ways of making a seed weak, you can also find weak seeds of many different shapes and sizes in the literature. Most commonly, you will find
• Square weak seeds (e.g., Gray and Pysklywec, 2013)
• Rectangular weak seeds (e.g., Huismans et al., 2005) with different aspect ratios
• Fault-shaped weak seeds (e.g., Currie et al., 2007, and Currie and Beaumont, 2011)
Modified from Gray & Pysklywec, 2013: Model setup with a square, frictionally weak zone of dimensions 10×10 km.
Seeding through geometrical discontinuity
Another method to create a zone of different rheological strength is by varying the thickness of the crust or lithosphere. A thickened lithosphere could represent a remnant of a previous mountain building phase, whereas a thinned lithosphere would represent a remnant of a previous rifting phase. Studies using this method of weakening include Gac et al. (2014) and Burg and Schmalholz (2008).
Modified from Gac et al. (2014): Model setup with an inherited thin crust.
Influence of weak seeds on the model evolution
As mentioned at NetherMod 2017, you would ideally either have a generic model for which you should determine the influence of different initial weak seeds to check how robust your model is, or you would have a region-specific model for which you find the optimal initial conditions to get your desired model output. Only few studies have investigated the former in detail. Dyksterhuis et al. (2013) found that a single weak seed typically produces symmetric narrow rifts; multiple seeds produce a wide rift; and an initial fault-shaped weak zone produces an asymmetric rift.
It also raises the question about whether or not there are differences between similar codes when the same initiation method is used.
To shine a very preliminary light on this problem, I ran some models of continental extension using the SULEC and ELEFANT codes with different initial conditions. As both codes are based on the same physics, and have similar implementation, they should show a high degree of similarity when using the same deformation initiation method. The results show that different initiation methods indeed result in different results, particularly with respect to the timing of the deformation (see figure below). Besides that, SULEC tends to show more asymmetric behaviour than ELEFANT. For a more complete overview of the results and the model setup, please look here (my old (first!) poster for GeoMod 2014).
Models of continental extension after 10 Myr of extension for SULEC (top) and ELEFANT (bottom) for different initial, frictionally weak, weak seeds with aspect ratios of 6×6 elements, 12×3 elements, and 3×12 elements (i.e., the weak seed consists of the same amount of elements in each model).
In conclusion, I hope to add to the discussion of ‘how we start our models’ with this post by affirming that the model evolution is affected by our choice of weak seed (if only by the amount of waiting until deformation starts) and its effect can differ slightly between codes, even if the codes are very similar. Taking into account the vast variability of methods to initiate deformation, one really should be careful when assessing model results.
Allken, V., Huismans, R., Fossen, H., and Thieulot, C. (2013). 3D numerical modelling of graben interaction and linkage: a case study of the Canyonlands grabens, Utah. Basin Research.
Allken, V., Huismans, R., and Thieulot, C. (2011). Three-dimensional numerical modeling of upper crustal extensional systems. Journal of Geophysical Research, page doi:10.1029/2011JB008319.
Beaumont, C., Jamieson, R., Butler, J., and Warren, C. (2009). Crustal structure: A key constraint on the mechanism of ultra-high-pressure rock exhumation. EPSL, 287:116-129.
Beaumont, C., Munoz, J., Hamilton, J., and Fullsack, P. (2000). Factors controlling the alpine evolution of the central pyrenees inferred from a comparison of observations and geodynamical models. Journal of Geophysical Research, 105:8121-8145.
Braun, J. and Beaumont, C. (1995). Three-dimensional numerical experiments of strain partitioning at oblique plate boundaries: Implications for contrasting tectonic styles in the southern Coast Ranges, California, and central South Island, New Zealand. Journal of Geophysical Research, 100(B9):18,059-18,074.
Braun, J. and Yamato, P. (2009). Structural evolution of a three-dimensional, finite-width crustal wedge.Tectonophysics, 484:181-192.
Brune, S. and Autin, J. (2013). The rift to break-up evolution of the Gulf of Aden: Insights from 3D numerical lithospheric-scale modelling. Tectonophysics, 607(0):65-79. The Gulf of Aden rifted margins system: Special Issue dedicated to the YOCMAL project (Young Conjugate Margins Laboratory in the Gulf of Aden).
Buiter, S., Babeyko, A., Ellis, S., Gerya, T., Kaus, B., Kellner, A., Schreurs, G., and Yamada, Y. (2006). The numerical sandbox: comparison of model results for a shortening and an extension experiment. Analogue and Numerical Modelling of Crustal-Scale Processes. Geological Society, London. Special Publications, 253:29-64.
Burg, J.-P. and Podladchikov, Y. (1999). Lithospheric scale folding: numerical modelling and application to the Himalayan syntaxes. International Journal of Earth Sciences, 88(2):190-200.
Burg, J.-P. and Schmalholz, S. (2008). Viscous heating allows thrusting to overcome crustal-scale buckling: Numerical investigation with application to the Himalayan syntaxes. Earth and Planetary Science Letters, 274(1):189-203.
Chenin, P. and Beaumont, C. (2013). Influence of offset weak zones on the development of rift basins: Activation and abandonment during continental extension and breakup. JGR, 118:1-23.
Currie, C. and Beaumont, C. (2011). Are diamond-bearing Cretaceous kimberlites related to low-angle subduction beneath western North America. EPSL, 303:59-70.
Currie, C., Beaumont, C., and Huismans, R. (2007). The fate of subducted sediments: a case for backarc intrusion and underplating. Geology, 35(12):1111-1114.
Dyksterhuis, S., Rey, P., Mueller, R., and Moresi, L. (2013). Effects of initial weakness on rift architecture. Geological Society, London, Special Publications, 282:443-455.
Ellis, S., Fullsack, P., and Beaumont, C. (1995). Oblique convergence of the crust driven by basal forcing: implications for length-scales of deformation and strain partitioning in orogens. Geophys. J. Int., 120:24-44.
Frederiksen, S. and Braun, J. (2001). Numerical modelling of strain localisation during extension of the continental lithosphere. Earth and Planetary Science Letters, 188(1):241-251.
Gac, S., Huismans, R. S., Simon, N. S., Faleide, J. I., and Podladchikov, Y. Y. (2014). Effects of lithosphere buckling on subsidence and hydrocarbon maturation: A case-study from the ultra-deep East Barents Sea basin. Earth and Planetary Science Letters, 407:123-133.
Gray, R. and Pysklywec, R. N. (2010). Geodynamic models of archean continental collision and the formation of mantle lithosphere keels. Geophysical Research Letters, 37(19).
Gray, R. and Pysklywec, R. (2012a). Geodynamic models of mature continental collision: Evolution of an orogen from lithospheric subduction to continental retreat/delamination. JGR, 117(B03408).
Gray, R. and Pysklywec, R. N. (2012b). Influence of sediment deposition on deep lithospheric tectonics. Geophysical Research Letters, 39(11).
Gray, R. and Pysklywec, R. (2013). Influence of viscosity pressure dependence on deep lithospheric tectonics during continental collision. JGR, 118.
Hansen, D. and Nielsen, S. (2003). Why rifts invert in compression. Tectonophysics, 373(1):5-24.
Huismans, R. and Beaumont, C. (2007). Roles of lithospheric strain softening and heterogeneity in determining the geometry of rifts and continental margins. Geological Society, London, Special Publications, 282(1):111-138.
Huismans, R., Buiter, S., and Beaumont, C. (2005). Effect of plastic-viscous layering and strain softening on mode selection during lithospheric extension. Journal of Geophysical Research, 110:B02406.
Kaus, B. (2009). Factors that control the angle of shear bands in geodynamic numerical models of brittle deformation. Tectonophysics, 484(1), 36-47.
Kaus, B. and Podlachikov, Y. (2006). Initiation of localized shear zones in viscoelastoplastic rocks. Journal of Geophysical Research: Solid Earth 111.B4.
Kneller, E. A., Albertz, M., Karner, G. D., , and Johnson, C. A. (2013). Testing inverse kinematic models of paleocrustal thickness in extensional systems with high- resolution forward thermo-mechanical models.
Lavier, L., Buck, W., and Poliakov, A. (2000). Factors controlling normal fault offset in an ideal brittle layer. 105(B10):23,431--23,442.
Mishin, Y. (2011). Adaptive multiresolution methods for problems of computational geodynamics. PhD thesis, ETH Zurich.
Petrunin, A. and Sobolev, S. (2008). Three-dimensional numerical models of the evolution of pull-apart basins. Physics of the Earth and Planetary Interiors, 171:387-399.
Pysklywec, R., Beaumont, C., and Fullsack, P. (2000). Modeling the behavior of continental mantle lithosphere during plate convergence. Geology, 28(7):655-658.
Pysklywec, R., Beaumont, C., and Fullsack, P. (2002). Lithospheric deformation during the early stages of continental collision: Numerical experiments and comparison with South Island, New Zealand. JGR, 107(B72133).
Thieulot, C., Fullsack, P., and Braun, J. (2008). Adaptive octree-based finite element analysis of two- and three-dimensional indentation problems. Journal of Geophysical Research, 113:B12207.
Thieulot, C. (2011). FANTOM: two-and three-dimensional numerical modelling of creeping
flows for the solution of geological problems. Physics of the Earth and Planetary Interiors, 188(1):47-68.
Tirel, C., Brun, J.-P., and Burov, E. (2008). Dynamics and structural development of metamorphic core complexes. Journal of Geophysical Research: Solid Earth (1978-2012), 113(B4).
Warren, C., Beaumont, C., and Jamieson, R. (2008a). Formation and exhumation of ultra-highpressure rocks during continental collision: Role of detachment in the subduction channel. Gcubed.
Warren, C., Beaumont, C., and Jamieson, R. (2008b). Modelling tectonic styles and ultra-high pressure (UHP) rock exhumation during the transition from oceanic subduction to continental collision. EPSL, 267:129-145.
Willett, S. D. (1999). Rheological dependence of extension in wedge models of convergent orogens. Tectonophysics, 305:419--435.
(Part of) Charitra Jain’s poster at NetherMod 2017 on the generation of primordial continental crust
Being a scientist is more than just doing research and science. You also need to be able to communicate your findings to your peers and/or the general public (outreach). At conferences, you usually have two options for presenting your work: a talk or a poster (although at EGU, you also have the PICO sessions). A poster is often preferred if you would like to start a discussion and get lots of feedback on your work. So how do you ensure that people will come to your poster, stay to read it, and take the most important messages home? Charitra Jain, PhD student at ETH Zurich, Switzerland and winner of the Outstanding Student Poster Award at NetherMod 2017, gives some tips.
Some things I consider important while making posters:
• Break down the text in concise bullet points
• Use a non-white background to make your poster stand out among hundreds of posters
• Find the right balance between text and figures (depending on if you are planning to stay at your poster)
• Make sure your poster is easy to navigate
• Highlight the keywords
• Use 2-3 font sizes to represent hierarchy
• Think about “breathability”: don’t overcrowd your poster
• Demarcate different sections clearly
• Use perpetually-uniform color scales (also see this post by Fabio Crameri. I am trying to integrate these colour scales in my future plots/figures)
• Zoom 100% in on your poster on your screen and try to read it from 2 meters away to get an impression of what the poster will look like eventually
Besides tips from Charitra Jain, it is also useful to know what the jury deems important in a poster. Therefore, the list of criteria that Susanne Buiter presented at the Outstanding Student Poster Award Ceremony at NetherMod 2017 is reproduced here:
• Aim and motivation
• Key findings
• Large figures
• Readable text
• More figures than text
Presentation and knowledge of the subject
• The story
• Figures supportive of the story
• Discussion/ability to answers questions
Charitra Jain’s winning poster at NetherMod 2017 on the generation of primordial continental crust (click to enlarge)
Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. Following from the first entry which showcased the Eastern Mediterranean, we move further east, and back in time, to the realm of the Tethys. The post is by postdoctoral researcher Sabin Zahirovic of the EarthByte Group and Basin GENESIS Hub, The University of Sydney.
The southern and southeastern region of Eurasia (Fig. 1) represents one of the most tectonically complex areas in the world, with active deformation that appears as flurries of sometimes-deadly earthquakes, as well as the slower processes of mountain building. This region truly represents geophysical extremes, with the world’s tallest mountain ranges (namely, the Himalayas), and the planet’s deepest surface depressions (namely, the Mariana Trench), which collectively demonstrate the ongoing tectonic processes that are shaping, and re-shaping, our planet’s surface. Embedded in every rock, grain, mineral, and fossil in the region is the record of ancient tectonic activity, with a mosaic of exotic terranes sutured onto the continental margin. The Tethys was a vast oceanic domain at the southern margin of Eurasia and parts of its eastern history are now preserved in sutures onshore. The sutures are often peppered with ophiolites (Fig. 2), which are fragments of ancient ocean basins and gateways, the rest of which have been lost to subduction and now reside deep in the mantle. The opening and closure of these ocean basins has also been implicated in fundamental shifts of oceanic circulation and long-term sea level, which highlights the importance of better understanding the tectonic evolution of our planetary surface and deep churning interior.
Figure 1. Shuttle Radar Topography Mission data of the eastern Tethyan region displayed in the GPlates web portal, with 50X vertical exaggeration of elevation. (EarthByte Group and Scripps Institution of Oceanography)
Figure 2. Regional map highlighting the sutures (pink and blue lines) demarcating the terranes of the eastern Tethyan tectonic domain, as well as the ancient ocean basins that have since been consumed by subduction. Figure from Zahirovic et al. (2016b).
The first step to rewinding the tectonic clock requires undoing seafloor spreading in preserved ocean basins. Coincidentally, the geoscience community is celebrating the 50th anniversary of plate tectonics this week, with the understanding of seafloor spreading being a critical component of the plate tectonics paradigm. Through the work of numerous scientists such as Fred Vine, Harry Hess, Drummond Matthews, Dan McKenzie, amongst others, the ideas of seafloor spreading began to take shape. Another visionary who has received less acknowledgement is Marie Tharp, who painstakingly collected and manually plotted sonar measurements from voyages criss-crossing the Atlantic. Marie pieced together the morphology and topology of the mid-oceanic ridge systems (Fig. 3), which were mysterious bathymetric features at the time. When Marie superimposed earthquake epicentres on her maps, and suggested that this could be the missing piece of the continental drift puzzle, her significant insights were initially dismissed. However, Marie’s perseverance and determination laid much of the foundations in our understanding of these bathymetric features representing regions of seafloor spreading, from which a rush of subsequent work established the principles of plate tectonics.
Following decades of data collection, including magnetic polarity reversal mapping of the oceans, the age of the oceanic crust and other features of the seafloor fabrics were compiled into global digital community models (e.g., Müller et al., 1997; Matthews et al., 2011). With these two ingredients, one can generate digital models of plate tectonic reconstructions, where the seafloor spreading of the ocean basins can be reversed to reveal the ancient configuration of continents (e.g., the reconstruction of Seton et al., 2012 using open-source community plate reconstruction software, GPlates, (Boyden et al., 2011)). However, due to the conservation of surface area (i.e., the Earth is not expanding), large gaps emerge that represent ancient subducted ocean basins.
As the Atlantic, Southern and Indian oceans opened, it occurred at the expense of the youngest (the Neo-Tethys), which was subducted along the southern margin of Eurasia. This process left a vast chain of arc volcanoes and orogens, stretching from the Alps in Europe to Sundaland in Southeast Asia (Fig. 1). However, only patchy evidence remains of how the continental terranes traversed the Tethyan ocean basins (Fig. 2), with existing methods largely relying on sedimentary affinities, fossil co-occurrences, and paleo-latitudinal estimates from paleomagnetic data (e.g., Metcalfe, 1994; Torsvik and Cocks, 2004). With the advent of modern seismic techniques (e.g., Li et al., 2008), it was possible to “image” the Earth’s mantle using seismic tomography (akin to a medical scan, Animation 1). This led teams of researchers to interpret the mantle structure (Fig. 4), and infer the history of subduction in the absence of preserved data on the surface (e.g., Hafkenscheid et al., 2006; van der Voo et al., 1999). In order to test interpretations of the India-Eurasia convergence that consumed the eastern Neo-Tethyan ocean basin, efforts were then launched to try and reproduce the India-Eurasia mantle structure using numerical models of mantle convection (e.g., Jarvis and Lowman, 2005).
Figure 4. Schematic interpretations of the India-Eurasia subduction history (A) using inferences from seismic tomography (van der Voo et al., 1999) (B), which were then tested using simple numerical experiments of mantle convection (Jarvis and Lowman, 2005) (C). Figures modified from Zahirovic et al. (2016b).
Animation 1. East-west sweep through the eastern Tethyan mantle, highlighting cold subducted slabs (blue) representing ancient Tethyan oceanic lithosphere (tomography model from Li et al., 2008).
With advances in community software platforms to model the evolution of entire plates and their boundaries through time, as well as a near-exponential increase in supercomputing resources, it became easier to model the coupled plate-mantle system in a time-evolving 3D spherical shell (e.g., using tools such as Aspect, CitcomS, and others). These advances enabled us, our colleagues, and the wider geo-community, to track subducting slabs from a complex and evolving global network of plate boundaries, including those associated with the eastern Tethyan tectonic domain. The new models provided an additional insight into the interaction of the deep Earth and surface processes, especially in such a tectonically-complex region like Southeast Asia.
For more than a decade, research has highlighted that Australia’s northern continental margin and the Sundaland continental promontory were low-lying and partially-flooded regions, likely due to the influence of sinking lithospheric plates from the eastern Tethyan slab graveyard (DiCaprio et al., 2009; DiCaprio et al., 2011; Spasojevic and Gurnis, 2012). What the models of mantle flow have revealed is that this effect, known as “dynamic topography”, is a transient signal through space and time, depending on the position of the continents in relation to large-scale mantle upwellings or downwellings. In the case of the eastern Tethys, these mantle downwellings were ephemeral, affecting the relative contribution of global sea level change and regional uplift or subsidence that drove shoreline retreat and advance over the continents. For Southeast Asia, we now think that the whole region was elevated and entirely emergent from about 80 to 40 million years ago, largely due to a hiatus in subduction along southern Eurasia between ~80 and 60 million years ago (Fig. 5). These notions complement traditional ideas of tectonic topography (where the surface experiences uplift or subsidence due to collisional or rifting processes), and allow us to consider the additional role of mantle flow (upwellings and downwellings contributing to several hundred meters in elevation change) over geological time in shaping the surface evolution of our planet.
Figure 5. Palaeogeographic reconstructions highlight that Southeast Asian topography was very different in the past, one dominated by a contiguous and emergent landmass about 80 to 40 million years ago, despite higher sea levels than today. Numerical models of the eastern Tethyan region suggests that the absence of subduction between 80 and 60 million years ago led to a “dynamic rebound” of the Sundaland continental promontory. However, the region became flooded again from about 40 million years ago because of a resumption in subduction, which led to regional subsidence, even with falling long-term sea levels. Figure adapted from Zahirovic et al. (2016a).
The plate tectonic reconstructions (Animation 2), along with the mantle flow models, have added significant insights into the evolution of the plate-mantle system. However, one emerging area of study is the exploration of how the shifting tectonic regimes have influenced other Earth systems – whether it be climate, ocean circulation, or biological evolution. In terms of the eastern Tethys, further research will help us uncover the role of tectonics and mantle flow in understanding the deep carbon cycle (at the plate-mantle scale), which has the potential to reveal the flux of carbon between shallow and deep Earth reservoirs, and their influence on deep-time climate (e.g., Jagoutz et al., 2016; Kent and Muttoni, 2013). For example, as the Neo-Tethys ocean basin was consumed, huge amounts of CO2 were emitted through a vast network of convergent plate boundaries that hosted arc volcanoes, leading to warmer greenhouse conditions. However, once India crashed into these subduction systems, the buoyant continental crust jammed and shut down subduction, turning off a major input of CO2 into the atmosphere. In addition, the mountain building processes in the Alpine-Himalayan mountain belt sequestered enormous amounts of CO2 from the atmosphere through chemical weathering of silicate rocks, influencing (and perhaps controlling) long-term cooling of the planet in the last 45 million years. These components collectively highlight the need for more integrative work that unpacks the complexity of regions like the eastern Tethys in order to understand the interaction of physical, chemical, and biological processes that have shaped planetary evolution.
Images from Animation 2 – please click the link below to download the movie file (10MB)
Animation 2 (please click the link above to download and view). The latest plate tectonic reconstructions of the eastern Tethys consider both the evolution of the continents, as well as the oceanic plate that carried it, much like a conveyor belt on the Earth’s surface. The reconstructions are embedded in global models with evolving plate boundaries (GPlates software), and are easier linked to numerical models of mantle convection (Zahirovic et al., 2016b), which provide an additional avenue to test our interpretations of past continental and oceanic arrangements
Note from author: This summary is by no means comprehensive, and represents a personal perspective of plate tectonics and eastern Tethyan geodynamics.
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