El Edificio Antiguo de la isla de San Vicente (Cabo Verde): características del sector occidental

Abstract

A more than 1000m thick pile of basaltic lava flows and pyroclasts dipping seawards rises on the western sector of San Vicente. Several systems of dikes cut across the succession that is composed of two different sequences. The lower sequence (Lazareto Formation), about 400 m thick, is formed mainly by pahoe-hoe, frequently ankaramitic, basalt lavas and some breccias intercalated. This sequence is exposed in small hills and low rounded ridges of gentle slopes. The upper sequence (Monte Cara Formation), some 700 m thick, consisting in individually thicker basaltic lava flows with typical scoria levels is well exposed in high scarps. Numerous subvertical dikes cross the entire succession. The dikes vary increasingly their strike from south to north. Frequent sills (one every 3-5 m) also intrude the lower sequence. The distribution of the lava flow dip strikes as well as the dike strikes suggests that the succession as a whole was part of a central volcanic edifice, of which the eruptive centre was once located south of the city of Mindelo. This giant volcanic edifice, 16-20 km in diameter and 1600-2000m high, would have extended off the present coast. According to the radiometric ages obtained the volcano built up between 6,1Ma and 4,6 Ma with maximum activity concentrated between 5,5 and 5 Ma.