Deformation of the Hidden Creek Lake ice dam: evidence for faulting through the entire thickness of a glacier

As a glacier-dammed lake fills, a large volume of water may be stored beneath the ice dam (the part of the glacier adjacent to the lake) and then released during a subsequent jökulhlaup. The surface of the ice dam rises and falls accordingly. At Kennicott Glacier, Alaska, the kinematics of such motion during the annual jökulhlaup cycle of Hidden Creek Lake provide strong evidence that the ice dam is pervasively faulted through its entire thickness, and that "mega-crevasses" extending across the entire width of the ice dam are in fact the surficial expression of these faults. The surface of the ice dam rises by a distance h >= 20 m every year as the lake fills to a volume of about 25 million m³, then drops by about the same amount as the lake drains over a period of 2 to 3 days. The change in h with distance x from the lake is abrupt: h decreases from ca. 20 m to less than 1 m as x increases by only a few hundred m. Such a large gradient in vertical movement cannot be explained using any plausible model of flexural deformation. Horizontal movement of targets on the ice dam is steady and oblique to the face of the ice dam as the lake fills, but during drainage, movement speeds up greatly and target trajectories become nearly perpendicular to the face of the ice dam. Outside the zone in which |dh/dx| is large, the strain rate ˙ {e}_xx is always positive (extensional), but within the zone of large |dh/dx|, ˙ {e}_xx <0 as the lake fills, then reverses sign as the lake drains. Survey targets separated from one another (in x) by large crevasses move relative to one another in a nearly reversible fashion as the lake fills and drains, with trajectories of relative displacement plunging at an angle of about 75 to 80 degrees towards the lake. These relative-movement trajectories evidently reflect movement along high-angle faults that extend from the surface to the base of the ice dam through as much as 250 m if ice. Fault movement is in the reverse sense (˙ {e}_xx<0) as the lake fills and the normal sense (˙ {e}_xx>0) as the lake drains. The extensional strain rate during lake drainage is as much as 1 a^-1. The onset of both vertical downdrop and accelerated horizontal motion lags the onset of lake drawdown, with the lag time increasing with distance from the ice-dam face. The existence of such a lag may reflect the residual strength and complex geometry of the fault surfaces. Although some sections of the ice dam break apart as they collapse into the lake basin during lake drainage, and thereby serve as the source of icebergs during the subsequent lake-filling episode, the overall pattern of crevasses is regenerated from year to year, presumably reflecting bedrock control on the stress field within the ice.