Abstract
The Al Madinah area bimodal dike swarms underwent detailed field mapping, measurements of dikes' attitudes, structural analyses, remote sensing data interpretation, and petrology study. The oldest rock unit is a greenish intermediate dioritic unit that varies in its petrologic composition between microdiorite and andesite porphyry which is intruded by alkali granite. The dioritic and alkali granite units have been cut by intermediate to mafic dikes, ranging in texture and composition from micromonzodiorite and micromonzonite to microgabbro, microdolerite, and basalt or basaltic andesite, as revealed by thin-section work. A stereographic projection and rose diagrams of the mafic dikes formed a regional conjugate dike system, following probably a typical andersonian style, consisting of three major sets, namely, the WNW set which trends between N295 and N330, E-W set between N265 and N270, and finally the ENE (N065) set. The WNW set has its own local system, but it is considered as a set in the overall regional conjugate dike system. The WNW and E-W sets are the conjugate pair of the WNW Najd fault system maximum horizontal shearing, where the ENE set reflects the minimum regional horizontal shearing stress of the Najd system. The interpretation of this dike system is a WNW-ESE sinistral faulting and NE-SW dilation that formed Wadi Al-Hamd Ediacaran continental basin, bounded by two Najd faults and Thurwah-Bir Um suture. The study area underwent final thrust faulting that tilted the dikes towards the east. For the consequent magmatisms during the Neoproterozoic period, followed by Cenozoic volcanism, the Al Madinah area is considered a large igneous province.