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Abstract

A palaeomagnetic and rock magnetic study has been performed on serpentinized peridotite rocks recovered at Ocean Drilling Program (ODP) Leg 149 and 173 sites, off the west coast of Portugal (Sites 897, 899 and 1070). Stable components of magnetization are revealed after detailed thermal and alternating field demagnetization. Results from a series of rock magnetic measurements with advanced instrumentation all corroborate the demagnetization behaviour and show that (titano)magnetites and maghemite are present in the peridotites. At Leg 149 and 173 sites, the inclinations of characteristic magnetization in the ‘fresher’ lower part of the serpentinized peridotite section show a predominantly reversed polarity in a depth zone of c. 25 m, which is compatible with a reversed event (probably correlated with marine Anomaly MO at c. 121 Ma) before the Cretaceous Long Normal Superchron (83–120Ma) and is incompatible with a Holocene field direction. In contrast, the inclinations of samples from the more ‘altered’ upper part are predominantly normal. In view of the polarity of magnetization identified from the overlying Cretaceous and Tertiary sediments and the newest interpretation of the drilling results for the Iberia margin, our results suggest that the Iberian peridotites were emplaced during Aptian-Barremian time and recorded the middle Cretaceous geomagnetic field during Anomaly M0 time (c. 121 Ma). Our data are consistent with new radiometric dates for the Iberian peridotites at Leg 173 drill sites.

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