Abstract

Glacial striae, recently exposed on carbonate bedrock adjacent to Saskatchewan Glacier, Alberta, provide information about the mechanics of abrasion beneath temperate glaciers and along fault surfaces. The striae consist of plowed grooves that vary smoothly in width and depth but are located where similar variations in normal stress between the bed and subglacial rock fragments were unlikely. Finite-element calculations of stresses in bedrock beneath abrading fragments suggest that under constant normal loading, plowing elements with steep leading edges will plow progressively deeper into the bed with sliding, whereas those with more gently inclined leading edges will climb out of their grooves. Results of simple experiments in which limestone slabs were pushed beneath fixed plowing elements indicate a similar dependence on contact geometry. This dependence, together with rotation of fragments during glacier sliding, provides a plausible origin for the three characteristic shapes of striae in the area.

Shapes of striae indicate that abrading fragments commonly rotate, affecting depths of plowing and the shear stress supported by abrading debris. An important refinement in models of glacial abrasion would be the incorporation of fragment rotations into estimates of abrasion rates and debris-influenced glacier sliding velocities. Some striae become progressively deeper and wider down-glacier and then end abruptly, similar to some fault striae that have been linked to stick-slip phenomena. They may form as a result of tensile fracture at depth in front of plowing asperities that is not necessarily dependent on stick-slip motion. Other minor structures along faults that are used as shear-sense indicators may result from the same process.

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