Gravity data from detailed marine surveys were used to investigate the density structure of the crust and upper mantle beneath the northern Red Sea, a continental rift considered by many to be in the process of establishing a mid-ocean ridge type spreading center. A gravity and bathymetry profile across the sea near 27-degrees-N was analyzed in detail. Model constraints include single-channel and multi-channel seismic reflection lines coincident with the gravity profile, which provide detailed information on sedimentary layers, a series of ESP seismic experiments centered nearly on the gravity line which constrain the sediment and crustal thickness, and a series of 58 closely spaced heat flow measurements nearly coincident with the gravity line which closely constrain the mantle thermal structure and thus density variations within the mantle. The calculated gravity anomalies based on the known crustal and mantle structure fail to match the observed data. Specifically, the model anomalies become steadily more negative from the margins toward the center of the sea with the result that the calculated gravity is approximately 25 mGal more negative near the axis relative to the marginal areas than the observed anomalies. Two possible solutions were investigated. One is to introduce a lower crustal layer resulting from crustal underplating within what was originally considered to be the upper mantle beneath the marginal areas. The other is to introduce an area of slightly denser crust resulting from intrusion of new mafic crustal material within the axial depression. Both solutions can reproduce the observed gravity anomalies within the Red Sea. However, the complete set of geophysical data, and in particular the heat flow in the Red Sea and gravity and seismic refraction data on land, suggests that the most satisfactory model is that the crust of the axial depression is denser than that of the marginal areas. This implies that crustal extension in the northern Red Sea is presently occurring in large part by intrusion of new mafic material.
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