Olivine rheology, shear stress, and grain growth in the lithospheric mantle: Geological constraints from the Kaapvaal craton

The rheology of Earth's mantle is a complex function that depends on, at least, temperature, pressure, stress, grain size, and water content. Understanding this functionality is of fundamental importance in mantle dynamics, but constraints from laboratory experiments entail considerable extrapolation. Here we propose a new observational approach based on a unique tectonic setting of the Kaapvaal craton, which allows us to exploit its continental lithosphere as a natural laboratory. Mantle xenoliths brought to the surface by kimberlites in the Kaapvaal craton delineate a representative thermal profile, and the spatial distribution of their ages provides a tight constraint on a lithospheric-scale deformation history. Combined with a simple modeling of the thermal and dynamic evolution of the upper mantle subject to continental drift, these xenolith-based observations can test the validity of existing flow laws at geological strain rates, estimate the magnitude of shear stress in the upper mantle, and constrain the effective grain-growth rate at lithospheric conditions. Calculated displacements relative to surface over the last 3.5 billion years, with flow-law parameters recommended by Korenaga and Karato (2008), are found to be consistent with geochronological observations. Our results also indicate that shear stress has been increasing in the past 3.5 billion years, owing to the cooling of the ambient mantle, within the range of 0.01-0.1 MPa. Though this range of shear stress is about two orders of magnitude lower than that predicted by grain-size-based piezometers, greater stress would lead to too large displacement across the lithosphere to be consistent with its geochronological pattern. The grain size of olivine in mantle xenoliths is typically 5-10 mm, which is too small to be at a dynamical equilibrium with the estimated stress, suggesting that grain growth is considerably suppressed by some mechanism such as orthopyroxene pinning.

► Olivine flow-law parameters are tested with a new observational approach. ► Secular evolution of shear stress in the upper mantle is estimated. ► The stress estimate is utilized to derive robust bounds on effective grain growth.