Strain Localization at Constant Strain Rate and Changing Stress Conditions: Implications for Plate Boundary Processes in the Upper Mantle

We present results from a natural deformed shear zone in the Turon de Técouère massif of the French Pyrenees that directly addresses the processes involved in strain localization, a topic that has been investigated for the last 40 years by structural geologists. Paleopiezometry indicates that differential stresses are variable both spatially across the zone, and temporally during exhumation. We have, however, also calculated strain rate, which remains constant despite changes in stress. This result appears to be at odds with recent experimental deformation on monophase (olivine) rocks, which indicate that strain localization occurs dominantly as a result of constant stress. We hypothesize that in the Turon de Técouère massif—and many natural shear zones—strain localization occurs as a result of reactions, which decrease the grain size and promote the activation of grain size sensitive deformation mechanisms. From a tectonics perspective, this study indicates that the deformation rate in a particular plate boundary is relatively uniform. Stress, however, varies to accommodate this deformation. This viewpoint is consistent with deformation at a plate boundary, but it is not the typical way in which we interpret strain localization.

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Ductile deformation and microstructures

10.1093/oso/9780192843876.003.0004 ◽

2021 ◽

pp. 58-85

Author(s):

Jean-Luc Bouchez ◽

Adolphe Nicolas

Keyword(s):

Plastic Deformation ◽

Grain Size ◽

Elastic Deformation ◽

Applied Stress ◽

Deformation Mechanism ◽

Chemical Elements ◽

Shear Zones ◽

Ductile Deformation ◽

Deformation Mechanisms ◽

Solid Materials

In contrast to the elastic deformation, which is reversible, usually neglected by field geologists but important for geophysicists working in seismology, ductile deformation is irreversible. This chapter is restricted to solid materials. Materials containing a melt fraction will be examined in Chapter 7. In the geological literature, ‘ductile’ is often used as a synonym for ‘plastic’. The latter is rather used, and will be used to specify deformation mechanisms that dominantly involve the action of dislocations. In contrast to brittle deformation, which by essence is discontinuous and highly localized (see Chapter 3), ductile deformation is generally continuous and affects large volumes of rock. However, ductile deformation may be concentrated into restricted rock volumes (or domains). Such localization is common in shear zones and/or when superplastic deformation mechanism is involved. Plastic deformation mechanisms naturally depend on temperature, magnitude of the applied stress, mineral nature and grain-size of the rocks. In upper parts of the crust, fluids are able to carry chemical elements over large distances and influence the deformation mechanisms. Micrographs of several microstructural types as well as deformation maps for olivine and calcite are given at the end of this chapter.

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Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

Solid Earth ◽

10.5194/se-8-1211-2017 ◽

2017 ◽

Vol 8 (6) ◽

pp. 1211-1239 ◽

Cited By ~ 3

Author(s):

Thomas van der Werf ◽

Vasileios Chatzaras ◽

Leo Marcel Kriegsman ◽

Andreas Kronenberg ◽

Basil Tikoff ◽

...

Keyword(s):

Grain Size ◽

Upper Mantle ◽

Lower Crust ◽

Baja California ◽

Plate Boundary ◽

Volcanic Field ◽

Strike Slip ◽

Low Viscosity ◽

The Pacific ◽

Flow Laws

Abstract. The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

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Statistical characteristics for the type and length of deformation-induced cracks in columnar-grain ice

Journal of Glaciology ◽

10.3189/s0022143000003269 ◽

1997 ◽

Vol 43 (144) ◽

pp. 311-320 ◽

Cited By ~ 1

Author(s):

Lorne W. Gold

Keyword(s):

Grain Size ◽

Grain Boundary ◽

Strain Rate ◽

Relative Proportion ◽

Constant Strain Rate ◽

Statistical Characteristics ◽

Columnar Grain ◽

Log Normal ◽

Almost All ◽

Log Normal Distribution

AbstractObservations are reported on cracks formed during compressive, unidirectional, constant-strain-rate deformation of columnar-grain ice. The axis of hexagonal crystallographic symmetry of each grain tended to be in the plane perpendicular to the long direction of the grains and to have a random orientation in that plane. For stress applied perpendicular to the long direction of the grains, the deformation was practically two-dimensional. It was found that the relative proportion of grain-boundary cracks increased with increasing strain rate, decreasing temperature and, for strain rate greater than 7 × 10−5 s−1, with decreasing grain-size. Almost all the grain-boundary cracks had at least one edge at a triple point. For each test, the grain-boundary and transcrystalline crack lengths tended to have a log-normal distribution. The logarithmic mean crack length (LMCL) decreased with increasing strain rate, decreasing grain-size and decreasing temperature and tended to a constant value of 0.75 mm at 10°C. For grain-size of 3 mm or greater, the LMCL had a maximum at a strain rate of 10−5 to 10−6 S−1 at −10°C. The LMCLs and the relative proportion of grain-boundary cracks tended to be normally distributed for given load conditions.

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Mechanical anisotropy control on strain localization in upper mantle shear zones

Tectonics ◽

10.1002/2015tc004007 ◽

2016 ◽

Vol 35 (5) ◽

pp. 1177-1204 ◽

Cited By ~ 4

Author(s):

Marco Herwegh ◽

Ivan Mercolli ◽

Jolien Linckens ◽

Othmar Müntener

Keyword(s):

Strain Localization ◽

Upper Mantle ◽

Shear Zones ◽

Mechanical Anisotropy

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Effect of Changing Strain Rate during Warm Deformation on Static Recrystallisation of IF Steel

Materials Science Forum ◽

10.4028/www.scientific.net/msf.550.3 ◽

2007 ◽

Vol 550 ◽

pp. 3-12

Author(s):

C. Prentice ◽

C.M. Sellars

Keyword(s):

Grain Size ◽

Strain Rate ◽

Subgrain Size ◽

Constant Strain Rate ◽

Salt Bath ◽

Interstitial Free ◽

Compression Tests ◽

Constant Rate ◽

Interstitial Free Steel ◽

Kinetics Of

Plane strain compression tests have been carried out on Ti stabilised interstitial free steel at 700oC with constant and changing strain rates. Specimens were annealed in a salt bath at 750oC to determine the effects of changing strain rate on the kinetics of static recrystallisation and on the recrystallised grain size. After relatively slow changes in rate, the recrystallisation behaviour at the end of the change was the same as for tests at constant strain rate with the final value. For faster changes in rate, there were transients in recrystallisation rate and recrystallised grain size at the end of the change in strain rate at a strain of 1.0. These were removed by a further increment of 0.1 strain at constant rate. In all cases the recrystallised grain size correlated with the subgrain size present at the end of deformation.

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Microstructures and Tensile Properties of Electrodeposited Cu Sheets with Grain Sizes from Nanocrystalline to Ultrafine Scale

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.538-541.1611 ◽

2012 ◽

Vol 538-541 ◽

pp. 1611-1614

Author(s):

Han Zhuo Zhang ◽

Huiping Zhang ◽

Lei Liu

Keyword(s):

Grain Size ◽

Plastic Strain ◽

Strain Rate ◽

Tensile Properties ◽

Tensile Deformation ◽

Tensile Ductility ◽

Deformation Mechanisms ◽

Grain Sizes ◽

Low Strain Rate

Four types of Cu sheets, with average grain sizes of 200 nm, 90 nm, 33 nm and 11 nm respectively, were electrodeposited and tested by tension at both high and low strain rate. Typically, a higher strength with lower tensile ductility was obtained by increasing the strain rate or reducing the grain size till 33 nm. An inverse Hall-Petch result was found in 11 nm Cu, while 200 nm Cu exhibited an increase of both strength and plastic strain by the increment of strain rate. Tensile deformation mechanisms of the Cu sheets were also discussed with their microstructural features.

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Relationship between microstructures and resistance in mafic assemblages that deform and transform

Solid Earth ◽

10.5194/se-11-2141-2020 ◽

2020 ◽

Vol 11 (6) ◽

pp. 2141-2167

Author(s):

Nicolas Mansard ◽

Holger Stünitz ◽

Hugues Raimbourg ◽

Jacques Précigout ◽

Alexis Plunder ◽

...

Keyword(s):

Grain Size ◽

Shear Strain ◽

Phase Distribution ◽

Shear Bands ◽

Constant Strain Rate ◽

Peak Stress ◽

Confining Pressure ◽

Shear Zones ◽

Reaction Products ◽

Fine Grained

Abstract. Syn-kinematic mineral reactions play an important role for the mechanical properties of polymineralic rocks. Mineral reactions (i.e., nucleation of new phases) may lead to grain size reduction, producing fine-grained polymineralic mixtures, which have a strongly reduced viscosity because of the activation of grain-size-sensitive deformation processes. In order to study the effect of deformation–reaction feedback(s) on sample strength, we performed rock deformation experiments on “wet” assemblages of mafic compositions in a Griggs-type solid-medium deformation apparatus. Shear strain was applied at constant strain rate (10−5 s−1) and constant confining pressure (1 GPa) with temperatures ranging from 800 to 900 ∘C. At low shear strain, the assemblages that react faster are significantly weaker than the ones that react more slowly, demonstrating that reaction progress has a first-order control on rock strength. With increasing strain, we document two contrasting microstructural scenarios: (1) the development of a single throughgoing high-strain zone of well-mixed, fine-grained aggregates, associated with a significant weakening after peak stress, and (2) the development of partially connected, nearly monomineralic shear bands without major weakening. The lack of weakening is caused by the absence of interconnected well-mixed aggregates of fine-grained reaction products. The nature of the reaction products, and hence the intensity of the mechanical weakening, is controlled by the microstructures of the reaction products to a large extent, e.g., the amount of amphibole and the phase distribution of reaction products. The samples with the largest amount of amphibole exhibit a larger grain size and show less weakening. In addition to their implications for the deformation of natural shear zones, our findings demonstrate that the feedback between deformation and mineral reactions can lead to large differences in mechanical strength, even at relatively small initial differences in mineral composition.

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Temperature and strain controls on ice deformation mechanisms: insights from the microstructures of samples deformed to progressively higher strains at −10, −20 and −30 °C

10.5194/tc-2020-2 ◽

2020 ◽

Cited By ~ 1

Author(s):

Sheng Fan ◽

Travis Hager ◽

David J. Prior ◽

Andrew J. Cross ◽

David L. Goldsby ◽

...

Keyword(s):

Grain Size ◽

Grain Boundary ◽

Strain Rate ◽

Microstructural Evolution ◽

Deformation Mechanisms ◽

Opening Angle ◽

Ice Flow ◽

Grain Rotation ◽

Subgrain Rotation ◽

Small Grains

Abstract. Understanding ice deformation mechanisms is crucial for understanding the dynamic evolution of terrestrial and planetary ice flow. To understand better the deformation mechanisms, we document the microstructural evolution of ice with increasing strain. We include data from deformation at relatively low temperature (−20 and −30 °C) where the microstructural evolution has never before been documented. Polycrystalline pure water ice was deformed under a constant displacement rate (equal to the strain rate of ~1.0×10−5 s−1) at temperatures of −10, −20 and −30 °C to progressively higher true axial strains (~ 3, 5, 8, 12 and 20 %). Mechanical data show peak and steady-state stresses are larger at colder temperatures as expected from the temperature dependency of creep. Cryo-electron backscattered diffraction (EBSD) analyses show distinct sub-grain boundaries in all deformed samples, suggesting activation of recovery and subgrain rotation. Deformed ice samples are characterised by big grains interlocking with small grains. For each temperature series, we separated big grains from small grains using a threshold grain size, which equals to the square mean root diameter at ~ 12 % strain. Big grains are more lobate at −10 °C than at colder temperatures, suggesting grain boundary migration (GBM) is more prominent at warmer temperatures. The small grains are smaller than subgrains at −10 °C and they become similar in size at −20 and −30 °C, suggesting bulge nucleation facilitates the recrystallization process at warmer temperature and subgrain rotation recrystallization is the nucleation mechanism at colder temperatures. At temperatures warmer than −15 °C, c-axes develop a crystallographic preferred orientation (CPO) characterized by a cone (i.e., small circle) around the compression axis. We suggest the c-axis cone forms as a result of selective growth of grains at easy slip orientations (i.e., ~ 45° to shortening direction) by strain-energy driven GBM. This particular finding is consistent with previous works. The opening-angle of the c-axis cone decreases with strain, suggesting strain-induced GBM is balanced by grain rotation. Furthermore, the opening-angle of the c-axis cone decreases with temperature. At −30 °C, the c-axis CPO transits from a narrow cone to a cluster, parallel to compression, with increasing strain. This closure of the c-axis cone is interpreted as the result of a more active grain rotation together with a less effective GBM. As the temperature decreases, the overall CPO intensity decreases, facilitated by the CPO weakening in small grains. We suggest the grain size sensitivity of grain boundary sliding (GBS) favours a faster strain rate in small grains and leads to the CPO weakening at cold temperatures. CPO development cannot provide a uniform explanation for the mechanical weakening (enhancement) after peak stress. Grain size reduction, which can be observed in all deformed samples, is most likely to cause weakening (enhancement) and should be considered to have a significant control on the rheology of natural ice flow.

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Formation of Ultramylonites in an Upper Mantle Shear Zone, Erro-Tobbio, Italy

Minerals ◽

10.3390/min11101036 ◽

2021 ◽

Vol 11 (10) ◽

pp. 1036

Author(s):

Jolien Linckens ◽

Sören Tholen

Keyword(s):

Grain Size ◽

Shear Zone ◽

Upper Mantle ◽

Microstructural Analysis ◽

Shear Zones ◽

Geodynamic Setting ◽

Mixing Processes ◽

Phase Mixing ◽

Fine Grained ◽

Order Of Magnitude

Deformation in the upper mantle is localized in shear zones. In order to localize strain, weakening has to occur, which can be achieved by a reduction in grain size. In order for grains to remain small and preserve shear zones, phases have to mix. Phase mixing leads to dragging or pinning of grain boundaries which slows down or halts grain growth. Multiple phase mixing processes have been suggested to be important during shear zone evolution. The importance of a phase mixing process depends on the geodynamic setting. This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting. The first stage of deformation occurred under melt-free conditions, during which clinopyroxene and olivine porphyroclasts dynamically recrystallized. With ongoing extension, silica-undersaturated melt percolated through the shear zones and reacted with the clinopyroxene neoblasts, forming olivine–clinopyroxene layers. Furthermore, the melt reacted with orthopyroxene porphyroclasts, forming fine-grained polymineralic layers (ultramylonites) adjacent to the porphyroclasts. Strain rates in these layers are estimated to be about an order of magnitude faster than within the olivine-rich matrix. This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones.

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Superplasticity in Nanocrystalline Ni3Al and Ti Alloys

MRS Proceedings ◽

10.1557/proc-634-b1.3.1 ◽

2000 ◽

Vol 634 ◽

Cited By ~ 4

Author(s):

Sam X. Mcfadden ◽

Alla V. Sergueeva ◽

Tomas Kruml ◽

Jean-Luc Martin ◽

Amiya K. Mukherjee

Keyword(s):

Grain Size ◽

Strain Rate ◽

Strain Hardening ◽

Nanocrystalline Materials ◽

Tensile Elongation ◽

Constant Strain Rate ◽

Grain Boundary Sliding ◽

Simple Extension ◽

Size Dependent ◽

Enhanced Properties

ABSTRACTThe advent of nanocrystalline materials has provided new opportunities to explore grain size dependent phenomenon. Superplasticity is such a grain size dependent phenomenon defined by the ability to attain tensile elongation of 200% or more. Superplasticity in microcrystalline materials has been well characterized. The constitutive equations that describe microcrystalline superplasticity predict enhanced properties for nanocrystalline materials. Enhanced properties in such nanocrystalline material include lower superplastic temperature at constant strain rate, higher superplastic strain rate at constant temperature, and lower flow stresses. Investigations with nanocrystalline Ni3Al and ultra-fine grained Ti-6Al-4V alloy have shown a reduction in the superplastic temperature. However, the flow stresses in these materials are significantly higher than expected. The high flow stresses are accompanied by strong strain hardening.Transmission electron microscopy in situ straining of nanocrystalline Ni3Al has shown that grain boundary sliding and grain rotation occurred during straining. The sliding and rotation decreased with strain. Dislocation activity was observed but was not extensive. There was no observable dislocation storage. The parameters of the generalized constitutive equation for superplasticity for nanocrystalline Ni3Al and Ti-6Al-4V are in reasonable agreement with the parameters for microcrystalline material. The rate parameters suggest that nanocrystalline superplasticity shares common features with microcrystalline superplasticity. In contrast, the observed flow stresses and strong strain hardening indicate that nanocrystalline superplasticity is not a simple extension of microcrystalline behavior scaled to finer grain size.

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