Quantifying diapir ascent velocities in power-law viscous rock under
far-field stress: Integrating analytical estimates, 3D numerical
calculations and geodynamic applications
Abstract
Diapirism is crucial for heat and mass transfer in many geodynamic
processes. Understanding diapir ascent velocity is vital for assessing
its significance in various geodynamic settings. Although analytical
estimates exist for ascent velocities of diapirs in power-law viscous,
stress weakening fluids, they lack validation through 3D numerical
calculations. Here, we improve these estimates by incorporating combined
linear and power-law viscous flow and validate them using 3D numerical
calculations. We focus on a weak, buoyant sphere in a stress weakening
fluid subjected to far-field horizontal simple shear. The ascent
velocity depends on two stress ratios: (1) the ratio of buoyancy stress
to characteristic stress, controlling the transition from linear to
power-law viscous flow, and (2) the ratio of regional stress associated
with far-field shearing to characteristic stress. Comparing analytical
estimates with numerical calculations, we find analytical estimates are
accurate within a factor of two. However, discrepancies arise due to the
analytical assumption that deviatoric stresses around the diapir are
comparable to buoyancy stresses. Numerical results reveal significantly
smaller deviatoric stresses. As deviatoric stresses govern
stress-dependent, power-law, viscosity analytical estimates tend to
overestimate stress weakening. We introduce a shape factor to improve
accuracy. Additionally, we determine characteristic stresses for
representative mantle and lower crustal flow laws and discuss practical
implications in natural diapirism, such as sediment diapirs in
subduction zones, magmatic plutons or exhumation of ultra-high-pressure
rocks. Our study enhances understanding of diapir ascent velocities and
associated stress conditions, contributing to a thorough comprehension
of diapiric processes in geology.