Thermally induced brittle deformation in oceanic lithosphere and the spacing of fracture zones

Brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model, with an emphasis on the spacing of fracture zones. We use a parallel FEM code, SNAC, which explicitly solves the momentum balance and heat energy equations. The problem domain is 500 km long and 50 km deep. The 2-D plane strain condition is imposed such that the domain represents a vertical ridge-parallel cross-section. Effects of cooling on momentum balance are two-fold: Viscosity is temperature-dependent and volumetric strain has a contribution from thermal contraction. Brittle deformation is represented by localized plastic strain within a material having an elasto-visco-plastic rheology with strain softening. Fracture zones are brittle zones that extend to the surface creating topographic features. Crustal thickness, creep strength, and the plastic flow rule are shown to control the formation of fracture zones. There is a threshold crustal thickness such that fracture zones are created when crust is thinner than the threshold. Creep strength shifts the threshold in such a way that lowered creep strength can completely prohibit the formation of fracture zones even if crustal thickness remains the same. Through finite versus zero dilatation in plastic strain, associated and non-associated flow rule results in nearly vertical and V-shaped primary cracks, respectively. Changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness, and thus related to brittle deformation. The slow- and oblique-spreading Reykjanes ridge is free of transform faults and fracture zones. It is known to have a uniform and larger-than-average crustal thickness (≥10km) due to the vigorous magma supply from the Iceland hotspot. The Australian-Antarctic Discordance is an anomalous portion of the intermediate-spreading Southeast Indian Ridge (SEIR), spanning from 120°E to 128°E. Compared to the normal parts of the SEIR, its notable characteristics include highly rugged topography, deeper residual depth, increased number of fracture zones, and thin crust (~4km). These syntheses are consistent with increased brittle deformation of oceanic lithosphere when the crust is thin and vice versa.