This paper presents a new model for anisotropic damage in bedrock under the combined influences of biotite weathering, regional stresses, and topographic stresses. We used the homogenization theory to calculate the mechanical properties of a rock representative elementary volume made of a homogeneous matrix, biotite inclusions that expand as they weather, and ellipsoidal cracks of various orientations. With this model, we conducted a series of finite element simulations in bedrock under gently rolling topography with two contrasting spatial patterns in biotite weathering rate and a range of biotite orientations. In all simulations, damage is far more sensitive to biotite weathering than to topographic or regional stresses. The gradient of damage follows that of the imposed biotite weathering rate and does not extend beyond the weathering zone. The direction of micro-cracks tends to align with that of the biotite minerals. Relative to the stress field imparted by topographic and regional stresses, the stress field after 1,000 years of biotite weathering exhibits higher magnitudes, wider shear stress zones at the feet of hills, more tensile vertical stress below the hilltops, and more compressive horizontal stress concentrated in the valleys. These behaviors are similar in simulations of slowing eroding topography and static topography. Over longer periods of time (500 kyr), the combined effects or weathering and erosion result in horizontal tensile stress under the hills and vertical tensile stress under and in the hills. These simulations illustrate how this model can help elucidate the influence of mineral weathering on Critical Zone evolution.

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.