Ice-marginal thrusting of drift and bedrock: thermal regime, subglacial aquifers, and glacial surges

Abstract

Glacial thrust systems composed of blocks of drift and bedrock, associated with hummocky stagnation moraine along the margin of the Rainy lobe of the Laurentide Ice Sheet in Minnesota, are used in conjunction with paleoecological studies to constrain a numerical model of the ice-marginal thermal regime. Subglacial meltwater production in the thawed-bed zone was at least two orders of magnitude greater than the amount that could refreeze to the base of the glacier near the margin. The excess water recharged a thick subglacial aquifer, and thrust-system development was enhanced by the presence of a frozen toe and high pore-water pressures beneath the outer 2 km the glacier. The pore-water pressure required for thrusting is calculated from overburden pressures and basal shear stresses determined by numerical modeling. The heat generated by flow of water through the subglacial aquifer substantially affects the ice-marginal thermal regime, making a steady-state frozen toe 1 .O-2.0km in width unstable. Thrusting apparently occurred during multiple oscillations, or surges, when the ice was advancing over permafrost.Les systhmes de chevauchement glaciaire, composCs de blocs de drift et du substratum rocheux, associCs aux moraines de stagnation mamelonnCes le long du lobe Rainy de l'inlandsis laurentidien dans le Minnesota, sont utilists de pair avec les Ctudes palCoCcologiques pour imposer des bornes numCriques dans un modkle de rCgime thermique de bordure de glacier. La production d'eau de fonte sous-glaciaire dans la zone OD se trouve la couche fondante Ctait au moins de deux ordres de magnitude plus grande que la masse d'eau qui pouvait recongeler a la base du glacier prks de la marge. Le surplus d'eau rechargait un Cpais aquif&re sous-glaciaire, et le dtveloppement du systkme de chevauchement Ctait facilitt par la presence d'une langue de glace et des pressions d'eau interstitielles ClevCes sous les deux premiers kilomktres du front du glacier. La pression d'eau interstitielle nCcessaire pour crCer le chevauchement est calculte a partir des pressions gkostatiques et des forces de cisaillement basales dBterminBes par la modtlisation nurnkrique. La chaleur engendrke par 1'Ccoulement de l'eau au travers l'aquifhre sous-glaciaire affecte substantiellement le rCgime thermique de la bordure du glacier, rendant instable une langue de glace rCguliere sur une largeur de 1,O-2,O km. Le chevauchement est apparu vraisemblablement durant les multiples oscillations, ou avancCes rapides, du glacier sur le pergCliso1.[Traduit par la revue]Can, J. Eanh Sci. 27, 849-862 (1990) Introduction Studies of glacial sediments and landforms provide valuable information on the physical nature and dynamics of Pleistocene ice ,sheets. The list of researchers who have used glacial geological mapping to interpret ice characteristics is long, and it is not the purpose of this paper to review the extensive literature. Within the midcontinent region of North America, the studies of Moran (1971), Moran (1974), andMathews (1974) are notable, as they est...

Climate-Controlled Glacial Erosion in the Unconsolidated Sediments of Northwestern Europe, Based on a Genetic Model for Tunnel Valley Formation

“…There are many cited examples where basal meltwater is important in explaining subglacial morphological features with respect to glacier beds of unlithified sediments, such as the deformable bed (Boulton and Jones, 1979), channelized subglacial meltwater transport (Shoemaker, 1986;Alley, 1989), drumlin formation (Boulton, 1987) and glaciotectonic features (van der Wateren, 1992;Mooers, 1990). Others have pointed out the eroding power of subglacial meltwater and its relation to the formation of tunnel valleys (Nye, 1976;Wright, 1973;Boulton and Hindmarsh, 1987) and, to a certain extent, to the origin of glacial basins (van der Wateren, 1992).…”

“…Others have pointed out the eroding power of subglacial meltwater and its relation to the formation of tunnel valleys (Nye, 1976;Wright, 1973;Boulton and Hindmarsh, 1987) and, to a certain extent, to the origin of glacial basins (van der Wateren, 1992). We follow the method applied by Shoemaker (1986), Boulton and Jones (1979) and Mooers (1990) for modelling subglacial hydrology, but we put emphasis on longitudinal ice sheet trends.…”

Glaciotectonic deformation and reinterpretation of the Worth Point stratigraphic sequence: Banks Island, NT, Canada

“…However, as ice persisted at Worth Point, permafrost would have warmed from the bottom-up by geothermal heating (Dyke,1993), favouring the addition of water as well as more ductile deformation (cf. Mooers, 1990;Waller et al, 2012). Therefore, the variable physical properties of the permafrozen sediment at Worth Point, and their consequent response to changeable glacial stress regimes, would have evolved over the course of multiple phases of glaciotectonism.…”

The Last Cordilleran Ice Sheet in Southern Yukon Territory

“…pressures involve restricting subglacial drainage through the freezing of the glacial sole to its substrate in advance of wetbased ice flow (Moran era/., 1980;Mooers, 1990) or freezing of the glacier sole to its bed due to ice accumulation and flow dynamics (Eyles, 1983a, p. 20-23). The former conditions would be expected during glacial onset as the relatively thin and easily chilled margins of glacial ice advanced over permafrost terrain (Eyles, 1983a, p. 21).…”