New data on the structure of the Bahia seamounts (Western Portion of the Brazil Basin,South Atlantic)

During the 28th trip of the R/V Akademik Sergei Vavilov undertaken in 2009, exploration was carried out using a Parasound acoustic profilograph that permitted examining the upper portion of the sedimentary cover and getting transverse bathymetric profiles through a number of seamounts within the northern range of the Bahia group of seamounts, which were also sampled by dredging. The studies performed indicated that this linear structure consists of paleovolcanoes and formed under impulse conditions of volcanic activity. A number of seamount types have been distinguished, the morphostructural settings and the positions of which reflect the intensity of volcanic impulses and the association with deep permeable zones. The strongest impulse led to the formation of a northwest striking arched uplift, on which there are several central type paleovolcanoes of different heights. The largest of these paleovolcanoes, including the Stocks and Groll Seamounts, have flattened tops formed as a result of marine abrasion. Isolated seamounts have formed during shorter and lower power impulses of volcanic activity. The northern range of the Bahia seamounts, which have a major northwesterly strike, is an ensemble of structures with northwesterly, nearly east-west, and nearly north-south strikes, which indicates the presence of relevant deep permeable zones for the mantle matter; that mantle matter is the source of the melts, eruptions of which led to the formation of the volcanic edifices. The prevailing zone of weakness has a northwesterly strike. Volcanic activity in the Bahia group of seamounts ended in the Paleocene, but tectonic movements continued there up until the Holocene. Neotectonic structures are represented by horsts, diapirlike piercement structures, and folded zones. The paleovolcanoes are areas where slope processes of transport of sedimentary material occurred.