Olivine Grain Size Distributions in Faults and Shear Zones: Evidence for Nonsteady State Deformation

Abstract

The grain size distribution of deformed rocks may provide valuable information about their deformation history and the associated mechanisms. Here we present a unique set of olivine grain size distributions from ultramafic rocks deformed under a wide range of stress and strain rate conditions. Both experimentally deformed and naturally deformed samples are included. We observe a surprisingly uniform behavior, and most samples show power law grain size distributions. Convincing lognormal distributions across all scales were only observed for samples experimentally deformed at high temperature (1200 °C) and for some mantle‐deformed natural samples. Single power law distributions were observed for natural samples deformed by brittle mechanisms and by samples deformed experimentally in the regime of low‐temperature plasticity. Most natural samples show a crossover in power law scaling behavior near the median grain size from a steep slope for the larger grain fraction to a more gentle slope for the smaller grains. The small grain fraction shows a good data collapse when normalized to the crossover length scale. The associated power law slope indicates a common grain size controlling process. We propose a model that explains how such a scaling behavior may arise in the dislocation creep regime from the competition between the rate involved in the dislocation dynamics and the imposed strain rate. The common departure from lognormal distributions suggests that naturally deformed samples often have a deformation history that is far from a steady state scenario and probably reflects deformation under highly variable stress and strain rates.

Dynamic earthquake rupture in the lower crust

“…−1.5 and −1.8 and a cross-over to steeper slopes (−2.2 and − 2.6) for the largest clasts [insets of Fig. 2, C and D; see ( 16 ) and Methods].…”

“…Some of the garnet clasts from the images showed minor growth along their rims. The grain size distributions were fitted with either one or two power laws following the same procedure as in ( 16 ).…”

The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

“…Grains larger than the median size show a power law probability distribution with a large (negative) exponent (−3.6, Figure e), reflecting a very narrow grain size distribution. Grain boundaries are dominated by triple junctions and 120° angles (Aupart et al, ), reflecting significant recovery and grain growth in the wake of the fragmentation process. The fragmented garnet furthermore contains a large number of inclusions distributed along the grain boundaries and triple junctions of the new grains and subgrains, occupying in total ~7% of the garnet volume (Austrheim et al, ).…”

“…(e) Probability density function (PDF) of the area of grains defined by low‐angle boundaries in b) in blue, and d) in red. Gray bar denotes the scale at which there is a cross‐over to lower power law exponents for small grains (see Aupart et al, for methods and discussion of the significance of the cross over scale).…”

“…Figure shows the contrasting alteration observed in wall rock garnets from the two situations described above. One garnet cut by the high‐P,T pseudotachylyte (Figure a) experienced pervasive fragmentation into grains and subgrains (Figure b) with a mean grain size of 31 μm 2 (sample AF2‐4 of Aupart et al, ) and a strong crystallographic preferred orientation (CPO; J‐ and M‐index values of 6.0 and 0.3 respectively, cf. Mainprice et al, ).…”