Crevasses, Fractures and Folds within Firn and Marine Ice of the McMurdo Shear Zone, Antarctica interpreted from GPR Profiles acquired with an Unmanned Rover

Abstract:

The firn of the McMurdo shear zone within the Ross Ice Shelf, Antarctica is intensively crevassed, and may also contain crevasses within its meteoric and marine ice. However, the surface crevassing prevents ordinary vehicle access to investigate its structure geophysically. We used a lightweight robotic vehicle to tow 200 and 400 MHz ground-penetrating radar antennas simultaneously along 100 evenly spaced transects over a 28 km² grid spanning the shear zone width. Transects were mainly orthogonal to ice flow. Total firn and meteoric ice thickness was about 160 m. Firn crevasses profiled at 400 MHz were up to 16 m wide, under snow bridges up to 10 m thick, and with strikes near 50–55° to general flow direction. From top down, 200-MHz profiles reveal firn diffractions originating to about 40 m depth, no discernible structure within the meteoric ice, a discontinuous transitional horizon, and at least 20 m of disturbed and stratified marine ice. Freeboard ranging from 28–31 m shows more marine ice exists. We interpret the transitional horizon to be a thin saline layer, and marine ice hyperbolic diffractions and reflections to be responses to localized fractures and to crevasses of various widths, filled with unstratified marine ice, and at strikes mainly between 41–63°. We interpret off-nadir, marine ice horizons to be responses to linear and folded faults, the structure and orientation of which are similar to some in firn. The coinciding and synchronously folded areas of fractured firn and marine ice suggest the visibly unstructured meteoric ice is also fractured, but either never crevassed, or crevassed and sutured without sea water penetration, and that any bottom crevasses that occurred near grounding lines have been thermally eroded. Consequently any fractures in the meteoric ice beneath our grid are likely to have formed far from any grounding area, but it is unclear why there is no evidence of sea water penetration given the fractured and crevassed marine ice.