Vein development during folding in the upper brittle crust: The case of tourmaline-rich veins of eastern Elba Island, northern Tyrrhenian Sea, Italy.

Detailed structural analysis of tourmaline-rich veins hosted in the contact aureole of the 6 Ma Porto Azzurro granite in southeastern Elba Island, northern Tyrrhenian Sea is presented. Using geometric features of the veins, the physical conditions at the time of vein formation are estimated, namely the stress ratio, driving stress ratio and fluid overpressure. Two vein sets (A veins and B veins) have been recognized based on orientation and thickness distributions and infilling material. Analysis of vein pole distributions indicates stress ratio = 0.57 and driving stress ratio = 0.24 for the A veins andstress ratio = 0.58 and driving stress ratio = 0.47 for the B veins, and fluid pressures less than the intermediate stress magnitude. Analysis of geometric features of the veins gives estimated fluid overpressuresof between 16 MPa (A veins) and 32 MPa (B veins). We propose a model for the tectonic environment of vein development, in which formation of secondary permeability in the deforming thermal aureole of the Porto Azzurro pluton was controlled by ongoing development of fracture systems in the hinge zone of a regional NNW-SSE trending fold that favored transport and localization of hydrothermal fluids.