Understanding how the mantle transition-zone 'valve' controls slab fate
Lead Research Organisation:
CARDIFF UNIVERSITY
Department Name: School of Earth and Ocean Sciences
Abstract
Subduction is the process where tectonic plates descend into Earth's deep interior, the mantle. Subduction is critically important since it drives (i) plate tectonics (the ultimate process behind seismicity and mountain building); (ii) melting, (critical for volcanism, and producing crust and atmosphere) and (iii) mantle circulation. Yet, we do not fully understand how it 'works'.
Subducting plates ('slabs') form the downwelling limb of mantle convection. Mantle convection differs in several important ways from the familiar convection of water boiling in a saucepan on a stove. Firstly, mantle rocks are solid, but they can creep on long time scales. Secondly, in the 'transition zone', 400 to 800 km down into the 3000 km deep mantle, mantle minerals undergo high-pressure phase changes to more tightly-packed and denser structures.
Creep varies strongly with temperature, making cold subducting plates much stiffer than the surrounding warm mantle. Exact creep style varies with, amongst others, pressure and stress, and controls how rapidly slabs lose their strength as they heat up while sinking. How easily a slab deforms again influences its sinking speed. The style of creep is also affected by the changes in mineral structure and grain size that occur at phase transitions. The interaction between creep and phase changes in the transition zone complicate the subduction of plates from the upper mantle into the mantle below the transition zone. Of special importance is the transition around 660 km depth where mantle viscosity increases by a factor of 10-100 and a delay of the phase transformation in the cold slabs makes them temporarily lighter than the mantle. This can lead to stalling of slabs in the transition zone. In this way, the transition zone controls how efficiently heat and material are cycled through the mantle, (including water and CO2 which have affected the evolution of climate).
Observed rapid changes in plate motions indicate that there are episodes in which slabs sink through the transition zone quite readily ('valve' open), and others in which they stall there and pile up ('valve' shut). Seismic tomography images of the Earth's interior, reconstructed from seismogram recordings, show that at the moment, many slabs, including those below Tonga, Japan and Sumatra pool in the transition zone, while a few others, for example below Central America, descend straight to great depths.
Different explanations have been proposed. One end-member hypothesis (put forward by co-I Dr. Goes) is that the oldest, coldest plates are stiffest and tend to flatten at the base of the transition zone rather than sink straight through, while young warm slabs form piles that sink through the transitions more easily. Partner Karato in contrast hypothesises that slabs emerge from the major phase transition at 400 km consisting of small, weak new grains. While in young slabs, warm temperatures encourage grain growth and the slabs quickly regain strength allowing them to push through, old slabs remain weakened and are hence unable to open the valve.
Recently, co-I Davies, together with colleagues at Imperial developed a numerical code that allows models with grids that adapt to the scale of model complexity, i.e. high resolution in regions with changes over small scales, like near changes in phase or creep mechanism, and, computationally-less-expensive, coarser resolution in regions with low variability. This allows us to model for the first time, the complex interplay between the thermal, phase and creep effects on subducting slabs.
We will make a set of subduction models incorporating the most recent data on phase change properties (from co-I Lithgow-Bertelloni) and creep laws (from partner Karato). By comparing model predictions with geophysical observations we will be able to determine if either of the two end-member hypotheses or combined or alternative mechanism explains the crucial workings of the transition zone 'valve'.
Subducting plates ('slabs') form the downwelling limb of mantle convection. Mantle convection differs in several important ways from the familiar convection of water boiling in a saucepan on a stove. Firstly, mantle rocks are solid, but they can creep on long time scales. Secondly, in the 'transition zone', 400 to 800 km down into the 3000 km deep mantle, mantle minerals undergo high-pressure phase changes to more tightly-packed and denser structures.
Creep varies strongly with temperature, making cold subducting plates much stiffer than the surrounding warm mantle. Exact creep style varies with, amongst others, pressure and stress, and controls how rapidly slabs lose their strength as they heat up while sinking. How easily a slab deforms again influences its sinking speed. The style of creep is also affected by the changes in mineral structure and grain size that occur at phase transitions. The interaction between creep and phase changes in the transition zone complicate the subduction of plates from the upper mantle into the mantle below the transition zone. Of special importance is the transition around 660 km depth where mantle viscosity increases by a factor of 10-100 and a delay of the phase transformation in the cold slabs makes them temporarily lighter than the mantle. This can lead to stalling of slabs in the transition zone. In this way, the transition zone controls how efficiently heat and material are cycled through the mantle, (including water and CO2 which have affected the evolution of climate).
Observed rapid changes in plate motions indicate that there are episodes in which slabs sink through the transition zone quite readily ('valve' open), and others in which they stall there and pile up ('valve' shut). Seismic tomography images of the Earth's interior, reconstructed from seismogram recordings, show that at the moment, many slabs, including those below Tonga, Japan and Sumatra pool in the transition zone, while a few others, for example below Central America, descend straight to great depths.
Different explanations have been proposed. One end-member hypothesis (put forward by co-I Dr. Goes) is that the oldest, coldest plates are stiffest and tend to flatten at the base of the transition zone rather than sink straight through, while young warm slabs form piles that sink through the transitions more easily. Partner Karato in contrast hypothesises that slabs emerge from the major phase transition at 400 km consisting of small, weak new grains. While in young slabs, warm temperatures encourage grain growth and the slabs quickly regain strength allowing them to push through, old slabs remain weakened and are hence unable to open the valve.
Recently, co-I Davies, together with colleagues at Imperial developed a numerical code that allows models with grids that adapt to the scale of model complexity, i.e. high resolution in regions with changes over small scales, like near changes in phase or creep mechanism, and, computationally-less-expensive, coarser resolution in regions with low variability. This allows us to model for the first time, the complex interplay between the thermal, phase and creep effects on subducting slabs.
We will make a set of subduction models incorporating the most recent data on phase change properties (from co-I Lithgow-Bertelloni) and creep laws (from partner Karato). By comparing model predictions with geophysical observations we will be able to determine if either of the two end-member hypotheses or combined or alternative mechanism explains the crucial workings of the transition zone 'valve'.
Planned Impact
Specific users this work might be of interest to and how they will benefit
The Hydrocarbon industry are interested in a better understanding of subduction and more broadly mantle dynamics and how it has affected surface motions - vertical and horizontal - and near-surface temperatures - through Earth history. These are of value in understanding basin evolution and prospectivity.
Shell International Exploration and Production (SEIP) are already funding a geodynamic project at Cardiff testing plate motion histories. The Global Frontiers group of SEIP will be directly informed of the new knowledge at our ongoing regular meetings. We also propose to attend the leading exploration geophysics conference (Society of Exploration Geophysicists Annual Meeting - SEG) in the final year of the project, to disseminate our results and tools to a wider audience from industry
Geodynamics modellers
The numerical code, Fluidity, will be formally released at the end of this project and made available to the global Earth Science community. We note that numerically this is a leading edge tool, especially in being able to dynamically adapt the grid effectively on parallel architectures. In addition though it will incorporate even more leading physics, partly as a result of this project. A proper manual will be written for the code, and the AMCG web-site extended to release the code. A proper stable release will be prepared for a number of platforms; and all the necessary libraries will be provided also. The AMCG group at Imperial College already have much experience and success in this with their other codes. We will hold a 3-day workshop at Imperial College London, covering all necessary aspects for use of the code (i) overview; (ii) obtaining and building Fluidity; (iii) mesh-generation; (iv) the input file/Graphical User Interface; (v) running in parallel; (vi) visualization; (vii) pedagogical examples. Through this we expect the projects impact to build through the academic community. In this way not only will our science outputs but also the methodology developments will have a long term impact.
Wider user interest
Secondary School Students - The clarity that this project will bring to our understanding of the interior dynamics will ultimately allow textbooks to present an exciting and accurate picture of the driving processes. It shows students the power and need for interdisciplinary science (in this case Physics, Chemistry (Mineralogy), Computer Science, Maths, Geology) to answer significant questions. It also shows a 'big' application of 'simple' Physics. In particular to aid teachers, we will prepare and upload a series of power-point presentations, providing a simplified summary of the latest scientific results.
Public - While this project clearly addresses part of the Earth (ocean crust and mantle) that is out of sight it is so fundamental and intrinsically fascinating and visual (detailed thermodynamics aside possibly!) that with the correct presentation - this work will help us forge an appreciation in the public of the significance of the deep interior.
Over recent years, all project investigators have been contacted by journalists (e.g. New Scientist, S4C, BBC) in relation to their research. For example PI Davies' work has recently been covered by major popular publications New Scientist, Earth (American Geological Institute's news stand publication); and he has taken part in TV news and documentaries for S4C and BBC. We will engage politicians / policy makers by attending the Annual 'Science and the Assembly' meeting organized by the Royal Society of Chemistry which occurs in May every year at the Wales Millennium Centre and Senedd, Cardiff Bay. The PI has already presented his work to Wales's First Minister.
We will continue to pursue such openings. We will use the web-site to not only release Fluidity but also to present the exciting results to the public as well as the academic beneficiaries.
The Hydrocarbon industry are interested in a better understanding of subduction and more broadly mantle dynamics and how it has affected surface motions - vertical and horizontal - and near-surface temperatures - through Earth history. These are of value in understanding basin evolution and prospectivity.
Shell International Exploration and Production (SEIP) are already funding a geodynamic project at Cardiff testing plate motion histories. The Global Frontiers group of SEIP will be directly informed of the new knowledge at our ongoing regular meetings. We also propose to attend the leading exploration geophysics conference (Society of Exploration Geophysicists Annual Meeting - SEG) in the final year of the project, to disseminate our results and tools to a wider audience from industry
Geodynamics modellers
The numerical code, Fluidity, will be formally released at the end of this project and made available to the global Earth Science community. We note that numerically this is a leading edge tool, especially in being able to dynamically adapt the grid effectively on parallel architectures. In addition though it will incorporate even more leading physics, partly as a result of this project. A proper manual will be written for the code, and the AMCG web-site extended to release the code. A proper stable release will be prepared for a number of platforms; and all the necessary libraries will be provided also. The AMCG group at Imperial College already have much experience and success in this with their other codes. We will hold a 3-day workshop at Imperial College London, covering all necessary aspects for use of the code (i) overview; (ii) obtaining and building Fluidity; (iii) mesh-generation; (iv) the input file/Graphical User Interface; (v) running in parallel; (vi) visualization; (vii) pedagogical examples. Through this we expect the projects impact to build through the academic community. In this way not only will our science outputs but also the methodology developments will have a long term impact.
Wider user interest
Secondary School Students - The clarity that this project will bring to our understanding of the interior dynamics will ultimately allow textbooks to present an exciting and accurate picture of the driving processes. It shows students the power and need for interdisciplinary science (in this case Physics, Chemistry (Mineralogy), Computer Science, Maths, Geology) to answer significant questions. It also shows a 'big' application of 'simple' Physics. In particular to aid teachers, we will prepare and upload a series of power-point presentations, providing a simplified summary of the latest scientific results.
Public - While this project clearly addresses part of the Earth (ocean crust and mantle) that is out of sight it is so fundamental and intrinsically fascinating and visual (detailed thermodynamics aside possibly!) that with the correct presentation - this work will help us forge an appreciation in the public of the significance of the deep interior.
Over recent years, all project investigators have been contacted by journalists (e.g. New Scientist, S4C, BBC) in relation to their research. For example PI Davies' work has recently been covered by major popular publications New Scientist, Earth (American Geological Institute's news stand publication); and he has taken part in TV news and documentaries for S4C and BBC. We will engage politicians / policy makers by attending the Annual 'Science and the Assembly' meeting organized by the Royal Society of Chemistry which occurs in May every year at the Wales Millennium Centre and Senedd, Cardiff Bay. The PI has already presented his work to Wales's First Minister.
We will continue to pursue such openings. We will use the web-site to not only release Fluidity but also to present the exciting results to the public as well as the academic beneficiaries.
People |
ORCID iD |
J Davies (Principal Investigator) | |
Martin Wolstencroft (Researcher) |
Publications
Le Voci G
(2014)
A systematic 2-D investigation into the mantle wedge's transient flow regime and thermal structure: Complexities arising from a hydrated rheology and thermal buoyancy
in Geochemistry, Geophysics, Geosystems
Hedjazian N
(2017)
Age-independent seismic anisotropy under oceanic plates explained by strain history in the asthenosphere
in Earth and Planetary Science Letters
Kramer S
(2012)
An implicit free surface algorithm for geodynamical simulations
in Physics of the Earth and Planetary Interiors
Jones T
(2016)
Do mantle plumes preserve the heterogeneous structure of their deep-mantle source?
in Earth and Planetary Science Letters
Beall A
(2021)
Influence of Subduction Zone Dynamics on Interface Shear Stress and Potential Relationship With Seismogenic Behavior
in Geochemistry, Geophysics, Geosystems
Garel F
(2014)
Interaction of subducted slabs with the mantle transition-zone: A regime diagram from 2-D thermo-mechanical models with a mobile trench and an overriding plate
in Geochemistry, Geophysics, Geosystems
Davies JH
(2015)
Overriding plate deformation during subduction evolution (invited)
Perrin A
(2016)
Reconciling mantle wedge thermal structure with arc lava thermobarometric determinations in oceanic subduction zones
in Geochemistry, Geophysics, Geosystems
Greve S
(2014)
Shear-velocity structure of the Tyrrhenian Sea: Tectonics, volcanism and mantle (de)hydration of a back-arc basin
in Earth and Planetary Science Letters
Description | We have discovered that there are four distinct modes of behaviour for the descending tectonic plate in regions of plate convergence, like the 'ring of fire' of earthquakes and volcanoes. We have found that the material properties are critical, as is the possibility for the point of convergence of the plates to migrate freely. |
Exploitation Route | Our findings will guide future workers in this field in regards of what aspects need to be considered and what values are reasonable. This will improve researchers abilities to develop models which better allow us to understand geological evolution in such settings. This will also ultimately help workers to understand the drivers for the dynamics. |
Sectors | Energy Environment |
Description | Our findings have been used by scientists interested in the dynamics of the 'ring of fire' with its earthquakes and volcanoes, and similar settings globally. Our findings have shown that the motion of tectonic plates at Earth's surface are important for the resultant dynamics in the interior. |
First Year Of Impact | 2014 |
Sector | Energy,Environment |
Impact Types | Societal |
Description | ARC Future Fellowship |
Amount | $683,700 (AUD) |
Funding ID | FT140101262 |
Organisation | Australian Research Council |
Sector | Public |
Country | Australia |
Start | 01/2015 |
End | 12/2018 |
Description | Research Leave Scheme |
Amount | £15,000 (GBP) |
Organisation | Cardiff University |
Sector | Academic/University |
Country | United Kingdom |
Start | 08/2014 |
End | 09/2015 |
Description | Research Programme - Volatiles, Geodynamics & Solid Earth Controls on the Habitable Planet - Mantle volatiles: processes, reservoirs and fluxes |
Amount | £221,000 (GBP) |
Funding ID | NE/M000397/1 |
Organisation | Natural Environment Research Council |
Sector | Public |
Country | United Kingdom |
Start | 06/2014 |
End | 06/2019 |
Description | Research Programme - Volatiles, Geodynamics & Solid Earth Controls on the Habitable Planet - Volatile legacy of the Early Earth |
Amount | £123,512 (GBP) |
Funding ID | NE/M000400/1 |
Organisation | Natural Environment Research Council |
Sector | Public |
Country | United Kingdom |
Start | 08/2014 |
End | 08/2019 |
Description | Crystal2Plate Workshop-Plate Tectonics on a Convective Mantle: From Crystal Scale Processes to Global Data and Models |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Participants in your research and patient groups |
Results and Impact | Presentation at Crystal to Plate Workshop - Frejus: Investigating the relationship between the mantle transition zone and the fate of subducted slabs: an adaptative-mesh numerical approach. Played a key role in Dr. Fanny Garel securing a continuing position as a lecturer in Montpellier. |
Year(s) Of Engagement Activity | 2013 |
Description | Earth Science Teacher Association Conference - Presentation |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | National |
Primary Audience | Schools |
Results and Impact | A presentation was made to around 40 Secondary School teachers attending the Earth Science Teacher Association's Conference, using the 3D projection facility, Helix, to describe mantle geodynamics research and the way that all STEM subjects contribute to such research. The presentation was followed by a lot of questions and discussion. I was also requested to prepare an article related to the presentations to ESTA's Journal. The presentation was also attended by Iain Stewart who twitted the event to his significant followers. |
Year(s) Of Engagement Activity | 2015 |
Description | Ffeil - Contribution to TV Programme produced by BBC for S4C |
Form Of Engagement Activity | A press release, press conference or response to a media enquiry/interview |
Part Of Official Scheme? | No |
Geographic Reach | National |
Primary Audience | Media (as a channel to the public) |
Results and Impact | I was answered questions to camera regarding sinkholes for the Welsh language TV Programme - Ffeil - aired on S4C - produced by BBC. Target audience is 8 to 16 years old. |
Year(s) Of Engagement Activity | 2016 |
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 presentation to whole Research School at Australian National University - presentation to whole School - subduction dynamics |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | Local |
Primary Audience | Professional Practitioners |
Results and Impact | Talk was followed by extensive questions and discussion which continued for the weeks of my visit. Increased awareness of this project in one of the leading Earth Science research Universities in Australia |
Year(s) Of Engagement Activity | 2014 |
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 | 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: Simula Research Laboratories, Olso, Norway. |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | National |
Primary Audience | Professional Practitioners |
Results and Impact | Overview of research and future plans: discussion on potential future collaboration. Ongoing collaboration with Simula. |
Year(s) Of Engagement Activity | 2012 |
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 | invited presentation Gordon Research Conference on Deep Earth - 'Slab Interaction with the Mantle Transition Zone' |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Other academic audiences (collaborators, peers etc.) |
Results and Impact | talk triggered questions and discussion. The talk helped to bring our project and its outcomes to the attention of a broad group of colleagues. This increased the academic impact of the work. |
Year(s) Of Engagement Activity | 2013 |
URL | http://www.grc.org/programs.aspx?year=2013&program=interior |