Crustal Structure and Composition of the Arabian Shield: Examples Using GIS Analysis, Kingdom of Saudi Arabia
Peter R. Johnson and Maher R. Al-Subhi
Analysis of seismic and geologic data are examples of earth science GIS applications in Saudi Arabia that elucidate the basic crustal structure and composition of part of the Precambrian crystalline basement that underlies the entire Arabian Plate. The basement crops out in the west as the Arabian shield, the focus of this study, and in the east in small parts of Oman. It is up to 50 km thick in the east, close to the Zagros subduction zone, 33-53 km thick beneath the Arabian shield and the interior parts of the Peninsula, where it is covered by as much as 10 km of Phanerozoic sedimentary rocks of the Arabian Platform, and less than 15 km in the Red Sea basin, west of the shield. The surface area of Proterozoic exposures in the Saudi Arabian part of the shield is 441,181 km2. A first-order lithologic classification of the exposures divides the rocks into layered, plutonic, and high-grade metamorphic and(or) strongly deformed units (schist and gneiss). The layered and plutonic rocks each cover just over 47 percent, and the high-grade rocks and gneissic rocks up to 5.5 percent of the Proterozoic outcrop area. The layered rocks are further divisible into volcanic, sedimentary, and mixed volcanic and sedimentary units, of which the mixed volcanic and sedimentary grouping is the most widespread, underlying 18 percent of the outcrop area of the shield. Granitoids are the most abundant plutonic rock types, forming about 41 percent of the exposed surface area in the shield. Thirty-two percent of the plutonic rocks are alkali-feldspar-rich granitoids (alkali granite, syenogranite, and monzogranite) and 49 percent are plagioclase-rich granitoids (granodiorite and tonalite). Late- to posttectonic intrusions are typically alkali-feldspar-rich granite, particularly in the northeastern, northwestern, and eastern parts of the shield. The shield rocks in Saudi Arabia range from late Paleoproterozoic to late Ediacaran, but are predominantly Cryogenian. They formed in a variety of tectonic environments including ocean-floor spreading centers, volcanic arcs, and downwarped and fault-controlled basins, and are divisible into at least nine tectonostratigraphic terranes. Postamalgamation volcanic and sedimentary basins make up 9 percent of the surface of the shield. The highest metamorphic grade is the amphibolite facies, and very locally, the granulite facies. Most of the rocks in the shield, however, are in the greenschist or lower facies, and many of the younger rocks are unmetamorphosed. The metamorphic classification is preliminary because available information is commonly unreliable, and considerable additional work is necessary to make a robust metamorphic analysis of the shield.
Johnson, P.R. and Al-Subhi, M.R., 2007, Crustal structure and composition of the Arabian shield: examples using GIS analysis, Kingdom of Saudi Arabia: Saudi Geological Survey Open-File Report SGS-OF-2006-5, 30 p., 16 figs., 1 table.