Abstract:
We evaluate the applicability of plagioclase and gabbro flow laws by comparing
predicted and observed deformation mechanisms in gabbroic shear zones. Gabbros and
layered gabbro mylonites were collected from the Southwest Indian Ridge (SWIR), ODP
Hole 735B. Deformation temperatures are constrained by two-pyroxene thermometry,
stress is estimated from grain size, and deformation mechanisms are analyzed by
microstructure and the presence or absence of a lattice preferred orientation (LPO). Our
analyses indicate that mylonite layers deformed at a strain rate in the range of 10-12 to 10-
11 s-1, while coarse-grained gabbro deformed at a strain rate of approximately 10-14 to 10-
13 s-1. Plagioclase in pure plagioclase mylonite layers exhibit strong LPOs indicating they
deform by dislocation creep. Plagioclase grain size in mixed plagioclase-pyroxene
mylonite layers is finer than in pure plagioclase layers, and depends on the size and
proportion of pyroxenes. Progressive mixing of pyroxene and plagioclase within gabbro
mylonite layers is accompanied by weakening of the LPO indicating that phase mixing
promotes a transition to diffusion creep processes that involve grain boundary sliding.
Our results indicate that experimental flow laws are accurate at geologic strain rates,
although the strain rate for diffusion creep of fine-grained gabbro may be underestimated.
At the conditions estimated for the SWIR crust, our calculations suggest that strain
localization leads to a factor of two to four decrease in lower crustal viscosity. Even so,
the viscosity of lower gabbroic crust is predicted to be similar to that of dry upper mantle.
