Influence of grain size and geothermal gradient on the ductile-to-brittle transition in arenaceous sedimentary rocks: implications for fault structure and fluid flow

Abstract

Abstract To investigate the interaction between the rheology of arenaceous sedimentary rocks (sand and sandstone) and stress conditions during burial we have coupled published results from deformation experiments with a simple quartz cementation model. The model provides valuable insights into controls on sandstone deformation consistent with observations from nature. A transitional zone exists in subsiding sedimentary basins, here referred to as the ductile-to-brittle transition (DBT), above which faults in normally pressured arenites will tend to form fluid flow barriers, and below which they will tend to form conduits. The DBT depth in sandstone is dependent upon geothermal gradient, burial rate and grain size. Low geothermal gradients, rapid sedimentation rates and coarse grain sizes favour a deep DBT and vice versa. Fine-grained arenites may only deform in a brittle manner for most natural burial rates and geothermal gradients, explaining why they do not usually contain thick deformation band zones. Coarser-grained arenites may deform in a brittle–ductile or ductile manner, which is why they often contain thick deformation band zones and occasionally experience pervasive porosity collapse. Sandstones within high geothermal gradient areas may deform to produce fluid flow conduits at shallow depths when porosities in the sequence as a whole are high; this possibly favours fault-related mineralization.