GLACIERS - EXAMPLES OF COMPLETE TECTONIC SYSTEMS
Glaciers and ice caps can be considered complete tectonic systems. The driving force is gravity, and the kinematics can be established by direct measurement. All glaciers develop from the accumulation of sedimentary layers, with bedding, marked by variation in bubble and dirt content, that can often be traced into the most strongly deformed ice. Foliation develops from the modification by deformation of sedimentary layering and other features initially present or introduced into the ice. Prominent among the latter are veins that form from filled fractures. The intensity of foliation development reflects the intensity of strain, foliation becoming parallel to the base and sides of glaciers where shear strain may be very large.
Folds are common and result from perturbations in flow over irregular bedrock and from perturbations due to the development of fractures. The lack of strong rheological contrast in glaciers means that most folds are passive with similar fold geometry. In a small valley glacier in Sweden, under a simple overall kinematic frame involving eastward flow, folds of various orientations develop with end members being - horizontal and perpendicular to flow in weakly deformed ice near the equilibrium line; horizontal and parallel to flow in strongly deformed ice near the base and front of the glacier; and vertical, associated with fractures near the margins of the glacier. In the basal ice, sheath folds develop. There may be analogs of all these features in rocks.