Role of subaerial volcanic rocks and mantle plumes in creation of South Atlantic margins: implications for salt tectonics and source rocks

摘要

Seaward-dipping re¯ectors (SDRs) represent ¯ood basalts rapidly extruded during either rifting or initially subaerial sea-¯oorspreading. Evaporites can form on this basaltic proto-oceanic crust, as in the Afar Triangle today. Evidence for SDRs in SouthAtlantic deep-water regions comes from proximity to the uniquely large Paranaà ±Etendeka volcanic province onshore, theTristan and Gough hot spots, drilled volcanic rocks, and seismic pro®les showing SDR provinces more than 100 km wide, asmuch as 7 km thick, and thousands of kilometers long. SDRs are clearest adjoining the Aptian salt basins. However, wespeculate that SDRs are also present but seismically obscured below the salt basins. We argue that the conjugate Aptian saltbasins are post-breakup, not pre-breakup; they were separated from the start by a mid-oceanic ridge; distal salt accumulated onproto-oceanic crust, not rift basins. This hypothesis is supported by: seismic stratigraphy and structure; magnetic anomalies;plate reconstructions; and hydrothermal potash evaporites. An important implication for exploration is that thick basalts, ratherthan rift-age source rocks, may underlie distal parts of the salt basins.