Spatial phase distribution and deformation processes

Spatial phase distributions can be grouped into random, clustered or anticorrelated/distributed based on the probability to encounter a given nearest neighbour. This property can also be probed with respect to intervals of directions, frequently revealing an anisotropy in the spatial phase distribution. In this study, several high temperature ultramylonites with variable composition from felsic to ultramafic as well as coarse grained deformed rocks (e.g. eclogite from Münchberg, Germany) were investigated. Measurements of phase distribution anisotropy frequently manifest in a pronounced direction of phase clustering and one direction of anticorrelation. Especially in the investigated ultramylonites but also in deformed eclogites and amphibolites, those two directions are found not to be orthogonal and not to coincide with finite strain axes (as far as manifested by foliation and stretching lineation). Clustering phases (e.g. qtz, plg or grt, depending on the rock type) form stacks antithetically tilted against the sense of shear. These stacks are separated by phases such as kfs, bt or cpx. Below a certain volume threshold of the stack-forming phase, stacking is not observed.

In addition to the phase distribution, truncated chemical zonations and/or indications of directed growth are frequently observed. On the other hand, there is a lack of microstructures which can reasonabley be associated with steady state dislocation creep.

It is suggested that the observed microstructures in combination indicate deformation by a mechanism best described by dissolution-precipitation accommodated granular flow (or "diffusion creep" in the broadest sense). Stack-forming phases undergo mostly rigid-body rotation and translation temporarily forming transient force chains before being disintegrated again. Since these stacks can be observed in the rock record, the residence time in the force chain position must be greater than in a randomly distributed position, compatible with jamming of particles during granular flow.

The presence of this particular type of anisotropic spatial phase distribution may not only serve as a shear sense indicator but could in general be useful for the identification of deformation mechanisms.