3D Structure of NEGIS shear margins from radar stratigraphy

The North East Greenland Ice Stream (NEGIS) is delineated by well-defined shear margins, which are evident in the gradient of surface velocity field as well as in the surface topography, where they form troughs up to ten meters deep. In the upper part of the ice stream the margins appear not to be linked to bedrock topography. To understand this efficient system of mass transport towards the ocean it is essential to investigate the nature of the shear margins, as here very localized deformation decouples the inner ice stream from the slower flowing surrounding ice sheet. This process is influenced by several factors and feedback mechanisms, including the crystal fabric orientation, strain heating and localization of meltwater. In summary, the shear margins are area-wise a small part of the ice stream itself, but the processes leading to the localization of deformation are of similar importance for ice discharge as the processes enabling fast flow of the main trunk over the bed.We present results from an airborne radar survey with the AWI Ultra-Wide Band Radar system, covering an area 150 km upstream and 100 km downstream of the deep drilling site on the ice stream (EGRIP). Over the survey area the ice stream accelerates from 12 m/a to 75 m/a. We focus on the signatures of the shear margins in the radar data. In the regions of localized shear, the internal reflections in the radargrams show disturbances in the form of steep undulations, or chevron folds, which are intensified with ongoing shear. As the ice stream has been covered with 36 flow-perpendicular radar sections we are able to show the evolution of these characteristic signatures over the survey area, and thus, as an analog, over time. 3D-representations of the folded stratigraphic layers reveal how new folds are formed when the ice stream widens and how older structures are preserved in the outer part of the main trunk, where they are no longer subject to shear. Furthermore, we link the change of the shape of the internal reflections in the shear zones to a strain rate field calculated from high resolution flow velocities derived by TerraSAR-X data.

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Interior of an ice stream: 3-D geometry of distorted radar stratigraphy of upstream NEGIS and vicinity.

10.5194/egusphere-egu21-15382 ◽

2021 ◽

Author(s):

Daniela Jansen ◽

Steven Franke ◽

Tobias Binder ◽

Paul Bons ◽

Dorthe Dahl-Jensen ◽

...

Keyword(s):

Wide Band ◽

Shear Zones ◽

Surface Velocity ◽

Ice Flow ◽

North East ◽

The North ◽

Ice Surface ◽

Ice Stream ◽

Dense Grid ◽

Over Time

The North East Greenland Ice Stream clearly stands out in the surface velocity field of the ice flow of Greenland, with its sharp and narrow shear margins visible in the flow field almost up to the central divide. While the current extent and strength of the streaming can be determined from remotely sensed velocities of the ice surface, it is less known how the ice stream is affecting the deeper layers of ice in its catchment area, and how it may have evolved over time. The deformation of the ice due to streaming can be made visible by mapping the distortion of the isochronous stratigraphy of the ice. This has been done by an airborne radar survey centering on the location of the EGRIP drilling camp, carried out with the ultra wide band &#160;radar system (AWI UWB). The dense grid of profiles arranged mainly perpendicular to the ice flow reveals the imprint that the strong shearing leaves within the layering of the ice. Although the layers are tightly folded and distorted within the shear zones, it is possible to continuously trace reflections within the upper half of the ice column throughout the entire survey area. It can be shown that the intensity of the folding is linked to the strain rate field derived from the surface velocities, and that the deformation history of the ice is preserved in the folded layers, even after it is no longer affected by high strain rates.&#160; The advection patterns of the mapped stratigraphic features reveal how the streaming of the ice and the resulting local changes of surface topography may have affected the inflow into the stream and the position of the shear margins over time.

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Consistent comparison of full-Stokes and higher-order approximation in the central North East Greenland Ice Stream

10.5194/egusphere-egu21-2568 ◽

2021 ◽

Author(s):

Martin Rückamp ◽

Thomas Kleiner ◽

Angelika Humbert

Keyword(s):

Ice Sheet ◽

Higher Order ◽

Surface Velocity ◽

Numerical Comparison ◽

Geophysical Research ◽

East Greenland ◽

North East ◽

Aspect Ratios ◽

Ice Stream ◽

Depth Analysis

Modelling ice sheet flow is at best modelled using the full-Stokes (FS) equations. However, it rarely sees application even in recent years due to its high computational demand and problems in numerical convergence due to the thin aspect ratio of ice sheets. For this reason, the modelling community has relied on simplified mathematical models, such as the three-dimensional Higher-Order (HO) approximation which neglects horizontal gradients of vertical velocities and bridging effects. Here, we conduct an analysis of the difference in stresses and velocity fields solving the FS system of equation and using two different types of HO approximations equivalent to LMLa (known as Blatter-Pattyn type) and LTSML according to Hindmarsh (2004). Our intention was to avoid any bias from a difference in discretization or implementation, therefore we implemented it in a single ice sheet model to avoid effects arising from discretization and implementation. We selected a subset of the North East Greenland Ice Stream (NEGIS) as investigation area. As differences between FS and HO emerge in regions with steep bedrock gradients or high aspect ratios, we step-wise increase spatial resolution from 12.8 km down to 0.1 km. Our analysis reveals that surface velocity differences between the FS and HO solution emerge below 1km horizontal resolution and increase with resolution. Compared to the absolute ice flow velocity, the relative error between FS and HO remains small. We present an in-depth analysis, that reveals that different factors are affected by the approximation, such as basal drag and rheologyReferences: Hindmarsh, R. C. A.: A numerical comparison of approximations to the Stokes equations used in ice sheet and glacier modeling, Journal of Geophysical Research: Earth Surface, 109, https://doi.org/10.1029/2003JF000065, 2004

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A confined–unconfined aquifer model for subglacial hydrology and its application to the North East Greenland Ice Stream

10.5194/tc-2017-221 ◽

2017 ◽

Cited By ~ 2

Author(s):

Sebastian Beyer ◽

Thomas Kleiner ◽

Vadym Aizinger ◽

Martin Rückamp ◽

Angelika Humbert

Keyword(s):

Computational Cost ◽

Ice Sheet ◽

Unconfined Aquifer ◽

Ocean Modeling ◽

Model Parameters ◽

Efficient System ◽

East Greenland ◽

North East ◽

The North ◽

Ice Stream

Abstract. Subglacial hydrology plays an important role in the ice sheet dynamics as it determines the sliding velocity of ice sheets and also drives freshwater into the ocean. Modeling subglacial water has been a challenge for decades, and only recently new approaches have been developed such as representing subglacial channels and thin water sheets by separate layers of variable permeability. We extend this concept by modeling a confined and unconfined aquifer system (CUAS) in a single layer. The advantage of this formulation is that it prevents unphysical values of pressure at reasonable computational cost. We also performed sensitivity tests to investigate the effect of different model parameters. The strongest influence of model parameters was detected in terms governing the opening and closure of channels. Furthermore, we applied the model to the North East Greenland Ice Stream, where an efficient system independent of seasonal input was identified about 500 km downstream from the ice divide. Using the effective pressure from the hydrology model in the Ice Sheet System Model (ISSM) shows considerable improvements of modeled velocities in the coastal region.

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Complex basal conditions influence flow at the onset of the North East Greenland Ice Stream

10.5194/egusphere-egu2020-1173 ◽

2020 ◽

Author(s):

Steven Franke ◽

Daniela Jansen ◽

John Paden ◽

Olaf Eisen

Keyword(s):

Shear Zones ◽

Surface Flow ◽

Ultra Wideband ◽

Ice Flow ◽

Systematic Analysis ◽

East Greenland ◽

North East ◽

The North ◽

Ice Surface ◽

Ice Stream

The onset and high upstream ice surface velocities of the North East Greenland Ice Stream (NEGIS) are not yet well reproducible in ice sheet models. A major uncertainty remains the understanding of basal sliding and a parameterization of basal conditions. In this study, we assess the slow-flowing part of the NEGIS in a systematic analysis of the basal conditions and investigate the increased ice flow. We analyze the spectral basal roughness in correlation with basal return power from an airborne radar survey with AWIs ultra-wideband radar system in 2018 and compare our results with current ice flow geometry and ice surface flow. We observe a roughness anisotropy where the ice stream widens, indicating a change from a smooth and soft bed to a harder bedrock as well as the evolution of elongated subglacial landforms. In addition, at the upstream part of the NEGIS we find a clear zoning of the bedrock return power, indicating an increased water content at the base of the ice stream. At the downstream part, we observe an increased bedrock return power throughout the entire width of the ice stream and outside its margins, indicating enhanced melting and the distribution of basal water beyond the shear zones.

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Decision letter for "Early Palaeozoic deformation features and tectonic implications in the eastern Jiangnan Orogen, South China: Constraints from structural analysis of north–north‐east ductile shear zones and relevant dating"

10.1002/gj.4009/v4/decision1 ◽

2020 ◽

Keyword(s):

Structural Analysis ◽

South China ◽

Shear Zones ◽

North East ◽

Tectonic Implications ◽

Ductile Shear Zones ◽

Deformation Features ◽

Ductile Shear ◽

Early Palaeozoic

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Of isbræ and ice streams

Annals of Glaciology ◽

10.3189/172756403781816347 ◽

2003 ◽

Vol 36 ◽

pp. 66-72 ◽

Cited By ~ 88

Author(s):

Martin Truffer ◽

Keith A. Echelmeyer

Keyword(s):

Ice Sheet ◽

Shear Zones ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Bed Topography ◽

Ice Stream ◽

Fast Ice ◽

Fast Flow ◽

End Members

AbstractFast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.

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Are longitudinal ice-surface structures on the Antarctic Ice Sheet indicators of long-term ice-flow configuration?

Earth Surface Dynamics Discussions ◽

10.5194/esurfd-2-911-2014 ◽

2014 ◽

Vol 2 (2) ◽

pp. 911-933 ◽

Cited By ~ 1

Author(s):

N. F. Glasser ◽

S. J. A. Jennings ◽

M. J. Hambrey ◽

B. Hubbard

Keyword(s):

Flow Line ◽

Ice Sheet ◽

Surface Structures ◽

Surface Velocity ◽

Ice Flow ◽

Antarctic Ice Sheet ◽

Ice Surface ◽

Ice Stream ◽

The Antarctic

Abstract. Continent-wide mapping of longitudinal ice-surface structures on the Antarctic Ice Sheet reveals that they originate in the interior of the ice sheet and are arranged in arborescent networks fed by multiple tributaries. Longitudinal ice-surface structures can be traced continuously down-ice for distances of up to 1200 km. They are co-located with fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of m yr-1 and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. Associating these structures with the AIS' surface velocity field reveals (i) ice residence times of ~ 2500 to 18 500 years, and (ii) undeformed flow-line sets for all major flow units analysed except the Kamb Ice Stream and the Institute and Möller Ice Stream areas. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow and ice-velocity configuration of the ice sheet may have remained largely unchanged for several thousand years, and possibly even since the end of the last glacial cycle. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

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The effect of the north-east ice stream on the Greenland ice sheet in changing climates

The Cryosphere Discussions ◽

10.5194/tcd-1-41-2007 ◽

2007 ◽

Vol 1 (1) ◽

pp. 41-76 ◽

Cited By ~ 10

Author(s):

R. Greve ◽

S. Otsu

Keyword(s):

Large Scale ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Simulation Code ◽

North East ◽

The North ◽

Ice Stream ◽

Basal Sliding ◽

Speed Up ◽

Fast Flow

Abstract. The north-east Greenland ice stream (NEGIS) was discovered as a large fast-flow feature of the Greenland ice sheet by synthetic aperture radar (SAR) imaginary of the ERS-1 satellite. In this study, the NEGIS is implemented in the dynamic/thermodynamic, large-scale ice-sheet model SICOPOLIS (Simulation Code for POLythermal Ice Sheets). In the first step, we simulate the evolution of the ice sheet on a 10-km grid for the period from 250 ka ago until today, driven by a climatology reconstructed from a combination of present-day observations and GCM results for the past. We assume that the NEGIS area is characterized by enhanced basal sliding compared to the "normal", slowly-flowing areas of the ice sheet, and find that the misfit between simulated and observed ice thicknesses and surface velocities is minimized for a sliding enhancement by the factor three. In the second step, the consequences of the NEGIS, and also of surface-meltwater-induced acceleration of basal sliding, for the possible decay of the Greenland ice sheet in future warming climates are investigated. It is demonstrated that the ice sheet is generally very susceptible to global warming on time-scales of centuries and that surface-meltwater-induced acceleration of basal sliding can speed up the decay significantly, whereas the NEGIS is not likely to dynamically destabilize the ice sheet as a whole.

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

10.5194/egusphere-egu21-3266 ◽

2021 ◽

Author(s):

Meghana Ranganathan ◽

Brent Minchew ◽

Colin Meyer ◽

Matej Pec

Keyword(s):

Grain Size ◽

Ice Sheet ◽

Shear Zones ◽

Dislocation Creep ◽

Model Parameters ◽

Localized Deformation ◽

Ice Shelves ◽

Fine Grained ◽

Initiation And Propagation ◽

Rapid Deformation

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

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Unravelling heterogeneities in magnetic fabric record of the strain: a combined AMS and ApARM data analysis applied to intraplate shear zones.

10.5194/egusphere-egu21-1481 ◽

2021 ◽

Author(s):

Claudio Robustelli Test ◽

Elena Zanella ◽

Andrea Festa ◽

Francesca Remitti

Keyword(s):

Cluster Analysis ◽

Hanging Wall ◽

Shear Zones ◽

Magnetic Fabric ◽

Plate Boundaries ◽

Thrust Faults ◽

Localized Deformation ◽

Main Thrust ◽

Shear Sense ◽

And Cluster Analysis

Deciphering the stress and strain distribution across plate boundary shear zones is critical to understanding the physical processes involved in the nucleation of megathrust faults and its behaviour. Plate boundaries at shallow depth represent complex and highly deformed zones showing structures from both distributed and localized deformation.As magnetic minerals are sensitive to stress regime, the investigation of the magnetic fabric has proven to be an effective tool in studying faulting processes at intraplate shear zones.Anisotropy of magnetic susceptibility (AMS) provides insights into the preferred orientation of mineral grains and the qualitative relationships between petrofabrics and deformation intensity.We present an approach of combined Contoured Diagram and Cluster Analysis to isolate the contribution of coexisting petrofabrics to the total AMS and evaluating the significance of magnetic fabric clusters.Our results reveal distinct subfabrics with reasonably straightforward correlations with structural data. Specific AMS pattern may be associated to the intensity of the reworking related to tectonic shearing and the structural position within the shear zone (i.e., the proximity to the main thrust faults).Close to the main thrust the magnetic fabric is dominantly oblate with magnetic foliation consistent to the S-C fabric and/or m&#233;lange foliation and the magnetic lineation parallel to the shear sense.Away from the thrust faults the degree of anisotropy as well as the ellipsoids oblateness gradually diminishes. Thus, the presence of subfabrics related to previous tectonic events or less intense deformation (i.e. intersection lineation fabric) became dominant. The discrimination of subfabrics also allowed to unravel the presence of minor thrust plane and qualitatively evaluate the heterogeneous registration of strain (i.e. distributed versus localized deformation).An abrupt change in magnetic ellipsoid shape and parameters is also observed below the basal d&#233;collements showing purely sedimentary magnetic fabric or previous deformation history with minor to absent evidences of shearing in the hanging wall.Then, the integration with anisotropy of magnetic remanence experiments in different coercivity windows (ApARM) allow to separate the contribution of different ferromagnetic subpopulation of grains, constraining the significance of the different magnetic pattern/clusters detected through the AMS analysis.In conclusion, our results show the potential of a combination of density diagrams and cluster analysis validated by ApARM experiments in distinguishing the superposition of deformation events, unravelling strain partitioning/concentration and thus to better understand the geodynamic evolution of subduction-accretion complexes.

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