Modelling the loading stresses associated with active continental rift systems

Abstract

Abstract The loading stress associated with an ideal active continental rift system underlain by a hot, low density upper mantle, such as that of Baikal or East Africa, is modelled at various possible stages of evolution using elastic/viscoelastic finite element analysis. The first set of models excludes faulting. An incipient hot upper mantle with anomalously low density of only - 6 kg/m 3 between 400 and 200 km depths produces a small uplift of 300 m and deviatoric tension of up to 70 MPa, possibly sufficient to initiate a rift system. The stress is transferred upwards into the upper elastic lithosphere by a combination of excess pressure and shearing stress produced by the deep load. Much larger deviatoric tension of up to 230 MPa occurs when the hot, low density region extends upwards to the base of the lithosphere so as to produce 1.5 km isostatic uplift. The tension is further increased by an asthenospheric upwelling but a small opposing compression is caused by crustal thinning. In contrast, local loading stresses associated with simple passive rifting are small. Symmetrical faulting is incorporated in the next set of models, and a graben forms in response to the deviatoric tension. The rate of faulting is critically influenced by the underlying viscosity distribution. Faulting slightly reduces the deviatoric tension except within the graben where it is generally slightly increased. It is shown that the local loading tension can produce subsidence of a sediment loaded rift graben of 8 km or more. By constraining the distant edge of a model with a faulted or otherwise weak central rift, it is shown that a substantial rift push force can develop which may be of importance in the continental splitting mechanism. The modelling explains the occurrence of tension in the present-day uplifted rift systems (which were initiated in the Tertiary) in presence of predominant continental compression. It suggests that a hot upper mantle produced by a plume can initiate continental rifting, in association with directional plate interior stresses and basement weaknesses.