Density Structure of the Upper Mantle in the Middle East and Surroundings: Interaction of Diverse Tectonic Processes

Abstract:

The Middle East is a very complex region combining several tectonic regimes, which are linked together. Density heterogeneity of the upper mantle, which is related to temperature and compositional variations, is one of the principal factors governing tectonic processes. Therefore, a comprehensive density model of the upper mantle is a key for understanding of these processes. Here we use seismic, gravity and tomography data to construct a 3D density model of the lithosphere and upper mantle and to identify main factors responsible for density variations. At the first stage we use a recent crustal model (Stolk et al., 2013) to estimate gravity effect of the crust and to remove it from the observed fields. As a result, the residual mantle gravity anomalies and residual topography are calculated. In addition we remove the impact of deep density variations below 325 km as estimated by a recent instantaneous dynamic model of the mantle (Kaban et al., 2014). We invert the residual fields jointly with seismic tomography data to image density distribution within the crust and upper mantle. The inversion technique accounts for the fact that the residual gravity and residual topography are controlled by the same factors but in a different way, e.g. depending on depth and wavelength. This provides a possibility for remarkably better vertical resolution of the resulting density model. As the initial approximation, we employ the seismic tomography model of Schaeffer and Lebedev (2013). Velocity variations are converted to density by applying mineral physics constrains. This model is adjusted in the inversion to fit both residual mantle gravity and topography. The obtained density variations are very significant; their amplitude somewhere exceeds 60 kg/m³ relative to a reference model. The most pronounced decrease of the mantle density corresponds to the Gulf of Aden spreading axis, the Red sea and the Afar zone. The maximum density of the upper mantle is associated with the continental collision zones, in particularly to the northeast from the Zagros fold belt and especially under South Caspian.