DAMAGE ZONE DEVELOPMENT DURING FOLDING OF A WELL CEMENTED QUARTZ ARENITE

Damage zones are suites of brittle structures often spatially associated with fault zones and developed in porous sandstones deformed at low P/T conditions. In contrast, we examine a damage zone in a fold hinge in the well cemented Silurian Tuscarora Sandstone of the central Appalachians. The fold hinge is one of two anticlines that form the core of the Cave Mountain anticline, an asymmetric mapscale fold in the roof sequence above the Alleghanian Wills Mountain duplex that formed at subgreenschist conditions in the central Appalachian Valley and Ridge province. The Cave Mountain anticline also contains three backthrust systems with greater total layer-parallel shortening than the two anticline hinges. The damage zone is developed where a lesser backthrust system and an anticlinal hinge intersect.

The two anticlinal hinges in the core of the Cave Mountain anticline are geometrically different. One hinge is a chevron-style fold that has a bulk dip change of 70º from a shallow to a steep NW dip. The second hinge is an open fold with only a 20º dip change from a shallow SE dip to a shallow NW dip. Given their geographic proximity and similarity in stratigraphic position, the two anticlines likely formed at the same P/T conditions, yet the two folds differ not only in style, but in internal structures and, hence, deformation behavior during formation of the Cave Mountain anticline.

The chevron anticline has a narrow hinge and straight limbs, local hinge contraction faults, and an array of microstructures including deformation lamellae, grain suturing, and fluid inclusion planes that are typical of Alleghanian deformation of the Tuscarora Sandstone. In contrast, folding in the open hinge is accomplished through a network of NW-dipping thrusts, or backthrusts, with relatively small displacements that are linked by multiple relay fracture sets. Dilational breccias and porous zones along with very intense joint sets are developed within the hinge region. Microscale brittle structures, such as fluid inclusion planes, microveins, and dilational microcracks are also abundant as compared to the microstructures in the chevron fold. The abundance of these structures suggests that fluid played a role in triggering formation of this unusual damage zone.