Abstract
The Ad Damm shear zone (ADSZ) is a major mylonitic right-lateral structure that bounds the Jeddah terrane to the north from the Asir terrane to the south. High-resolution field mapping coupled with petrological and geochemical analyses indicate that Jeddah terrane is characterized by heterogeneous magmatism with extensive meta-basalt intruded by silicic plutons of varying size. South of the ADSZ, Asir terrane is characterized by larger scale granitic batholiths. A younger generation of Eocene-Miocene basaltic dikes cut the mylonitic shear zone at a high angle. Petrographic analyses of the ADSZ mylonitic rocks indicate dynamic recrystallization and grain-size reduction, suggesting high-temperature recrystallization. Field observations also found a lack of low-temperature fault zone rocks (e.g. gouge) except for isolated brittle slickensides. Spider diagrams of Jeddah, Asir terranes, and ADSZ rocks are characterized by an arc-related signature, which related to the amalgamation of Jeddah and Asir terranes and defined ADSZ as Neoproterozoic structure. In contrast, Eocene-Miocene basaltic dikes and southern basaltic flow are represented by a rift-related signature, which associated with the development of the Red Sea rift system. Offshore, south of the ADSZ, the Red Sea rift exhibits well-developed linear magnetic anomalies and large topographic escarpment perpendicular to the rift margin, but they are not present north of it. In addition, recent seismicity recorded along the ADSZ and differences in the crustal thickness and characteristics of Asir and Jeddah terranes, collectively, make ADSZ acted as an active crustal boundary and still influence the ongoing tectonic evolution of the Red Sea rift.