STRUCTURAL EVOLUTION OF AD DAMM SHEAR ZONE, WESTERN SAUDI ARABIAN MARGIN AND ITS RELATION TO RED SEA RIFT SYSTEM

This project seeks to determine how the opening of the Red Sea rift was influenced by pre-existing tectonic features and to place constraints on the structural evolution of the western Saudi Arabia margin. In detail, we investigate whether or not the Ad Damm Shear Zone (ADSZ) was solely a Neoproterozoic structure or if it has been reactivated during the Cenozoic. The Ad Damm shear zone is a major mylonitic right-lateral structure that bounds the Jiddah terrane to the north from the Asir terrane to the south.

South of the ADSZ, the Red sea rift exhibits well-developed linear magnetic anomalies, but north of it, they are not present. On land, the ADSZ bounds a large topographic escarpment perpendicular to the rift margin, with higher elevations to the south. In addition, recent studies show active seismicity within the Jeddah terrane bounded by the ADSZ. These observations collectively suggest some type of tectonic reactivation.

Four techniques were used to test this hypothesis: field mapping, satellite image processing (ASTER, SPOT-5), microstructural studies, and Rb?Sr dating of shear zone biotite. Field observations show various meta-plutonic and volcanic rocks that range in age from ≈ 800-540 Ma. North of the ADSZ, the Jeddah terrane is characterized by heterogeneous magmatism with exensive meta-basalt intruded by silicic plutons of varying size. South of the ADSZ, the Asir terrane is characterized by larger-scale granitic batholiths. In addition, a large synform, likely a km-scale drag fold, is present along the northern edge of the ADSZ. Felsic dikes intrude a package of metabasalt and are folded in the synform. A younger generation of Eocene to Miocene basaltic dikes cut the mylonitic shear zone at a high angle.

Petrographic analyses of 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. Although Rb?Sr dating of biotite will determine the absolute age of ADSZ, initial observations suggest that high temperature ADSZ deformation is Neoproterozoic in age. However, differences in the crustal characteristics of the Asir and Jiddah terrane still influence the ongoing tectonic evolution of the Red sea rift.