GEMOC ARC National Key Centre

PRELIMINARY RESULTS OF LITHOSPHERE MAPPING OF THE EASTERN PART OF THE SIBERIAN PLATFORM

Yvette H. Poudjom Djomani¹ , William L. Griffin^1,2, Lev Natapov¹, Suzanne Y. O'Reilly¹ and  Yuriy Erinchek³

1. GEMOC Key Centre,
2. CSIRO EM,
3. VSEGEI, Sredny pr.
4. St Petersburg 199026, Russia

The Siberian craton consists of several major Archean terranes that have been mapped from surface geology on the Anabar shield and projected under the platform cover using magnetic data. A 1000-km long kimberlite field with age ranging from Paleozoic to Mesozoic runs in a NNE trend from the centre of the craton to its northern margin. Mantle material from these kimberlites has been analysed to construct mantle sections showing translithospheric discontinuities in the stratigraphy. On this traverse, the boundary between the Archean root and the Proterozoic mantle is marked by the abrupt disappearance of harzburgitic and wehrlitic rocks.
 
In this study, we have first used gravity and topography data on 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 density models used in the geophysical modelling are constrained by rock stratigraphy derived from geochemical data. Secondly, we have processed and enhanced the available magnetic data and applied spectral analysis to estimate the depths to the magnetic source bodies and distribution of magnetisation. The gravity and magnetic results can then be integrated with petrological and geochemical data to produce a map of distribution of major lithospheric blocks related to variations in lithospheric mantle composition.

The gravity and topo inversion results show that the lithosphere on the Siberian platform is relatively weak compared to values usually observed on cratonic areas. These low values of Te may be explained by the fact that the topographic relief, which is an important parameter in the inversion program, is very low and therefore there isn't enough power in the signal to resolve the range of elastic thickness in this area. Although these values of Te are low, it is evident that the Archean domains within the kimberlite corridor are stronger than the Proterozoic areas, suggesting a cold lithospheric root beneath the southern part of the kimberlite corridor. The depth to magnetic source bodies, which could be approximated to the depth to the Curie isotherm where the rocks lose their magnetisation, are of the same order of magnitude as the elastic thickness values in each lithospheric block. Comparison of Te and the depth to the magnetic bodies suggests that the two parameters may follow the same isotherm.

Acknowledgments: This project is supported by a collaborative grant between Western Mining Corporation and Macquarie University.