Seismic Imaging of Submarine Volcanoes and Volcanic Ridges at the Southeastern Azores Plateau

Abstract

There is an ongoing debate about the water depths to which explosive submarine volcanism is possible. In this study, we present high-resolution reflection seismic data from cone-shaped intraplate volcanoes on the southeastern Azores Plateau that indicate explosive submarine volcanism at water depths greater than 2 km. In addition, the data illustrate the early stages of volcanic ridge evolution at similar water depths, which were mainly formed by fissure eruptions. The volcanic cones are ca. 4 km wide and 500 m high above the ocean floor. They are characterised by stratified flanks that are onlapped by sub-horizontal, hummocky high-amplitude reflections. We interpret the layered flanks as evidence of rather unconsolidated volcanic deposits from submarine explosive eruptions, where the magma emerging from the seafloor was fragmented by expanding volatiles. Guided by diffraction imaging, we interpret the high-amplitude reflections as the top of lava flows from effusive eruptions, that postdated the explosive eruptions. Distinct upward-bent reflections beneath the volcanic cones are interpreted to be velocity pull-ups caused by the presence of dense, high p-wave velocity material in the central part of the volcanic cones. We apply depth-stretching to correct for these artefacts and to access the true geometry of the volcanic cones and the underlying features. Upward concave reflections within the basaltic basement might represent funnel shaped conduits or diatremes. The seismic signature of a volcanic ridge between two of the volcanic cones suggests that stacked effusive eruptions dominated its evolution. Both the cones and ridges superimpose hemipelagic sediments, which in turn overlie the basaltic basement. Hence, the evolution of the volcanic features postdated the main magmatic evolution of the western part of the eastern Azores Plateau that started in the middle Miocene. Generally, our study sheds new light on the complex volcanic evolution of the southeastern Azores Plateau and highlights the potential of seismic imaging as a tool for submarine volcanology.