Abstract. During the Geophysical Measurements Across the Continental Margin of Namibia (MAMBA) experiments, offshore and onshore refraction and reflection seismic as well as magnetic data were collected. Together with the existing free-air gravity data, these were used to derive two crustal sections across the ocean-

S U M M A R YThe geology of the wide shelves surrounding the South Atlantic is closely linked to the kinematics and history of the opening of the ocean. However, several wide sedimentary basins, which developed along the margins show peculiarities that are not yet understood in the context of the evolution of the South Atlantic.The Colorado Basin, a wide sedimentary basin on the broad shelf of Argentina, extends in EW direction. The basin's evolution oblique or orthogonal to the continent-ocean boundary indicates that it is not a product of simple progressive extension and crustal thinning. In addition a basement high, paralleling the continental margin and separating the Colorado Basin from the deep-sea basin is a common interpretation. These findings are hardly in accordance with the idea that the Colorado Basin is an extensional basin that developed in conjunction with the early E-W opening phase of the South Atlantic in the Late Jurassic/Early Cretaceous. The composition, type, and structure of the basement, key points for the evaluation of the basins evolution, are widely speculative.In this context multichannel seismic reflection data from the Argentine Shelf and a 665-kmlong onshore-offshore refraction profile, running across the Colorado Basin onto the coast are discussed in combination with gravity data. The stratigraphy for the sedimentary successions was adopted from the literature and the reflection seismic marker horizons formed besides the interval velocities the input for the starting model for refraction seismic traveltime modelling. The modelling strategy was an iterative procedure between refraction seismic traveltime and gravity modelling. The preparation of the density models was coarsely orientated on published velocity-density relations. The modelling results are in favour of a continuation of the main onshore geological features beneath the sedimentary infill of the Colorado Basin. We interpret the basement along the line from west to east as offshore continuation of the Ventana Hills, the Claromecó depocentre, and of Palaeozoic to Middle Mesozoic rocks of the Patagonia terrane. In the deepest part of the pre-/ synrift graben within the Colorado Basin a volcanic/igneous intrusion was interpreted forming an injection into an extensional fault.According to our interpretation most of the Colorado Basin developed in conjunction with an early opening phase of the South Atlantic (150-130 Ma) and thus represents a typical rift basin instead of an intracontinental sag basin. The origin of the oblique rift most probably resulted from extensional stress, acting either through or interfering with the prevailing Palaeozoic basement fabric, oriented NW-SE. Although there was certainly a strike-slip component in the basins evolution and it may be interpreted as pull-apart basin we suggest that the Colorado Basin represents a failed rift structure: The basin's floor is more or less flat across the shelf, shows a slow rise at the shelf break and deepens towards the deep-sea basin where it finally merges wi...

The discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate.

No abstract

Wide-angle reflection and refraction data acquired as part of the URSEIS '95 geophysical experiment across the southern Uralide orogen provide evidence for a 12- to 15-kilometer-thick crustal root, yielding a total crustal thickness of 55 to 58 kilometers. Strong reflections from the Mohorovičić discontinuity (Moho) at relatively small precritical distances suggest that the crust-mantle transition beneath the crustal root is a sharp feature. The derived P - and S -wave velocity models constrain key physical properties of the crust, including the depth of the mafic rocks of the Magnitogorsk volcanic arc and the existence of a lower crustal zone of possible basic rock enrichment beneath the East Uralian zone.