Microstructure modeling

Microstructure model (left) of early-stage, asymmetric crenulation cleavage. Phyllosilicate (P)- and Quartz/Feldspar (QF)-domains indicated. Polygons identified as muscovite (lighter) and quartz (darker). Both minerals have complete 3D elasticity tensor components as attributes. Muscovite oriented so that C-axes are perpendicular to long dimensions, and parallel to plane of page. Quartz grains all have a single crystallographic orientation (for simplicity in this instance). Microstructure was subjected to elastic strain. Model was shortened by 1% in the X-direction (horizontal), and allowed to stretch in the Y direction (vertical). Mean stress and shear strain shown – dark colors represent relatively high values for both. Deformation is plane strain, but 3D stresses are calculated.

This simplified model shows that the quartz-rich regions of QF domains have higher mean stress than in the P-domains. This may contradict the pressure solution hypothesis for crenulation cleavage development. High elastic shear strains localized in the P-domains may be responsible for dissolution of quartz and feldspar in these zones, and dilatational strains in the QF domains may provide sites of deposition for the dissolved material. Thus, the mass transfer so characteristic of crenulation cleavage development may be driven by strain as opposed to stress.

We are currently funded by NSF to investigate this problem as part of a larger program to investigate the coupling of chemical and mechanical processes in fabric development. We'll be preparing the first publications near the end of 2007, so stay tuned!