The Cretaceous Peri-Atlantic Alkaline Pulse (PAAP): Deep mantle plume origin or shallow lithospheric break-up?

Although the Early Jurassic initial phases of Atlantic rifting were marked by the tholeiitic basalt flows and dikes of the Central Atlantic Magmatic Province (CAMP), the Cretaceous Era was characterised by widespread alkaline igneous activity on widely separated peri-Atlantic continental margins. Nearly half the peri-Atlantic alkaline rocks dated between 250 to 50 Ma fall within the 125–80 Ma range. This period of intense alkaline activity during the mid-Cretaceous formed the Peri-Atlantic Alkaline Pulse (PAAP). The mechanisms for the opening of the Atlantic Ocean, as well as for the CAMP and later PAAP magmatic events, have been attributed either to deep mantle plumes or to a combination of tensional forces, lithospheric rifting and structural controls. We describe the PAAP event for the first time and test the plume and structural models.The PAAP displays evidence of strong structural control and appears to be related to reactivation of pre-existing lithospheric structures throughout the peri-Atlantic zone. Early stages of continental separation and abrupt changes in plate motion were the most likely mechanisms for the reactivation of favourably oriented pre-existing structures. Mid-Cretaceous pulses in alkaline magmatic activity were closely associated with major Atlantic tectonic events. The opening of the South Atlantic, the change in intensity and direction of both seafloor spreading and plate motion, and the consequent fluctuation in intra-plate stresses were responsible for two spikes in peri-Atlantic alkaline magmatism at ~ 125 Ma and ~ 85–80 Ma.The general lack of evidence for a deep hotspot associated with the peri-Atlantic zone seems incompatible with a mantle plume origin. On the other hand, the spatial distribution of the PAAP along passive continental margins suggests an ideal setting for the development of edge-driven convection. We propose that the periodic reactivation of deep-seated pre-existing zones of weakness during the major stages of Atlantic tectonic evolution, combined with coeval asthenospheric upwelling due to edge-driven convection and continental insulation flow, enhanced the ascent of alkaline magmas. This mechanism of shallow, small-scale upwelling during periodic structural reactivation provides a more likely explanation for the PAAP than the commonly proposed deep mantle plume scenario.