Open-File Report 2006-1259

Published 2006
Online Only

Structural Geology

Two periods of folding were distinguished in the area: an older tightly folded sequence in the rocks of the intermediate to felsic volcanic complex (Zv, Zvab, Zvrd, and Zvdf, fig. 2, fig. 3, fig. 4A, fig. 4B, and fig. 5) and several younger broad open folds in the Reedy Branch Tuff (Zr, Zra, Zrb, and Zrc). Subsequently, the Reedy Branch Tuff was itself deformed into several open folds. We interpret several narrow outcrop areas of the young Reedy Branch Tuff (sectors G, J, and K) to be the keels of doubly plunging synclinal folds.

The volcanic and volcaniclastic rocks older than the Reedy Branch Tuff are cleaved to a greater or lesser degree. Cleavage tends to be more intense near major shear zones, near some of the faults, and presumably near fold axes. Cleavage is poorly developed in most of the Reedy Branch Tuff.

Evidence of faults or fracture zones is common. Several straight northeast-trending stream courses, recognizable on Landsat images, topographic maps, and aerial photographs, were found during field work to be zones of fracturing. The Snow Camp fault and the South Fork fault system are interpreted to be major structural features that divide the area into the northwestern Cane Creek Mountain structural block, the downfaulted central Major Hill structural block, and the southeastern Chestnut Hill structural block (fig. 2, fig. 3, fig. 4A, and fig. 5).

Both the Snow Camp fault and the South Fork fault system (fig. 2, fig. 3, fig. 4A, and fig. 5) are interpreted to include many separate strands. Northeastward, where the structural block is wider and displacement was less, the older rocks within the graben become indistinguishable from those of the bounding areas, and the structure cannot be recognized. The bounding faults of the graben appear to splay outward from the Major Hill structural block as it is traced northeastward, suggesting that the block moved in that direction.

The highly complex Major Hill structural block consists of two distinct areas: the broad northeastern part is underlain by rocks of the intermediate to felsic volcanic complex (Zv, Zvab, Zvrd, and Zvdf) that have been intruded by many granitic and gabbroic plutons (Ztm, Zcg, Zlf, Zg, and ZCambrian symbold), especially adjacent to the bounding faults. They are unconformably overlain in the southwest-narrowing end of the block by the younger Reedy Branch Tuff, indicating that the hinged graben structure plunges southwest.

A few north- or northwest-trending faults are suggested by offsets of rocks and alteration zones. The northwest-trending fault inferred to be present north of Major Hill (fig. 2, fig. 3, fig. 4A, and fig. 5, sector F) is not supported by linear topographic features, but is based on the abrupt juxtaposition of strongly hydrothermally altered rocks and unaltered rocks. The trend of the inferred fault 2.3 mi (3.6 km) southeast of Saxapahaw (fig. 2, sectors D and H) follows a line of abrupt change in rock type as well as by local straight segments of stream courses. It also parallels the trend of individual strands of the South Fork fault system.

In the area south and southeast of Sheeprock (fig. 2, fig. 3, fig. 4A, and fig. 5, sector J), a sequence of northeast-trending faults of the South Fork fault system separates intensely altered rocks of the Major Hill block from much less altered rocks of the Chestnut Hill block. The different segments, in part inferred, appear to die out or are unrecognized to the northeast. Where faults can be examined in stream courses northeast of Ore Hill (fig. 2, fig. 3, fig. 4A, and fig. 5, sector N), on the Pine Hill Branch fault strand (sector K) and in a streambed close to the Braxton Mine (sector L), zones of brecciation, quartz-sericite alteration, and sulfide mineralization can be seen.


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