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Some continental margins appear to be structurally featureless; however, those which are not now, but once may have been, orogenic commonly offer some evidence of subsurface tensional or transcurrent faulting. The modern borders of the Atlantic are typical. Where such faults have not been defined, differential vertical uplift along the coast suggests local faulting.

Orogenic-Taphrogenic Margins

Where a coast has been an orogenic margin, as along much of eastern North America, the typical structures, intrusives, and mineral belts are present and the taphrogenic features (Fig. 41) are superimposed. Tensional relaxation of compression to produce rift valleys is typical, and transcurrent displacement of hinterland and belt segments may be related, in this mixed environment, the earlier orogenic mineralization has a profound effect on later taphrogenic mineralization.

During orogeny the zoned mineralization belt is established. All the elements of low threshold mobility are mobilized and reestablished under new stability conditions in the various zones. The hinterland, especially, is flushed of low-temperature elements that might have been present originally.

Postorogenic taphrogeny occurs mostly in the “flushed” hinterland, and usually stimulates mineralization of low temperature and low intensity (Gabelman, 1976a). In several belts, taphrogeny has caused remobilization of earlier deposits but has not changed the established element zones (Gabelman, 1968a, b; 1976a) - e.g., in the Appalachian Triassic grabens. In such areas it is believed that the presence of low-temperature minerals, including those of uranium, indicates taphrogeny, and that their zonal configuration illustrates the fault zones or lineaments which were the possible feeding fractures. Investigation of the grabens in Pennsylvania (Gabelman, 1974) showed that most of the taphrogenic mineral belts enclosed the graben margins or crossed the adjacent orogenic fold belt at a high angle.

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