Magnetic studies of the Late Cretaceous magmatism in Portugal : from Iberian plate kinematics to magnetic fabrics

Tese de doutoramento (co-tutela), Ciências Geofísicas e da Geoinformação (Geofísica), Université de Toulouse, Universidade de Lisboa, 2013

The Jurassic-Cretaceous Iberian kinematics is still not well understood, mainly due to limitations on reconstructions based on marine magnetic anomalies (uncertainties about the nature of the crust and presence of the Cretaceous Normal Superchron) and on paleomagnetic data (insu cient and sometimes low-quality data, low age resolution, tectonic in uences and remagnetizations). In this thesis, we rst provide new high-quality paleomagnetic poles for the Late Cretaceous of Iberia, calculated from two ma c sills (Foz da Fonte, 88 Ma et Pa co de Ilhas, 94 Ma), which enable better calibrating the Iberian apparent polar wander path (APWP) at the Late Cretaceous. A detailed study of magnetic mineralogy and microscopic observations (electronic and optic) con rm a primary magnetization carried by titanomagnetite. We then present a new compilation of the published paleomagnetic data for the Late Jurassic-Cretaceous of Iberia from which we calculate mean poles for six time periods. By rotating these mean poles to the African frame using Euler rotation poles from kinematic models we evaluate their position with respect to the global APWP in African coordinates. We verify that while post-rift mean poles (from 70 to 120 Ma) are in agreement with the APWP, poles corresponding to pre-rift times (older than 120 Ma) do not t the APWP, revealing an incompatibility between the di erent types of data. We next seek for the cause of this incompatibility, which may be due to the APWP (we show that this is unlikely since di erences between di erent APWPs are not signi cative), euler poles (we show that a discard is veri ed using both end-member models of Olivet [1996] and Vissers & Meijer [2012]) and/or paleomagnetic data. We solve the inverse problem of nding the Euler poles that t the pre-rift mean paleomagnetic poles with the GAPWP and then test their implications on Iberian reconstructions. We nd that Iberian poles from the Early Cretaceous (mean poles for 123 and 130 Ma) are incompatible with the GAPWP, bringing into question their validity. Contrarily, Late Jurassic data (mean pole at 151 Ma) are compatible with the GAPWP and thus can be considered reliable. These results enable us to constrain a reconstruction of Iberia and surrounding plates at 150 Ma. These results highlight that more high-quality paleomagnetic data are needed for the Iberian plate particularly at the Early Cretaceous, and that kinematic models based on the interpretation of marine magnetic anomalies should be reviewed. In the last part of this thesis we present a study of the anisotropy of magnetic susceptibility of the Foz da Fonte sill, 7-8 m width. A detailed vertical sampling of the sill allowed us to distinguish three domains with respect to the magnetic fabric, which we relate with distinct regimes of magma ow. The chilled margins, 50 cm apart from the margins, where low anisotropies suggest low velocity gradients and heterogeneous ow paths during initial emplacement stages; in the center of the sill, where undisturbed magma ow is expected, low anisotropies suggest low shear gradients and magma displacement close to pure translation; and intermediate zones, where high anisotropy values are ascribed to maximum shear gradient zones. A mean orientation at N310 of the magnetic lineation agrees with the direction of elongation of vesicles and is interpreted as magma ow direction. In addition, a sense of ow toward the southeast is inferred from the mirror imbrication of the magnetic foliation and lineation at the borders. Implications of these results are discussed with respect to the West Iberian Late Cretaceous magmatism, by integrating magnetic anomalies, isotope chronology and tectonics. The Cabo Raso anomaly is proposed as the magmatic source of the Foz da Fonte sill, and a 350 km long magmatic structure is proposed to connect the region of Sintra-Cabo Raso to the Tore seamount.

Fundação para a Ciência e a Tecnologia (FCT, projetos PEst-OE/CTE/LA0019/2013, PTDC/CTE-GIX/117298/2010, PTDC/CTEGIN/68462/2006); TOPOMED (TOPOEUROPE/0001/2007); Prémio Fundação Calouste Gulbenkian