Characteristics of VRM in Oceanic Basalts
Laboratory experiments, each lasting several weeks, have been conducted to establish the characteristics of viscous remanent magnetization (VRM) in oceanic basalts from many sites of the Deep Sea Drilling Program (DSDP). VRM is most pronounced in low-coercivity basalts whose natural remanences (NRM) have low median destructive fields, less than 100 Oe. A simple logarithmic acquisition law is rarely obeyed, but two or three distinct stages are instead observed, in each of which a logarithmic dependence of VRM intensity on acquisition time may be assumed. This observation leads to a simple interpretational model for the nature of VRM in DSDP basalts, but also implies that extrapolation of laboratory observations to geological times is not meaningful. Instead, the ratio of laboratory VRM (acquired in a 1 Oe field during 1000 h) to NRM is used as a minimum indicator of the potential seriousness of VRM. Experiments show that VRM acquired in the presence of NRM is more serious than VR M acquired in alternating field (AF) demagnetized samples. As most published VRM data in DSDP basalts were obtained after AF demagnetization, these are regarded also as minimum estimates of the significance of VRM acquired by oceanic basalts in situ. The consequences of the common occurrence of such an unstable component of magnetization in the oceanic basalt layer are considered in relation to the nature and distribution of oceanic magnetic quiet zones. The Cretaceous, and possibly the Jurassic, magnetic quiet zones are considered adequately explained by constant paleomagnetic field polarity. However, if VRM is a substantial and widespread magnetization component in the oceanic crust, it may not always be appropriate to interpret oceanic magnetic anomalies (or their absence) as an exact record of paleomagnetic field behavior. Remagnetization of the oceanic crust by VRM acquisition may be a viable alternative explanation of the origin of the marginal magnetic quiet zones.
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Also Published In
- Journal of Geophysics