GEMOC ARC National Key Centre

Yvette Poudjom Djomani, Suzanne Y. O'Reilly, William L. Griffin and Lev Natapov
GEMOC Macquarie

Mantle-derived xenoliths and xenocrysts can be used to obtain information on the thermal state and composition of the lithosphere using techniques developed by O'Reilly and Griffin (1996) and Ryan et al. (1996). These techniques provide an estimate of the paleogeotherm, which serves as a reference for determining the depth of origin of individual mineral grains (garnet, chromite) for which temperatures are determined by trace-element thermometers (Ryan et al., 1996). The depth to the base of the chemically defined lithosphere is determined by the change from depleted (lithospheric) to undepleted (asthenospheric) trace-element signatures in garnet (Griffin and Ryan, 1995). For the geophysical analysis, we use a method based on the wavelength relationship between gravity and topography data (Forsyth, 1985, Poudjom Djomani et al., 1995) to estimate the flexural rigidity (D) of the lithosphere, or equivalently its effective elastic thickness (Te). This technique is used to map major lithospheric domains with different geophysical properties.

On the Eastern Siberian platform, mantle sections constructed from the analysis of garnet and chromite concentrates reveal that 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. The estimation of the Te of the lithosphere refines these lithospheric boundaries, and reveals a major zone, ~ 150 km wide, of very weak lithosphere (Te < 10 km) running N-S across the western part of the craton. This zone coincides with 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 this zone of weak lithosphere. We suggest that the low Te reflects a mantle shear zone which has been a preferred conduit for magma fluids (eg magmas) into the lower crust, and has controlled the location of kimberlite emplacement in the study area.

In Fennoscandia, the geophysical analysis shows a regional variation in elastic plate thickness from 8 km in relatively "young" areas, to 70 km in "older" areas. These results suggest that the lithosphere is strongest in the relatively stable Archaean Province, weaker in the regions characterised by Proterozoic crustal formation, and lowest in the tectonically reworked and deformed Caledonian belt. Furthermore, the results show that there is a direct correlation between lithosphere strength (Te), the age of the last major tectonothermal event registered in the crust and lithospheric mantle composition. These broad correlations reflect thinner and more fertile lithosphere, and higher geothermal gradients, beneath regions of progressively younger crust.

REFRENCES :

O'Reilly SY & Griffin WL 4-D lithosphere mapping: a review of the methodology with examples, Tectonophysics 262 (1996) 3-18.

Ryan CG, Griffin WL & Pearson NJ Garnet geotherm: a technique for derivation of P-T data from Cr-pyrope garnets, J. Geophys. Res. 101 (1996) 5611-5625

Griffin WL & Ryan CG Trace elements in indicator minerals: area selection and target evaluation in diamond exploration, J. Geochem. Explor. 53 (1995) 311-337.

Forsyth DW Subsurface loading and estimate of flexural rigidity of continental lithosphere, J. Geophys. Res. 90 (1985) 12,623-12,632.

Poudjom Djomani YH, Nnange JM, Diament M, Ebinger CJ & Fairhead JD, Effective elastic thickness and crustal thickness variations in West-Central Africa inferred from gravity data, J. Geophys. Res. 100 (1995) 22,047