Joint modeling of seismic, magnetic and gravimetric data unravels the extent of the Late Cretaceous Magmatic Province on the Estremadura Spur offshore West Iberia

This work consists on the interpretation of multichannel seismic profiles complemented and supported by gravimetric and magnetic forward modeling, on the region surrounding the underwater volcano Fontanelas (Estremadura Spur, west of Lisbon).

The Fontanelas seamount (FSM) is a volcanic cone about 3000 m high from its top to its submerged base that coincides with a strong magnetic anomaly (~350 nT). From dredged samples it is known that it is consists of altered pillow-lavas of ultrabasic and basic alkaline composition (foidites and alkaline basalts) (Miranda et al., 2010). It has been associated with onshore Upper Cretaceous alkaline magmatic events due to its enrichment in incompatible elements and similar isotopic elementary signatures (Miranda et al., 2009 and 2010). The FSM is located halfway between the onshore Sintra intrusive complex and the Tore seamount, between which a 300 km long tectono-magmatic lineament of intrusive/extrusive alkaline bodies of Upper Cretaceous age has been proposed, based on the existence of several other magnetic anomalies (Neres et al., 2014).

Magnetic and gravimetric modeling allowed to constrain the location, depth, extension and geometry of the magmatic bodies in the seismic reflection profiles that were used to map and dating the magmatic bodies and tectonic events.

The joint modeling of these three geophysical methods (seismic, magnetic and gravimetric) allowed for the production of an integrated tectono-magmatic-sedimentary model of the Estremadura Spur. The existence of a complex volcanic and subvolcanic system in the Estremadura Spur was confirmed, including several intrusive bodies, besides the Fontanelas volcano: sills, secondary volcanic cones, large laccolith-type intrusions in the Upper Jurassic. Some extensional rift faults were used as magma conduits for sills plugs and volcanoes.  Magmatic bodies localized compressive strain during the tectonic inversion of the Lusitanian basin during the Alpine compression.

The age of the magmatic bodies is constrained by seismic stratigraphy as prior to the Campanian (83.9 Ma), which allows to associate them with the onshore Upper Cretaceous alkaline magmatic event (Sintra, Sines, Monchique, Lisbon Volcanic Complex, minor intrusive bodies), also correlative of the alkaline magmatism existing offshore along the Madeira-Tore Rise (Merle et al., 2018).

This work will be the basis of future studies regarding the heat dissipation from the intrusion of the magmatic bodies over time in order to estimate the temperatures that surrounding rocks have reached.

Support by Landmark Graphics Corporation, Oasis Montaj (Geosoft), FCT (project UID/GEO/50019/2019- Instituto Dom Luiz) and DGEG is acknowledged.      

 

Merle, R., et al. (2018). Australian Journal of Earth Sciences, 65(5), 591-605. https://doi.org/10.1080/08120099.2018.1471005

Miranda R., et al. (2009). Cretaceous Research, 30, Elsevier, 575-586. https://doi.org/10.1016/j.cretres.2008.11.002.

Miranda, R., et al. (2010). In X Congresso de Geoquímica dos Países de Língua Portuguesa e XVI Semana de Geoquímica, 28 de Março a 1 de Abril de 2010. http://hdl.handle.net/10400.9/1246

Neres, M., et al. (2014). Geophysical Journal International, 199(1), 78-101. https://doi.org/10.1093/gji/ggu250

Pereira, R., et al. (2016). Journal of the Geological Society, 174(3), 522-540. https://doi.org/10.1144/jgs2016-050