COMPARING ONSHORE AND OFFSHORE VOLUMES OF LARGE IGNEOUS PROVINCES ASSOCIATED WITH PASSIVE CONTINENTAL MARGINS

The rifting of continents can lead to the initiation of seafloor spreading and the formation of passive margins. Volcanic (magma-rich) passive margins, which are defined as being underlain by enormous volumes of igneous rocks, are often associated with large igneous provinces (LIPs). However, the relationship in space and time between LIPs on land and marine igneous units found along adjacent passive margins is poorly understood. The Mid- and South Atlantic Ocean breakups, and the resulting offshore volcanic passive margins, are temporally and spatially associated with the Central Atlantic Magmatic Province (CAMP) and Paraná-Etendeka LIP. We explore these issues using the VOLMIR (VOLcanic passive Margin Igneous Rocks; Gallahue et al. Marine & Petroleum Geology, 2020) dataset based on seismic reflection profiles in which the volumes and geometries of both shallow seaward dipping reflector (SDR) and deeper high velocity lower crustal (HVLC) units can be measured. We find a relatively consistent ratio of SDR to HVLC volumes, with SDR volumes about 1/3 that of HVLC. This consistency suggests that as magmas rise and erupt to the surface to form SDRs, the remaining high-density residuum or cumulate becomes the HVLC. We extend the approach to estimate the amount of igneous material in offshore passive margins and compare it to estimates for adjacent on-land LIPs. The results indicate that a significant volume of volcanics exist offshore, increasing the inferred amount of volcanic output associated with the LIPs. Further studies will provide insight into the relationship between offshore passive margins and on-land LIPs, and perhaps provide further understanding on the role of magmatism in rifting processes.