HR: 1330h
AN: V22A-0565    [PDF]
TI: Internal structure and evolution of a North-Atlantic rift system: the Desertas rift zone, Madeira Archipelago
AU: * Schwarz, S
EM: sschwarz@uni-bremen.de
AF: University of Bremen, Department of Geosciences, Klagenfurter Strasse, Bremen, 28359 Germany
AU: Kl\"ugel, A
EM: akluegel@uni-bremen.de
AF: University of Bremen, Department of Geosciences, Klagenfurter Strasse, Bremen, 28359 Germany
AB: The three Desertas Islands represent the top of a 60 km long submarine ridge southeast of Madeira. They dominantly consist of lava flows, cinder cones stacked one on another and steeply dipping, partly sheeted dike swarms and thus represent the deeply eroded interior of a well-developed volcanic rift zone. In order to reconstruct the evolution of the Desertas rift and to understand its volcanotectonic origin, we have undertaken detailed field studies of volcanic structures. It was found that the abundance of scoria cones, dikes and sills indicating proximity to eruptive centers increase along the rift towards south: The northern end is composed mainly of lava flows up to several meter thickness, whereas the central part shows an alternation of proximal pyroclastics and series of subhorizontal lava flows. The southern part is dominated by scoria and cinder cones and minor thin lava flows. Dikes, isolated or as swarms, were observed all along the islands' coasts. The most distinctive and most dense dike swarm, however, occurs in the southernmost region, and sills up to 30 m thickness are also more common in this area. We did not find any volcanological or petrological indication of a central caldera underlain by a shallow magma reservoir feeding the rift zone. Because of the distribution of volcanics and intrusive rocks, we propose that a major volcanic center was located at the southern end of the Desertas ridge, and that the rift zone successively developed towards Madeira. This model implies that the Desertas ridge represents a discrete volcanic system that became interconnected with Madeira by growth to the north-northeast. The evolution of the Desertas rift hence differs from that of caldera-centered, two-armed rift systems typical for Hawaiian volcanoes such as Kilauea. A conceivable explanation for the formation of the Desertas rift zone is a local gravitative stress field causing preferred extension in-between the two overlapping Madeira and Desertas edifices.
DE: 5480 Volcanism (8450)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting