Effects of lateral dyke propagation and pre-existing fractures on shallow deformation: Data from the Etna 1947 eruption and analogue models

Mount Etna, one of Europe's most active volcanoes, has experienced a variety of eruption settings throughout its history, including summit, lateral, and eccentric eruptions. In this study, we provide a detailed analysis of the structures formed during the 1947 lateral eruption, along the NE Rift, using historical aerial photos and accounts, archival images, and contemporary field and drone data. Photogrammetric processing of 1932 and 1954 aerial photos enabled us to map the structures formed before and during the eruption, in order to examine the effects of pre-existing fractures on the 1947 deformation pattern. With the aid of field surveys, we studied 90 normal faults, 194 dry fractures and 17 eruptive fissures, collecting data on the structures' length, azimuth, vertical offset, vectors and amount of opening. The reconstruction of a detailed chronology of the eruption's day-by-day development, allowed us to characterize the event as a NE-directed, lateral propagation of magma along a N-S to NE-SW-striking dyke, which followed the path of a previous similar event. Based on the reconstruction of the fault-slip profiles at both sides of various 1947 grabens, we were able to show that fault scarps taper towards NE. Analogue models were also used to simulate the lateral intrusions of dykes and the consequent formation of shallow structures. All the data suggest that the formation of the various structures, with different geometry and deformation, depends upon a combination of magma overpressure, local topography, external stresses, pre-existing structures, and direction of magma propagation.