Lithospheric structure of the Arabian Shield and Platform from complete regional waveform modelling and surface wave group velocities

Regional seismic waveforms reveal significant differences in the structure of the Arabian Shield and the Arabian Platform. We estimate lithospheric ve locity structure by modelling regional waveforms recorded by the 1995-1997 Saudi Arabian Temporary Broadband Deployment using a grid search scheme. We employ a new method whereby we narrow the waveform modelling grid search b y first fitting the fundamental mode Love and Rayleigh wave group velocitie s. The group velocities constrain the average crustal thickness and velocit ies as well as the crustal velocity gradients. Because the group velocity f itting is computationally much faster than the synthetic seismogram calcula tion this method allows us to determine good average starting models quickl y. Waveform fits of the Pn and Sn body wave arrivals constrain the mantle v elocities. The resulting lithospheric structures indicate that the Arabian Platform has an average crustal thickness of 40 km, with relatively low cru stal velocities (average crustal P- and S-wave velocities of 6.07 and 3.50 km s(-1), respectively) without a strong velocity gradient. The Moho is sha llower (36 km) and crustal velocities are 6 per cent higher (with a velocit y increase with depth) for the Arabian Shield. Fast crustal velocities of t he Arabian Shield result from a predominantly mafic composition in the lowe r crust. Lower velocities in the Arabian Platform crust indicate a bulk fel sic composition, consistent with orogenesis of this former active margin. P - and S-wave velocities immediately below the Moho are slower in the Arabia n Shield than in the Arabian Platform (7.9 and 4.30 km s(-1), and 8.10 and 4.55 km s(-1), respectively). This indicates that the Poisson's ratios for the uppermost mantle of the Arabian Shield and Platform are 0.29 and 0.27, respectively. The lower mantle velocities and higher Poisson's ratio beneat h the Arabian Shield probably arise from a partially molten mantle associat ed with Red Sea spreading and continental volcanism, although we cannot con strain the lateral extent of a zone of partially molten mantle.