Interesse patrimonial dos aspectos geológicos e geomorfológicos da região de Aveleda-Baçal (Parque Natural de Montesinho, NE Portugal)

Abstract

O Parque Natural de Montesinho (PNM) fica situado no Nordeste de Portugal, abrange a parte norte dos concelhos de Vinhais e Bragança e engloba as serras da Coroa e Montesinho. Geologicamente o PNM situa-se nas unidades autóctones da Zona Centro Ibérica e nas unidades parautóctones e alóctones da Zona Galiza-Trás-os-Montes. Nesta primeira abordagem acerca do Património Geológico do PNM, faz-se a analise da região situada entre Aveleda e Baçal, cuja clareza das formas de relevo e a definição das relações com a geologia e com a tectónica, no seio de uma área protegida, são razões para a sua valorização. Assim, no presente trabalho descrevem-se e interpretam-se os aspectos geomorfológicos desta região. A partir de locais de observação estabelecidos é possível constatar que a região de Aveleda-Baçal corresponde ao bloco abatido de um graben controlado por falhas de orientação NNE-SSW, com destaque para a falha de Portelo que origina uma escarpa pelo soerguimento do bloco ocidental e abatimento a leste. No bloco ocidental deste acidente tectónico estão representados restos de uma superfície de aplanamento a cotas superiores a 900 metros (superfície de Espinhosela) e para norte a serra de Montesinho (1481 m), um bloco mais elevado de topos relativamente aplanados.

The Montesinho Natural Park (MNP) is located in NE Portugal (Vinhais -Bragan~a region), comprising the Coroa and Montesinho mountains. The geology of the area is characterised by the occurrence of mafic and ultramafic rocks defining the Bragança massif, one of the four massifs in the northwest Iberia. Geologically the MNP is placed both in the autochthonous sequences of the Central Iberian Zone and in the pile of thrust units of the "Galiza-Trás-os-Montes" Zone. These nappes comprises from bottom to the top: 1) parautochthonous thrust complex, including metasediments of Silurian-Devonian age having lithological affinities with the subautochthonous; 2) lower allochthonous thrust complex (H-P metamorphism, peralcaline bimodal volcanism) of Lower Palaeozoic for the bottom to Upper Palaeozoic age for the top of the unit; 3) a fragmented Palaeozoic ophiolite complex; 4) an upper continental allochthonous terrane of Precambrian to Lower Cambrian age. They are thrusted over a Silurian-Devonian subautochthonous metasediment sequence. Ordovician quartzites and slates mainly form the autochthonous. Finally, Cenozoic tectono-sedimentary events left their imprints on the landscape, namely with the presence of three unconforrnity-bounded sedimentary sequences. The late variscan tectonic episodes break up the Iberian Meseta plateau in several parts. The Aveleda-Baçal surface is a graben controlled by the NNE-SSW (Portelo fault) and N-S (Aveleda fault) fault systems. The Portelo fault belongs to a major lineament of the late-variscan tectonic episode of the NW Iberia (Bragança-Vilariça fault system). This is a left -lateral strike -slip fault with the uplift of the western block. The Aveleda fault is reactivated as dextral strike slip fault in late-variscan episodes. This complex geology is responsible for unique landscape features, having an important role on the natural heritage of the park. In this first overview, the Espinhosela area was selected for its distinguishing landscape features, clearly controlled by geological and tectonical events. The geomorphological features are described and interpreted from distinct observation sites. It is possible to define several local surfaces, traces of the Iberian Meseta plateau. In the western uplift block three surfaces are defined: 1) Espinhosela surface (900 m) tectonically controlled by the major thrusts between the allochthonous units; lithologically this surface is defined on the Espinhosela gneisses; 2) the Soutelo surface (1000-1100 m) bordered by the two major thrusts between parautochthonous and subautochthonous units; 3) to the north of this thrust, the Montesinho upper surface (1400 m) is defined on the granite intrusion. In the Aveleda-Baçal graben (locally known as "Baixa Lombada") two surfaces can be defined: Aveleda surface (800-900 m) defined on Cenozoic sediments and Baçal surface (600-700 m) defined on gneisses. The eastern limit of the graben is a complex conjugation of N/S and WNW/ESE faults rising the eastern block up to 900 m ("Alta Lombada").