Crustal doming and the mechanism of rifting Part I: Rift formation

Abstract

Continental rifts are observed to be associated with crustal doming. This is well-established by both the elevation of structural boundary layers as well as that of the crustmantle boundary. Rift regions show typical fault systems corresponding to the stage of rifting. Moreover faulting is accompanied by a temporal and geographical sequence of volcanic activity. The attempt has been made here to consider the uplift of subcrustal material to be responsible for the rift formation process. For this purpose numerical calculations have been done by means of finite-element approximation of crustal dynamics. This procedure allows to combine structural features of the crust, crustal rheology, kinematics of uplift and appropriate boundary conditions in particular models. Thus the effect of the shape of prescribed subcrustal uplift on the dynamic response of crustal models has been examined for elastic as well as elasto-viscoplastic behaviour. The calculated crustal stress fields make it possible to predict regions of crustal failure and their development. On account of data of the Upper Rhinegraben rift region the imposed mechanism of crustal doming allows to trace the lines of the graben. Additionally the modelled riftforming mechanism seems to ‘explain’ the apparent sequence of volcanic activity in space and time. Hence, the numerical results lend support to the idea that the mechanism of rifting is caused by epeirogenetic upwarping of the crust. Neither a horizontal ‘pull’ nor a viscous drag seems to be required to initiate rifting.