Impact of grain size evolution on necking in calcite layers deforming by combined diffusion and dislocation creep

Natural pinch-and-swell structure in centimetre-thick calcite layers shows a reduction of grain size from swell towards pinch. However, the impact of grain size evolution on necking and pinch-and-swell formation is incompletely understood. We perform zero-dimensional (0D) and 2D thermo-mechanical numerical simulations of calcite layer extension to quantify the impact of grain size evolution on necking for bulk extension rates between 10−12 s−1 and 10−14 s−1 and temperatures around 350 °C. For a combination of diffusion and dislocation creep we calculate grain size evolution according to the paleowattmeter (grain size is proportional to mechanical work rate) or the paleopiezometer (grain size is proportional to stress). Numerical results fit three observations: (i) significant thickness variations along the layer after extension, (ii) grain size reduction from swells towards pinches, and (ii) dislocation creep dominated deformation in swells and significant contribution of diffusion creep in pinches. Modelled grain size in pinches (10-60 μm) and swells (70-800 μm) is close to observed grain size in pinches (21 ± 6 μm) and in swells (250-1500 μm). In the models, grain size evolution has a minor impact on necking, and viscous shear heating and grain size evolution have a negligible thermal impact.