Changes in anelasticity and grain boundary processes with stress cycling
in semibrittle salt-rocks
Abstract
The coupled operation of fracture, diffusion, and
intracrystalline-plastic micromechanisms during semibrittle deformation
of rock is directly relevant to understanding crustal processes such as
earthquake rupture at the base of the seismogenic zone and failure of
salt caverns for energy storage. Triaxial stress-cycling experiments are
used to investigate elastic-plastic and viscoelastic behaviors in two
synthetic salt-rocks deformed at room temperature and low confinement.
During semibrittle flow at high differential stress, porous, granular,
work-hardened samples deform predominantly by grain boundary sliding and
opening accompanied by minor intragranular cracking and dislocation
glide. In contrast, fully annealed, near-zero porosity samples deform at
lower differential stress by dislocation glide, grain-boundary sliding
and opening accompanied by minor intragranular cracking. During
high-stress cycling and semibrittle flow, grain boundary sliding is
predominantly frictional; but, associated dispersal of water previously
trapped in fluid inclusions can activate fluid-assisted diffusional
sliding along grain boundaries at low strain rates. Young’s modulus and
Poisson’s ratio are largely controlled by the behavior of closed grain
boundaries. Grain boundary sliding accommodated by fluid-assisted
diffusion leads to nearly complete stress relaxation after semibrittle
flow, and in subsequent low-stress cycling both viscoelasticity and
pronounced hysteresis are observed. However, such time-dependent effects
vanish with grain boundary healing over days-long holds at low
differential stress. Experimental results suggest that within the
semibrittle regime, high-stress events can lead to significant transient
reduction in viscosity and related phenomena.