Tectónica da Ponta de São Lourenço, Madeira Island

The tectonics of Madeira Island is, in its generality, still quite unknown. A structural characterization together with the formulation of hypothesis contributing to this problematic is, in our opinion, of extreme importance to relate this context with the regional and global tectonic models. The Ponta de São Lourenço, in Madeira Island, exhibited an unusual situation for studies of this nature, due to the diversity of lithologies found there, as well as the presence of excellent natural cross-sections originated by marine erosion. In terms of tectonics, the studied area is dominated by accidents of distensive nature, organized in two outstanding families: E-W and NW-SE. From these structures the most important are: a dyke swarm, very dense and with an approximate WNW-ESE direction; and a fracture pattern where we highlight the Seixal-Machico (N50°W, subvertical), Pedra Furada (E-W, 70°N) and Estreito (N50°W, 70°N) faults. The analysis of those structures suggests a distensive tension field with minimum compression axis (s3) oriented with NNE-SSW direction, and maximum compression axis (s1) subvertical, related to the weight of the lythostatic column (gravitical origin). This tension field suggests a hotspot rift model, similar to the one proposed by WALKER (1987), for shield volcanoes of the Hawaiian kind. This tension field seems to have a constant space-temporal orientation, with probable neotectonic activity. It is also proposed a tectonic control of volcanism, confirmed by the presence of aligned volcanic structures with the same directions previously described. It is equally proposed the existence of a structural control of the coast line and the landforms in the area.