COMPRESSIONAL TECTONICS IN A POLYTHERMAL GLACIER TERMINUS

Glacier motion has often been viewed as a convenient analog for the tectonic emplacement of thrust sheets. Because glaciers are driven entirely by gravity, their consideration in tectonics has been limited primarily to gravity-driven gliding or gravitational spreading. On a local scale, however, variations in basal drag beneath glaciers cause basal shear stresses to be distributed non-uniformly, resulting in a variety of local stress fields in the ice. Where polythermal glaciers have margins at sub-freezing temperatures, enhanced ice-bed coupling under the cold, non-sliding margin can cause longitudinal compression as sliding ice at its melting temperature approaches slow ice downstream at the basal thermal transition (BTT). As a result, the glacier bed at the BTT may support substantially greater basal shear stresses than the sliding portions up-glacier. A possible consequence of this stress gradient is the development of thrust faults or shear zones across which basal ice is deflected toward the surface as it crosses the BTT. A better understanding of the behavior of glaciers under these conditions may yield insight into analogous processes in crustal tectonics driven by horizontal compression.

Using a two-dimensional, steady-state, finite-element model of ice flow, we have investigated the internal stress and strain-rate fields in a glacier with a frozen margin. The model is driven and tested with velocities and strain rates measured on the terminus of a polythermal glacier in northern Sweden where past kinematic analyses of debris-bearing ice have suggested that discrete shear zones permit uplift of basal ice over the BTT. Our model results combined with direct measurements of stresses and displacements in and beneath the glacier serve to better constrain the role of the shear zones in accommodating horizontal compression at the glacier margin.