What drives and resists plate sinking through the transition zone?

Lead Research Organisation: Imperial College London
Department Name: Earth Science and Engineering

Abstract

Mantle circulation is largely driven by the sinking ('subduction') of cold and dense tectonic plates. When these cold slabs reach the transition zone between the upper and lower mantle (from 400 to 800 km depth), their progress is hampered by rapid increases in mantle density and viscosity, as mantle minerals change phase. X-ray type images of the interior of the Earth made using earthquake waves ('seismic tomography') reveal that this zone only forms a barrier for some slabs, while others seem to pass through unhindered. Furthermore, when the seismic images are compared with plate motions through time, it becomes clear that slabs penetrated the lower mantle in the past, where shallower parts of the plate are trapped in the transition zone today. This, as well as evidence of fast and slow subduction phases in plate motions (Goes et al., Nature 2008) indicate that this is a time dependent process, where plate material may pond until a critical mass of material has accumulated, and then flush rapidly into the lower mantle.

It is important to understand this fundamental part of mantle circulation as it controls how efficiently the Earth cools and how well heterogeneities like sediments, crust, fluids and CO2 are mixed into it, or brought back up. Furthermore, sudden slab flushing events into the lower mantle have been linked to periods of continental crust formation, changes to the early atmosphere and reorganisations of plate motions.

We will investigate slab behaviour in the transition zone using 3D dynamic models of subduction, and evaluate which of the modelled mechanisms are consistent with observational data from the Pacific. Previous numerical models have investigated how individual factors like slab strength, slab density, coupling to the upper plate, and mantle phase transitions affect whether a slab goes straight through the transition zone or stalls there. However, none of these individual parameters can explain the observed variations of plate-transition zone interaction. Nor is it clear for how long slabs may be stalled, and hence on which time scales the upper and lower mantle mix, and on which time scales plate motions and accompanying surface deformation vary.

As single properties do not explain the variability of slab-transition-zone interaction, the interplay between mantle, downgoing and upper-plate properties must be crucial. Studying the interaction between these different factors requires 3D dynamic models that let plate motions and slab morphology develop freely, something that is numerically challenging. In recent years, our groups developed such dynamic models and elucidated how combinations of plate density, strength and width control upper-mantle slab morphology.

The newest generation of these dynamic models, developed by the group of the PI at Durham, are now capable of modelling all potentially relevant plate and mantle parameters. With these models, we will explore a wide range of parameters to determine which combinations lead to slab ponding and penetration. Next we will compare modelled conditions for stalling and release with those that can be inferred for major subduction zones throughout the last 100-200 million years of Earth history from seismic tomography, plate motion histories and earthquakes in downgoing slab. This will be done using the expertise in model-data comparison in the group of the Co-I at Imperial, in collaboration with partners Prof. Spakman and Prof. Torsvik, leading experts in seismic imaging and plate motion reconstructions, respectively.

With these new models and this interdisciplinary team, we will be able to answer the fundamental question of how the transition zone traps and releases subducting slabs, a process that plays a pivotal role in the Earth's internal and plate-tectonic evolution.

Planned Impact

Who will benefit from this research?

The main beneficiaries of this research will be the geodynamic and other Earth scientific communities. The UK has a long record of excellence in almost all aspects of subduction related science, including geophysical imaging, geochemical characterization, earthquake and volcano research and hazard assessment, climate science, and modelling of the lithosphere and mantle. This has led to recent efforts to set up a national program that would link together the subduction work from different groups within the UK (led by PIs from Universities of Durham, Bristol, Cardiff and the British Antarctic Survey). Our work complements existing modelling efforts in the U.K., with its fully dynamic and regional mantle-scale approach. Results from the proposed work will facilitate future efforts to integrate U.K. subduction work.

The primary non-academic beneficiaries are the students from secondary to undergraduate level and the general public in the UK and elsewhere. The operation of plate tectonics and subduction is of primary importance for the thermal evolution of the Earth, the composition of the oceans and atmosphere, and the occurrence of the largest earthquakes and most hazardous volcanic eruptions on Earth. These topics naturally fascinate school children and students, and therefore form an excellent way to engage them with science. The changing surface of the Earth through time forms part of the secondary-school national science curriculum, and A-levels in geography and geology.

There is substantial interest from the general public in plate tectonics, and in how it shaped the surface of our Earth, including Britain (which was assembled by subduction about 450 million years ago), and in how it generates mountains, earthquakes and volcanic arcs. This was demonstrated by the invitation to explain our Nature results on BBC radio 4's Material World (broadcast 3 March 2008).

How will they benefit from this research?

We will set up a website which features movies and images from our research with broadly accessible explanations. Through outreach programmes to schools, as coordinated by Dr. Paula Martin in Durham (some salary requested), and including Imperial's involvement in the BGS-led Seismology-in-Schools project, students (as well as their teachers) will directly benefit from this research. In addition, we propose to develop a publicly available and intuitive user interface to the codes (see 'Pathways to Impact' for more information), This will allow secondary school children and undergraduate students, at different levels of difficulty, to perform their own experiments to obtain insight in why plates move. In this era where sustainability of our natural environment plays such an important role, it is essential that the next generation of scientists will be raised with a proper concept of how our planet operates, and plate tectonics and subduction are key processes in our changing planet. In addition, through public lectures, the interested attendant will be provided with cutting-edge research and state-of-the-art insights in the plate tectonics

Publications

10 25 50
 
Description As part of the motion of plates, when two plates converge, the denser of the two slides below the other in a process called subduction. The lower of the two plates then sinks into the depths of the Earth's mantle, where about a fourth of the way down they encounter a zone which apparently provides further resistance to plate sinking. This zone is called the mantle transition zone, as at these depths mantle minerals go through a set of transitions to denser an probably higher-viscosity phases. It is observed that some subducting plates sink straight through the transition zone with some thickening and buckling, while other plates flatten an appear to stagnate there. It is important to understand why subducting plates do not all behave the same way, because these different modes of behaviour appear linked to phases of high or low plate velocities as well as to the stress in the surface plates. Zones of stagnation are often associated with regions where new ocean basins get formed behind the subduction zone, while in many other zones the plates get compressed above the subduction zone and mountains are formed. In the project, we dynamically modelled the effects of a range of proposed viscosity increases and different types of phase transitions and a range of subducting plate ages (some old, some young). It had previously been proposed that one factor would be responsible for the diverse behaviour, but we found that the interplay between all was required to match observations,
Exploitation Route The review paper that we wrote at the end of the project is already getting significant attention (e.g., our many invitations to present our results at conferences, a graduate school, other departments). We expect that these insights will be used in further work for understanding how different regions evolved tectonically, to help understand how plate motions evolved through time. These insights are useful for understanding the plate forces that drive and have driven uplift and subsidence of the Earth's surface and that govern where large earthquakes and landslides occur. Uplift and subsidence histories are also used for finding locations with potential for forming mineral deposits.
Sectors Energy,Environment,Other

 
Description Trond Torsvik 
Organisation University of Oslo
Country Norway 
Sector Academic/University 
PI Contribution numerical modelling of subduction
Collaborator Contribution comparison with plate motions
Impact in progress
Start Year 2012
 
Description Wim Spakman 
Organisation Utrecht University
Country Netherlands 
Sector Academic/University 
PI Contribution numerical modelling of subduction
Collaborator Contribution comparison with tomographic models
Impact in progress
Start Year 2012
 
Description How can initially stagnant slabs sink into the lower mantle? XIV International Workshop on Modelling of Mantle and Lithosphere Dynamics, OrĂ©lons, France 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Poster on How can initially stagnant slabs sink into the lower mantle?
Year(s) Of Engagement Activity 2015
 
Description Invited Blog on project outcomes on European Geophysical Union website 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Invited blog about our research on subduction - transition zone interaction for EGU membership and general public with science/plate tectonic interest.
Year(s) Of Engagement Activity 2018
URL https://blogs.egu.eu/divisions/gd/2018/03/14/subduction-through-the-mantle-transition-zone-sink-or-s...
 
Description Invited Geophysics Seminar Univ. Cambridge Oct 2016 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Invited research seminar at University of Cambridge
Year(s) Of Engagement Activity 2016
 
Description Invited department seminar Durham Jan 2017 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Invited departmental seminar at Durham university
Year(s) Of Engagement Activity 2017
 
Description Invited lectures at Les Houches graduate school Oct. 2017 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Lectures for a graduate school for wide range of postgraduates in solid-Earth geoscience, mostly from European universities but students originally from all over the world (Asia, South America, Europe)
Year(s) Of Engagement Activity 2017
URL https://epti.isterre.fr
 
Description Invited seminar Oxford University May 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Other audiences
Results and Impact invited seminar Oxford University
Year(s) Of Engagement Activity 2018
 
Description Invited seminar University of Leicester May 2017 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Invited departmental seminar
Year(s) Of Engagement Activity 2017
 
Description Invited talk undergraduate seminar Cambridge March 2019 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Undergraduate students
Results and Impact invited presentation in student-organised workshop in Cambridge
Year(s) Of Engagement Activity 2019
 
Description On the advantage of a divergence-free velocity interpolation for particle-in-cell method, Poster at EGU, 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Poster on the divergence-free velocity interpolation for particle-in-cell method
Year(s) Of Engagement Activity 2015
 
Description Presentation at William Smith 50 years Plate Tectonics Workshop Geological Society 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact This 3-day workshop celebrated the 50 years since plate tectonics was discovered, with many prominent international speakers. Our submitted abstract got selected for a talk amongst these presentations.
Year(s) Of Engagement Activity 2017
URL https://www.geolsoc.org.uk/wsmith17
 
Description Presentation on "Slab-transition zone Interaction: What drives and resists plate sinking through the transition zone?" at the University of Roma 3, Italy 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact A scientific presentation on the interaction of subducting slabs with the Earth's mantle transition zone.
Year(s) Of Engagement Activity 2017
URL http://www.scienze.uniroma3.it/seminars/posts/88
 
Description Sinking slabs, sliding plates and subduction enigmas - keynote talk British Geophysical Association 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact talk led to questions and subsequent discussions with students.

networking with some of the promising young scientists in the field.
Year(s) Of Engagement Activity 2013
URL http://bullard.esc.cam.ac.uk/~bga2013/abstractViewer.shtml?al=20_0
 
Description Talk at Geol. Soc. William Smith Meeting on Plate Tectonics 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Presentation in International Workshop to celebrate 50 years of Plate Tectonics
Year(s) Of Engagement Activity 2017
URL https://www.geolsoc.org.uk/wsmith17
 
Description The effect of metastable pyroxene on the slab dynamics. Talk at 2014 AGU Fall Meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Talk on the effect of metastable pyroxene on the slab dynamics

none
Year(s) Of Engagement Activity 2014
 
Description The influence of mantle phase transformations on the slab dynamics. GeoBerlin 2015, Berlin, Germany 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk on The influence of mantle phase transformations on the slab dynamics
Year(s) Of Engagement Activity 2015
 
Description The influence of metastable phases on the dynamics of subduction. Talk at EGU 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk on the influence of mantle phase on slab dynamics
Year(s) Of Engagement Activity 2015