Paleomagnetic study of the As Sarat volcanic field, southwestern Saudi Arabia

Abstract

Four stratigraphic sections through alkali basalt flows of Oligocene-Miocene age (24 to 29 m.y.) in the As Sarat volcanic field, southwestern Saudi Arabia, were sampled for paleomagnetic study. Forty-two magnetically acceptable flows (139 samples) yield a mean direction of magnetization of D=-4.7°, 1=15.2° ( α 95=4.3°), which defines a paleomagnetic pole at 78.8°N, 247.8°E. Of these acceptable flows, 24 are normally magnetized and 18 are reversed. Parts of two sections of flows erupted apparently during the early phases of a polarity reversal of the earth's field. Although the mean paleomagnetic directions determined from each flow do not collectively define a strict Fisherian distribution, the fact that the mean directions for each of the four sections are nearly identical indicates that the overall paleomagnetic pole position is reliable. The magnetostratigraphy can be only generally correlated from one section to another apparently because of a lateral thickening and pinching out of many flows between sections. The dispersion of directions due to secular variation of the upper Oligocene and lower Miocene field (standard angular deviation = 16.2°) is about the same as that produced by the recent field. If the rate of secular change during the time of As Sarat eruption was approximately that of the present, the rate of eruption during several sequences of As Sarat flows is estimated to be one flow about every 100 years. The fact that the mean direction derived from 24 normally magnetized flows is significantly different from that derived from the 18 reversed flows supports the hypothesis of a displaced dipole source for the earth's field in late Oligocene to early Miocene time. The results from As Sarat, as well as those from Aden, compared with paleomagnetic results from Late Tertiary rocks in Africa indicate with 95 percent confidence that the Red Sea has opened at least 2° in the past 5 m.y., and at least 4° in the past 25 m.y. The paleomagnetic results also indicate that the Arabian Peninsula 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.