Evidence for a recently abandoned shear margin adjacent to ice stream B2, Antarctica, from ice-penetrating radar measurements

Satellite imagery of the area between ice streams B1 and B2, Antarctica, shows a lineation on the surface of the ice sheet of uncertain origin. Ice motion in the area (2 m yr(-1)) is 2 orders of magnitude slower than that of the surrounding streams and shows no significant variation that could explain the feature. A low-power, high-resolution radar system was used to image the upper 80 m of the ice sheet between the two ice streams; the survey shows that the lineation is associated with what is likely an abandoned shear margin. The radar data show that a set of chaotic diffractors lies buried beneath two thirds of the area, while the remaining one third is undisturbed to SO m depth. The chaotic ice ends abruptly along a boundary that is parallel to, but offset 2.5 km from, the surface lineation. Also, isolated linear diffractors are commonly observed in the otherwise undisturbed ice immediately adjacent to the boundary. The depth, location, orientation, and curved form of the diffractors strongly suggest they are the tops of crevasses that were active at the time the chaotic ice was being strained and that they were formed by left-lateral shear. This is the same sense of shear that is presently active in the B2 margin, 6 km away. The depth to the chaotic diffractors suggests that the shear margin abandoned its prior position similar to 190 years B.P.; the burial depth decreases toward the B2 margin and suggests a migration rate of similar to 100 m yr(-1). In addition, a separate high-power radar system was used to image the entire thickness of the ice sheet (similar to 1 km) in the same area. These data show numerous linear diffractors near the base of the ice sheet. It is very likely that at least some of these diffractors are entrained morainal debris. Others may be bottom crevasses or zones of wet, reflective ice that developed in the high-strain environment of the now-abandoned shear margin.