Magnetic anisotropy delineator calibration error

Abstract

Summary. Four stratigraphic sections through alkali basalt flows of Oligocene to Miocene age (29-24Ma) in the As Sarat volcanic field, south-western Saudi Arabia, were sampled for palaeomagnetic study. After systematic alternating-field demagnetization, 42 magnetically acceptable flows (139 samples) yield a mean direction of magnetization of D = 355.3, I= 15.2 (ag5 = 4.3), which defines a palaeomagnetic pole at 78.8N, 247.8E. Of these acceptable flows, 24 are normally, and 18 reversely, magnetized. Part of one section was apparently erupted during the early phases of a polarity reversal of the Earth's field. The mean direction derived from the 24 normally magnetized flows is significantly different (after inverting 180) from that derived from the 18 reversed flows, supporting the hypothesis of a displaced dipole source for the Earth's field in late Oligocene to early Miocene time. A comparison of the results from As Sarat with palaeomagnetic results from upper Tertiary rocks in Africa indicates that the Red Sea has opened 12 (estimated +8 at the 95 per cent confidence level). These results indicate that most of the counter-clockwise rotation of Arabia relative to Africa occurred since the eruption of the As Sarat volcanics, in disagreement with proposed models for main-stage Red Sea spreading from late Eocene to early Oligocene time. The palaeomagnetic results also indicate that the Arabian Peninsuia was more equatorial in late Oligocene to early Miocene time than it is now, by an amount compatible with the opening of the Gulf of Aden.