scholarly journals Microstructure and Crystallographic Preferred Orientations of an Azimuthally Oriented Ice Core from a Lateral Shear Margin: Priestley Glacier, Antarctica

2021 ◽  
Vol 9 ◽  
Author(s):  
Rilee E. Thomas ◽  
Marianne Negrini ◽  
David J. Prior ◽  
Robert Mulvaney ◽  
Holly Still ◽  
...  
Keyword(s):  
Large Scale ◽  
Electron Backscatter Diffraction ◽  
Boundary Migration ◽  
Ice Core ◽  
Vertical Axis ◽  
Structural Heterogeneity ◽  
The Core ◽  
Terra Nova Bay ◽  
Subgrain Rotation ◽  
A Minor

A 58 m long azimuthally oriented ice core has been collected from the floating lateral sinistral shear margin of the lower Priestley Glacier, Terra Nova Bay, Antarctica. The crystallographic preferred orientations (CPO) and microstructures are described in order to correlate the geometry of anisotropy with constrained large-scale kinematics. Cryogenic Electron Backscatter Diffraction analysis shows a very strong fabric (c-axis primary eigenvalue ∼0.9) with c-axes aligned horizontally sub-perpendicular to flow, rotating nearly 40° clockwise (looking down) to the pole to shear throughout the core. The c-axis maximum is sub-perpendicular to vertical layers, with the pole to layering always clockwise of the c-axes. Priestley ice microstructures are defined by largely sub-polygonal grains and constant mean grain sizes with depth. Grain long axis shape preferred orientations (SPO) are almost always 1–20° clockwise of the c-axis maximum. A minor proportion of “oddly” oriented grains that are distinct from the main c-axis maximum, are present in some samples. These have horizontal c-axes rotated clockwise from the primary c-axis maximum and may define a weaker secondary maximum up to 30° clockwise of the primary maximum. Intragranular misorientations are measured along the core, and although the statistics are weak, this could suggest recrystallization by subgrain rotation to occur. These microstructures suggest subgrain rotation (SGR) and recrystallization by grain boundary migration recrystallization (GBM) are active in the Priestley Glacier shear margin. Vorticity analysis based on intragranular distortion indicates a vertical axis of rotation in the shear margin. The variability in c-axis maximum orientation with depth indicates the structural heterogeneity of the Priestley Glacier shear margin occurs at the meter to tens of meters scale. We suggest that CPO rotations could relate to rigid rotation of blocks of ice within the glacial shear margin. Rotation either post-dates CPO and SPO development or is occurring faster than CPO evolution can respond to a change in kinematics.

scholarly journals Annual layering in the NGRIP ice core during the Eemian

2011 ◽  
Vol 7 (1) ◽  
pp. 749-773 ◽  
Author(s):  
A. Svensson ◽  
M. Bigler ◽  
E. Kettner ◽  
D. Dahl-Jensen ◽  
S. Johnsen ◽  
...  
Keyword(s):  
Boundary Migration ◽  
Ice Core ◽  
Interglacial Period ◽  
Grain Boundary Migration ◽  
Model Estimate ◽  
The Core ◽  
Impurity Analysis ◽  
Annual Layer ◽  
Pressure Melting ◽  
Mis 5E

Abstract. The Greenland NGRIP ice core continuously covers the period from present day back to 123 ka before present, which includes several thousand years of ice from the previous interglacial period, MIS 5e or the Eemian. In the glacial part of the core annual layers can be identified from impurity records and visual stratigraphy, and stratigraphic layer counting has been performed back to 60 ka. In the deepest part of the core, however, the ice is close to the pressure melting point, the visual stratigraphy is dominated by crystal boundaries, and annual layering is not visible to the naked eye. In this study, we apply a newly developed setup for high-resolution ice core impurity analysis to produce continuous records of dust, sodium and ammonium concentrations as well as conductivity of melt water. We analyzed three 2.2 m sections of ice from the Eemian and the glacial inception. In all of the analyzed ice, annual layers can clearly be recognized, most prominently in the dust and conductivity profiles. Part of the samples is, however, contaminated in dust, most likely from drill liquid. It is interesting that the annual layering is preserved despite a very active crystal growth and grain boundary migration in the deep and warm NGRIP ice. Based on annual layer counting of the new records, we determine a mean annual layer thickness close to 11 mm for all three sections, which, to first order, confirms the modeled NGRIP time scale (ss09sea). The counting does, however, suggest a longer duration of the climatically warmest part of the NGRIP record (MIS5e) of up to 1 ka as compared to the model estimate. Our results suggest that stratigraphic layer counting is possible basically throughout the entire NGRIP ice core provided sufficiently highly-resolved profiles become available.

scholarly journals Stratigraphy, stable isotopes and salinity in multi-year sea ice from the rift area, south George VI Ice Shelf, Antarctic Peninsula

1991 ◽  
Vol 37 (127) ◽  
pp. 357-367
Author(s):  
J.-L. Tison ◽  
E. M. Morris ◽  
R. Souchez ◽  
J. Jouzel
Keyword(s):  
Stable Isotopes ◽  
Sea Ice ◽  
Fresh Water ◽  
Large Scale ◽  
Sea Water ◽  
Ice Core ◽  
Ice Shelf ◽  
The Core ◽  
Brackish Ice ◽  
Ice Production

AbstractResults from a detailed profile in a 5.54 m multi-year sea-ice core from the rift area in the southern part of George VI Ice Shelf are presented. Stratigraphy, stable isotopes and Na content are used to investigate the growth processes of the ice cover and to relate them to melting processes at the bottom of the ice shelf.The thickest multi-year sea ice in the sampling area appears to be second-year sea ice that has survived one melt season. Combined salinity/stable-isotope analyses show large-scale sympathetic fluctuations that can be related to the origin of the parent water. Winter accretion represents half of the core length and mainly consists of frazil ice of normal sea-water origin. However, five major dilution events of sea water, with fresh-water input from the melting base of the ice shelf reaching 20% on two occasions, punctuate this winter accretion. Two of them correspond to platelet-ice production, which is often related to the freezing of ascending supercooled water from the bottom of the ice shelf.Brackish ice occurs between 450 and 530 cm in the core. It is demonstrated that this results from the freezing of brackish water (Jeffries and others, 1989) formed by mixing of normal sea water with melted basal shelf ice, with dilution percentages of maximum 80% fresh water.

scholarly journals The enigmatic curvature of Central Iberia and its puzzling kinematics

Solid Earth ◽  
2020 ◽  
Vol 11 (4) ◽  
pp. 1247-1273 ◽  
Author(s):  
Daniel Pastor-Galán ◽  
Gabriel Gutiérrez-Alonso ◽  
Arlo B. Weil
Keyword(s):  
Large Scale ◽  
Vertical Axis ◽  
Variscan Orogeny ◽  
Eastern North America ◽  
The Core ◽  
Northwest Africa ◽  
Tectonic Events ◽  
Single Process ◽  
Kinematic Models ◽  
Geometry And Kinematics

Abstract. The collision between Gondwana and Laurussia that formed the latest supercontinent, Pangea, occurred during Devonian to early Permian times and resulted in a large-scale orogeny that today transects Europe, northwest Africa, and eastern North America. This orogen is characterized by an “S” shaped corrugated geometry in Iberia. The northern curve of the corrugation is the well-known and studied Cantabrian (or Ibero–Armorican) Orocline and is convex to the east and towards the hinterland. Largely ignored for decades, the geometry and kinematics of the southern curvature, known as the Central Iberian curve, are still ambiguous and hotly debated. Despite the paucity of data, the enigmatic Central Iberian curvature has inspired a variety of kinematic models that attempt to explain its formation but with little consensus. This paper presents the advances and milestones in our understanding of the geometry and kinematics of the Central Iberian curve from the last decade with particular attention to structural and paleomagnetic studies. When combined, the currently available datasets suggest that the Central Iberian curve did not undergo regional differential vertical-axis rotations during or after the latest stages of the Variscan orogeny and did not form as the consequence of a single process. Instead, its core is likely a primary curve (i.e., inherited from previous physiographic features of the Iberian crust), whereas the curvature in areas outside the core is dominated by folding interference from the Variscan orogeny or more recent Cenozoic (Alpine) tectonic events.

scholarly journals Quartz Microstructures from the Sambagawa Metamorphic Rocks, Southwest Japan: Indicators of Deformation Conditions during Exhumation

Minerals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1038
Author(s):  
Toru Takeshita
Keyword(s):  
Large Scale ◽  
Boundary Migration ◽  
Metamorphic Rocks ◽  
Grain Boundary Migration ◽  
Structural Level ◽  
Southwest Japan ◽  
Physical Conditions ◽  
Subgrain Rotation ◽  
Garnet Zone ◽  
Quartz Grains

The Sambagawa metamorphic rocks in central Shikoku, southwest Japan consist of an inverted metamorphic sequence from the upper chlorite to oligoclase-biotite zones at the lower structural level (LSL), which is overlain by a normal metamorphic sequence consisting of the albite-biotite and garnet zones at the upper structural level (USL). These sequences form a large-scale recumbent fold called the Besshi nappe. To unravel the mechanism of recrystallization and physical conditions in quartz, and their relation to exhumation tectonics, microstructures of recrystallized quartz grains in quartz schist from the Asemi-Saruta-Dozan River traverse were analyzed. The recrystallized quartz grain size increases with increasing structural level from 40 µm in the upper chlorite zone to 160 µm in the garnet zone of the USL. Further, the mechanism of dynamic recrystallization of quartz changes from subgrain rotation to grain boundary migration with increasing structural level across the uppermost garnet zone of the LSL. From these data, the deformation temperatures in quartz schist are calculated to increase with increasing structural level within the range between 300 and 450 °C using paleopiezometers and experimental flow laws. It could be interpreted that a rapid cooling of the Besshi nappe from above is responsible for the deformation temperatures recorded in quartz schist.

scholarly journals Stratigraphy, stable isotopes and salinity in multi-year sea ice from the rift area, south George VI Ice Shelf, Antarctic Peninsula

1991 ◽  
Vol 37 (127) ◽  
pp. 357-367 ◽  
Author(s):  
J.-L. Tison ◽  
E. M. Morris ◽  
R. Souchez ◽  
J. Jouzel
Keyword(s):  
Stable Isotopes ◽  
Sea Ice ◽  
Fresh Water ◽  
Large Scale ◽  
Sea Water ◽  
Ice Core ◽  
Ice Shelf ◽  
The Core ◽  
Brackish Ice ◽  
Ice Production

AbstractResults from a detailed profile in a 5.54 m multi-year sea-ice core from the rift area in the southern part of George VI Ice Shelf are presented. Stratigraphy, stable isotopes and Na content are used to investigate the growth processes of the ice cover and to relate them to melting processes at the bottom of the ice shelf.The thickest multi-year sea ice in the sampling area appears to be second-year sea ice that has survived one melt season. Combined salinity/stable-isotope analyses show large-scale sympathetic fluctuations that can be related to the origin of the parent water. Winter accretion represents half of the core length and mainly consists of frazil ice of normal sea-water origin. However, five major dilution events of sea water, with fresh-water input from the melting base of the ice shelf reaching 20% on two occasions, punctuate this winter accretion. Two of them correspond to platelet-ice production, which is often related to the freezing of ascending supercooled water from the bottom of the ice shelf.Brackish ice occurs between 450 and 530 cm in the core. It is demonstrated that this results from the freezing of brackish water (Jeffries and others, 1989) formed by mixing of normal sea water with melted basal shelf ice, with dilution percentages of maximum 80% fresh water.

scholarly journals Techniques for measuring high-resolution firn density profiles: case study from Kongsvegen, Svalbard

2008 ◽  
Vol 54 (186) ◽  
pp. 463-468 ◽  
Author(s):  
Robert L. Hawley ◽  
Ola Brandt ◽  
Elizabeth M. Morris ◽  
Jack Kohler ◽  
Andrew P. Shepherd ◽  
...  
Keyword(s):  
High Resolution ◽  
Electrical Properties ◽  
Density Profile ◽  
Large Scale ◽  
Ice Core ◽  
Fine Scale ◽  
Density Profiles ◽  
The Core ◽  
Core Section

AbstractOn an 11 m firn/ice core from Kongsvegen, Svalbard, we have used dielectric profiling (DEP) to measure electrical properties, and digital photography to measure a core optical stratigraphy (COS) profile. We also used a neutron-scattering probe (NP) to measure a density profile in the borehole from which the core was extracted. The NP- and DEP-derived density profiles were similar, showing large-scale (>30 cm) variation in the gravimetric densities of each core section. Fine-scale features (<10 cm) are well characterized by the COS record and are seen at a slightly lower resolution in both the DEP and NP records, which show increasing smoothing. A combination of the density accuracy of NP and the spatial resolution of COS provides a useful method of evaluating the shallow-density profile of a glacier, improving paleoclimate interpretation, mass-balance measurement and interpretation of radar returns.

scholarly journals Using borehole logging and electron backscatter diffraction to orient an ice core from Upper Fremont Glacier, Wyoming, USA

2011 ◽  
Vol 57 (205) ◽  
pp. 832-840 ◽  
Author(s):  
R.W. Obbard ◽  
T. Cassano ◽  
K. Aho ◽  
G. Troderman ◽  
I. Baker
Keyword(s):  
Electron Backscatter Diffraction ◽  
Maximum Shear Stress ◽  
Ice Core ◽  
Ice Cores ◽  
Borehole Logging ◽  
The Core ◽  
Electron Backscatter ◽  
Borehole Logs ◽  
Glacier Flow ◽  
Backscatter Diffraction

AbstractWhile glacier fabric reflects the accumulated strain, detailed azimuthal information is required to link the microstructure to the flow, and this is not easily gathered at depth. Borehole logging provides a way to obtain a log of azimuthal orientation of tilted stratigraphic features that can be used to orient the core with respect to glacier flow. We demonstrate this using acoustic borehole logs and the ice core from a 162 m borehole in Upper Fremont Glacier, Wind River Range, Wyoming, USA. We measured the dip of tilted dust and bubble layers in the actual ice core, identified them on the borehole log, then used their strike to orient the core sections containing them. We examined the crystal orientation fabric of our samples, using electron backscatter diffraction in a scanning electron microscope. When we compared the orientation of the tilted layers in some samples with the fabric, we found that the normal to the foliation and the c-axes maxima both pointed in the direction of maximum shear stress. This illustrates the usefulness of borehole logs for orienting ice cores after removal from the borehole, and for developing a better understanding of fabric development.

scholarly journals Texture and Shape Analysis of Quartzite Mylonites of the Metamorphic Sole of the Samail Ophiolite (Oman): Evidence for Syn- and Post-Obduction Deformation

Geosciences ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 111
Author(s):  
Peter Hallas ◽  
Wilfried Bauer
Keyword(s):  
Grain Boundary ◽  
Electron Backscatter Diffraction ◽  
Boundary Migration ◽  
Arabian Plate ◽  
Grain Boundary Migration ◽  
Main Role ◽  
Subgrain Rotation ◽  
Shear Sense ◽  
Subduction System ◽  
Backscatter Diffraction

The metamorphic sole, tectonically welded to the base of the Samail ophiolite in a supra-subduction system, is assumed to play the main role in strain accumulation during later thrusting onto the Arabian Plate (i.e., during obduction). The present study deals with five quartzite samples representative of the upper amphibolite and lower greenschist facies parts of the sole. Whole-rock textures obtained by neutron time-of-flight technique were coupled with microstructural observation using electron backscatter diffraction analyses. The quartz microstructural fabrics and textures in the upper and lower parts of the sole represent grain boundary migration and [c]-in-Y textures and subgrain rotation recrystallization and {r}-in-Z textures, respectively. The shear sense in these samples points to top-to-the-SW to SSW shear. One sample of the upper part, yielding a higher calcite amount, is later overprinted by bulging and displays top-to-the-NNE shear. We postulate to differentiate two main deformation steps. The first is the overall present subgrain rotation and grain boundary migration recrystallization combined to top ~SW shear is related to the sole accretion to the ophiolite and the eventually following thrusting onto the Arabian Plate. The second is correlated to a post-obduction extensional top-to-the-NNE shearing, which is associated with tectonic thinning of the ophiolite and mainly documented in the underlying autochthonous units.

scholarly journals Elucidating the Onset of Plasticity in Sliding Contacts Using Differential EBSD Tomography

2021 ◽  
Author(s):  
Stefan Eder ◽  
Philipp G Grützmacher ◽  
Manel Rodríguez Ripoll ◽  
Jim Belak
Keyword(s):  
Grain Boundary ◽  
Large Scale ◽  
Surface Deformation ◽  
Electron Backscatter Diffraction ◽  
Boundary Migration ◽  
Material Design ◽  
Lattice Rotation ◽  
Time Resolved ◽  
Near Surface ◽  
Color Saturation

Abstract Depending on the mechanical and thermal energy introduced to a dry sliding interface, the near-surface regions of the mated bodies may undergo plastic deformation. In this work, we use "differential EBSD tomography" to highlight changes to the microstructure near tribological fcc alloy surfaces, allowing us to detect subtle differences in lattice orientation and small distances in grain boundary migration. The analysis approach compares electron backscatter diffraction images generated from large-scale molecular dynamics simulations with their undeformed counterparts via a simple image analysis filter. We use our visualization method to discuss the acting microstructural mechanisms in a load- and time-resolved fashion, focusing on sliding conditions that lead to twinning, partial lattice rotation, and grain boundary dominated processes. Extracting and laterally averaging the color saturation value of the generated tomographs allows us to produce quantitative time- and depth-resolved maps that give a good overview of the progress and severity of near-surface deformation. Corresponding maps of the lateral standard deviation in the color saturation show evidence of homogenization processes occurring in the tribologically loaded microstructure, frequently leading to the formation of a well-defined separation between deformed and undeformed regions. When integrated into a computational materials engineering framework, our approach could help optimize material design for tribological and other deformation problems.

Static Recrystallization of Magnesium Alloy during TRC-HC Deformation Followed by Annealing

2011 ◽  
Vol 295-297 ◽  
pp. 730-733 ◽  
Author(s):  
Yong Wang ◽  
Shou Ren Wang ◽  
Ru Ma ◽  
Li Ying Yang
Keyword(s):  
Magnesium Alloy ◽  
Grain Boundary ◽  
Static Recrystallization ◽  
Electron Backscatter Diffraction ◽  
Boundary Migration ◽  
Transmission Electron ◽  
Subgrain Rotation ◽  
Twin Roll Cast ◽  
Backscatter Diffraction ◽  
Twin Roll

Using optical microscopy, electron backscatter diffraction and transmission electron microscopy, the static recrystallization (SRX) mechanisms of ZK 60 magnesium alloy was examined under twin-roll-cast and hot compression (TRC-HC, 350 °C/0.1 s-1) and subsequent annealing (1000 second at 250-400°C). The static recrystallization (SRX) mechanisms, such as grain boundary migration (GBM), grain boundary bulging (GLB) and subgrain rotation (SGR), were discussed.

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