Morphology from subaerial erosion of a Mediterranean seamount

Abstract

The morphology of a seamount in the Valencia Trough of the Western Mediterranean has been investigated with side-looking sonar (Sea MARC I) and swath-mapping sonar (Sea Beam). The seamount is characterized by deep reentrants on its flanks, radiating spurs and a broad summit plateau. Presently its summit lies 1020 m below sea level. Avalanche scars and debris flow deposits are detected on re-entrant floors and slopes. Fragments of aphanitic basalt and andesitic tuff, sampled at DSDP Site 122 attest to the volcanic origin of the Valencia Seamount. First constructed as a subsea volcano about 20–25 m.y. ago, its flanks became almost entirely emerged 6 m.y. ago at the time of the evaporitic drawdown of the Mediterranean. Valencia Seamount has some features in common with young and old seamounts on the floor of the Pacific Ocean that have never emerged above sea level and other features which may be attributed to subaerial exposure. Two working hypotheses, erosional and constructional, are presented to explain the observed morphology. The erosional hypothesis better accounts for the known paleogeographic and paleoceanographic history of the Mediterranean. The mapped spurs are interpreted as flank rift zones which give many seamounts a starfish-like appearance. Fountain eruptions of hyaloclastites from vents could explain the broadly arched summit plateau. Peaks might be secondary parasitic cones or remanent erosional features of stocks and dykes which are more resistent to mass-wasting than an extrusive cover of vesicular lava flows and deposits of tuff and ash. Gullies are most abundant on the middle and lower flanks. Whereas the gullies might be breached calderas, it is more reasonable to expect that they are dominantly subaerial erosional in origin, considering the emergence history of this particular seamount.