Philippe Yamato

Dynamic constraints on the crustal-scale rheology of the Zagros fold belt, Iran

Thin-skinned fold-and-thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer. The resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner. It is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales. Here we use the Zagros Mountains of Iran as a case study, as it is one of the most spectacular, well-studied thin-skinned fold-and- thrust belts in the world. Both analytical and numerical models are employed to study what con- trols fold spacing and under what conditions folding dominates over thrusting. The models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover, deformation is dominated by thrusting, which is inconsistent with the data of the Zagros fold belt. If we instead take into account additional decollement layers that have been documented in the fi eld, a switch in deformation mode occurs and crustal-scale folding is obtained with the correct spacing and time scales. We show that fold spacing can be used to constrain the friction angle of the crust, which is ~5° the Zagros fold belt. This implies that on geological time scales, the upper crust is signifi cantly weaker than previously thought, possibly due to the effect of fl

Eclogite breccias in a subducted ophiolite: A record of intermediate-depth earthquakes?

Understanding processes acting along the subduction interface is crucial to assess lithospheric-scale coupling between tectonic plates and mechanisms causing intermediate-depth seismicity. Despite a wealth of geophysical studies aimed at better characterizing the subduction interface, we still lack critical data constraining processes responsible for seismicity within oceanic subduction zones. We herein report the finding of eclogite breccias, formed at ∼80 km depth during subduction, in an almost intact 10-km-scale fragment of exhumed oceanic lithosphere (Monviso ophiolite, Western Alps). These eclogite breccias correspond to meter-sized blocks made of 1–10 cm fragments of eclogite mylonite cemented by interclast omphacite, lawsonite, and garnet, and were later embedded in serpentinite in a 30–150-m-wide eclogite facies shear zone. At the mineral scale, omphacite crack-seal veins and garnet zoning patterns also show evidence for polyphased fracturing-healing events. Our observations suggest that a possible seismic brecciation occurred in the middle part of the oceanic crust, accompanied by the input of externally derived fluids. We also conclude that these eclogite breccias likely mark the locus of an ancient fault zone associated with intraslab, intermediate-depth earthquakes at ∼80 km depth.

Subducting slabs: Jellyfishes in the Earth's mantle

The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

Passive margins getting squeezed in the mantle convection vice

[1] Passive margins often exhibit uplift, exhumation, and tectonic inversion. We speculate that the compression in the lithosphere gradually increased during the Cenozoic, as seen in the number of mountain belts found at active margins during that period. Less clear is how that compression increase affects passive margins. In order to address this issue, we design a 2-D viscous numerical model wherein a lithospheric plate rests above a weaker mantle. It is driven by a mantle conveyor belt, alternatively excited by a lateral downwelling on one side, an upwelling on the other side, or both simultaneously. The lateral edges of the plate are either free or fixed, representing the cases of free convergence, and collision (or slab anchoring), respectively. This distinction changes the upper mechanical boundary condition for mantle circulation and thus, the stress field. Between these two regimes, the flow pattern transiently evolves from a free-slip convection mode toward a no-slip boundary condition above the upper mantle. In the second case, the lithosphere is highly stressed horizontally and deforms. For a constant total driving force, compression increases drastically at passive margins if upwellings are active. Conversely, if downwellings alone are activated, compression occurs at short distances from the trench and extension prevails elsewhere. These results are supported by Earth-like models that reveal the same pattern, where active upwellings are required to excite passive margins compression. Our results substantiate the idea that compression at passive margins is in response to the underlying mantle flow that is increasingly resisted by the Cenozoic collisions.

The Minimized Power Geometric model: An analytical mixing model for calculating polyphase rock viscosities consistent with experimental data

Here we introduce the Minimized Power Geometric (MPG) model which predicts the viscosity of any polyphase rocks deformed during ductile flow. The volumetric fractions and power law parameters of the constituting phases are the only model inputs required. The model is based on a minimization of the mechanical power dissipated in the rock during deformation. In contrast to existing mixing models based on minimization, we use the Lagrange multipliers method and constraints of strain rate and stress geometric averaging. This allows us to determine analytical expressions for the polyphase rock viscosity, its power law parameters, and the partitioning of strain rate and stress between the phases. The power law bulk behavior is a consequence of our model and not an assumption. Comparison of model results with 15 published experimental data sets on two-phase aggregates shows that the MPG model reproduces accurately both experimental viscosities and creep parameters, even where large viscosity contrasts are present. In detail, the ratio between experimental and MPG-predicted viscosities averages 1.6. Deviations from the experimental values are likely to be due to microstructural processes (strain localization and coeval other deformation mechanisms) that are neglected by the model. Existing models that are not based on geometric averaging show a poorer fit with the experimental data. As long as the limitations of the mixing models are kept in mind, the MPG model offers great potential for applications in structural geology and numerical modeling.

Fluid pathways and high-P metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

The Mt. Emilius klippe (Western Alps, Italy) corresponds to a segment of the stretched Adriatic continental margin metamorphosed in granulite-facies during Permian. This slice was subducted during the early Cenozoic Alpine subduction with the underlying eclogite-facies remnants of the Tethyan seafloor (Zermatt-Saas zone). Near the base of the Mt. Emilius massif, a shear zone presents eclogite-facies hydrofracture systems associated with deformation-induced re-equilibration of granulites during high pressure metamorphism. We herein focus on a pluri-hectometer-length domain of sheared mafic boudins hosted in the granulitic paragneiss. These mafic boudins exhibit garnetites and garnet veins, clinopyroxenites, clinozoisite and calcite veins. We report multiple events of fracture opening, brecciation, boudinage and parallelization of structures coevally with fluid-rock interaction, metasomatism and volume change. Our integrated petrological, micro-textural and geochemical investigations illustrate the multiplicity and the chemical variability of fluid sources during prograde to peak metamorphic evolution in the lawsonite-eclogite-facies field (at ~ 2.15-2.4 GPa, 500-550°C). The calcite veins crosscutting the garnetites have relatively low δ18OVSMOW values (c. +6.5 per mil) near those for marble layers (and nearby calc-silicates) embedded within the metasomatized granulites (+8 to +10 per mil). It is proposed that infiltration of externally-derived H2O-rich fluids derived from the plate interface flushed the marbles, promoting decarbonation followed by short-distance transport and re-precipitation along garnetite fractures. This study highlights the importance of inherited structural heterogeneities (such as mafic bodies or sills) in localizing deformation, draining fluids from the downgoing plate, and creating long-lasting mechanical instabilities during subduction zone deformation.

Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)

Metamorphic soles are tectonic slices welded beneath most large-scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as forming during the first million years of intra-oceanic subduction following heat transfer from the incipient mantle wedge towards the top of the subducting plate. This study reappraises the formation of metamorphic soles through detailed field and petrological work on three key sections from the Semail ophiolite (Oman and United Arab Emirates). Based on thermobarometry and thermodynamic modelling, it is shown that metamorphic soles do not record a continuous temperature gradient, as expected from simple heating by the upper plate or by shear heating as proposed in previous studies. The upper, high-temperature metamorphic sole is subdivided in at least two units, testifying to the stepwise formation, detachment and accretion of successive slices from the down-going slab to the mylonitic base of the ophiolite. Estimated peak pressure-temperature conditions through the metamorphic sole, from top to bottom, are 850°C and 1 GPa, 725°C and 0.8 GPa and 530°C and 0.5 GPa. These estimates appear constant within each unit but differing between units by 100 to 200°C and ~0.2 GPa. Despite being separated by hundreds of kilometres below the Semail ophiolite and having contrasting locations with respect to the ridge axis position, metamorphic soles show no evidence for significant petrological variations along strike. These constraints allow us to refine the tectonic–petrological model for the genesis of metamorphic soles, formed via the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover. Metamorphic soles result not so much from downward heat transfer (ironing effect) as from progressive metamorphism during strain localization and cooling of the plate interface. The successive thrusts originate from rheological contrasts between the sole, initially the top of the subducting slab, and the peridotite above as the plate interface progressively cools. These findings have implications for the thickness, the scale and the coupling state at the plate interface during the early history of subduction/obduction systems. This article is protected by copyright. All rights reserved.

Using Observed Fold Spacing and Models to Constrain the Dynamics and Rheology of the Zagros Mountains.

Fold belts are generally considered as the result of thrusting and folding of a sedimentary cover over a weak decollement layer. As the resulting anticlines are typically spaced in a fairly regular manner, it is tempting to use this spacing along with other geological constraints to obtain insights in the dynamics and rheology of the crust on geological time scales. Here we use the Zagros Mountains as a case study as it is one of the most spectacular, well-studied thin-skinned fold-and-thrust belts in the world (e.g., Stocklin 1968; Mouthereau et al., 2007). Geologically, the Zagros consists of crustal-scale folds with an average spacing of around 14 km, which have been formed in around 5.5 Myrs.