LINKING DEEP AND SHALLOW STRUCTURE: LATERAL HETEROGENEITY AND VERTICAL STRATIFICATION OF CRATONIC LITHOSPHERIC KEELS

The presentation summarizes geophysical models for Precambrian cratons, with a focus on structure and thermo-compositional heterogeneity of the lithospheric mantle as constrained by a set of different geophysical data. The latter include thermal structure of the Precambrian lithosphere based on surface heat flow data, lithosphere density heterogeneity as constrained by free-board and satellite gravity data, and non-thermal part of upper mantle seismic velocity heterogeneity based on a joint analysis of thermal and seismic tomography data. Precambrian cratons show a significant variability in the depth extent of the lithospheric keels, which locally may reach the depth of 300-350 km or more in the cratons of Laurasia. In contrast, the lithosphere of the cratons which were a part of the Gondwanaland does not presently exceed 250 km depth. This conclusion is supported by a worldwide compilation of cratonic xenolith P-T arrays. An analysis of temperature-corrected seismic velocity structure indicates strong vertical and lateral heterogeneity of the cratonic lithospheric mantle, with a pronounced stratification in many Precambrian terranes; the latter is, in particular, supported by xenolith data from the Slave craton and the Baltic Shield. Importantly, the lateral extent of depleted lithospheric keels diminishes with depth and, below a 150-200 km depth, is significantly smaller than geological boundaries of the cratons exposed at the surface. A comparison of density structure of the cratonic lithosphere, based on free-board and satellite gravity, with crustal structure and surface tectonics indicates a significant correlation between the deep, shallow, and near-surface structure of the lithosphere. The latter observation is illustrated by examples from the East European and the Siberian cratons, where Proterozoic sutures and intracratonic basins are manifested by an increase in mantle density as compared to light and strongly depleted lithospheric mantle of the Archean nuclei. Lateral and vertical heterogeneity of the cratonic lithosphere is discussed in connection to regional tectono-thermal evolution.