The Experts below are selected from a list of 306 Experts worldwide ranked by ideXlab platform
John W Rudnicki - One of the best experts on this subject based on the ideXlab platform.
-
Shear Heating of a fluid saturated slip weakening dilatant fault zone 2 quasi drained regime
Journal of Geophysical Research, 2003Co-Authors: Dmitry I Garagash, John W RudnickiAbstract:[1] This paper analyzes slip on a fluid-infiltrated dilatant fault for imposed (tectonic) strain rates much slower than the rate of fluid exchange between the gouge zone and the surroundings and for exchange of heat slower than of fluid, typical of interseismic or most laboratory loading conditions. The limiting solution, corresponding to the infinitely rapid drainage rate, developed in the companion paper by Garagash and Rudnicki [2003], yielded unbounded slip acceleration for sufficiently large slip weakening of the fault. Analysis for rapid but finite drainage rates (quasi-drained condition) reveals two types of possible instability. The first is inertial instability (a seismic event) characterized by an unbounded slip rate and attributed to the destabilizing effects of Shear Heating and fault slip weakening. The second corresponds to loss of uniqueness and arises from the small pressure increase caused by Shear Heating for rapid but finite rates of drainage. This instability emerges for even small amounts of thermomechanical coupling and a large range of dilatancy and slip weakening. Its resolution probably requires a more elaborate frictional description (e.g., rate and state), but within the slip-weakening framework here, leads to either slip arrest or inertial instability. The response is always unstable (in one of these two ways) for sufficiently large thermal coupling or initial stress regardless of the amount of slip weakening and dilatancy. The counterintuitive stabilizing effect of increased slip weakening or decreased dilatancy, similar to the effect in the companion paper, also occurs for nearly drained slip.
-
Shear Heating of a fluid‐saturated slip‐weakening dilatant fault zone: 2. Quasi‐drained regime
Journal of Geophysical Research, 2003Co-Authors: Dmitry I Garagash, John W RudnickiAbstract:[1] This paper analyzes slip on a fluid-infiltrated dilatant fault for imposed (tectonic) strain rates much slower than the rate of fluid exchange between the gouge zone and the surroundings and for exchange of heat slower than of fluid, typical of interseismic or most laboratory loading conditions. The limiting solution, corresponding to the infinitely rapid drainage rate, developed in the companion paper by Garagash and Rudnicki [2003], yielded unbounded slip acceleration for sufficiently large slip weakening of the fault. Analysis for rapid but finite drainage rates (quasi-drained condition) reveals two types of possible instability. The first is inertial instability (a seismic event) characterized by an unbounded slip rate and attributed to the destabilizing effects of Shear Heating and fault slip weakening. The second corresponds to loss of uniqueness and arises from the small pressure increase caused by Shear Heating for rapid but finite rates of drainage. This instability emerges for even small amounts of thermomechanical coupling and a large range of dilatancy and slip weakening. Its resolution probably requires a more elaborate frictional description (e.g., rate and state), but within the slip-weakening framework here, leads to either slip arrest or inertial instability. The response is always unstable (in one of these two ways) for sufficiently large thermal coupling or initial stress regardless of the amount of slip weakening and dilatancy. The counterintuitive stabilizing effect of increased slip weakening or decreased dilatancy, similar to the effect in the companion paper, also occurs for nearly drained slip.
-
Shear Heating of a fluid saturated slip weakening dilatant fault zone 1 limiting regimes
Journal of Geophysical Research, 2003Co-Authors: Dmitry I Garagash, John W RudnickiAbstract:[1] The one-dimensional model of Rudnicki and Chen [1988] for a slip-weakening dilating fault is extended to include Shear Heating. Because inertia is not included, instability (a seismic event) corresponds to an unbounded slip rate. Shear Heating tends to increase pore pressure and decrease the effective compressive stress and the resistance to slip and consequently tends to promote instability. However, the decrease of effective compressive stress also reduces the magnitude of Shear Heating. Consequently, in the absence of frictional weakening and dilation, there exists a steady solution for slip at the tectonic rate in which the pressure does not change and the Shear Heating is exactly balanced by heat flux from the fault zone. In the absence of Shear Heating, dilatancy tends to decrease pore pressure and inhibit instability; more rapid slip weakening promotes instability. Analysis of undrained, adiabatic slip (characteristic of rapid slip or hydraulically and thermally isolated faults) reveals that the interaction of these effects can cause increased slip weakening to be stabilizing and increased dilatancy to be destabilizing. These counterintuitive effects are due to the dependence of the Shear Heating on the total Shear stress (not just its change). They occur for small thermal expansivity and for material parameters within a plausible range for 0–10 km depth.
-
Shear Heating of a fluid‐saturated slip‐weakening dilatant fault zone 1. Limiting regimes
Journal of Geophysical Research, 2003Co-Authors: Dmitry I Garagash, John W RudnickiAbstract:[1] The one-dimensional model of Rudnicki and Chen [1988] for a slip-weakening dilating fault is extended to include Shear Heating. Because inertia is not included, instability (a seismic event) corresponds to an unbounded slip rate. Shear Heating tends to increase pore pressure and decrease the effective compressive stress and the resistance to slip and consequently tends to promote instability. However, the decrease of effective compressive stress also reduces the magnitude of Shear Heating. Consequently, in the absence of frictional weakening and dilation, there exists a steady solution for slip at the tectonic rate in which the pressure does not change and the Shear Heating is exactly balanced by heat flux from the fault zone. In the absence of Shear Heating, dilatancy tends to decrease pore pressure and inhibit instability; more rapid slip weakening promotes instability. Analysis of undrained, adiabatic slip (characteristic of rapid slip or hydraulically and thermally isolated faults) reveals that the interaction of these effects can cause increased slip weakening to be stabilizing and increased dilatancy to be destabilizing. These counterintuitive effects are due to the dependence of the Shear Heating on the total Shear stress (not just its change). They occur for small thermal expansivity and for material parameters within a plausible range for 0–10 km depth.
Riaz A. Mufti - One of the best experts on this subject based on the ideXlab platform.
-
Shear-Heating Effects on Piston Skirts Lubrication in the Initial Engine Start Up
2020Co-Authors: M. Afzaal Malik, S. Adnan Qasim, Mumtaz Ali Khan, Riaz A. MuftiAbstract:A 2-D hydrodynamic piston skirts lubrication model during initial engine start up is developed by incorporating Shear Heating effects due to lubricant flow between piston skirt and liner surfaces. Numerical analysis is presented based on 2D thermal energy equation having adiabatic conduction and convective heat transfer with no source term effects. Viscous dissipation coupled with piston motion, pressure field generation, temperature effects on oil viscosity and subsequent oil film profiles in the contact region are examined and influence of Shear Heating on hydrodynamic film thickness at the time of engine initial start up are investigated. This study suggests that oil film temperature rise due to Shear Heating adversely affects lubricant film thickness and hydrodynamic pressures, which in turn affect Newtonian lubricant
-
modeling Shear Heating in piston skirts ehl considering different viscosity oils in initial engine start up
Journal of Engineering for Gas Turbines and Power-transactions of The Asme, 2012Co-Authors: Adnan S Qasim, Afzaal M Malik, Ali M Khan, Riaz A. MuftiAbstract:A fully established elastohydrodynamic lubricating (EHL) film between the piston and the liner surfaces during normal engine operation minimizes piston slap and prevents adhesive wear. Wear cannot be prevented in the initial engine start up due to the absence of EHL film. During normal engine operation, thermal loading due to combustion dominates piston skirts lubrication. However, in a few initial cold engine start-up cycles, Shear Heating affects the lubricant viscosity and other characteristics considerably. This study models 2D piston skirts EHL by incorporating Shear Heating effects due to lubricant flow between the skirts and liner surfaces. The hydrodynamic and EHL film profiles are predicted by solving the 2D Reynolds equation and using the inverse solution technique, respectively. The temperature distribution within the oil film is given by using the 2D transient thermal energy equation with heat generated by viscous Heating. The numerical analysis is based on an energy equation having adiabatic conduction and convective heat transfer with no source term effects. The study is extended to low and high viscosity grade engine oils to investigate the adverse effects of the rising temperatures on the load carrying capacity of such lubricants. Numerical simulations show that piston eccentricities, film thickness profiles, hydrodynamic and EHL pressures visibly change when using different viscosity grade engine lubricants. This study optimizes the viscosity-grade of an engine lubricant to minimize the adhesive wear of the piston skirts and cylinder liner at the time of initial engine start up.
-
low viscosity Shear Heating in piston skirts ehl in the low initial engine start up speeds
Tribology International, 2011Co-Authors: S. Adnan Qasim, Mumtaz Ali Khan, Afzaal M Malik, Riaz A. MuftiAbstract:Absence of elastohydrodynamic lubricating (EHL) film causes piston wear in low speed cold initial engine start up, while Shearing of low viscosity lubricant in few cycles affects its load carrying capacity. Shear Heating effects are incorporated in 2-D hydrodynamic and EHL model by solving 2-D heat equation. EHL pressures are calculated using inverse solution technique. Comparative analysis is based on viscous dissipation coupled with piston motion, changes in pressure, film thickness and viscosity. This study suggests that the increase in temperature varies with speed to affect piston eccentricities, viscosity and film thickness. This optimizes low start up speed for a few engine cycles.
-
Modeling of Shear Heating in Elastohydrodynamic Lubrication of Piston Skirts by Considering Different Viscosity Oils in Initial Engine Start Up
Volume 7: Fluid Flow Heat Transfer and Thermal Systems Parts A and B, 2010Co-Authors: M. Afzaal Malik, S. Adnan Qasim, Mumtaz Ali Khan, Riaz A. MuftiAbstract:The presence of fully established elastohydrodynamic lubricating (EHL) film between piston skirt and cylinder liner during normal engine operation prevents adhesive wear, piston noise and slap. The absence of EHL in the initial engine start up fails to prevent the same. During normal engine operation, thermal loading due to combustion dominates piston skirts lubrication. However, in a few initial cold engine start up cycles, Shear Heating may be assumed to considerably affect the lubricant viscosity and other characteristics. This study undertakes a 2-D EHL modeling of piston lubrication by incorporating Shear Heating effects due to lubricant flow between skirts and liner surfaces. The EHL film profile is predicted by solving the 2-D Reynolds equation using inverse solution technique and the finite difference method in fully flooded lubrication conditions. The temperature distribution within oil film is given by using the 2-D transient thermal energy equation with heat generated by viscous Heating. The numerical analysis is based on energy equation having adiabatic conduction and convective heat transfer with no source term effects. The study is extended to a number of engine lubricants having different viscosities to investigate the extent of adverse effects due to temperature rise on load carrying capacity of lubricants. Numerical simulations show that piston eccentricities, film thickness profiles, hydrodynamic and EHL pressures visibly change when using different viscosity grade engine lubricants. This study suggests that a lubricant of appropriate viscosity can be optimized keeping in view the vulnerability of piston skirts and cylinder liner to adhesive wear at the time of initial engine start up.Copyright © 2010 by ASME
Dmitry I Garagash - One of the best experts on this subject based on the ideXlab platform.
-
Shear Heating of a fluid saturated slip weakening dilatant fault zone 2 quasi drained regime
Journal of Geophysical Research, 2003Co-Authors: Dmitry I Garagash, John W RudnickiAbstract:[1] This paper analyzes slip on a fluid-infiltrated dilatant fault for imposed (tectonic) strain rates much slower than the rate of fluid exchange between the gouge zone and the surroundings and for exchange of heat slower than of fluid, typical of interseismic or most laboratory loading conditions. The limiting solution, corresponding to the infinitely rapid drainage rate, developed in the companion paper by Garagash and Rudnicki [2003], yielded unbounded slip acceleration for sufficiently large slip weakening of the fault. Analysis for rapid but finite drainage rates (quasi-drained condition) reveals two types of possible instability. The first is inertial instability (a seismic event) characterized by an unbounded slip rate and attributed to the destabilizing effects of Shear Heating and fault slip weakening. The second corresponds to loss of uniqueness and arises from the small pressure increase caused by Shear Heating for rapid but finite rates of drainage. This instability emerges for even small amounts of thermomechanical coupling and a large range of dilatancy and slip weakening. Its resolution probably requires a more elaborate frictional description (e.g., rate and state), but within the slip-weakening framework here, leads to either slip arrest or inertial instability. The response is always unstable (in one of these two ways) for sufficiently large thermal coupling or initial stress regardless of the amount of slip weakening and dilatancy. The counterintuitive stabilizing effect of increased slip weakening or decreased dilatancy, similar to the effect in the companion paper, also occurs for nearly drained slip.
-
Shear Heating of a fluid‐saturated slip‐weakening dilatant fault zone: 2. Quasi‐drained regime
Journal of Geophysical Research, 2003Co-Authors: Dmitry I Garagash, John W RudnickiAbstract:[1] This paper analyzes slip on a fluid-infiltrated dilatant fault for imposed (tectonic) strain rates much slower than the rate of fluid exchange between the gouge zone and the surroundings and for exchange of heat slower than of fluid, typical of interseismic or most laboratory loading conditions. The limiting solution, corresponding to the infinitely rapid drainage rate, developed in the companion paper by Garagash and Rudnicki [2003], yielded unbounded slip acceleration for sufficiently large slip weakening of the fault. Analysis for rapid but finite drainage rates (quasi-drained condition) reveals two types of possible instability. The first is inertial instability (a seismic event) characterized by an unbounded slip rate and attributed to the destabilizing effects of Shear Heating and fault slip weakening. The second corresponds to loss of uniqueness and arises from the small pressure increase caused by Shear Heating for rapid but finite rates of drainage. This instability emerges for even small amounts of thermomechanical coupling and a large range of dilatancy and slip weakening. Its resolution probably requires a more elaborate frictional description (e.g., rate and state), but within the slip-weakening framework here, leads to either slip arrest or inertial instability. The response is always unstable (in one of these two ways) for sufficiently large thermal coupling or initial stress regardless of the amount of slip weakening and dilatancy. The counterintuitive stabilizing effect of increased slip weakening or decreased dilatancy, similar to the effect in the companion paper, also occurs for nearly drained slip.
-
Shear Heating of a fluid saturated slip weakening dilatant fault zone 1 limiting regimes
Journal of Geophysical Research, 2003Co-Authors: Dmitry I Garagash, John W RudnickiAbstract:[1] The one-dimensional model of Rudnicki and Chen [1988] for a slip-weakening dilating fault is extended to include Shear Heating. Because inertia is not included, instability (a seismic event) corresponds to an unbounded slip rate. Shear Heating tends to increase pore pressure and decrease the effective compressive stress and the resistance to slip and consequently tends to promote instability. However, the decrease of effective compressive stress also reduces the magnitude of Shear Heating. Consequently, in the absence of frictional weakening and dilation, there exists a steady solution for slip at the tectonic rate in which the pressure does not change and the Shear Heating is exactly balanced by heat flux from the fault zone. In the absence of Shear Heating, dilatancy tends to decrease pore pressure and inhibit instability; more rapid slip weakening promotes instability. Analysis of undrained, adiabatic slip (characteristic of rapid slip or hydraulically and thermally isolated faults) reveals that the interaction of these effects can cause increased slip weakening to be stabilizing and increased dilatancy to be destabilizing. These counterintuitive effects are due to the dependence of the Shear Heating on the total Shear stress (not just its change). They occur for small thermal expansivity and for material parameters within a plausible range for 0–10 km depth.
-
Shear Heating of a fluid‐saturated slip‐weakening dilatant fault zone 1. Limiting regimes
Journal of Geophysical Research, 2003Co-Authors: Dmitry I Garagash, John W RudnickiAbstract:[1] The one-dimensional model of Rudnicki and Chen [1988] for a slip-weakening dilating fault is extended to include Shear Heating. Because inertia is not included, instability (a seismic event) corresponds to an unbounded slip rate. Shear Heating tends to increase pore pressure and decrease the effective compressive stress and the resistance to slip and consequently tends to promote instability. However, the decrease of effective compressive stress also reduces the magnitude of Shear Heating. Consequently, in the absence of frictional weakening and dilation, there exists a steady solution for slip at the tectonic rate in which the pressure does not change and the Shear Heating is exactly balanced by heat flux from the fault zone. In the absence of Shear Heating, dilatancy tends to decrease pore pressure and inhibit instability; more rapid slip weakening promotes instability. Analysis of undrained, adiabatic slip (characteristic of rapid slip or hydraulically and thermally isolated faults) reveals that the interaction of these effects can cause increased slip weakening to be stabilizing and increased dilatancy to be destabilizing. These counterintuitive effects are due to the dependence of the Shear Heating on the total Shear stress (not just its change). They occur for small thermal expansivity and for material parameters within a plausible range for 0–10 km depth.
Simon M Peacock - One of the best experts on this subject based on the ideXlab platform.
-
The importance of blueschist → eclogite dehydration reactions in subducting oceanic crust
Geological Society of America Bulletin, 1993Co-Authors: Simon M PeacockAbstract:The metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure-temperature ( P-T ) paths with a model of metabasalt phase equilibria. In steady-state subduction zones with high rates of Shear Heating, the upper parts of the subducting oceanic crust progress through the greenschist → amphibolite → granulite → eclogite facies, whereas lower parts of the subducting oceanic crust progress through the blueschist → eclogite facies. In steady-state subduction zones with moderate rates of Shear Heating, most of the subducting oceanic crust passes through the blueschist → eclogite transition. In steady-state subduction zones with low rates of Shear Heating, the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km. For oceanic crust containing 1-2 wt% H 2 O, dehydration will not begin until the onset of eclogite- or amphibolite-facies metamorphism, depending on the P-T path. For many subduction zones, the most important dehydration reactions in the subducting oceanic crust occur at the blueschist → eclogite facies transition associated with the breakdown of lawsonite (or clinozoisite), glaucophane, and chlorite. Large amounts of H 2 O released by blueschist → eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas.
-
blueschist facies metamorphism Shear Heating and p t t paths in subduction Shear zones
Journal of Geophysical Research, 1992Co-Authors: Simon M PeacockAbstract:The low temperatures recorded by blueschist-facies metamorphic rocks place upper bounds on the magnitude of Shear stresses in subduction zones at depths of 15–50 km. In this paper, steady state and transient pressure-temperature-time (P-T-t) paths followed by subducted material are calculated using analytical expressions presented by Molnar and England (1990), supplemented by two-dimensional numerical calculations. In the absence of Shear Heating, calculated steady state P-T-t paths are very cold, intersecting 1 GPa (10 kbar) at T < 200°C for a wide range of subduction parameters. For the case of constant Shear stress (τ) in the subduction Shear zone and typical thermal parameters, steady state P-T-t paths intersect the blueschist facies for τ = 10–60 MPa and 0–100 MPa for convergence rates of 10 and 3 cm yr−1, respectively. Blueschist-facies metamorphism will not occur in rapid subduction zones if Shear stresses exceed 60 MPa. For the case of Shear stress increasing linearly with depth, steady state P-T-t paths intersect the blueschist facies when τ = 1–9% P and 0–14% P for convergence rates of 10 and 3 cm yr−1, respectively. Subduction zone Shear stresses may be considerably lower, and possibly zero, if blueschist terrains represent material metamorphosed during the early stages of subduction (first 5–10 m.y.) while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation. Subduction zone Shear stresses of several tens of MPa (at 35 km depth) or a few percent of lithostatic pressure are consistent with a steady state model of Franciscan metamorphism, a progressive underplating model for the formation of the Pelona Schist inverted metamorphic gradient, and surface heat flow measurements in the NE Japan subduction zone.
-
Blueschist‐facies metamorphism, Shear Heating, and P‐T‐t paths in subduction Shear zones
Journal of Geophysical Research, 1992Co-Authors: Simon M PeacockAbstract:The low temperatures recorded by blueschist-facies metamorphic rocks place upper bounds on the magnitude of Shear stresses in subduction zones at depths of 15–50 km. In this paper, steady state and transient pressure-temperature-time (P-T-t) paths followed by subducted material are calculated using analytical expressions presented by Molnar and England (1990), supplemented by two-dimensional numerical calculations. In the absence of Shear Heating, calculated steady state P-T-t paths are very cold, intersecting 1 GPa (10 kbar) at T < 200°C for a wide range of subduction parameters. For the case of constant Shear stress (τ) in the subduction Shear zone and typical thermal parameters, steady state P-T-t paths intersect the blueschist facies for τ = 10–60 MPa and 0–100 MPa for convergence rates of 10 and 3 cm yr−1, respectively. Blueschist-facies metamorphism will not occur in rapid subduction zones if Shear stresses exceed 60 MPa. For the case of Shear stress increasing linearly with depth, steady state P-T-t paths intersect the blueschist facies when τ = 1–9% P and 0–14% P for convergence rates of 10 and 3 cm yr−1, respectively. Subduction zone Shear stresses may be considerably lower, and possibly zero, if blueschist terrains represent material metamorphosed during the early stages of subduction (first 5–10 m.y.) while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation. Subduction zone Shear stresses of several tens of MPa (at 35 km depth) or a few percent of lithostatic pressure are consistent with a steady state model of Franciscan metamorphism, a progressive underplating model for the formation of the Pelona Schist inverted metamorphic gradient, and surface heat flow measurements in the NE Japan subduction zone.
Boris J. P. Kaus - One of the best experts on this subject based on the ideXlab platform.
-
Shear Heating and subduction initiation
2020Co-Authors: Marcel Thielmann, Boris J. P. KausAbstract:Despite it’s importance in geodynamics, the processes that result in subduction initiation remain incompletely understood. Shear Heating has been put forward as a mechanism to create lithospheric-scale Shear zones (e.g. Ogawa 1987, Regenauer-Lieb et al. 2001). A scaling analysis highlighted the governing parameters that control Shear localization (Kaus and Podladchikov 2006), and showed that the boundary between localization and no localization is quite sharp. Recently, this scaling analysis was extended to include more realistic lithospheric rheologies and structures and it could be demonstrated that Shear-Heating induced lithospheric scale localization might occur for Earth-like parameters (Crameri and Kaus, submitted). It however is unclear if all lithospheric-scale Shear zones evolve into self-sustaining subduction zones. Here, we therefore use viscoelastoplastic 2D geodynamical numerical simulations to investigate under which conditions lithospheric failure results in the formation of an evolved subduction zone. The results are compared with analytical scaling laws for Shear localization in the lithosphere.
-
Grain size assisted formation of pseudotachylites: A numerical study
2020Co-Authors: Marcel Thielmann, Boris J. P. Kaus, Antoine Rozel, Yanick RicardAbstract:The processes resulting in the formation of lithospheric-scale Shear zones are still poorly understood. Among others, Shear Heating and grain size reduction have been proposed to be viable weakening mechanisms to localize deformation and form lithospheric-scale Shear zones. The interplay between both mechanisms is particularly interesting, as both compete for a part of the deformational work. High temperatures favor grain growth, therefore one would expect larger grain sizes in Shear zones that have been formed by Shear Heating. However, larger temperatures increase strain rates, thus also the amount of deformational work, which in turn would favor grain size reduction. Here we investigate the interplay between both mechanisms using numerical models of a viscoelastic slab deforming in simple Shear, employing a viscous rheology composed of dislocation and diffusion creep. Grain size evolution is governed by a recently developed physics-based evolution law. We develop scaling laws for the peak stress and the dominating deformation mechanisms depending on various material parameters and boundary conditions. We find that grain size reduction alone does not localize deformation in simple Shear. In conjunction with Shear Heating however, a localized Shear zone is formed due to thermal runaway. During this process, grain size is significantly reduced. Depending on grain growth parameters, a mylonitic Shear zone is formed in which deformation is permanently localized and which deforms in diffusion creep. Additionally, the stress required to initiate thermal runaway is reduced compare to cases with Shear Heating alone, thus facilitating the formation of a narrow localized Shear zone in the ductile regime. These results have several implications ranging and from simultaneous pseudotachylite and mylonite formation at depths below the seismogenic depth to subduction initiation.
-
intermediate depth earthquake generation and Shear zone formation caused by grain size reduction and Shear Heating
Geology, 2015Co-Authors: Marcel Thielmann, Boris J. P. Kaus, Antoine Rozel, Yanick RicardAbstract:The underlying physics of intermediate-depth earthquakes have been an enigmatic topic; several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation. Here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on Shear zone and pseudotachylite formation. Our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway, making it more likely to result in intermediate-depth earthquakes at shallower depths. Furthermore, grain size is reduced in and around the Shear zone, which agrees with field and laboratory observations where pseudotachylites are embedded in a simultaneously formed mylonite matrix. The decrease in critical stress to initialize localization has important implications for large-scale geodynamics, as this mechanism might induce lithosphere-scale Shear zones and subduction initiation. We suggest that the combination of grain size reduction and Shear Heating explains both the occurrence of intermediate-depth earthquakes and the formation of large-scale Shear zones.
-
Shear Heating induced lithospheric scale localization does it result in subduction
Earth and Planetary Science Letters, 2012Co-Authors: Marcel Thielmann, Boris J. P. KausAbstract:Abstract Even though it is a well-established fact that the Earth is currently in a plate-tectonics mode, the question on how to “break” lithospheric plates and initiate subduction remains a matter of debate. Here we focus on Shear Heating as a potential mechanism to cause lithospheric Shear localization and subsequent subduction initiation in oceanic plates. It is shown that Shear Heating under some conditions (i) facilitates the formation of a lithospheric-scale Shear zone and (ii) is capable of stabilizing a lithospheric-scale Shear zone, thus creating the necessary condition for subduction initiation to occur. Furthermore, we demonstrate that not only the localization process is of importance, but also the post-localization stage, where rapidly growing convective instabilities might prevent an incipient subduction zone from developing. We develop scaling laws for both the localization and the post-localization stages. In the case of lithospheric localization, the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere, thus of the order of the competent layer thickness. This shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller-scale heterogeneities. We compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that, for Earth-like conditions, a dry olivine rheology is more likely to result in subduction initiation. A large plate age does not always increase the potential for subduction initiation, as it increases the potential for convective instabilities to occur. Instead, an optimal plate age exists for Shear Heating induced subduction initiation, which is around 40 Ma for a wet olivine rheology.
-
Potential causes for the non‐Newtonian rheology of crystal‐bearing magmas
Geochemistry Geophysics Geosystems, 2011Co-Authors: Y. Deubelbeiss, Boris J. P. Kaus, James A. D. Connolly, Luca CaricchiAbstract:Experimental studies indicate that crystal-bearing magma exhibits non-Newtonian behavior at high strain rates and solid fractions. We use a zero-dimensional (0-D) inversion model to reevaluate rheological parameters and Shear Heating effects from laboratory data on crystal-bearing magma. The results indicate non-Newtonian behavior with power law coefficients of up to n = 13.5. It has been speculated that finite strain effects, Shear Heating, power law melt rheology, or plasticity are responsible for this non-Newtonian behavior. We use 2-D direct numerical crystal-scale simulations to study the relative importance of these mechanisms. These simulations demonstrate that Shear Heating has little effect on aggregate (bulk) rheologies. Finite strain effects result in both strain weakening and hardening, but the resulting power law coefficient is modest (maximum n = 1.3). For simulations with spherical crystals the strain weakening and hardening behavior is related to rearrangement of crystals rather than strain rate related weakening. Finite strain effects were insignificant in a numerical simulation with naturally shaped crystals. Strain partitioning into the melt phase may induce microscopic stresses that are adequate to provoke a nonlinear viscous response in the melt. Large differential stresses and low effective stresses revealed by the simulations are sufficient to cause crystals to fail plastically. Numerical experiments that account for plastic failure show large power law coefficients (n ≈ 50 in some simulations). We conclude that this effect is the dominant cause of the strong nonlinear viscous response of crystal-bearing magmas observed in laboratory experiments.
