Upper mantle structure beneath eastern Siberia: Evidence from gravity modeling and mantle petrology

Abstract

The spatial distribution of large‐scale lithospheric domains and the boundaries between them may control the emplacement of large ore bodies, and as such, regional mapping of the lithosphere is relevant to mineral exploration. In this study we combine potential‐field geophysical data and mantle petrology to map major lithospheric structures on the eastern part of the Siberian platform. The platform consists of several Archean and Proterozoic terranes that have been mapped from regional magnetic data and basement exposures in the Anabar shield. We use garnet and chromite concentrates from a chain of Paleozoic to Mesozoic kimberlites across the platform to construct mantle sections, which show significant lateral variation in rock type distribution within the lithospheric mantle. These lateral variations correspond to the terranes mapped at the surface and indicate that the terrane boundaries are translithospheric. Archean terranes are underlain by depleted Archean lithosphere more than 200 km thick, while the Proterozoic terranes are underlain by thinner and less depleted lithosphere. Geophysical data show more strongly negative Bouguer anomalies and a more heterogeneous magnetic anomaly pattern over the Archean terranes than on the Proterozoic terranes. The pattern of the gravity data reflects the lateral variation in mantle composition beneath the terranes, as shown by mantle‐petrology studies. We invert gravity and topography data to estimate the flexural strength, or elastic thickness (Te), of the lithosphere across the area. Although on a stable Precambrian craton, the Te is relatively low (<30 km) across most of the area, suggesting a relatively weak lithosphere comparable to that of tectonically much younger areas around the world. A 150‐km‐wide zone of very weak lithosphere (Te < 10 km) runs N‐S across the western part of the study area. This weak zone coincides with a zone of thickened lower crust, and abnormally high sub‐Moho P wave velocities which suggest anisotropy in the upper mantle. The kimberlite fields in the Archean part of the platform are localized on the flanks of this zone of weak lithosphere. We suggest that the low‐Te zone may be a mantle shear zone which has been a preferred conduit for the emplacement of magmas into the lower crust and later has controlled the emplacement of kimberlites in the study area.