The shaping of a volcanic ridge in a tectonically active setting: The Pico-Faial Ridge in the Azores Triple Junction

Abstract

The Pico-Faial ridge is a steep WNW-ESE volcanic ridge that has developed within the Nubia-Eurasia diffuse plate boundary, close to the Azores Triple Junction. The ridge comprises two islands, Pico and Faial, separated by a shallow (<100 m depth) and narrow (<8 km) channel. Despite some similarities, the two islands show contrasting features still deserving explanation: (1) meaning of GPS data; (2) meaning of palaeomagnetic data; (3) the island-scale Faial Graben does not have a counterpart in Pico; (4) both islands comprise a main central volcano each, but the one in Faial is symmetrical and the one in Pico is asymmetrical; (5) Pico shows evidence of at least two large-scale flank collapses, but none has been recognized so far in Faial. The new data reported here lead to the following probable answers: (1) the GPS data show much larger vertical and horizontal ground motions in Faial, which could be the result of recent volcanism and tectonics; (2) the cyclicity inferred from the paleomagnetic data in Pico could be related to inflation/deflation cycles that could have triggered the inferred flank collapses; (3) the Faial Graben ends abruptly at the eastern edge of the island, which we interpret as inhibited propagation to the east because of the load and stresses imposed by the nearby large Pico Volcano; (4) we attribute the asymmetry of the Pico Volcano to partial flank collapse recognizable in new offshore seismic profiles; (5) either the Faial island is actually gravitationally more stable, or there have been flank collapses not yet recognized. All these processes have concurred to shape the two islands and the ridge to their current morphology, but the current morphologies of Pico and Faial result mainly from three of these processes: volcanism, tectonics and large-scale landslides.