DELAYED MAGMATISM AND TECTONIC ACTIVITY ON “PASSIVE” MARGINS

Passive continental margins created during the oceanization rift-drift transition stage are commonly regarded as volcanic or non-volcanic according to the amount of igneous products emplaced during the break-up stage. In this respect, the Atlantic Ocean is acknowledged as bounded by Paleocene volcanic margins in the north, and Jurassic-Cretaceous non-volcanic margins in the central part. Here, however, both western and eastern conjugated rifted margins are the site of small-volume magmatism much younger than the rift-drift stage.

A prime example of this type of magmatism is found in Senegal, whose continental margin developed in Middle-Late Jurassic and is affected by Oligocene to Quaternary alkaline magmatism. The igneous products are scattered over the Cap-Vert peninsula as lava flows and shallow level intrusions. Additional saucer-shaped sills have been detected offshore from Senegal on the basis of integrated seismic, magnetic and gravimetric surveys. The occurrence of sill-related hydrothermal vents and forced folds allows dating of the igneous event to the Miocene. The age of magmatism displays a minimum delay of 130 Ma with respect to rifting, with consequent decoupling of any cause-effect link between rifting and magmatism. Additional examples of similar delayed alkaline magmatism come from other Atlantic margins such as Newfoundland-Grand Banks and its conjugate Iberian margin along the Tore-Madeira Rise. Southwards, cases of delayed magmatism are also found in the Canary Islands and along the Cameroon line.

In the Senegal margin, onshore structural data, offshore seismic sections, and gravimetrically-magnetically defined shapes and arrays of offshore sills all indicate that magma emplacement followed the geometric impact of Atlantic oceanic fracture zones into the transitional-continental crust. Therefore, we propose that the engine for the generation of alkaline magmas on passive margins tens of Ma after the rift-drift transition is linked to the reactivation of oceanic fracture zones, with an active example represented by the Antarctic rift-Southern Ocean fracture zone system. The inventory of passive margin types has to include a "new" type of margin: besides volcanic and non-volcanic margins, "delayed-volcanic" passive margin with alkaline magmatism should be accounted for.