HypothèseTOTALFINA

Presalt Breakup Hypothesis

In this geological model, the breakup of the lithosphere occurs at the top of the rift-type basins. The breakup occurs in a thin and highly intruded continental crust when the amount of intruded mantle material is big enough to individualize two lithospheric plates separated by a rift-valley. Since then, the continental crust cannot be lengthened anymore. The extensional tectonic regime, responsible of the formation of the rift-type basins, ends. The large majority of the normal faults bordering the half-grabens of the rift-type basins are sealed by the breakup unconformity (BUU). The initial valley-rift (breakup) occurs in a continental environment. It initiates a sub-aerial accretion of the individualized lithospheric plates, which start to move away from each other (continent drifting). The mantle material arriving at surface flows outward, in opposite directions. The lava-flows thin away from the successive rift-valleys (sub-aerial expansion centers) form the sub-aerial volcanic crust, which can be considered as the volcanic sector of the margin. In the later stages of the volcanic sector of the margin, a shallow marine or lacustrine depocenters can be deposited between the valley-rift (expansion centers) and the continental crust (where rift-type basins are likely). However, with time, the weight of the stacking of lava flows push-down the sub-aerial expansion centers until they become submerged. When the expansion centers are put under the water, the mantle material arriving at surface (through sheeted dykes) cannot flow anymore. It is frozen in pillow-lavas forming the conventional oceanic crust. Such a subsidence associated with the seafloor spreading (volume reduction of the oceanic basin) creates a significant absolute (eustatic) sea level rise inducing the deposition of the transgressive phase (aggradational geometry) of the post-Pangea continental encroachment stratigraphic cycle, i.e., the trigger of marine ingression responsible for the deposition sediments of the clastic sector of the margin. The regressive phase of the post-Pangea continental encroachment stratigraphic cycle, which is characterized by a progradational geometry, is associated with a significant absolute (eustatic) sea level fall induced by the volume increasing of the oceanic basins (total water volume is keep constant) created by the gathering of the continents (B- and A-type subduction zones).