The single-slip hypothesis revisited: Crystal-preferred orientations of sheared quartz aggregates with increasing strain in nature and numerical simulation

This study discusses the fabric development in naturally sheared quartz aggregates in comparison to results from texture modeling according to the polycrystalline plasticity theory with particular emphasis on the formation of a single c-axis maximum. The investigated natural shear zone samples were deformed at about 650 ± 50 °C with increasing strain up to γ ≈ 14 and show dynamic recrystallization microstructures of grain boundary migration recrystallization. Neutron diffraction texture analysis results in c-axis pole figures with a single maximum at the periphery of the pole figure. This maximum does not align with the shear plane normal towards higher strain, but rotates towards an inclined orientation in accordance with the sense of shear. Such a rotation is inconsistent with the single-slip hypothesis and suggests that the formation of this c-axis pattern is controlled by multi-slip on several slip systems. Based on the polycrystalline plasticity theory, this quartz fabric can develop if combined { 10 1 ¯ 1 } 〈 1 2 ¯ 10 〉 {r}<a>, { 1 ¯ 011 } 〈 1 2 ¯ 10 〉 {z}<a> and { 10 1 ¯ 1 } 〈 1 2 ¯ 10 〉 prism<a> slip dominates and must not be related to the commonly proposed ( 0001 ) 〈 1 2 ¯ 10 〉 basal<a> slip. The multi-slip texture development is in agreement with the shear sense interpretation from the asymmetry between well-defined quartz fabrics and the foliation. For dominant ( 0001 ) 〈 1 2 ¯ 10 〉 basal<a> slip in quartz and γ > 2, numerical simulations predict a single peripheral maximum perpendicular to the shear plane and two a-maxima with a ∼30°-inward position parallel to the shear plane. This simulation corresponds to naturally observed CPO patterns of quartz formed at different deformation conditions and it is in agreement with the single-slip hypothesis. Hence, our combined natural and numerical data suggest that the single-slip hypothesis is a possible explanation for a single c(0001)-maximum but not universally true.