Paleomagnetic results from Oman ophiolites related to their emplacement

Abstract

The paleomagnetic results from the Cretaceous Oman ophiolites suggest large rotations of the ophiolitic nappe during the intraoceanic thrusting and continental obduction which preceded its last emplacement onto the Arabian margin, and they confirm that the ophiolites were formed and thrust at equatorial latitudes. Furthermore, they may indicate the occurrence of short reversals during the Cretaceous long normal polarity period. Our paleomagnetic study was mainly conducted on metalliferous sediments interbedded within the volcanics of the ophiolites. Ninety-nine cores from 31 samples from fifteen outcrops distributed along the nappe were analysed. Alternating field and stepwise demagnetisation revealed complex multicomponent magnetisations carried by hematite and magnetite. Among the various components, a stable component was commonly found in each hand specimen from the same outcrop, with varying unblocking temperature or field spectra because of varying composition from one sample to another. After tilt correction, the inclinations of these stable components were all found to be low, and the declinations grouped around NW-SE and NNE-SSW axes, each group containing roughly antipodal directions. The occurrence of antipodal directions is inconsistent with the ages of the sediments situated in the middle of the Cretaceous long normal polarity period, but is consistent with previously published findings of dual polarity in gabbros and dikes. The two NW-SE and NNE-SSW paleomagnetic groups may correspond to two successive episodes of magnetisation acquired during diagenesis, or during high- or low-temperature hydrothermal activity. The large rotations of the nappe that these magnetisations suggest, would be consistent with the transport directions recorded within the metamorphic soles. In further studies, the magnetic mineralogy and the paleomagnetic results will have to be related to hydrothermal activity, and the particular magnetic properties of our manganese-rich sediments will need to be better understood.