Plagioclase preferred orientation in layered mylonites: Evaluation of flow laws for the lower crust

@article{citeulike:2883518, 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), Ocean Drilling Program 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 that they deformed 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 2–4 decrease in lower crustal viscosity. Away from shear zones, the viscosity of lower gabbroic crust is predicted to be similar to that of dry upper mantle.}, author = {Mehl, Luc and Hirth, Greg }, citeulike-article-id = {2883518}, doi = {http://dx.doi.org/10.1029/2007JB005075}, journal = {Journal of Geophysical Research}, month = {May}, pages = {B05202+}, posted-at = {2008-06-11 19:13:34}, priority = {2}, title = {Plagioclase preferred orientation in layered mylonites: Evaluation of flow laws for the lower crust}, url = {http://dx.doi.org/10.1029/2007JB005075}, volume = {113}, year = {2008} }

TY - JOUR ID - citeulike:2883518 L3 - citeulike-article-id:2883518 TI - Plagioclase preferred orientation in layered mylonites: Evaluation of flow laws for the lower crust JF - Journal of Geophysical Research VL - 113 SP - B05202 N2 - 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), Ocean Drilling Program 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 that they deformed 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 2–4 decrease in lower crustal viscosity. Away from shear zones, the viscosity of lower gabbroic crust is predicted to be similar to that of dry upper mantle. AU - Mehl, Luc AU - Hirth, Greg PY - 2008/05/07/ UR - http://dx.doi.org/10.1029/2007JB005075 ER -