Influence of cone morphology on dykes, Stromboli, Italy

The island of Stromboli represents the emerged part of a 2.6-km-high composite volcano elongated in a NE direction. Over the period 100–13 ka BP the morphological evolution of the volcano was characterised by a series of summit caldera collapses between major building phases. From 13 ka BP onward, a series of large lateral collapses occurred northwestwards in one sector of the cone. The results of the field study of the total (94) dyke population is discussed in this paper. They show that during the last 100 ka the majority of dykes were injected with a NNEGSSW to ENE–WSW strike along a NE–SW weakness zone crossing the volcano summit. This is interpreted as a volcanic rift the geometry of which is controlled by regional tectonic stresses. Dykes injected prior to ∼13 ka BP at lower altitudes are generally steeper than those injected at higher elevations. After 13 ka BP, dykes were injected along the NE–SW weakness zone and parallel and close to the margins of the earliest sector collapse, at dips of 20–80° inclined towards the collapse depression. I suggest that this is linked to unbuttressing of the collapse depression or to seaward gravity instability of the depression infill deposits. Extensive emplacement of long dykes along the NE–SW weakness zone occurred only when the volcano regained a conical shape. Some of the dykes emplaced close to the original surface of the cone bent to assume a dip direction parallel to the local slope. I propose that this local change of dyke geometry can be explained by the direct influence of the free surface of the cone leading to a rotation of the least principal stress which assumes an orientation perpendicular to the slope.