Imprints of volcanic, erosional, depositional, tectonic and mass-wasting processes in the morphology of Santa Maria insular shelf (Azores)

Abstract

The present-day morphology of volcanic island shelves is the result of several factors, namely the original shape and structure of the volcanic edifice, the nature and age of the materials that compose its flanks, and the erosional, volcanic, tectonic and sedimentary processes that acted to form these shelves. By combining detailed seafloor mapping with onshore studies it is possible to gain a unique insight onto the evolution of volcanic island edifices and the processes that helped to shape their shelves. Here we look at Santa Maria Island in the Azores Archipelago, which is a 6 Ma old ocean-island volcano surrounded by a 135 km2 shelf, characterized by an history of subsidence since its formation c. 6 Ma ago, followed by uplift from 3.5 Ma onwards. By analyzing the geomorphic parameters of the insular shelf (such as the shelf width and the erosive edge depth), the island's history of vertical movements, and by relating the morphology of the outcrops found on the shelf with those onshore, we contribute to a better knowledge of the evolutionary history of Santa Maria with particular focus on its earlier stages of growth. We infer that the outer part of the northern shelf is carved on units older than the oldest volcanic sequences exposed on land; the western and southern shelves are almost entirely carved in the products of the oldest shield volcano (Anjos Volcanic Complex), whilst the eastern shelf is the youngest in formation, being related to units of the Pico Alto volcanic complex. We also discuss the distribution of sediments on the shelf. High wave energy concomitant with sea level drops contributed to the erosion and transport of older sequences offshore, which were ultimately lost to the slopes of the island, resulting in a shelf that is sediment-stripped for the most part. Presently, wide shelves further contribute to wave-energy dissipation, resulting in a significantly diminished sediment production. The shelf edges around Santa Maria exhibit several erosive scars, interpreted as the head of a submarine drainage network carved on the slopes of the island. On the western, southern and eastern shelves, these mass-wasting features are mostly controlled by tectonics since the extension of faults can be followed from onshore to the shelf edge. On the northern shelf mass-wasting appears to be mostly correlated with presence of unstable sediments on the shelf edge that triggered landsliding. Overall, this work shows how detailed mapping of the shelves surrounding old volcanic islands can still provide important complementary information about the evolution of these islands, which could not be acquired solely by the study of their subaerial and deeper submarine parts.