The radial dyke swarm of the Lower Old Volcanic Edifice of La Gomera (Canary Islands): evidence for a possible eroded E-W volcanic rift.

Márquez, Álvaro; Herrera, Raquel; Izquierdo, Tatiana; Martin, Fidel; López, Iván; Martín, Silvia

Abstract

The rough topography of La Gomera (Canary Islands) offers an excellent window for the study of the magmatic feeder system of an oceanic volcano. Ancochea et al. (2008) proposed the existence of several radial dyke swarms in the island, due to the migration southwards of three successive subaerial volcanic edifices. We have focused in the dyke swarms of the oldest (10.5 – 8.6 Ma) subaerial volcano of La Gomera, the Lower Old Edifice (LOE), analyzing dykes in 20 outcrops radially distributed at several distances of the proposed volcanic centre. We have measured the location, attitude (strike and dip) and width of a total of 415 dykes. We have separated dykes of the LOE mainly using field crosscutting relationships, and estimated the minimum horizontal stress direction and the strain accommodated by LOE dyke intrusions, following the method proposed by Marinoni (2001). In agreement with previous work of Ancochea et al. (2008) we find in the LOE two different dyke swarms: one formed by outward shallow dipping sheets, and a later swarm of (quasi)vertical dykes. Our results of the second swarm show a roughly radial stress field but with a clear asymmetrical strain distribution and that the analysis of altitude and distance to the volcano centre of each outcrop is essential to detect this radial asymmetry. In the NE region of the island (Hermigua sector) we find a possible E-W volcanic rift, since the dense network of dykes pointing to the proposed centre of the LOE shows densities of 10 dykes per 100 meter involving a 15% of horizontal extension, similar to volcanic rifts in other islands. This proposed E-W rift, however, is not present in the outcrops located at the NW region, at the west side of the LOE centre.

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Fecha de publicación: 2010