Magma flow pattern in dykes of the Azores revealed by anisotropy of magnetic susceptibility

Type Article
Date 2015-02
Language English
Author(s) Moreira M. A.1, 2, Geoffroy Laurent3, Pozzi J. P.4
Affiliation(s) 1 : Inst Super Engn Lisboa, Lisbon, Portugal.
2 : Inst Dom Luis, Lisbon, Portugal.
3 : Univ Brest, Inst Univ Europeen Mer, CNRS, Lab Domaines Ocean,UMR 6538, Plouzane, France.
4 : Ecole Normale Super, CNRS, Lab Geol, UMR 8538, Paris, France.
Source Journal Of Geophysical Research-solid Earth (2169-9313) (Amer Geophysical Union), 2015-02 , Vol. 120 , N. 2 , P. 662-690
DOI 10.1002/2014JB010982
WOS© Times Cited 7
Keyword(s) AMS, Azores, dykes, lithosphere, magma flow, volcanism
Abstract

The localization of magma melting areas at the lithosphere bottom in extensional volcanic domains is poorly understood. Large polygenetic volcanoes of long duration and their associated magma chambers suggest that melting at depth may be focused at specific points within the mantle. To validate the hypothesis that the magma feeding a mafic crust, comes from permanent localized crustal reservoirs, it is necessary to map the fossilized magma flow within the crustal planar intrusions. Using the AMS, we obtain magmatic flow vectors from 34 alkaline basaltic dykes from SAo Jorge, SAo Miguel and Santa Maria islands in the Azores Archipelago, a hot-spot related triple junction. The dykes contain titanomagnetite showing a wide spectrum of solid solution ranging from Ti-rich to Ti-poor compositions with vestiges of maghemitization. Most of the dykes exhibit a normal magnetic fabric. The orientation of the magnetic lineation k(1) axis is more variable than that of the k3 axis, which is generally well grouped. The dykes of SAo Jorge and SAo Miguel show a predominance of subhorizontal magmatic flows. In Santa Maria the deduced flow pattern is less systematic changing from subhorizontal in the southern part of the island to oblique in north. These results suggest that the ascent of magma beneath the islands of Azores is predominantly over localized melting sources and then collected within shallow magma chambers. According to this concept, dykes in the upper levels of the crust propagate laterally away from these magma chambers thus feeding the lava flows observed at the surface.

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