Over-ice Deep Seismic Soundings (refraction and reflection seismic experiments) were carried out during three seasons across the Filchner and Ronne Ice Shelves using arrays of ground geophones and explosive charges. Seismic arrivals were received from boundaries in the crust and upper mantle. Ray-trace modelling shows that the crustal section has different characteristics along the seismic line. Normal or moderately stretched continental crust is observed on the flanks of the section with some evidence of igneous underplating and mafic intrusions beneath the East Antarctic margin, whereas the central part exhibits anomalous features. In this area, an up to 15 km thick, presumably Late Mesozoic-Cenozoic layer overlies an 8-10 km thick medium-velocity (5.5-5.6 km/s) layer interpreted as metamorphosed Paleozoic deposits which, in turn, rest directly on the lower "mafic" crust characterized by velocities of 6.9-7.5 km/s. According to suggested interpretation the entire thickness of the sedimentary basin strata is as much as 20-22 km. Two models are proposed to explain the anomalous crustal structure of the studied region. The first model suggests that the lower, high-velocity layer is a strongly stretched continental crust, contaminated by mafic (mantle-derived) rocks which was developed in the settings of Late Neoproterozoic rifting and Paleozoic to Mesozoic back-arc basin. The second model considers the lower crustal layer as a sligtly stretched Neoproterozoic or Early to mid-Palaeozoic relict oceanic crust which either was retained after the transformation of the Antarctic segment of the Gondwana margin from a rifted to a subducted one or originated due to sea-floor spreading in a back-arc setting.