SYNTAXIAL QUARTZ VEINS ACT AS MECHANICAL ANISOTROPY DURING LAYER-PARALLEL SHORTENING, LOCALIZING FOLDING IN GNEISS: TWO ADDITIONAL EXAMPLES FROM THE INNER PIEDMONT OF SOUTH CAROLINA

Extensional syntaxial quartz veins in Late Ordovician orthogneiss in the Inner Piedmont, South Carolina, have localized the formation of some meso-scale folds of the Walhalla thrust sheet. Once localized, continued compression deformed the veins. Previously Clendenin and Garihan (JSG, 28, 2006) had recognized this vein-fold development relationship in Six Mile schist, where progressive folding evolved into fault propagation laterally away from vein ends.

Coarse quartz crystals are 90° to margins in the lenticular veins, which are a few cm wide and tens of cm to ~1 m long. A middle ‘seam’ marks the position of new quartz growth. Vein characteristics include: parallelism with or at a slight angle to adjacent gneissic foliation; locally overlapping, nested veins; composite veins with several growth ‘seams’; generally no open voids within the vein, indicating quartz growth at ‘seams’ matched vein opening rate(s); a bleached zone adjacent to some vein margins; and vein formation prior to regional extensional, collapse-related structures.

In one example, a localized W-vergent, overturned fold initiated at the vein during layer-parallel shortening, which then continued to deform the vein and adjacent gneiss. Slip along later small-scale normal faults truncated the overturned fold limb and followed straight margins of the folded vein. In the second example, syntaxial veins localized folding and then were deformed into tight, chevron-style S and Z parasitic folds about a developing isocline.

Vein history indicates: 1) a pre-vein episode of tight folding of gneiss foliation; 2) development of syntaxial veins by compression nearly parallel to shallow foliation attitudes, and their subsequent, continued tight-isoclinal folding, likely during regional emplacement of the Walhalla and Six Mile thrust sheets; and 3) later local slip along margins of folded syntaxial veins during normal faulting associated with regional, SW-directed gravitational collapse of the thrust stack.