THE EFFECTS OF THE SPINEL-GARNET PHASE-TRANSITION ON THE FORMATION OFRIFTED SEDIMENTARY BASINS

We have examined the effects of the spinel-garnet phase transition on subsidence of extensional sedimentary basins. For a constant positive Clapeyron slope (dP/dT), the phase boundary moves downwards in the syn -rift and upwards in the post-rift phase. For a non-linear Clapeyron c urve (dP/dT>0 above 900 degrees C and dP/dT<0 below 900 degrees C), th eory predicts for the reaction of the spinel-garnet phase transition, the direction of:phase boundary movement is dependent on the stretchin g factor, the position of the Clapeyron curve and the lithospheric thi ckness. A smaller syn-rift and larger post-rift subsidence are predict ed for a deeper phase boundary and a thicker lithosphere. The model wi th a non-linear Clapeyron curve is applied to the subsidence histories of a, young extensional basin (Gulf of Lion) and an old continental m argin (eastern Canada). The observed syn-rift uplift and the larger po st-rift subsidence can be reasonably explained by this model, where th e optimum depth of the phase boundary for eastern Canada (similar to 9 0 km) is consistent with the estimate from seismic observations and is larger than that for the Gulf of Lion (similar to 50 km). The depth o f the spinel-garnet phase boundary is sensitive to the composition of mantle rocks and increases with the extraction of basaltic components from the lithosphere, compatible with our result that the phase bounda ry is deeper for an older and thicker lithosphere. Thus the surface mo vement associated with the rifting for these areas may reflect the che mical evolution of the continental lithosphere.