Air photographs studied over areas that had been glaciated during the Pleistocene show a lineament pattern on the surface. Most of these can be attributed to ice-sheet scouring, fluting, and stream-lining. Certain lineaments, however, can not be attributed to glacial action. Only those in excess of 10 mi in length and consisting of subtle tonal changes commonly associated with stream alignments are herein considered. For these non-glacial lineaments, several theories explaining fault reflection have been proposed. The volume of ice present over North America during the Pleistocene was very great compared with the usual rate of sedimentation; unloading of the crust, deglaciation, took about 5000 years. Ice front retreat was not uniform and ice stagnation occurred in large areas. It is here proposed that initial rapid crustal rebound and tilting alone, and not necessarily accompanied by a high incidence of earthquake activity, affected the existing faults and lines of weakness in the bedrock. Another possible mechanism for differential rebound is that if a lithological variation exists across a fault, with one side more susceptible to compaction due to ice loading than the other, the change in density combined with differential depression, could affect the isostatic balance of that unit; this could result in different isostatic adjustment upon rebound after deglaciation.

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