Paleomagnetic Study of Azores Archipelago: Volcano-Tectonic Implications

Abstract:

Oceanic islands are by nature unstable volcanic buildings generally marked by rapid growth that alternates with destruction due to a variety of mass-wasting processes, including giant sector collapses, vertical caldera subsidence, fault generation/propagation, shallow landslides and coastal erosion. Due to its diverse volcanic and tectonic frameworks, the Azores archipelago represents an excellent case study for geophysical and geological proposes. Main results of a paleomagnetic study, conducted on oriented samples from approximately 60 accessible lava piles of three islands of Azores archipelago central group (Faial, Pico and Terceira islands) and covering as much as possible spatially and temporally these islands are: i) The paleomagnetic polarity is in close agreement with the radiometric results known for these islands; ii) Onshore volcanic activity began during the Matuyama geochron for Faial and during Brunhes for Terceira and Pico; iii) The mean ChRMs from Terceira and Pico islands result in a paleomagnetic pole similar to the ones retrieved from the studies of Johnson et al (1998) and Silva et al (2012), from S. Miguel and S. Jorge islands (other islands of Azores archipelago), respectively; iv) ChRMs from Faial show an elliptical distribution perpendicular to the WNW-ESE grabben that is the major structure of the island, suggesting tilting towards SSW of the southern wall and towards NNE of the northern one. The presence of listric faults plunging towards the middle of the grabben and aligned along the N110º azimuth could explain the elliptical distribution of paleomagnetic directions. This study is a contribution for the research project REGENA (PTDC/GEO-FIQ/3648/2012).

References

Johnson, C.L., Wijbrans, J.R., Constable, C.G., Gee, J., Staudigel, H., Tauxe, L., Forjaz, V.-H., Salgueiro, M., 1998. 40Ar/39Ar ages and paleomagnetism of S. Miguel lavas,Azores, Earth planet. Sci. Lett., 160, 637–649.

Silva, PF; Henry, B; Marques, FO; Hildenbrand, A., Madureira, P., Mériaux, C; Kratinová, Z. Palaeomagnetic study of a sub-aerial volcanic ridge (São Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectono-magmatic implications. Geophysical Journal International, 188, 3, 959-978, 2012