Chapter 2 Mechanisms of rifting: Geodynamic modeling of continental rift systems

Abstract

This chapter discusses the theoretical studies relevant to the geodynamics of continental rifting, omitting the petrological intricacies of magma genesis and ascent. Hypotheses for the initiation of rift structures depends on the primary source of extensional stress which initiates the faulting. The passive hypothesis attributes the primary extension to plate interior deviatoric tension of distant origin, such as produced by the trench suction plate boundary force, with volcanism and vertical movement being a secondary response to lithospheric extension. The active hypothesis attributes volcanism, doming and faulting to the influence of an underlying hot upper mantle which may be generated by a plume. Finite element modeling is used to show that quite a small upper mantle hot spot gives rise to substantial deviatoric tension in the strong regions of the lithosphere. The present stress regime is applicable during the Tertiary, indicates low-lying continental regions are subject to compression rather than tension. As a consequence, the active hypothesis may be relevant to the Tertiary rift systems, whereas the passive hypothesis may have been dominant during the more tensional stress regime of the Mesozoic. Lithospheric boundinage may occur where the deformation is periodic. Thinning of the lithosphere may also take place without stretching as a result of conversion to asthenosphere; a number of mechanisms are proposed based on thermal, magmatic, and diapiric processes.