Geophysical constraints on a modified crust below the Rayn Anticlines, Eastern Saudi Arabia

The Gravity anomaly signature for a regional Trans-Rayn Geophysical Traverse (TRGT) across Rayn Anticlines (RA) illustrates alternating Bouguer-Anomaly (B.A.) 'highs' and 'lows', highs over the anticline axes and lows in flanking sub-basins. We demonstrate for the first time that the bipolar gravity signatures actually camouflage a more extensive B.A. low of amplitude similar to 63 mgal and wavelength similar to 750 km, developed between the Qatar and Amar Arches. 2D-gravity modeling for TRGT infers a 3-layer crustal model for RA: (i) a regionally extensive upper crustal layer similar to 10-15 km thick of lighter density (by -0.02 g/cm(3)) compared to normal upper continental crust, (ii) a mid-crustal similar to 10 km thick transition layer of intermediate density and (iii) the lower maflc crust of higher than normal density. The Moho dips westward, from 37.3 below Ghawar-Anticline (GA) to 47 and 47.6 km below the KBS-Amar Arches, with a lateral density change in the lower crust following the same direction. Crustal thinning by as much as 22% occurred below GA; the altered crust directly correlating to the basement uplift on its top and mantle uplift at its base. An inverse correlation between crustal thicknesses and Vp/Vs is observed for the entire RA, displaying highest Vp/Vs (1.93) and Poisson's ratio (0.443) for the GA crust.