KINEMATIC SIGNIFICANCE OF MINGLING-ROLLING STRUCTURES IN LAVA FLOWS -A CASE-STUDY FROM PORRI VOLCANO (SALINA, SOUTHERN TYRRHENIAN SEA)

A basaltic andesite lava flow from Porri Volcano (Salina, Southern Tyr rhenian Sea) is composed of two different magmas. Magma A (51 vol.% of crystals) has a dacitic glass composition, and magma B (18 vol.% of c rystals), a basaltic glass composition. Magma B is hosted in A and con sists of sub-spherical enclaves and boudin-like. banding and rolling s tructures (RS). Four types of RS have been recognized: sigma-type: del ta-type; complex sigma-delta-types and transitional structures between subspherical enclaves and rolling structures. An analysis of the RS h as been performed in order to reconstruct the flow kinematics and the mechanism of flow emplacement. Rolling structures have been selected i n three sites located at different distances from the vent. In all sit es most RS show the same sense of shear. Kinematic analysis of RS allo ws the degree of flow non-coaxiality to be determined. The non-coaxial ity is expressed by the kinematic vorticity number Wk, a measure of th e ratio SI between pure shear strain rate and simple shear strain rate . The values of Wk calculated from the measured shapes of microscopic RS increase with increasing distance from the vent, from approximately 0.5 to 0.9. Results of the structural analysis reveal that the RS for med during the early-intermediate stage of flow emplacement. They repr esent originally sub-spherical enclaves deformed at low shear strain. At higher strain, RS deformed to give boudin-like and stretched bandin g structures. Results of the kinematic analysis suggest that high visc osity lava flows are heterogeneous nonideal shear flows in which the d egree of non-coaxiality increases with the distance from the vent. In the vent area, deformation is intermediate between simple shear and pu re shear. Farther from the vent, deformation approaches ideal simple s hear. Lateral extension processes occur only in the near-vent zone, wh ere they develop in response to the lateral push of magma extruded fro m the vent. Lateral shortening processes develop in the distal zone an d record the gravity-driven movement of the lava. The lava flow advanc ed by two main mechanisms? lateral translation and rolling motion. Lat eral translation equals rolling near the vent, while rolling motion pr evailed in the distal zones.