A gravity survey of the Jiddah area between lat 21°23' and 21°48' N., long 39°03' and 39°20' E., carried out in early 1977, covers an area of 744 km2 and includes 215 stations, at an average spacing of 1.9 km. The resulting simple Bouguer gravity anomaly map shows a series of four large (7-15 mgal) gravity anomaly highs along a north-trending gravity gradient located immediately west of outcrops of Precambrian rocks. The regional Bouguer gravity anomaly field of the map area decreases eastward at an approximately constant rate of about 1.1 mgal-km-1. Several anomalies suggest northeast-trending, right-lateral strike-slip faulting throughout the area.
Interpretation of the Bouguer gravity anomaly map, combined with geologic, aeromagnetic, and paleomagnetic data, leads to an interpretive model in which the gravity gradient marks the boundary zone between Precambrian continental crust to the east and oceanic sea-floor crust overlain by marine sedimentary rocks to the west. The strike-slip faults in the area are probably the landward extension of offshore Red Sea transform faults, and, because they penetrate and offset continental crust, their study may yield useful information about the mechanism of transform faulting. The gravity anomaly highs are interpreted to result from shallow intrusive bodies, probably gabbroic in composition, that are buried a few hundred meters below the surface and that are approximately 2 km thick.