A paleomagnetic and rock magnetic study has been performed on the serpentinized peridotites recovered from Ocean Drilling Program Leg 149 Sites 897 and 899 across the ocean/continent transition in the Iberia Abyssal Plain to estimate the ages of their emplacement and subsequent alteration, which have a direct relation to the early opening of the North Atlantic. Stepwise thermal and alternating-field demagnetization experiments on 69 10-cm3 minicore samples from Holes 897D and 899B show that the natural remanent magnetization (NRM) of many samples is composed mainly of a single stable component. The mean NRM intensity is typically on the order of 0.2 A/m for samples from Hole 897D, but about 1.3 A/m for samples from Hole 899B. The much stronger magnetization intensity of Site 899 is apparently in excellent agreement with a magnetic anomaly high observed in the vicinity of Site 899, suggesting that the serpentinized peridotite body under Site 899 contributes significantly to the magnetic anomaly. Conversely, the fact that Site 897 is located more oceanward but has a mean NRM intensity less than one-third of the average intensity (~4 A/m) of dredged and drilled oceanic basalts in the Atlantic may be used to explain the weakly negative anomaly data observed nearby.
At both sites, the upper part of the basement section immediately underlying the Pleistocene to Lower Cretaceous sedimentary cover is a zone of alteration in which rocks are pervasively veined and altered to brown-colored, altered, serpentinized peridotites. The lower part of the basement section consists of "fresher" greenish gray serpentinized peridotites. In this fresher lower part, the inclinations of characteristic magnetization show a consistent polarity pattern in a depth zone of about 20 m that is correlative between the two sites. Within this zone, the inclinations of samples are predominantly reversed, which is compatible with a reversed event (probably correlated to marine Anomaly M0 at about 118 Ma) prior to the Cretaceous Long Normal Superchron (84-118 Ma) and is incompatible with a Holocene field direction. In contrast, the inclinations of samples from the more "oxidized" upper part are almost all normal and the directions are similar throughout this part regardless of depth and lithology. This polarity pattern would suggest that the emplacement of these peridotites took place sometime during the middle Cretaceous (probably at the M0 time) and alteration of the upper part probably occurred during the Cretaceous Long Normal Superchron by convecting seawater during the opening of the North Atlantic. Paleomagnetic results from this study provide an example of the potential value of magnetic polarity dating of tectonic processes that accompanied continental breakup and the onset of steady-state seafloor spreading.
Date of initial receipt: 5 December 1994
Date of acceptance: 26 June 1995
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