COEXISTENCE OF PSEUDOTACHYLYTE VEINS AND MYLONITIC ZONES AT THE BASE OF THE DEEP CRUST: AN EXAMPLE FROM THE CALABRIA (SOUTHERN ITALY)

Natural examples indicate that pseudotachylytes cohexist in the same outcrop with mylonites and ultramylonites. Pseudotachylytes intimately associated with mylonites and ultramylonites can develop in high strain zone close to the brittle-ductile transition (e.g. Passchier, 1982) or entirely within the ductile regime as result of plastic instabilities (e.g. Hobbs et al., 1986; Handy & Brun, 2004). This study report microstructural investigations on two pseudotachylyte veins found within the felsic granulites at the base of the ~20-25 km thick Variscan crustal section outcropping in the Serre Massif (southern Calabria). Felsic granulites consist of quartz, plagioclase, K-feldspar, biotite, garnet, sillimanite and accessory minerals. Stretched minerals and S-C composite foliations are detected in zones crystal-plastic deformation. In places, felsic granulites exhibit an alternance of mylonitic and ultramylonitic bands (a few millimeters thick). Pseudotachylyte fault-veins develop along planes, which have a parallel orientation to the mylonitic and/or ultramylonitic foliation. Locally, pseudotachylyte fault-veins occur along the S-C composite foliations of the felsic granulite. On the other hand, the pseudotachylyte injection-veins cross cut the mylonitic and/or ultramylonitic bands. Microstructural observations indicate that the felsic granulite exhibits a strong grain-size reduction along the S-C composite foliations and near the contact with the pseudotachylyte veins. Back scattered electron (BSE) images, obtained by scanning electron microscopy (SEM) and Field-Emission Gun SEM, show that in proximity of the contact with the pseudotachylyte the garnet of the felsic granulite is fractured and shows rims characterized by a new cristallization of very small euhedral garnet (3-4 m). On the other hand, the ultramylonitic bands display a very fine-grained matrix and have a dark appearance. However, the BSE images reveal a strong penetrative foliation, which is defined by the alignment of biotite and by the shape preferred orientation of quartz, plagioclase and garnet. Moreover, the ultramylonitic bands are characterized by a new crystallization of very small crystals (a few microns in length) of sillimanite and K-feldspar, aligned along the foliation planes. Matrix of the pseudotachylytes is microcrystalline and contains abundant clasts (>50%) made up of quartz, plagioclase, K-feldspar, garnet and rare biotite. Clasts in the matrix are aligned with a parallel orientation to oblique foliation of the mylonitic granulite. At the margin of the vein, garnet of the pseudotachylyte may occur in two habits: 1) garnet microlites with very small sizes (3-4 m) and an idiomorph habit, which formed by direct crystallization from the frictional melt, and 2) garnet clasts (a few ten micrometres in size), with rims characterized by a new crystallization of very small garnets (<2 m) and with a similar aspect to the garnet rims of the host rock. In the vein centre, the matrix is mainly composed of skeletal plagioclase and biotite (a few microns in lenght). Plagioclase and biotite microlites often nucleated on rounded clasts of quartz or plagioclase. Garnet microlites are absent in the vein centre. These data, combined with the indications for the formation depth of the pseudotachylytes (21-23 km) obtained by Altenberger et al. (2010), indicate that during propagation of the seismic rupture the shear deformation was highly heterogeneous and took place through the development of alternating pseudotachylyte and ultramylonite, as result of plastic instabilities. References Altenberger, U., Prosser, G. & Grande, A. (2010): Workshop Physico-chemical processes in seismic faults, 11 Handy, M.R. & Brun, J.P. (2004): Earth Planet. Sc. Lett., 223, 427-441 Hobbs, B.E., Ord, A. & Teyssier, C. (1986): Pure Appl. Geophys., 124, 309-336 Passchier, C.W. (1982): J. Struct. Geol., 4, 69-79