39 Structural Geology (Posters)

Added due to amount of discipline abstracts submitted.

Wednesday, 11 March 2026
9:00 AM - 12:00 PM
RCC, Lower Level Hall

Effect of Stress Concentrations on Quartz-Coesite Transition During Dislocation Creep

Paper number: 39-2
Program: Technical Sessions
Time: 9:00 AM – 9:00 AM

Authors

Abstract

The reaction of quartz to coesite may occur in subduction zones and lead to deep focus earthquakes. This reaction occurs at high hydrostatic confining pressure but several studies including Hirth and Tullis (1994) and Richter et al. (2016) found that coesite formed during deformation by semi-brittle creep when confining pressure and mean stress were significantly below the coesite stability field. They concluded that the reaction occurs as soon as the maximum principal stress (σ1) reaches the coesite stability field, not the mean stress. However, stress concentrations about cracks that form during semi-brittle creep may cause locally high mean stresses that drive this reaction. To determine if the reaction is caused by mean stress or σ1, we experimentally deformed a natural novaculite by dislocation creep in the absence of cracking using the D-DIA at confining pressures of 2 to 5 GPa, a temperature of 900̊ C, and a strain rate of 10-5 s-1. In these experiments, σ1 and mean stress were varied relative to the 3 GPa threshold: σ1 and mean stress below the quartz-coesite reaction pressure, σ1 above the reaction pressure while mean stress remained below, and both σ1 and mean stress above the reaction pressure. In all the experiments, the novaculite cylinder deformed at a constant strain rate and steady-state strength after reaching ~5% shortening. Coesite was not observed in X-ray spectra collected during the experiments, but the volume fraction of coesite may be beneath the detection limit. However, we will perform microstructural analyses to determine if coesite formed, the volume fraction of coesite produced and if it coesite formation is related to the mean stress or σ1.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411731

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