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II. Rock Products of Thrusting |
Department of Geology, The University Dundee, Scotland, DD1 4HN
One of the salt glaciers in S Iran is interpreted here as an analogue of a gravity-driven, thrust nappe complex which has overridden its own debris. Fabrics from different parts of the body are interpreted in terms of grain boundary diffusion and dynamic recrystallization, together with tensional separation and sliding along the cubic cleavages of porphyroclasts of clear halite.
Slides generate within the salt as ramps, where the salt glacier slows and thickens to surmount the bedrock obstructions to its general downslope flow. Zones of plastic flow-folds and smaller zones of large-scale crenulation cleavage form upstream of the obstructions and the slides emerge from the downstream end of such zones. Few, if any, ramp directly to the surface and most turn to flats within the salt sheet where they may heal as a result of stress-drop annealing.
The competence of the salt mass is assumed to decrease down its length in response to a decrease in grain size and a relative increase in insoluble grains. Such a decrease in competence accounts for the change in character of the internal deformation of the salt sheet when traced downstream. First the slides and then the crenulation cleavage fail to develop upstream of successive obstructions and eventually all internal folding ceases. Instead, the salt glacier moves over the last few bedrock ridges as accommodation folds and another type of slide appears to form by layer parallel slip.
All the slides in the salt glacier originate near irregularities in its bedrock channel. The slide propagation zones remain stationary and the salt tears as it passes through, and beyond them. Instead of eroding its bedrock, the salt glacier effectively smooths irregularities in its channel by shearing over the infilling bodies of static salt.
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