RHOMB-DOMINATED CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS IN INCIPIENTLY DEFORMED QUARTZ SANDSTONES

Incipiently deformed quartz sandstones yield low-intensity but unambiguous quartz crystallographic preferred orientations (CPOs) in which the most distinctive characteristic is alignment of the poles to positive (r) and/or negative (z) rhombs. Most commonly, the r-poles form three point maxima parallel to the inferred principal strain directions whereas the z-poles either form symmetrically disposed point maxima or small-circle girdles suggesting rotation about one of the r-poles. Surprisingly, these weak but distinctive CPOs (with maxima commonly <2.5 multiples uniform density) appear even at minimal strains with scarcely modified original detrital grain boundaries. These CPOs contrast with most patterns in naturally deformed quartz-bearing rocks; typically, quartz c-axes show the highest intensity preferred orientations simply because there is only one c-axis whereas other crystallographic directions have three or more symmetrically equivalent axes. Experimental work shows that Dauphiné twinning (a 60° rotation about the c-axis) will occur when the z-rhombs are oriented perpendicular to the maximum principal stress direction, allowing the elastically-softer r-rhombs to accommodate more elastic deformation. To test this hypothesis, we have mapped the spatial distribution and orientation of twinned and untwinned quartz grains from these sandstones. In each case, we find about half of all grains (by area) to be twinned, and untwinned grains are consistently oriented with an r-rhomb perpendicular to the measured or inferred maximum shortening direction. We document this pattern from low-grade quartzites from three locations: the Ordovician Eureka Quartzite in the Pequop Mountains of northeastern Nevada; the Cambrian Antietam Formation of the Blue Ridge of central Virginia; and the Cambrian Mesón Group of the Eastern Cordillera in northwestern Argentina. The widespread presence of these CPOs in minimally deformed quartz rocks suggests that they can be used to define the deformational regime where measurable strains have not yet accumulated. One example of such a regime could be the layer-parallel shortening regime in the foreland of propagating thrust systems.