Model for glaciotectonism

Dislocated, thrusted, and folded masses of soft bedrock and drift have been described throughout the glaciated portions of central North America and northern Europe, and the origin of these surficial deformations has mainly been settled in favor of glacioteconic causes. The model for g]aciotectonic deformation described here consists of two stages: (1) initial stage of pro glacial or ice-marginal thrusting and stacking of large floes during glacier advance over permafrozen terrain, and (2) subsequent stage of subglacial shearing and penetrative deformation of previously-thrusted floes, when ice overrides the area. Ice thrusting takes place along a basal permafrost decollement, where high pore-water pressure is de­veloped, and floes are pushed as permafrozen slabs forward and upward in front and beneath the toe of the advancing glacier to form a topographic ridge (Stauchmoranen or kuppelbakke ). Once the ice overrides the ridge of thrusted floes, subglacial deformation by basal shearing may create glaciodynamic melange. Most glaciotectonic disturbances are thought to develop in a largely frozen state, and the possibility of large-scale subglacial thrusting beneath thick ice or far back from the ice margin is generally believed negligible. The majority of severe glaciotectonic deformations in central North America and northern Europe are found in transitional zones, which separate the central glaciated zones characterized by deep glacial erosion from marginal zones, where glacial deposition was the rule. Glacial thrusting provides a mechanism for stripping sedimentarycover off the Canadian and Fennoscandian shields during repeated Pleistocene glaciations.