Restoring volcanic ocean island palaeotopography to uncover island-scale buried sector collapses

Abstract

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 aim to 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 a submarine complex, shows a large eastward concave depression coincident with the asymmetric distribution of volcanic complexes east and west of the eastward concave main divide of the island. This concave depression is here interpreted as the scar of an island-scale, east-directed, sector collapse. Given the position of Santiago relative to the nearby Maio Island, which could work as a buttress in the east, we conclude that the debris generated by the inferred eastward collapse might have been diverted to the northeast. A west-directed sector collapse can be recognised from the topography of the island and marine geophysical data, which supports, by similarity, the sector collapse inferred for eastern Santiago. This methodology could be replicated in many other oceanic islands worldwide where granular rocks currently outcrop above sea level.