THE EFFECT OF CRUSTAL THICKNESS AND LITHOSPHERIC RHEOLOGY ON THE WIDTH AND SUBSIDENCE HISTORY OF RIFTED MARGINS

The width of the sedimentary basins in the eastern Brazilian margin and their amount of rift related magmatism generally decrease from south to north. This trend coincides with changes in geological domains onshore Brazil, from the Neoproterozoic Ribeira and Araçuaí mobile belts to the Archean Sao Francisco Craton. It seems there is a relationship between the width of the sedimentary basins and both the age of the lithosphere and the initial thickness of the crust. We use numerical experiments to test if there is a physical relationship between these factors and their effect on the evolution of rifted continental margins. Our parameter space is a range of composition, temperature structure, initial thickness and velocity of extension. Roughly summarizing our results, narrow rifts can be consequence of relatively cold geotherm and dry rheologies, where localized extension leads to rapid initial subsidence of a rigid wedge-shaped crustal block (block H) in the mantle. Thinning of block H is delayed until its base is heated up by the increased geotherm. Therefore, thinning of the lithosphere occurs from bottom to top (mantle to crust). Part of the rift basin is uplifted during crustal thinning forming shallow water environments and even subaerial erosion. Hyperextension of the crust is mostly limited to the initial block H domain and each conjugate margin is 100km or less wide. On the other hand, wide rift occurs when the upper mantle is weak and/or crust is much thicker than the brittle lithosphere. Crust-mantle coupling decreases with increasing crustal thickness because increase in ductile lower crust thickness causes deformation to delocalize. Unlike the evolution of narrow rifts, the hyperextended domain greatly exceeds the edges of the initial graben, with lateral migration of the maximum strain with time. A young lithosphere/hot geotherm also helps to widen the rift basins. Syn-rift subsidence is mostly shallow (300m depth or less), locally reaching 1-1.5km depth more than 10Myr after the onset of rifting.