Robyn Gardner | Macquarie University (original) (raw)

Papers by Robyn Gardner

Research paper thumbnail of The critical role of deformation-assisted melt migration in the formation of oceanic core complexes: mineral chemistry

Research paper thumbnail of Tectonic evolution and continental fragmentation of the southern West Australian margin

Research paper thumbnail of The critical role of deformation-assisted melt migration in the formation of oceanic core complexes

Australian Journal of Earth Sciences, Oct 8, 2023

Research paper thumbnail of The Effects of viscosity on the formation of boudins : comparison of simulation results with field data from Fiordland, New Zealand

Research paper thumbnail of Characterising rocks dredged from the Gulden Draak microcontinent

Empirical thesis."GEOS899 Research Project" --Bibliography: pages 81-84.Chapter 1. Intr... more Empirical thesis."GEOS899 Research Project" --Bibliography: pages 81-84.Chapter 1. Introduction -- Chapter 2. Regional geology -- Chapter 3. Fieldwork -- Chapter 4. Petrography -- Chapter 5. Mineral chemistry -- Chapter 6. Major & minor elements -- Chapter 7. Zircon morphology and composition -- Chapter 8. U-Pb dating of zircons -- Chapter 9. Hafnium isotopes -- Chapter 10. Conclusions.Gulden Draak is a bathymetric feature 2-3 kilometres below the surface of the Indian Ocean 2,000 kilometres west of Perth, Western Australia. Four samples were selected from the continental rocks dredged from the knoll, three orthogneisses and one paragneiss. Minerals and textures indicate the samples were subjected to amphibolite to granulite facies metamorphism in multiple metamorphic events. U-Pb and Lu-Hf isotope analysis shows the samples have undergone a complex history of emplacement, metamorphism and partial remelting. The orthogneisses were originally emplaced prior to 2800 Ma, 1290...

Research paper thumbnail of The effects of a second phase on rates of grain boundary diffusion creep and grain boundary migration

EGU General Assembly Conference Abstracts, Apr 1, 2018

Research paper thumbnail of Development of William’s Ridge, Kerguelen Plateau and Broken Ridge: tectonics, hotspot magmatism, microcontinents, and Australia’s Extended Continental Shelf

EPIC3Australian Earth Science Convention, Australia, 2021-02-09-2021-02-12, Feb 1, 2021

William’s Ridge, a ~300-km-long salient extending southeast from the Central Kerguelen Plateau, a... more William’s Ridge, a ~300-km-long salient extending southeast from the Central Kerguelen Plateau, and Broken Ridge are conjugate divergent margins in the southern Indian Ocean that separated at ~43 Ma. In early 2020, scientists aboard Australia’s Marine National Facility, RV Investigator, acquired multichannel seismic reflection (MCS), sub-bottom profiling, multibeam bathymetry, and gravity data on these margins, as well as dredged rock samples, on a 57-day voyage. The research project constitutes the first-ever case study of conjugate oceanic plateau end-member tectonic plates, with the goal of advancing knowledge of lithospheric rifting, breakup, and initial plate separation processes. The first-ever dedicated multibeam mapping of William’s and Broken ridges encompassed ~52,000 km2 and ~43,000 km2, respectively. Four new RV Investigator MCS profiles (500 line-km) across William’s Ridge complement one legacy RV Rig Seismic and three new RV Sonne MCS profiles; five new RV Investigator MCS profiles (603 line-km) across the conjugate portion of Broken Ridge are the first to be acquired on that feature. Multibeam bathymetry and MCS transects of William’s Ridge show multiple linear ridges and troughs interpreted as horst and graben. In contrast, multibeam bathymetry and MCS transects of Broken Ridge show a prominent E-W scarp (Diamantina Escarpment) with a complex morphology of emanating en echelon crustal blocks and depressions at the base of the scarp. Prominent angular unconformities (middle Eocene hiatus?) characterize the sedimentary section on some ridges, and dipping reflection sequences within interpreted igneous basement suggest subaerial basalt flows. Rock dredges on the facing conjugate margin fault scarps targeted all stratigraphic levels exposing basement rocks. Nine on William’s Ridge yielded both oceanic and (in situ?) continental rocks; eight on Broken Ridge yielded solely oceanic rocks. The new geophysical data and geological samples may justify a new or revised submission to the United Nations Commission on the Limits of the Continental Shelf to extend Australia’s marine jurisdiction on and around William’s Ridge under the United Nations Convention on the Law of the Sea

Research paper thumbnail of Data for: Microstructures reveal multistage melt present strain localisation in mid-ocean gabbros

Electron microprobe mineral data for sample 148R2 at 953.7 mbsf from IODP expedition 176 to the A... more Electron microprobe mineral data for sample 148R2 at 953.7 mbsf from IODP expedition 176 to the Atlantis Bank oceanic core complex, South West Indian Ridge.

Research paper thumbnail of Comprehensive Automated Thin-Section Characterization Combined with Quantitative Major-Trace Element Analysis on a Single SEM

Microscopy and Microanalysis, 2021

An analytically equipped scanning electron microscope (SEM) is an exceptionally powerful tool, ab... more An analytically equipped scanning electron microscope (SEM) is an exceptionally powerful tool, able to bring together information from multiple techniques to enable in-depth characterization of materials. We usually have a specific set of analytical requirements for geological samples that can be addressed by a combination of in-situ approaches in a SEM. These requirements often lead to varying levels and types of information being obtained from different parts of a thin section. There is frequently a need to understand how mineral composition/abundance varies over the scale of a thin section and the specific chemistry of the minerals present (including high sensitivity measurements of trace element concentrations/compositional variations). It may also be desirable to know the textural/crystalline properties of the rock to make links between what is observed compositionally and the physical processes that have affected it during its history. These requirements often need analyses covering large proportions of the sample(s), so that a statistically representative dataset can be generated.

Research paper thumbnail of A review of numerical modelling of the dynamics of microstructural development in rocks and ice: Past, present and future

Journal of Structural Geology, 2018

This review provides an overview of the emergence and current status of numerical modelling of mi... more This review provides an overview of the emergence and current status of numerical modelling of microstructures, a powerful tool for predicting the dynamic behaviour of rocks and ice at the microscale with consequence for the evolution of these materials at a larger scale. We emphasize the general philosophy behind such numerical models and their application to important geological phenomena such as dynamic recrystallization and strain localization. We focus in particular on the dynamics that emerge when multiple processes, which may either be enhancing or competing with each other, are simultaneously active. Here, the ability to track the evolving microstructure is a particular advantage of numerical modelling. We highlight advances through time and provide glimpses into future opportunities and challenges.

Research paper thumbnail of Micro-continents offshore Western Australia: implications for East Gondwana reconstructions

Research paper thumbnail of Discovery of a microcontinent (Gulden Draak Knoll) offshore Western Australia: Implications for East Gondwana reconstructions

Gondwana Research, 2015

Analysis of dredged samples from the Gulden Draak Knoll demonstrates that it is a submarine rifte... more Analysis of dredged samples from the Gulden Draak Knoll demonstrates that it is a submarine rifted continental fragment that lies at the boundary between the western Perth Abyssal Plain and Wharton Basin, Indian Ocean. The Knoll comprises a granulite facies basement, including pelitic paragneiss and mafic orthogneiss, with a Cambrian granite inferred to intrude the other rocks. Boulders and cobbles of felsic gneiss with Mesoproterozoic and Cambrian protolith ages were also sampled likely reflecting a complex basement to variable sedimentary and volcanic rocks. The U-Pb isotopic system in the Archean and Mesoproterozoic zircon is significantly disturbed, reflecting Cambrian orogenesis that affected all samples. The protolith to garnet-sillimanite-biotite paragneiss has a maximum deposition age of 1163 ± 24 Ma and includes older detrital zircon grains with populations at c. 2.65 Ga and between 1.4 and 1.1 Ga. A younger population in this sample is interpreted as a mix of newly grown metamorphic zircon and isotopically reset zircon, implying that the granulite facies metamorphism occurred at c. 511 ± 5 Ma. Protracted Cambrian orogenesis is indicated by a metamorphic age in the mafic orthogneiss of 530 ± 6 Ma and isotopic disturbance shortly following emplacement of granite (c. 540 Ma with zircon ages disturbed to 509 ± 7 Ma) and the protolith to the felsic orthogneiss (c. 528 Ma with zircon ages disturbed to 510 ± 3 Ma). Xenocrystic zircon grains in the Cambrian rocks include Archean (c. 2839 ± 9 Ma) and Mesoproterozoic (1230-1370 Ma) populations also isotopically disturbed during Cambrian orogenesis. Igneous Cambrian zircon grains have less radiogenic Hf-isotope compositions (Hf i = 0.281821-0.281367) than Mesoproterozoic xenocrysts (Hf i = 0.282267-0.281993), indicating limited involvement of the Mesoproterozoic crust in granite production. A more likely source includes Archean crust represented by xenocrysts with Hf i = 0.281399-0.280863. The Gulden Draak Knoll is reconstructed in Gondwana ('Leeuwin' full-fit model) along strike of a major structure termed the Indo-Australo-Antarctic Suture (IAAS), recently mapped from geophysical interpretations in Wilkes Land, Antarctica. New isotopic data suggest that basement rocks from the Gulden Draak Knoll have affinity to crust exposed either side of the IAAS. Determining if this structure is a suture zone sensu stricto remains to be tested.

Research paper thumbnail of Oxide enrichment by syntectonic melt-rock interaction

Research paper thumbnail of Ductile deformation without localization supplementary codes

This data includes the Elle geometry files, the Elle control script, Basil boundary conditions an... more This data includes the Elle geometry files, the Elle control script, Basil boundary conditions and the C++ code used for the stress threshold test in "Ductile deformation without localization: Insights from numerical modelling".<br>The Elle and Basil code can be installed from: <br>http://www.microstructure.info/elle<br&gt;

Research paper thumbnail of Pinch and swell structures: evidence for strain localisation by brittle–viscous behaviour in the middle crust

Research paper thumbnail of What can asymmetry tell us? Investigation of asymmetric versus symmetric pinch and swell structures in nature and simulation

Research paper thumbnail of Pinch and swell structures: evidence for brittle-viscous behaviour in the middle crust

Solid Earth Discussions, 2015

The flow properties of middle to lower crustal rocks are commonly represented by viscous flow. Ho... more The flow properties of middle to lower crustal rocks are commonly represented by viscous flow. However, examples of pinch and swell structures found in a mid-crustal high strain zone at St. Anne Point (Fiordland, New Zealand) suggest pinch and swell structures are initiated by brittle failure of the more competent layer in conjunction with material softening. On this basis we develop a flexible numerical model using brittle-viscous flow where Mohr–Coulomb failure is utilised to initiate pinch and swell structure development. Results show that pinch and swell structures develop in a competent layer in both Newtonian and non-Newtonian flow provided the competent layer has enough viscosity contrast and initially fails brittlely. The degree of material softening after initial failure is shown to impact pinch and swell characteristics with high rates of material softening causing the formation of thick necks between swells by limiting the successful localisation of strain. The flow regim...

Research paper thumbnail of Strain localisation during diffusion creep: influence of grain coarsening and grain boundary sliding

&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;... more &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Diffusion creep and the wet low temperature version, pressure solution, are major deformation mechanisms in the Earth. Pressure solution operates in many metamorphosing systems in the crust and may contribute to slow creep on fault surfaces. Diffusion creep prevails in areas of the upper mantle deforming slowly, and possibly in most of the lower mantle. Both mechanisms contribute to localisation since small grain sizes can deform faster.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;However, there has been limited attention paid to the evolution of microstructure during diffusion creep. In some experiments grains coarsen; in some but not all experiments grains remain rather equant. We have developed a grain-scale numerical model for diffusion creep, which indicates that those processes are very important in influencing evolving strength. Our models illustrate three behaviours.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;ol&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Strain localises along slip surfaces formed by aligned grain boundaries on all scales. This affects overall strength.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Diffusion creep is predicted to produce elongate grains and then the overall aggregate has intense mechanical anisotropy. Thus strength during diffusion creep, and localisation on weak zones, is influenced not just by grain size but by other aspects of microstructure.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Grain coarsening increases grain size and strength. Our most recent work shows how it interacts with ongoing deformation. In particular grain growth can lead to particular grain shapes which are directly related to strain rate, and influence strength. Consequently, understanding localisation during diffusion creep must encompass the effects of diffusion itself, grain boundary sliding and grain coarsening.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/ol&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;

Research paper thumbnail of Ductile deformation without localization: Insights from numerical modelling

Geochemistry, Geophysics, Geosystems

Research paper thumbnail of Determining relative bulk viscosity of kilometre-scale crustal units using field observations and numerical modelling

Tectonophysics

This is a repository copy of Determining relative bulk viscosity of kilometre-scale crustal units... more This is a repository copy of Determining relative bulk viscosity of kilometre-scale crustal units using field observations and numerical modelling.

Research paper thumbnail of The critical role of deformation-assisted melt migration in the formation of oceanic core complexes: mineral chemistry

Research paper thumbnail of Tectonic evolution and continental fragmentation of the southern West Australian margin

Research paper thumbnail of The critical role of deformation-assisted melt migration in the formation of oceanic core complexes

Australian Journal of Earth Sciences, Oct 8, 2023

Research paper thumbnail of The Effects of viscosity on the formation of boudins : comparison of simulation results with field data from Fiordland, New Zealand

Research paper thumbnail of Characterising rocks dredged from the Gulden Draak microcontinent

Empirical thesis."GEOS899 Research Project" --Bibliography: pages 81-84.Chapter 1. Intr... more Empirical thesis."GEOS899 Research Project" --Bibliography: pages 81-84.Chapter 1. Introduction -- Chapter 2. Regional geology -- Chapter 3. Fieldwork -- Chapter 4. Petrography -- Chapter 5. Mineral chemistry -- Chapter 6. Major & minor elements -- Chapter 7. Zircon morphology and composition -- Chapter 8. U-Pb dating of zircons -- Chapter 9. Hafnium isotopes -- Chapter 10. Conclusions.Gulden Draak is a bathymetric feature 2-3 kilometres below the surface of the Indian Ocean 2,000 kilometres west of Perth, Western Australia. Four samples were selected from the continental rocks dredged from the knoll, three orthogneisses and one paragneiss. Minerals and textures indicate the samples were subjected to amphibolite to granulite facies metamorphism in multiple metamorphic events. U-Pb and Lu-Hf isotope analysis shows the samples have undergone a complex history of emplacement, metamorphism and partial remelting. The orthogneisses were originally emplaced prior to 2800 Ma, 1290...

Research paper thumbnail of The effects of a second phase on rates of grain boundary diffusion creep and grain boundary migration

EGU General Assembly Conference Abstracts, Apr 1, 2018

Research paper thumbnail of Development of William’s Ridge, Kerguelen Plateau and Broken Ridge: tectonics, hotspot magmatism, microcontinents, and Australia’s Extended Continental Shelf

EPIC3Australian Earth Science Convention, Australia, 2021-02-09-2021-02-12, Feb 1, 2021

William’s Ridge, a ~300-km-long salient extending southeast from the Central Kerguelen Plateau, a... more William’s Ridge, a ~300-km-long salient extending southeast from the Central Kerguelen Plateau, and Broken Ridge are conjugate divergent margins in the southern Indian Ocean that separated at ~43 Ma. In early 2020, scientists aboard Australia’s Marine National Facility, RV Investigator, acquired multichannel seismic reflection (MCS), sub-bottom profiling, multibeam bathymetry, and gravity data on these margins, as well as dredged rock samples, on a 57-day voyage. The research project constitutes the first-ever case study of conjugate oceanic plateau end-member tectonic plates, with the goal of advancing knowledge of lithospheric rifting, breakup, and initial plate separation processes. The first-ever dedicated multibeam mapping of William’s and Broken ridges encompassed ~52,000 km2 and ~43,000 km2, respectively. Four new RV Investigator MCS profiles (500 line-km) across William’s Ridge complement one legacy RV Rig Seismic and three new RV Sonne MCS profiles; five new RV Investigator MCS profiles (603 line-km) across the conjugate portion of Broken Ridge are the first to be acquired on that feature. Multibeam bathymetry and MCS transects of William’s Ridge show multiple linear ridges and troughs interpreted as horst and graben. In contrast, multibeam bathymetry and MCS transects of Broken Ridge show a prominent E-W scarp (Diamantina Escarpment) with a complex morphology of emanating en echelon crustal blocks and depressions at the base of the scarp. Prominent angular unconformities (middle Eocene hiatus?) characterize the sedimentary section on some ridges, and dipping reflection sequences within interpreted igneous basement suggest subaerial basalt flows. Rock dredges on the facing conjugate margin fault scarps targeted all stratigraphic levels exposing basement rocks. Nine on William’s Ridge yielded both oceanic and (in situ?) continental rocks; eight on Broken Ridge yielded solely oceanic rocks. The new geophysical data and geological samples may justify a new or revised submission to the United Nations Commission on the Limits of the Continental Shelf to extend Australia’s marine jurisdiction on and around William’s Ridge under the United Nations Convention on the Law of the Sea

Research paper thumbnail of Data for: Microstructures reveal multistage melt present strain localisation in mid-ocean gabbros

Electron microprobe mineral data for sample 148R2 at 953.7 mbsf from IODP expedition 176 to the A... more Electron microprobe mineral data for sample 148R2 at 953.7 mbsf from IODP expedition 176 to the Atlantis Bank oceanic core complex, South West Indian Ridge.

Research paper thumbnail of Comprehensive Automated Thin-Section Characterization Combined with Quantitative Major-Trace Element Analysis on a Single SEM

Microscopy and Microanalysis, 2021

An analytically equipped scanning electron microscope (SEM) is an exceptionally powerful tool, ab... more An analytically equipped scanning electron microscope (SEM) is an exceptionally powerful tool, able to bring together information from multiple techniques to enable in-depth characterization of materials. We usually have a specific set of analytical requirements for geological samples that can be addressed by a combination of in-situ approaches in a SEM. These requirements often lead to varying levels and types of information being obtained from different parts of a thin section. There is frequently a need to understand how mineral composition/abundance varies over the scale of a thin section and the specific chemistry of the minerals present (including high sensitivity measurements of trace element concentrations/compositional variations). It may also be desirable to know the textural/crystalline properties of the rock to make links between what is observed compositionally and the physical processes that have affected it during its history. These requirements often need analyses covering large proportions of the sample(s), so that a statistically representative dataset can be generated.

Research paper thumbnail of A review of numerical modelling of the dynamics of microstructural development in rocks and ice: Past, present and future

Journal of Structural Geology, 2018

This review provides an overview of the emergence and current status of numerical modelling of mi... more This review provides an overview of the emergence and current status of numerical modelling of microstructures, a powerful tool for predicting the dynamic behaviour of rocks and ice at the microscale with consequence for the evolution of these materials at a larger scale. We emphasize the general philosophy behind such numerical models and their application to important geological phenomena such as dynamic recrystallization and strain localization. We focus in particular on the dynamics that emerge when multiple processes, which may either be enhancing or competing with each other, are simultaneously active. Here, the ability to track the evolving microstructure is a particular advantage of numerical modelling. We highlight advances through time and provide glimpses into future opportunities and challenges.

Research paper thumbnail of Micro-continents offshore Western Australia: implications for East Gondwana reconstructions

Research paper thumbnail of Discovery of a microcontinent (Gulden Draak Knoll) offshore Western Australia: Implications for East Gondwana reconstructions

Gondwana Research, 2015

Analysis of dredged samples from the Gulden Draak Knoll demonstrates that it is a submarine rifte... more Analysis of dredged samples from the Gulden Draak Knoll demonstrates that it is a submarine rifted continental fragment that lies at the boundary between the western Perth Abyssal Plain and Wharton Basin, Indian Ocean. The Knoll comprises a granulite facies basement, including pelitic paragneiss and mafic orthogneiss, with a Cambrian granite inferred to intrude the other rocks. Boulders and cobbles of felsic gneiss with Mesoproterozoic and Cambrian protolith ages were also sampled likely reflecting a complex basement to variable sedimentary and volcanic rocks. The U-Pb isotopic system in the Archean and Mesoproterozoic zircon is significantly disturbed, reflecting Cambrian orogenesis that affected all samples. The protolith to garnet-sillimanite-biotite paragneiss has a maximum deposition age of 1163 ± 24 Ma and includes older detrital zircon grains with populations at c. 2.65 Ga and between 1.4 and 1.1 Ga. A younger population in this sample is interpreted as a mix of newly grown metamorphic zircon and isotopically reset zircon, implying that the granulite facies metamorphism occurred at c. 511 ± 5 Ma. Protracted Cambrian orogenesis is indicated by a metamorphic age in the mafic orthogneiss of 530 ± 6 Ma and isotopic disturbance shortly following emplacement of granite (c. 540 Ma with zircon ages disturbed to 509 ± 7 Ma) and the protolith to the felsic orthogneiss (c. 528 Ma with zircon ages disturbed to 510 ± 3 Ma). Xenocrystic zircon grains in the Cambrian rocks include Archean (c. 2839 ± 9 Ma) and Mesoproterozoic (1230-1370 Ma) populations also isotopically disturbed during Cambrian orogenesis. Igneous Cambrian zircon grains have less radiogenic Hf-isotope compositions (Hf i = 0.281821-0.281367) than Mesoproterozoic xenocrysts (Hf i = 0.282267-0.281993), indicating limited involvement of the Mesoproterozoic crust in granite production. A more likely source includes Archean crust represented by xenocrysts with Hf i = 0.281399-0.280863. The Gulden Draak Knoll is reconstructed in Gondwana ('Leeuwin' full-fit model) along strike of a major structure termed the Indo-Australo-Antarctic Suture (IAAS), recently mapped from geophysical interpretations in Wilkes Land, Antarctica. New isotopic data suggest that basement rocks from the Gulden Draak Knoll have affinity to crust exposed either side of the IAAS. Determining if this structure is a suture zone sensu stricto remains to be tested.

Research paper thumbnail of Oxide enrichment by syntectonic melt-rock interaction

Research paper thumbnail of Ductile deformation without localization supplementary codes

This data includes the Elle geometry files, the Elle control script, Basil boundary conditions an... more This data includes the Elle geometry files, the Elle control script, Basil boundary conditions and the C++ code used for the stress threshold test in "Ductile deformation without localization: Insights from numerical modelling".<br>The Elle and Basil code can be installed from: <br>http://www.microstructure.info/elle<br&gt;

Research paper thumbnail of Pinch and swell structures: evidence for strain localisation by brittle–viscous behaviour in the middle crust

Research paper thumbnail of What can asymmetry tell us? Investigation of asymmetric versus symmetric pinch and swell structures in nature and simulation

Research paper thumbnail of Pinch and swell structures: evidence for brittle-viscous behaviour in the middle crust

Solid Earth Discussions, 2015

The flow properties of middle to lower crustal rocks are commonly represented by viscous flow. Ho... more The flow properties of middle to lower crustal rocks are commonly represented by viscous flow. However, examples of pinch and swell structures found in a mid-crustal high strain zone at St. Anne Point (Fiordland, New Zealand) suggest pinch and swell structures are initiated by brittle failure of the more competent layer in conjunction with material softening. On this basis we develop a flexible numerical model using brittle-viscous flow where Mohr–Coulomb failure is utilised to initiate pinch and swell structure development. Results show that pinch and swell structures develop in a competent layer in both Newtonian and non-Newtonian flow provided the competent layer has enough viscosity contrast and initially fails brittlely. The degree of material softening after initial failure is shown to impact pinch and swell characteristics with high rates of material softening causing the formation of thick necks between swells by limiting the successful localisation of strain. The flow regim...

Research paper thumbnail of Strain localisation during diffusion creep: influence of grain coarsening and grain boundary sliding

&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;... more &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Diffusion creep and the wet low temperature version, pressure solution, are major deformation mechanisms in the Earth. Pressure solution operates in many metamorphosing systems in the crust and may contribute to slow creep on fault surfaces. Diffusion creep prevails in areas of the upper mantle deforming slowly, and possibly in most of the lower mantle. Both mechanisms contribute to localisation since small grain sizes can deform faster.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;However, there has been limited attention paid to the evolution of microstructure during diffusion creep. In some experiments grains coarsen; in some but not all experiments grains remain rather equant. We have developed a grain-scale numerical model for diffusion creep, which indicates that those processes are very important in influencing evolving strength. Our models illustrate three behaviours.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;ol&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Strain localises along slip surfaces formed by aligned grain boundaries on all scales. This affects overall strength.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Diffusion creep is predicted to produce elongate grains and then the overall aggregate has intense mechanical anisotropy. Thus strength during diffusion creep, and localisation on weak zones, is influenced not just by grain size but by other aspects of microstructure.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Grain coarsening increases grain size and strength. Our most recent work shows how it interacts with ongoing deformation. In particular grain growth can lead to particular grain shapes which are directly related to strain rate, and influence strength. Consequently, understanding localisation during diffusion creep must encompass the effects of diffusion itself, grain boundary sliding and grain coarsening.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/li&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/ol&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;

Research paper thumbnail of Ductile deformation without localization: Insights from numerical modelling

Geochemistry, Geophysics, Geosystems

Research paper thumbnail of Determining relative bulk viscosity of kilometre-scale crustal units using field observations and numerical modelling

Tectonophysics

This is a repository copy of Determining relative bulk viscosity of kilometre-scale crustal units... more This is a repository copy of Determining relative bulk viscosity of kilometre-scale crustal units using field observations and numerical modelling.