Microstructures and deformation mechanisms

The cataclastic rocks encompass the whole spectrum between clast-supported protocataclasite (matrix <10%) and matrix-supported ultracataclasite (matrix ~90%). There is a general decrease in the size and increase in roundness of clasts and increase in matrix proportion when passing from protocataclasite to ultracataclasite. This observation suggests the increasing importance of cataclasis, which consists in microfracturing and microbrecciation of quartz and feldspar crystals, together with intergranular sliding and rotation of the resulting fragments as suggested by Fabbri et al. (2000). Although cataclasis played the most dominant role in the evolution of this fault rock diffusive mass transfer also played a subordinate but important role in the genesis of cataclasite. The signature of dissolution is stylolite-like surfaces crossing clasts of quartz and, in some instances, the matrix. Along with mechanical cataclasis, recrystallization of the dissolved material occurred in the matrix part where aphanitic silica precipitation is pervasive. In the matrix dominated cataclasite the long clasts show a tendency to be aligned preferentially indicating occurrence of flow in matrix-supported cataclasite.

The mechanisms of deformation which led to the formation of the randomly oriented cataclasite are faulting, cataclasis, and diffusive mass transfer processes. The dark seams, in fact, consist of residual accumulations of insoluble biotite and iron oxides. Newly recrystallized polygonal quartz grains or subgrains surround larger quartz or feldspar crystals and fill the intragranular fractures spaces in a similar way described by Hippertt and Egydio-Silva (1996). Plastic deformation mechanisms played a subordinate role in the process and are represented by undulatory extinction, recrystallization and subgrain formation of quartz grains and sigmoidally bent biotite crystals. Silicification is pervasive and all samples studied under cathodoluminescence show silicification along fractures down to hairline cracks.