The late rifting phase and continental break-up of the southern South Atlantic: the mode and timing of volcanic rifting and formation of earliest oceanic crust

Abstract

AbstractMultichannel seismic and potential field data shed light on the final rifting stage in the southern South Atlantic. This was associated with major episodes of magmatism during the Early Cretaceous continental break-up. An asymmetrical simple shear-dominated variable strain rifting model is proposed with the margin asymmetry visible in shelf width, amplitude of magnetic anomalies, orientation of break-up-related sedimentary basins and basement slope angle. Along-margin rotation in spreading- and later rifting-direction from north–south to west–east are of great importance for the asymmetries. Such rotational opening may also explain why the southernmost segments of the South Atlantic are magma starved, with a sharp transition to a volcanic-rifted margin type northwards. Interpretation of pre-M5 (c.130 Ma) magnetic seafloor spreading lineations constrains the timing of excess break-up-related volcanism and transition to ‘normal’ seafloor spreading. Termination of magnetic anomalies within seaward-dipping reflector sequences points towards a deposition of the volcanics from south to north prior to and during the early rift and opening stages. Identification of previously unknown pre-M5 magnetic lineations offshore Argentina completes the lineation pattern in the southern South Atlantic. The oldest magnetic anomaly related to oceanic spreading is M9 (c.135 Ma). Older anomalies, previously identified as M11 (c.137 Ma) offshore Cape Town, are related to structural or magnetization variations within seaward-dipping reflector sequences.