Strain weakening enables continental plate tectonics

International audienceMuch debate exists concerning the strength distribution of the continental lithosphere, how it controls lithosphere-scale strain localization and hence enables plate tectonics. No rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid-like behaviour of plate interiors. Here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution. Geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle, respectively. The positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remai

Evolution in H2O contents during deformation of polycrystalline quartz: An experimental study

Accepted manuscript version, licensed CC BY-NC-ND 4.0. Published version available at https://doi.org/10.1016/j.jsg.2018.05.021.Shear experiments were performed in a Griggs-type apparatus at 800 °C and 1.5 GPa, at a strain rate of 2.1 × 10−5s−1 using different starting materials: (i) Powder (grain size 6–10 μm) of dry Brazil quartz with 0.15 wt% added H2O, (ii) “dry” Brazil quartz porphyroclasts (grain size ∼100–200 μm), devoid of fluid inclusions embedded in the same fine grained powder, and (iii) “wet” porphyroclasts (grain size ∼100–200 μm), containing initially a high density of μm-scale fluid inclusions embedded in the same powder. After hot pressing, samples were deformed to large shear strains (γ∼3 to 4.5), in order for the microstructures and H2O distribution to approach some state of “equilibrium”. The H2O content and speciation in quartz were analyzed by Fourier Transform Infra-Red (FTIR) spectroscopy before and after the experiments. Mechanical peak strength is generally lower in experiments with 100% hydrated matrix, intermediate in experiments incorporating wet porphyroclasts (with a proportion of 30 or 70%) and highest in those with dry porphyroclasts. All experiments with porphyroclasts show pronounced strain weakening, and the strengths of most samples converge to similar values at large strain. Wet porphyroclasts are pervasively recrystallized during deformation, while dry porphyroclasts recrystallize only at their rims and remain weakly deformed. Recrystallization of the initially fluid-inclusion-rich porphyroclasts results in a decrease in inclusion abundance and total H2O content, while H2O content of initially dry clasts increases during deformation. H2O contents of all high strain samples converge to similar values for matrix and recrystallized grains. In samples with wet porphyroclasts, shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free-fluid phase. These high porosity sites are sinks for collecting H2O in excess of the storage capacity of the grain boundary network of the recrystallized aggregate. The H2O storage capacity of the grain boundary network is determined as a H2O-boundary-film of ∼0.7 nm thickness

La localisation de la déformation dans le manteau sous-continental:<br />origine à travers l'étude du massif de Ronda (Espagne) et implications sur la résistance de la lithosphère.

The predicted rheology for the sub-continental mantle does not account for both the weak mantle strength beneath deforming regions and processes of lithosphere-scale strain localization. In order to better constrain this rheology, we used structural geology, tectonics and numerical modeling through the study of the Ronda peridotites. The structural study of these peridotites shows that their deformations were formed beneath a back-arc continental rift, before they were inserted into the internal Betics at the early Miocene. Just below the extending crust, a ductile strain gradient occurs in response to a process that implies the dominance of the dryGBS creep during dynamic olivine grain size reduction. Numerical quantifications demonstrate that this ductile process can trigger large strain localization and a drop of strength for the highly deformed peridotites at low temperature (< 800 °C). At larger scale, the results of a 2-D numerical model show also that such a weakening process promotes high strain localization in the sub-continental mantle, which leads to trigger continental necking. This strain localization provokes consistently an important drop of strength in the sub-continental mantle, as observed beneath deforming regions. Based on this “localizing” rheology, our results allow us to propose a new definition of the mantle rheology that accounts for the evolution of the mantle strength during the deformation of the lithosphere.L'actuelle définition rhéologique du manteau sous-continental ne rend ni compte de sa faible résistance estimée sous les régions déformées, ni des processus de localisation de la déformation qui le caractérisent. Pour mieux contraindre sa rhéologie, nous avons donc utilisé la tectonique et la modélisation numérique à travers l'étude des péridotites de Ronda. L'étude structurale de ces péridotites montre que leurs déformations sont liées à une extension continentale arrière-arc, juste avant qu'elles soient intégrées dans les Bétiques internes au Miocène inférieur. Au cours de cette extension intervient la formation d'un gradient de déformation ductile et sous-continental, qui serait initié par un processus impliquant l'action dominante du fluage dryGBS pendant la réduction dynamique de taille de grains de l'olivine. La quantification numérique de ce processus ductile montre qu'il peut provoquer, à basse température (< 800 °C), une intense localisation de la déformation et une chute de résistance des péridotites intensément déformées. À plus grande échelle, ce processus est aussi capable de localiser la déformation dans le manteau sous-continental, permettant, d'après nos résultats numériques 2-D, d'initier la formation d'un rift continental étroit. La conséquence majeure de cette localisation se traduit par une intense chute de résistance du manteau au cœur du rift, comme observée dans les régions déformées. Cette rhéologie « localisante » et ces résultats nous permettent donc de proposer une nouvelle définition de la rhéologie du manteau, qui tient compte de l'évolution de sa résistance pendant la déformation de la lithosphère

On the Origin of Ultramylonites

International audienceDeformation of lithospheric rocks regularly localizes into high-strain shear zones that include fine-grained ultramylonites. Occurring as quasi-straight layers of intimately mixed phases that often describe sharp transitions with the host rock, these structures may channelize fluid flow[1,2] and could serve as precursors for deep earthquakes[3]. However, although intensively documented, ultramylonites originate from still unknown processes. Here I focus on a mylonitic complex that includes numerous mantle ultramylonites in the Ronda peridotite (Spain). Among them, I was able to highlight one of their precursors that I better describe as a long and straight grain boundary, along which four-grain junctions are observed with randomly oriented grains of olivine and pyroxenes. This precursor starts from a pyroxene porphyroclast and extends to an incipient, weakly undulated ultramylonite, where intimate phase mixing arises with asymmetrical grain size distribution. While the finer grain size locates on one side, describing a sharp - but continuous - transition with the host rock, the grain size gradually increases towards the other side, giving rise to a smooth transition. All phases have a very weak lattice preferred orientation (LPO) in the ultramylonite, which strongly differs from the host rock where olivine is highly deformed with evidence of high dislocation densities and a strong LPO. Altogether, these features shed light on the origin of mantle ultramylonites that I attribute to a migrating grain boundary, the sliding of which continuously produces new grains by phase nucleation, probably at the favor of transient four-grain junctions. Nucleated grains then grow and progressively detach from the precursor as it keeps on migrating depending on the dislocation densities in the host rock. Although such an unusual grain boundary remains to be understood in terms of source mechanism, these findings provide new constraints on the appearing and development of ultramylonites. [1] Fusseis, F., Regenauer-Lieb, K., Liu, J., Hough, R. M. & De Carlo, F. Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones. Nature 459: 974-977 (2009)[2] Précigout, J., Prigent, C., Palasse, L. & Pochon, A. Water pumping in mantle shear zones. Nat. commun. 8: 15736, https://doi.org/10.1038/ncomms15736 (2017)[3] White, J. C. Paradoxical pseudotachylyte - Fault melt outside the seismogenic zone. J. Struct. Geol. 38: 11-20 (2012

Modes of continental rifting as a function of ductile strain localization in the lithospheric mantle

International audienceAnalogue and numerical models have shown that the strength of the lithospheric mantle controls the mode of lithosphere deformation. In extension, the presence or absence of a high strength brittle mantle respectively leads to localized or distributed rifting. However, first order geophysical data question the existence of such a brittle mantle. Here we use 2-D finite-element large strain modelling to quantify the impact of a ductile localizing mantle - instead of brittle - in triggering continental rifting. As a novelty, the mantle rheology considers the effect of grain boundary sliding during strain-induced grain size reduction, which may promote a significant strength drop and subsequent strain localization at low mantle temperature (< 700-800 °C). Our results reveal that such ductile localizing mantle implies varying modes of continental rifting that mainly depend on both the amount of weakening in the ductile mantle and the strength of the lower ductile crust. A medium to strong lower crust implies coupling between the upper crust and ductile localizing mantle, yielding to narrow continental rifting. In contrast, a weak lower crust implies decoupling between the upper crust and ductile localizing mantle, giving rise to a switch from distributed faulting at incipient strain to localized faulting at large strain. Ductile strain localization in the lithospheric mantle is therefore sufficient to trigger continental rifting, although a critical amount of weakening is required. Such ductile localizing mantle provides a relevant geological and mechanical alternative to the brittle mantle. It moreover provides a wider variety of modes of upper crustal faulting that are commonly observed in nature

B-type olivine fabric induced by grain boundary sliding

International audienceOlivine fabric, or Lattice Preferred Orientation (LPO), in naturally deformed peridotite largely contributes to the seismic anisotropy of the upper mantle. LPO usually results from motion of intra-crystalline dislocations during dislocation creep. In this case, experimental and numerical data indicate that the degree of mineral alignment (fabric strength) increases with increasing finite strain. Here, we show an opposite trend suggesting that olivine fabric can also result from a different deformation mechanism. Based on documentation of olivine LPOs in peridotites of a kilometer-scale mantle shear zone in the Ronda massif (Spain), we highlight a transition from a flow-parallel [a]-axis LPO (A-type fabric) to a flow-normal [a]-axis LPO (B-type fabric). While dislocation sub-structures indicate that A-type fabric results from dislocation motion, we conclude that the B-type fabric does not originate from dislocation creep, but instead from grain boundary sliding (GBS) because: (1) dislocation sub-structures remain consistent with the A-type slip system in all samples; (2) the fabric transition from A-type to B-type correlates with decreasing fabric strength despite increasing finite strain; and (3) our observations are supported by experiments that document B-type fabric in olivine aggregates where deformation involves a component of GBS. The B-type olivine fabric has a specific signature in term of seismic anisotropy, and hence, our results may have important implications for interpreting upper mantle structures and deformation processes via seismic observations

Fluid pumping induced by strain localization during high-pressure experiment

International audienceStrain localization commonly affects naturally deformed rocks of the crust and mantle. In the ductile regime, this gives rise to shear zones where the deformation of fine-grained phases involves creep cavitation to occur with possible fluid pumping (Ref. 1). As recently highlighted in mantle shear zones (Ref. 2), this latter process may have strong implications for ores deposits, partial melting or rock rheology. However, as creep cavitation potentially involves some dilatancy, it remains challenging for this mechanism to be relevant at high pressure, typically when differential stresses are far below the lithostatic pressure, i.e., below the Goetze criterion. Based on ion probe analyzes, we here document the water content of a fine-grained (1-2 μm) olivine matrix deformed experimentally in presence of water added (1300 ppm) and coarse-grained (> 100 μm) diopside (30 %). The experiments were carried out at a pressure of 1.2 GPa, a temperature of 900 °C, and the differential stress never exceeded 0.9 GPa. The development of a narrow shear zone (50 μm thick) also gave rise to pronounced sample-scale strain localization. The olivine matrix has been then probed post-mortem across the resulting strain gradient, each spot (10*10*4 μm) including several grain boundaries. Through data interpolation, our ion probe dataset reveals high water concentration - higher than initially added (> 2000 ppm) - where strain has been localized. Furthermore, using high-resolution EBSD maps to precisely measure the olivine grain size, we show that water content does not follow the trend of the expected porosity related to grain size reduction. Instead, we document a deficit of water in the low-strain region and an excess of water in the high-strain zone. Together with TEM observations of some micro-cracks in the shear zone, these features indicate the presence of strain-induced cavities where water has been pumped and trapped as fluid inclusions. Our findings therefore provide evidence of experimental creep cavitation at high pressure and below the Goetze criterion, suggesting that related fluid pumping may persist at great depths despite increasing pressure. Ref. 1: Fusseis, F., Regenauer-Lieb, K., Liu, J., Hough, R. M., and De Carlo, F. (2009) Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones. Nature 459: 974-977 Ref. 2: Précigout, J., Prigent, C., Palasse, L., and Pochon A. (2017) Water pumping in mantle shear zones. Nature communications 8: 1573

Fluid pumping induced by strain localization during high-pressure experiment

International audienceStrain localization commonly affects naturally deformed rocks of the crust and mantle. In the ductile regime, this gives rise to shear zones where the deformation of fine-grained phases involves creep cavitation to occur with possible fluid pumping (Ref. 1). As recently highlighted in mantle shear zones (Ref. 2), this latter process may have strong implications for ores deposits, partial melting or rock rheology. However, as creep cavitation potentially involves some dilatancy, it remains challenging for this mechanism to be relevant at high pressure, typically when differential stresses are far below the lithostatic pressure, i.e., below the Goetze criterion. Based on ion probe analyzes, we here document the water content of a fine-grained (1-2 μm) olivine matrix deformed experimentally in presence of water added (1300 ppm) and coarse-grained (> 100 μm) diopside (30 %). The experiments were carried out at a pressure of 1.2 GPa, a temperature of 900 °C, and the differential stress never exceeded 0.9 GPa. The development of a narrow shear zone (50 μm thick) also gave rise to pronounced sample-scale strain localization. The olivine matrix has been then probed post-mortem across the resulting strain gradient, each spot (10*10*4 μm) including several grain boundaries. Through data interpolation, our ion probe dataset reveals high water concentration - higher than initially added (> 2000 ppm) - where strain has been localized. Furthermore, using high-resolution EBSD maps to precisely measure the olivine grain size, we show that water content does not follow the trend of the expected porosity related to grain size reduction. Instead, we document a deficit of water in the low-strain region and an excess of water in the high-strain zone. Together with TEM observations of some micro-cracks in the shear zone, these features indicate the presence of strain-induced cavities where water has been pumped and trapped as fluid inclusions. Our findings therefore provide evidence of experimental creep cavitation at high pressure and below the Goetze criterion, suggesting that related fluid pumping may persist at great depths despite increasing pressure. Ref. 1: Fusseis, F., Regenauer-Lieb, K., Liu, J., Hough, R. M., and De Carlo, F. (2009) Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones. Nature 459: 974-977 Ref. 2: Précigout, J., Prigent, C., Palasse, L., and Pochon A. (2017) Water pumping in mantle shear zones. Nature communications 8: 1573

Role of ductile strain localization in the lithospheric mantle on continental rifting

International audienceThe strength of the lithospheric mantle controls the mode of lithosphere deformation: the presence or absence ofa high strength brittle mantle respectively leads to localized or distributed rifting. However, first order geophysicaldata question the existence of such a brittle mantle. Here we use 2-D finite-element large strain modelling toquantify the impact of a ductile localizing mantle – instead of brittle – in triggering continental rifting. As a novelty,the mantle rheology considers the effect of grain boundary sliding during strain-induced grain size reduction,which may promote a significant strength drop and subsequent strain localization at low mantle temperature (<700-800°C). Our results reveal that such ductile localizing mantle implies varying modes of continental rifting thatmainly depend on both the amount of weakening in the ductile mantle and the strength of the lower ductile crust. Amedium to strong lower crust implies coupling between the upper crust and ductile localizing mantle, yielding tonarrow continental rifting. In contrast, a weak lower crust implies decoupling between the upper crust and ductilelocalizing mantle, giving rise to a switch from distributed faulting at incipient strain to localized faulting at largestrain. Ductile strain localization in the lithospheric mantle is therefore sufficient to trigger continental rifting,although a critical amount of weakening is required. Such ductile localizing mantle provides a relevant geologicaland mechanical alternative to the brittle mantle. It moreover provides a wider variety of modes of upper crustalfaulting that are commonly observed in nature

The Ronda peridotite (Spain): A natural template for seismic anisotropy in subduction wedges

International audienceThe origin of seismic anisotropy in mantle wedges remains elusive. Here we provide documentation of shear wave anisotropy (AVs) inferred from mineral fabric across a lithosphere-scale vestige of deformed mantle wedge in the Ronda peridotite. As predicted for most subduction wedges, this natural case exposes a transition from A-type to B-type olivine fabric that occurs with decreasing temperature and enhanced grain boundary sliding at the expense of dislocation creep. We show that B-type fabric AVs (maximum of 6%) does not support geophysical observations and modeling, which requires 8% AVs. However, an observed transitional olivine fabric (A/B) develops at intermediate temperatures (800–1000°C) and can generate AVs ≥ 8%. We predict that the A/B-type fabric can account for shear wave splitting in contrasting subduction settings, exemplified by the Ryukyu and Honshu subduction wedges. The Ronda peridotite therefore serves as a natural template to decipher the mantle wedge deformation from seismic anisotropy