The influence of seismically-induced deform­­ation on the microstructure of quartz in ductile regime

Abstract:

Large seismic events can induce transient elevated stress and strain rates in underlying ductile extensions of fault zones. As shear zone rocks are exhumed through the ductile crust and approach the seismogenic zone, they may experience stress variations from hundreds of earthquakes. To investigate the rheology and microstructure of quartz aggregates deformed under cyclic high-stress events, we carried out a series of Griggs type general shear and axial compression experiments on quartz aggregates subjected to multiple stress pulses. We designed the strain and relaxation time per pulse so that the integrated average strain rate of the experiments matched those of control experiments deformed at constant strain rate ( s^-1, s^-1). EBSD analyses indicate: (1) deformation involving stress-pulses generates grain size distributions indistinguishable from control samples deformed at the same average strain rate, regardless of whether samples were frozen immediately before or after pulses. (2) Grains in the stress-pulse samples have significantly lower aspect ratios compared to those in the control samples. (3) Stress pulse samples deformed at relatively high temperature (900 ℃) showed a strong Y-max fabric (i.e., easy slip on prism <a> system). At lower temperature (800 ℃), stress-pulse samples show CPO suggesting activity on both prism <a> and basal <a> slip systems. A weak CPO relative to controls in 800° C samples suggests enhancement of dislocation-accommodated grain boundary sliding (disGBS). (4) Stress values calculated from recrystallized grain sizes of stress pulse samples match average stress values of the final cycles. These observations suggest that peak stresses associated with transient seismic events are not reflected in recrystallized grain sizes. In turn, we argue that the grain size paleopiezometry can “see-through” the effect of seismicity and reflect the long-term stress history of middle crustal shear zones.