GLACIERS - EXAMPLES OF COMPLETE TECTONIC SYSTEMS

Glacial ice develops structures and fabrics that have their counterparts in rocks, and glaciers provide natural laboratories for structural geologists. Glaciers have the advantage of being monomineralic. Ice is well understood rheologically, deforming close to its melting point and behaving as a power-law fluid involving crystal plastic deformation mechanisms and dynamic recrystallization under the stresses encountered in nature. It has low tensile strength, resulting in extensional fractures common at the surface of glaciers. Ductile and brittle processes may operate at the same time in the same volume of ice. The existence of faults in glacier ice has been controversial. Under most conditions it can be shown that ice will flow before shear failure occurs, and what are often interpreted as faults in glaciers have other origins.

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.