SUBGLACIAL BEDFORMS - BASAL IMPRINTS OF INTERNAL ICE FOLDS?

Subglacial bedforms contain information about the physical nature of interactions between ice masses and their substrata. However, it is still uncertain what quantitative framework should be used to interpret processes leading to development of subglacial bedforms and their varied characteristics. Internal ice folds imaged within Antarctic ice streams have similar wavelengths and elongation ratios as mega-scale glacial lineations observed on a number of paleo-ice stream beds. We propose that these highly elongated subglacial bedforms are a result of basal imprinting of internal ice folds onto deformable sediments. To illustrate our argument quantitatively we draw an analogy to 3D folding of layered viscous rock masses. This analogy suggests that development of mega-scale glacial lineations (elongation ratios > 10) should be favored under conditions of cross-flow compression and along-flow extension, particularly for horizontal strain rate ratio greater than -2. Drumlins (elongation ratios <10) will be favored by less negative, or positive, horizontal strain-rate ratios. Featureless plains of deformable till may develop either where there is no (weak?) vertical viscosity layering in ice (temperate ice?) or where horizontal strain rate ratio is highly negative (< -2), which favors negative growth of folds with all elongation ratios. We speculate that our conceptual model for bedform generation may be able to yield ribbed moraines if significant vertical shear on horizontal plains is involved in internal ice folding. The most fundamental difference between the model proposed here and other models of subglacial bedform generation is that the latter focus on processes taking place subglacially. Our model places the origin of instability leading to bedform patterning in the ice itself, with subglacial bedforms representing a muted, basal expression of englacial folding. If the proposed model is correct, it may be possible to put constraints on strain-rate and stress fields of past ice sheets and ice streams based on geometry of subglacial bedforms and to predict what bedforms may be forming beneath modern ice masses based on observed strain-rate and stress conditions.