Static deformation of silica and silicates

This paper reviews the static ductile deformational processes in naturally and experimentally deformed quartz, olivine, pyroxenes, and plagioclase as determined by many workers primarily in the last decade. The orientations of deformation lamellae in experimentally deformed quartz vary with temperature, pressure and strain rate but, as predicted by the experiments, lamellae of only one orientation (sub-basal I) are similar in all respects to most natural lamellae. The slip systems in olivine change with increasing temperature and pressure and decreasing strain rate from (100), {110}[001] through {0kl}[100] to (010)[100]. At a natural strain rate of 10−14 sec−1 the {0kl} [100] system observed in most peridotites should operate at temperatures above about 300°C at low (3–5 kb) pressures. Orthopyroxenes in rocks commonly deform by kinking on the system {100}[001], but this mechanism is observed in experiments only at very high temperatures and low rates, since deformation under other conditions is accompanied by a martensitic inversion to clinopyroxene. The slip-versus-transformation boundary is strongly dependent on temperature and rate, as well as on stress, and the transformation should not take place in nature at temperatures above about 500°C. Experimentally deformed clinopyroxenes twin on {001}[100] at the lower temperatures and higher rates, giving way at higher temperatures and lower rates to twin and translation gliding (in opposite sense) on {100} [001]; the latter mechanisms are dominant in tectonites. Plagioclases are rarely deformed in tectonites, but experiments on those of various composition and structural states have produced albite and pericline twinning, {010} slip, and subbasal lamellae. The best understood of the recovery processes, polygonization, has been observed in both experimentally and naturally deformed quartz, olivine, and enstatite. Polygonization, leading to the formation of subgrains having small disorientations, occurs by the diffusion-controlled process of dislocation climb. Dry creep experiments on peridotite have shown the polygon size in olivine to be dependent on stress. The analysis leads to an estimate of 1 kb stress difference during polygonization of olivine in certain xenoliths, although small concentrations of H2O might lower this estimate appreciably. Syntectonic recrystallization of α quartz aggregates in axially symmetric experiments give c ‖ σ1 at low temperatures and high rates, changing continuously to small circle girdles (25° to 50°) about σ1 at the higher temperatures and lower rates. Crossed-girdle fabrics, similar to those commonly observed in quartz tectonites, were produced in the a field at high temperatures in experiments in which the stress and strain fields were not axially symmetric. Quartz aggregates recrystallized in the β field develop composite c ‖ σ1 + c ⊥ σ1 fabrics. Experimental syntectonic recrystallization of both olivine and enstatite aggregates gives b ‖ σ1 with a and c girdles in the σ2=σ3 plane, fabrics also common to naturally deformed peridotites. However, in natural deformations in which σ1 ≠ σ2 ≠ σ3, a (ol) probably develops parallel to σ3 as may a (opx), in accord with predictions of the elastic anisotropy hypothesis. The deformational processes induced by static and dynamic deformation of silica and silicates are compared briefly; the processes operative in tectonites are readily distinguished from those produced by shock deformation.