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Stress pattern in Portugal mainland and the adjacent Atlantic region, West Iberia
Author(s) -
Ribeiro A.,
Cabral J.,
Baptista R.,
Matias L.
Publication year - 1996
Publication title -
tectonics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.465
H-Index - 134
eISSN - 1944-9194
pISSN - 0278-7407
DOI - 10.1029/95tc03683
Subject(s) - geology , clockwise , seismology , stress field , tectonics , borehole , subduction , continental margin , lithosphere , neogene , paleontology , rotation (mathematics) , structural basin , physics , geometry , mathematics , finite element method , thermodynamics
The Portuguese mainland territory is located close to the Azores‐Gibraltar plate boundary, in a tectonic setting responsible for significant neotectonic and seismic activities. However, few data concerning the present regional lithospheric stress field were available, as testified by recently published maps of stress indicators for the Europe and Mediterranean regions. One of the authors already presented a synthesis on this subject [ Cabral , 1993], where geological and geophysical stress indicators were considered. In this paper we introduce new information, mainly a considerable amount of borehole breakout data. The updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145° (standard deviation 21°) for the maximum horizontal stress direction (S Hmax ). On the average, the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth, while the borehole elongation results are consistent with the mean S Hmax trend. These differences in stress trend suggest a regional progressive rotation of the S Hmax direction from NNW‐SSE to WNW‐ESE since the upper Pliocene. To estimate stress trajectories, new and published stress indicators in the adjacent Atlantic area and northern Africa were also investigated, showing a very uniform NW‐SE S Hmax trend in west Iberia. A high level of horizontal compressive stress acting oblique to the western Portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model, of activation of this passive margin, with the nucleation of a subduction zone in the Atlantic SW of Iberia, at the Gorringe submarine bank, which is propagating northward along the base of the continental slope, at the transition between thinned and normal continental crust.