Author: Sébastien Merkel

A new publication for TIMEleSS ! This week, former TIMEleSS post-doc John Keith Magali, has a new paper, along with Angelo Pisconti,  James Wookey , former TIMEleSS PhD student Jeffrey Gay, and former TIMEleSS PI’s Sébastien Merkel and Christine Thomas. The work was published in Nature Communications on  05 October 2026: Mid-mantle discontinuities beneath subduction zones linked to depth-dependent deformation of bridgmanite.

In this work, we look at bridgmanite, the main mineralogical phase in the Earth’s lower mantle. Bridgmanite is intrinsically anisotropic and may therefore generate seismic anisotropy in actively-deforming regions, particularly subduction zones. The bulk of the lower mantle is widely regarded as well-mixed but recent seismic observations suggest structures at mid-mantle depths whose origin remains unclear. Here, we propose a mechanism that could generate seismic discontinuities beneath subduction zones with depth distributions consistent with some of the observed mid-mantle reflectors.

Eight years after we started, the work is a nice conclusion to the overall TIMEleSS projet. High-pressure experiments we performed identified a gradual transition in plastic deformation mechanisms in bridgmanite with depth. This was the work of PhD student Jeff Gay. Then post-doc John Keith Magali coupled thermo-mechanical subduction models with pressure-dependent plasticity calculations and showed it could manifest as an anisotropic discontinuity near 1000 km. Finally, he used full waveform simulations to show that the predicted discontinuity is indeed detectable with array seismological methods.

These results suggest that mid-mantle reflectors near subduction zones may be strain-induced, providing an additional mechanism that links mantle flow, mineral physics, and seismic observations, which is what we wanted to get to when we started the project!

We have a new publication ! In his work published in AGU’s Geochemistry, Geophysics, Geosystems, J.K. Magali investigates the Influence of Pressure, Phase Transitions, and Water on Large-Scale Seismic Anisotropy Underneath a Subduction Zone.

In fact, subduction deep inside the Earth’s mantle causes the surrounding mantle to deform according to the movement and pressure exerted by the plates. This influences the alignment of minerals making up the mantle, which in turn, affects the speed and direction of seismic waves known as seismic anisotropy.

In this publication, we investigate the role of pressure in the upper mantle and water in the transition zone on large‐scale seismic anisotropy across a subduction zone. In the upper mantle, we show that the patterns of anisotropy at places where deformation is presumed to be large are affected by changes in pressure. In the transition zone, anisotropy tends to favor a wetter environment.

Want to know more ? The full reference is here : Magali, J. K., Thomas, C., Ledoux, E. E., Capdeville, Y., & Merkel, S. (2025). On the influence of pressure, phase transitions, and water on large-scale seismic anisotropy underneath a subduction zone. Geochemistry, Geophysics, Geosystems, 26, e2024GC011827. https://doi.org/10.1029/2024GC011827.

TIMEleSS PhD student Federica Rochira has a new paper! Along with TIMEleSS PI C. Thomas, Federica is the first author of On the Importance of Using Directional Information in the Search for Lower Mantle Reflectors. The paper is available in Gold Open Access in The Seismic Record.

In this work, Federica focuses on SS precursor signals. SS precursors are signals from S-waves reflected from structures in the Earth’s mantle. Those waves are interesting because they are sensitive to the local thermal and compositional structure deep in the Earth’s mantle.

She analyses how to model the location of the reflections of the wave and shows that a full account of the wave travel path can lead to differences of up to ∼150 km in regard to more simple geometrical calculations. These results hence indicate that the travel‐path deviations of SS precursors should be considered to avoid misinterpretation of mantle discontinuities and potentially reduce previously observed scatter in discontinuity depth.

Congratulations to Federica for the nice work!

We have a new paper release ! On May 3rd, 2022, TIMEleSS student Matthias Krug, PI Carmen Sanchez-Valle, and PI Sébastien Merkel published a paper in Physical Review Materials, along with Binbin Yue and Fang Hong from the Center for High Pressure Science & Technology in Beijing, China.

The paper focuses on deformation in MnO₂ and particularly its high pressure phase, in the CaCl₂ structure. Why do we care about MnO₂ ? MnO₂ is not a deep Earth material, but it can be used as an analogue for SiO₂ stishovite, which crystalizes in a rutile structure at low pressure and transform to a denser CaCl₂ structure under high pressure. In SiO₂, the stishovite to post-stishovite transformations occurs at ~ 50 GPa and is not always easily reached in deformation experiments. In MnO₂, this same structural transition occurs at much lower pressure, around 4 GPa.

Our results show that, after phase transition to a ${\mathrm{CaCl}}^{}$₂ structure above 3.5 GPa, the dominant (010)[100] and secondary {110}[001] and {011}[0-11] slip systems induce a 121 texture in compression. Further compression increases the activity of the $\left\{011\right\}⟨0\text{−}11⟩$ slip system, with an enhanced 001 texture at $\sim 50\mathrm{GPa}$. Finally, ${\mathrm{MnO}}_2$ transforms back to a rutile structure upon pressure release, with a significant orientation memory, highlighting the martensitic nature of the ${\mathrm{CaCl}}_2$ to rutile structural transformation. Overall, these results help us understanding plasticity and microstructures of ${\mathrm{CaCl}}_2$-structured dioxides, with implications in materials and Earth and planetary science.

The full reference for the study: B Yue, M. Krug, C. Sanchez-Valle, S. Merkel, and F. Hong, Deformation and slip systems of CaCl2-type MnO2 under high pressure, Phys. Rev. Mater., 6, 053603 (2022) [doi: 10.1103/PhysRevMaterials.6.053603]