skip to main content


Title: Inferring Ice Fabric From Birefringence Loss in Airborne Radargrams: Application to the Eastern Shear Margin of Thwaites Glacier, West Antarctica
Abstract

In airborne radargrams, undulating periodic patterns in amplitude that overprint traditional radiostratigraphic layering are occasionally observed, however, they have yet to be analyzed from a geophysical or glaciological perspective. We present evidence supported by theory that these depth‐periodic patterns are consistent with a modulation of the received radar power due to the birefringence of polar ice, and therefore indicate the presence of bulk fabric anisotropy. Here, we investigate the periodic component of birefringence‐induced radar power recorded in airborne radar data at the eastern shear margin of Thwaites Glacier and quantify the lateral variation in azimuthal fabric strength across this margin. We find the depth variability of birefringence periodicity crossing the shear margin to be a visual expression of its shear state and its development, which appears consistent with present‐day ice deformation. The morphology of the birefringent patterns is centered at the location of maximum shear and observed in all cross‐margin profiles, consistent with predictions of ice fabric when subjected to simple shear. The englacial fabric appears stronger inside the ice stream than outward of the shear margin. The detection of birefringent periodicity from non‐polarimetric radargrams presents a novel use of subsurface radar to constrain lateral variations in fabric strength, locate present and past shear margins, and characterize the deformation history of polar ice sheets.

 
more » « less
Award ID(s):
1739027
NSF-PAR ID:
10420752
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Earth Surface
Volume:
126
Issue:
5
ISSN:
2169-9003
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract. The crystal orientation fabric (COF) of ice sheets records the past history ofice sheet deformation and influences present-day ice flow dynamics. Though notwidely implemented, coherent ice-penetrating radar is able to detect bulkanisotropic fabric patterns by exploiting the birefringence of ice crystals atradar frequencies, with the assumption that one of the crystallographic axesis aligned in the vertical direction. In this study, we conduct a suite ofquad-polarimetric measurements consisting of four orthogonal antennaorientation combinations near the Western Antarctic Ice Sheet (WAIS) Divideice core site. From these measurements, we are able to quantify the azimuthalfabric asymmetry at this site to a depth of 1400 m at abulk-averaged resolution of up to 15 m. Our estimates of fabricasymmetry closely match corresponding fabric estimates directly measured fromthe WAIS Divide ice core. While ice core studies are often unable to determinethe absolute fabric orientation due to core rotation during extraction, we areable to identify and conclude that the fabric orientation is depth-invariantto at least 1400 m, equivalent to 6700 years BP (years before1950) and aligns closely with the modern surface strain direction at WAISDivide. Our results support the claim that the deformation regime at WAISDivide has not changed substantially through the majority of theHolocene. Rapid polarimetric determination of bulk fabric asymmetry andorientation compares well with much more laborious sample-based COFmeasurements from thin ice sections. Because it is the bulk-averaged fabricthat ultimately influences ice flow, polarimetric radar methods provide anopportunity for its accurate and widespread mapping and its incorporation intoice flow models. 
    more » « less
  2. Abstract Recent acceleration and thinning of Thwaites Glacier, West Antarctica, motivates investigation of the controls upon, and stability of, its present ice-flow pattern. Its eastern shear margin separates Thwaites Glacier from slower-flowing ice and the southern tributaries of Pine Island Glacier. Troughs in Thwaites Glacier’s bed topography bound nearly all of its tributaries, except along this eastern shear margin, which has no clear relationship with regional bed topography along most of its length. Here we use airborne ice-penetrating radar data from the Airborne Geophysical Survey of the Amundsen Sea Embayment, Antarctica (AGASEA) to investigate the nature of the bed across this margin. Radar data reveal slightly higher and rougher bed topography on the slower-flowing side of the margin, along with lower bed reflectivity. However, the change in bed reflectivity across the margin is partially explained by a change in bed roughness. From these observations, we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties. Given the potential for future increases in ice flux farther downstream, the eastern shear margin may be vulnerable to migration. However, there is no evidence that this margin is migrating presently, despite ongoing changes farther downstream. 
    more » « less
  3. Abstract

    The southern Patagonian Andes record Late Cretaceous–Paleogene compressional inversion of the Rocas Verdes backarc basin (RVB) and development of the Patagonian fold-thrust belt (FTB). A ductile décollement formed in the middle crust and accommodated underthrusting, thickening, and tectonic burial of the continental margin (Cordillera Darwin Metamorphic Complex (CDMC)) beneath the RVB. We present new geologic mapping, quartz microstructure, and crystallographic preferred orientation (CPO) fabric analyses to document the kinematic evolution and deformation conditions of the décollement. Within the CDMC, the décollement is defined by a quartz/chlorite composite schistose foliation (S1-2) that is progressively refolded by two generations of noncylindrical, tight, and isoclinal folds (F3–F4). Strain intensifies near the top of the CDMC, forming a >5 km thick shear zone that is defined by a penetrative L-S tectonite (S2/L2) and progressive noncylindrical folding (F3). Younger kink folds and steeply inclined tight folds (F4) with both north- and south-­dipping axial planes (S4) overprint D2 and D3 structures. Quartz textures from D2 fabrics show subgrain rotation and grain boundary migration recrystallization equivalent to regime 3, and quartz CPO patterns indicate mixed prism <a> and [c] slip systems with c-axis opening angles indicative of deformation temperatures between ~500° and >650°C. Approximately 40 km toward the foreland, the shear zone thins (~1 km thick) and is defined by the L-S tectonite (S2/L2) and tightening of recumbent isoclinal folds (F3). Quartz textures and CPO patterns indicate subgrain rotation recrystallization typical of regime 2 and dominantly basal <a> slip, and c-axis opening angles are consistent with deformation temperatures between ~375° and 575°C. Deformation occurred under greenschist and amphibolite facies conditions in the foreland and hinterland, respectively, indicating that the shear zone dipped shallowly toward the hinterland. The Magallanes décollement is an example of a regional ductile shear zone that accommodated distributed middle to lower crustal thickening below a retroarc FTB.

     
    more » « less
  4. Abstract

    The architecture and mechanical properties of the subduction interface impact large‐scale subduction processes, including mass and volatile recycling, upper‐plate orogenesis, and seismic behavior. The nature of the deep subduction interface, where a dominantly frictional megathrust likely transitions to a distributed ductile shear zone, is poorly understood, due to a lack of constraints on rock types, strain distribution, and interface thickness in this depth range. We characterized these factors in the Condrey Mountain Schist, a Late Jurassic to Early Cretaceous subduction complex in northern California that consists of an upper and lower unit. The Lower Condrey unit is predominantly pelagic and hemipelagic metasediment with m‐to km‐scale metamafic and metaserpentinitic ultramafic lenses all deformed at epidote blueschist facies (0.7–1.1 GPa, 450°C). Major and trace element geochemistry suggest tectonic erosion of the overriding plate sourced all ultramafic and some mafic lenses. We identified two major ductile thrust zones responsible for Lower Condrey unit assembly, with earlier strain distributed across the structural thickness between the ductile thrusts. The Lower Condrey unit records distributed deformation across a sediment‐dominated, 2+ km thick shear zone, possibly consistent with low velocity zones observed in modern subduction zones, despite subducting along a sediment poor, tectonically erosive margin. Periodic strain localization occurred when rheological heterogeneities (i.e., km‐scale ultramafic lenses) entered the interface, facilitating underplating that preserved 10%–60% of the incoming sediment. Modern mass and volatile budgets do not account for erosive margin underplating, so improved quantification is crucial for predicting mass and volatile net flux to Earth′s interior.

     
    more » « less
  5. Abstract

    Firn thickness across the NE Greenland Ice Stream is a function of accumulated strain, with thinner firn in the high‐strain margins of the ice stream. We present a novel technique for extracting firn density from previously collected seismic reflection profiles and apply this technique across both shear margins of NE Greenland Ice Stream. Firn is up to 30 m thinner in the vicinity of the ice stream shear margins. Snow accumulation rates across the ice stream were calculated from airborne ice‐penetrating radar data, calibrated with ground‐based firn density measurements from a shallow core. We find that accumulation is ~20% higher in the shear margins compared to the surroundings. The higher density firn adjacent to shear margins is due to high along‐flow stresses that accelerate firn densification and develops despite the higher accumulation rate favoring lower density. These firn density variations influence subglacial hydropotential by changing the ice surface slope and overburden pressure and may influence subglacial water flow. These results demonstrate the importance of high‐resolution firn surveys in studies of shear‐margin dynamics.

     
    more » « less