Fabric transitions in quartz via viscoplastic self-consistent modeling part I: Axial compression and simple shear under constant strain

Luiz F.G Morales,Geoffrey E Lloyd,David Mainprice

doi:10.1016/j.tecto.2014.08.011

Abstract

Quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures, leading to the development of different types of crystallographic preferred orientation (CPO) patterns. In this contribution we present the results of an extensive modeling of quartz fabric transitions via a viscoplastic self-consistent (VPSC) approach. For that, we have performed systematic simulations using different sets of relative critical resolved shear stress of the main quartz slip systems. We have performed these simulations in axial compression and simple shear regimes under constant Von Mises equivalent strain of 100% (γ=1.73), assuming that the aggregates deformed exclusively by dislocation glide. Some of the predicted CPOs patterns are similar to those observed in naturally and experimentally deformed quartz. Nevertheless, some classical CPO patterns usually interpreted as result from dislocation glide (e.g. Y-maxima due to prism <a> slip) are clearly not developed in the simulated conditions. In addition we reported new potential preferred orientation patterns that might happen in high temperature conditions, both in axial compression and simple shear. We have demonstrated that CPOs generated under axial compression are usually stronger that those predicted under simple shear, due to the continuous rotation observed in the later simulations. The fabric strength depends essentially on the dominant active slip system, and normally the stronger CPOs result from dominant basal slip in <a>, followed by rhomb <a> and prism [c] slip, whereas prism <a> slip does not produce strong fabrics. The opening angle of quartz [0001] fabric used as a proxy of temperature seems to be reliable for deformation temperatures of ~400°C, when the main slip systems have similar behaviors.

Highlights

Quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures, from very low metamorphic grades just above diagenesis, to sub-solidus temperatures
Our observations are in indirect agreement with fabric studies in calcite deformed under simple shear, where the authors demonstrated that the strength of crystallographic preferred orientation (CPO) becomes stronger with the imposed strain even when the aggregates are completely recrystallized (e.g. Barnhoorn et al, 2004)
Numerical simulations of the crystallographic preferred orientation transition in quartz via viscoplastic self-consistent approach under axial compression and simple shear in constant strain allowed us to conclude the following: (i) CPOs are strongly developed in axial compression rather than in simple shear

Summary

Introduction

Quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures, from very low metamorphic grades just above diagenesis, to sub-solidus temperatures. Such evolution was due mainly to improved understanding of the physical processes underpinning the development of CPO in crystalline materials, combined with a significant improvement in computational processes that allowed the development of complex codes capable of incorporating a number of different parameters that influence CPO development Chastel et al, 1993), isotropic models (Takeshita et al, 1990), n-site models (Wenk et al, 1991), and viscoplastic self-consistent models (Lebensohn and Tomé, 1993) All of these methods have in common the assumption that deformation is accommodated by dislocation glide on a limited number of crystallographic planes in specific crystallographic directions. The lower bound state assumes that the stress in the aggregate is homogeneous, whereas in the upper bound state it is the strain in an aggregate that is homogeneous (Sachs, 1928; Taylor, 1938)

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Discussion

Conclusion