Volcano-stratigraphic and structural evolution of Brava Island (Cape Verde) based on 40Ar/39Ar, U-Th and field constraints

Abstract

Three volcano-stratigraphic units were identified at Brava Island in the Cape Verde Archipelago on the basis of field relationships, geologic mapping and 40Ar/39Ar and U–Th ages. The Lower Unit comprises a 2-to-3 Ma-old submarine volcanic sequence that represents the seamount stage. It is composed of nephelinitic/ankaramitic hyaloclastites and pillow lavas, which are cut by abundant co-genetic dikes. Plutonic rocks of an alkaline–carbonatite complex, which intruded the submarine sequence 1.8 to 1.3 Ma ago, constitute the Middle Unit. A major erosional surface developed between 1.3 and ~ 0.25 Ma. The post-erosional volcanism recorded in the Upper Unit started 0.25 Ma ago and is dominated by phonolitic magmatism. This phase is characterised by explosive phreato-magmatic and magmatic activity that produced block and ash flow, surge, and pyroclastic fall deposits and numerous phreato-magmatic craters. Effusive events are represented by lava domes and coulées. One peculiarity of Brava is the occurrence of carbonatites in both the plutonic complex and the post-erosional phase as extrusive volcanics. The intrusive carbonatites are younger than those occurring on Fogo, Santiago and Maio islands. Young (Upper Pleistocene to Holocene) extrusive carbonatites occurring in the late stages of volcanism are unknown in other Cape Verde islands.

The occurrence of pillow lavas and hyaloclastites above the present sea level (up to 400 m) and raised Upper Pleistocene beaches indicates continuous uplift of Brava since the seamount stage. By dating raised marine markers, uplift rates were estimated at between 0.2 and 0.4 mm/a. The evolution of Brava was controlled by faults with directions similar to those described for Fogo, suggesting a common stress field.