REGIONAL PATTERNS OF DAUPHINÉ TWINNING IN QUARTZ TRACK VARIATIONS IN PALEOSTRESS DIRECTION: AN EXAMPLE FROM THE PENNSYLVANIA SALIENT

Experimental and empirical studies demonstrate that Dauphiné twins in quartz may develop as a response to stress. The elastic properties of quartz are anisotropic, with the negative rhomb z-face being stiffer than the positive rhomb {r}. When loaded perpendicular to {z}, quartz may undergo Dauphiné twinning (equivalent to a 180° rotation around the c-axis) that aligns the more compliant r-face perpendicular to the stress. Although it has long been suggested that this phenomenon could form the basis of a paleostress tool, few empirical studies have explored the viability of this approach. Here, I present new quartz crystallographic data from sandstones collected around the Pennsylvanian salient, a large (> 200 km along-strike) map-scale structure in the Appalachians. Calcite twinning data from nearby limestones document regional variation in paleostress direction, making this an ideal setting within which to test and develop the quartz Dauphiné twin paleostress tool. Crystallographic orientations of quartz were collected using electron backscatter diffraction. All studied sandstone samples exhibit weak to non-existent preferred orientations of the quartz c-axis but clear patterns for the rhombs. Samples display a preferred alignment of the positive rhombs parallel to the direction of maximum compression inferred from published calcite twin data, consistent with the idea that Dauphiné twinning patterns reflect paleostress. However, due to the trigonal symmetry of quartz, one or two additional rhomb maxima are generally present that could represent alternative compressive stress directions. Thus, I explore whether or not other aspects of the crystallographic fabric, such as twin density or variations in peak twin intensity, can be used to discriminate between these possibilities.