Sub-critical fracture and associated retrogression of garnet during mylonitic deformation

Garnets from the Alpine Fault mylonites are fractured. Most fractures have a retrogressive fracture fill; a small number have garnet fill. A detailed SEM study has been carried out to elucidate the mechanisms of fracture and associated retrogression. Wide fracture zones contain free floating islands of garnet in a retrogressive matrix. Electron channelling patterns show that segments of garnet across such fracture zones rarely show any relative rotations. Where rotations are observed, they are small and systematic. Some large fractures show clearly that fracture and retrogression occurs by a replacement process. Fracture surface morphologies are indicative of surface dissolution. A model is established where crack tips advance by a process of dissolution at a high stress crack tip whilst precipitation occurs as the tip and its associated stress field advance. Precipitation of retrogressive fracture fill occurs if fracture occurs out of the field of stability of garnet.