A three-dimensional gravity modeling combined with integrated heat flow and elevation modeling is conducted to map out the crustal and lithospheric mantle thickness in the Alboran Basin, in the westernmost Mediterranean. A sediment-corrected Bouguer anomaly has been derived using a depth-to-the-basement map and densities determined from well logs and seismic data. The gravity effect of the base of the lithosphere has been removed from the sediment-corrected Bouguer anomaly to obtain a crustal Bouguer anomaly, which has been inverted for crustal thickness. The resulting lithospheric structure is further constrained by elevation data under the assumption of local isostasy. The low residual elevation anomalies obtained (¿100 m in average) suggest that the area is in local isostasy, particularly the medium- and long-wavelength topography features. Variations in crustal thickness range from 36 km underneath the Betic and Rif Chains to <12 km beneath the easternmost part of the Alboran Sea Basin, in the transition to the South Balearic Basin. In western Alboran the Moho lies at a rather constant depth of ~18 km, deepening sharply toward the Gibraltar Strait down to 30--32 km. The base of the lithosphere shallows from 140 km depth in the Gibraltar Strait to <45 km depth in the easternmost Alboran Sea. Lithospheric thinning penetrates to the southeastern side of the Iberian margin crosscutting topographic highs of the central and eastern Betic Chain. Our results favor mantle delamination produced by detachment and subsequent peeling away of the lithospheric mantle rather than convective removal of the lithospheric mantle either by orogenic collapse or detachment and sinking of a lithospheric slab. ¿ 2000 American Geophysical Union