Erosion and rift dynamics: new thermomechanical aspects of post-rift evolution of extensional basins

In this study, we investigate thermo-mechanical consequences of erosion of rift shoulders. Conventional models imply post-rift cooling that results in subsidence and strengthening of the lithosphere. Existing models neglect geodynamic consequences of surface processes (erosion and sedimentation). According to sedimentologic and geomorphologic data, surface processes modify the topography and thickness of sedimentary infill at rates comparable with the rates of the tectonic uplift/subsidence (few 0.1 mm/y). Consequently, a coupling between the surface and tectonic processes can be expected. An increase of the sedimentary load leads to localised inelastic weakening of the lithosphere. At the same time, erosional unloading of rift shoulders leads to local strengthening and flexural rebound. Subsidence of the rift ''neck'' (strongest layer of the thinned lithosphere) and uplift of the rift shoulders create pressure gradients sufficient to drive ductile flow in the low-viscosity lower crust. This flow, directed outward from the centre of the basin might facilitate uplift of the rift shoulders. It may even drive some post-rift ''extension''. In the limiting case of slow erosion and sedimentation rates, gravitational stresses can reverse the flow, resulting in a retardation of basin subsidence rate, homogenisation of the crustal thickness, accelerated collapse of the shoulders and in some post-rift ''compression''. These effects significantly change predictions of basin evolution inferred from the conventional back-stripping models. (C) 1997 Elsevier Science B.V.