A natural example of brittle-to-viscous strain localization under constant-stress conditions: a case study of the Kellyland fault zone, Maine, USA

Abstract This article integrates field, powder X-ray diffraction and microstructural data to constrain deformation mechanisms in and the rheology of granite-derived fault rocks exposed along the SE side of the crustal-scale, strike-slip Kellyland fault zone. Deformation in this area of the Kellyland fault zone localized during cooling and is marked by (1) foliated granite, (2) a ∼50 m wide band of pulverized foliated granite, (3) a ∼2.8 m wide breccia zone hosting coeval shear zones, and (4) a >100 m wide ultramylonite zone. The earliest fabric in the foliated granite is defined by elongated quartz grains, and quartz dislocation creep was the rate-controlling deformation mechanism. Seismogenic deformation initiated when recorded flow stresses reached 96–104 MPa at temperatures of 400–450 °C and is marked by coeval pulverization and formation of breccia. Interseismic viscous creep at similar flow stresses is recorded by mutual cross-cutting relationships between breccia-hosted shear zones, brittle fractures and pseudotachylyte. Field and microstructural observations indicate that breccia-hosted shear zones are low-strain equivalents of the >100 m wide ultramylonite zone, and seismogenic deformation abated as the ultramylonite formed. The rheology of ultramylonites was governed by grain-size-sensitive creep at 112–124 MPa flow stresses. Hence, from the onset of seismogenesis, the Kellyland fault zone was likely a constant-stress system wherein the rate-controlling mechanism shifted from episodic seismogenic slip and interseismic viscous creep to steady state grain-size-sensitive creep in ultramylonites derived from brittle fault rocks. Flow stresses recorded by these rocks also imply that the whole zone was relatively weak if the brittle–viscous transition and uppermost viscous zone are the strongest part of the crust.

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Recrystallization of ice enhances the creep and vulnerability to fracture of ice shelves

10.5194/egusphere-egu21-3266 ◽

2021 ◽

Author(s):

Meghana Ranganathan ◽

Brent Minchew ◽

Colin Meyer ◽

Matej Pec

Keyword(s):

Grain Size ◽

Ice Sheet ◽

Shear Zones ◽

Dislocation Creep ◽

Model Parameters ◽

Localized Deformation ◽

Ice Shelves ◽

Fine Grained ◽

Initiation And Propagation ◽

Rapid Deformation

The initiation and propagation of fractures in floating regions of Antarctica has the potential to destabilize large regions of the ice sheet, leading to significant sea-level rise. While observations have shown rapid, localized deformation and damage in the margins of fast-flowing glaciers, there remain gaps in our understanding of how rapid deformation affects the creep and toughness of ice. Here we derive a model for dynamic recrystallization in ice and other rocks that includes a novel representation of migration recrystallization, which is absent from existing models but is likely to be dominant in warm areas undergoing rapid deformation within the ice sheet. We show that, in regions of elevated strain rate, grain sizes in ice may be larger than expected (~15 mm) due to migration recrystallization, a significant deviation from solid earth studies which find fine-grained rock in shear zones. This may imply that ice in shear margins deforms primarily by dislocation creep, suggesting a flow-law exponent of n=4 in these regions. Further, we find from existing models that this increase in grain size results in a decrease in tensile strength of ice by ~75% in the margins of glaciers. Thus, we expect that this increase in grain size makes the margins of fast-flowing glaciers less viscous and more vulnerable to fracture than we may suppose from standard model parameters.

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Stress fields of ancient seismicity recorded in the dynamic geometry of pseudotachylyte in the Outer Hebrides Fault Zone, UK

Journal of the Geological Society ◽

10.1144/jgs2020-101 ◽

2020 ◽

Vol 178 (1) ◽

pp. jgs2020-101

Author(s):

L.R. Campbell ◽

G.E. Lloyd ◽

R.J. Phillips ◽

R.C. Walcott ◽

R.E. Holdsworth

Keyword(s):

Fault Zone ◽

Dynamic Geometry ◽

Shear Zones ◽

Stress Fields ◽

Tectonic Activity ◽

Sensitivity Testing ◽

Fault Rocks ◽

Outer Hebrides ◽

Supplementary Material

Heterogeneous sequences of exhumed fault rocks preserve a record of the long-term evolution of fault strength and deformation behaviour during prolonged tectonic activity. Along the Outer Hebrides Fault Zone (OHFZ) in NW Scotland, numerous pseudotachylytes record palaeoseismic slip events within sequences of mylonites, cataclasites and phyllonites. To date, the kinematics and controls on seismicity within the long active history of the OHFZ have been poorly constrained. Additional uncertainties over the relative location of a meteorite impact and possible pre-OHFZ brittle faulting also complicate interpretation of the diffuse seismic record. We present kinematic analyses of seismicity in the OHFZ, combining observations of offset markers, en echelon injection veins and injection vein geometry to reconstruct slip directions and stress fields. This new dataset indicates that a range of fault orientations, slip directions and slip senses hosted seismicity in the OHFZ. Such complexity requires several stress field orientations, in contrast with the NW–SE Caledonian compression traditionally attributed to frictional melting along the OHFZ, indicating that seismicity had a long-term presence across the fault zone. Persistence of strong frictional failure alongside the simultaneous development of weak fault rocks and phyllonitic shear zones in parts of the OHFZ has significant implications for understanding seismic hazard along mature continental faults.Supplementary material: Tables listing analysed orientation measurements plus further information and sensitivity testing of palaeostress analysis parameters are available at https://doi.org/10.6084/m9.figshare.c.5134797

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On the straight and narrow: extreme localization of brittle fault damage and displacement along the Glade Fault Zone, Eastern Fiordland, New Zealand

10.5194/egusphere-egu2020-4878 ◽

2020 ◽

Author(s):

Michael Ofman ◽

Steven Smith

Keyword(s):

New Zealand ◽

Fault Zone ◽

Damage Zone ◽

Shear Zones ◽

Strike Slip ◽

Fault Rock ◽

Fault Rocks ◽

X Ray ◽

Fracture Damage ◽

Host Rocks

The southern Glade Fault Zone is a crustal-scale, subvertical dextral strike-slip fault zone on the eastern margin of Fiordland, New Zealand. For a distance of c. 40 km between Lake Te Anau and the Hollyford Valley, the fault cuts plutonic host rocks and has an estimated total dextral separation of c. 6-8 km. We report previously unidentified mylonites, cataclasites, pseudotachylites and fault gouge subparallel to pervasive sets of planar cooling joints in the Hut Creek-Mistake Creek area plutonic suites. The outcropping assemblage of joints and fault rocks record thermal, seismic and rheological conditions in the southern Glade Fault. Here we integrate methods to characterise the fault rocks and fracture damage zone of the southern Glade Fault from Glade Pass to Mt Aragorn. We use (i) EDS (Energy Dispersive x-ray Spectroscopy), XRD (X-Ray Diffraction) and EBSD (Electron Backscatter Diffraction) analysis to describe the mineralogy, kinematics and microstructures of fault rocks and, (ii) drone orthophotography and traditional structural measurements to detail geometrical relationships between structural features. Field mapping of glacially polished outcrops identifies the zone of brittle fault-related damage (i.e. damage zone + fault rock sequence) is up to one order of magnitude narrower than documented along other strike-slip faults with similar displacements, suggesting that the Glade Fault Zone represents an &#8220;end-member&#8221; of extreme localization of brittle deformation and fault displacement. This is interpreted to result from linkage of pre-existing cooling joints (and mylonitic shear zones), which allowed the younger brittle fault zone to establish its length and planarity relatively efficiently compared to the case of fault nucleation and growth in more isotropic host rocks.

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Self-Consistent Grain Size Evolution controls Strain Localization during Rifting

10.21203/rs.3.rs-561920/v1 ◽

2021 ◽

Author(s):

Jonas Ruh ◽

Leif Tokle ◽

Whitney Behr

Keyword(s):

Grain Size ◽

Upper Mantle ◽

Numerical Models ◽

Shear Zones ◽

Dislocation Creep ◽

Diffusion Creep ◽

Plate Boundaries ◽

Mantle Dynamics ◽

Size Evolution ◽

Dominant Deformation Mechanism

Abstract Geodynamic numerical models often employ solely grain-size-independent dislocation creep to describe upper mantle dynamics. However, observations from nature and rock deformation experiments suggest that shear zones can transition to a grain-size-dependent creep mechanism due to dynamic grain size evolution, with important implications for the overall strength of plate boundaries. We apply a two-dimensional thermo-mechanical numerical model with a composite diffusion-dislocation creep rheology coupled to a dynamic grain size evolution model based on the paleowattmeter. Results indicate average olivine grain sizes of 3–12 cm for the upper mantle below the LAB, while in the lithosphere grain size ranges from 0.3–3 mm at the Moho to 6–15 cm at the LAB. Such a grain size distribution results in dislocation creep being the dominant deformation mechanism in the upper mantle. However, deformation-related grain size reduction below 100 μm activates diffusion creep along lithospheric-scale shear zones during rifting, affecting the overall strength of tectonic plate boundaries.

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Syn-kinematic hydration reactions, grain size reduction, and dissolution–precipitation creep in experimentally deformed plagioclase–pyroxene mixtures

Solid Earth ◽

10.5194/se-9-985-2018 ◽

2018 ◽

Vol 9 (4) ◽

pp. 985-1009 ◽

Cited By ~ 8

Author(s):

Sina Marti ◽

Holger Stünitz ◽

Renée Heilbronner ◽

Oliver Plümper ◽

Rüdiger Kilian

Keyword(s):

Grain Size ◽

Confining Pressure ◽

Shear Zones ◽

Grain Boundary Sliding ◽

Size Reduction ◽

Deformation Mechanisms ◽

Dislocation Creep ◽

Exact Nature ◽

Crystallographic Preferred Orientation ◽

Mafic Rocks

Abstract. It is widely observed that mafic rocks are able to accommodate high strains by viscous flow. Yet, a number of questions concerning the exact nature of the involved deformation mechanisms continue to be debated. In this contribution, rock deformation experiments on four different water-added plagioclase–pyroxene mixtures are presented: (i) plagioclase(An60–70)–clinopyroxene–orthopyroxene, (ii) plagioclase(An60)–diopside, (iii) plagioclase(An60)–enstatite, and (iv) plagioclase(An01)–enstatite. Samples were deformed in general shear at strain rates of 3×10−5 to 3×10−6 s−1, 800 °C, and confining pressure of 1.0 or 1.5 GPa. Results indicate that dissolution–precipitation creep (DPC) and grain boundary sliding (GBS) are the dominant deformation mechanisms and operate simultaneously. Coinciding with sample deformation, syn-kinematic mineral reactions yield abundant nucleation of new grains; the resulting intense grain size reduction is considered crucial for the activity of DPC and GBS. In high strain zones dominated by plagioclase, a weak, nonrandom, and geometrically consistent crystallographic preferred orientation (CPO) is observed. Usually, a CPO is considered a consequence of dislocation creep, but the experiments presented here demonstrate that a CPO can develop during DPC and GBS. This study provides new evidence for the importance of DPC and GBS in mid-crustal shear zones within mafic rocks, which has important implications for understanding and modeling mid-crustal rheology and flow.

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Dynamic weakening mechanism in Earth’s mantle - A comparison between damage-dependent weakening and grain-size sensitive rheologies

10.5194/egusphere-egu2020-17979 ◽

2020 ◽

Author(s):

Lukas Fuchs ◽

Thorsten W. Becker

Keyword(s):

Grain Size ◽

Fault Zone ◽

Strain Localization ◽

Effective Viscosity ◽

Shear Zones ◽

Plate Boundaries ◽

Tectonic Inheritance ◽

Parameter Combination ◽

Size Evolution ◽

Viscous Shear

The creation and maintenance of narrow plate boundaries and their role in the thermo-chemical evolution of Earth remain one of the major problems in geodynamics. In particular, the cause and consequences of strain localization and weakening within the upper mantle remain debated, even though strain memory and tectonic inheritance, i.e. the ability to preserve and reactivate inherited weak zones over geological time, and strain localization appear to be critical features in plate tectonics.Frictional-plastic faults in nature and brittle shear zones in the lithosphere may be weakened by high transient, or static, fluid pressures, or mechanically by gouge, or mineral transformations. Weakening in ductile shear zones in the viscous domain may be governed by a change from dislocation to diffusion creep caused by grain-size reduction. In mechanical models, strain weakening and localization in the shallow parts of the lithosphere has mainly been modeled by an approximation of brittle behavior using a pseudo visco plastic rheology. This has often been implemented by a linear decrease of the yield strength of the lithosphere with increasing deformation. Strain weakening in viscous shear zones, on the other hand, may be described by a linear dependence of the effective viscosity on the accumulated deformation.Here, we analyze how a parameterized, apparent-strain, or &#8220;damage&#8221;, dependent weakening (SDW) rheology governs strain localization and weakening as well as healing in the lithosphere. The weakening and localization due to the SDW rheology has been related to a grain-size sensitive (GSS) composite rheology (diffusion and dislocation creep). While we focus on GSS rheology to constrain the parameters of SDW, the analysis is not limited to grain-size evolution as the only possible microphysical mechanism. We explore different types of strain weakening (plastic- (PSS) and viscous-strain (VSS) softening) and compare them to the predictions from different models of grain-size evolution for a range of temperatures and a step-like variation of total strain rate with time. PSS leads to a weakening and strengthening of the effective viscosity of about the same order of magnitude as due to a GSS rheology, while the rate depends on the strain-weakening parameter combination. In addition, the SDW weakening rheology allows for memory of deformation, which weakens the fault zone for a longer period. Once activated, the memory effect and weakening of the fault zone allows for a more frequent reactivation of the fault for smaller strain rates, depending on the strain-weakening parameter combination.

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Deformation of shale and dolomite in the Lewis thrust fault zone, northwest Montana, U.S.A.

Canadian Journal of Earth Sciences ◽

10.1139/e94-127 ◽

1994 ◽

Vol 31 (9) ◽

pp. 1440-1448 ◽

Cited By ~ 11

Author(s):

S. Gregg Erickson

Keyword(s):

Grain Size ◽

Fault Zone ◽

Rock Type ◽

Hanging Wall ◽

Thrust Fault ◽

Size Reduction ◽

Fault Rocks ◽

Slip Surfaces ◽

Northwest Montana ◽

Planar Fabric

The Lewis thrust fault zone at Marias Pass, northwest Montana, is an example of a fault zone in which hanging-wall dolomite and footwall shale deformed at relatively shallow levels (~7 km). Fabric in the fault zone depends on the rock type. Deformation of dolomite involved coalescence and widening by cataclasis of fractures, formation of anastomosing cataclasite zones that isolate less deformed clasts, and rounding and reduction in size of clasts to produce random-fabric cataclasite. Whereas dolomite deformed by progressive widening of cataclasite zones, shale deformation localized along ultracataclasite zones and slip surfaces that bound shale duplexes. Fault rocks that include both footwall shale and hanging-wall carbonate are characterized by isoclinal, intrafolial folds and a foliation that is defined by alternating shale- and carbonate-rich bands, elongate lenses of carbonate, and preferred orientation of phyllosilicates. Calcitization and subsequent solution of hanging wall rocks incorporated in the shale contributed to the development of this planar fabric. Lenses of hanging-wall carbonate were isolated in footwall shale by the emplacement of shale tongues into the hanging wall along mesoscopic faults. Displacement on the Lewis fault was accommodated by deformation of both dolomite and shale. Grain-size reduction of dolomite, mixing of dolomite and shale, and calcitization of dolomite in the fault zone may have enhanced diffusional processes in the carbonate and thereby weakened the fault zone.

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Self-consistent grain size evolution controls lithospheric shear zone formation during continental rifting

10.5194/egusphere-egu21-9600 ◽

2021 ◽

Author(s):

Jonas B. Ruh ◽

Leif Tokle ◽

Whitney M. Behr

Keyword(s):

Grain Size ◽

Upper Mantle ◽

Shear Zones ◽

Size Reduction ◽

Numerical Code ◽

Dislocation Creep ◽

Diffusion Creep ◽

Dominant Mechanism ◽

Size Evolution ◽

Growth Laws

In geodynamic numerical models, grain-size-independent dislocation creep often solely defines the governing crystal-plastic flow law in the upper mantle. However, grain-size-dependent diffusion creep may become the dominant deformation mechanism if grain size is sufficiently small. Previous studies implying composite diffusion-dislocation creep rheologies and fixed grain size suggest that the upper mantle is stratified with the dominant mechanism being dislocation creep at shallow depths and diffusion creep further down. Studies with variable grain size in the upper mantle depending on common grain-size evolution models demonstrate that the contrary might be the case, where diffusion creep is acting within the mantle lithosphere and dislocation creep in the asthenosphere below. Diffusion creep as a dominant mechanism has important implications for the overall strength of the lithosphere and therefore for the dynamic evolution of lithospheric-scale extension and orogeny.To investigate the importance of grain size and the effects of resulting crystal-plastic creep within the upper mantle, we developed a two-dimensional thermo-mechanical numerical code based on the finite difference method with a fully staggered Eularian grid and freely advecting Lagrangian markers. The model implies a composite diffusion-dislocation creep rheology and a dynamic grain-size evolution model based on the paleowattmeter including recently published olivine grain growth laws.Results of upper mantle extension indicate olivine grain sizes of ~7 cm for large parts of the upper mantle below the LAB, while in the lithosphere grain size ranges from ~1 mm at the Moho to ~5 cm at the LAB. This grain size distribution indicates that dislocation creep dominates deformation in the entire upper mantle. However, diffusion creep activates along lithospheric-scale shear zones during rifting where intense grain size reduction occurs to local stress increase. We furthermore test the implications of wet and dry olivine rheology and respective crystal growth laws and interpret their effects on large-scale tectonic processes. Our results help explain strain localization during extension by strength loss related to grain size reduction and consequent diffusion creep activation.

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Texture and Microstructure Effects on Electromigration Behavior of Aluminum Metallization

MRS Proceedings ◽

10.1557/proc-225-21 ◽

1991 ◽

Vol 225 ◽

Cited By ~ 10

Author(s):

D. B. Knorr ◽

K. P. Rodbell ◽

D. P. Tracy

Keyword(s):

Grain Size ◽

Standard Deviation ◽

Pure Aluminum ◽

Time To Failure ◽

X Ray Diffraction ◽

Aluminum Films ◽

Failure Data ◽

Transmission Electron ◽

Aluminum Metallization ◽

Microstructure Effects

ABSTRACTPure aluminum films are deposited under a variety of conditions to vary the crystallographic texture. After patterning and annealing at 400°C for 1 hour, electromigration tests are performed at several temperatures. Failure data are compared on the basis of t50 and standard deviation. Microstructure is quantified by transmission electron microscopy for grain size and grain size distribution and by X-ray diffraction for texture. A strong (111) texture significantly improves the electromigration lifetime and decreases the standard deviation in time to failure. This improvement correlates with both the fraction and sharpness of the (111) texture component.

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Preparation of high-entropy carbides by different sintering techniques

Journal of Materials Science ◽

10.1007/s10853-021-06004-y ◽

2021 ◽

Vol 56 (19) ◽

pp. 11237-11247 ◽

Cited By ~ 1

Author(s):

Johannes Pötschke ◽

Manisha Dahal ◽

Mathias Herrmann ◽

Anne Vornberger ◽

Björn Matthey ◽

...

Keyword(s):

Grain Size ◽

Single Phase ◽

X Ray Diffraction ◽

Vacuum Sintering ◽

X Ray ◽

High Entropy ◽

Mean Grain Size ◽

The Mean ◽

Gas Pressure Sintering ◽

Hardness Values

AbstractDense (Hf, Ta, Nb, Ti, V)C- and (Ta, Nb, Ti, V, W)C-based high-entropy carbides (HEC) were produced by three different sintering techniques: gas pressure sintering/sinter–HIP at 1900 °C and 100 bar Ar, vacuum sintering at 2250 °C and 0.001 bar as well as SPS/FAST at 2000 °C and 60 MPa pressure. The relative density varied from 97.9 to 100%, with SPS producing 100% dense samples with both compositions. Grain size measurements showed that the substitution of Hf with W leads to an increase in the mean grain size of 5–10 times the size of the (Hf, Ta, Nb, Ti, V,)C samples. Vacuum-sintered samples showed uniform grain size distribution regardless of composition. EDS mapping revealed the formation of a solid solution with no intermetallic phases or element clustering. X-ray diffraction analysis showed the structure of mostly single-phase cubic high-entropy carbides. Hardness measurements revealed that (Hf, Ta, Nb, Ti, V)C samples possess higher hardness values than (Ta, Nb, Ti, V, W)C samples.

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