Geophysical evidence for a magmatic intrusion in the ocean-continent transition of the SW Iberia margin

Abstract

A prominent positive gravimetric and magnetic anomaly stands out in the region of the Guadalquivir-Portimão Banks, Gulf of Cadiz, off SW Iberia. The bathymetric and gravimetric highs have been interpreted as resulting from basement uplift occurred during Mesozoic rifting followed by overthrusting during the Alpine orogeny. However, we show that basement uplift alone does not account for the observed magnetic anomaly, implying that an additional magnetic source must be considered for an integrated model that fits with all the data. We present a new magnetic study that calls for the existence of a large magmatic body intruded beneath the Guadalquivir and Portimão Banks, the Guadalquivir-Portimão intrusion. We perform 3D inversion of magnetic data to obtain a susceptibility model that estimates the shape and extension of the intrusion. The modeled volume is constrained by estimation of regional Curie depth and the susceptibility model is limited to a realistic range, allowing interpretation in terms of rock nature. To clarify the relationship between the intrusion and the local lithospheric structure, we build 2D cross-sections integrating the modeled intrusion with recent crustal models and use 2D forward modeling to ensure that both the predicted magnetic and gravimetric fields fit the observed data. These sections show the Guadalquivir-Portimão intrusion emplaced along a lithospheric decollement within the necking domain of the Algarve rifted margin, across the lower and upper crust. It is suggested that is crosscuts Jurassic sedimentary sequences, suggesting that it intruded after continental rifting. Given the similarities with other onshore magnetic anomalies corresponding to Late Cretaceous alkaline intrusions, it is likely that the Guadalquivir-Portimão intrusion represents the southernmost expression of the Late Cretaceous alkaline magmatic event that affected both onshore and offshore West Iberia, becoming the fourth element of the enigmatic NNW-SSE trending alignment of Sintra-Sines-Monchique plutons, whose significance remains to be fully understood.