M. Moreira

Ecoulement magmatique dans les dykes du point chaud des Açores: étude préliminaire par anisotropie de susceptibilité magnétique (ASM) dans l'île de San Jorge

We present a preliminary study of magmatic flow in dykes of San Jorge Island, Azores, using anisotropy of magnetic susceptibility (AMS). Oriented samples were taken from dyke margins. All dykes show well-grouped minimum susceptibility axes near the pole of the dykes. In addition magnetic foliation planes present an imbricated arrangement relative to each dyke margin. Magmatic flow direction is determined independently, from each margin, and is the result of relative orientations of the magnetic foliation plane and the dyke margin plane. Digital processing of thin sections has shown that maximum and intermediate susceptibility axes may be correlated with alignments of large crystals. Moreover, in most cases, orientation of the intermediate susceptibility axes is in agreement with the flow sense derived from analysis of the imbricated magnetic foliation planes. Overall results show a predominance of horizontal or sub-horizontal flow over vertical flow.

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

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 k1 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.

On the applicability of joint inversion of gravity and resistivity data to the study of a tectonic sedimentary basin in Northern Portugal

Abstract The Chaves basin is a pull-apart tectonic depression implanted on granites, schists, and graywackes, and filled with a sedimentary sequence of variable thickness. It is a rather complex structure, as it includes an intricate network of faults and hydrogeological systems. The topography of the basement of the Chaves basin still remains unclear, as no drill hole has ever intersected the bottom of the sediments, and resistivity surveys suffer from severe equivalence issues resulting from the geological setting. In this work, a joint inversion approach of 1D resistivity and gravity data designed for layered environments is used to combine the consistent spatial distribution of the gravity data with the depth sensitivity of the resistivity data. A comparison between the results from the inversion of each data set individually and the results from the joint inversion show that although the joint inversion has more difficulty adjusting to the observed data, it provides more realistic and geologically meaningful models than the ones calculated by the inversion of each data set individually. This work provides a contribution for a better understanding of the Chaves basin, while using the opportunity to study further both the advantages and difficulties comprising the application of the method of joint inversion of gravity and resistivity data.