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

Abstract

Abstract. 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-•) 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 80 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 leftlateral 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 -190 years B.P.; the burial depth decreases toward the B2 margin and suggests a migration rate of -100 m yr -•. In addition, a separate high-power radar system was used to image the entire thickness of the ice sheet (-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. IntroductionThe ice streams that drain West Antarctica into the Ross Ice Shelf carry 20-25% of the total outflow and thus form an important ingredient to understanding the dynamics and stability of the West Antarctic ice sheet as a whole (Figure 1 Figures 2a and 2b). The interstream ridge is commonly referred to as the "Unicorn" (Figures 1 and 2 Two separate radar systems were used to image the ice sheet. The first was a low-power impulse-type system that provided high-resolution images of the upper 80 m of the ice sheet. The second was a high-power modulated-type system, which provided images of the entire depth of the kilometerthick ice sheet at low resolution. The purpose of the highresolution radar survey was to image both near-surface and 13,409

Formation mechanisms for ice‐stream lateral shear margin moraines

“…There is no convincing evidence of their occurrence but Clarke et al (2000) identified several features described as entrained morainal debris at the base of a recently abandoned shear margin of Ice Stream B in West Antarctica. Further offshore in West Antarctica, there are also descriptions of 'laterally accreting ridges' marking the position of the lateral margins of palaeo-ice stream tracks (Shipp et al, 1999).…”

Ice stream shear margin moraines

“…This area is characterized by buried crevasses but also reveals several features at the base of the ice sheet, directly below the inferred former margin. The origin of these features is unknown, but Clarke et al (2000) speculate that some of them may represent 'entrained morainal debris'. This is largely based on the fact that a drilling experiment encountered abnormal resistance on reaching such a feature and that the drill tip showed signs of fresh scratches on arriving at the surface.…”

“…Using ice-penetrating radar, Clarke et al (2000) have found a recently abandoned shear margin of Ice Stream B. This area is characterized by buried crevasses but also reveals several features at the base of the ice sheet, directly below the inferred former margin.…”

Decadal-scale variations in ice flow along Whillans Ice Stream and its tributaries, West Antarctica

“…Fast inward margin migration (as seen at G4) is best explained by the advection of colder ice from adjacent ridges into the ice stream and is not unprecedented (Van der Veen and Whillans, 1996;Jacobson and Raymond, 1998;Raymond and others, 2001). Clarke and Bentley (2000) estimated an inward migration rate of 100 m a -1 for the south margin of WIS (near G1n/G2n) as it migrated from the 'Fishhook' starting $200 years ago. Similarly, Jacobel and others (2000) suggested that the multiple scars on the north margin of Kamb Ice Stream indicate that the margin moved inward in rapid jumps.…”