GEMOC - Department of EPS - Faculty of Science

MAPPING LITHOSPHERE-SCALE STRUCTURES IN THE EASTERN SIBERIAN PLATFORM

Y.H. Poudjom Djomani¹, W.L. Griffin^1,2, L.M. Natapov¹, S.Y. O'Reilly¹ and Y. Erincheck³

1. GEMOC Macquarie

2. CSIRO Exploration and Mining

3. VSEGEI, St. Petersburg, Russia

The Siberian platform consists of several major Archean and Proterozoic basement terranes that have been mapped from regional magnetic data and basement exposures in the Anabar shield. Garnet and chromite concentrates from mantle xenoliths from a chain of Paleozoic to Mesozoic kimberlites across the platform have been used to construct mantle sections showing rock type distribution. These mantle 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 > 200 km thick, while the Proterozoic terranes are underlain by thinner and less depleted lithosphere.

Enhancement of geophysical data shows more strongly negative Bouguer anomalies and a more noisy magnetic anomaly pattern over the Archean terranes than on the Proterozoic terranes. This reflects the difference in the mantle composition beneath the terranes as revealed by mantle-petrology studies. The elevation across the platform is not more than 900 meters. Gravity and topography data have been inverted to estimate the flexural strength, or elastic thickness (Te) of the plate. The results show that although the study area is a geologically stable Precambrian craton, Te is relatively low (< 30km) across most of the area suggesting a weak lithosphere comparable to that of tectonically much younger areas around the world. These results also show a major zone, ~ 150 km wide, of very weak lithosphere (Te < 10 km) running N-S across the western part of the study area which coincides with a zone of thicker lithosphere, lower surface heat flow and thicker lower crust, as well as abnormally high sub-Moho P-wave velocities suggesting an anisotropy in the upper mantle. The kimberlite fields in the Archean part of the platform are localised on the western flank of the zone of weak lithosphere. We suggest that the low Te zone is a mantle shear zone which has been a preferred conduit for magma into the lower crust, and has controlled the emplacement of kimberlites in the study area.