GEODYNAMICS OF RIFTING AND IMPLICATIONS FOR HYDROCARBON HABITAT

Rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub-lithospheric upper mantle, combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system, and by far-filed stresses controlling the interaction of lithospheric plates moving relative to each other. Mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting. Therefore, it is questioned whether a distinction between ''active'' and ''passive'' rifts is still justified. Lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere. Magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust. This could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data. The duration of the rifting stage of palaeorifts and passive margins is highly variable. Depending on plate interaction, crustal separation can be achieved after as little as 7 Ma and as much as 280 Ma of rifting activity. The availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved. The structural style of rifts is controlled by the physical state of the lithosphere, the mode and amount of its extension and the lithological composition of pre- and syn-rift sediments. Subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere. Post-rift basin subsidence is mainly governed by the decay of rift-induced thermal anomalies; densification of the lower crust and intra-plate stresses provide overprinting effects. In view of this it is unlikely that quantitative analyses of post-rift basin subsidence give a direct measure of lithospheric ''tension. Tectonically active rifts, palaeorifts and passive margin basins contain major hydrocarbon provinces. Their hydrocarbon charge can rely exclusively on pre-rift, syn-rift or post-rift sedimentary sequences or a combination thereof. Maturation of source-rocks can be achieved during the syn- and/or post-rift stage of basin evolution. During rifting, conductive and convective heat transfer accounts for elevated geothermal gradients; these play an important role in the maturation of pre- and syn-rift source-rocks; thermal gradients decrease asymptotically during the post-rift stage.