Constraints on the structure of the crust and lithosphere beneath the Azores Islands from teleseismic receiver functions

Abstract

The Azores Archipelago is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, resting on both sides of the ridge. Various methods including seismic reflection, gravity and passive seismic imaging have previously been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 and 30 km, but until now models of the more fine-scale crustal structure have been lacking. Pending questions include the thickness of the volcanic edifice beneath the islands and whether crustal intrusions or even underplating can be observed beneath any island. In this study, we use data from nine seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. Our results indicate that the base of the volcanic edifice is located approximately 1 to 4 km depth beneath the different islands and that the crust–mantle boundary has an average depth of ∼17 km. There is strong evidence for magmatic underplating beneath the island of Sao Jorge, and indications that the underplating is also present beneath Sao Miguel and possibly Santa Maria. Additionally, the seismological lithosphere–asthenosphere boundary, defined as a seismic velocity drop in the uppermost mantle, seems to deepen with increasing distance from the MAR. It has a depth of ∼45 km beneath the islands close to the MAR, compared to depths >70 km beneath the more distal islands.