Lithospheric boundaries on the eastern Siberian platform

Abstract

The regional mapping of large lithospheric blocks can provide guidance for mineral exploration, because such large-scale structures, and particularly the boundaries between them, may control the emplacement of several types of large ore bodies. This study uses a combined analysis of potential-field geophysical data and mantle petrology, to map major lithospheric structures on the eastern part of the Siberian platform. Garnet and chromite concentrates from a chain of Paleozoic to Mesozoic kimberlites across the platform have been used to construct mantle sections. These sections reveal that the lithospheric mantle shows significant differences, corresponding to tectonic terranes mapped at the surface, and indicating that the terrane boundaries are translithospheric. Furthermore, the Archean terranes are underlain by typical depleted Archean lithosphere > 200km thick, while the Proterozoic terranes are underlain by thinner and less depleted lithosphere. We inverted gravity and topography data from the Siberian platform to determine the flexural rigidity or elastic plate thickness (Te), which is a measure of the mechanical strength of the lithosphere. The results of this inversion reveal a major zone, ~150km wide, of very weak lithosphere (Te < 10km) running N-S across the western part of the craton. This zone coincides with thicker lithosphere, lower surface heat flow and abnormally high sub-Moho P-wave velocities suggesting an anisotropy in the upper mantle. The kimberlite fields in the Archaean part of the platform are localised on the western flank of this zone of weak lithosphere. We suggest that the low Te reflects a mantle shear zone which has been a preferred conduit for fluids (eg magmas) into the lower crust, and has controlled the location of kimberlite emplacement in the area.