The strain path and emplacement mechanism of lava flows: an example from Salina (southern Tyrrhenian Sea, Italy)

Abstract

A lava flow from Salina (southern Tyrrhenian Sea) consists of subcircular to ellipsoidal basaltic enclaves dispersed in a dacitic host. A 2D strain and kinematic analysis of the enclaves has been performed in order to determine (a) the relative contribution of the coaxial ( α) and non-coaxial ( γ) components to the bulk flow deformation, (b) the flow vorticity Wk, (c) the strain path and (d) the mechanism of flow emplacement. The axial ratio R f and the angle φ f between the long axis of the enclaves and the transport direction have been measured in 196 sites located at different distances from the vent. In the near vent zone α> γ whereas further from the vent γ> α. α values were always larger than 1, ranging from 1.6 in the near vent zone to 1 at the front. γ values were between 0.2 (at the back) and 2.6 (at the front). Wk was between 0.13 and 1 and increased from the back to the front. The enclaves result from the injection of basalt into dacite. Most of the deformation was acquired during the lava flow emplacement and not during the rise in the conduit (plug flow). The strain path depicted by the enclaves is consistent with that resulting from experimental analogue models and reveals that the lava suffers lateral extension near the vent. Further from the vent, the lava deforms according to an ideal non-coaxial model. The lava emplacement is mainly controlled by the gravity. Evidence of deformation induced by the magma pressure is lacking. Near the vent, the lava behaves as a hyperbolic flow whereas at the front it behaves as a simple shear flow. The mechanism of flow emplacement is consistent with a mixed ‘viscous gliding–spreading’ transport model at the back and with a ‘viscous gliding’ model at the front.