Santiago Island, Cape Verde: Evidence for island-scale collapse from paleotopography onshore and bathymetry offshore

A volcanic edifice much larger than the current one must have existed in Santiago Island, Cape Verde, because the granular rocks and dyke-in-dyke complex representing magma chambers and deep feeders currently outcrop up to 700 m altitude. Therefore, we must find an explanation for the massive destruction of the original edifice. We developed a new tool for the quantitative reconstruction of ancient topographies in a volcanic ocean island to address this problem, because it allows us to estimate the shape and volume of volcanic rock removed at a certain time. The reconstruction of the topography of the basement complex at ca. 6 Ma ago, before the unconformable deposition of the submarine complex, shows a concave depression coincident with the asymmetric distribution of volcanic complexes east and west of the main divide of the island. This concave depression is here interpreted as the remnant of an island-scale, summit collapse. Instituto do Mar de Cabo Verde bathymetry and RRS Charles Darwin (8/85) seismic reflection profile data suggest that the west side of Santiago is characterised by a narrow insular shelf, a major debris avalanche deposit with scattered blocks and at least one lateral sector collapse structure. Data, however, east of Santiago are limited and so the full extent of mass wasting on the east side of the island is not known. Maio Island, which is similar in age to Santiago, would have acted as a buttress in the east, and it is possible that any eastward collapse might have rotated and travelled to the northeast. Irrespective, one or more mass wasting events west or east of Santiago are consistent with a major destruction of the original volcano edifice which removed the summit, exposed the basement complex of the island, and redistributed volcano-clastic material over a large area of the adjacent seafloor.