Pamplona J.,Dias R. & Ribeiro A.
Presented in the XII Reunião de Geologia do Oeste Peninsular, Évora, 1993
Terra nova Abstracts, nº 6, vol.5, p.4-5.
Abstract
The Praia Norte-Viana do Castelo region is litologically composed by Slate and Graywacke Complex, Quartzite-Psammitic's Formation (Lower Psammites, Compact Quartzites, Upper Psammites) and by Valongo's Formation (Chiastolite Schist with some iron layers and Garnet Andalusite Schist).
The first Hercynian deformation phase (F1) is clearly preserved on Ordovician Quartzite-Psammitic Formation. The presence of sinistral shear corridor N20W/90 is decisive to the Quartzite sigmoidal geometry, as well as the sinistral and dextral shear senses observed, having as result a sinistral domino structure. On this sinistral shear corridor the fold axial planes are subvertical and with strike N22-40W, while the fold axes are undulants (N36-50W and S22-31E with plunge values ranging from, respectively, 4 to 88 and 5 to 60). The stretching lineation-X1 are according to the kinematic axis b, plunging to NW preferentially between 2 and 12 and to SE between 8 and 24.
The second Hercynian deformation phase (F2) is obserable on Slate and Graywacke Complex and Valongo's Formation being easily characterized through microtextural analysis: minerals rotation (andalusite and garnet sin-F1); micas crystallization defining new schistosity surfaces; refolding of S1 (schistosity of F1), viewed on a few microfolds, by S2.
The third Hercynian deformation phase (F3) is defined by the presence of: 1. shears, sometimes conjugates, sinistral N35-45W subvertical to vertical, and dextral N20W-N5E subvertical to vertical. The strike and dip of F3 shears can coincide with those of F1, because, frequently, these are reactivated.
2. crenulation (14-22/132-155 and flattened/322-349) and schistosity (N16-36W/60W-subvertical and N18-23W/80-85E) wich are well defined.
The geological-structural study at regional and detail scales, in outcrop or thin section, make possible a better understanding of the tectonic of Viana do Castelo and also of the tectonic style variation to the North.