A gravity survey of parts of quadrangles 26E, 26F, 27E, and 27F, northeastern Arabian Shield, Kingdom of Saudi Arabia

Abstract

A gravity survey using nearly 800 stations was conducted over an area of about 13,400 km 2 located in the northeast part of the Arabian Shield. The stations were set on spot elevations of relative high density and shown on high-quality l:50,000-scale topographic base maps. The error in a gravity reading due to uncertain elevation is estimated to be less than 0.33 mgal. Determination of station coordinates was aided by helicopter-mounted LORAN-C navigation units. The cost/time factors involved in the survey compared favorably with commercial surveys done with inertial-guidance systems but without the l:50,000-scale maps. As topographic-map coverage becomes available, gravity surveys should be run over the entire Arabian Shield. The Arabian Shield is generally comprised of Proterozoic sedimentary rocks metamorphosed to varying degrees and intruded by plutonic rocks. The northeastern Shield is traversed by the north-trending Nabitah mobile belt, a zone of flexing, faulting, shearing, and mineralization. The complete Bouguer gravity field defines the boundary of the mobile belt, as well as a tectonic platelet that has undergone especially conspicuous thrusting and other types of deformation within the mobile belt. Many plutons intruded the metamorphic rocks in the survey area. The youngest rocks are peraluminuous, contain anomalously high tin and tungsten concentrations, and are similar to other tin and tungsten-bearing granites elsewhere. Pronounced gravity lows are associated with these plutons, but not all of a pluton or its associated mineralized rocks may crop out. Thus, gravity surveys of areas with similar plutonism are important in the study of plutonic mineralization. Quaternary deposits are distributed along mountain fronts and wadis. Gravity lows are commonly associated with the wadis, but their low amplitude suggests alluvial thicknesses of less than 25 m. This implies that the valleys are not structurally controlled by extension and that the alluvial aquifers, from which nearly all domestic water is produced, are relatively thin. The greatest complete Bouguer anomaly is associated with basaltic lava flows located in the northeastern part of the survey area. The thickness of the basalt in outcrop does not account for the anomalies with the highest amplitudes, but the latter may be due to the presence of a basalt-filled vent. Those anomalies that are present do not define the basalt flows well, but the largest free-air anomaly occurs over the southwestern margin of the Salma Caldera, located about 15 km from the basalt flows. The source of the free-air anomaly is unknown, but it may be related to another hidden basaltic vent.