Crevasse Extent and Lateral Shearing of the McMurdo Shear Zone, Antarctica, Using GPR and GPS Observations, and Numerical Modeling

Abstract:

Sub-ice-shelf circulation plays a fundamental role in ice shelf mass budget. The shape of the underside of an ice shelf is important, such that the presence of basal crevasses can significantly modulate the transfer of heat at the ice-ocean interface. In situ observations of basal crevasses are challenging to obtain, but surface-based ground penetrating radar (GPR) surveys can be used to determine crevasse location and orientation. Here, we use GPR methods to map the internal structures in the McMurdo Shear Zone (SZ) which marks the boundary between the Ross Ice Shelf and the slower-moving McMurdo Ice Shelf. Radar surveys with 200 MHz and 400 MHz antennas reveal the presence of crevasses both in the upper firn and within a zone of accreted marine ice at a depth of approximately 170 meters. A spatial correspondence between near-surface and basal crevasses suggests that both are formed locally by lateral shearing. A combination of three dimensional higher order and Shallow Shelf Approximation ice flow equations within the Ice Sheet System Model (ISSM) are used to test this hypothesis. This model estimates the detailed velocity field of the SZ and is constrained by GPS-derived observations of surface motion. The distribution and orientations of surface crevasses is consistent with the gradients in velocity field predicted by the model. Though a wider range of orientation angles exists for crevasses within the basal regime, the average strike angle is consistent with firn crevassing and we conclude that the marine ice coevally fractured with the firn layer.