The Bransfield Strait, located between the South Shetland Islands and the north-western end of the Antarctic Peninsula, is a back-arc basin transitional between rifting and spreading. We compiled a geomorphological structural map of the Bransfield Basin combining published data and the interpretation of bathymetric images. Several analogue experiments reproducing the interaction between the Scotia, Antarctic, and Phoenix plates were carried out. The fault configuration observed in the geomorphological structural map was well reproduced by one of these analogue models. The results suggest the establishment of a transpressional regime to the west of the southern segment of the Shackleton Fracture Zone and a transtensional regime to the south-west of the South Scotia Ridge by at least c. 7 Ma. A probable mechanism for the opening of the Bransfield Basin requires two processes: 1) Significant transtensional effects in the Bransfield Basin caused by the configuration and drift vector of the Scotia Plate after the activity of the West Scotia Ridge ceased at c. 7 Ma. 2) Roll-back of the Phoenix Plate under the South Shetland Islands after cessation of spreading activity of the Phoenix Ridge at 3.3 ± 0.2 Ma, causing the north-westward migration of the South Shetland Trench.

Examination of the holotype of Sipho gaini Lamy, 1910, attributed by recent authors to the genus Chlanidota (Buccinulidae), revealed that this species belongs to the genus Belaturricula and is a senior synonym of Belaturricula antarctica Dell, 1990. A redescription of the shell, anterior alimentary system, and radular morphology of Belaturricula gaini is provided. The genus Belaturricula is transferred to the family Conidae (sensu Taylor et al. 1993) on the basis of its radula, which is composed of hollow marginal teeth. The presence of a large shell, prominent operculum, acinous salivary glands, radular teeth with narrow bases in B. gaini, as well as the absence of buccal lips and rhyncodeal introvert, all indicate affinities with the conid subfamily Clathurellinae. Because the radula of Pontiothauma ergata Hedley (1916) is nearly indistinguishable from that of Belaturricula gaini, we reassign Hedley's species to the genus Belaturricula as Belaturricula ergata (Hedley, 1916).