Formation mechanisms for ice‐stream lateral shear margin moraines

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractThe lateral shear margins of palaeo-ice stream beds are occasionally marked by large curvi-linear moraines but little work has addressed how they might form. In this paper we review the characteristics of 'lateral shear margin moraines' and present two quantitative models for their formation; (i) differential erosion rates related to lateral variations in the stream velocity; and (ii), lateral incursion from inter-stream ridges after the thinner stream retreats, where the shear margin moraine is consequently just a terminal moraine of the inter-stream ridge. Other possible mechanisms of formation are addressed qualitatively. Both of our quantitative theories are, in principle, feasible, but the incursion model is less plausible and inconsistent with observations. The differential erosion model can predict moraines tens of metres thick and several kilometres wide which can be formed in a thousand years. The width of the moraine depends on the competition between a term which varies across the ice-stream because it is dependent on ice velocity and which always causes erosion, and a term related to the local accumulation rate which causes erosion if is positive and deposition if negative. These terms combine to create the possibility of deposition at the shear margins of ice streams in ablation areas. The width of the moraine has a tendency to increase with the width of the stream. The conditions which permit shear margin moraines to form are quite constrained; too much ablation and deposition occurs everywhere rather than just near the lateral margins of the ice streams; too little ablation and there is erosion everywhere. This is consistent with the infrequent observations of lateral shear margin moraines.

Palaeo‐ice streams and the subglacial landscape mosaic of the North American mid‐continental prairies

“…For example, the western margin 2 follows the Basin Lake Ridge, which is the 200 km long till ridge that forms the eastern limit of A2 and that extends further to the south (Figs 4, 7B, 8). This ridge is interpreted as an ice‐stream shear‐margin moraine (Dyke & Morris 1988; Stokes & Clark 2002b; Hindmarsh & Stokes 2008) that likely formed during an early stage of the Buffalo palaeo‐ice stream. Its northern end shows evidence of lateral migration and stillstands in the position of the shear margin.…”

Large-scale reorganization and sedimentation of terrestrial ice streams during late Wisconsinan Laurentide Ice Sheet deglaciation

“…These features are similar to "shear margin moraines" described in the literature and are inferred to have formed in the shear zone between the rapidly moving ice stream and adjacent slower moving ice (cf. Raymond et al, 2001;Stokes and Clark, 2002;Hindmarsh and Stokes, 2008). Megascale glacial lineations in the trunk zone are overprinted locally by a rectilinear network of sediment ridges, which are 1-4.5 m high, 0.5-2 km wide, and 1-5 km long, exceptionally on August 6, 2015 gsabulletin.gsapubs.org Downloaded from reaching 8 km in length (Fig.…”

Processes controlling the downstream evolution of ice rheology in glacier shear margins: case study on Rutford Ice Stream, West Antarctica

“…These studies have illuminated numerous physical processes related to the thermomechanics of ice flow (e.g., Schoof, 2004Schoof, , 2012Hindmarsh, 2004), the hydromechanical properties of subglacial till (e.g., Kyrke-Smith and others, 2013;Robel and others, 2016;others, 2016, 2017;Lipovsky and Dunham, 2017), the transport of subglacial till to form shear-margin moraines (Stokes and Clark, 2002;Hindmarsh and Stokes, 2008) and the detailed mechanics of the shear margin as a boundary-layer between a fast-flowing ice stream and the slow, convergent flow of an adjacent ice ridge (e.g., Schoof, 2012;Haseloff and others, 2015). Most of these studies highlight the importance of heating through viscous dissipation, a prominent control on ice rheology because the softness of ice varies exponentially with temperature (Cuffey and Paterson, 2010;Hudleston, 2015).…”