Internal structure and evolution of a volcanic rift system in the eastern North Atlantic: the Desertas rift zone, Madeira archipelago

Abstract

The three elongated Desertas Islands form the top of a 60-km-long NW–SE-striking submarine ridge southeast of Madeira (NE Atlantic). The alignment of eruptive centres and parallel dyke swarms indicates that the islands represent a deeply eroded volcanic rift zone. Detailed field studies combined with 40Ar/ 39Ar age determinations and geochemical analyses reveal the internal structure and evolution of this rift, which may serve as a type example of North Atlantic rift zones. The northern rift end is composed mainly of lava flows up to several meter thickness and few scoria. The central part is characterised by a 100- to 200-m-thick basal sequence of tuff as a result of explosive eruptions due to magma–sea water interaction during the early subaerial stage. These layers are overlain by subhorizontal lava flows reflecting the change from dominantly explosive to more effusive volcanism during island growth. The southern part is dominated by a basal tuff sequence, scoria and cinder cones, minor lava flows and sills up to 30 m in thickness. In general, bomb- and scoria-rich deposits, dykes and sills increase in frequency towards the south. In contrast to rift systems of Hawaiian volcanoes such as Kilauea, we did not find any volcanological or petrological indication of a central caldera underlain by a shallow magma reservoir feeding the rift. 40Ar/ 39Ar ages show that subaerial Desertas volcanism lasted from >5.1 to 1.9 Ma overlapping with the shield-building phases of Madeira island. Desertas lavas show no temporal or spatial variation in chemistry, except for samples from the top of the islands, erupted between ∼3.4 and 2 Ma, which are strongly REE and Y enriched. We suggest that the Desertas ridge represents a discrete volcano that became interconnected with Madeira by growth to the north–northeast. Major volcanic centres may have been located at the southern end of the ridge and/or near its central part. A conceivable explanation for the formation of the elongated Desertas rift is a local gravitative stress field causing preferred extension between the Madeira and Desertas edifices as they progressively overlapped. Our model thus emphasises the importance of edifice amalgamation and gravitative stress in the evolution of large volcanoes and their rift zones. The initiation of Desertas volcanism could be the result of a weak pulsating mantle plume and irregular motion of the African plate, combined with flexural stresses within the lithosphere due to the loads of Madeira and Porto Santo islands. Cessation of volcanism coincides with the exclusive occurrence of Y- and REE-enriched lavas that may be related to sediment assimilation. Our results indicate that models for Hawaiian rifts cannot necessarily be transferred to hotspot volcanoes associated with weak plumes and/or irregular plate motion.