Crack-seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures

An analysis of crack-seal fibre growth mechanisms has shown two important implications of the crack-seal process for the interpretation of microstructures in low-grade metamorphic rocks: 1. (1) The ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms. 2. (2) Fibrous crystals may develop cristallographic preferred orientations in certain crack-seal situations. The microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate (001) and grain shape preferred orientation can develop during crack-seal deformation by oriented growth mechanisms. During successive crack-seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history, resulting in preferential rejoining, by syntaxial overgrowth, of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack. Preferred orientation may also be developed and enhanced in crack-seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls. In this case the crystallographic preferred orientation need not be simply related to the displacement history during crack-seal fibre growth. Since crack-seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock, such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes.