scholarly journals Creeping gabbro: dissolution-precipitation creep facilitating deformation in mafic rocks

2021 ◽  
Author(s):  
Amicia Lee ◽  
Holger Stünitz ◽  
Mathieu Soret ◽  
Matheus Ariel Battisti ◽  
Jiří Konopásek
Keyword(s):  
Transformation Process ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
General Transformation ◽  
Preferential Growth ◽  
Mafic Rocks ◽  
The North ◽  
Dominant Deformation Mechanism ◽  
Textural Data ◽  
Extension Direction

<p>Mafic rocks consist of strong minerals (e.g. clinopyroxene, plagioclase) that can only be deformed by crystal plastic mechanisms at high temperatures (>800°C). Yet, mafic rocks do show extensive deformation by non-brittle mechanisms when they have only reached lower temperatures (~650°C). In many of such cases, the deformation is accommodated by an interaction of deformation with simultaneous mineral reactions. Here we show that dissolution-precipitation creep plays a major role in deformation of gabbro lenses at mid and upper amphibolite facies conditions.</p><p>The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C, and depths of ∼26-34 km. The Kågen gabbro on south Arnøya is comprised of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and metamorphism from the low strain gabbro lenses to high strain mylonites at margins of the lenses. Microstructural and textural data indicate that dissolution-precipitation creep is the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: new plagioclase and clinopyroxene, amphibole, and garnet. Amphibole shows a strong CPO that is primarily controlled by its preferential growth in the extension direction. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. The weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.</p>

scholarly journals Dissolution precipitation creep as a process for the strain localisation in mafic rocks

2021 ◽  
Author(s):  
Amicia Lee ◽  
Holger Stünitz ◽  
Mathieu Soret ◽  
Matheus Battisti
Keyword(s):  
Transformation Process ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
General Transformation ◽  
Preferential Growth ◽  
Crystallographic Preferred Orientation ◽  
Mafic Rocks ◽  
Dominant Deformation Mechanism ◽  
Lower Temperature ◽  
Extensive Deformation

Unaltered mafic rocks consist of mechanically strong minerals (e.g. pyroxene, plagioclase and garnet) that can be deformed by crystal plastic mechanisms only at high temperatures (>800°C). Yet, many mafic rocks do show extensive deformation by non-brittle mechanisms when they have been subjected to lower temperature conditions. In such cases, the deformation typically is assisted by mineral reactions. Here we show that dissolution-precipitation creep (as a type of diffusion creep) plays a major role in deformation of gabbro lenses at upper amphibolite facies conditions. The Kågen gabbro exposed on south Arnøya is comprised of almost undeformed gabbro lenses with sheared margins wrapping around them. The shearing has taken place at temperatures of 690 ± 25 °C and pressures of 1.0 to 1.1 GPa. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and crystallographic preferred orientation (CPO) data indicate that dissolution-precipitation creep is the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: new plagioclase and clinopyroxene (with different composition), amphibole, and garnet. Amphibole shows a strong CPO that is primarily controlled by its preferential growth in the stretching direction. Synchronous deformation and mineral reactions of clinopyroxene suggest that mafic rocks can become mechanically weak during a general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. The weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation; the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses and low stress exponents.

scholarly journals Dissolution precipitation creep as a process for the strain localisation in gabbro

2020 ◽  
Author(s):  
Amicia Lee ◽  
Holger Stunitz ◽  
Matheus Ariel Battisti ◽  
Jiri Konopasek
Keyword(s):  
Creep Deformation ◽  
Driving Forces ◽  
Transformation Process ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Strain Localisation ◽  
General Transformation ◽  
Active Deformation ◽  
The North ◽  
Extension Direction

<p>Strain localisation and fabric development in the lower crust is controlled by the active deformation mechanisms. Understanding the driving forces of such deformation aids in quantifying the stresses and rates of the deformation processes. Here we show that diffusion creep plays a major role in deformation of gabbro lenses at upper amphibolite facies conditions. The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C (depths of ∼26-34 km). The Kågen gabbro on south Arnøya is made up of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and textural data indicate that preferential crystal growth of amphibole grains in the extension direction has produced the deformation microstructure and the CPO. Dissolution precipitation creep is inferred to be the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: amphibole, garnet and zoisite. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. Deformation and metamorphic reaction were both important transformation processes during diffusion creep deformation of the margins of the gabbro lenses. The weakening is directly connected to a transformation process that facilitates diffusion creep deformation of strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.</p>

scholarly journals Self-Consistent Grain Size Evolution controls Strain Localization during Rifting

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.

Macro- and microstructural analysis of the North Tea Lake Mylonite Zone: an extensional shear zone in the Central Gneiss Belt, Grenville Province, Ontario

2015 ◽  
Vol 52 (11) ◽  
pp. 1027-1044 ◽  
Author(s):  
Nicholas Culshaw ◽  
Christopher Gerbi ◽  
Laura Ratcliffe
Keyword(s):  
Shear Zone ◽  
Microstructural Analysis ◽  
High Stress ◽  
Shear Zones ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
The North ◽  
Mylonite Zone ◽  
Central Gneiss Belt

The North Tea Lake Mylonite Zone is a late extensional ductile fault that is concordant with and has reworked fabrics of the North Tea Lake Shear Zone, the frontal thrust shear zone of the upper amphibolite–granulite facies Kiosk domain within the interior of the Central Gneiss Belt. North Tea Lake Mylonite Zone fabric is an anomalously fine-grained mylonite compared to Central Gneiss Belt gneisses, and consists of three microstructural domains that display progressive recrystallization and grain size refinement of the protolith granitoid. On the basis of petrography and electron backscatter diffraction, these microdomains are inferred to represent a transition from dominantly dislocation creep to diffusion creep and diffusion-accommodated grain boundary sliding at elevated stress (>100 MPa), low fluid activity, and temperatures ∼500 °C. The North Tea Lake Mylonite Zone is interpreted to mark a step in the progressive transition in deformation mode during late- to post-Ottawan extension and cooling of the Grenville orogen from weak, wide, wet, and warm shear zones to Rigolet-phase cooler, narrow, ultrafine, high-stress shear zones reworking dry protoliths.

scholarly journals Subduction of non-Newtonian plates: Thin-sheet dynamics of slab necking and breakoff

2019 ◽  
Author(s):  
Neil M Ribe ◽  
Bingrui Xu
Keyword(s):  
Convergence Rate ◽  
Initial Period ◽  
Thin Sheet ◽  
Boundary Integral ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Characteristic Ratio ◽  
Lithospheric Thinning ◽  
Slab Breakoff ◽  
Stress Resultant

Summary We use a hybrid boundary-integral/thin-sheet (‘BITS’) model to investigate the subduction of 2-D viscous sheets with composite diffusion creep/dislocation creep rheology, with a focus on the conditions required for slab necking and breakoff. To validate the model, we show that its predictions of the sinking speed of the slab follow a universal scaling law identical to one previously derived for purely Newtonian sheets. We obtain analytical expressions for the fiber stress resultant and bending moment of the sheet during deformation by pure stretching and pure bending, respectively, and show that the non-Newtonian dislocation creep component of the rheology significantly weakens the sheet for both types of deformation. We solve the BITS equations for two distinct situations: “free” subduction, in which the slab pulls an attached negatively buoyant plate without hindrance; and “arrested” subduction that slows or stops when a positively buoyant (continental) portion of the attached plate arrives at the trench. Strong lithospheric thinning is difficult to obtain in free subduction and requires dominantly non-Newtonian rheology, i.e. large (>5) values of the characteristic ratio λ of the Newtonian to the non-Newtonian viscosity. However, during arrested subduction strong thinning leading to breakoff occurs for much lower values of λ, and the point of maximum thinning is at shallower depths. These results are explained by a one-dimensional viscous dripping model (‘DRIP’) of a vertical slab with a composite rheology and an arbitrary kinematically prescribed time-dependent convergence rate U0(t). By injecting into DRIP convergence histories predicted by BITS, we find that DRIP reproduces closely the results of the more complicated BITS model. The DRIP model shows that the convergence rate controls slab breakoff in two distinct ways. On the one hand, the rate of lithospheric thinning is proportional to the accumulated convergence, i.e., the time integral of U0(t). The breakoff depth, on the other hand, is controlled by the convergence rate history, being shallower when U0 rapidly decreases (arrested subduction) after an initial period of oceanic subduction.

scholarly journals Ductile saline ice

1994 ◽  
Vol 40 (136) ◽  
pp. 566-568
Author(s):  
G. A. Kuehn ◽  
E. M. Schulson
Keyword(s):  
Fresh Water ◽  
Tensile Ductility ◽  
Compressive Deformation ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Water Ice ◽  
The Matrix ◽  
And Diffusion

AbstractExperiments have shown that tensile ductility of about 5% or more can be imparted to columnar, saline ice by pre-compressing the material by about 3.5%. This effect is similar to that observed in granular, fresh-water ice and is attributed to the operation of both dislocation creep and diffusion creep within that part of the matrix which recrystallized during the pre-compressive deformation.

scholarly journals Myrmekite and strain weakening in granitoid mylonites

2018 ◽  
Author(s):  
Alberto Ceccato ◽  
Luca Menegon ◽  
Giorgio Pennacchioni ◽  
Luiz Fernando Grafulha Morales
Keyword(s):  
Grain Size ◽  
Heterogeneous Nucleation ◽  
Eastern Alps ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Phase Mixing ◽  
Fine Grained ◽  
Fine Grain ◽  
Creep Cavitation

Abstract. At mid-crustal conditions, deformation of feldspar is mainly accomplished by a combination of fracturing, dissolution/precipitation and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we investigate with EBSD the microstructure of a granodiorite mylonite, developed at 420–460 °C during cooling of the Rieserferner pluton (Eastern Alps), to assess the microstructural processes and the role of weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of the plagioclase of pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain size reduction and ordered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws calculated for monomineralic quartz, feldspar, and quartz + plagioclase aggregates (sheared myrmekite), show that during mylonitization at 450 °C, grain-size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the mid-upper continental (felsic) crust.

Dislocation Creep in Al-22.2, 53.6 and 101 at.ppm Fe Solid Solution Alloys

2014 ◽  
Vol 922 ◽  
pp. 749-754
Author(s):  
K. Takeshima ◽  
Tokuteru Uesugi ◽  
Yorinobu Takigawa ◽  
Kenji Higashi
Keyword(s):  
Solid Solution ◽  
High Temperature ◽  
Creep Resistance ◽  
Dislocation Creep ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
Stress Range ◽  
Creep Tests ◽  
Ultra High Purity ◽  
Dominant Deformation Mechanism

Creep tests of ultra-high-purity (99.999%) Al and Al-22.2, 53.6, 101 at.ppm Fe solid solution alloys were conducted at 773 K in the stress range of 2-6 MPa in order to investigate effect of solute Fe on high temperature deformation of Al. Creep resistance was enhanced by addition of Fe in solid solution. The stress exponents of the samples exhibited values of about 5, which indicate that climb-controlled dislocation creep was dominant deformation mechanism. It could be suggested that Fe atoms segregating in dislocations due to the strong interaction between solute Fe atoms and the dislocation enhanced the creep resistance.

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