The deformation of basement massifs in orogenic belts presents several fundamental problems—what is their mechanical response to deformation, how does the strain vary through them, and how does one explain the “missing” basement suggested by palinspastic reconstructions? The Blue Ridge anticlinorium in northern Virginia has at its core Precambrian granitic basement deformed during the late Paleozoic orogeny; there is a major cleavage fan across this anticlinorium. The deformation in the basement is inhomogeneous, being concentrated along narrow zones of ductile deformation where rocks are mylonitized. These zones may range in size from microscopic dimensions to tens of metres wide, and they tend to develop preferentially in certain basement rock types. Narrow deformation zones in coarse granitic rocks often occur in conjugate sets whose bisector is the regional cleavage. Integration of strain across these zones added to the penetrative strain, calculated from the rotation of these zones, and the shortening due to displacement on thrust faults, give the total shortening of the basement. Such shortening must be taken into account in attempting any kind of geologic reconstructions.

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