Grain Size Sensitive Creep Research Articles

Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip

Faults weaken during the propagation of earthquakes due to the onset of thermally-activated mechanisms, which vary depending on the rock type. Recent experimental work suggests that carbonate-hosted faults are lubricated by viscous flow in nano-granular aggregates having ultramylonitic textures. However, their frail nature has often hindered unbiased characterisation of the textures and deformation mechanisms operating at such extreme conditions (strain rates as high as 104), which remain so far poorly investigated and understood.We explore the formation, evolution and deformation mechanisms of coseismic ultramylonites in carbonate-hosted faults generated during high velocity (1.4 ms−1), displacement-controlled shear experiments in a rotary apparatus. Microstructures were analysed using integrated SEM and TEM imaging while detailed crystallographic fabrics were investigated using the electron back-scattered diffraction (EBSD) technique.Mechanical data show that the strength of the experimental fault decays dynamically with slip, according to a characteristic four stage evolution; each stage is associated with characteristic textures. Microstructural observations show that brittle processes dominate when the fault is strong (friction coefficients >0.6). Cataclasis, aided by twinning and crystal plasticity, operates forming an extremely comminuted shear band (mean grain size ∼200 nm). As the fault starts weakening, shear localises within a well-defined principal slip zone. Here, thermally-activated grain size sensitive (GSS) and insensitive (GSI) creep mechanisms compete with brittle processes in controlling fault strength. GSI mechanisms produce strong monoclinic crystallographic preferred orientations in the slip zone, while textures and crystallographic orientations in adjacent locations do not evolve from the previous deformation stage. By the end of the transient weakening stage, the slip zone has reached a steady state thickness (30 μm) and shows a nanogranular ultramylonitic texture. The intensity of the crystallographic preferred orientation in the coseismic ultramylonite is reduced compared to the previous stage, due to grainsize sensitive creep mechanisms becoming gradually more dominant. As the experimental fault re-strengthens, upon deceleration to arrest, the ultramylonite may be partially reworked by brittle deformation.Our findings show that the crystallographic orientations of transient microstructures are preserved in the slip zone of coseismic ultramylonites and in narrow, adjacent deactivated layers, where mirror-like surfaces are located. This shows that EBSD techniques can usefully be employed to determine the deformation mechanisms of coseismic ultramylonites and their evolution during earthquake slip in both experimental and, potentially, natural faults.

The architecture of long-lived fault zones: insights from microstructure and quartz lattice-preferred orientations in mylonites of the Median Tectonic Line, SW Japan

We combine field mapping with quartz microstructure and lattice preferred orientations (LPO) to constrain the mechanisms and spatio-temporal distribution of deformation surrounding the Median Tectonic Line (MTL), SW Japan. In the study area, the MTL occurs either as a narrow gouge zone or as a sharp contact between hanging-wall quartzofeldspathic mylonites to the north and footwall pelitic schists to the south. Along the northern margin of the MTL, there exists a broad zone of mylonitic rocks, overprinted by cataclastic deformation and a damage zone associated with brittle deformation. The mylonitic shear zone is dominated by coarse-grained protomylonite up to ~ 100 m from the MTL, where fine-grained ultramylonite becomes dominant. We observe a systematic variation in quartz LPO with distance from the MTL. In protomylonites, quartz LPOs are dominantly Y-maxima patterns, recording dislocation creep by prism<a> slip at ~ 500 °C. Closer to the MTL, we observe R- and Z-maxima, and single and crossed girdles, reflecting dislocation creep accommodated by mixed rhomb<a> and basal<a> slip, likely under cooler conditions (~ 300 °C–400 °C). Some ultramylonite samples yield weak to random LPOs, interpreted to result from the influx of fluid into the shear zone, which promoted deformation by grainsize-sensitive creep. Following cooling and uplift, deformation became brittle, resulting in the development of a narrow cataclasite zone. The cataclasite was weakened through the development of a phyllosilicate foliation. However, healing of fractures strengthened the cataclasites, resulting in the development of anastomosing cataclasite bands within the protomylonite.

Mesoscopic Scale Modeling of "Superplastic" Flow in Geological and Glacial Materials

A viewpoint that suggests that grain/ interphase boundary sliding (GBS) that develops to a mesoscopic scale (“cooperative boundary sliding”) controls optimal superplastic (SP) deformation is able to explain superplasticity in metals and alloys, ceramics, intermetallics, composites and bulk metallic glasses of grain sizes ranging from a few microns down to a few nanometers. It is extended here to describe grain-size-sensitive (GSS) flow in minerals, rocks and ice within narrow experimental ranges. In this approach the accommodation processes of grain boundary diffusion, dislocation emission from sliding boundaries and/ or grain rotation accompanying boundary sliding are present overextremely short distancesand are assumed to be faster than GBS. Analysis shows that GSS creep in geological and glacial materials can be accounted for in terms of four “universal”, mesoscopic scale constants of average values, = 0.197, = 0.415 J.m-2, = 8.9 and = 0.166, where is the average shear strain associated with a basic boundary sliding event at the level of the atomistics, is the specific grain boundary energy (assumed to be isotropic), is the number of boundaries that align to form a mesoscopic boundary glide plane and “” is a constant that obeys the condition 0&lt;a&lt;0.5, whose magnitude depends on grain shape and size distribution in the material. It is demonstrated that with the help of these four constants and the Frost-Ashby equations for estimating the shear modulus, it is possible to predict steady state GSS creep flow in any geological or glacial material, including those whose mechanical response was not used to obtain the “universal” constants. Whether these observations are evidence for “superplasticity” in these materials can be known only if the findings are reproduced in tensile deformation also.

The ultimate fate of a synmagmatic shear zone. Interplay between rupturing and ductile flow in a cooling granite pluton

The Neoarchean Cundimurra Pluton (Yilgarn Craton, Western Australia) was emplaced incrementally along the transpressional Cundimurra Shear Zone. During syndeformational cooling, discrete networks of cataclasites and ultramylonites developed in the narrowest segment of the shear zone, showing the same kinematics as the earlier synmagmatic structures. Lithological boundaries between aplite/pegmatite veins and host granitic gneiss show more intense pre-cataclasite fabrics than homogeneous material, and these boundaries later became the preferred sites of shear rupture and cataclasite nucleation.Transient ductile instabilities established along lithological boundaries culminated in shear rupture at relatively high temperature (∼500–600 °C). Here, tensile fractures at high angles from the fault plane formed asymmetrically on one side of the fault, indicating development during seismic rupture, establishing the oldest documented earthquake on Earth.Tourmaline veins were emplaced during brittle shearing, but fluid pressure probably played a minor role in brittle failure, as cataclasites are in places tourmaline-free. Subsequent ductile deformation localized in the rheologically weak tourmaline-rich aggregates, forming ultramylonites that deformed by grain-size sensitive creep. The shape and width of the pluton/shear zone and the regime of strain partitioning, induced by melt-present deformation and established during pluton emplacement, played a key role in controlling the local distribution of brittle and then ductile subsolidus structures.

Recrystallisation, phase mixing and strain localisation in peridotite during rapid extrusion of sub-arc mantle lithosphere

The Anita Peridotite, in southwestern New Zealand, is a ∼1 × 20 km ultramafic massif that was rapidly extruded from beneath a Cretaceous arc within the 4 km wide mylonitic Anita Shear Zone. The peridotitic body contains a spectacular array of textures that preserve evidence for changing temperature, stress, and deformation mechanisms during the exhumation process. Olivine and orthopyroxene microstructures and lattice-preferred orientations (LPO) record a three-phase deformation history. Dislocation glide on the C- and E-type slip systems is recorded by coarse pre-mylonitised olivine grains, and occurred under hydrous conditions at T ∼650 °C, stress ∼200–700 MPa and strain rate ∼10−15 s−1, probably within hydrated sub-arc mantle lithosphere. Rare protomylonite pods record deformation by dislocation creep in porphyroclasts and dislocation-accommodated grain boundary sliding in the matrix on {0kl}[100] in olivine and (100)[001] in orthopyroxene, under conditions of T ∼730–770 °C, stress ∼52–700 MPa and strain rate ∼10−15 s−1. The massif, however, is dominated by mylonite and ultramylonite that wrap the protomylonite pods, comprising mostly fine-grained olivine neoblasts that lack internal distortions and have uniform LPOs. These textures indicate deformation occurred by grain-size sensitive (GSS) creep at T ∼650 °C, stress ∼69–137 MPa and strain rate ∼10−15 s−1, and thus during conditions of cooling and decreasing stress. GSS creep became more dominant with time, as the proportion of randomly-oriented neoblasts increased and formed interlinked networks that accommodated much of the strain. Grain boundary pinning allowed GSS creep to be maintained in polyphase regions, following mixing of olivine and orthopyroxene, which may have occurred by grain boundary transport in a fluid phase during a “creep cavitation” process. The results indicate that the Anita Peridotite recrystallised and underwent rheological weakening at a constant strain rate, with strain distributed across the entire section. This widespread deformation caused rapid exhumation of the peridotite from the lithospheric mantle into the overlying arc crust. The massif therefore records multiple overprinting phases of deformation under mantle and crustal conditions associated with the rapid exhumation of a large orogenic peridotite.

The link between strength of lattice preferred orientation, second phase content and grain boundary migration: A case study from the Alpine Fault zone, New Zealand

We analyse the microstructure and quartz LPOs of 36 layers of varying composition from a several-meter thick sequence of amphibolite-facies metacherts and related mica-garnet-plagioclase-quartz bearing schists from the central Southern Alps in the mylonite zone related to the Alpine Fault. Quartz contents vary from ∼10 to 100% and all of the LPO fabric skeletons are similar, featuring an asymmetric single girdle of [c]-axes inclined ∼30° away from the ZY plane. LPO strength is typically low at quartz contents <70% (M Index of ∼0.05) whereas it may be very high for nearly pure quartz rocks (M Index of up to 4.0). We attribute this change to a sparseness of interphase boundaries in the more quartzose rocks, a reduction in grain-boundary pinning, and a corresponding efficiency of grain boundary migration during dynamic recrystallization. The transition corresponds to a Zener parameter of approximately 700. In layers poor in quartz and rich in mica, the quartz grain size was kept small, and phase-boundary density, high. This may have promoted grain-size sensitive creep and dislocation glide in mica. Dislocation creep in the interspersed quartz grains was correspondingly reduced, and weaker quartz LPOs were produced. In highly quartzose layers, quartz grain boundaries experienced little drag or pinning from impurity phases and were able to migrate quickly into higher strain-energy grains. Preferential consumption of poorly oriented grains strengthened quartz LPOs, geometrically softened the dislocation creep process in these quartzose layers, and contributed to grain coarsening. The lack of evidence for instabilities in the thinly layered (<1 mm, quartz-rich vs. mica-rich) mylonite implies that a combination of deformation mechanisms, grain-size sensitive flow and dislocation creep, in the layers were able to accommodate a nearly homogeneous deformation between the different composition layers.

Rheology, microstructure, and fabric in a large scale mantle shear zone, Ronda Peridotite, southern Spain

The Ronda peridotite massif of southern Spain is a slice of mantle peridotite that records in its mineral assemblages several stages of its thermal and textural evolution during exhumation to crustal levels. The uppermost zone is a garnet- and spinel-bearing mylonite that grades downward into spinel-bearing tectonites. Overall grain-size increases downward, while the proportion of dynamically recrystallized olivine grains decreases downward. Olivine recrystallized grain-size throughout these two upper zones is ∼130 μm. Olivine neoblasts are relatively free of substructure and have curved or lobate grain boundaries, suggesting recrystallization by grain boundary migration. Crystallographic orientations of stretched orthopyroxene porphyroclasts indicate a top-SW sense of shear for the marginal mylonites. Crystallographic lattice-preferred orientations (LPO) of olivine grains indicate a dominant slip system of (010)[100]. We propose that these upper two zones of the Ronda peridotite massif represent two stages in the evolution of the shear zone that exhumed the peridotite body through the spinel stability field (∼2.0–1.0 GPa) at a differential stress of ∼40 MPa. We estimate a strain rate of ∼5 × 10−13 based on the rheology of wet olivine at 1500 MPa and 860 °C. Under these conditions olivine is likely to deform by a grain-size sensitive creep mechanism, which explains the progressive localization of strain into the mylonite zone.

Creep behavior during the eutectoid transformation of albite: Implications for the slab deformation in the lower mantle

In order to investigate effects of the post-spinel transformation on rheological behavior of descending slabs, we conducted simultaneous deformation and eutectoid reaction experiments using an analogue reaction from albite to jadeite and quartz by a deformation-DIA apparatus. Flow stress and transformed fraction were quantitatively obtained by in-situ X-ray observations during the constant strain rate deformation. The microstructures of recovered samples as well as the flow and kinetic data suggest sequential variation of the creep mechanism from dislocation creep of the transformed eutectoid colony followed by the grain-size sensitive creep in the degenerated eutectoid structure. Our study demonstrated that the creep behavior during the eutectoid transformation involves various processes than previously thought. The slabs may not be weaken promptly after entering into the lower mantle when the size of eutectoid colony is enough large, and keep their strength (or harden) over a period of time depending on the degeneration kinetics of the colony.

Strain Localization in Pyroxenite by Reaction-Enhanced Softening in the Shallow Subcontinental Lithospheric Mantle

We report structural evidence of ductile strain localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the base of the lithospheric section are systematically located within thin pyroxenite layers, suggesting that the pyroxenite was locally weaker than the host peridotite. Strain localization is associated with a sudden decrease of grain size and increasing volume fractions of plagioclase and amphibole as a result of a spinel to plagioclase phase transformation reaction during decompression. This reaction also fostered hydrogen extraction (‘dehydroxylation’) from clinopyroxene producing effective fluid saturation that catalyzed the synkinematic net-transfer reaction. This reaction produced fine-grained olivine and plagioclase, allowing the onset of grain-size sensitive creep and further strain localization in these pyroxenite bands. The strain localization in the pyroxenites is thus explained by their more fertile composition, which allowed earlier onset of the phase transition reactions. Geothermobarometry undertaken on compositionally zoned constituent minerals suggests that this positive feedback between reactions and deformation is associated with cooling from at least 1000°C to 700°C and decompression from 1·0 to 0·5 GPa.

Transition from fracturing to viscous flow in granulite facies perthitic feldspar (Lofoten, Norway)

Recrystallization of perthites in granulite facies (T = 700–730 °C, P = 0.65–0.8 GPa) shear zones in mangerite-charnockite rocks from Lofoten (Norway) is localized along intracrystalline bands parallel to fractures. Fracturing preferentially occurred along the cleavage planes (010) and (001). EBSD analysis of perthite porphyroclasts indicates a very low degree of internal misorientation (within 5°) and the lack of recovery features. Recrystallized grains show coarsening with increasing width of the bands, and chemical changes with respect to the host grains. Crystallographic orientation of the new grains does not show a host-control relation to the parent perthite grains. In summary, the microstructure and CPO data consistently indicate intragranular recrystallization by nucleation and growth from fractured grains. Perthite porphyroclasts are surrounded by a matrix of recrystallized plagioclase + K-feldspar ± amphibole ± biotite. There is extensive evidence of syndeformational nucleation of new phases and of phase boundary migration in the matrix, with plagioclase grains forming bulges and protrusions towards K-feldspar. The spatial distribution of K-feldspar and plagioclase in the recrystallized matrix is characterized by the predominance of phase boundaries over grain boundaries. All these observations are consistent with diffusion creep as the dominant deformation mechanism in the matrix, associated with grain boundary sliding. Accordingly, recrystallized plagioclase and K-feldspar show a very weak crystallographic preferred orientation, which is interpreted in terms of oriented growth during diffusion creep. Fracturing of perthites promoted extensive grain size reduction, recrystallization, fluid infiltration, and operation of grain-size sensitive creep, resulting in strain localization.

Deep structure of lithospheric fault zones

[1] We calculate the cumulative width w of ductile shear zones accommodating plate motion in continental lithosphere, based on the assumptions that (1) the flow stress is controlled by the yield strength of intact rock at any given depth; (2) the yield strength profile through the crust can be constrained from observations in exhumed shear zones; and (3) strain localization is primarily caused by grainsize reduction leading to a switch to grainsize-sensitive creep. We use a mid-crustal stress-temperature profile measured in the Whipple Mountains, California, and calculate stress profiles at depth from published flow laws for feldspar and olivine. We conclude thatwfor a plate boundary shear zone accommodating 50 mm/yr displacement (comparable to the San Andreas Transform) may reach 180 km in the quartz-rich mid-crust, depending on water fugacity and thermal gradient. It narrows to a few meters in feldspathic lower crust and in the uppermost mantle, and then widens rapidly with depth in the lower lithosphere. We explore the effects of differences in crustal thickness and composition, thermal gradient, and water activity.

Crystal preferred orientation in peridotite ultramylonites deformed by grain size sensitive creep, Étang de Lers, Pyrenees, France

In naturally deformed upper mantle rocks a strong olivine crystallographic preferred orientation (CPO) occurs in rocks with grain sizes larger than about 15 μm. Finer grained peridotites tend to have weak to random olivine CPO. The different types of olivine CPO are usually interpreted in relation to the dominant deformation mechanisms: with a strong CPO indicating dislocation creep and a random CPO indicating grain size sensitive (GSS) mechanisms involving grain boundary sliding and diffusion creep. Here we report the occurrence of a weak but systematic olivine CPO in ultra-fine-grained (0.5–10 μm) ultramylonites from the Étang de Lers lherzolite, Pyrenees, France. The microstructures, with elongated grains and mixed distribution of phases in the ultramylonites indicate deformation by dominant GSS creep. In theory, an olivine CPO may develop in rocks deformed by GSS creep during dislocation accommodated boundary sliding or phase boundary controlled diffusion creep in pyroxene-rich rocks. Alternatively, the CPO may be a relict texture preserved because the elongated grain shapes limit rotation during grain boundary sliding. Our observations confirm theoretical and experimental studies, which predict that systematic CPO and an associated seismic anisotropy can occur in naturally deformed upper mantle rocks during deformation by grain boundary sliding and diffusion creep.

Grainsize evolution in ductile shear zones: Implications for strain localization and the strength of the lithosphere

At high stresses and low temperatures, grainsize reduction by dynamic recrystallization profoundly modifies rock rheology. Strain energy driven grain-boundary migration (ρGBM) is involved both in the nucleation of new grains by the grain-boundary bulging mechanism (BLG), and in the subsequent evolution of the microstructure. Above the D min line, which is a line in stress/grainsize space that defines the minimum size of nucleus that can form by BLG, ρGBM dominates the microstructure, and grain growth by surface energy driven grain-boundary migration (γGBM) is inhibited. The recrystallized grainsize is therefore dominated by the nucleation process, possibly controlled by the size of subgrains or dislocation cells within the old grains. This provides a first-order explanation for the experimentally observed grainsize-stress relationship. ρGBM is an important agent of recovery in rocks deformed by dislocation creep, sweeping out dislocations and counteracting work-hardening. We have derived a new flow law (DRX-assisted dislocation creep) based on this process, which exhibits grainsize sensitivity as a result of the role of ρGBM. If grainsize obeys the empirically-determined grainsize-stress relationship, DRX creep has an effective stress exponent of a little over 4, consistent with experimental observations and inferences from naturally deformed rocks. DRX creep may be an important agent in weakening quartz at low temperatures, whereas current flow law data suggest it may not be important in olivine. Rocks deformed and dynamically recrystallized above the D min line may switch from climb-assisted dislocation creep to grainsize-sensitive creep (Coble creep, DRX creep, or creep dominated by grain-boundary sliding), resulting in weakening. Lithospheric-scale shear zones are likely to evolve at approximately constant stress; under these conditions weakening results in an increase in strain rate, not a stress drop. The rate of dislocation motion, the dislocation density, and the dynamically recrystallized grainsize all remain the same, and grain growth will be inhibited by the activity of ρGBM. Hence the switches in deformation mechanism and the weakening they cause will be permanent, so long as the tectonic boundary conditions remain unchanged. Grainsize reduction caused by dynamic recrystallization may therefore play a fundamental role in lithospheric weakening, and may be a key process in the maintenance of plate tectonics.

Grain growth systematics for forsterite ± enstatite aggregates: Effect of lithology on grain size in the upper mantle

Grain growth kinetics of forsterite (Fo) and enstatite (En) in fine grain aggregates of Fo ± En are examined as a function of volume fraction of En ( f En = 0.00 to 0.42). Growth rates fit d n ∝ k · t ( d: mean grain diameter; n: grain growth exponent; k: growth constant; t: time), where n ∼ 5 for both forsterite and enstatite grains in the enstatite-bearing samples. A negative correlation between k Fo and f En can be expressed as k Fo ≈ 0.06 exp [30 f En ( f En − 1.1)], whereas k En takes almost constant values for different f En. In addition, the ratio of d Fo/ d En is almost constant during grain growth, and its value becomes smaller with increasing f En, such that d Fo/ d En ≈ 0.74/ f En 0.59. Our obtained grain growth parameters and the microstructural characteristics in experimental and natural samples all indicate that the rate-controlling process for grain growth in both experiments and in nature is grain boundary diffusion of Si through grain boundaries of olivine. Thus, we are able to propose a semi-empirical relationship of d Ave / d dun ≈ { exp [ 30 f px ⋅ ( f px − 1.1 ) ] } 1 5 ⋅ ( 1 + 1.35 f px 1.59 − f px ) ( d Ave: average grain size in the rock; d dun: average grain size in dunite; f px: fraction of pyroxenes), which can predict how lithology affects grain size in the upper mantle. Grain size in lherzolite (pyroxenes content of 40%) can be ∼ 4 times smaller than that in dunite (pyroxenes content of 5%). Such grain size reduction through change in lithology can significantly affect viscosity of mantle deformation via grain-size sensitive creep.

Deformation-induced polymorphic transformation: experimental deformation of kyanite, andalusite, and sillimanite

Torsion experiments were performed on the Al 2SiO 5 polymorphs in the sillimanite stability field to determine basic rheological characteristics and the effect of deformation on polymorphic transformation. The experiments resulted in extensive transformation of andalusite and kyanite to sillimanite. No transformation occurred during the hot-press (no deformation) stage of sample preparation, which was carried out at similar P– T conditions and duration as the torsion experiments. Experiments were conducted on fine-grained (< 15 µm) aggregates of natural andalusite, kyanite and sillimanite at 1250 °C, 300 MPa, and a constant shear strain rate of 2 × 10 − 4 /s to a maximum shear strain of 400%. Electron back-scattered diffraction (EBSD) analysis of the experiments revealed development of lattice-preferred orientations, with alignment of sillimanite and andalusite [001] slightly oblique to the shear plane. The kyanite experiment could not be analyzed using EBSD because of near complete transformation to sillimanite. Very little strain (∼ 30%) is required to produce widespread transformation in kyanite and andalusite. Polymorphic transformation in andalusite and kyanite experiments occurred primarily along 500 µm wide shear bands oriented slightly oblique and antithetic to the shear plane and dominated by sub-µm (100–150 nm) fibrolitic sillimanite. Shear bands are observed across the entire strain field preserved in the torsion samples. Scanning transmission electron microscope imaging shows evidence for transformation away from shear bands; e.g. fibrolitic rims on relict andalusite or kyanite. Relict grains typically have an asymmetry that is consistent with shear direction. These experimental results show that sillimanite is by far the weakest of the polymorphs, but no distinction can yet be made on the relative strengths of kyanite and andalusite. These observations also suggest that attaining high bulk strain energy in strong materials such as the Al 2SiO 5 polymorphs is not necessary for triggering transformation. Strain energy is concentrated along grain boundaries, and transformation occurs by a dynamic recrystallization type process. These experiments also illustrate the importance of grain-size sensitive creep at high strains in a system with simultaneous reaction and deformation.

On the role of grain topology in dynamic grain growth — 2D microstructural modeling

Microstructural modification processes like dynamic recrystallization and grain growth can have a major effect on the transient and (semi-)steady state flow behaviour of deforming materials. Work on metals and ceramics suggests that deformation-enhanced changes in grain topology and the corresponding increase in fraction of non-hexagonal grains, called cellular defect fraction, can promote grain growth during deformation. The present study tests this hypothesis, by investigating the evolution of the cellular defect fraction during deformation, accompanied by grain growth, of aggregates with distributed grain sizes. For this purpose, we made use of the ELLE 2D microstructural modeling package. We simulated and quantified microstructural evolution under conditions where both surface energy driven grain boundary migration (GBM) and homogeneous deformation or grain size sensitive (GSS) straining were allowed to occur. The simulations show that contemporaneous GBM and simple geometrical straining of grain aggregates with distributed grain size and coordination number lead to extra grain neighbor switching, an increase in defect fraction, and enhanced grain growth. An increase in defect fraction was also found in a selected set of natural calcite mylonites that, with increasing temperature, show an increase in grain size and contribution of GSS creep. Analysis of defect fraction thus appears to be a good microstructural tool to establish whether or not a material has experienced normal static (defect fraction ∼ 0.7) or dynamic grain growth (defect fraction ∼ 0.8).

Formation and maintenance of shear zones

Research Article| February 01, 2006 Formation and maintenance of shear zones Caleb W. Holyoke, III; Caleb W. Holyoke, III 1Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA Search for other works by this author on: GSW Google Scholar Jan Tullis Jan Tullis 1Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA Search for other works by this author on: GSW Google Scholar Geology (2006) 34 (2): 105–108. https://doi.org/10.1130/G22116.1 Article history received: 31 Jul 2005 rev-recd: 10 Oct 2005 accepted: 12 Oct 2005 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Caleb W. Holyoke, Jan Tullis; Formation and maintenance of shear zones. Geology 2006;; 34 (2): 105–108. doi: https://doi.org/10.1130/G22116.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The processes that initiate and maintain prograde ductile shear zones are not well understood. We have performed shear experiments (γ = 1 to 3) on a fine-grained (100– 150 μm) gneiss (59% quartz, 28% plagioclase, 13% aligned but not interconnected biotite) to determine the evolution of deformation mechanisms that produce dramatic strain weakening and localization, at conditions (1.5 GPa, 800 °C, \({\dot{{\gamma}}}\) = 2 × 10−5/s) where pure quartz aggregates deform homogeneously by dislocation creep. Initial yield occurs where stress concentrations at the tips of weak biotite grains produce semibrittle deformation in intervening quartz or plagioclase, allowing local biotite interconnection by slip on (001) and initiating strain weakening. After yield, the interconnected biotites kink and thus strengthen, but the highly strained parts of grains react to a fine-grained, mixed-phase assemblage which deforms by grain-size–sensitive creep, allowing further strain weakening and localization. At γ = 3.3, the strain and strain rate in the ∼100-μm-thick shear zone are 100 times that of the enclosing host rock, similar to the localization observed in natural shear zones. Thus, the processes that initiate strain localization are not necessarily the same as those that preserve the weak shear zone, and once formed, a shear zone may be permanently weakened. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

STRUCTURES AND MICROSTRUCTURES OF MANTLE ROCKS: KEYS TO THE RHEOLOGY OF THE LITHOSPHERE

Upper mantle peridotites from ophiolitic and alpine-type massifs, as well as those brought up as basalt-borne xenoliths, show a spectrum of structures and microstructures that potentially reflect the mechanical behaviour of the upper mantle during flow. It is uncertain, however, if microstructures are unambiguous in this respect. Based on experimental studies and materials science know-how, we use microstructure in its broadest sense to infer the nature of the microphysical processes once operating in the upper mantle rocks in question. Aside the many uncertainties involved in such extrapolation of experimentally determined mechanical data via the microstructure, there is the even more difficult question if the flow and fine structure of a natural rock, deformed at depth and high temperature, really reflects the flow of interest. Inherent to the fact that these rocks are now found at the Earth’s surface, the microstructures seen in naturally deformed mantle rocks may to a certain extent represent the waning stages of deformation and recrystallization in the mantle, i.e., the microstructure frozen in the rock may not appropriately reflect the main flow. The question thus arises in how far the deformation microstructures in mantle rocks really represent a significant part of the ductile flow history. In this context it seems appropriate to use the concept of a steady-state microstructure, i.e., a microstructure that continuously evolves during flow without changing its principal characteristics such as average grain size, grain shape and grain orientation, and that likely reflects the microphysical processes during flow. These concepts are particularly important at high homologous temperatures where deformation-induced rotation recrystallization (development of new grains from subgrains during dislocation creep) may or may not be balanced by migration recrystallization (recrystallization due to the migration of grain boundaries). It may be envisaged that high-temperature flow in the asthenosphere or lower lithosphere thus leads to seemingly undeformed granular rocks, even protogranular in the sense of commonly used peridotite microstructure classifications, or that the samples recovered preserve more clearly deformed, porphyroclastic microstructures including coarse to fine-grained tectonites. A distinct class of microstructures in peridotites concerns very fine to ultrafine-grained deformation microstructures in sometimes spectacular mylonitic shear zones. The mechanical aspects of the initiation and growth of these structures is surrounded with uncertainties, but there is growing consensus regarding their mechanical significance once these structures have developed: they are mechanically extremely weak due to the fact that flow in such ultrafine-grained rocks will be dominated by grainsize-sensitive diffusion creep mechanisms. Mylonites in upper mantle rocks often overprint earlier flow structures, probably because they tend develop at lower temperatures and higher stresses inherent in the later stages of a mantle fragment’s history in the Earth’s upper mantle, e.g., related to emplacement at crustal levels in rifts, or near the ocean floor in ophiolites.

Dynamic Recrystallization of Dense Polycrystalline NaCl: Dependence of Grain Size Distribution on Stress and Temperature

Only few models explain the development of a steady state grain size during dynamic recrystallization, and their microphysical basis is poorly understood. In this study, we investigate mechanical and microstructural data on dry and wet NaCl, deformed at a range of stresses and temperatures at elevated pressure, with the aim to evaluate the different models. The results show that dry NaCl continuously work hardens and shows evidence for recrystallization dominated by progressive subgrain rotation, while the wet material shows, at similar conditions, oscillating stressstrain behaviour and recrystallization dominated by grain boundary migration. Taking into account the distribution of grain size, deformation of wet NaCl is best described by flow laws based on composite grain size sensitive (GSS) solution-precipitation creep and grain size insensitive (GSI) dislocation creep. The recrystallized grain size data in wet NaCl can be modeled with the hypothesis that recrystallized grain size organises itself in the boundary between the GSS and GSS creep domains.

On the role of melt-rock reaction in mantle shear zone formation in the Othris Peridotite Massif (Greece)

A 1-km-wide peridotite mylonite shear zone is exposed in the Othris peridotite massif in central Greece. The mylonites contain lenses of relatively coarse olivine crystals, which are interpreted as remnants of the tectonite microstructure in the adjacent wall rocks. Microstructure and texture analysis using light and SEM microscopy suggests that the dominant deformation mechanism in the tectonites was dislocation creep, whereas the deformation in the mylonites was probably controlled by grain-size sensitive (GSS) creep in fine-grained (<50μm) bands consisting of a mixture of olivine and orthopyroxene. The development of the fine-grained material in the mylonites can be explained by a melt-present reaction taking place in the tectonite protolith. This reaction led to the replacement of orthopyroxene porphyroclasts by fine-grained olivine and orthopyroxene. Tectonites adjacent to the mylonite zone preserve evidence for this reaction in the form of rims of fine-grained olivine and orthopyroxene around orthopyroxene porphyroclasts. This study illustrates the significance of rheological weakening of oceanic mantle lithosphere as a result of a change from dislocation to GSS creep.