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Magnetic studies of the Late Cretaceous magmatism in Portugal : from Iberian plate kinematics to magnetic fabrics
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Resumo(s)
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.
Descrição
Tese de doutoramento (co-tutela), Ciências Geofísicas e da Geoinformação (Geofísica), Université de Toulouse, Universidade de Lisboa, 2013
Palavras-chave
Paleomagnetismo Cretácico Teses de doutoramento (co-tutela) - 2013