Geochronological dating and stratigraphic sequences of Harrat Lunayyir, NW Saudi Arabia

Harrat Lunayyir is a basaltic volcanic field in NW Saudi Arabia. Lava flows are basaltic to basanitic in composition, and the Holocene flows are alkali olivine basalts. The volcanic field contains about 50 cones that were constructed on Precambrian crystalline rocks along an N-S axis. The dominantly basaltic lavas of Harrat Lunayyir have been divided based on their erosion characteristics into two major units – an older Tertiary unit (Jarad basalt) and a younger Quaternary unit (Maqrah basalt). The Quaternary Maqrah basalt has been subdivided into five stratigraphic subunits, Qm1 and Qm2 forming the Lower Maqrah basalt and Qm3, Qm4, and Qm5 forms the Upper Maqrah basalt. In 2009, a 3-km-long rupture in Harrat Lunayyir and lengthened to 8 km. Satellite radar images suggested that the most likely cause of this fault was magma intruding vertically along a 10-km-long fracture. The regional stress field deduced from the orientation of the observed dike indicates NE SW tension, which is also indicated by focal mechanism solutions with normal faulting with two major structural trends of NE-SW and NW-SE. These trends are consistent with the orientation of the opening of the Red Sea and with regional tectonics of Arabian-African rifting. Moreover, these results show that the Red Sea ridge stress field extends at least 200 km from the rift axis into the Arabian plate. Prior to this study, estimates of the ages of the samples ranged from Tertiary to Quaternary. This work places the ages of all samples firmly in the Quaternary period. 40 Ar/ 39 Ar age determinations have been produced for six samples. Most of these are fine-grained enough that whole-rock fragments were chosen for irradiation. The precision of the ages calculated in this study are limited by the small amounts of radiogenic argon compared to the larger amounts of non-radiogenic, or background argon typical in basaltic whole rocks of this age. The discrepancies in ages and stratigraphic position are due to the small amounts in accumulated argon over the relatively very short period of time. In general, this study shows that volcanic activity at Harrat Lunayyir started about 500,000 years ago. There must have been a substantial time between the lowest unit (which has previously been assigned a tertiary age) and youngest so that an unconformity has been developed. The latest activity could have taken place at about 5000 years ago.