The mechanism of flow and fabric development in mechanically anisotropic trachyte lava

Anisotropy of magnetic susceptibility (AMS) and Electron back-scattered diffraction (EBSD) of magnetite and sanidine fabrics throughout an eroded trachyte lava dome in Tertiary volcanic province of the NW Czech Republic revealed two fabric types. The high degree of AMS fabric is associated with sanidine textural domains similar to normal kink bands (Type I fabric) and occupies the whole body except the SW margin. Folded fabric and low anisotropy of AMS also in the SW margin reveals sanidine alignment domains resembling reverse kink-bands (Type II fabric). The flow of trachyte lava occurred via simultaneous slip of sanidine crystals along their (010) planes and also by readjustment of the textural domain boundaries according to the fibre-slip theory. This microfabric study suggests that the Type II fabrics resulted from collapse of vertically anisotropic trachyte crystal mush above the feeding conduit. Type I fabric is interpreted to originate from Type II fabrics by further stretching of highly attenuated fold limbs. Asymmetric Type I fabrics along margins of the dome are related to outflow of trachyte lava away from the conduit region due to divergent flow. The trachyte fabric zonality is interpreted to reflect the process of successive emplacement of progressively rotated trachyte lava lobes within a lava dome that locally preserves the collapsed and folded vertical fabrics.