Recover Volcanic Ocean Island Palaeotopography to Uncover Old Unsuspected Large-Scale Flank Collapses

The quantitative reconstruction of ancient topographies of a volcanic ocean island is critical to the understanding of the evolution of a volcanic edifice, because it allows to estimate the volumes of volcanic complexes, and uncover flank collapses. We use the Santiago Island in Cape Verde as case study, because (1) no flank collapses have yet been recognised there, (2) its bean-shape and geological asymmetry hint for a major eastward flank collapse, (3) it comprises a great variety of rock types, and (4) it has superb exposure. Granular rocks and a dyke-in-dyke complex (Basement Complex) currently outcrop up to 700 m altitude, covered by submarine lavas outcropping up to 410 m, and both with an asymmetric distribution relative to the main divide of the island. The recovery of the topography of the Basement Complex at ca. 6 Ma ago, before the deposition of the submarine complex, shows a large eastward concave depression here interpreted as the scar of an island-scale, east-directed, major full-flank collapse. This is consistent with outcropping magma chambers and dykes (Basement Complex) that fed an early major volcanic edifice well above the current topography. The flank collapse removed the sub-aerial and submarine slopes of the former edifice, and the collapse scar was massively, but not totally, buried by a submarine volcanic complex.