The kinematics of lava flows inferred from the structural analysis of enclaves: A review

Abstract

Most lavas are viscous laminar flows, and the roles played by different physical, chemical, and geometric parameters on the mechanism of flow emplacement are relatively well known. However, the kinematics of lavas and the style of flow deformation are still poorly known. Strain indicators of lavas include stretched vesicles, enclaves, crystal and Anisotropy of magnetic susceptibility fabrics, and folds. Enclaves may show different shapes: subcircular to ellipsoidal enclaves, rolling-like structures, boudin-like structures, banding-like structures, and folds. Here, we discuss the constraints that must be satisfied for the use of enclaves as strain indicators, and review the analytical techniques that can be used to determine the finite pure shear α and simple shear strain γ. We review three lava flows cropping on the Porri Volcano (Salina Island, southern Tyrrhenian Sea, Italy) as case study. In these lavas, enclaves of different shapes have been selected at different places (vent, intermediate, and front zone). The structural analysis of the enclaves allows us to determine the ratio between α and γ. The kinematic vorticity number, Wk, which is a measure of noncoaxiality, ranges between 0.1 and 0.7 in the vent zone, and increases up to 0.9–1 at the front. The prevailing pure shear deformation at the vent is due to the lateral extension of the flow, whereas the simple shear increments are acquired during the lava flow motion owing to gravity. Lava flows are extensional to hyperbolic flows at the vent and intermediate zones, whereas they may be considered as ideal simple shear flows near the front zone.