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Hydration and dehydration reactions play pivotal roles in plate tectonics and the deep water cycle, yet many facets of (de)hydration reactions remain unclear. Here, we study (de)hydration reactions where associated solid density changes are predominantly balanced by porosity changes, with solid rock deformation playing a minor role. We propose a hypothesis for three scenarios of (de)hydration front propagation and test it using one‐dimensional hydro‐mechanical‐chemical models. Our models couple porous fluid flow, solid rock volumetric deformation, and (de)hydration reactions described by equilibrium thermodynamics. We couple our transport model with reactions through fluid pressure: the fluid pressure gradient governs porous flow and the fluid pressure magnitude controls the reaction boundary. Our model validates the hypothesized scenarios and shows that the change in solid density across the reaction boundary, from lower to higher pressure, dictates whether hydration or dehydration fronts propagate: decreasing solid density causes dehydration front propagation in the direction opposite to fluid flow while increasing solid density enables both hydration and dehydration front propagation in the same direction as fluid flow. Our models demonstrate that reactions can drive the propagation of (de)hydration fronts, characterized by sharp porosity fronts, into a viscous medium with zero porosity and permeability; such propagation is impossible without reactions, as porosity fronts become trapped. We apply our model to serpentinite dehydration reactions with positive and negative Clapeyron slopes and granulite hydration (eclogitization). We use the results of systematic numerical simulations to derive a new equation that allows estimating the transient, reaction‐induced permeability of natural (de)hydration zones. Plain Language Summary: We investigate reactions of hydration, which is the incorporation of water into a rock, and dehydration, which is the liberation of water from a rock, with simple mathematical models. These reactions are critical in understanding processes like plate tectonics, but many aspects of how hydration or dehydration fronts move through a rock are unclear. Our research focuses on reactions where changes in density are mostly balanced by changes in pore space, termed porosity, rather than the deformation of the solid rock. We developed mathematical models that combine fluid flow, rock deformation, and hydration/dehydration reactions. We derived simple equations that predict changes in porosity during hydration and dehydration, even when the solid rock deforms simultaneously. We found that whether a rock hydrates or dehydrates depends on how its solid density changes with increasing pressure during the reaction. By systematically studying our model, we discovered that the speed of hydration and dehydration is not influenced by the interval of fluid pressure over which the reaction occurs or the relationship between porosity and permeability. We present an equation that can be used to estimate permeability from natural (de)hydration zones. Key Points: (De)hydration fronts propagate into zero‐permeability rock if the solid density of the reactant is smaller than the one of the productExternal fluid flux compensates the imbalance between fluid generated/consumed by reaction and fluid needed to fill generated porosityResults of systematic numerical simulations allow estimating the transient, reaction‐induced permeability of natural (de)hydration zones [ABSTRACT FROM AUTHOR]

In the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh‐pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well‐preserved phengite‐bearing eclogite sample from the Nordfjord region. The sample was investigated using phase‐equilibrium modelling, trace‐element analyses of garnet, trace‐ and major‐element thermobarometry and quartz‐in‐garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallization conditions in the amphibolite facies at 510–600°C and 11–16 kbar, whereas chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600°C, followed by near‐isobaric heating to 660–680°C. Near‐isothermal decompression to 10–14 kbar is recorded in fine‐grained clinopyroxene–amphibole–plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite facies metamorphism. Two end‐member tectonic scenarios are proposed to explain such an isothermal compression: Either (1) the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny. [ABSTRACT FROM AUTHOR]

Deformations of the colder regions of the lithosphere mainly occur in the frictional regime. In geodynamic models, frictional plastic deformations are often highly localized (shear bands) and are used as proxies for faults. However, capturing the generation and evolution of shear bands in geodynamic models is troublesome. Indeed, mesh dependency and lack of convergence affect, to some extent, the results of geodynamic models. Here we extend the most common plasticity implementation used in geodynamic codes (effective viscosity approach [EVA]) to include the combined effects of elastic compressibility, plastic dilatancy, strain softening, and viscoplasticity. The latter acts as a regularization that cures most of the known issues of geodynamic models related to frictional plasticity. Using regularized models based on the M2Di MATLAB routines, we show that volumetric elasto-plastic deformations can significantly impact crustal-scale shear banding. We also show that the artificial overstress caused by viscoplasticity can be mitigated by employing power-law models. Furthermore, we demonstrate that plasticity algorithms common in geodynamics (based on the EVA) can be as accurate as those obtained with algorithms typically used in engineering (return mapping with a consistent tangent operator). Finally, we show examples of long-term tectonic deformations using the state-of-the art geodynamic code MDoodz. They indicate that viscoplastic regularization can be used efficiently to obtain reliable simulations in geodynamics. [ABSTRACT FROM AUTHOR]

Pressure‐to‐depth conversion is a crucial step toward geodynamic reconstruction. The most commonly used pressure‐to‐depth conversion method assumes that pressure corresponds to the lithostatic pressure. However, deviatoric stresses can cause pressure to deviate from the lithostatic case strongly, thus adding considerable uncertainty to pressure to‐depth conversion. First, we rederive formulas of pressure‐to‐depth conversion that take into account deviatoric stresses. Then, we estimate the range of possible depth independently for each point in a data set containing peak and retrograde metamorphic pressure data (one‐point method). In a second time, we use both the peak and retrograde pressure of a rock sample together, assuming that both pressures were recorded at the same depth (two‐point method). We explore different cases to explain the transition from peak to retrograde pressure by varying the direction and magnitude of stresses. This alternative model is consistent with all data points but for a more restricted range of stress state and depth than the one‐point model. Our results show that (1) even small deviatoric stresses have a significant impact on depth estimates, (2) the second principal stress component σ2 plays an essential role, (3) several models can explain the pressure evolution of the data but lead to different depth estimates, and (4) strain data offer a mean to falsify our proposed two‐point pressure‐to‐depth conversion. The maximum predicted depth at peak pressure is 170 km using the assumption that pressure is lithostatic, compared to <75 km for our two‐point model, which could correspond to the crustal root Moho's depth. Plain Language Summary: During the formation of mountain belts, rocks are buried deep in the Earth and then exhumed. In this journey, rocks undergo transformations that record the pressure. We use the pressure to estimate the depth at which a rock was buried to reconstruct the history of mountain belts. The pressure is the sum of the weight of the overlying column of rock and shear/volumetric stresses. However, since these stresses cannot be measured, there has been a long‐standing debate on how much they influence the record of pressure in rocks. Here, we use mathematics and computer code to recalculate to recalculate the burial depth of a set of rock from pressure data. Two extreme scenarios emerge: (1) when ignoring tectonic forces (classical approach), we interpret the pressure history as the result of deep burial (up to 160 km) followed by fast exhumation (1 −10 cm/yr) to ∼20 km. The mechanism of such fast exhumation is itself intensely debated; (2) when considering tectonic forces, an alternative scenario is that the rock was buried to an intermediate depth (<75 km), followed by a change in tectonic forces without exhumation. If this second scenario is verified, then we must re‐evaluate the history of mountain belts. Key Points: We present several pressure‐to‐depth conversion models based on mechanics and apply them to a data set of metamorphic pressureThe lithostatic pressure assumption gives the upper estimate of depth at peak pressure. The maximum depth for our data set is >150 kmA change in stress state at a constant depth <75 km is enough to trigger the commonly observed peak to retrograde pressure decrease [ABSTRACT FROM AUTHOR]

Strain localization is a fundamental characteristic of plate tectonics. The resulting deformation structures shape the margins of continents and the internal structure of tectonic plates. To model the occurrence of faulting, geodynamic models generally rely on frictional plasticity. Frictional plasticity is normally embedded in visco‐plastic (V‐P) or visco‐elasto‐plastic (V‐E‐P) rheologies. This poses some fundamental issues, such as the difficulty, or often inability, to obtain a converged equilibrium state and a severe grid sensitivity. Here, we study shear banding at crustal‐scale using a visco‐elasto‐viscoplastic (V‐E‐VP) model. We show that this rheology allows to accurately satisfy equilibrium, leads to shear band patterns that converge upon mesh refinement, and preserves characteristic shear band angles. Moreover, a comparison with analytic models and laboratory data reveals that V‐E‐VP rheology captures first‐order characteristics of frictional plasticity. V‐E‐VP models thus overcomes limitations of V‐P and V‐E‐P models and appears as an attractive alternative for geodynamic modeling. Key Points: Visco‐elasto‐viscoplasticity allows for robust modeling of crustal‐scale shear bandingSimulations satisfy equilibrium, and results converge to a physically realistic solution upon mesh refinementModels are verified using basic analytical relations and validated using laboratory rheological data [ABSTRACT FROM AUTHOR]

The important role played by the upper plate in convergence zones dynamics has long been underestimated but is now more and more emphasized. However, the influence of its thickness and/or strength on orogenic systems evolution remains largely unknown. Here we present results from 3D thermo‐mechanical numerical simulations of convergence zones (including oceanic subduction followed by continental subduction/collision), in which we vary the rheological profile of the overriding plate (OP). For this, we systematically modify the crustal thickness of the overriding lithosphere and the temperature at the Moho to obtain a thermal thickness of the overriding lithosphere ranging from 80 to 180 km. While all models share a common global evolution (i.e., slab sinking, interaction between slab and the 660 km discontinuity, continental subduction/collision, and slab breakoff), they also highlight first‐order differences arising from the variations in the OP strength (thermal thickness). With a thin/weak OP, slab rollback is favored, the slab dip is low, the mantle flow above the slab is vigorous, and the trench migrates at a high rate compared to a thick/strong OP. In addition, slab breakoff and back‐arc basin formation events occur significantly earlier than in models involving a thick OP. Our models therefore highlight the major role played by the thickness/strength of the OP on convergence zone dynamics and illustrate its influence in a quantitative way. Key Points: We present 3D thermo‐mechanical lithospheric‐scale models of oceanic subduction followed by continental subduction/collisionWe show that the overriding plate thickness strongly impacts the convergence zone dynamics and the kinematics of the subduction eventsThicker overriding plate decreases mantle flow and trench mobility; it also controls overriding plate deformation and topography [ABSTRACT FROM AUTHOR]

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 intraoceanic 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- T 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 100-200°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. [ABSTRACT FROM AUTHOR]

Mountain building and landscape evolution are controlled by interactions between river dynamics and tectonic forces. Such interactions have been extensively studied, however a quantitative evaluation of tectonic/geomorphic feedbacks, which is imperative for understanding sediments routing within orogens and fold-and-thrust belts, remains to be undertaken. Here, we employ numerical simulations to assess the conditions of uplift and river incision necessary to deflect an antecedent drainage network during the growth of one, or several, folds. We propose that a partitioning of the river network into internal (endorheic) and longitudinal drainage arises as a result of lithological differences within the deforming crustal sedimentary cover. Using examples from the Zagros Fold Belt (ZFB), we show that drainage patterns can be linked to the non-dimensional incision ratio R between successive lithological layers, corresponding to the ratio between their relative erodibilities or incision coefficients. Transverse drainage networks develop for uplift rates smaller than 0.8 mm yr−1 and low incision ratios (−10 < R < 10). Intermediate drainage networks are obtained for uplift rates up to 2 mm yr−1 and large incision ratios ( R > 20). Parallel drainage networks and the formation of sedimentary basins occur for large values of incision ratio ( R > 20) and uplift rates between 1 and 2 mm yr−1. These results have implications for predicting the distribution of sediment depocenters in fold-and-thrust belts, which can be of direct economic interest for hydrocarbon exploration. They also put better constraints on the fluvial and geomorphic responses to fold growth induced by crustal-scale tectonics. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The thermal evolution around shear zones is controlled by three major thermal processes: diffusion, advection and shear heating. We present a dimensional analysis to quantify, to first-order, the relative contributions of these three processes to the thermal evolution around lithospheric-scale shear zones. We consider 11 parameters that control the kinematics, the three-dimensional (3-D) geometry, the initial thermal structure and the average strength of the shear zone. Three dimensionless parameters are presented to quantify the relative contributions of the three thermal processes. We validate the dimensional analysis with 2-D thermo-kinematic numerical models. The applicability of the dimensional analysis to any kind of shear zone (i.e. thrust, normal-slip and strike-slip shear zones) makes it a useful tool that is complementary to previous numerical and analytical studies. Finally, thrust-type shear zones are used to illustrate how the three thermal processes control the thermal evolution. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]