Paleomagnetic evidence bearing on Tertiary tectonics of the Tihamat Asir coastal plain, southwestern Saudi Arabia

Paleomagnetic directions determined for an upper Oligocene to lower Miocene dike swarm and from two lower Miocene layered gabbros in the Tihamat Asir coastal plain of southwestern Saudi Arabia are used to test several hypotheses concerning the tectonics of rifting along the eastern margin of the Red Sea. The dikes and gabbros were emplaced during the initial phases of Red Sea rifting and may mark the transition between continental and oceanic crust. Although these rocks have been hydrothermally altered to varying degrees, reliable remanent directions after alternating-field demagnetization were obtained for 23 dikes and for gabbros at Jabal at Tirf and Wadi Liyyah. Twelve of the dikes are reversely magnetized. After the directions of the reversely magnetized dikes are inverted 180?, the mean direction calculated for the normal dikes is approximately 24? more downward than that calculated for the reversed dikes. This result is similar to that found for the As Sarat volcanic field, 100 km to the north, and may be due to a displaced dipole source for the field.

The unrotated mean remanent direction for the dikes (inverting reversed dike directions 180?) is D (declination) = 353.2? and I (inclination) = 6.8? with a95 (radius of the cone of 95 percent confidence) = 8.9? whereas directions from the Jabal at Tirf and Wadi Liyyah gabbros lie at D = 176.2?, I = -1.6? (a95 = 7 1 ?) and D = 17.1?, I = 16.3? (a 95 = 8.7?), respectively. Comparing these results with the results from the As Sarat volcanic field, all the paleomagnetic evidence supports a model for approximately 20 ? of westward tilting of the Wadi Damad and Wadi Jizan areas after the emplacement of the Jabal at Tiff gabbro. The Wadi Liyyah area may have been tilted even more toward the Red Sea.

The paleomagnetic directions from three widely separated localities in the Jabal at Tirf gabbro are not significantly different, a fact which indicates that the body cooled in approximately its present bowl shape.

Evidence suggests that the ratio of normal to reversed dikes may change significantly along a 6-km-long traverse normal to the trend of the dike swarm, possibly reflecting migration of .a spreading axis.