Evolution of fracture networks in shear zones: Insights from see-through experiments on biphenyl aggregates

Evolution of fracture porosity in mid-crustal shear zones can be simulated in in-situ experiments. Evolution of fracture networks was monitored during simple shearing of 2-mm-wide zones in wet and dry aggregates of polycrystalline biphenyl (C₆H₅C₆H₅) in a Urai-Means see-through deformation apparatus. At low effective confining pressure in wet samples, mixed brittle-viscous deformation occurred at all strain rates (5.6 x 10^-4-5.8 x 10^-6 s^-1) at 94-97% of the absolute melting temperature. At the fastest strain rate, progressive shearing is rapidly localized to produce a narrow fault zone along a shear zone boundary. In contrast, at the slowest strain rate, fractures develop throughout the shear zone and connect to form continuous fracture systems at low shear strains (γ ≈2). These fracture systems accommodate most of the subsequent displacement in contrast to little fracturing and predominantly viscous deformation in a nominally dry experiment. Jogs, as parts of stairstepping fracture networks in wet samples, resemble in shape and distribution veins found in mid-to lower crustal shear zones. The experiments indicate that from low strains onwards, the presence of fluids close to confining pressure in creeping shear zones can lead to development of connected fracture networks that also localize most of the displacement.