Abstract

We describe quartz crystallographic preferred orientations (CPOs) from incipiently deformed quartz sandstones characterized by low-intensity but unambiguous alignment of the poles to positive {r} and/or negative {z} rhombs. These distinctive CPOs appear at minimal strains and in grains with scarcely modified original detrital boundaries. We consider the hypothesis that these patterns reflect Dauphiné twinning (a 180° misorientation about the c-axis) that preferentially affects grains oriented with the elastically stiffer z-rhombs at high angle to the maximum principal stress direction. Twinning facilitates elastic deformation by aligning the more compliant r-rhombs at high angle to the greatest principal stress. Crystallographic maps show that about two-thirds of all grains (by area) are twinned, and untwinned grains are oriented with an r-rhomb perpendicular to the inferred shortening direction. We document this pattern from low-grade quartzite from three locations: the Eureka Quartzite of northeastern Nevada (USA); the Mesón Group of northwestern Argentina; and the Antietam Formation of the Blue Ridge of central Virginia (USA). The widespread presence of these CPOs in minimally deformed quartz rocks suggests that they may be useful in defining paleostress trajectories.

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