Small-scale instability processes affecting volcanic island shelves: the case study of the southern shelf of Porto Santo Island (Madeira Archipelago)

Abstract

Shelves surrounding volcanic islands represent only a small portion of the entire submarine edifice that can range a few thousand meters below sea level. Despite their small area, the erosional, depositional, tectonic and volcanic processes affecting the subaerial island leave a significant imprint on the shelf morphology when compared to the slope or the submarine base of the islands (Quartau et al., 2014). Therefore, shelf morphology can be used to improve our knowledge about the island evolution, especially the more recent events. In this study, we use multibeam bathymetry, side scan sonar and seismic reflection data of the southern shelf of Porto Santo Island in Madeira Archipelago to map its seabed morphology and seismic stratigraphy from the nearshore to the shelf edge (~100m b.s.l.), aiming to better understand the evolution of mass-movements affecting the area. In general, the shelf consists of an erosive rocky surface mostly covered by sediments, and locally outcropping on the seafloor. The sedimentary cover on the shelf is highly variable both in thickness and internal architecture, mainly due to different onshore sediment supply and available accommodation space. Two main seismic units have been recognized: the lower one (U1) is interpreted as a transgressive deposit, formed during sea level rise after the Last Glacial Maximum (LGM); the upper one (U2) is interpreted as present-day highstand deposits. Based on the morphological and seismo-stratigraphic differences the study area has been divided in three sectors (Western, Central and Eastern). On the Western and Eastern sectors two sets of submerged erosional terraces have been mapped at different depths (35-45 m, 50-75 m b.s.l.). The overall arcuate shape of the southern shelf edge of Porto Santo Island strongly matches the coastline shape, suggesting the occurrence of an old large-scale landslide event (LS1), prior to the shelf formation (Quartau et al., 2018a). Further incisions on the shelf edge of the Central sector, mapped at and upslope the shelf edge, imply more recent mass-movement processes, LS2 at least older than seismic unit U1 and LS3 formed on the late Holocene, indicating a retrogressive evolution of the instability process. The absence of erosional terraces in the Central sector supports this hypothesis, being probably removed by the LS2 event. The terraces depths and their destruction by the LS2 event suggests that these were formed/modified at least by the MIS 5a-5d stillstands or even by older ones. The estimated volume of the LS2 event (0.6 km3) suggests that it might have caused a hazardous tsunami if it failed as a single event. The recent LS3 events, due to their small magnitude were probably not hazardous. In summary, three different mass-movements were defined according to their dimension and position on the shelf: from LS1 to LS3, the scars are increasingly smaller and further upslope. These observations provided a more comprehensive understanding of the recent evolution of Porto Santo insular shelf in terms of mass-movement processes.