scholarly journals Evolution of Quartz and Calcite Microstructures Exhumed From Deep Brittle-Ductile Shear Zones in the Southern Alps of New Zealand

2021 ◽  
Author(s):  
◽  
Matthew P Hill
Keyword(s):  
New Zealand ◽  
Grain Boundary ◽  
Shear Zones ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Differential Stress ◽  
Alpine Fault ◽  
Ductile Shearing ◽  
Boundary Sliding ◽  
Ductile Shear

<p><b>Arrays of brittle-ductile shears exposed in the Southern Alps of New Zealand, haveprovided a superb natural laboratory for insight into the microstructural evolution of lowercrustal shear zones during exhumation. Shears are exposed in the central section of theSouthern Alps at Sam Peak, Chancellor Ridge, and Baumann Glacier in a zone ~2 kmwide that is located 6–8 km structurally above the Alpine Fault. An array ofsystematically spaced shear zones that formed by embrittlement and faulting ofquartzofeldspathic schist took place at the same time as ductile shearing of quartzcarbonateveins embedded within the schist. This study has used field-based structuralmapping along with optical microscopy and universal stage measurements ofcrystallographic preferred orientations (CPO) to resolve the shear zone kinematics andrheology. On the basis of these data, the strain path can be reconstructed for the shearedveins during their progressive deformation. This began with their incidence as backshearsat the base of the Alpine Fault ramp and ended with their subsequent recrystallisation,uplift, and exhumation.</b></p> <p>The near-vertical shear planes have mean orientation of 221@89 NW ± 1o (n =780). They are inferred to have formed as backshears accommodating uplift of the PacificPlate as it was translated onto the oblique footwall ramp of the Alpine Fault during lateCenozoic oblique convergence. Detailed fault offset transect surveys across the shears atChancellor Ridge and Baumann Glacier reveal a mean spacing between the shear zones of25 ± 5 cm (n = 410). Quartz-carbonate marker veins are displaced in a dextral west-sideupshear sense. Fault offset geometry and a consistent arrangement of mineral fibrelineations that decorate fault surfaces, indicate that the mean displacement vector pitches35o SW in the shear plane (trend and plunge of: 262, 35 ± 7o). Ductilely deformed markerveins have been subject to a mean displacement of 9.9 ± 1.4 cm (n = 344) and a meanfinite ductile shear strain of 4.8 ± 0.3 (n = 219). A strain-rate for the ductile deformationof the veins is estimated at 3 x 10-11 sec-1 based on the observed finite ductile shear strain,an escalator kinematic model, and assumptions about the width of the deforming zone.</p> <p>Five deformation phases have affected the sheared veins during their transport upthe fault ramp: 1) initial brittle faulting and ductile shearing; 2) grain boundary sliding ofmylonitic quartz in response to a post-ramping differential stress drop; 3) recrystallisationand grain growth; 4) renewed late-stage dislocation creep; and 5) semibrittle deformationand exhumation. In the schist, the shears initiated as planar brittle faults at lower crustal depths of~21 km at a temperature of 450 ± 50oC. They developed in a zone of transiently highshear strain-rates near the base of the Alpine Fault ramp. Dislocation creep caused a CPOof quartz and calcite to develop in sheared veins. Using the flow law of Hirth et al. (2001)and the estimated strain-rate, a differential stress of ~165 MPa is inferred for ductiledeformation of the veins. Near-lithostatic (λ = 0.85) fluid pressures would have causedthe rocks to undergo brittle failure, a situation that is confirmed by a late component ofbrittle deformation that over prints the ductilely sheared veins. Syntectonic quartz-calciteveins infill the shear fractures, and these themselves have been sheared. The deformationof the veins was not a simple shear process but one with triclinic flow symmetry. This isinferred from discordance between the shear direction and the near-vertical principleextension direction that is revealed by the pattern and symmetry of quartz and calcite CPOfabrics.</p> <p>After the shears move away from the ramp-step, grain boundary sliding (GBS)accommodated by solid-state diffusion creep is inferred to have affected quartz veins.</p> <p>This deformation mechanism takes place because of 1) the small 8 μm grain size inheritedfrom Phase 1; 2) the presence of fluid in the shear zone; and 3) a stress drop to ~22 MPathat followed the initial up-ramping. Quartz CPO fabrics in the sheared veins areremarkably weak considering their large shear strains. GBS is inferred to have been achief deformation mechanism that caused the weakening of quartz CPO fabrics in thehighly sheared sections of deformed veins. Calcite has also affected the quartz fabricstrength as those veins containing >5% calcite have very weak quartz CPO fabrics. Incontrast to quartz, the CPO fabrics for the co-existing calcite remained strong andcontinued to develop by dislocation creep.</p> <p>The third phase of deformation, a process that may have contributed to subsequentweakening of quartz CPO fabrics, was recrystallisation and grain growth to 126 μm and anequigranular-polygonal grain shape fabric. This fabric was overprinted by late-stagedislocation creep microstructures in the fourth deformation phase in response increaseddifferential stress encountered by the rocks at lower temperatures in the upper crust. Thefinal phase of deformation to affect the sheared veins was semibrittle deformation atdifferential stresses of <189 MPa and temperatures of 200–280oC as the rocks passedthrough the steady-state brittle-ductile transition zone at depths of 8–10 km before beingexhumed at the surface.</p>

scholarly journals Evolution of Quartz and Calcite Microstructures Exhumed From Deep Brittle-Ductile Shear Zones in the Southern Alps of New Zealand

2021 ◽  
Author(s):  
◽  
Matthew P Hill
Keyword(s):  
New Zealand ◽  
Grain Boundary ◽  
Shear Zones ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Differential Stress ◽  
Alpine Fault ◽  
Ductile Shearing ◽  
Boundary Sliding ◽  
Ductile Shear

<p><b>Arrays of brittle-ductile shears exposed in the Southern Alps of New Zealand, haveprovided a superb natural laboratory for insight into the microstructural evolution of lowercrustal shear zones during exhumation. Shears are exposed in the central section of theSouthern Alps at Sam Peak, Chancellor Ridge, and Baumann Glacier in a zone ~2 kmwide that is located 6–8 km structurally above the Alpine Fault. An array ofsystematically spaced shear zones that formed by embrittlement and faulting ofquartzofeldspathic schist took place at the same time as ductile shearing of quartzcarbonateveins embedded within the schist. This study has used field-based structuralmapping along with optical microscopy and universal stage measurements ofcrystallographic preferred orientations (CPO) to resolve the shear zone kinematics andrheology. On the basis of these data, the strain path can be reconstructed for the shearedveins during their progressive deformation. This began with their incidence as backshearsat the base of the Alpine Fault ramp and ended with their subsequent recrystallisation,uplift, and exhumation.</b></p> <p>The near-vertical shear planes have mean orientation of 221@89 NW ± 1o (n =780). They are inferred to have formed as backshears accommodating uplift of the PacificPlate as it was translated onto the oblique footwall ramp of the Alpine Fault during lateCenozoic oblique convergence. Detailed fault offset transect surveys across the shears atChancellor Ridge and Baumann Glacier reveal a mean spacing between the shear zones of25 ± 5 cm (n = 410). Quartz-carbonate marker veins are displaced in a dextral west-sideupshear sense. Fault offset geometry and a consistent arrangement of mineral fibrelineations that decorate fault surfaces, indicate that the mean displacement vector pitches35o SW in the shear plane (trend and plunge of: 262, 35 ± 7o). Ductilely deformed markerveins have been subject to a mean displacement of 9.9 ± 1.4 cm (n = 344) and a meanfinite ductile shear strain of 4.8 ± 0.3 (n = 219). A strain-rate for the ductile deformationof the veins is estimated at 3 x 10-11 sec-1 based on the observed finite ductile shear strain,an escalator kinematic model, and assumptions about the width of the deforming zone.</p> <p>Five deformation phases have affected the sheared veins during their transport upthe fault ramp: 1) initial brittle faulting and ductile shearing; 2) grain boundary sliding ofmylonitic quartz in response to a post-ramping differential stress drop; 3) recrystallisationand grain growth; 4) renewed late-stage dislocation creep; and 5) semibrittle deformationand exhumation. In the schist, the shears initiated as planar brittle faults at lower crustal depths of~21 km at a temperature of 450 ± 50oC. They developed in a zone of transiently highshear strain-rates near the base of the Alpine Fault ramp. Dislocation creep caused a CPOof quartz and calcite to develop in sheared veins. Using the flow law of Hirth et al. (2001)and the estimated strain-rate, a differential stress of ~165 MPa is inferred for ductiledeformation of the veins. Near-lithostatic (λ = 0.85) fluid pressures would have causedthe rocks to undergo brittle failure, a situation that is confirmed by a late component ofbrittle deformation that over prints the ductilely sheared veins. Syntectonic quartz-calciteveins infill the shear fractures, and these themselves have been sheared. The deformationof the veins was not a simple shear process but one with triclinic flow symmetry. This isinferred from discordance between the shear direction and the near-vertical principleextension direction that is revealed by the pattern and symmetry of quartz and calcite CPOfabrics.</p> <p>After the shears move away from the ramp-step, grain boundary sliding (GBS)accommodated by solid-state diffusion creep is inferred to have affected quartz veins.</p> <p>This deformation mechanism takes place because of 1) the small 8 μm grain size inheritedfrom Phase 1; 2) the presence of fluid in the shear zone; and 3) a stress drop to ~22 MPathat followed the initial up-ramping. Quartz CPO fabrics in the sheared veins areremarkably weak considering their large shear strains. GBS is inferred to have been achief deformation mechanism that caused the weakening of quartz CPO fabrics in thehighly sheared sections of deformed veins. Calcite has also affected the quartz fabricstrength as those veins containing >5% calcite have very weak quartz CPO fabrics. Incontrast to quartz, the CPO fabrics for the co-existing calcite remained strong andcontinued to develop by dislocation creep.</p> <p>The third phase of deformation, a process that may have contributed to subsequentweakening of quartz CPO fabrics, was recrystallisation and grain growth to 126 μm and anequigranular-polygonal grain shape fabric. This fabric was overprinted by late-stagedislocation creep microstructures in the fourth deformation phase in response increaseddifferential stress encountered by the rocks at lower temperatures in the upper crust. Thefinal phase of deformation to affect the sheared veins was semibrittle deformation atdifferential stresses of <189 MPa and temperatures of 200–280oC as the rocks passedthrough the steady-state brittle-ductile transition zone at depths of 8–10 km before beingexhumed at the surface.</p>

scholarly journals Syn-kinematic hydration reactions, dissolution-precipitation creep and grain boundary sliding in experimentally deformed plagioclase-pyroxene mixtures

2018 ◽  
Author(s):  
Sina Marti ◽  
Holger Stünitz ◽  
Renée Heilbronner ◽  
Oliver Plümper ◽  
Rüdiger Kilian
Keyword(s):  
Grain Boundary ◽  
Confining Pressure ◽  
Shear Zones ◽  
Grain Boundary Sliding ◽  
Deformation Mechanisms ◽  
Dislocation Creep ◽  
Crystallographic Preferred Orientation ◽  
Mafic Rocks ◽  
Boundary Sliding ◽  
Crustal Shear Zones

Abstract. While it is widely observed that mafic rocks are able to exeprience high strains by viscous flow, details on their rheology and deformation mechanisms are poorly constrained. Here, 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. 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, non-random 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 on understanding and modelling of mid-crustal rheology and flow.

scholarly journals The role of grain size evolution in the rheology of ice: implications for reconciling laboratory creep data and the Glen flow law

2021 ◽  
Vol 15 (9) ◽  
pp. 4589-4605
Author(s):  
Mark D. Behn ◽  
David L. Goldsby ◽  
Greg Hirth
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Stress Exponent ◽  
Ice Sheets ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Ice Flow ◽  
Size Evolution ◽  
Boundary Sliding ◽  
Flow Law

Abstract. Viscous flow in ice is often described by the Glen flow law – a non-Newtonian, power-law relationship between stress and strain rate with a stress exponent n ∼ 3. The Glen law is attributed to grain-size-insensitive dislocation creep; however, laboratory and field studies demonstrate that deformation in ice can be strongly dependent on grain size. This has led to the hypothesis that at sufficiently low stresses, ice flow is controlled by grain boundary sliding, which explicitly incorporates the grain size dependence of ice rheology. Experimental studies find that neither dislocation creep (n ∼ 4) nor grain boundary sliding (n ∼ 1.8) have stress exponents that match the value of n ∼ 3 in the Glen law. Thus, although the Glen law provides an approximate description of ice flow in glaciers and ice sheets, its functional form is not explained by a single deformation mechanism. Here we seek to understand the origin of the n ∼ 3 dependence of the Glen law by using the “wattmeter” to model grain size evolution in ice. The wattmeter posits that grain size is controlled by a balance between the mechanical work required for grain growth and dynamic grain size reduction. Using the wattmeter, we calculate grain size evolution in two end-member cases: (1) a 1-D shear zone and (2) as a function of depth within an ice sheet. Calculated grain sizes match both laboratory data and ice core observations for the interior of ice sheets. Finally, we show that variations in grain size with deformation conditions result in an effective stress exponent intermediate between grain boundary sliding and dislocation creep, which is consistent with a value of n = 3 ± 0.5 over the range of strain rates found in most natural systems.

Grain Boundary Sliding during Low Temperature Creep of Ultrafine and Coarse Grained Aluminum

2012 ◽  
Vol 735 ◽  
pp. 17-21
Author(s):  
Eiichi Sato ◽  
Kaoru Ishiwata ◽  
Tetsuya Matsunaga
Keyword(s):  
Grain Boundary ◽  
Cell Structure ◽  
Grain Boundary Sliding ◽  
Coarse Grained ◽  
Dislocation Creep ◽  
Fcc Metals ◽  
Boundary Sliding ◽  
The Difference ◽  
Low Temperature Creep ◽  
Structure Lead

HCP metals show new dislocation creep at temperatures below 0.3 Tm with stresses below σ0.2, while FCC metals show it above σ0.2. In the former, grain boundaries absorb the dislocations through slip-induced grain-boundary sliding, while in the latter dislocations are accommodated by cross slip at cell walls. The difference comes from the difference in the crystal symmetry. In UFG-Al at low temperatures, it is anticipated that grains without cell structure lead creep deformation similar to CG HCP metals rather than CG Al. UFG Al specimens were fabricated by ARB method. They showed remarkable creep behavior at less than σ0.2 similary to CG HCP metals. It posseses stress exponent of about three, grain-size exponent of almost zero, and very low apparent activation energy of 20 kJ/mol, and also grain boundary sliding behavior is obserbed by AFM.

Reaction-enhanced ductile deformation during carbonation of serpentinized peridotite

2021 ◽  
Author(s):  
Manuel D. Menzel ◽  
Janos L. Urai ◽  
Peter B. Kelemen ◽  
Greg Hirth ◽  
Alexander Schwedt ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Preferred Orientation ◽  
Ductile Deformation ◽  
Dislocation Creep ◽  
Crystallographic Preferred Orientation ◽  
Serpentinized Peridotite ◽  
Boundary Sliding ◽  
Penetrative Foliation ◽  
Drilling Project

&lt;p&gt;Carbonated serpentinites record carbon fluxes in subduction zones and are a possible natural analogue for carbon capture and storage via mineralization, but the processes by which the reaction of serpentinite to listvenite (magnesite-quartz rocks) goes to completion are not well understood. Large-scale hydration and carbonation of peridotite in the Oman Ophiolite produced massive listvenites, which have been drilled by the ICDP Oman Drilling Project (OmDP, site BT1) [1]. Here we report evidence for localized ductile deformation during serpentinite carbonation in core BT1B, based on observations from optical microscopy, cathodoluminescence microscopy, SEM, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in segments of the core that lack a brittle overprint after listvenite formation [2].&lt;/p&gt;&lt;p&gt;Microstructural analysis of the serpentinized peridotite protolith shows a range of microstructures common in serpentinite with local ductile deformation manifested by a shape and crystallographic preferred orientation and kinking of lizardite. Listvenites with ductile deformation microstructures contain a penetrative foliation due to a shape preferred alignment of magnesite spheroids and/or dendritic magnesite, bending around Cr-spinel porphyroclasts. Locally the foliation can be due to aligned dendritic overgrowths on euhedral magnesite grains. Magnesite grains have a weak but consistent crystallographic preferred orientation with the c-axis perpendicular to the foliation, and show high internal misorientations. Locally, the microcrystalline quartz matrix also shows a crystallographic preferred orientation with the c-axes preferentially oriented parallel to the foliation. Folding and ductile transposition of early magnesite veins indicates that carbonation initiated before the ductile deformation stage recorded in listvenites with penetrative foliation. On the other hand, dendritic magnesite overgrowths on folded veins and truncated vein tips suggest that folding likely occurred before complete carbonation, when some serpentine was still present. TEM analysis of magnesite revealed that subgrain boundaries oriented at high angle to the foliation can consist of nano-cracks sealed by inclusion-free magnesite precipitates. High dislocation densities are not evident suggesting that dislocation creep was minor or negligible, in agreement with very low predicted strain rates for magnesite dislocation creep at the low temperatures (100 &amp;#8211; 200 &amp;#176;C) of serpentinite carbonation. This points to dissolution-precipitation, possibly in addition to grain boundary sliding, as the main mechanism for the formation of the shape preferred orientation of magnesite. The weak magnesite crystallographic preferred orientation may be explained by a combination of initial growth competition in an anisotropic (sheared) serpentine medium with subsequent preferred dissolution of smaller, less favorably oriented grains. We infer that transient lithostatic pore pressures during listvenite formation promoted ductile deformation in the reacting medium through grain boundary sliding accommodated by dilatant granular flow and dissolution-precipitation. Because the reaction product listvenite is stronger than the reacting mass, deformation may be preferentially partitioned in the reacting mass, locally enhancing transient fluid flow and, thus, the carbonation reaction progress.&lt;/p&gt;&lt;p&gt;[1] Kelemen et al., 2020. Site BT1: fluid and mass exchange on a subduction zone plate boundary. In: Proceedings of the Oman Drilling Project: College Station, TX&lt;/p&gt;&lt;p&gt;[2] Menzel et al., 2020, JGR Solid Earth 125(10)&lt;/p&gt;

Mechanisms-based constitutive equations for the superplastic behaviour of a titanium alloy

1996 ◽  
Vol 31 (3) ◽  
pp. 187-196 ◽  
Author(s):  
M Zhou ◽  
F P E Dunne
Keyword(s):  
Titanium Alloy ◽  
Grain Boundary ◽  
Grain Growth ◽  
Constitutive Equations ◽  
Grain Boundary Sliding ◽  
Deformation Mechanisms ◽  
Diffusional Creep ◽  
Dislocation Creep ◽  
Boundary Sliding

Mechanisms-based constitutive equations are proposed for the high-temperature behaviour of a class of titanium alloys, for which the deformation mechanisms include diffusional creep, grain boundary sliding, dislocation creep and grain growth. A computational procedure has been developed for the determination of the constitutive equations from a material database. The constitutive equations and the procedure for their determination have been validated by modelling the behaviour of the titanium alloy Ti-6Al-4V at 927°C. It is shown that the procedure developed for the determination of the mechanisms-based constitutive equations can be used to identify the important deformation mechanisms in operation for particular stress, temperature and strain rate conditions. For the case of the Ti-6Al-4V material, the procedure developed correctly predicts the material hardening due to grain growth and indicates that an additional hardening mechanism operates. In addition, the procedure is able to identify grain boundary sliding as a predominant deformation mechanism. The constitutive equations, which are generic in nature, and the procedure for their determination are applicable over a range of materials and are suitable for modelling the macroscopic and the important microscopic aspects of material behaviour during processing. The equations may be readily determined using the procedure presented, which is highly suitable for development as an expert system, to completely automate the process.

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