To investigate the effect of faulting on isotopic ratios, six K/Ar and 14 Rb/Sr mica, feldspar, and whole rock analyses were made on samples from two units of the Blue Ridge crystalline rock “Complex” and the adjacent Valley and Ridge sedimentary bedrock area near Blacks-burg in Southwestern Virginia. Included are “Older Precambrian” gneiss(es); Cambrian shale-siltstone; and sheared, mylonitized, and brecciated (fault) equivalents of these rocks. The oldest isotopic “ages” found are 687 + 15 m.y. (K/Ar, biotite) and 1320 ± 100 m.y. (Rb/Sr, whole-rock isochron) for the Grayson and Little River gneiss unit(s); the youngest are 345 ± 16 m.y. (K/Ar, whole-rock) and 330 ± 15 m.y. (Rb/Sr, mineral and whole-rock isochron) on samples from two fault zones.

Among the tentative conclusions reached are: (1) Some activity, probably the development of frictional heat that accompanies faulting, may cause adjustments of isotopic ratios so that the resulting “ages” indicate time of the faulting; and (2) isotopic data on rocks from such fault zones may also give clues as to the depth of the faulting.

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